How to Find Buried
Objects:
The Ground Penetrating Radar
[The text in this article has been extracted from the United States Environmental Protection Agency]
Ground penetrating radar (GPR) can be a very useful
geophysical method for UST sites because it is appropriate for a broad
range of investigations and is only rarely affected by cultural
interferences (e.g., buildings, fences, power lines).
GPR uses high frequency electromagnetic waves (i.e.,
radar) to acquire subsurface information. The waves are radiated into
the subsurface by an emitting antenna. When a wave strikes a suitable
object, a portion of the wave is reflected back to a receiving antenna.
Measurements are continuously recorded with a resolution that is
significantly higher than most other surface geophysical methods,
providing a profile (i.e., cross-section) of subsurface
conditions.
The GPR method utilizes antennas that emit a single
frequency between 10 and 3000 MHz. Higher frequencies within this range
provide better subsurface resolution at the expense of depth of
penetration. Lower frequencies in this range allow for greater
penetration depths but sacrifice subsurface target resolution.
In UST investigations, the working frequency range is
generally 100 to 900 MHz. Frequencies above 900 MHz are typically used
for investigations less than 2 feet below ground surface (bgs).
GPR Profile Crossing 4 UST's
In addition to the antenna frequency, the depth of wave
penetration is controlled by the electrical properties of the media
being investigated. In general, the higher the conductivity of the
media, the more the induced radar wave is attenuated (absorbed),
lessening the return wave.
Electrically conductive materials (e.g., many mineral
clays and soil moisture rich in salts and other free ions) rapidly
attenuate the radar signal and can significantly limit the usefulness of
GPR. For example, in shallow, wet clays with high conductivity values
(30 millimhos per meter or greater), the depth of penetration may be
less than 2 feet.
In contrast, in dry materials that have electrical
conductivity values of only a few millimhos per meter, such as clay-free
sand and gravel, penetration depths can be as great as 90 feet.
Penetration depths typically range between 3 and 15 feet bgs.
As a result, it is important to research the likely
subsurface materials in an area before deciding to use this method. Test
surveys are also commonly used to help predict the success of GPR.
The depths to reflecting interfaces can be calculated from
the two-way travel times of the reflected waves. Travel times are
measured in nanoseconds (i.e., 1 billionth of a second). Because
the velocity of electromagnetic radiation through various materials is
well established, one can calculate the depth of penetration with
various techniques. Estimations can also be made if the nature of the
subsurface materials is only generally known.
GPR measurements are usually made along parallel lines that
traverse the area of interest. The spacing of the lines depends on the
level of detail sought and the size of the target(s) of interest.
Typically, an average walking pace of 2 to 3 miles per hour
is used. Some very detailed investigations can be as slow as 0.1 mile
per hour, and newer systems can be mounted on vehicles and used at
speeds up to 65 miles per hour for reconnaissance of the shallow
subsurface.
The data can be recorded for processing off-site, or they
can be produced in real-time for analysis in the field.
GPR is relatively unaffected by above surface cultural
interferences if the GPR antennas are shielded. For antennas that are
not shielded, an experienced operator can often distinguish and ignore
reflections from overhead objects.
Subsurface cultural interferences include densely packed
rebar used in reinforced concrete (the density at which rebar is a
problem is site specific), wire mesh (often used for concrete floors in
buildings), and pipes and utilities (if geology is the target).
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