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
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






 Home | About Us | Network | Services | Support | FAQ | Control Panel | Order Online | Sitemap | Contact

quality web hosting


Our partners: PHP: Hypertext Preprocessor Cheap Web Hosting JSP Web Hosting Ontario Web Hosting  Jsp Web Hosting

Cheapest Web Hosting Java Hosting Cheapest Hosting Business web hosting division of Vision Web Hosting Inc.. All rights reserved