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2D and 3D Land Seismic Data Acquisition

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					2D and 3D Land Seismic Data Acquisition and
          Seismic Data Processing



           Kiran Kumar Talagapu
                M.Sc.(Tech.) Geophysics
                Department of Geophysics
            College of Science and Technology
                    Andhra University
                 Visakhaptanam - 530003
                  Andhra Pradesh, India
                            CERTIFICATE


This is to certify that Mr. Kiran Kumar Talagapu, a final year student of M. Sc.

(Tech.) Geophysics, Department of Geophysics, Andhra University, Waltair,

Visakhapatnam has participated in the project work “2-Dimensional and 3-

Dimensional Land Seismic Data Acquisition and Seismic Data Processing”

from 10th December 2004 to 31st January 2005, at Oil and Natural Gas

Corporation (ONGC) Chennai.




                                                      ( R.V.S. MURTHY )
                                                CHIEF GEOPHYSICIST (S)
                            CERTIFICATE


This is to certify that this training project report is bonafide work of

Mr. T. Kiran Kumar, submitted in partial fulfillment of M.Sc. (Tech.) degree in

Geophysics during the final year degree course.




                                       (PROF. A. LAKSHMIPATHI RAJU)
                                         HEAD OF THE DEPARTMENT,
                                        DEPARTMENT OF GEOPHYSICS,
                                           ANDHRA UNIVERSITY,
                                              VISAKHAPATNAM.
                           Acknowledgements

Every work we do is linked directly or indirectly to many different aspects, circumstances
and people. Aspects which we try to understand, work on and come to a conclusion,
circumstances which motivate us and people who help us and guide us to achieve what we
are intend to.
        Recollecting the near past events of my training period I am deeply indebted to the
people who were responsible for the successful completion of my work.
        To begin with I am thankful to our Head of the Department Prof. A. Lakshmipathi
Raju for the initiation. He took all the pains of shuffling students and assigning the projects.
My equivocal thanks are due to the General Manager – Head Geophysical Services, Chennai,
Mr. D. Dutta who considered our request and allowed us to go through the training in this
organization.
        I am thankful to Mr. R. V. S. Murthy who has prepared a flawless schedule. It was
him who has insisted us to go through the rigorous field work and gave us an insight of what
is called “the maximum utilization of resources”.
        I express my special sense of gratitude to the party chiefs of the geophysical parties
we have visited. The immense cooperation given by them is unforgettable. Not to forget, the
party members who have been equally helpful.
        I am also grateful to Mr. C.M. Varadarajan ,DGM(GP) who prepared the schedule of
the second phase of the project (Seismic Data Processing). My sincere thanks are due to Mr.
Kailash Prasad and his group members for guiding me all through the different dimensions of
the “processing” aspects. They are pin point right when they say, “Information is Power”.
This was the punchline which motivated me towards going through the excellent library
facility of ONGC.
        My heartfelt thanks are due to my other project colleagues also as they had been great
all through the period of project work.
        Last but not the least, I am thankful to my beloved parents and my brother. The fact
that I am a part of my family forms firm good reason to be thankful.

                                                                    –   Kiran Kumar Talagapu
                                        Preface
       This is a project report submitted to the Department of Geophysics, Andhra
University in partial fulfillment of the M. Sc. (Tech.) degree in Geophysics. The project work
forms a paper which is evaluated for a maximum of 100 marks as a part of the academic
curriculum.
       Under this programme, the students of M. Sc. (Tech.) Geophysics undergo training at
premier organizations like, Oil and Natural Gas Corporation (ONGC), National Geophysical
Research Institute (NGRI), National Institute of Oceanography (NIO), Indian Institute of
Geo-Magnetism (IIGM), etc., engaged in geophysical activities. Under the above program, I
had training at Oil and Natural Gas Corporation (ONGC) Chennai, CMDA Towers, Egmore,
Chennai, from 10th December, 2004 to 31st January, 2005.
       During my training at ONGC Chennai, I had associated with the 2D and 3D Land
Seismic Data Acquisition undertaken by three of the geophysical parties (identity not
revealed) exploring Oil and Natural Gas in the operational areas of the Krishna-Godavari
basin. One of the three field parties GP ‘X’ is acquiring 2D land seismic data in West
Godavari Sub-basin and. The second and third parties (GP ‘Y’ and GP ‘Z’) are deployed in
the East-Godavari sub basin of the KG basin, acquiring 3D land seismic data. I have been
associated with all these parties on a tentative schedule of 10 days (from 13th to 22nd of
December, 2004) with GP ‘X’, 10 days (from 23rd to 31st of December, 2004) with GP ‘Y’
and 6 days (from 1st to 6th January, 2005) with the GP ‘Z’.
       Further I had training in Data Processing at Regional Computer Center, ONGC
Chennai, from 10th to 31st of January, 2005 thus completing the full schedule of 7 weeks
training. All through these 7 weeks, I was exposed to many aspects of the seismic data
acquisition and processing. With first hand information on certain technical terms, I was
taken through the deeper aspects of seismic survey designing, parameters considerations
based on the previous data, experimental surveys for finalizing the parameters from stages of
the production work, uphole survey technique, regular survey and through various steps of
processing like De-convolution, Stacking, Migration etc.
       This report consists of all these aspects in a brief. Starting from the Introduction to
the Seismic Methods in Chapter 1, the Next chapter deals with the Modern Seismic Data
Acquisition. In Fundamentals of Seismic Prospecting (Chapter 3), I tried to give a brief
introduction about the basics of the different types of waves. Chapter 4 is about the
Reflection Field Equipment where I described about the seismic sources, seismic receivers
and the instrumentation. Chapters 5 and 6 deal with the 2D Survey Designing and 3D Survey
Designing respectively.
       Reflection Field Layouts are discussed in chapter 7 while chapter 8 focuses on the
Reflection Field Method for the land survey. In this I have included the field parameters that
were assumed in the field with a brief theoretical background of each parameter. The end part
of this chapter deals with the starting of the production work with parameters decided based
on the experimental studies.
       The next chapter (chapter 9) deals with some of the basic concepts of Seismic Data
Processing. The subsequent chapters (chapter 10, 11, 12 and 13) deal with some of the
important aspects of the seismic data Processing in detail. In these I have given a detailed
description of the various stages of data processing with the results shown in the form of
seismic records. At the end of the report there is an appendix touching some of the important
aspects which could not be explained in the due course of the chapters.


                                                                    – Kiran Kumar Talagapu
                                    Contents
Chapter 1    Introduction
1.1   Introduction
1.2   Historical Perspective
      1.2.1 Milestones in Seismic Industry

Chapter 2    Modern Seismic Data Acquisition
2.1   Land Data Acquisition
2.2   Marine Data Acquisition
2.3   Transition – Zone Recording

Chapter 3    Fundamentals of Seismic Prospecting
3.1   Seismic Wave Fundamentals
      3.1.1 Compressional Waves (P-waves)
      3.1.2 Shear Waves (S-waves)
      3.1.3 Air Wave
      3.1.4 Mode-Converted Waves
      3.1.5 Rayleigh Waves
      3.1.6 Love Waves
      3.1.7 Direct and Head Waves
      3.1.8 Ground Waves
3.2   Characteristics of Seismic Events
      3.2.1 Reflections
      3.2.2 Critical Reflection
      3.2.3 Refractions
      3.2.4 Diffractions
      3.2.5 Multiples
3.3   Seismic Noise
      3.3.1 Coherent noise
      3.3.2 Incoherent noise

Chapter 4    Reflection Field Equipment
4.1   Seismic Sources
      4.1.1 Land Energy sources
      4.1.2 Explosive Sources
             4.1.2.1 Charge Size
             4.1.2.2 Charge Depth
      4.1.3 Vibrators
      4.1.4 Other sources
4.2   Seismic Receivers
      4.2.1 Geophones
             4.2.1.1 Electrical Characteristics
      4.2.2 Hydrophones
      4.2.3 Dual Sensors
4.3   Seismic Instrumentation
      4.3.1 Basic components
             4.3.1.1 Roll-along switch
             4.3.1.2 Pre-amplifier
             4.3.1.3 Multiplexer
             4.3.1.4 Main Amplifier
             4.3.1.5 A/D Converter
             4.3.1.6 Gain Controller
             4.3.1.7 Formatter
             4.3.1.8 Tape Drive
      4.3.2 Telemetry System
      4.3.3 Storage

Chapter 5    Basics of 2D Survey Design
5.1   Basic Concepts in 2D Surveys

Chapter 6    Basics of 3D Survey Design
6.1   Why 3D Seismic Survey?
6.2   Basic Concepts in 3D Surveys
      6.2.1 Preliminary Parameters
6.3   3D Survey Design Sequence
6.4   Land 3-D Layouts

Chapter 7    Reflection Field Layouts
7.1   Split-Dip and Common Midpoint Recording
7.2   Spread Types
7.3   Arrays
7.4   Resolution

Chapter 8    Reflection Field Method for Land Survey
8.1   The Seismic Field Party
8.2   Seismic Data Acquisition
8.3   2D Survey Parameters (Before the Experimental Survey) (GP ‘X’)
8.4   3D Survey Parameters (Before the Experimental Survey)
8.5   Experimental Survey
      8.5.1 Uphole Survey (Depth Optimization)
       8.5.2 Noise Experiment (Determination of NTO and Array Length)
       8.5.3 Fold Back Experiment (Element Spacing Determination )
       8.5.4 Shot Depth and Charge Size Optimization
8.6    2D Survey Parameters (After the Experimental Survey) (GP ‘X’)
8.7    3D Survey Parameters (After the Experimental Survey)
       (GP ‘Y’ and GP ‘Z’)

Chapter 9     Seismic Data Processing
9.1    Introduction
9.2    Why Processing?
9.3    Seismic Data Processing
9.4    Objectives of Data Processing
9.5    Basic Data Processing Sequence

Chapter 10 Seismic Data Processing Stage I
                 (Pre-Processing)
10.1  Preprocessing
      10.1.1 De-Multiplexing
      10.1.2 Reformatting
      10.1.3 Re-sampling
      10.1.4 Editing
      10.1.5 Geometry Merging (Labeling)
      10.1.6 Static Corrections
      10.1.7 Amplitude Recovery (Geometric Spreading Correction)
      10.1.8 Muting
10.2 Sorting
10.3 Filtering

Chapter 11 Seismic Data Processing Stage II
                     (De-convolution)

11.1   Introduction:
11.2   Convolutional Model
11.3   De-convolution
11.4   De-convolution Methods
       11.4.1 Deterministic De-convolution
       11.4.2 Statistical De-convolution

Chapter 12 Seismic Data Processing Stage III
           (Velocity Analysis, NMO, DMO
           and Residual Static Corrections)

12.1   Velocity Analysis
12.2   Normal Moveout Correctons (NMO)
12.3   Dip Moveout Correctons (DMO)
12.4   Residual Statics Corrections

Chapter 13 Seismic Data Processing Stage IV
           (Stacking, Time Variant Filtering
                      and Migration)
13.1 Stacking
13.2 Time Variant Filtering
13.3 Migration

Appendix

Bibliography
                      List of Figures
Figure 1(a)    P – Wave Motion.
Figure 1(b)    S – Wave Motion.
Figure 2(a)    Surface Wave Motion.
Figure 2(b)    The particle motion in the wave front of a Rayleigh wave
               consists of a combination of P – Wave and SV – vibrations in
               the vertical plane. The particles move in retrograde sense
               around an ellipse that has its major axis vertical and minor
               axis in the direction of wave propagation.
Figure 2(c)    In a Love wave the particle motion is horizontal and
               perpendicular to direction of propagation. The amplitude of
               the wave decreases with depth below the free surface.
Figure 3       Reflection of plane compress ional wave at interface.
Figure 4       Refraction of plane compress ional wave across interface.
Figure 5       Diffraction from the edge. The source a of diffracted radiation
               has been set into oscillation by waves generated on surface.
               Radial lines with arrows are ray paths; circular arcs are
               wave-fronts.
Figure 6       Internal structure of a moving magnet geophone.
Figure 7(a)    Surface Geometry and Sub surface Nature and Behavior in
2D
                  Layout.
Figure 7(b)    Fresnel Zone.
Figure 8(a)    3D Layout.
Figure 8(b)    3D Cable Configuration used by GP “Y”.
Figure 8(c)    Shows the generalized stratigraphy of KG Basin.
Figure 9       Field Geometry and the Structure of the Bore Hole dug for
               doing the UPHOLE Survey (at GP X and GP Y).
Figure 10      Field Geometry and the Structure of the Bore Hole dug for
               doing the UPHOLE Survey (at GP Z).
Figure 11      Shows a field record as obtained by the uphole survey team
               of GP ‘X’ for a source (1m of detonating cord) at a depth of
               60m into a spread of geophones. The first four geophones
               are at an offset of 1m, 3m, 5m, 25m, and 50m.
Figure 12      Geometric Correction for the Uphole Data.
Figure 13(a)   t-d plot drawn for the data obtained by the first three
               receivers of uphole A of GP ‘X’.
Figure 13(b)   t-d plot drawn for the data obtained by the last two receivers
               of uphole A of GP ‘X’.
Figure 14      Noise Spread (Transposed Spread), G1, G2, … G108
               represent the Geophones.
Figure 15(a)   Noise Section for Shot point 1 at GP ‘X’.
Figure 15(b)   Noise Section for Shot point 2 at GP ‘X’.
Figure 15(c)   Noise Section for Shot point 3 at GP ‘X’.
Figure 15(d)   Noise Section for Shot point 4 at GP ‘X’.
Figure 16      Noise Section prepared by GP ‘Y’.
Figure 17      Layout for the Fold Back Experiment, G1, G2, … G216
               represent the Geophone strings (12 Geophones per string).
Figure 18      Seismic Section obtained by conducting Fold Back
               Experiment by GP “X”.
Figure 19(a)   Seismic Section obtained by conducting Fold Back
               Experiment, without the application of filter, by GP “Y”.
Figure 19(b)   Seismic Section obtained by conducting Fold Back
               Experiment, after the application of filter, by GP “Y”.
Figure 20(a)   1st Shot gather obtained during the regular production work
               by GP ‘X’
Figure 20(b)   2nd Shot gather obtained during the regular production work
               by GP ‘X’.
Figure 21(a)   Seismic data volume in processing coordinates – midpoint,
               offset and time.
Figure 21(b)   Conventional processing flowchart.
Figure 22(a)   Raw Field record in SEG – D format.
Figure 22(b)   Raw Field record in SEG – Y format.
Figure 23(a)   Editing (Raw Field Record).
Figure 23(b)   Record Obtained after Editing.
Figure 24      Seismic Data Merging.
Figure 25      Amplitude Decay with time/depth.
Figure 26(a)   Uncorrected Record.
Figure 26(b)   Seismic Record obtained after doing the Spherical
               Divergence Correction.
Figure 26(c)   Seismic Record obtained after doing the Amplitude
               Correction.
Figure 26(d)   Seismic Record obtained after doing the Amplitude
               Correction and applying filter.
Figure 27(a)   Time – Variant Filtering (Record without the application of
               filter).
Figure 27(b)   Time – Variant Filtering (Application of High Pass Filter – 8-
               16Hz).
Figure 28(a)   Spiking Deconvolution.
Figure 28(b)   Specturm of the Raw Data and the Decon Data.
Figure 29      Velocity Analysis.
Figure 30      Selection of Velocity Function.
Figure 31      NMO Stack.
Figure 32(a)   Surface – consistent statics model to establish the travel
               time model equation.
Figure 32(b)   Picking travel time deviations from NMO corrected gathers.
Figure 33      Decon and Residual Stack.
Figure 34      Brute Stack.
Figure 35      Final Stack.
Figure 36      Geometrical representation of Migration.
Figure 37      Migration Stack.
Figure 38      Aerial Network Principle (Master Slave Relation).
Figure 39(a)   A Hypothetical stacking chart – Each dot represents a single
               trace with the time axis perpendicular to the plane of the
               page.
Figure 39(b)   Different types of Gathers.
                                       Chapter 1
                                  Introduction
1.1       Introduction

The seismic method hasnear-surface geology for engineering studies, and coal and mineral
    a. Delineation of
                       three important/principal applications


             exploration within a depth of up to 1km: the seismic method applied to the near –
             surface studies is known as engineering seismology.
          b. Hydrocarbon exploration and development within a depth of up to 10 km: seismic
             method applied to the exploration and development of oil and gas fields is known
             as exploration seismology.
          c. Investigation of the earth’s crustal structure within a depth of up to 100 km: the
             seismic method applies to the crustal and earthquake studies is known as
             earthquake seismology.
Definition by Robert E. Sheriff: Seismic survey is a program for mapping geologic
structure by observation of seismic waves, especially by creating seismic waves with
artificial sources and observing the arrival time of the waves reflected from acoustic-
impedance contrasts or refracted through high velocity members.
1.2       Historical Perspective
      •   A.D. 100 – The earliest known seismic instrument, called the seismoscope, was
          produced in China to indicate the direction form which the tremor came during an
          earthquake motion.
      •   1848 – In France, Mallet began studying the Earth’s crust by using Acoustic waves.
          This science developed into earthquake seismology, solid earth or crustal geophysics.
      •   1914 – In Germany, Mintrop devised the first seismograph, it was used for locating
          enemy artillery during World War I.
      •   1917 – In the United States, Fessendon patented a method and apparatus for locating
          ore bodies.
      •   1920 – The introduction of “refraction methods” for locating salt domes in the Gulf
          Coast region of the United States began.
    •   1923 – A German seismic service company known as Seismos went international (to
        Mexico and Texas) using the refraction method to locate oil traps.
1.2.1 Milestones in Seismic Industry
        As the search for oil moved to deeper targets, the technique of using reflected seismic
waves, known as the “seismic reflection method”, became more popular during World War
II, because it aided delineation of other structural features apart from simple salt domes.
        During 1960’s the so-called digital revolution ushered in what some historians now
are calling the Information Age. This had a tremendous impact on the seismic exploration
industry. The ability to record digitized seismic data on magnetic tape, then process that data
in a computer, not only greatly improved the productivity of seismic crews but also greatly
improved the fidelity with which the processed data imaged earth structure. Modern Seismic
Data Acquisition could not have evolved without the digital computer.
        The late 1970’s saw the development of the 3D seismic survey, in which the data
imaged not just a vertical cross-section of earth but an entire volume of earth. The technology
improved during the 1980’s, leading to more accurate and realistic imaging of earth. In
1990’s depth section preparation got focused from the prevailing time section preparation
after processing the data. In 2000’s data is being acquired with an additional parameter of
“time” as the 4th dimension of the existing 3D data acquisition system. This is called 4D data
acquisition.
        As the seismic industry made one breakthrough after another during its history, it also
created new challenges for itself. Now we record not just p-waves but also converted s-waves
for a wide range of objectives. Using the multi-component seismic method, commonly
known as the 4-C seismic method, we are now able to see through gas plumes caused by the
reservoir below. We are able to sometimes better image the sub-salt and sub-basalt targets
with the 4C seismic method. Using the converted s-waves, we are able to detect the oil-water
contact, and the top or base of the reservoir unit that we sometimes could not delineate using
only p-waves.
                                     Chapter 2
      Modern Seismic Data Acquisition

   ubsurface geologic structures containing hydrocarbons are found beneath either land
S sea. So there is a land data-acquisition method and a marine data-acquisition method. The
or
two methods have a common-goal, imaging the earth. But because the environments differ,
so each required unique technology and terminology.


