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Seismic Acquisition

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					        Land and Marine Seismic
       Acquisition from 2D to 3D




From chapters 7-12 “Elements of 3D Seismology” by
Chris Liner
                   Outline-1
CMP METHOD (Harry Mayne)
Seismic sensors
    •geophones
    •hydrophones
    •gimballed geophones and hydrophones
    •accelerometers
Sources
    •Explosives
    • Vibroseis
    •aIr gUNs
SEGY data
                   Outline-2
Acquisition Parameters
    •Time Sample Rate
    •Offset Range
    •Listen Time
    •Sample Rate and Temporal Aliasing
    • Geophone Spacing and Spatial Aliasing
    •Shooting geometry
        •inline
        •cross-line
   Common Midpoint Method (CMP Method)




  Please take a look at the powerpoint presentation for the
       radio-telemetry field trip at the following link:
http://www.geol.lsu.edu/Faculty/Juan/ReflectSeismol05/labs
                  /ppt/Radio-telemetry.ppt
 This link has information to complement the explanation on
                      the CMP method.
         Common Midpoint Method (CMP Method)

  Hydrophone groups        Shotpoint # 1
#6 #5 #4 #3 #2        #1




                 Midpoints


            Separation between midpoints is
        1/2 separation between hydrophone groups
        Common Midpoint Method (CMP Method)

 Hydrophone groups    Shotpoint # 2
#6 #5 #4 #3 #2       #1




               Midpoints
       Common Midpoint Method (CMP Method)

Hydrophone groups    Shotpoint # 3
#6 #5 #4 #3 #2       #1




              Midpoints
       Common Midpoint Method (CMP Method)

Hydrophone groups    Shotpoint # 4
 #6 #5 #4 #3 #2           #1




              Midpoints
       Common Midpoint Method (CMP Method)

Hydrophone groups    Shotpoint # 5
   #6 #5 #4 #3 #2         #1




              Midpoints
       Common Midpoint Method (CMP Method)

Hydrophone groups    Shotpoint # 6
     #6 #5 #4 #3 #2        #1




              Midpoints
       Common Midpoint Method (CMP Method)

Hydrophone groups    Shotpoint # 7
        #6 #5 #4 #3 #2       #1




              Midpoints
       Common Midpoint Method (CMP Method)

Hydrophone groups    Shotpoint # 8
          #6 #5 #4 #3 #2       #1




              Midpoints
       Common Midpoint Method (CMP Method)

Hydrophone groups    Shotpoint # 8
          #6 #5 #4 #3 #2       #1




              Midpoints
         Common Midpoint Method (CMP Method)

  Hydrophone groups
#6 #5 #4 #3 #2        #1
                              Shotpoint # 1




               Midpoints
         Common Midpoint Method (CMP Method)

  Hydrophone groups
#6 #5 #4 #3 #2        #1
                              Shotpoint # 1
                                Shotpoint # 2


                               Shotpoint # 1
                                 Shotpoint # 2

               Midpoints
         Common Midpoint Method (CMP Method)

  Hydrophone groups
#6 #5 #4 #3 #2        #1
                              Shotpoint # 1
                                Shotpoint # 2
                                  Shotpoint # 3

                               Shotpoint # 1
                                 Shotpoint # 2
                                  Shotpoint # 3
               Midpoints
         Common Midpoint Method (CMP Method)

  Hydrophone groups
#6 #5 #4 #3 #2        #1
                              Shotpoint # 1
                                Shotpoint # 2
                                  Shotpoint # 3
                                    Shotpoint # 4
                               Shotpoint # 1
                                 Shotpoint # 2
                                  Shotpoint # 3
                                    Shotpoint # 4



                Midpoints
       Common Midpoint Method (CMP Method)

Hydrophone groups


                                                     Shotpoints # 1-8




                    1
                        2           Midpoints
                            3
                                45 6 7 8   8    13
       Common Midpoint Method (CMP Method)
   Fold or Multiplicity is the number of times that the same
midpoint is sampled by different shots and different receivers
   Signal-to-Noise increases as the square root of the fold


Fold




                 1
                     2           Midpoints
                         3
                             45 6 7 8   8    13
       Common Midpoint Method (CMP Method)


         Maximum Fold is achieved after the 6th shot




Fold




                 1
                     2           Midpoints
                         3
                             45 6 7 8   8    13
          Common Midpoint Method (CMP Method)


  When shotpoint spacing and group spacing are equal then
    Maximum fold = number of geophones or hydrophones
   Midpoint separation = 1/2 distance between geophones


                  In a more general case:
Maximum Fold = #recording groups * distance between groups
                2 * distance between shots
Midpoint separation = 1/2 smaller of the two: receiver group
                 spacing or shot spaceing
            Gather Types




A gather i.e. “a subset of the traces from the entire
data set” can be of different types:



        •Shotpoint gather
        •Common source-receiver offset gather (COS)
        •Common midpoint gather
Shotpoint Gather


e.g. Shotpoint gather #3
                 Shotpoint Gather
                   Shotpoint #3




       Hydrophone groups
      #6 #5 #4 #3 #2            #1




A shotpoint gather samples various midpoints and a variety of angles
What happens to the reflecting points in a shotpoint
   gather when the reflecting interrface dips?

