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Ground Penetrating Radar (commonly called GPR) is one of the
most widely used geophysical instruments today but reliance
on just GPR during geophysical surveys can lead to major
mistakes. GPR is a geophysical method that has been
developed over the past thirty years for shallow,
high-resolution, subsurface investigations of the earth. GPR
uses high frequency pulsed electromagnetic waves (generally
10 MHz to 1,000 MHz) to acquire subsurface information.
Energy is propagated downward into the ground and is
reflected back to the surface from boundaries at which there
are electrical property contrasts. GPR is a method that is
commonly used for environmental, engineering, archeological,
and other shallow investigations. GPR can be a wonderful
method when site conditions are optimal but there are many
misconceptions about GPR. GPR penetration depths as well as
accuracy are directly proportional to the conductivity of
the materials below. If you are working in an area with
increased soil conductivity then the likely hood of signal
attenuation (absorption) is high. In some cases depth of
penetration can be less than two feet. Below is a small list
of materials that may cause significant signal loss:
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1. Clay Soils
2. Reinforced Concrete
3. Highly Saline Soils
4. Foundry Slag |
5. Fly Ash
6. Saturated Soils
7. Certain Paving Materials
8. Metallic Debris
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The signal loss from the
GPR in these materials can cause an operator to misinterpret
data or overlook an anomaly completely. Other instruments
should be used in these circumstances to help resolve the
situation. For example, if an investigation for underground
storage tanks is being conducted in an area with high
concentrations of clay then an electromagnetic survey
(commonly called EM) can help locate the UST’s. GPR is
still one of the most effective tools when it comes to
shallow investigations. But reliance on this method alone in
particular circumstances can lead to negative results. If
there are any questions about how geophysical surveys are
conducted and the proper techniques used during these
surveys please feel free to contact us. Thank you for taking
the time to read our technical bulletin.
Below are GPR
Transects of Underground Storage Tanks in a Sandy
Environment and in a Clay Environment. Also included is an
EM-61 Plot of a UST in the same Clay Environment.
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| (GPR Transect of
UST in Sandy Environment) |
(GPR Transect of
UST in Clay Environment) |
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(EM-61 Plot of UST in Clay Environment) |
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