2.1    Land Data Acquisition: In land acquisition, a shot is fired (i.e., energy is
transmitted) and reflections from the boundaries of various Lithological units within the sub-
surface are recorded at a number of fixed receiver stations on the surface. These geophone
stations are usually in-line although the shot source may not be. When the source is in-line
with the receivers – at either end of the receiver line or positioned in the middle of the
receiver line – a two-dimensional (2D) profile through the earth is generated. If the source
moves around the receiver line causing reflections to be recorded form points out of the plane
of the in line profile, then a three-dimensional (3D) image is possible (the third dimension
being distance, orthogonal to the in-line receiver-line). The majority of land survey effort is
expended in moving the line equipment along and / or across farm fields or through
populated communities. Hence, land operations often are conducted only during daylight thus
making it a slow process.


2.2    Marine Data Acquisition: In a marine operation, a ship tows one or more
energy sources fastened parallel with one or more towed seismic receiver lines. In this case,
the receiver lines take the form of cable called Steamer containing a number of hydrophones.
The vessel moves along and fires a shot, with reflections recorded by the streamers. If a
single streamer and a single source are used, a single seismic profile may be recorded in like
manner to the land operation. If a number of parallel sources and/or streamers are towed at
the same time, the result is a number of parallel lines recorded at the same time. If many
closely spaced parallel lines are recorded, a 3D data volume is recorded. More than one
vessel may be employed to acquire data on 24-hour basis, since there is no need to curtail
operations in nights.


2.3     Transition – Zone Recording: Because ships are limited by the water depth in
which they safely can conduct operations, and because land operations must terminate when
the source approaches the water edge, or shore lines, transition-zone recording techniques
have been developed to provide a continuous seismic coverage required over the land and
then into the sea. Geophones that can be placed on the sea bed or used with both marine and
land shots fired into them.
        Techniques have been developed to use both Geophones and hydrophones in the
surface area where the shore line / water edge is likely to migrate towards land and sea
depending on the tide of sea a day. The combination of such hydrophone / geophones is
called a “Dual Sensor”. The advantage of why this is to see that either of the receiver of Dual
Sensor pickups the surveyed from the slots recorded using a land or marine source and data
gaps all along the coast within the area of prospect.
        In this report, though the principle of all sorts of seismic operations like land, marine
and TZ are discussed, the ultimate emphasis is given on the land acquisition only as the
training has been in this regard.
                                      Chapter 3
Fundamentals of Seismic Prospecting

3.1      Seismic Wave Fundamentals


                                      earth can be          by assuming
T he transmission of energy ainto the The Earth’s explainedconsidered as that the Earthelastic
has the elastic properties of solid.               crust is              completely
(except in the immediate vicinity of the shot), and hence the name given to this type of
acoustic wave transmission is elastic wave propagation. Several kinds of wave phenomenon
can occur in an elastic solid. They are classified according to how the particles that make up
the solid move as the wave travels through the material.


3.1.1 Compress ional Waves (P-waves): On firing an energy source, a compress
ional force causes an initial volume decrease of the medium upon which the force acts. The
elastic character of rock then caused an immediate rebound or expansion, followed by a
dilation force as shown in figure 1(a). This response of the medium constitutes a primary
“compress ional wave” or P-wave. Particle motion in a P-wave is in the direction of wave
propagation. The P-wave velocity is a function of the rigidity and density of the medium. In
dense rock, it can vary from 2500 to 7000 m/sec, while in spongy sand, form 300 to 500
m/sec.


3.1.2 Shear Waves (S-waves): Shear strain occurs when a sideways force is exerted
on a medium;(figure 1(b)) a shear wave may be generated that travels perpendicularly to the
direction of the applied force. Particle motion of a shear wave is at right angles to the
direction of propagation. A shear wave’s velocity is a function of the resistance to shear
stress of the material through which the wave is traveling and if often approximately half of
the material’s compress ional wave velocity. In liquids such as water, there is no shear wave
possible because shear stress and strain cannot occur in liquids.
3.1.3 Air Wave: On land the energy source (shot) generates an airwave known as the air
blast, which itself can set up an air-coupled wave, a secondary wave-front in the surface
layer. This wave generally travels by about 350 m/sec velocity slower, than the compress
ional wave, the speed of the airwave depends mainly on temperature and humidity, as shown
below:
                V = 1051 + 1.1F ft/sec where F = Fahrenheit temperature
                V = 331.5 + 0.607C m/sec      where C = Celsius temperature.


3.1.4 Mode-Converted Waves: Each time a wave impinges on a boundary, a portion
of the energy is reflected and the remaining transmitted. Depending upon the elastic
properties of the boundary, the incident P-wave or S-wave may convert to one or the other or
to a proportion of each. Such converted waves sometimes, degrade the signal-to-noise ratio.
This degradation causes problems during data processing.


3.1.5 Raleigh Waves: It is a type of seismic surface wave propagated along the free
surface of a semi-infinite medium. Figure 2(a) shows surface wave motion.This medium is
known as weathering layer or low-velocity layer (LVL). Raleigh waves are of low frequency
nature, traveling horizontally with retrograde elliptical motion and away from the energy
source (shot). The particle motion of this wave reduces (amplitude) with increase in depth,
eventually reversing in direction. This point is in the vicinity of the base of the weathering
layer. Because the motion of the ground appears to roll, this wave is commonly known as
ground roll (figure 2(b)).


3.1.6 Love Waves: The Love wave (figure 2(c)) is a surface wave borne within the
LVL, which has horizontal motion perpendicular to the direction of propagation with,
theoretically no vertical motion. Also known as the horizontal SH-wave, Q-wave, Lq-wave
or G-wave in crustal studies, such waves often propagate by multiple reflection within the
LVL, dependent upon the LVL material. If such waves undergo mode conversion, a number
of noise trains appear across the seismic record, obscuring reflected energy content even
further.


3.1.7 Direct and Head Waves: The expanding energy wave front that moves along
the air-surface interface outward form shot commonly is observed as the direct wave and has
the velocity of the surface layer through which it travels. Head waves are the portions of the
initial wavefront that are transmitted down to the base of the weathering layer or the water
bottom and are refracted along the weathering base. They return to the surface as refracted
energy or refractions. Sometimes the refracted velocity is higher than the velocity of
propagation in the surface layer. In that case, refracted head waves appear in the mid-to-far-
offset traces before arrival of the direct wave.


3.1.8 Ground Waves: When a layer of the Earth has an extreme density or velocity
contrast at both its upper and lower boundaries, a wave traveling along the layer may undergo
internal reflection (i.e., stay within the layer, reflecting from upper interface to lower, back
up again, and so on). Such waves are called guided waves and exhibit mainly vertical particle
motion. They appear as short shingled waves, repeating on the shot record.


3.2      Characteristics of Seismic Events


Seismic wave created by an explosive source emanate outward from the shot point in a 3D
sense. Huygen’s principle is commonly used to explain the response of the wave. Every point
on an expanding wave front can be considered as the source point of a secondary wave front.
The envelope of the secondary wave fronts produces the primary wave fronts after a small
time increment. The trajectory of a point moving outward is known in optics as a ray, and
hence in seismics as a raypath.


3.2.1 Reflections: The phenomenon in which the energy or wave from a seismic source
has been returned from an interface having acoustic impedance contrast (reflector) or series
of contrasts within the earth is called reflection. This phenomenon is pictorially represented
in figure 3. The amplitude and polarity of reflections depend on the acoustic properties of the
material on both sides of discontinuity. Acoustic impedance is the product of density and
velocity. The relationship among incident amplitude Ai, reflected amplitude Ar, and reflection
coefficient Rc, is:



                                                             ,
where,
Where velocity is constant, a density contrast will cause a reflection and vice versa. In other
words, any abrupt change in acoustic impedance causes a reflection to occur.
        Energy not reflected is transmitted. With a large Rc, less transmission occurs and,
hence signal-to-noise ratio reduces below such an interface.


3.2.2 Critical Reflection: When an impinging wave arrives at such an angle of
incidence that energy travels horizontally along the interface at the velocity of the second
medium, then critical reflection occurs. The incident angle ic, at which critical reflection
occurs can be found using Snell’s Law.




3.2.3 Refractions: The change in direction of a seismic ray upon passing into a medium
with a different velocity, is called refraction. Snell’s law describes how waves refract. It
states that the sine of the incident angle of a ray, (sin i), divided by the initial medium
velocity V1 equals the sine of the refracted angle of a ray (sin r), divided by the lower
medium velocity V2, that is:




when a wave encounters an abrupt change in elastic properties, part of the energy is reflected,
and part is transmitted or refracted (figure 4) with a change in the direction of propagation
occurring at the interface.
3.2.4 Diffractions: Diffractions (figure 5) occur at sharp discontinuities, such as at the
edge of a bed, fault, or geologic pillow. When the wave front arrives at the edge, a portion of
the energy travels through into the higher velocity region, but much of it is reflected. The
reflected wave front arrives at the receivers get aligned along the trajectory of a parabola on
the seismic record.
        In conventional in-line recording, diffractions may arrive from out of the plane of the
seismic line / profile. Such diffractions are considered as noise and reduce the signal-to-noise
ratio. However, in 3D recording, in which specialized data processing techniques are used
(i.e.,, the 3D seismic migration), the diffractions are considered as useful scattered energy
because the data-processing routines transfer the diffracted energy back to the point from
which is generated, thereby enhancing the subsurface image. Hence in 3D surveys, out-of-the
plane diffractions events are considered part of the signal.
3.2.5 Multiples: Seismic energy that has been reflected more than once is called multiple
while virtually all seismic energy involves some multiples. The important distinction between
long-path and short-path multiples is that a long-path multiple arrives as a distinct event
whereas a short-path multiple arrives soon after the primary and changes the wave shape.
3.3     Seismic Noise
        The reliability of seismic mapping is strongly dependent on the quality of the records
/ data. We use the term “signal” to denote any event on the seismic record from which we
wish to obtain information. Everything else is “noise”, including coherent events that
interfere with the observation and measurement of signals.
        The signal-to-noise ratio (S/N), is the ratio of the signal energy in a specified portion
of the record to the total noise energy in the same portion. Poor records result whenever the
signal-to-noise ratio is small. Seismic noise may be either
                a) Coherent or
                b) Incoherent
Another important distinction is between
                a) noise that is repeatable and
                b) noise that is non repeatable.
The properties – coherence, travel direction and repeatability – form the basis of most
methods of improving record quality.
3.3.1 Coherent noise includes surface waves, reflections or reflected refractions from
near-surface structures such as fault planes or buried stream channels, refractions carried by
high-velocity stringers, noise caused by vehicular traffic or farm tractors, multiples and so
forth. All the preceding except multiples travel essentially horizontally and all except
vehicular noise are repeatable on successive shots. Coherent noise is sometimes subdivided
into:
                a) energy that travels essentially horizontally and
                b) energy that reaches the spread more or less vertically
3.3.2 Incoherent noise is often referred to as random noise (spatially random), which
implies not only non-predictability but also certain statistical properties.
Incoherent noise is due to scattering from near-surface irregularities and in homogeneities
such as boulders, small-scale faulting, and so froth. Non repeatable random noise may be due
to wind shaking a geophone or causing the roots of trees to move, which generates seismic
waves, stones ejected by the shot and falling back on the earth near a geophone, ocean waves
beating on a seashore, distant earthquakes, a person walking near a geophone, and so on.
                                        Chapter 4
               Reflection Field Equipment

4.1       Seismic Sources


      eismic sources can be broadly divided into two categories: land energy sources and
S
marine energy sources.


4.1.1 Land Energy sources: The choice of energy source is critical in land data
acquisition because resolution and signal-to-noise ratio quality are limited by the source
characteristics. A geophysicist should select a source based on the following five criteria:
      •   Penetration to the required depth: Knowing what the exploration objectives are, the
          geophysicist should select a source that has adequate energy to illuminate the target
          horizons. Past experience can help here.
      •   Bandwidth for the require resolution: If high resolution reflections are required to
          delineate subtle geological features such as a stratigraphic traps, the source must
          transmit a broad range of frequencies, both high and low. For very shallow targets, a
          detonator may possess adequate energy and frequency bandwidth. For deeper
          reflections, the longer travel path to a deep reflector requires the selection of a source
          that has enough energy at the higher frequencies to maintain a broad reflection
          bandwidth.
      •   Signal-to-noise- characteristics: Different areas have different noise problems. They
          may dictate the source selection.
      •   Environment: When working in populated areas, there are special safety requirements
          to which geophysicists must adhere.
      •   Availability and Cost: The time of arrival of a crew can be extremely important.


Land Energy Sources are of two types: Explosive sources and Non Explosive sources.
4.1.2 Explosive Sources: Explosive sources produce robust P-waves. The selection of
explosives as the sources of choice depends primarily on near-surface conditions and the
accessibility of other energy sources. If drilling is fast and efficient, single shot hole filled
with explosives might be the most economical source option. The explosive source consists
of a detonator and an explosive charge. In the seismic industry, the explosive charge is
commonly referred to as ‘powder’ and the detonators are referred to as ‘caps’ or ‘primers’.
4.1.2.1 Charge Size: The choice of charge size depends largely on the depth to the
horizon of interest. The best charge size is that which achieved the maximum signal-to-noise
ratio (S/N) at the target depth. Deeper targets usually require larger charge sizes. Generally,
larger charge sizes cause more ground roll and air blast contamination of the record.
Alternatively, smaller charge sizes mean higher frequency content, but less energy going into
the ground. De convolution enhances the frequency content such that the bandwidth will be
higher and have an improved S/N ratio compared to a record with a smaller charge size.
4.1.2.2 Charge Depth: The charge depth depends on the depth of the weathering layer
and the level of noise interference one encounters when testing. Generally, the shallower the
source, the stronger the air-blast and the ground-roll. On the other hand, it is usually not
economical to go much beyond 50m depth. If the drilling is really tough and expensive, one
may have to limit the shot hole depth to as little as 2m or a surface shot may be used instead.
4.1.3 Vibrators: Vertical vibrators produce and asymmetric radiation pattern of P-waves
and S-waves. Horizontal vibratos produce weak P-waves and robust S-waves. If multiple
dynamite patterns do not pump enough energy into the ground, vibrators may be preferred on
technical grounds, regardless of relative cost. Vibrators are designed in two basic groups:
        Buggy-mounted and truck-mounted units.
4.1.4 Other sources: Although dynamite and Vibroseis are used in majority of surveys,
other sources can be and are used in the field 3D surveys, such as:
            •   Airguns and mud guns (used in transition zone surveys)
            •   Shotgun (Betsy)
            •   Mini-Seis (Thumper)
            •   Land air gun
            •   Dinoseis
            •   Elastic wave generator (EWG)
            •   Mini-vibes
4.2     Seismic Receivers
4.2.1 Geophones: Conventional geophones are based on Faraday’s law of
electromagnetic induction. This law states that relative motion of a conductor through a
magnetic field induces an electromagnetic force (EMF) which causes a current to flow
through the conductor, if the conductor is an element of an electrical circuit. The two types of
geophones widely used in geophysical surveys are
        1. moving coil geophone and 2. moving magnet geophone(figure 6)
The essential ingredients to make a geophone are a permanent magnet, a conductor and a
spring which positions either the conductor in the magnetic field space (in moving coil
geophone) or the permanent magnet in the electric field space (as in moving magnet
geophone). The conductor in reality is a length of copper wire wrapped into a cylindrical coil
shape. It is often referred to as the coil or element.
        The conductor’s or the magnet’s motion through the magnetic/electrical field,
according to Faraday’s law, causes an EMF to be induced that is proportional to the velocity
of the earth’s motion. Hence, such a geophone is called a velocity phone because its output is
proportional to the velocity of the earth’s motion.
        The large amount of subsurface information carried by seismic signal would be fully
available for interpretation only if the geophones follow ground movement faithfully with
minimum distortion.