                                Shotpoint #3




       Hydrophone groups
      #6 #5 #4 #3 #2            #1




A shotpoint gather samples various midpoints and a variety of angles
     What happens to the reflecting points in a shotpoint
        gather when the reflecting interrface dips?

                                    Shotpoint #3




           Hydrophone groups
           #6 #5 #4 #3 #2           #1



   Reflecting points
         Midpoints
A shotpoint gather samples different reflecting points at a variety of angles
Common Midpoint Method (CMP Method)

           Hydrophone group #4
     Common source-receiver offset and
       common receiver, shotpoints 1-8
                   Hydrophone group #4
             Common source-receiver offset and
              common receiver, shotpoints 1-8




COS means equal reflection angle




                              Midpoints
  In the case of a COS gather where are the true
 midpoints when the reflecting, geological interface
                     has a dip?




COS means equal reflection angle




                             Midpoints
       COS NO LONGER implies equal reflection angles




Actual reflecting points

                              Midpoints
Common Midpoint Method (CMP Method)

           Hydrophone group #4
           Common mid-points and
               shotpoints 1-8




                         Midpoints
                  Hydrophone group #4
                  Common mid-point and
                     shotpoints 1-8




                  group
         8 76 5 4 3 2 1




                          Midpoint #6

CMP gathers sample varying angles but a common geological midpoint
What happens to a common midpoint gather when the
         reflecting interface has a dip?




                  group
         8 76 5 4 3 2 1




                          Midpoint #6

CMP gathers sample varying angles but a common geological midpoint
 CMP gathers SAMPLE varying angles but with
         a relatively smaller spread of
   reflecting points than the shotpoint and
            common-offset gathers




                 group
        8 76 5 4 3 2 1




        Midpoint #6

True Reflecting Points
A common midpoint gather minimizes the effect of dip while it
          helps increase the signal-to-noise ratio
                   Outline-1
CMP METHOD (Harry Mayne)
Seismic sensors
    •geophones
    •hydrophones
    •gimballed geophones and hydrophones
    •accelerometers
Sources
    •Explosives
    • Vibroseis
SEGY data
                     Geophones

       Convert ground motion into electricity
         at a rate of about 1 Volt/inch/sec




                            Natural Resonance Frequency 100 Hz


GS-100 from Geospace
Geophone layout
Geophone layout
                Seismic Sensors


•Hydrophones convert changing pressure into Volts
                  (Volts/bar)




   e.g. Preseis 2517 from I/O 1V/microPascal
•Gimballed Geophone-hydrophone combinations for
                sea-bottom work



                        Sea-Array 4 from Geospace
Streamer layout
                   •Accelerometers


    Convert ground acceleration into Volts
                  d(dx/dt)
                     dt




             E.g. VectorSeis from I/O
3-component digital accelerometer (requires battery)
 full-scale at 3.3 m/s2; noise level 0.44 microm/s2
           140db = 20 log (3.3/4*10^-7)
                   Outline-1
CMP METHOD (Harry Mayne)
Seismic sensors
    •geophones
    •hydrophones
    •gimballed geophones and hydrophones
    •accelerometers
Sources
    •Explosives
    • Vibroseis
SEGY data
 Vibroseis Method (Liner, 2004;
         p.157, para. 4, )

An output sweep
(e.g., 10-80 Hz)


enters the earth




                   …..and undergoes various reflections
          +
                      +               =
                                   ...something too complicated to draw




Field correlation “unravels” the raw data into ….
Vibroseis images from the Lithoprobe Project, Canada
                 www.lithoprobe.ca




                    A vibrator truck




             “12 elephants dancing in unison” (LITHOPROBE, CANADA)
Explosives

    Noble Explochem Limited
         GI Watergun Array




NSF R/VIB NBPalmer- February/March 2003
                   Outline-1
CMP METHOD (Harry Mayne)
Seismic sensors
    •geophones
    •hydrophones
    •gimballed geophones and hydrophones
    •accelerometers
Sources
    •Explosives
    • Vibroseis
SEGY data
                 SEGY data       3200 byte
One line at a time             EBCDIC header



                             400 byte tape header

                             240 byte trace header

                                         DATA
                             240 byte tape header

                                          DATA

                              400 byte tape header

                                           DATA
                   Outline-2
Acquisition Parameters
    •Time Sample Rate
    •Offset Range
    •Listen Time
    •Sample Rate and Temporal Aliasing
    • Geophone Spacing and Spatial Aliasing
    •Shooting geometry
        •inline
        •cross-line
           Sample Rates




What is the fewest number of times I need to sample
             this waveform per second?

