GG 450

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GG 450 Lecture 19 February 26, 2006 Ground
Penetrating Radar
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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.
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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.
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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.
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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 (lt 50 m) subsurface evaluations.
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• 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. 

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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. 
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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.
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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.
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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.
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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.
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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.
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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/introd
uc.htm)
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HIGH RESOLUTION REQUIRED? YES use high
frequencies DEEP PENETRATION REQUIRED? YES use
low frequencies
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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.
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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.
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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.
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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."
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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.
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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.
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Whats the wavelength of the signal at 100
MHz? Velocity distance / time Wavelength
distance / cycle Frequency cycles / time
Wavelength velocity / frequency 3108
m/sec / 108 cycles/sec 3 meters
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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.
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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.
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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 materials 12 - 20 ns/m
Damp geological materials 20-35 ns/m
Water 60,000 ns/m

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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.
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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
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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.
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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.
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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.
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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
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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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