4.2.1.1 Electrical Characteristics:
    •   Sensitivity: Geophones are available with a wide range of sensitivities. For
        example, at one end of the sensitivity scale, a geophone can produce 0.1V output for
        a 2.5cm/sec velocity, while another geophone can produce as much as 0.4mV output
        for a tiny movement of 2.5 X 10 m/sec.
    •   Tolerances: Geophones have typical tolerances. That are as follows:
            o Natural frequency within + 0.5Hz. of the manufacturer stated value
            o Natural frequency distortion with a maximum 20 tilt, + 0.1Hz.
            o Sensitivity within + 5% of the manufacturer stated value.
        A large variety of modern geophones are available today to meet the specific
requirement of the user. Close tolerance digital grade geophones have distortions as low as
0.03%, tolerance of 2-2.5% on frequency, sensitivity and damping, and very high geophone-
to-geophone uniformity. To modern geophones has done away with the shunt resistance,
resulting in very low distortion and high spurious response up to 250Hz. These geophones
maintain their natural frequency specifications with high tilt angles.
         The particular receiver type depends on the characteristics of the data to be recorded
and the environment where the data acquired. In normal land operations, geophones have a
resonant frequency of 10 or 14Hz., but in some parts of the world it is still normal practice to
use 6 to 8Hz phones. However, geophones with resonant frequencies up to 40Hz are being
manufactured.
         Receivers are usually wired in groups of 1, 4, 6, 9, 12 or 24. While the trend is
towards higher number of phones (9, 12, 24 on even 72 in the Middle East), however
numbers (e.g., 6) are still used in certain areas, e.g., South America. In hilly, terrain, where
the height difference between the ends of any receiver group exceeds 2m, geophones may be
clustered in a small area. In steep terrain (over 5m. elevation difference) one can spread the
phones out parallel to topographic contours to minimize inter-array statics smear. Three-
component 3D recording requires three times the number channels of recording capacity
since each component is recorded separately. This increased number of channels may make it
difficult to create a patch that creates sufficient fold. Since shear-wave reflections contain a
lower frequency bandwidth, phones with lower resonant/natural frequencies are used.


4.2.2 Hydrophones: The hydrophone is an electro acoustic transducer that converts a
pressure pulse into an electrical signal by means of the piezoelectric effect. If mechanical
stress is applied on tow opposite faces of a piezoelectric crystal, then electrical charges
appear on some other pair of faces. If such a crystal is placed in an environment experiencing
changes in pressure, it will produce a voltage proportional to that variations in pressure.


4.2.3 Dual Sensors: For ocean bottom cable (OBC) applications, combining the output
of geophones and a hydrophone is now widely accepted technique for reducing the ghosting
effect caused by the water/air interface. To overcome the disadvantage of using two separate
sensors, both geophone and hydrophone are available in a single unit known as dual sensors
or the 4-component (4C) receivers consist of a hydrophone, two horizontal geophones and a
vertical geophone installed in a single water proof enclosure for recording P, SV and SH
waves.
4.3 Seismic Instrumentation
Once a seismic signal is transmitted and received, it must be recorded. The different types
signals are as follows:
    •    Source Signal: The pressure field created by the seismic source.
    •    Reflectivity Signal: The earth’s reflection sequence convolved with the source
         wavelet.
    •    Seismic Signal: Everything received as a result of the source firing. The seismic
         signal includes the reflectivity signal as well as ground roll, refractions, diffractions,
         sidesweep, channel waves etc.
    •    Received Signal: The electrical output of the receiver group. This is the seismic
         signal plus all environmental noise.
    •    Recorded Signal: The data, that is the instrument filtered signal plus any addition
         instrument noise, which goes onto the tape.
The information contained in a signal can be characterized by three quantities:
         Signal-to-noise ratio,
         Bandwidth and
         Duration
In seismic exploration, the recorded signal bandwidth is usually 0-250Hz. or lower. Often,
data are processed in a narrower band, say 5-80Hz., even though they may be recorded in a
broader band. The duration of recorded signals depends on the nature of the source and target
depth.
         A reflection is a physical event caused by a change in the acoustic impedance of the
earth. It is recorded signal, that event is represented by a wavelet that has two components –
the earth filter and the acquisition wavelet. The wavelet can be described in the time domain
or alternately, in the frequency domain. The Fourier transform can be used to move form one
representation to the other. If a wavelet has a short extent in a time and appears like a spike, it
is likely to be composed of a broad band of frequencies, each separate frequency having its
own phase value. The amplitude and phase of a wavelet contain all the spectral information
of a wavelet. These spectra are called the frequency-domain representation of the wavelet,
whereas the wavelet in time is considered to be in the time domain.
         When seismic recording first began in a 1920s the recording systems consisted of
heavy, metal cased geophones connected by wire cables to a recording truck. The signal was
recorded on a rotating photographic drum. Drums were replaced by analog magnetic tape
recorders during the late 1950s but these often failed to operate well. In the early 1960s they
were replaced by digital tape recorders, each of which had an analog-to-digital converter at
the input stage to the tape drive. The individual analog amplifiers also were unreliable, and
by late 1960s, they were being replaced in recording devices by a single multiplexed analog
amplifier.
         In late 1970s, distributed systems were introduced that performed amplification,
filtering, digitization and multiplexing at or near the receiver stations. By the mid 1980s
distributed systems were in wide use throughout the industry.


4.3.1 Basic components: The basic components of the land recording systems are:
4.3.1.1 Roll-along switch: It allows the observer to record a selected subset of the
geophones connected to the recording truck. It minimizes the need to move the recording
truck.
4.3.1.2 Pre-amplifier: This is a fixed gain amplifier that raises the incoming seismic
signal above the background instrument noise level. The preamplifier has low noise, high
input impedance and low distortion. Its input impedance is equal to or greater than the cable
impedance to the farthest station so that no signal amplitude is lost because of mismatching
of impedances. The amplifier must be completely linear throughout its operating range.


4.3.1.3 Multiplexer: This is an electronic switch that time shares data form multiple
channels. It changes multiple parallel inputs to a serial output relay for amplification,
digitization and recording. The multiplexer cycles through all of the inputs during each
digital sampling interval.


4.3.1.4 Main Amplifier: This amplifier receives all analog signals input to it and passes
then on to the A/D converter with an amount of gain determined by the gain controller.


4.3.1.5 A/D Converter: Analog signals are converted to digital signals with this device.
It allows the analog stream of data to be recorded in digital form. The received incoming
signal must be filtered to prevent aliasing prior to conversion to a digital form.
4.3.1.6 Gain Controller: The received signal includes, reflections, refractions, ground
roll and environmental noise, all of which may have amplitudes varying in a range from
microvolts to volts. A fixed form of amplification with only a relatively small number of data
bits cannot handle that range without some dipping at the most significant bit end of the
converter. Instead a variable or automatic gain control (AGC) level is determined for
application by the main amplifier in the feedback loop with the A/D converter to reduce or
amplify incoming signal to keep signal levels within the desired converter range. The
controller sets the amount of gain while the amplifier applies it to the incoming signal. The
AGC level set at each sample is recorded on tape as part of the gain word.
4.3.1.7 Formatter: The formatter arranges the data stream (in the form of voltage and
gain levels) into a binary code for writing onto magnetic tape. In addition, instrument
operational commands are distributed by the formatter to all the other components, making
the formatter the “brain” of the recording operation.


4.3.1.8 Tape Drive: Data finally are recorded on tape in digital form, ready to be passed
on to the processing center for further processing. Magnetic tape may be replaced by floppy
disks, depending upon the system in use. In land using recording a non-distributed system, an
analog seismic signal travels from the geophones along electrical conductors (the cable) to a
roll-along switch in the recording truck (or “doghouse” or “dog box”), after which it is
converted to a digital signal and recorded on tape or disk.
        In contrast, in a distributed system, the seismic signal passes from the geophone
string directly into an amplifier and/or A/D converter, after which it travels in digital form
along a cable to the recording truck. Because digital transmission of multiplexed data uses
many fewer cables than analog transmission, layout of the large receiver spreads often used
for 3D acquisition became considerably simpler.
        Today, majority of the acquisition systems provide 24-bit recording technology. A
24-bit technology system offers high fidelity because it records data over a large dynamic
range. Peculiarities for each system need to be examined for the task at hand. In land
operations, these recording units are usually truck or buggy mounted and can, therefore,
travel easily to areas of data acquisition. Lower channel count systems with higher sampling
rates, such as the DMT/SUMMIT and the 24-bit OYO DAS, can be used for small, near-
surface 3D surveys. In the case of very low channel count systems (e.g. less than 120), it is
normal practice for several recorders to be used together in a master-slave pattern to reach
sufficient channel capacity even for small 3D surveys.
         If a 3D survey crosses a variety of terrains (e.g., mountain, plain, transition zone), it
is desirable to use one type of recorder to cover all the survey areas. Thus shots of different
types in the mountains or in the swamp can be recorded by the same instrument. If more than
one recorder is used, amplitude and phase matching will be required to compensate for the
recorder differences. “Seam less” receiver coverage from a variety of sources enables
application of surface-consistent processes as de convolution, statics, and amplitude
correction.


                         Different Types of Seismic Recorders
Manufacturer    System          T/D/R              Boxes          Stations   Line Units       Central
                                                                  per Box                      System
    Sercel       SN388     Distributed System    Station Unit       1-6       Crossing      Central Control
                                                     (SU)                    Station Unit        Unit
                                                                                (SU)             (SU)
    Sercel       408UL         Telemetry        Field Digitizer      1          Line        Central Module
                           System/Distributed        Unit                    Acquisition       (CMU)
                            System/Remote           (FPU)                    Unit Cross
                           Seismic Recording                                  (LAUX)



4.3.2 Telemetry System
         True telemetry system has no physical connection between the station recording unit
and the control system in the recording truck. These systems should be used where access is
limited due to rugged terrain, permit problems, or any other reason. Sercel Eagle system is an
example of such systems. The SAR (Seismic Acquisition Remote unit) records the signal and
sends it via radio frequencies to the CRS (Central Recording Station).
         Some telemetry systems can receive data in real time. Other telemetry systems have a
disadvantage over distributed system in that the radio transmission of the data from the boxes
to the recording unit takes longer than real time. For some systems, data transmission time
may be on the order of minutes per source point, which may slow down the shooting crew.
Tree cover may also cause a problem for the signal transmission, and FM interference may be
significant in populated areas. Mixed systems may be used to cross-rivers or roads at select
locations.
4.3.3 Storage: The data obtained in the seismic field survey is stored on magnetic tapes
or cartridges. While the conventional storage devices are the tape drives the latest equipment
uses the cartridges with 10 GB memory capacity for storing the data. The data is stored in
SEG D format. Previously it used to get recorded in SEG B format or SEG C formats.
                                      Chapter 5
                2D Survey Design Basics

5.1     Basic Concepts in 2D Surveys


                                            seismic            will image the
The guiding principle should be to design afor costssurvey thatResolution parameters, such
selected target in the most economical way           and time.
as the frequency required to image the target, are starting design factors. Shallow horizon of
interest and deeper horizons may be interpretational needs; thus, the definition of the
representative horizons is the beginning of the design.
5.1.1 Near Surface layer: The velocity of the surface layer is used as a factor in
computing offsets and determining the effect of ground roll. Usually the weathered layer is
very low velocity because of exposure and erosion, but it may be quite complex and have
several layers of variant velocity. The velocity and maximum dip of each layer are initial
parameters. This information can be obtained in approximate form from existing well logs or
seismic data in the area. If the area is frontier area, then noise tests, experience, or geologic
theory can be the source of this information.
5.1.2 Shallow Layer: While the target layer is most important for imaging, a shallow
layer may be necessary for processing or interpretation. Good data in the shallow part is
needed to use the velocity analysis with confidence. The velocity Vs and the approximate
arrival time are the needed parameters. These parameters allow computation of depth of the
layer Zs by the familiar time-distance formula



where: t        =       two-way travel time to the shallow horizon
        Vs      =       average velocity to the layer, and
        Zs      =       depth to the shallow layer
This formula provides the information needed for the near offset, i.e.,
5.1.3 Target Layer: The layer is the horizon of primary interest for the survey. When
parameters conflicts arise during the design, the requirements for the target layer should
prevail. For imaging the target horizon, geologic knowledge of the expected thickness and
reflectivity is needed to estimate the frequency range


5.1.4 Group Interval: Group Interval is the basic sampling on the earth’s surface by the
survey. It is the distance on the ground between receiver stations. Group interval represents
which largest spatial sampling shall prevent aliasing during migration:




where, Vmin     = minimum velocity,
        θ       = maximum dip of the target horizon in degrees, and
        Fmax    = maximum frequency expected


5.1.5 Fresnel Zone: Fresnel zone (figure 7 (b)) is the smallest part of the reflector
making an unambiguous image of the individual event and is circular at zero offset but
elliptical with offset. Fresnel zone is given by:




where, tz       =       two-way record time of the target horizon

Maximum group interval       =



5.1.6 Far Offset: Far offset is a function of the depth modified by the velocity field. The
far offset required should be computed first for the target horizon and then for the deep
horizon. The velocity of the surface layer is involved because of the initial angular influence
on the down going seismic waveform at the depth of the horizon. Once the maximum offset
is computed in combination with the near / offset, the group interval, the ideal parameters can
be evaluated within the framework of the available equipment.




where, Z is the depth of horizon, Vs is the velocity of the surface layer, and V the average
velocity to the target.
If the horizon is dipping, then the distance, Hmax should be extended by:



where, Z        =         depth of horizon and
        θ        =        dip
This extension is quite important in 3D exploration (Migration aperture). Neglect of this
factor can result in underestimating field costs. The well-founded rule of thumb says that the
spread length should be equal to or a little greater than the depth of the reflection being
imaged. The far-trace distance should preserve full fold on the target horizon. Data
processing often requires considerable muting of the shallow data on the greater offsets
because of NMO stretch, noise trains, and other factors. The target horizon should be
protected by the survey for the mute. The custom is to automatically mute below the “20 to
30 percent stretch factor” the formula most used for this step is:




where H = offset distance, V= velocity at time Tz and Tz = arrival time of the event at H = 0.
When Tm exceeds 0.3, then data processing will probably form an automatic mute.
        Too large on offset range for a given number of receiver stations may result in
inadequate fold for the shallow layer or even the target layer. On the other hand, if the offset
range is not long enough, accurate velocity analysis and the suppression of multiples during
the processing can be endangered.


5.1.7 Record Length: Part of the survey design is to determine the required sampling
rate in time and the record length is a function of depth and velocity of the deepest horizon.



Where Td = two-way arrival time of the deepest horizon of interest at the maximum offset
        Tr = required record length in time, and
        L = length in time of the longest processing filter.
Normally 200ms is adequate for the filter length. The extra time in recording is balanced
against the possible benefits from data from very deep horizons. Signal length becomes more
important when the source is vibratory in nature. Some allowance should also be made for
migration.


5.1.8 Sample Rate: The sampling rate in time is more or less standard, ranging from 2
to 4 ms depending on the resolution needed. The rule is




A sample rate of 2ms is used for most seismic surveys.


5.1.9 Group Interval and Field Equipment: The group interval possible with a
particular recording equipment given by




where, Hmax = far offset,
        Hmin = near offset, and
        NC = the number of channels available for recording.
5.1.10 Fold Coverage: Each source position yields a certain amount of subsurface
coverage. For flat layers, the sampled point is half the distance from source to receiver. The
subsurface sampling is half the interval of the surface coverage. Foldage is defined as the
number of times a particular sub-surface sampling point (CMP) is covered by different
sources receiver locations. The maximum fold of coverage is given by




Where, S is the number of units of group interval in the source spacing and NC the number of
channels available.


5.1.11 Source Interval: The source interval in the distance between source positions.
The source interval is function of the desired fold coverage and the number of channels
available.




Where, NC is the number of channels and F is the fold.


5.1.12 Source Power: There is a decision to be made in some cases on the source power.
For dynamite, the charge size in kilograms if the unit. For vibratory sources, the available
power (in pounds per square inch) is specified by equipment model. For instance, a large
vibrator can generate 50,000 psi. Marine sources such as air guns and water guns are defined
in terms of their volume and peak-to-peak strength. The power needed is function of target
depth and the environmental noise. As the earth has a natural attenuation, target depth is the
primary consideration. Noise is also involved since more power has the potential to generate
more noise.
        The amount of energy generated in a shot hole is proportional to the quality of
dynamite. It is well known that an explosive source in a cylindrical enclosure generates
pressure waves and shear waves of both polarizations.


5.1.13 Line Location and Orientation: The geometry of the survey is not
independent of the target. The location, direction, and length of the lines are important
considerations in the survey design. Dip lines for instance, are favored over strike lines.
Some of basic concepts generally accepted for lines locations/orientations are:
             •   The lines should, when possible, be perpendicular to fault planes. Since
                 definition of the fault plane is best on the seismogram when the lines are
                 perpendicular to the plane.
             •   Line ties are important to interpretation. When there is existing seismic data
                 nearby, new lines are planned in such a manner that they can be ties with the
                 existing data. One very helpful ties is to a well. The closer the line can
                 approach the well, the more useful the tie of the seismic data to the well log.
             •   When there is no conflict with other needs, lines should be planned to
                 minimize elevation and terrain problems. Sometimes a small shift in the line
                 location can avoid a troublesome obstacle.
          Figure 7(a) shows the Surface Geometry and Sub surface Nature and Behavior of 2D
Layout.
                                     Chapter 6
               3D Survey Design Basics

6.1    Why 3D Seismic Survey?
   ub-surface geological features of interest in hydrocarbon exploration are 3-dimensional in
S
nature. A 2D seismic section is a cross-section of 3D seismic response. Despite the fact that
2D seismic section contains signal from all directions, including out-of-plane of the profile,
2D migration normally assumes that all the signal comes from the plane of the profile itself.
Although out of plane reflections (side-sweeps) are often recognizable by the experienced
seismic interpreter, the out of plane signal sometimes causes 2D migrated section to mistie.
These misties are due to inadequate imaging of the subsurface resulting from the use of 2D
rather than 3D migration. On the other hand, 3D migration of 3D data provides adequate and
detailed 3D image of the subsurface, leading to a more reliable interpretation. When
integrated with well logs, core and other petrophysical and production data, 3D data permits
reservoir characterization. The integrity of any 3 D data set leans heavily upon the suitability
of acquisition geometry.