                                             ?



                                             ?


                                             ?
Sample Rates
Sample Rates
Sample Rates
           Sample Rates




What is the fewest number of times I need to sample
             this waveform per second?


      At least twice per wavelength or period!



                     OTHERWISE ….
Undersampled waveforms

                           f   f
Amplitude




Reconstructed frequency          True frequency (f -true)
            (f -aliased)
    Oversampled waveforms

                         Nyquist frequency
  Amplitude




  Reconstructed frequency = True frequency (f -true)
      frequency is unaliased


    Nyquist frequency = 1 / twice the sampling rate

Minimum sampling rate must be at least twice the desired frequency
              E.g., 1000 samples per second for 500Hz,
               2000 samples per second for 1000 Hz
      Oversampled waveforms

                            Nyquist frequency
    Amplitude




In practice we are best oversampling by double the required minimum
            i.e. 1000 samples per second for a maximum of 500 Hz
         i.e., 2000 samples per second for a maximum of 1000 Hz
                      Oversampling is relatively cheap.
                   Outline-2
Acquisition Parameters
    •Sample Rate and Temporal Aliasing
    •Offset Range
    •Listen Time
    • Geophone Spacing and Spatial Aliasing
             Offset Range

            Maximum shot-receiver
                     offset




                                                 Target depth



      One-layer earth of a semi-infinite layer
Maximum shot-receiver offset >= target depth.
           Near critical distance
 Offset Range

Maximum shot-receiver
        offset




                        Target depth




 Multi-layered earth
                   Outline-2
Acquisition Parameters
    •Time Sample Rate
    •Offset Range
    •Listen Time
    •Sample Rate and Temporal Aliasing
    • Geophone Spacing and Spatial Aliasing
    •Shooting geometry
        •inline
        •cross-line
        Listen Time


….Twice target time to be sage
                   Outline-2
Acquisition Parameters
    •Time Sample Rate
    •Offset Range
    •Listen Time
    •Sample Rate and Temporal Aliasing
    • Geophone Spacing and Spatial Aliasing
    •Shooting geometry
        •inline
        •cross-line
                        Spatial aliasing

Spatial frequency, or wavenumber (k) is the number of cycles per
unit distance.


One spatial cycle or wavenumber = frequency/velocity.


 Each wavenumber must be sampled at least twice per wavelength
                  (two CMP’s per wavelength)



                         1
               Nk 
                    2(CMPspacing )
IN PRACTICE each wavenumber must be sampled at least four times per minimum
                   wavelength (two CMP’s per wavelength)
                  Spatial aliasing




However, dip (theta) as well as frequency and
velocity event changes the number of cycles per
distance, so


                         lambda
          CMP interval                    Liner, 9.7,p.192
                          4sin 
                  Spatial aliasing




                     lambda
      CMP interval 
                      4sin 

                       x
                                    
                           V t

                 V t
 sin  limit   
                  x

For aliasing NOT to occur, delta(t) must be less than T/2
    Spatial aliasing


                VT
sin  limit   
                2 x

               VT
  xlim it   
              2sin 
Geophone Spacing and Spatial Aliasing




                                   K=0
    1/4 wavelength shift per trace
total shift across array=3/4 wavelength




                                K=+ or -ve?
    1/4 wavelength shift per trace
total shift across array=3/4 wavelength




                                     K=?
    1/2 wavelength shift per trace
total shift across array=3/2 wavelength




                                     K=0
     3/4 wavelength shift per trace
total shift across array=2 1/4 wavelength
              Spatial aliasing




•Degrades (“string of pearls”) stacked sections
•Degrades migration
                Signal-to-Noise




Improves with stacking:
•greater fold
•greater repetition of shots



        S / N  CMP fold vertical stack
                   Outline-2
Acquisition Parameters
    •Time Sample Rate
    •Offset Range
    •Listen Time
    •Sample Rate and Temporal Aliasing
    • Geophone Spacing and Spatial Aliasing
    •Shooting geometry
        •inline
        •cross-line
Fundamental Parameters for land 3D shooting



           X geophone  ( xg , y g , z g )
           X shot  ( xs , ys , zs )
        Common Midpoint




         1
X CMP    ( X shot  X geophone )
         2
  Source-Receiver Offset




X offset  ( X shot  X geophone )   2D



X offset  X shot  X geophone       3D
        Azimuth (3D)



         xshot  xgeophone 
  tan 
        1
                            
        y y               
          shot    geophone 
Inline geometry




                  Matlab code
                   Outline-2
Acquisition Parameters
    •Time Sample Rate
    •Offset Range
    •Listen Time
    •Sample Rate and Temporal Aliasing
    • Geophone Spacing and Spatial Aliasing
    •Shooting geometry
        •inline
        •cross-line
Cross-line geometry




                      Matlab code
              Spatial aliasing




•Degrades (“string of pearls”) stacked sections
•Degrades migration