Refraction surveying is frequently necessary along the same transect line to resolve shallow

velocity relationships.

Ground Penetrating Radar (GPR)

Ground penetrating radar (GPR) technology uses repetitive, high frequency (80 1000

MHz), short time duration (nanoseconds) electromagnetic energy radiated into the ground to

acquire continuous subsurface profile data along a transect. The electromagnetic pulses are

emanated using a broad bandwidth radar antenna that is placed in close proximity to the

surface and is electromagnetically coupled to the ground (Morey, 1974). The antenna is

moved across the measurement surface along the line of the survey. The transmitted radar

signals are reflected from various subsurface interfaces in response to contrasts in the

dielectric properties of the subsurface materials and are received back at the transmitting

antenna where the signal is processed. The method is capable of producing a high quality

graphic profile at speeds of up to several kilometers per hour. GPR can resolve subsurface

conditions on the order of centimeters. Commonly, a printed record of the survey run is

produced in the field so that the applicability of the method to a particular site is quickly

determined. Interpretation skills of the operator are critical in obtaining reliable data.

GPR has been used to profile both the water table and the overburden/bedrock

interface, to locate buried objects including storage tanks and utilities, and to identify voids

and areas of soil subsidence; GPR also has had considerable utility in mine applications.

Beres and Haeni (1991) provide results of the application of GPR to stratified drift deposits in

Connecticut.

The depth of radar signal penetration is highly site specific and dependent on the

electrical conductivity properties of subsurface soil and rock. Morey (1974) reported

penetration depth of greater than 75 feet in water saturated sand and 230 feet in an Antarctic

ice shelf. Fountain (1976) states that this method has shown detection capacity only to depths

of approximately 2.4 meters in moist, clay rich soils. If the specific conductance of the pore

fluid is sufficiently low, however, data can commonly be obtained to a depth of 3 to 10

meters in saturated materials (Dobecki and Romig, 1985). Electrically conductive subsurface

materials such as wet clay, sea water, or extensively micaceous materials with high dielectric

permittivity properties can significantly attenuate radar signals. Signal attenuation for a

particular material is also dependent on the frequency of the radar pulse. In general, good

results can be obtained in dry, sandy, rocky areas.

The continuous nature of GPR offers a number of advantages over many other

geophysical methods and allows for a substantial increase in the detail obtained along a

traverse line. Additionally, the high speed of data acquisition permits many lines to be run

across a site, and in some cases, total site coverage is economically feasible (Benson et al.,

1982). The method is limited by the attenuative properties of many subsurface materials, by

November 1992

4 27