6.2    Basic Concepts in 3D Surveys
       The 2D surveys are as linear as the terrain allows. Source and receiver are normally
in-line with each other. Arrays may be multi-dimensional, but most often are also in the line
of survey. For 3D surveys, this is seldom the case.
       The source interval of a 2D survey must be extended to include a definition of the
source line. For 3D surveys, source line must be defined, since for most common designs, the
source line is orthogonal to the receiver lines. The receiver line becomes the receiver lines.
As many receiver lines are laid out as the equipment for acquisition allows. Also, the
receiver, layout may not be lines but circles, checkerboards and other patterns developed for
3D surveys. Thus the simple parameters that defined the traditional 2D line now must be
extended to include more geometry.
       The analysis of 2D designs centers on the subsurface coverage in the form of
common-depth points (CDPs). For 3D surveys, the CDP becomes two-dimensional and is
termed a “bin”. These bins may be square or rectangular and define the spatial resolution of
the data sampling. Indeed, deciding the bin size will be the first step in designing a 3D
template. Subsurface sampling will be, as with the CDP, half the surface size. The accent of
2D lines is on the fold of coverage and the offset range. For 3D survey the fold may less, but
the azimuth range is added to the offset range as a parameter. If structure is complex, then
good azimuths range becomes more important. Where structure is complex the velocity
analysis must include an azimuthal property. The range of azimuths in the bin is also a
consideration.
        Another new factor is the use of computers to do the design. Moreover, interpretation
is usually conducted on workstations. The multiple source and receiver lines, the difficulty of
computing, fold coverage, azimuthal distribution, and offset ranges in the bins make the use
of a computer program to aid in 3D design almost a necessity. The Fresnel zone assumes
some new characteristics in three dimensions. Essentially, the theoretical point source
expands as it propagates in depth, “illuminating” a circular area at vertical incidence. In a
seismic context, this is the reflecting surface constructively contributing to the reflection. A
good approximation to the radius of the zone is which shows that the zone increases in radius
with depth but decreases with higher frequency wave fronts. Migration serves to reduce the
zone to some minimal size when accurately done and the data fits the assumption.
        It should be noted that when the reflecting point is offset, the circle becomes
elliptical. This angular effect actually reduces the size of the zone along the minor axis of the
elliptical response. Dip and structure also are factors in the actual response..




where, V          =     average velocity to the event,
        T         =     arrival time and
        t         =     peak-to-zero crossing of the wavelet.
An important aspect of 3D data and Fresnel zones is the extra dimension of focusing possible
with migration.
6.2.1 Preliminary Parameters: There are some parameters that need to be estimated
as input when designing the 3D survey. The physics and concepts are somewhat independent
of whether the survey is to have two or three dimensions.


6.2.1.1 Offset: The imaging of shallow target and deep horizons still requires certain offset
of source and receiver. An approximation to the required offset for a given horizon is very
simple and used often when surveys are designed in the field:
                Offset = depth of the horizon.
New factors include the fact that the offset may now be measured at an angle and the depth is
now that of a plane rather than a line.


6.2.1.2 Fold: The fold required for noise suppression is a function of the S/N conditions.
This translates in 3D to the number of traces in a bin. Because of the extra focusing by
migration and the flexibility of binning, fold can be less than required in 2D surveys. Field
tests or existing 2D seismic data can yield an estimate of the needed fold for the 3D survey.


6.2.1.3 Frequency: The temporal frequency required is not much different from that of
2D surveys. The rules for the resolution of layer of given thickness are best determined by
modeling. The general rule is that the resolution of a thin bed requires it to be sampled twice
within a quarter wavelength of the highest frequency. As a field approximation, the
maximum frequency expected:




T       =       Two way time of the horizon


6.2.1.4 Objectives of the Survey: The most important information is defining the
objectives of the survey. Although this seems a rather obvious comment, many times the
objectives of the survey except for the aerial extent and approximate spatial sampling are not
part of the input to design. Requirements of good fold on a shallow reference layer or a deep
reflection for survey are clearly stated.
6.2.1.5 Migration Aperture: When the beds are dipping, the extent of the survey must
be increased by:
                                     D       =        Z tan θ
where, Z = depth,
       θ = dip
6.2.1.6 Seismic Data Input: The most useful direct input is existing seismic data. The
seismic sections give information abut many of the design parameters such as noise, source
power, weathering problems, and general structure.
       Field records and final stack should be checked for environmental and source
generated noise conditions. The array design should be studied for possible use in the 3D
survey. There are areas where neither source nor environmental noise is a problem, which
greatly simplifies the survey design and makes the whole project less expensive. Type of
source, the power used in either surveys, quality of reflections at depth, frequency content of
shallow data are some of the key factors in deciding the source power to be used. If extensive
static corrections made during processing indicate problems in the near surface, this should
be noted on the survey design. The design of the survey can reduce processing problems in
many cases.


6.3    3-D Survey Design Sequence
       There are many ways to begin and complete a survey design. The specific sequence
of steps that follow are general guide lines. Some design templates will dictate a different
sequence of other parameters.
       A summary of the proposed sequence for developing a design sequence is:
           •       Determine the subsurface bin size. Twice the chosen bin size is the source
                 and receiver station spacings.
           •       Compute the number of source stations per kilometer required to achieve
                 fold with available equipment. The number of stations per square kilometer
                 allows computation of the source line spacing.
           •     Compute the receiver line spacing.
           •     Find the number of receiver lines allowed by the field equipment, constrained
                 by the required offset ranges. The result is the template.
           •     Decide the in-line and cross-line roll alongs.
              •   Allow for obstacles and run analyses of the offset distribution ranges of
                  offsets and azimuthal properties of the bins.
              •   Estimate time and costs of the script and iterate until attributes, costs, and
                  time are satisfied.


6.3.1 Bin size: For 3-D data the bin is the basic building block for the rest of the survey.
Bin size depends on target size, spatial resolution needed, and economics. The traces when
their subsurface reflection point falls with in the bin, are treated as a CDP, and corrected and
summed to represent that bin position by a point. A bin can be any size but rectangles and
squares are the popular. The basic sampling theorem applies to the bin.




where, b is the bin size,
           Fm = Maximum frequency expected,
           θ = maximum dip in degrees and
           Vmin = minimum velocity.


6.3.2 Source line spacing: The bin size will, however, allow more design calculations
if the fold and number of channels on the equipment are known .




where, NS = shots per square kilometer,
           F = desired fold,
           R = number of channels
B = subsurface bin size.
Determining NS allows for the computation of the next important parameter source line
spacing.
6.3.3 Receiver line spacing: The new information required is the minimum offset and
the offset ranges needed. The controlling parameter will be the largest minimum offset within
a bin. The minimum offset has been previously established with preliminary calculations and
modeling. The approximation is that the maximum offset needs to be at least as long as the
depth of the most reflection to be imaged.
       Thus, a2 = (c2-b2)1/2 where ‘a’ is the receiver line spacing, c is the largest minimum
offset and b is the source line spacing. A smaller near offset would require ‘a’ smaller
receiver line spacing and be more expensive. As with the source line spacing, the receiver
line spacing from calculation may be reduced.


6.3.4 Number and length of the receiver lines in the template: The problem
is to be determine the number of receiver lines possible with the template. The number of
lines is constrained by the required maximum offset which sets the length of the lines. The
maximum offset found in the preliminary 2-D calculations or 3-D modeling is a function of
deepest horizon to be imaged. The field estimate is that the maximum offset should be a little
greater than the depth of the deep horizon, but exact formula include dip. The target
parameters are the number and length of the receiver lines. The source line shift is an
adjustable variable. The second constraint is the number of channels available with the
equipment.


6.3.5 Determining the template movement: Usually the field people prefer to
roll along the direction of receiver lines. The increment is at the source line spacing. At the
end of the coverage in in-line direction the next swath would be done in same manner
incremented in the source direction and continued until the coverage was completed.


6.3.6 Estimation of nominal fold: Stacking fold is the number of field traces that
contribute one stack trace. Fold controls the signal to noise ratio. Fold should be decided by
looking at previous 2-D and 3-D surveys in the area.
6.3.6.1 In-line fold: For an orthogonal straight-line survey, in-line fold is defined
similarly to the fold on 2-D data. The formula is as follows:
        In line fold = ___( no. of receivers x station interval )__ .
                         2 x Source interval along the receiver line


                                           (or)
        In line fold = ( number of receivers x receiver interval )
                                    2 x Shot line interval


6.3.6.2 Cross-line fold: Similar to the calculation of in-line fold, the cross-line fold is:
        Cross line fold =        source line length _.
                            2 x receiver line interval


6.3.6.3 Total Fold: The total 3D nominal fold is the produce of in-line fold and cross-line
fold:
        Total nominal fold = ( in-line fold ) x ( cross-line fold).


6.4     Land 3D Layouts: Numerous layout strategies have been developed for land 3D
surveys. One has to establish which features are important in the area of the survey in order
to select the best design option.


6.4.1 Full fold 3-D: A full fold 3D survey is one where source points and receiver
stations are distributed on an even two-dimensional grid with station spacings equal to the
line spacings. The grids are offset by one bin size. A full fold 3D survey has outstanding
offset and azimuth distributions as long as one can afford to record with a large number of
channels. All other 3D designs are basically subsets of such full-fold surveys, and the
designer has to decide which aspects of a 3D design are absolutely necessary and which can
be compromised.


6.4.2 Swath: The swath acquisition method was used in the earliest 3D designs. In this
geometry Source and receiver lines are parallel and usually coincident. While source points
are taken on one line, receivers are recording not only along the source line but also along
neighboring parallel receiver lines, creating swath lines halfway between pairs of source and
receiver lines. The offset distribution in all occupied bin lines is excellent. However
inadequate sampling in the cross-line direction makes this design a “poor man’s 3-D”,
because many bins are empty. The azimuth mix is very narrow and depends on the number of
live receiver lines in the recording patch and the line spacing. Most companies prefer to have
the source points at the half-integer positions. Parallel swaths are sometimes considered on
land when severe surface restrictions exist, or when costs have to be minimized. The
operational advantages are attractive, but are achieved at the cost of a poor azimuth mix and
poor cross-line sampling.


6.4.3 Orthogonal: Generally, source and receiver lines are laid out orthogonal to each
other. Because the receivers cover a large area, this method is sometimes referred to as the
patch method. This geometry is particularly easy for the survey crew and recording crew, and
keeping track of station numbering is straightforward. In an orthogonal design, the active
receiver lines form a rectangular patch surrounding each source point location creating a
series of cross spreads that overlap each other. This technique allows more surface area to be
acquired prior to receiver stations moves.
       This method is easy to lay out in the field and can accommodate the extra equipment
and roll along operation. Usually all the source points between adjacent receiver lines are
recorded. Then the receiver patch is rolled over one and the process is repeated. The azimuth
distribution for the orthogonal method is uniform as long as wide recording patch is used.
       Figure 8(a) shows a 3D layout and the subsurface nature while figure 8(b) shows the
3D cable layout used by GP “Y”.
                                     Chapter 7
                 Reflection Field Layouts
7.1
    Split-Dip and Common Midpoint Recording: Virtually all routine source
seismic work consists of continuous coverage (profiling), that is, the cables and
points are arranged so that there are no gaps in the data other than those due to the fact that
the geophone groups are spaced at intervals rather than continuously spaced. Single coverage
implies that each reflecting point is sampled only once, in contrast to common-midpoint, or
redundant, coverage where each reflecting point is sampled more than once. Areal or cross
coverage indicates that the dip components perpendicular to the seismic line have been
measured as well as the dip components along the line. Each of these methods can employ
various relationships between sources and geophone groups.
7.2    Spread Types: By spread we mean the relative locations of the source point and
the centers of the geophone groups used to record the energy form the source. In split-dip
shooting the source point is at the center of a line of regularly spaced geophone group often
results in a noisy trace (because of ground roll or truck noise with a surface source, or gases
escaping from the shot hole and ejection of tamping material); hence the source may be
moved 15 to 50 m. perpendicular to the seismic line. Often the geophone groups nearest the
source are not used, which creates a sourcepoint (shotpoint) gap.
       Often the source is at the end of the spread of active geophone groups to produce an
end-on spread, and in areas of exceptionally heavy ground roll the source point is offset by an
appreciable distance along the line from the nearest active geophone group to produce an in-
line offset spread. Alternatively, the sourcepoint may be offset in the direction normal to the
cable, either at one end of the active part to produce a broadside-L or opposite the center to
give a broadside-T spread. End-on and in-line offset spreads often employ sources off each
end to give continuous coverage and two records for each spread. The in-line and broadside
offsets permit recording reflection energy before the ground-roll energy arrives at the spread.
Cross spreads, which consist of two lines of geophone groups roughly at right angles to each
other, are used to record 3D dip information.
7.3    Arrays: The term array refers either to the pattern of geophones that feeds a single
channel or to a distribution of shotholes or surface energy sources that are fired
simultaneously; it also includes the different locations of sources for which the results are
combined by vertical stacking. A wave approaching the surface in the vertical direction will
affect each geophone or an array simultaneously so that the outputs will affect the various
geophones at different times so that there will be a certain degree of destructive interference.
Similarly, waves traveling vertically downward from a source array will add constructively
whereas waves traveling horizontally away from the source array will arrive at a geophone
with different phases and will be partially cancelled. Thus, arrays provide a means of
discriminating between waves arriving from different directions.
        The two popular types of array designs are the linear array and the areal array. Arrays
are linear when the elements are spread along the seismic line or areal when the group is
distributed over an area.
7.4     Resolution
7.4.1 Vertical Resolution: Resolution refer to the minimum separation between two
features such that we can tell that there are two separate features. If seismic wavelets were a
spike, the resolution would not have been a problem. Rayleigh criterion of resolution states
that two events can be resolved if their separation is half cycle, since events are recorded in
terms of two-way time, therefore real separation of the features must be quarter cycle. Thus
resolvable limit is wavelength/4.
7.4.2 Horizontal Resolution: Horizontal resolution depends on the radius of the first
“Fresnel Zone”. A “Fresnel Zone” is that portion of the reflector, which sends back energy to
the receiver within a half cycle delay, so that it will produce constructive interference. The
size of the zone depends on frequency, the higher the frequency the smaller the zone.
Effective radius of the first Fresnel zone is half of the actual radius. If we consider point
source, the effective radius of first Fresnel zone is




where, V        =       average velocity of the reflector,
        t       =       two way time and
        f       =       frequency.
                                          Chapter 8
          Reflection Field Method for Land
                                          Survey

8.1
     Tinto the following groups: The land seismic data acquisition team is
divided
        he Seismic Field Party:


Survey Crew
      •   Fixing the control points for the line based on the GPS points given before hand,
      •   Ranging/Filling team for putting the pickets of the specified intervals along the line
          based on the control points on the line,
      •   Leveling team for giving the elevations at the shot point location and the receiver
          point location.
Shot Hole Drilling Crew
      •   For drilling the holes up to the specified depth for putting the charge for blasting.
Uphole Survey Crew
      •   For measuring the velocity and thickness of the weathered layer (Low Velocity
          Layer) and velocity of the sub-weathered layer (in crude terms for depth
          optimization)
Recording Unit
      •   Shooting Crew: For filling the drilled holes with the charge of specified quantity
          and detonating it.
      •   Jug hustlers (the Cable laying crew): For laying the cable and planting the
          geophones at the specified pickets and for observing them all through the recording
          time for further corrections.
      •   Recording Crew: For recording the seismic signals received by the geophones
          after blasting the charge.
8.2       Seismic Data Acquisition
8.2.1 The Program: Usually the seismic crew receives the program in the form of lines
on a map that indicate where data are to be acquired. Before beginning a survey the following
questions should be asked: “Is it possible that the proposed lines will provide the required
information?” Data migration may require that lines be located elsewhere than directly on top
of features in order to measure critical aspects of a structure. Crustal areas may be so
extensively faulted that lines across them are nondefinitive. The structures being sought may
be beyond seismic resolving power. Near surface variations may be so large that the data are
difficult to interpret whereas moving the seismic line a short distance may improve data
quality. Obstructions along a proposed line may increase difficulties unnecessarily, whereas
moving the line slightly may achieve the same objectives at reduced cost. Where the dip is
considerable, merely running a seismic line to a wellhead may not extend sufficiently beyond
faults and other features to establish the existence of such placements. Lines may cross
features such as faults so obliquely that their evidences are not readily interpretable. Lack of
cross control may result in features located below the seismic line being confused by features
to the side of the line.
Objective of the Survey: The objective of the survey done by the GP ‘X’, GP ‘Y’ and
GP ‘Z’ is to map strati-structural features within the specified formation at an area in
Krishna-Godavari Basin of Andhra Pradesh which lie within the lower to upper cretaceous
section. The seismic equivalent of these geological objectives are as under:
For GP ‘X’
  Area            Depth (m)               Two Way Travel Time in (ms)                  Dip
 Area 1         1500 to 4300                        1250 to 3000                     10 to 15
 Area 2         1700 to 4200                        1400 to 2900                     10 to 15


For GP ‘Y’
  Area           Depth         Two Way Travel Time in        Average Velocity          Dip
                  (m)                    (ms)                      (m/sec)
 Area 1      1800 to 3600            1500 to 2500
 Area 2      1800 to 3400            1500 to 2500


For GP ‘Z’
  Area           Depth         Two Way Travel Time in        Average Velocity          Dip
                  (m)                    (ms)                      (m/sec)
 Area 1      1800 to 3400            1500 to 2500               2400 to 2720        100 to 120
Reasons for the Survey: Out of the wells drilled in the area some have proved the
presence of gaseous hydrocarbons from the formation and some have been dry. There by the
area assumed important for exploration from these targets


Geology of the Area: The Krishna-Godavari basin has been subdivided into three sub-
basins
         Krishna Sub-basin
         West Godavari Sub-basin and
         East Godavari Sub-baisn.
The area under investigation for the GP ‘X’ and GP ‘Y’ lies in West Godavari Sub-basin
while that of GP ‘Z’ lies in the East Godavari Sub-basin. Figure8(c) shows the generalized
stratigraphy of KG Basin.


