GPR - SOEST by n6y3Jc

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									        GG 450
       Lecture 19
   February 26, 2006


Ground Penetrating Radar
Background:

Contrary to popular belief, GPR or Ground
Penetrating/Probing Radar, is not a new
technology. The first uses were in Austria in
1929, but the technology was largely
abandoned until the late 1950's when U.S. Air
Force radars were seeing through ice as
planes tried to land in Greenland, misreading
the altitude and crashing into the ice.
 This started investigations into the ability of radar
to see into the subsurface not only for ice sounding
but also mapping subsoil properties and the water
table. GPR systems have been in commercial use
for over 30 years. It is only recently that the
environmental, construction and utility industries
have discovered the multiple uses and benefits of
performing GPR surveys to gain forehand
knowledge of what's underground and in walls.
GPR surveys are now being specified into
engineering designs, environmental assessments
and maintenance programs.
Description:

Ground penetrating radar has many similarities with
wave propagation methods in subsurface imaging
for oil exploration. This analogy has been used to
transfer technology from the petroleum industry to
the geotechnical arena. The approach does have its
limits, as the physical processes involved in signal
transmission are very different. Seismic methods
use acoustic waves, radar uses electromagnetic
waves.
Nonetheless, GPR has become one of the
instruments of choice for many small site
investigations where a metallic object that is
shallowly buried, such as an underground
gasoline storage tank, must be located.
Ground penetrating radar (GPR) has
established itself as a successful technique
for a wide range of shallow (< 50 m)
subsurface evaluations.
How does GPR work?

