by Lambert Dolphin
formerly Senior Research Physicist
SRI International, Menlo Park, California
In the early 1970's several different teams of scientists began to develop radars for viewing into the earth. Radars of this type were first developed for military applications-such as locating tunnels under the DMZ between North and South Korea. GPR use in locating and mapping utility lines has been the subject of much on-going research conducted by both military and commercial organizations.
GPR's usually operate in the VHF-UHF region of the electromagnetic spectrum. The frequency used is a compromise. One desires to use the lowest possible frequency because low frequencies give reasonably high penetration depths into the earth. But a sufficiently high frequency must be selected so that the radar wavelength is short, allowing detection and resolution of small objects such as pipes. For cart mounted radars, 150 MHz is a typical center frequency, however 300 and 500 MHz are sometimes used for shallow, high-resolution probing, and frequencies as low as 20 MHz are used for locating deep caves or mine tunnels.
GPR's are also known as "impulse radars" because the transmitted pulse is very short and is ordinarily generated by the transient voltage pulse generated from an overloaded avalanche transistor. Resistively-loaded antennas are employed because one can not tolerate antenna ringing. The distances in the ground where such radars look is measured from inches to tens of feet. This corresponds to travel times measured in nanoseconds, that is, billionths of a second. Short transmitted pulses imply wide radar bandwidths, so a GPR operating at a center frequency of 150 MHz actually radiates substantial energy from 75 to 300 MHz.
The performance capability of this type of radar is strongly dependent on the soil electrical conductivity at the site. If the soil conductivity is high, attenuation of the radar signal in the soil can severely restrict the maximum penetration depth of the radar signal. In California where soils in many areas are often high in clay content the soil absorptive losses can be quite high. Whereas maximum penetration depth achievable with these radars can be tens of feet in favorable environments, these numbers are reduced to a few feet or less at many sites in California.
GPR surveys should be performed in the dry season if at all possible, especially at California sites. Soil moisture, especially in high-clay soils, only increases the radar attenuation rates, further limiting the radar performance.
Spurious radar echoes (known as "clutter") can also be expected in many test areas because of buried debris such as old rails, wire scraps, boulders, and small metal objects. Usually a trained operator can interpret the desired radar signatures in the midst of a moderate amount of such clutter.
It is not possible to built GPR antennas so that the antenna beamwidth is narrow. The wide antenna beamwidth of cart-mounted ground-penetrating radars makes it difficult to resolve closely spaced objects, such as two parallel pipes in a common trench. In some cases the fill in the trench or the trench walls may be detected on the radar, but the pipes in the trench may not be radar discernible. Interpretation of GPR records is an art as well as a science, even with the best available state of the art radars.
Ground-penetrating radars in principal are capable of locating plastic pipes as easily as metallic pipes since the radar signal reflection from the pipe depends on contrasting dielectric properties of the soil and pipe, not just a high electrical conductivity for the pipe. In actual practice (1) soil attenuation may restrict the use of GPR to shallow depths. (2) The GPR antenna beamwidth is broad making it difficult for radar to discriminate between closely-spaced pipes. (3) In disturbed ground the radar may detect the walls of a trench but not the pipe it contains. Nevertheless GPR can be very useful when a thorough search of the site is required. GPR normally has an accuracy of several feet or less when measuring the depth of a buried object.
Cart GPR's rely on motion of the cart to generate a continuous radar record of traverse distance vs. depth in the earth. GPR data is ordinarily recorded on video tape with voice comments for archive purposes, and printed out on thermal printer paper on site for immediate analysis. The successful interpretation of GPR records is an art as well as a science requiring considerable operator experience for good results.
GPR's are also used in boreholes, and for point-to-point exploration in either a monostatic or bistatic mode. Monostatic sounding means the transmitter and receiver are located in the same borehole, bistatic sounding implies the transmitter and receiver are emplaced in separate boreholes.
revised April 3, 1995. November 7, 1997.
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Background on Ground Penetrating Radars
Geophysical Methods in Archaeology
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