8.2.2 Permitting: Once the seismic program has been decided o n, it is usually
necessary to secure permission to enter the land to be traversed. Permission to enter may
involve a payment, often a fixed sum per source location, as compensation in advance for
“damages that may be incurred”. Even where the surface owners do not have the right to
prevent entry, it is advantageous to explain the nature of the impending operations. Of
course, a seismic crew is responsible for damages resulting from their actions whether or not
permission is required to carry out the survey.


8.2.3 Layout of Line
         The survey crew lays out the lines to be shot, usually by using an Electronic Total
Station (refer Appendix), Compass Theodolite, and transit-and-chain survey that determines
the positions and elevations of both the source points and the centers of geophone groups.
Usually the survey crew is given a few GPS stations beforehand in the area of operation.
         The survey crew divides themselves into three main groups. The first group fixes the
control points (using the Electronic Total Station) which control the direction of the source
line or the receiver line. Usually these control points are given at an interval of about 1km.
along the line, on either side of the line. The second group does the ranging and the filling
(using the compass Theodolite) part on the line along the line at specified interval and
placing the pickets (made of flat bamboo sticks with the marking of the picket number on
them) at those stations. The third group does the leveling i.e., gives the elevation values at
each picket.
        Thus the survey crew lays the grid of source lines and receiver lines with specified
picket intervals, receiver line intervals and source line intervals on ground. What they do is to
project the details on the given onto ground very precisely.


8.2.4 Shothole Drilling: The next team of people to star their activities unit in the
scene is the drilling crew (when explosives are used as the energy source). Depending on the
number and depth of holes required and the case of drilling, a seismic crew deploys the
drilling crews. Whenever conditions permit, the drills are truck-mounted. Water trucks are
often required to supply the drills with water for drilling. In areas of rough terrain, the drills
may be mounted on tractors or portable drilling equipment may be used. Usually the drilling
crew places the explosive in the holes before leaving the site.


Seismic survey is divided into two main classes which are interlinked. These are:
                Experimental Surveys and
                Regular/Production Survey


8a.3 2D Survey Parameters (Before the Experimental Survey) (GP ‘X’)


        Instrument                      408 UL
        Source Type                     Dynamite
        Group Interval                  20m
        Field Season                    2004-05
        Type of Shooting                Asymmetrical spread (216 + 40)
        Channel/Foldage                 256/64
        Spread Length                   4300 + NTO
        Shot Interval                   40m
        No. of Geophones per group      12
        Geophone Pattern                Linear
        Shot Hole Pattern               Single
        Record Length                   6S
        Sample Rate                     2ms.
       Gain Mode                       24bit
       K – Gain dB                     0, 12
       Low Cut Filter (Hz/dB)          Out
       High Cut Filter (Hz/dB)         200/370
       Notch (50 Hz)                   NA




8.4    3D Survey Parameters (Before the Experimental Survey) (GP ‘Y’
and GP ‘Z’)


Parameters                       GP ‘Y’                     GP ‘Z’
Instrument                       408UL                      SN388
Source Type                      Dynamite
Group Interval                   40 m.                      40 m.
Field Season                     2004-05                    2004-05
Type of Shooting                 Asymmetric Split Spread    Asymmetric Split Spread
Channel/Foldage                  1008(168 per line)/6 X 6   1008( 168 per line )/6X6
Spread Length (m)                6680 m (each line)         6680 m (each line)
Shot Interval    (m)             40 m                       40 m
No. of Geophones per group       12
Geophone Pattern                 Areal                      Areal
Shot Hole Pattern                Orthogonal – Single        Orthogonal – Single
Record Length (sec.)             6                          5
Sample Rate      (m sec.)        2                          2
Gain Mode                        0
K – Gain (dB)                                               12
Low Cut Filter (Hz/dB)           Out                        Out
High Cut Filter (Hz)             200                        125
Notch (50 Hz)                    Out                        Out
Receiver Line Interval (m)       280                        280
Source Line Interval (m)
Bin size (m x m)                 20 x 20                    20 x 20
8.5      Experimental Survey


8.5.1 Uphole Survey (Depth Optimization)


Sheriff defines an uphole survey as follows
   •     Successive sources at varying depths in a borehole in order to determine the
         velocities of the near-surface formations, the weathering thickness, and (sometimes)
         the variations of record quality with source depth.
   •     Sometimes a string of geophones is placed in a hole of the order of 200 feet deep to
         measure the vertical travel times form a nearby shallow source.


8.5.1.1 Data Acquisition Method: Once the locations of the uphole survey have been
decided based on line intersections, at regular spacing along the lines, or in an anomalous
area any necessary paperwork must be completed prior to drilling. The depth of the hole to be
drilled depends on the area on the problem to be solved. Unless there are unusual problems in
the area, it is likely that a depth of 50-100 m. will be adequate. Although in extreme cases,
uphole depths have exceeded 500m. The type of drill used must be appropriate, or at least
acceptable, for the proposed depths and the type of drilling in the area. During drilling, it is
important that information be obtained about the penetrated geologic formations, specifically
their lithologies. Normally, this is done by cuttings from various depths in the borehole,
along with comments about hard or easy drilling or that circulation was lost at a specific
depth.
         The objective of an uphole survey is to estimate the thickness and times and hence
velocities, of the near surface layers. To obtain accurate time estimates, the source and
receiver must be as broadband as possible and the data have a good signal-to-noise ratio; that
is, the source should ideally be a short time-duration pulse. No delays should occur in the
recording system, which implies that the recording filters must be left open whenever
possible, apart form anti-alias filters used for digital recording. Checks must be made on the
whole timing system, from the time break through to the display, to ensure that any delays
are understood and accounted for in the interpretation. If detonators (caps) are used, for
example, their delay must either be very small or to be estimated for each shot so that the
detonation time is known.
        Picks should be normally be estimated to an accuracy of 0.5ms, meaning that the
recording speed must be fast enough to allow the picks to be interpreted with this precision.
In a high-resolution survey with a small depth increment between observations, the accuracy
should be better than this.
        Many systems now use a magnetic storage devise which allows several displays to be
made at different gains. The recording equipment should have the capability of stacking the
data to enhance the signal-to-noise ratio for low-power surface sources and, possibly in the
future, for nondestructive sources in the borehole.


The two basic approaches for conducting uphole surveys are:
        (i)     The source in the borehole and the receivers on the surfaces and
        (ii)    The source at the surface and the receivers in the borehole.


8.5.1.2 Source in Borehole and Receivers at the Surface: The basic field set up
is as shown in the figure 9. A succession of charges detonated at different depth are recorded
by one or more receivers at the surface located a few meters away from the hole. This is
generally preferred method and dynamite is used by the production crew. This mode of
operation can also be used in transition zone or shallow-water survey areas where it is
practical and safe to drill and load charges into the borehole.


8.5.1.3 Source: If dynamite, the size of the charge depends on the near-surface geology
and the depth of the shot; hence, tests must be conducted in the new area. As a guide, caps
(detonators) are normally sufficient to at least 20m. depth and primers to at least 50m.
Charges can be loaded and detonated independently, or a wiring harness can be used to load
many shots at one time. Regardless of which method is used, the deepest shot must be
detonated first. Woods and Patterson showed that the times are influenced by the charge size,
with larger sizes leading to anomalous times. Thus, to obtain seismic velocities form an
uphole survey, the charge size should be kept as small as possible yet still allow the signal
recorded at the surface to have sufficient signal-to-noise ratio.
        The wiring harness is composed of many pairs of wires each of which is used for one
of the charges; consecutive charges have a preset distance (Shot interval) between them. The
charges are attached to the harness and a weight attached beneath the deepest charge. The
whole assembly is then carefully loaded into the borehole to the correct range of depths,
normally with the weight at the base of the borehole.


8.5.1.4 Receiver: A number of receivers are positioned close to the top of the borehole; a
normal minimum is four located in a cross arrangement to record data from four azimuths.
The type of geophone used should have good low and high frequency responses to obtain the
desired broadband recording. Thus, a low-frequency geophone is generally required, with a
natural frequency of less than 10Hz. Each receiver should be located several meters away
from the top of the borehole. If a receiver is too close to the borehole, the recording will be
contaminated by arrivals through the drilling fluid and the invaded zone, where the drilling
fluid has entered the rock formation close to the borehole. In addition, the drilling process
disturbs the ground near the borehole, which can delay the arrival of an upcoming wave-field
by as much as several milliseconds.


8.5.1.5 Sample Interval: The near-surface detail required was related to both the
objectives of the survey and the complexity of the near surface. These vary from complex
near-surface areas where the targets of the main survey have limited area and closure, to
those in which the near surface changes slowly along the line and targets have a appreciable
time relief or the exact attitude of the target formations is not critical. With respect to uphole
survey sampling requirements, three aspects need to be considered:


    •   the sampling over the area and along any one line,
    •   the depth sampling of any one survey, and
    •   the digital sample rate for surveys that are recorded digitally.


        The other technical factor that impacts the spacing of uphole surveys is the method
that is used to interpolate the near-surface layers between the uphole survey locations to
define a near-surface model for the computation of datum static corrections.
         The spacing of uphole surveys depends on several factors and on the problems to be
solved. Overall, the system approach should be used, in which an analysis is done of which
components of the near-surface problems are to be solved by the various techniques
available, such as upholes, refraction, residual static corrections and interpretation. However,
it is generally desirable to locate uphole surveys at line intersections so that the information
can be used on the two or more intersecting lines and to sample different near-surface
lithologies. In critical areas, an uphole survey may be needed as often as every spread-length
in extreme cases, and even smaller spacing.
         The depth sampling must be sufficient to allow time-depth picks to define each of the
geologic formations adequately. Each formation requires a minimum of three and preferably
more picks for a reasonable velocity estimate. If taken to the limit, this implies a very fine
sample interval. However, the near-surface formations change both vertically and
horizontally away from the borehole, often on an irregular basis, so that having precise
measurements at one location will be of little practical value in interpolating a value midway
between two uphole locations. A depth sample interval of a few meters (2-3m) is generally
adequate for most areas and allows velocity estimates over an intervals of 5-10 m.
         Where the data is recorded digitally, the sample interval should be small enough to
retain as much high frequency signal as possible so that a good uphole break is obtained.
When dynamite is used as a source, this should be 0.5 ms. or less; for detailed shallow high
frequency surveys, a much smaller value may be appropriate. For non explosive sources,
1ms. should be more than adequate, and in some areas 2ms. or 4ms. sampling may be
appropriate.


8.5.1.6 Parameters in the Uphole Survey
         The parameters used by all the three parties in conducting the uphole surveys are as
under:
8.5.1.6.1         Used by GP ‘X’ and GP ‘Y’
Source Used             : Caps (detonators)
Receiver                : Geophone
Spread Length           : 50m.
No. of Geophones        : 5 (one at 5 different offset distances)
Offset Distance         : 1st Geophone is placed at a distance of 1m from the shotpoint
                         2nd Geophone is placed at a distance of 3m from the shotpoint
                         3rd Geophone is placed at a distance of 5m from the shotpoint
                         4th Geophone is placed at a distance of 25m from the shotpoint
                         5th Geophone is placed at a distance of 50m from the shotpoint
Depth of Shot Hole      : 68m.
Shot interval(depth)    : 2m.
Table (I) shows the description of the uphole used by GP ‘X’.


8.5.1.6.2         Uphole parameters Used by GP ‘Z’
Source Used             : Caps (detonators)
Receiver                : Geophone
Spread Length           : 50m
No. of Geophones        :9
Offset Distance         : four Geophones are placed at a distance of 1m with different
                         azimuths from the shotpoint
                         5th Geophone is placed at a distance of 3m. from the shotpoint
                         6th Geophone is placed at a distance of 5m. from the shotpoint
                         7th Geophone is placed at a distance of 10m. from the shotpoint
                         8th Geophone is placed at a distance of 25m. from the shotpoint
                         9th Geophone is placed at a distance of 50m. from the shotpoint
Depth of Shot Hole      : 68m.
Shot Interval (depth)   : 2m.
Figure 10 shows the field layout used for doing the uphole survey by GP Z.


8.5.1.7 Interpretation


The main components of uphole survey interpretation are:
    •   Picking the first arrivals from each depth level,
    •   Applying any necessary corrections to these times and
    •   Plotting the data and estimating the velocities and thicknesses of the various layers
        identified.
The interpretation of individual uphole surveys along a line should be followed by lateral
adjustments to ensure that the layer thickness and velocity profiles along the line are realistic.
8.5.1.7.1       Picking and Timing Data: The picked times should be estimated to the
nearest 0.5 ms or better. These times are used not only to measure times to specific depths
but also to estimate interval velocities over fairly small depth ranges. Prerequisites for
picking data at this accuracy are a sufficiently broad band signal bandwidth, adequate signal-
to-noise ratio, a fast paper speed display, and sufficient gain to show a good break on the
display. In addition, a time break test must be conducted with the signal displayed on the
display to obtain absolute time information. Any observed delay is measured and then
removed from each of the times picked form the display.
        In most acquisition systems (Smartseis, refer Appendix in this case), data are
recorded on a digital tape or disk. The gain of the display is important. With respect of the
picking of the first arrival, Ricker stated, “There is no sudden takeoff of the trace when the
disturbance arrives. The motion begins gradually as if a first kick arrival time is attempted,
the time picked will depend upon the over-all magnification of the seismograph”
        Figure 11 shows a field record as obtained by the uphole survey team of GP ‘X’ for a
source (1m of detonating cord) at a depth of 60m into a spread of geophones. The first four
geophones are at an offset of 1m, 3m, 5m, 25m, and 50m
        For a conventional uphole survey analysis, picks should be made for all near-offset
displays. When several traces are recorded at the same offset but with different azimuths,
variations in the time are often observed. This can be due to near-surface variations close to
the receivers, ground coupling of the receivers, variations in the invaded zone between the
source and receiver, or disturbance around the borehole form the drilling process. To
investigate these effects, traces out to an offset of about 15 – 25 m. should also be picked.
        Uphole times are picked from a peak, trough or zero crossing; these cannot give
absolute times, but the interval times can be used for interval velocity estimates. For this
approach to be sufficiently accurate, the waveform must not change form one depth level to
the next. However, the pulse width typically broadens with the distance traveled.


8.5.1.7.2       Arrival-Time Corrections: The two major corrections applied to
uphole survey data are conversion to absolute time and vertical time. The correction to
absolute time uses information obtained from the time break test, designed to measure minor
delays in the total system due to filters and other components of the recording instruments.
The measured times are from a known depth in the borehole to a specific offset from the top
of the borehole. In some cases, the surface elevation at the top of the borehole is different to
the receiver (or source) elevation located a few meters away. Corrections for this elevation
change the offset from the borehole need to be applied to the picked times to simulate a set of
arrival times that would be measured at the top of the borehole.
       A simple geometric correction is applied routinely to correct from the inclined ray-
path to a vertical one; the parameters are shown in the figure 12 which assumes that the
borehole is vertical. When the drill is on a slope, all the depth and times normally refer to a
reference system perpendicular to the ground; in other non-vertical situations, additional
corrections are required. The relationship between the vertical uphole time and the measured
(inclined) uphole time for the general case is given by




where, t is the measured uphole time corrected for any time delays,
T is the vertical uphole time,
       x is the offset of the receiver from the top of the borehole,
       z1 is the depth of the shot, z2 is the depth of the receiver, and
       ∆ E is the difference in elevation between the receiver and the top of the shot hole
When ∆ E = 0 and z2 = 0, the above equation simplifies to
Table I shows a worked example of the application of the second equation; the receivers are
at different offsets from the top of the borehole.
        The underlying assumption in the geometric correction of the above equations is that
no refraction of rays occurs at any velocity interfaces. This is a reasonable approximation
when the offset distance of the receiver (or source) form the top of the borehole is only a few
meters; however, it also implies that the dips of the interfaces are small.


8.5.1.7.3       Time – Depth Display: The absolute arrival times, corrected to vertical
travel, are plotted at the appropriate depths on a time-depth display or plot. The most
commonly used convention is for depths to be plotted vertically and times horizontally. Any
information about near-surface geology from the driller or geologist, as well as other relevant
information, should be included on the display. This can be used to help define the various
interfaces present and to provide an independent check of the depths noted.
        The time-depth display is then interpreted and interval velocities are estimated for the
layers identified. This is often subjective procedure, and several different interpretations can
often be made from one data set. The geologic information is often useful in deciding where
an interface is located; however, not all changes in geology give rise to a change in velocity
and the velocity can change within a geologic unit. A major point to consider is the error
associated with each plotted point; one must also remember that the objective is normally to
define the simplest model consistent with the data.


Figures 13 (a) and 13 (b) show the t-d plots drawn for the data obtained at uphole A of GP
‘X’.


8.5.1.7.4       Spatial Consistency: Each uphole survey is initially interpreted on its
own, but consideration must also be given to other uphole surveys in the area. It is then often
possible to adjust the velocities for a specific uphole survey to be more consistent with other
values along the line or in the area for a specific layer or formation. Any adjustment must be
within the error of the survey.


8.5.1.8 Conclusion: From figures 13 (a) and 13 (b) we can thus conclude that the
optimum depth to place the charge for uphole A is 36m .
8.5.2 Noise Experiment (Determination of Near Trace Offset and Array
Length)


At the start of the survey, a noise spread shot performed to measure the level of the noise. If
the noise is severe enough to hinder the survey objectives, the geophysicist must decide how
to handle the noise problem. One approach is to ignore the noise problems in the field and to
assume that various data processing steps subsequently remove the problem of high noise
levels. If the dynamic range between the amplitude of the noise and that of the underlying
seismic signal exceeds the dynamic range of the recording system, the signal will not be
recorded and the data processing methods have nothing to recover. In such situations it is best
to select survey parameters to attenuate noise prior to recording in filed.
        The most common methods of reducing noise in the filed are frequency filtering in
the recording instruments and wavelength filtering through use of directional source and
receiver arrays. The wavelength data needed to optimize array parameters may be measured
by performing noise-spread tests. For each kind of spread, the offset between geophones and
the source should extend from zero to maximum offset that will be used in production
recording. The receiver interval used during a noise test must be short enough to avoid spatial
aliasing of the short-wavelength noise.
        There are four methods of noise analysis:
                The normal spread,
                The transposed spread,
                The double-ended spread and
                The expanded spread.
Of all the above spread types the transposed spread is more suitable for the land surveys.


