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| What Is GPR? - Geophysical Equipment Descriptions - GPR | |||||||||||
| Background: | |||||||||||
| Contrary
to popular belief, GPR or Ground Penetrating/Probing
Radar, is not a new technology. GPR systems were
originally developed by the military and 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. GPR surveys are now being specified
into engineering designs, environmental assessments and
maintenance programs. The advantages of GPR are simple. 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 non-intrusive, non-destructive 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. Some specific applications include the detection of underground storage tanks (USTs) and drum piles; utility locating and mapping; pipeline leak detection and assessment; delineation of underground voids, near-surface rock and water tables; the extent and boundaries of landfills, historic fill, and borrow pits; delineation of contaminant product plumes; detection of historic features, such as foundations, trash pits, and burials; grave site investigations and mapping; concrete structural analysis including, rebar and rebar spacing delineation, floor/wall thicknesses, sub-base settlement and void detection, post-tensioned cable location; as well as nearly any other near-surface subsurface anomaly or target-of-concern. Some limiting factors in using GPR include site accessibility and penetration depths. While the GPR units are the size of laptops, the antenna systems can be fairly large. For an area to be surveyed with GPR, it must be relatively free from underbrush, debris and equipment. A Geo-Graf rule-of-thumb is, "If you want an area surveyed, it has to be clear enough that you could push a shopping cart through it." The second limiting factor is depth of signal penetration. GPR should be thought of as a near-surface technique. For most commercial or industrial project sites, the realistic depth of signal penetration of GPR within the mid-Atlantic U.S. is usually 8' to 15' below the surface. Depths are totally dependent upon the GPR antenna system used and the properties of the subsoil. However, for the majority of projects, these depth ranges are more than sufficient to delineate most utilities, tanks, foundations, and other features-of-concern. Even with recent computer advances in data collection and analysis, GPR data interpretation is highly dependent upon the knowledge and experience of the operator. GPR is an instrument that can not be taken out of the box and used effectively by anyone. It takes years of field experience in "real world" situations to determine the best data collection procedures and to accurately interpret the data. This is a major reason for the perceived slow growth of the GPR industry compared to other non-intrusive technologies. However, with proper training and experience, GPR is a valuable tool with multiple uses and applications in a wide variety of different engineering, environmental, and archaeological fields. There are several GPR manufactures in the world, some better than others, but all ground radar systems work on the following basic principles. |
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| Basic Principle of Operation: | |||||||||||
| Ground Penetrating Radar is analogous to the radar used by airports to track planes. The word radar is actually an acronym standing for radio detecting and ranging. A radar device is like a very sophisticated stopwatch. A transmitter will generate and emit a pulse (wave). If something is in the path of the pulse (a target), it will either deflect, reflect or absorb the wave. If the wave is reflected, the pulse is picked up by the receiver and processed. The time it takes for the wave to return can be calculated and equated to the distance the target is from the transmitter. Also, by analyzing some of the characteristic properties of the return pulse, additional information about the target can be obtained. | |||||||||||
| GPR works on the same principle. A transducer generates a broadband 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. 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. | |||||||||||
| 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. 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 400 MHz or 200 MHz antenna would be best. |
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| GPR Data Analysis: | |||||||||||
| In
theory, GPR data interpretation sounds easy, however,
there are several other factors that complicate things.
As mentioned in the basic principles, the frequency and
the medium (materials) through which the waves travel are
important properties. Unlike airport radar waves that
travel through the atmosphere in tight directional bands,
GPR waves travel through many different materials in the
ground, in wide-angle bands. Different types of soil,
fill material, debris, and varying amounts of water
saturation all have different dielectric and conductive
properties that effect the GPR waves, and thus GPR data
interpretation. In a "perfect world," all soil
would be homogenous, allowing the GPR operator to be able
to point to the data and determine that a target is 8'
below the surface. In the "real world," the
soil is a combination of pavement, rebar, fill material
and debris, all at varying degrees of saturation. As a
result, GPR data interpretation is considered more of an
art rather than a science. Following the proper procedures for data acquisition is important, but making correct data interpretations involves years of field experience that cannot be taught in school or found in a library. Currently, there are many new GPR post-processing software programs aimed to assist in the computerization of data interpretation, however, these only accelerate the interpretation process. The "expert's eye" will never be replaced. |
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| GPR Depth Determination: | |||||||||||
| Since
the subsoil between 0-20 feet are far from homogenous,
actual target depth determination is difficult, if not
impossible. However, for most cases an estimated depth
range can be determined with accuracy wholly dependent
upon the subsurface material. There are 4 methods for
target depth approximating, the two most common are
included here. Method (1): D = (5.9 t ) /sqrt of (er) where: D = depth of target in inches, t = wave travel time in nanoseconds, scaled from GPR data, 5.9 = a constant incorporating the speed of light and unit conversions, and er = dielectric constant of subsurface material. The following are dielectric constants of common materials found in the field. Air
....................................1 Thus, if a target is detected at 46 ns in dry sand (er = 4), according to the Equation, the target is approximately 11 feet below the surface. However, if the same target at 46 ns is located in wet clay (er = 12), from the Equation, the depth is approximately 6.5 feet below the surface. Method (2): Best Method:
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