•   Ground penetrating radar uses
electromagnetic wave propagation and
scattering to image, locate and quantitatively
identify changes in electrical and magnetic
properties in the ground. It may be
performed from the surface of the earth, in a
borehole or between boreholes, from
aircraft or satellites. It has the highest
resolution in subsurface imaging of any
geophysical method, approaching
centimeters under the right conditions.
A transducer generates a broadband (10-1000
MHZ) electromagnetic wave (impulse). A specially
directed antenna emits the pulse into the ground.
As the wave travels through the ground, it is
reflected, deflected and absorbed by varying
degrees of the material (soil, water) through which it
travels. As the radar reflects off of materials it ‘echo
locates' materials, or objects, of different
electromagnetic conductivity within a matrix, for
instance, a pipeline, storage tank, contaminant or
re-bar in a matrix of soil or concrete.
The receiver in the antenna will pick up the return
signal to be processed by the radar unit. The radar
unit will then plot a mark on a vertical scale based
on the time it took for each signal to return. The
radar unit will also analyze the characteristic
properties of the waves, mainly the amplitude. On
the same plot, the radar unit will assign a color to
the vertically-scaled mark based on the severity of
change in the return signal's amplitude and the
emitting signal's amplitude. This severity of change
in amplitude of the transmitted signal is based on
the conductivity and dielectric properties of the
reflective target.
The waves reflect off the subsurface interfaces as
if they are mirror-like. Because of this, the image
produced will not be a direct replica of the
subsurface – sloping reflectors will appear to
slope less than they really do, and point or
circular reflectors will appear as hyperbolas.
The pulse has to travel through the substrate
before it gets to the reflector, and again
through the substrate to get to the receiver.
Anything in the substrate that may block the
beam will affect the data. Because the beam
is a 45° cone, reflectors angled at greater
than 45° cannot be seen. Objects within the
matrix, such a pipeline or re-bar, show up
quite clearly as hyperbolas with amplitudes
depending on their conductivity contrast.
Because the propagation of electromagnetic
energy at radar frequencies is controlled by
dielectric properties in geologic materials, the
method is sensitive to changes in dielectric
permittivity of the bulk material. The dielectric
permittivity of a material is strongly related to its
resistivity. The higher the resistivity, the higher
the dielectric permittivity, and the farther an
electro-magnetic wave will propagate through
that material without absorption.
The bulk dielectric permittivity of a rock formation
is highly dependent upon the dielectric value of
any pore fluid present, the degree of saturation,
and the porosity. The presence of water filled
pores increases the bulk dielectric permittivity
from the value associated with the unsaturated
state. This characteristic allows GPR to detect the
water table under certain conditions. If pore water
is replaced by organic compounds, which typically
have a dielectric constant less than water,
electromagnetic energy will be reflected.
Depth of Investigation varies from less than a
meter to over 5,400 meters, depending upon
material properties. Detectability of a
subsurface feature depends upon contrast in
electrical and magnetic properties, and the
geometric relationship with the antenna.
Quantitative interpretation through modeling
can derive from ground penetrating radar data
such information as depth, orientation, size
and shape of buried objects, density and
water content of soils, and much more.
(http://www.g-p-r.com/introduc.htm)
HIGH RESOLUTION REQUIRED?
  YES: use high frequencies
DEEP PENETRATION REQUIRED?
  YES: use low frequencies
Sander et al. [1992] and Greenhouse et al. [1993]
describe the 1991 Borden experiment, in which GPR
was used, along with other geophysical techniques,
to monitor a controlled spill of percholorethylene
(PCE), a dense nonaqueous phase liquid (DNAPL).
This study points out the need for time-differential
measurements to remove background effects to
allow the detection of small dielectric changes. This
technique will be most useful for monitoring
contaminant movement during remediation efforts.
GPR Data Collection:
In order to generate an "image" of a buried object ,
a GPR profile must be obtained. A GPR profile is
generated when the antenna is moved along the
surface. This can be done by hand, by vehicle, or
even by air. The radar unit emits and receives
reflected signals up to a thousand times per second.
As a result, not only do the relative depths and
"strengths" of the targets appear, but the image or
shape of the target is "seen" on the monitor.
A number of these transect lines need to be
acquired to gain a precise location of the target in
one direction. The same process must be done in
the perpendicular direction to get a full picture of
where objects are in the matrix. The reflected
energy pulses are acquired only in a narrow line
directly below where the transects are taken and
the positions of objects have to be correlated from
line to line. The data can also be utilized in a 3-D
program to yield a sub-surface profile of the area
surveyed.
An obvious problem with GPR data acquisition is
site accessibility. Since the GPR antenna has to be
moved over the area to be investigated, the search
area has to be physically accessible. Heavily
wooded sites or areas containing cars, debris piles,
sharp inclines, etc. all limit the accessibility of GPR
data acquisition. A good analogy when considering
the accessibility of a GPR investigation (for most
applications) is to use Geo-Graf's rule of thumb, "
The desired search area has to be clear enough so
that you could push a shopping cart through it."
In addition to the medium through which the GPR
pulse travels, the frequency of the wave is a
contributing factor in depth of GPR signal
penetration. Typically, within the range of GPR
antenna frequencies, the lower the frequency of the
pulse, the deeper the signal penetration, but at the
"cost" of data image resolution. Conversely, the
higher the frequency, the greater the image
resolution, but at the "cost" of signal penetration.
This is due to the inherent properties of the
Earth, that typically allow lower-frequency waves
to travel farther within the subsurface. The type of
antenna used will depend on the particular
targets-of-concern. For instance, in measuring
concrete floor thickness or rebar spacing, a 900
to 1500 MHz antenna would provide the best
data. However, if the desired target is a UST or
bed rock layers, a 120 MHz or 80 MHz antenna
would be best.
What’s the wavelength of the signal at 100 MHz?
Velocity = distance / time
Wavelength = distance / cycle
Frequency = cycles / time


Wavelength = velocity / frequency
              = 3*108 m/sec / 108 cycles/sec = 3 meters
GPR works best in dry coarse-grained materials
like sand and gravel. It works poorly in moist fine-
grained sediments. Penetration in course grained
sediments may be as much as 20 m and as little
as 2 m in fine-grained materials.
Usually GPR can be used with several antennae sizes that
produce waves of different frequencies. High frequency
antennas (200 to 400 MHz) produce the highest resolution
images, but penetrate only to shallow depths because
waves are quickly attenuated. Low frequency (80 MHz)
antennae produce poorer resolution images, but can
penetrate more deeply into the subsurface.
The Radargram