8.5.2.1 Transposed Spread: The spread remains fixed in one location and the shot
moves away from the receiver spread one spread length after each shot is fired. This method
is more popular than the normal spread because it is easier to move the source than the
receivers. A problem with this method is that a shot static difference misaligns noise and
reflection traces when the individual spread is still the most popular type of noise analysis. It
is often called a walk away noise test because the shot or vibrator literally walks away from
the receiver spread during the recording horizons.
8.5.2.2 Instrument Parameters faced in the noise test:


8.5.2.2.1 (At GP ‘X’, GP ‘Y’)


        Instrument                      : CM 408 UL
        No. of Channels                 : 108
        Profile Length                  : 2160m
        Group Interval                  : 20m
        Number of Shots                 :6
        Record Length                   : 6sec
        Sampling Interval               : 2ms
        Pre Amplification Gain          : 0 dB
        Filters                         : Out


8.5.2.2.2 (At GP ‘Z’)


        Instrument                      : SN 388
        No. of Channels                 : 108
        Profile Length                  : 2160m
        Group Interval                  : 20m
        Number of Shots                 :6
        Record Length                   : 5sec
        Sampling Interval               : 2ms
        Pre Amplification Gain          : 12 dB
        Filters                         : Out


8.5.2.3 Noise Analysis: From field record time, distance (t-X) graph is plotted for each
noise wave traces observed in the record. Each wave will align along different straight lines,
slope of each straight line gives the apparent velocity of the corresponding wave. This can
also be done with the help of computer by simply plotting all the noise records in order of
their offset distances. Waves of different apparent velocities will align along different straight
lines having different slopes. Time period of the wave can be found by measuring the time
difference between two consecutive peaks or troughs where the wave shape / stand out is
clear and with least interference. Inverse of the same indicates frequencies and apparent
velocities of signal at different offsets and times can also be measured from the record.
Amplitudes of signals and noises can also be measured at different offsets and times.
          Figure 14 shows the geometry of the noise test carried out by the field party I have
visited. The spread length for this test is about 535m with a geophone interval of 5m. The
geophones are placed as bunch (a bunch contains 12 geophones). The shots are placed at an
interval of 400m and 8 shots were taken. Figures 15 (a), (b), (c) and (d) shows the noise
sections as obtained using the recording instrument Sercel CM408UL            by GP ‘X’ after
recording the data for four shot points and figure 16 shows the noise section obtained at GP
‘Y’.
          On seeing the section in figure 15 (a) and (b) we can observe the noise trends marked
as A, B, C, ….. Following are the various noise characteristics deduced from the record / plot
Trends / Events           Velocity (m/sec)       Frequency (Hz)          Wavelength (m)
A                         198                    9                       22
B                         210                    10                      21
C                         220                    11                      20
D                         252                    12                      21


Based on the maximum wavelength obtained from noise analysis the array length is fixed.
Thus the array length is fixed as 22m.
Analysis of amplitude spectra for different trace offsets and F-K plots in different transform
windows helps in calculating noise wavelength and amplitudes and also in deciding the near
offset.
8.5.3 Fold Back Experiment (Element Spacing Determination )
          Fold back experiment is conducted to select the suitable element spacing in an array,
which can effectively suppress the source-generated noise. Different types of geophone
arrays are tested depending upon the noise characteristics, observed in the noise experiment.
Four spreads, each with a different array are shot with constant charge size and depth. The
field monitors, simulated plots and frequency-amplitude spectra are evaluated with the
emphasis on the standout of events in the zone of interest.
          Actually the entire spread length decided for the regular survey is folded into four
arms as shown in the figure 17, each with equal length. Thus the name is “fold back”
experiment. As per the fold back experiment conducted by GP ‘X’ the length of each arm is
about 1060 m. with 54 channels. The first arm consists the channels from 1 to 54, the second
arm contains channels from 55 to 108, the third from 109 to 162 and the final limb from 163
to 216. Along the first arm the geophones are placed as bunches at an interval of 20 m. The
second arm contains the 12 geophones of each string spaced at 1.5m. again with the same
group interval of 20m. The geophones in the third line are spaced at 1.75m. The last arm has
the 12 geophones of a string spaced at an interval of 2m with a group interval of 20m. The
distance between the 1st arm and the second arm is 5m while the distance between the 2nd and
3rd arm is 10m and the distance between the 3rd and 4th limb is again 5m. The shot line is
placed exactly at the center of the four arms in the direction of the lines. Four shots with a
shot interval of 400 m. are taken with the first shot placed as shown in the figure 17.
        Figure 18 shows the section obtained by conducting the fold back experiment at GP
‘X’ and figure 19(a) and 19(b) shows the section obtained by conducting the fold back
experiment at GP ‘Y’, while 19 (a) shows the section without the application of any kind of
filter (b) shows the section after the application of a band pass filter (10 to 80 Hz). As seen
from 18, the limb with 1.75m element spacing appears to be better section with less
interference and better signal preservation compared to other arms. Thus the element spacing
is fixed as 1.75m. From figure 19 (a) and (b) we can fix the element interval as 2m.


8.5.4 Shot Depth and Charge Size Optimization
    After optimizing the geophone array, shot depth and charge size optimization is
conducted using the normal spread with
    •   Constant charge size and varying shot hole depth and
    •   Constant shot hole depth and varying charge size.
    The field setup is laid as per the requirement of regular production work. At first shot
location three shot holes are drilled with different depths (in the order of the optimum depth
suggested by the uphole survey i.e., if the depth suggested by the uphole survey is 36m. then
the three hole are dug with the depth variations as 34, 36 and 38m) and separated by a
distance of 5m. perpendicular to the receiver line. Equal amount of charge is placed in all the
three holes and recorded one after another. The records are observed and the depth with
which the best response is obtained is fixed as the depth for the regular survey. Similarly at
the second location, shot holes of same depth ( as optimized previoiusly) are drilled and
different charge sizes are placed in them and the shots are recorded. Again the best response
as is seen from the recorded sections size is fixed as charge size for conducting the regular
survey.
   The analysis of the processed outputs is done in a similar manner as for the other
experiments. The acquisition parameters optimized on the basis of above experiments are
adopted for production work.
A set of 2D acquisition parameters optimized on the basis of experimental work is given
below
8.6       2D Survey Parameters for production work by GP ‘X’


          Instrument                   CM 408 UL
          Group Interval               20m
          Field Season                 2004-05
          Type of Shooting             Asymmetrical spread (216 + 40)
          Channel/Foldage              256/64
          Spread Length                4500m
          Shot Interval                40m
          No. of Geophones per group   12
          Geophone Pattern             Linear
          Shot Hole Pattern            Single
          Record Length                6 secs.
          Sample Rate                  2ms
          Gain Mode                    24bit
          K – Gain dB                  0, 12
          Low Cut Filter (Hz/dB)       Out
          High Cut Filter (Hz/dB)      200/370
          Notch (50 Hz)                NA
___________________________________________________
          Shot Hole Depths             36 + 2m.
          Charge Size                  2.5Kg
          Near Trace Offset            200m
          Element Spacing              1.35 m.
          Array Length                 18 m.
8.7      3D Survey Parameters for production work by GP ‘Y’ and GP ‘Z’
Parameters                             GP ‘Y’                            GP ‘Z’
Instrument                             CM 408UL                          SN388
Source Type                            Dynamite                          Dynamite
Group Interval                         40m                               40m
Field Season                           2004-05                           2004-05
Type of Shooting                       Asymmetric Split Spread           Asymmetric Split Spread
Channel/Foldage                        1008(168 per line)/6 X 6          1008( 168 per line )/6X6
Spread Length                          6680m(each line)                  6680m (each line)
Shot Interval                          40m                               40m
No. of Geophones per group             12
Geophone Pattern                       Areal                             Areal
Shot Hole Pattern                      Orthogonal – Single               Orthogonal – Single
Record Length                          6sec                              5sec
Sample Rate                            2ms                               2ms
Gain Mode                              0dB
K – Gain dB                                                              12dB
Low Cut Filter (Hz/dB)                 Out                               Out
High Cut Filter (Hz)                   200Hz                             125Hz
Notch (50 Hz)                          Out                               Out
Receiver Line Interval                 280m                              280m
Source Line Interval                   560m                              560m
Bin size                               20 x 20                           20 x 20
Migration Aperture                     4300m                             4500m
------------------------------------   -------------------------------   --------------------------------
Charge Size                            2.5Kg                             2.5Kg
Shot Hole Depth                        36 + 2m                           36 + 2m
Element Spacing                        1.5m                              1.5m
Array Length                           20m                               20m
With all the above said parameters the regular survey is carried out. Figures 20 (a) and 20 (b)
shows the first two shot gathers obtained during the regular production work by GP ‘X’.
                                       Chapter 9
                  Seismic Data Processing

9.1       ntroduction: The seismic method has been greatly improved in the both in the
      I                   areas of data acquisition and processing. Digital recording along with
the CMP multifold coverage was introduced during the early 60’s. Data acquired from the
field are prepared for processing by the field party itself and then it is send to the processing
centre. Processing is required because the data collected from the field is not a true
representation of the subsurface and hence nothing of importance can be inferred from it.
With the advent of high end computing systems modern day processing has become a lot
easier than it really used to be. Turnaround times have therefore come down with lot of
processing taking place in-field or onboard.


9.2       Why Processing?
          Field record which we obtain contains:
             •   reflections,
             •   coherent noise, and
             •   random ambient noise.


9.2.1 Reflections: Reflections are recognized by the hyperbolic travel times. If the
reflection interface is horizontally flat, the reflection hyperbola is symmetric with respect to
zero offset. On the other hand if it is dipping interface, then the reflection hyperbola is
skewed in the up dip direction.


9.2.2 Coherent Noise: Under the coherent noise category there are several wave types.
             •   Ground roll is recognized by its low frequency, strong amplitude and low
                 group velocity. It is the vertical component of dispersive surface waves i.e.
                 Raleigh waves. Typically we try to eliminate ground roll in the field itself by
                 array forming of receivers.
            •     Guided waves are persistent, especially in shallow marine records in areas
                  with hard water bottom. Guided waves also are found in the land records.
                  These waves are largely attenuated by CMP stacking. Because of their
                  prominently linear move-out, in principle they also can be suppressed by dip
                  filtering techniques. One such filtering technique is based on 2D Fourier
                  transformation of the shot record.
            •     Side Scattered noise commonly occurs at the water bottom, where there is no
                  flat, smooth topography.
            •     Cable noise is another form of coherent noise which is linear and low in
                  amplitude and frequency. It appears on shot records as late arrivals.
            •     Another form of coherent noise is the air wave which has a velocity of 300
                  m/s. It can be a serious problem when shooting with surface charges. Notch
                  muting is the only way of removing them. Power lines also give rise to noisy
                  traces in the form of a mono frequency wave (50 or 60 Hz).
            •     Multiples are another type of coherent noise. They are secondary reflections
                  having inter- or intra- bed ray paths. They propagate both in sub and super-
                  critical regions.
            •     Power lines also cause noisy traces in the form of a mono-frequency wave. A
                  mono-frequency way may be 50 or 60 Hz, depending on where the field
                  survey was conducted. Notch filters of ten are used in the field to suppress
                  such energy.


9.2.3 Random Noise: Random noise has various sources. Poor planting of geophone,
wind, transient movements in the vicinity, wave motion in the water (marine) and finally
electrical noise of the recording instrument.
        One important aspect of data processing is to uncover genuine reflections by
suppressing all unwanted energies (noise of various types) .The objective of seismic data
processing is to convert the information recorded in the field to a form that can be used for
geological interpretation. Through processing we are enhancing the signal to noise
ratio, removing the seismic impulse from the trace (inverse filtering) and repositioning the
reflectors to its true location (NMO, DMO and migration), thereby making it into a more
palatable form.
9.3     Seismic Data Processing
        Seismic data processing is composed of basically five types of corrections and
adjustments:
    •   Time,
    •   Amplitude,
    •   Frequency-phase content,
    •   Data compressing (stacking), and
    •   Data positioning (migration)
        These adjustments increase the signal-to-noise ratio, correct the data for various
physical processes that obscure the desired (geologic) information of the seismic data, and
reduce the volume of data that the geophysicist must analyze.
        The geologic information desired form seismic data is the shape and relative position
of the geologic formations of interest. In areas of good data quality it is possible to produce
estimates of the litho logy based upon velocity information. From the amplitudes of
reflections, it is even possible to make estimates of the pore constituents, since gas
accumulations often generate amplitude anomalies. Knowing the shape of the structures at
depth allows oil company explorationists to assign probabilities of finding commercially
exploitable hydrocarbons in the area surveyed.
        The velocities of seismic waves in the earth can be derived from seismic data or
measured in wells, and they are used to convert the known reflection times into estimated
reflector depth.


9.3.1 Time Adjustments: Time adjustments fall into two categories:
                       Static and
                       Dynamic
Static time corrections (normal move-out) are a function of both time and offset and convert
the times of the reflections into coincidence with those that would have been recorded at zero
offset, that is, to what would have been recorded if source and receiver were located at the
same point.
9.3.2 Amplitude Adjustments: Amplitude adjustments correct the amplitude decay
with time due to spherical divergence and energy dissipation in the earth. There are two
broad types of amplitude gain programs:


                        Structural amplitude gaining or automatic gain control (ABC), and
                        Relative true amplitude gain correction


The first scales amplitudes to a nearly alike and is generally chosen for structural mapping
purposes. The second attempts to keep the relative amplitude information so that the
amplitude anomalies associated with facies changes, porosity variations, and gaseous
hydrocarbons are preserved.


9.3.3 Frequency-Phase Content: The frequency-phase content of the data is
manipulated to enhance signal and attenuate noise. Appropriate band-pass filters (one-
channel filtering) can be selected by reference to frequency scans of the data which aid in
determining the frequency content of the signals. De-convolution is the inverse filtering
technique used to compress an oscillatory (long) source waveform, often seen in marine data,
into as near a spike (unit-impulse function) as possible. Ghosts, seafloor multiples, and near-
surface reverberations can often be attenuated through de-convolution approaches. Many de-
convolution techniques use the autocorrelation of the trace to design an inverse operator that
removes undesirable, predictable energy.


9.3.4 Data Compressing (Staking): The data compression technique generally used
is the common midpoint (CMP) stack. It sums all offsets of a CMP gather into one trace.
Forty-eight to 96-fold stacks are common. Conventional 2D seismic data initially exist in a
3D space: the three axes are time, offset and a coordinate x along the line of survey. Three-
dimensional data consist initially of a 4D data set; the coordinates being time, offset and two
horizontal spatial coordinates, x and y, which lies on the midpoint axis.




9.3.5 Data Positioning (Migration): The data positioning adjustment is migration.
Migration moves energy form its CMP position to its proper spatial location. In the presence
of dip, the CMP location is not the true subsurface location of the reflection. Migration
collapses diffractions to foci, increases the visual spatial resolution, and corrects amplitudes
for geometric focusing effects and spatial smearing. Migration techniques have been
developed for application pre-stack, post-stack, or a combination of both.


9.4    Objectives of Data Processing


The objectives of data processing may be summarized as follows:
   •   To enhance the signal to noise ratio (S/N).
   •   To produce seismic cross section representative of geology.
   •   To meet the exploration objectives of the client.


9.5    Basic Data Processing Sequence


Since the introduction of digital recording, a routine sequence in seismic data processing has
evolved. There are three primary steps in processing seismic data
       1. De-convolution,
       2. Stacking, and
       3. Migration,
in their usual order of application. Figure 21(a) represents the seismic data volume in
processing coordinates – midpoint, offset and time.
All other processing techniques may be considered secondary in that they help improve the
effectiveness of the primary processes. The secondary processing steps include corrections
(statics, geometric, NMO, DMO, velocity analysis, filtering etc.). Many of the secondary
processes are designed to make data compatible with the assumptions of the three primary
processes. De-convolution assumes a stationary, vertically incident, minimum-phase, source
wavelet and white reflectivity series that is free of noise. Stacking assumes hyperbolic move-
out, while migration is based on a zero-offset (primaries only) wave field assumption.
Conventional processing of reflection seismic data yields an earth image represented by a
seismic section usually is displayed in time. A conventional processing flowchart is shown in
the figure 21(b) on the next page.
1. PRE-PROCESSING
      a. Demulitplexing
      b. Reformatting
      c. Resampling
      c. Editing
      d. Geometry Merging ( Labeling )
      e. Static Corrections
      f. True Amplitude Recovery
            i. Spherical Divergence Correction
            ii. Absorption/Attenuation Correction
      g. Muting
2.   TIME INVARIANT FILTERING
3. CMP SORTING
4. DECONVOLUTION
5. VELOCITY ANALYSIS
6. RESIDUAL STATIC CORRECTIONS
7. VELOCITY ANALYSIS
8. NMO CORRECTIONS
9. DMO CORRECTION
10. INVERSE NMO CORRECTION
11. VELOCITY ANALYSIS
12. NMO CORRECTION, MUTING AND STACKING
13. DECONVOLUTION
14. TIME VARIANT SPECTRAL WHITENING
15. TIME VARIANT FILTERING
16. MIGRATION
17. GAIN APPLICATION
                                     Chapter 10
       Seismic Data Processing Stage I
                           (Pre-Processing)

10.1
       Preprocessing: sequence and itiscommences with the receptionthe processing and
                      Preprocessing the first and foremost step in
                                                                     of field tapes
observers log. Field tape contains seismic data and observers log contains geographical data
(shot/receiver numbers, elevations, latitude, longitude etc).


10.1.1 De-Multiplexing: Field data are recorded in multiplexed mode (trace sequential)
using a certain type of format. So first de-multiplexing (time sequential) of the data has to be
done. Mathematically, de-multiplexing can be envisioned as transposing a big matrix so that
the rows of the resulting matrix can be read as seismic traces recorded at different offsets i.e.
changing time sequential form into a trace sequential form.


10.1.2 Reformatting: In this stage the data are converted to a convenient format which
is used through out processing. There are many standards available for data storage. Format
differs with the manufacturer, type of recording instrument and also with the version of
operating system. Since the processing software can not operate directly on the above
mentioned formats, the system internally converts its input data into a format which is
compatible to it. Data formatting defines –How data is arranged and what information is
stored as on magnetic media (tapes or drives) which will usually follow an industry standard
connection. Data from the field will not usually be in the format required by the processing
centre. The formats generally used for data recording are SEG-D (multiplexed/de-
multiplexed data), and SEG-B (multiplexed format). Hence they are called field formats. De-
multiplexing is not done on data recorded in SEG-D format. The out put of processing is in
SEG-Y format.
10.1.3 Re-sampling: Processing can be done at a sample rate different to that of
recording (e.g., 1, 2, or 4ms). Usually processing is performed at 4ms, if the accuracy is
sufficient, as the processing time and cost are less. If we are looking for an improvement in
resolution, or if we want more accuracy in measurements (velocity analysis, static
corrections), a sample rate of 4ms or even 1ms can be taken, provided that recording was at
this rate. Frequency aliasing effects can be avoided by high frequency (HF) filter, adopted to
the new sample rate:
                  Sample rate 4ms ------ cutoff 125Hz
                  Sample rate 2ms ------ cutoff 250Hz
                  Sample rate 1ms ------ cutoff 500Hz
For sample to go from a sample rate of 1ms to 4ms it is necessary to filter all the information
of frequency greater than 125Hz. Figure 22 (a) and 22(b) shows two different field records
one in SEG-D format, obtained in field and the other in SEG Y format readied for processing.


10.1.4 Editing: Editing involves leaving out the auxiliary channels & NTBC traces and
detecting and changing dead or exceptionally noisy traces. Bad data may be replaced with
interpolated values. Noisy traces, those with static glitches or mono-frequency high
amplitude signal levels are deleted. Polarity reversals are corrected. Out put after editing
usually include a plot of each file so that one can see what data need further editing and what
type of noise attenuation are required. Figure 23 (a) and (b) clearly shows the effect of
editing, wherein the removal of the occasional noisy traces gives the signal unmasked.
Figure 23 (a) shows the raw record and 23(b) shows the record after editing.


10.1.5 Geometry Merging (Labeling): No matter how meticulously processing
parameters are chosen, such as in de-convolution and velocity analysis, bad quality stack
section often is due to incorrect field geometry set up. So an important step in the
preprocessing is to apply the field geometry to the seismic data. The field geometry is
obtained from the observer’s log. The field geometry has to be incorporated with the seismic
traces. This was previously done manually. Nowadays this work is done by a module of the
processing software. This procedure is called Labeling or Merging. Figure 24 shows the
process of merging with the record being the cmp gather and the window with numbers being
the index which gives the details of the field parameters, using which the data on the record
was collected
10.1.6 Static Corrections: Sheriff’s definition of static corrections, often shortened to
statics, is as follows.
        “Corrections applied to seismic data to compensate for the effects of variations in
elevation, weathering thickness, weathering velocity, or reference to a datum”
Statics are time shifts applied to seismic data to compensate for:
    •   Variations in elevations in land,
    •   Variations in source and receiver depths (marine gun/cable, land source),
    •   Tidal effects (marine),
    •   Variations in velocity/thickness of near surface layers,
    •   Change in data reference times.
The objective is to determine the reflection arrival times which would have been observed if
all measurements had been made on a (usually) flat plane with no weathering or low-velocity
material present. These corrections are based on uphole data, refraction first-breaks, and/or
event shooting.
    •   Uphole-based statics involve the direct measurement of vertical travel-times form a
        buried source. This is usually the best static correction method where feasible.
    •   First-break based statics are the most common method of making field (or first
        estimate) static corrections.
    •   Data-smoothing statics methods assume that patterns of irregularity that most events
        have in common result from near-surface variations and hence static corrections trace
        shifts should be such as to minimize such irregularities. Most automatic statics-
        determination programs employ statistical methods to achieve the minimization.
The term ‘static’ is used to denote constant time shift of whole data traces, as opposed to
variable time shifts as applied by NMO corrections which are dynamic. The elevation needed
for shot/receiver time correction is obtained from labeling records. The velocity needed for
calculating the time shift is obtained from shot uphole times. The elevation corrections (also
called datum correction) may be used to bring all times in a seismic record to a fixed level in
subsurface which is the final processing datum. FPD could be any arbitrary level(depending
on the client requirement) or msl (mean sea level).
10.1.7 Amplitude Recovery (Geometric Spreading Correction): A field
record represents a wave field that is generated by a single shot. Conceptually a single shot is
thought of as a point source that generates a spherical wave field:
    •   In a homogeneous medium, energy density decays proportionately to 1/r2, where r is
        the radius of the wave front. Wave amplitude is proportional to the square root of
        energy density; it decays as 1/r. In practice, velocity usually increases with depth,
        which causes further divergence of the wave front and a more rapid decay in
        amplitudes with distance.
    •   The frequency content of the initial source signal changes in a time variant manner as
        it propagates. In particular, high frequencies are absorbed more rapidly than low
        frequencies. This is because of the intrinsic attenuation of the rocks.
Figure 25 shows a graph relating the Amplitude Decay with time/depth.
Newman’s Formula: The factor 1/r that describes the decay of wave amplitudes as a
function of the radius of the spherical wave front is valid for a homogeneous earth without
attenuation. For a layered earth, amplitude decay can be described approximately by 1/ [ v2
(t) · t ]. Here t is the two way travel time and v(t) is the rms velocity of the primary
reflections averaged over a survey area. Therefore the gain function for geometric spreading
compensation is defined by




where, v0 is the reflection velocity at a specified time t0 .


10.1.7.1      Spherical Divergence: For a spherically spreading wave in a ‘lossless’
material, the seismic pressure amplitude decreases as reciprocal of the distance traveled.




For a constant velocity medium,
But in the ‘layer cake’ model used in CMP stacking, the velocity increases between the
layers, and in practice it increases with depth within layers, this results in a TV2 relationship,
yielding




This property is used when compensating for amplitude decay. Figure 26(b) gives a
representation of the spherical divergence correction doing which we can see the clear
recovery of the amplitudes at the later times of the section that were not comparable with
those in figure 26(a) , which represents raw data.


10.1.7.2       Exponential Gain: Absorption is a process where by the energy of a seismic
wave is converted to heat while passing through a medium. The loss is a result of the elastic
movement. Absorption is very much a function of geology. Absorption can be expressed as a
function of the distance traveled by the seismic wave, implying it is also time variant.




where,     Ax = amplitude at distance x
           A0 = amplitude at reference point
           α    = attenuation factor (absorption coefficient)
The key point here is that amplitude decay due to absorption is exponential with distance.
Loss due to absorption seems to be nearly constant per cycle. Hence attenuation is lesser for
low frequencies and higher for higher frequencies. Again applying this gain correction we
can see the amplitudes recovering in figure 26(c) which were not available on figure 26(b).
figure 26(d) shows the record which is obtained by applying the filter the record 26(c).
10.1.8 Muting: The field data does not always necessarily contain the reflected data. It
may also contain first arrival, super critical reflections, ground coupled air waves, surface
waves (ground rolls) etc. So these effects have to be removed to improve the data quality. For
this purpose muting is done which involves arbitrarily assigning zero values to traces during
a desired interval selected by the processor.


10.2 Sorting: Seismic data acquisition with multifold coverage is done in shot-receiver
(s, g) coordinates. Seismic data processing, on the other hand, conventionally is done in
midpoint-offset (y, h) coordinates. The required coordinate transformation is achieved by
sorting the data into CMP gathers. Based on the field geometry information, each individual
trace is assigned to the midpoint between the shot and receiver locations associated with that
trace. Those traces with the same midpoint location are grouped together, making up a CMP
gather. Albeit incorrectly, the term Common Depth Point (CDP) and common midpoint
(CMP) often are used interchangeably. Figures 39(a) and 39(b)shows the superposition of
shot receiver (s, g) and midpoint-offset (y, h) coordinates, and raypath geometries for various
gather types. For most recording geometries, the fold of coverage nf for CMP stacking is
given by




where,     g and   s are the receiver-group and shot intervals, respectively, and ng is the
number of recording channels, by using this relationship, the following rules can be
established:


a. The fold does not change when alternating traces in each shot record are dropped.
b. The fold is halved when every other shot record is skipped, whether or not alternating
traces in each record are dropped.


10.3 Filtering: Filtering is done to remove unwanted frequencies from the seismic data.
Seismic frequencies have a range of 12 – 72 Hz and the frequencies other than this are
attenuated using various filtering techniques.
          The following tables give an idea on various types of noises & methods to attenuate
them.


                        Noise Attenuation Techniques
Random                                             Coherent
Band-pass filtering                                Band-pass filtering
Notch filtering                                    Velocity filtering i.e.F-K filtering
K-filtering e.g. Trace/shot summation              Muting
F-K filtering                                      Coherency filtering
Stacking
De-spike
F-X filtering
Coherency filtering
Editing (e.g. kill)


                        Land Data – Additional Type of Noises
Noise/problem                           Nature                       Solution
Hi-line                                 Random                       Kill, notch filter
Ground roll                             Coherent                     F-K filter
Air wave                                Coherent                     Hi-cut filter, surgical mute
Correlation noise                       Random                       Mute
Traffic(vehicles, people, animals)      Random                       Filter, stack
Falling debris                          Random                       Filter, stack
Wind noise                              Random                       Filter , stack
                                    Chapter 11
       Seismic Data Processing Stage II
                          (De-convolution)

11.1
       I ntroduction: De-convolution compresses the basic wavelet in the recorded
                      seismogram, attenuates reverberations and short-period multiples,
thus increases temporal resolution and yields a representation of subsurface reflectivity. The
processed normally is applied before stack; however, it also is common to apply de-
convolution to stacked data. De-convolution sometimes does more than just wavelet
compression; it can remove a significant part of the multiple energy from the section.
        Wavelet compression can be done using an inverse filter as a de-convolution
operator. An inverse filter, when convolved with the seismic wavelet, converts it to a spike.
When applied to a seismogram, the inverse filter should yield the earth’s impulse response.
An accurate inverse filter design is achieved using the least-squares method.
        The fundamental assumption underlying the de-convolution process (with the usual
case of unknown source wavelet) is that of minimum phase.      The Wiener filter converts the
seismic wavelet into any desired shape. For example, much like the inverse filter, a Weiner
filter can be designed to convert the seismic wavelet into a spike. However, the Weiner filter
differs from the inverse filter in that it is optimal in the least squares sense. Also, the
resolution (spikiness) of the output can be controlled by designing a Wiener production error
filter – the basis for predictive de-convolution. Converting the seismic wavelet into a spike is
like asking for a perfect resolution. In practice, because of noise in the seismogram and
assumptions made about the seismic wavelet and the recorded seismogram, spiking de-
convolution is not always desirable. Finally, the prediction error filter can be used to remove
periodic components – multiples, from the seismogram.


11.2    Convolutional Model: The recorded seismic trace may be modeled as a series of
interactions between the source signature (a finite, band limited wavelet) and the earth. The
convolutional model postulates that the above wavelet is the superposition of several
responses (the source wavelet, earth filter, ghosting, multiples, instruments etc.) to form a
complex pulse which then convolves with the reflectivity function to give the actual
seismogram. A seismic trace x(t) is given by the convolution of the basic seismic wavelet
w(t) with the reflectivity series r(t) plus random noise n(t).



                                                             -------------        (1)


11.3    De-convolution: The objective of de-convolution is to remove the effect of the
convolution of the basic wavelet with the reflectivity, output seismic trace to be the
reflectivity series. In practice it is to arrive at a better estimate of the reflectivity function. In
theory, we resolve the reflectivity r(t) from the equation given below.

                                                                             ---------- (2)
where, s(t) is the waveform component associated with source location
        e(t) represents the earth’s impulse response
Under the assumption the source waveform is known we have the following equation:

                                                          ---------- (3)
The basic seismic wavelet w (t) is actually made up of the convolution of source signature
with the propagation effects in the earth and the recording system sources.
In the frequency domain

                                                                           ------------   (4)
Where, X (f), S (f), E (f) and R (f) represent the amplitude spectra of the corresponding time
functions (ignoring the phase for now). We can remove the effect of the (S(f) ×E(f)) term in
this equation by making it equal to one (or any constant value). The function which has a
constant amplitude spectrum over all frequencies is a SPIKE. The de-convolution operator is
an inverse filter. In the time domain, de-convolution involves finding an inverse of the
wavelet which, when convoluted with the seismic trace, output the reflectivity series. The
seismic wavelet is converted to a spike.


11.4    De-convolution Methods: Generally de-convolution fall into one of the
following two categories
11.4.1 Deterministic De-convolution: De-convolution where part of the seismic
system is known. No random elements are involved. For e.g. where the source wavelet is
accurately known we can do source signature de-convolution. This is done when vibroseis is
used as the source.
11.4.2 Statistical De-convolution: Statistical De-convolution is a process where we:


•   Have no pre knowledge of the wavelet.
•   Derive information about the wavelet (either ‘source’, ‘system’, or combined wavelets)
    from the data itself, specifically from the auto correlation of the data.
•   Make certain assumptions about the data which justify the statistical approach.
•   Does not need to be used in conjunction with deterministic de-convolution.


Assumptions: To perform statistical de-convolution, the algorithm(s) used rely on the
following assumptions.
1. The earth is made up of horizontal layers of constant velocity.
2. The source generates a compress ional plane wave that impinges on layer boundaries at
    normal incidence. Under such circumstances, no shear waves are generated
3. The source waveform does not change as it travels in the subsurface – it is stationary (i.e.
    within the operator design window the shape of the wavelet is consistent. Multi-window
    design/application may be required to get optimum results for particular data sets where
    frequency content etc varies greatly with time).
4. The noise component is low enough to be ignored.
5. The source waveform is known.
6. Reflectivity is a random process. This implies that the seismic wavelet in that their
    autocorrelations and amplitude spectra are similar.
7. The input wavelet is minimum phase (i.e., before de-convolution a minimum phase
    conversion (source de-signature) step may be required), therefore, it has a minimum-
    phase inverse.
Assumptions 1, 2 and 3 allow formulating the convolutional model of the 1D seismogram by
equation 1. Assumption 4 eliminates the unknown noise term in equation 1 and reduces it to
equation 3. Assumption 5 is the basis for deterministic de-convolution – it allows estimation
of the earth’s reflectivity series directly from the 1D seismogram defined by equation 3.
Assumption 6 is the basis for statistical de-convolution – it allows estimates for the auto-
correlogram and amplitude spectrum of the normally unknown wavelet in equation 3 from
the known recorded 1D seismogram. Finally assumption 7 provides a minimum-phase
estimate of the phase spectrum, which is re-estimated from the recorded seismogram by way
of assumption 6.
Statistical de-convolution attempts to ‘spike’ the data and/or remove repetitive energy (e.g.
multipliers). ‘Spiking’ compresses the wavelet (by enhancing frequency content) but will
never result in ‘reflectivity’ series being output; mainly because


    •   Limited bandwidth
    •   Assumption not valid. E.g. not minimum phase, noise not zero etc.
Statistical de-convolution can be
    •   Spiking De-convolution
    •   Predictive De-convolution(Also ‘gap’ de-convolution)


11.4.2.1    Spiking De-convolution: The process by which the seismic wavelet is
compressed to a zero-lag spike is called spiking de-convolution. The filters that achieve this
goal are the inverse and the least-squares inverse filters. Their performance depends not only
on filter length, but also on whether the input wavelet is minimum phase. The spiking de-
convolution operator is strictly the inverse of the wavelet.
        Once the amplitude and phase spectra of the seismic wavelet are statistically
estimated from the recorded seismogram, its least-squares inverse – spiking de-convolution
operator, is computed using optimum Wiener filters. When applied to the seismogram, the
filter yields the earth’s impulse response.
        The Wiener filter applies to a large class of problems in which any desired output can
be considered, not just the zero-lag spike. Five choices for the desired output are
        Type 1: Zero-Lag Spike
        Type 2: Spike at arbitrary lag
        Type 3: Time-Advanced form of Input Series
        Type 4: Zero-Phase Wavelet
        Type 5: Any Desired Arbitrary Shape
The general form of the matrix equation for a filter of length ‘n’ is :




here, ri, ai and gi, I = 0,1,2,3    ,n-1 are the autocorrelation lags of the input wavelet, the
Wiener filter coefficients, and the cross-correlation lags of the desired output with the input
wavelet, respectively.
        The process with type 1 desired output is called spiking de-convolution. Cross
correlation of the desired spike, say (1, 0, 0, …..,0), with input wavelet, say (x0, x1, x2,
……, xn-1) yields the series (x0, 0, 0, ….., 0). The generalized form of the normal
equation1 takes the special form:




        This equation is scaled by (1/x0). The least-squares inverse filter has the same
form as the matrix equation (6). Therefore, spiking de-convolution is mathematically
identical least-squares inverse filtering.
        The autocorrelation matrix on the left side of equation 6 is computed from the
input seismogram (assumption 6) in the case of spiking de-convolution (statistical de-
convolution), whereas it is computed directly from the known source wavelet in the case
of least-squares inverse filtering (deterministic de-convolution).