GPR data are presented as a radargram. As the
antennas are moved across the surface, the
Transmitter radiates short sharp pulses, and the
Receiver records the echoes. The radar system
constructs amplitude vs. time traces as the
antennas are moved across the subsurface, very
much like a seismic reflection profile. These traces
are plotted next to each other showing recorded
amplitudes vs. distance along the profile, and time
(depth) into the ground.
 The resulting radargram appears in the form distance
(horizontal axis) vs. time (vertical axis). The simplest
conversion from time to depth requires that one know (or
estimate) the velocity of the pulse in the ground. Typical time-
to-depth conversion factors are given in the next table:


Medium                 Time-to-Depth Conversion, (two-
                       way travel-time)
Air                    6.6 nanoseconds/meter
Dry geological         12 - 20 ns/m
materials
Damp geological        20-35 ns/m
materials
Water                  60,000 ns/m
                 APPLICATIONS
ENVIRONMENTAL & ARCHAEOLOGICAL SURVEYS

mapping extent of contaminant plumes
determining direction of contaminant migration
locating buried storage tanks
locating buried artifacts, ruins or treasures
delineating boundaries of ancient cemeteries and locating
burial plots
Mapping gravel and sand deposits, determining depth and
quantifying volumes
Glacial ice thicknesses - in cold ice bedrock-ice contact
can be mapped.
Finding rebar or culverts during highway construction.
Finding caves or sinkholes.
Oil & Gas:

      Location of pipelines, utility lines, water and sewer
      pipes for gas / oil facility surveys
    Assessing depth of sediment cover over pipeline river
  crossings and rights of way
Determining depth to bedrock for proposed pipeline rights of
way

Aerial reconnaissance
 Survey depths up to 4 meters or more (depending on soil
conditions - see chart)
  GPR can be tied to a GPS to yield precise locations
 3-D software allows results to be obtained with x, y and z
coordinates
CIVIL SURVEYS

Accurate location of in-slab:
  structural steel (re-bar)
  stress cables
  electrical and communication conduits - including
PVC, fiber optics, telephone wiring and other non-
ferrous materials
  water and sewer pipes
It is essential to avoid hitting these features when
coring or drilling through a concrete slab during
construction renovations.
Advantages of GPR
As opposed to other locating techniques that are capable of
detecting only metallic or conductive utilities and underground
targets, GPR can locate and characterize both metallic and
non-metallic subsurface features. It is completely nonintrusive,
nondestructive and safe. GPR can be thought of as a
Subsurface Imaging System, similar to sonar used for
underwater applications. With GPR, surface conditions are not
a major factor. Targets can be "seen" beneath reinforced
concrete, asphalt, gravel, and most other common surfaces.
    High-resolution data in certain cases.
      Non-destructive and quiet.
     Requires only one or two people for field work.
      Fast and economic
.     Wide spatial coverage may be obtained, can
      be towed by a truck.
Disadvantages of GPR

Equipment is expensive.
Limited penetration depths.
Can be used in only specific sediment-bedrock
terrains.
Requires trained people for data collection and
interpretation.
   Post-processing of data requires sophisticated
   computer software
Information about dialectric properties must be
known in order to convert to wave return times to
depths.
Safety and Interference Concerns

During investigations, especially civil surveys, GPR surveys
are often performed near sensitive electronic equipment or
tenant occupied spaces. To address safety and interference
concerns GPR technology is quite benign. The energy
source is, as the name implies, Radar, or radio frequency. It
is relatively low power so there are no deleterious effects
from destructive radiation and no need to do locates after
hours.
The antennas used in civil surveys are fully shielded to
direct all the transmitted energy into the ground and to
eliminate surface reflection artifacts and radio frequency
interference common to an unshielded system
      The radar signal reflects off of any objects with a
      difference in conductivity so materials such as
      plastics or air voids, as well as steel, can be
      resolved. Distinguishing between different materials
      can only be done in a relative sense, and because
      concrete varies a great deal, a direct calibration must
      be done to get accurate depth measurements. The
      reflected signal from 3 mm steel reinforcement mesh
      can be more pronounced than 30 mm PVC
      conduit. GPR can accurately resolve objects such
      as re-bar, stress cables and conduit in concrete to a
      depth of 450 mm depending on how many other there
      are in between.

Commercial daily rate: ~$3,500.

								
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