11.4.2.2      Predictive De-convolution: The type 3 desired output (Time-Advanced
Form of Input Series) suggests a prediction process. Predictive de-convolution ‘predicts’
repetitive elements within the seismic trace (multiplier, ringing etc) and generates an operator
which will remove it leaving only the random element i.e. the reflection series. Given the
input series x(t), w want to predict its value at some future time (t + α), where α is prediction
lag. Wiener showed that the filter used to estimate x(t + α) can be computed by using a
special form of the matrix equation (5). Since the desired output x(t + α) is the time advanced
version of the input x(t), we need to specialize the right side of equation (6) for the prediction
problem. Following is the matrix showing an n-long prediction filter and an α-long prediction
lag:




Design of the predictive filters requires only autocorrelation of the input series. There are two
approaches to predictive de-convolution:


       •   The prediction filter may be designed using equation (7) and applied on input series.
       •   Alternately, the prediction error filter can be designed and convolved with the input
           series.
Predictive de-convolution is a general process that encompasses spiking de-convolution. In
general, the following statement can be made: “Given an input wavelet of length (n + α), the
prediction error filter contracts it to an α-long wavelet, where α is the prediction lag. When α
= 1, the procedure is called spiking de-convolution.


11.4.2.3       Predictive De-convolution in Practice
11.4.2.3.1           Operator Design: We start with a single, isolated minimum-phase wavelet.
Assumptions 1 through 5 are satisfied for this wavelet. The ideal result of spiking de-
convolution is a zero-lag spike. The action of spiking de-convolution on the seismogram
derived by convolving the minimum-phase wavelet with a sparse-spike series is similar to the
case of the single isolated wavelet. An increasingly better result should be obtained with
more and more coefficients are included in the inverse filter. Now consider the real situation
of an unknown source wavelet. Based on assumption 6, autocorrelation of the input
seismogram rather than that of the seismic wavelet is used to design the de-convolution
operator.
Auto Correlation: The result is a zero phase wave form with a maximum at zero lag. If
two wave forms are perfectly random then the auto correlation is a spike. Statistical de-
convolution filters(or operators) are most commonly derived from the auto correlation of the
input data using Wiener-Levinson algorithm.
Autocorrelation analysis: We can decay the point on our wavelet where our de-convolution
operator begins to operate - via the production ‘lag’ or ‘gap’. If the predictive gap or delay is
only one sample, we have spiking de-convolution. Or in other words, spiking de-convolution
may be considered as a special case of predictive de-convolution where the ‘gap’ is one
sample.
Following are some of the implications in designing the De-convolution Operator:
    •     The operator may be long enough to predict the multiples targeted.
    •     Design window usually at least five times operator length.
    •     Derivation windows slide behind the first break noise.
    •     Window over data representative of design criteria.
    •     Not too long an operator (less than 500 ms) – dependent on objective.
    •     Separate operators derived from multiple windows?
    •     One or two derivation windows at the most (multi window de-con).


11.4.2.3.2 Prediction Gap Length: Gap length will have an effect on:
    •     Pulse stabilization – to equalize the basic wavelet through out the data.
    •     Wavelet compression – degree of spiking.
    •     Occasionally, which multiple system is targeted – long gap length with short active
          operator to straddle long period multiples.
(Too long a gap may result in short period reverberations remaining)


11.4.2.3.3 Pre-Whitening: Addition of white noise to data (auto-correlogram) during
               operator design is to prevent:
    •     Operator instability (divisions by zero when calculating wavelet inverse).
    •     Equalizing the amplitude of noise in addition to the signal.
The amount of white noise to add will generally be in the range of 0.1 % to 1%.
Too little white noise may:
   •   Cause the de-convolution operator to become unstable.
   •   Decrease the S/N ratio of the data.
Too much white noise may:
   •   Decrease the effectiveness of the de-convolution process.
   •   Narrow the band width of data.
All these predictive de-convolution parameters are fixed from running de-convolution panels
by trial and error method.


11.4.2.3.4     De-convolution Panel: Operator length and amount is pre-whitening is
decided by trial and error method by applying different operator lengths and pre-whitening to
a CDP gather. The values of operator length and pre-whitening which yields the sharpest
output is taken as the optimum and de-convolution is done using these values.


       Figure 28 (a) shows the effect of application of the Spiking Deconvolution on the
raw data and we can see the events clearly marking their differences from the
neighbouring random reflections. The tempporal resolution is incresed and events show
the continuity in their behaviour. Figure 28 (b) shows the spectrum of the raw data and
the decon data. We can clearly see the removal of the incoherent noise caused by the
electric power lines in the decon spectrum.
                                     Chapter 12
    Seismic Data Processing Stage III
   (Velocity Analysis, NMO, DMO and
            Residual Static Corrections)

12.1
       V elocity Analysis: Velocity analysis is an interactive tool usedon 2D & 3D pre-
                           stacking or normal move out velocities
                                                                         to interpret


stack seismic data. Several techniques utilize the variation of normal move out with record
time to find velocity. Velocity analysis is usually done on common midpoint gathers where
the hyperbolic alignment is often reasonable. Where dips are large, a common reflecting is
not achieved. Typically the analysis procedure involves comparing a series of stacked traces
in which a range of velocities were applied in NMO. There are many methods for
determining correct velocities for the NMO equation. The methods that are being used by
RCC are given below.
Velocity Spectrum Analysis: Velocity spectrum analysis provides a means to
interactively pick the velocity which is correct for applying NMO corrections.
Multi Velocity Function Stacks: The multi velocity function stacks (mvfs) panel
displays a series of side by side stacked traces for a set of CDP’s. These traces are corrected
for NMO with a series of different velocities. The velocities can be a series of time variant
velocity functions as a function of time. Typically the test range is small at shallow times and
larger at deep times due to the nature of the NMO effect. This panel is used to pick velocities
by visually locating the maximum-stacked response.
In practice velocity analysis is done as follows:
         A reference velocity function is taken from the well data of the nearest well. A
number of velocity functions are then generated (in practice usually six). One half of them
will contain lesser velocity values and the other half will contain greater velocity values (as
compared to the reference velocity function) with a constant increment or decrement from
one velocity function to the other. Figure 29 shows a record displaying a section to be
analysed for velocity. A group of GDP’s (usually 21) which fall under full foldage area is
then taken and each of these CDP’s are stacked applying each one of the seven velocity
functions. The output is seven strips with 21 traces each, each strip corresponding to each
velocity function and each trace corresponding to each CDP. This is called a multi velocity
function stacks (mvfs) panel. From this we can interactively pick the correct velocity
function. Alternately a velocity spectrum is also generated. Mvfs are used generally to fine
tune the velocity picked using velocity spectrum. Figure 30 shows the velocity function
selection and thus how the velocity analysis is done.


12.2   Normal Moveout Correctons (NMO): NMO is the difference between
reflection arrival time at a geophone situated at a certain distance from the shot point and
arrival time at a geophone situated at the shot point. As offset increases, the seismic wavelet
arrives late at the geophone. This is not due to any anomalies in the subsurface, but due to the
additional distance traveled by the seismic wavelet. So a time correlation has to be applied
according to offset. NMO correction is the time correction which will ideally linearise the
alignment of primarily reflected signals in the CDP gather.
NMO is applied according to the formula




Where, Tx is the actual reflection time of the seismic event due to Normal Move Out effects.
T0 is the zero offset reflection time of the seismic event; x is the actual source receiver
distance; v is the normal Move Out velocity or stacking velocity of reflection event.
       While applying NMO the trace undergoes a slight non linear stretch which is called
NMO stretch. As a result of NMO correction a frequency distortion occurs particularly for
shallow events and at large offsets. The maximum permissible for the stretch is 10% and
signals where more stretch is observed is muted. It is quantified by
where, f is the dominant frequency
            f is change in frequency
            TNMO = Tx – T0
Figure 31 shows the NMO stack obtained after stacking the NMO corrected traces.
12.3       Dip Moveout Correctons (DMO): In the case of a dipping reflector, in
addition to NMO, another correction which takes into account is the dip of the reflector must
be applied. This following from the fact that, the move out will be greater when the reflector
is dipping. DMO correction is applied according to the formula




where, Tx = Two way travel time
           T0 = Zero offset travel time
           V = velocity above the reflector
           φ = dip angle
After applying the DMO correction the data is in CMP gather from a dipping interface model
do have a common reflector points. The terms CRP and CRP gather are accurate descriptions
of the data post DMO. Because DMO is a geometric correction that repositions seismic data
in a sense of migration scheme. The alternate name for DMO is pre-stack partial migration.


12.4       Residual Statics Corrections: ‘Field’ statics do not generally solve all delays
           within the data for a variety of reasons for example:
       •    Velocities vary both laterally and vertically within the layers.
       •    Weathering thickness varies rapidly.
       •    Undetectable thin, low velocity layers.
       •    Local anomalies (e .g ‘lenses’ of low velocity material near the surface)
       •    Vertical ray approximation is incorrect.
Residual statics correction attempts to fine-tune the field statics. Typical procedure is to
measure time-shifts between traces within a CMP and a ‘pilot’ trace (usually the stacked
CMP itself) and solve for the source and receiver static in a surface consistent manner .This
results in non surface consistent static values for every trace. Residual statics may be applied
to data as they are and known as ‘Trim’ statics. Residual statics can be, at times destructive.
         Residual statics corrections involve three phases:
         1. Picking travel time deviations tij based on cross-correlation of traces in a CMP
gather with a reference or pilot trace that needs to be defined in some fashion,
         2. Modelling tij by way of following equation and decomposing it into its
components: source and receiver statics, structural and residual moveout terms, and




where, the various terms are defined in the figure 32(a) while figure 32(b) shows how to
pick travel time deviations from NMO corrected gathers.
         3. Applying the derived source and receiver terms sj and rj, respectively, to travel
times on the pro – NMO – corrected CMP gathers.
The most common methods of deriving the time-shifts and the resultant static values are-
     •    Cross-correlation method
     •    Stack-power optimization
     •    Combination of above
The time-shifts produced using the cross-correlation technique may be decomposed into shot
and receiver statics by solving a set of simultaneous equations.
         Stack-power optimization, in simple terms, may be the result of applying multiple
sets of surface consistent values to the data and the set giving the maximum stack-power
chosen. Alternatively, the stack-power optimization may be used to determine the best
correlation coefficient prior to solving the final time-shifts using the simultaneous equation or
similar techniques.
         Figure 33 shows a field record on which represents the De-convoluted Stack and
Residual Stack.
                                     Chapter 13
    Seismic Data Processing Stage IV
(Stacking, Time Variant Filtering and
                                     Migration)
13.1       tacking: Stacking is basically summing of all the traces which has a common
       S             reflection point. By summing the S/N ratio is increased as signal gets
enhanced but random noise remains the same. Considering all the noises to be random, the
S/N ratio improvement by stacking will be √n times, where n is the foldage. The main point
in recording multifold data is to stack all the traces together. Stacking is ineffective in
suppressing multiples and diffractions. Before final stacking all the corrections viz.NMO,
DMO, Statics etc has to be made. Generally before decon and velocity analysis a gather is
stacked to have a rough idea about the different horizons, prevailing noises etc. This stack is
called BRUTE STACK. The velocity that has to be applied for NMO correction to prepare
brute stack is a reference velocity obtained from VSP data. Figure 34 shows real field record
with brute stacking. While the other figure 31 and 35 show the NMO Corrected Stack and
the Final Stack.
13.2 Time Variant Filtering:
Owing to the attenuation of seismic energy by the earth, the shallow reflections will have
high frequencies and the deeper reflections will have lower frequencies. Any departure from
this trend (ie high frequencies in lower part of the trace or low frequencies in the upper part
of the trace) indicates a noise which has to be removed so as to improve the S/N ratio. This is
done using time variant filtering. Time variant filtering is usually applied on stacked data.
        Figure 27 (b) represents the filtered record which is obtained by applying a high
pass filter (8 to 16 Hz) on the raw field record shown in figure 27 (a). This record clearly
shows the elimination of various noise components from the raw field record. The air
waves which are clearly visible on the raw record, at the mid portion of the record, are
eliminated in the filtered record.
13.3    Migration
        Migration is a process which attempts to correct the directions of the geological
structures inherent in the seismic section. Migration redistributes energy in the seismic
section to better image the true geological structures. Migration is done to rearrange seismic
data so that reflection events may be displayed at their true subsurface positions. It collapses
diffraction back to their point of origin. It improves resolution and collapses Fresnal zone. It
provides more accurate depth section.
        Zero offset stack section gives a false picture of dipping reflectors as events A`` and
B`` are plotted at true trace positions A` and B` respectively in figure 36. The apparent dip of
an event on a zero offset stack section is less than the true dip of the event.
13.3.1 Restrictions of 2D Migration: Migration must normally be carried out in the
plane of incidence relative to each horizon. It is only valid if this plane of incidence is fixed
for each horizon considered. The final section assembles all these planes of incidence to carry
out the migration. Migration requires that the velocity function at each of these planes of
incidence be known.
Migration is based on a 2D scheme with the following assumptions:
        1. All depth points of seismic horizons are in a single plane passing through the
            seismic line.
        2. This plane of incidence is vertical
        3. Structures can be represented by cylinders whose principal axes are perpendicular
            to the plane of section.
The ideas behind the above constraining assumptions also underlie the production of multi-
fold coverage stack sections, which give an inexact, deformed and displaced image of the
subsurface, as soon as there is any dip or velocity variation:
        1. Reflections originating anywhere are brought into the vertical plane of section
            (X,T). Geophones record the vertical component of the moment of the ground
            and the hydrophones record a pressure wave whatever the incidence of the
            wavefront is.
        2. After NMO and stack , the seismic section now represents               the theoretical
            acquisition configurations of coincident source and receiver, which only allows
            for only travel paths perpendicularly to the reflectors.
        3. Times are measured vertically along the CDP traces.
        4. The CDP is situated perpendicularly below the midpoint on the surface, which
            assumes horizontal beds.
 Geophysicists know well the simple examples of images in time deformed and / or
displaced in relation to the depth model:
        Depth model                           Time representation
     Diffracting point           Diffraction hyperbola
     Dipping reflector          Dipping reflector, shifted down dip
                                                   and dip decreased
      Tight syncline             “bow tie” shape


13.3.2 Migration in Fourier Domain: Migration in Fourier domain works with
dispersion relation which provides the relationship between the horizontal wavenumber and
the vertical wavenumber for any temporal frequency.
                                Kx2+ Kz2 = (ω/v)2
                                             /
If we consider the seismic as a sum of monochromatic plane waves, then all the same
frequency and the plane wave of the same frequency and dip are mapped on to a single point
in the F-K domain irrespective of their location in the original time section. So any operation
in F-K domain is localized to account for any lateral or vertical velocity variations.
13.3.3 Kirchoff Summation: The diffraction summation that incorporates the
obliquity, spherical spreading and wavelet shaping factors is called the Kirchhoff summation,
and the migration method based on this summation is called the Kirchhoff migration. To
perform this method, multiply the input data by the obliquity and spherical spreading factors.
Then apply the filter with the above specifications and sum along the hyperbolic path. Place
the result on the migrated section at time corresponding to the apex of the hyperbola.
        In practice, the order of the filter application specified by the wavelet shaping factor
(This wavelet shaping factor is designed with a 45-degree constant phase spectrum and an
amplitude spectrum proportional to the square root of the frequency for 2-D migration .
        For 3-D migration the phase shift is 90 degrees and the amplitude is proportional to
frequency.) and the summation can be interchanged without sacrificing accuracy because the
summation is a linear process and the filter is independent of time space. The velocity is
taken as the rms velocity, which can be allowed to vary laterally. However, lateral variation
in velocity distorts the hyperbolic nature of the diffraction pattern and somehow must be
considered. The value for the rms velocity typically is that of the output time sample.
13.3.4        Phase Shift Migration: Phaseshift migration is due to Gazdag and works in
F-K domain. For downward continuation, the phaseshift operator is computed at every depth
step allowing variation in velocity with depth. At every depth step, inverse Fourier transform
is taken to convert F-K domain for imaging at t=0, which is equivalent to summing over all
frequencies. This techniques can handles vertical velocity variation for dips while preserving
amplitude and phase, but cannot account for lateral velocity variations.
Phase Shift Plus Correction: This is an extension of phase shift migration to account for
lateral velocity variations. Here, the downward continuation is performed with a constant
average velocity function. After converting F-K to F-X, an additional phase shift is applied to
account for the difference between the average velocity function and the actual velocity
function at each X before applying the imaging principle. Here we cannot downward
continue the previous F-K domain data, because at each step an additional phase shift is
applied before imaging. Therefore the phase shifted data in F-K for the next depth steps and
hence is much move expensive. Still the method can account for only mild lateral velocity
variations.
13.3.5     Omega – X Migration (F – X Migration or Hybrid Migration): This
is similar to the finite difference migration in T-X domain and is developed by Kjartannson.
It is based on the 45 degree approximation to the one way scalar wave equation and is
formulated in the F-X domain.There are two terms in the computation: the diffraction term
that collapses the energy along the hyperbolic path to its apex and the thin lens shift term that
places the collapsed energy at its actual spatial position in the subsurface. This term is
velocity dependent for depth migration and velocity independent for time migration.
         Figure 37 shows a Migration stack.
                             Bibliography

Name of the Book                                        Author/Publisher
Introduction to Geophysical Prospecting                 Dobrin M.B.
Designing Seismic Surveys in two and three Dimensions   Dale G. Stone
A Handbook for Seismic Data Acquisition                 Evans B. J.
Static Corrections for Seismic Reflection Surveys       Mike Cox
ONGC Project Reports on KG Basin                        ONGC
Seismic Data Processing Quality Manual                  ONGC
Seismic Data Analysis                                   Oz Yilmaz
Acquiring Better Seismic Data                           Peter Carr Perchett
Encyclopedic Dictionary of Applied Geophysics           Robert E. Sheriff
Applied Geophysics                                      Telford W. M.

				
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Description: Seismic survey is a program for mapping geologic structure by observation of seismic waves, especially by creating seismic waves with artificial sources and observing the arrival time of the waves reflected from acousticimpedance contrasts or refracted through high velocity members.