Ground Penetrating Radar
Ground Penetrating Radar (GPR) uses high-frequency pulsed electromagnetic waves to map subsurface information. GPR uses transmitting and receiving antennae, which are dragged along the ground surface. The transmitting antenna radiates short pulses of high-frequency radio waves into the ground. The wave spreads out and travels downward. If it hits a buried object or a boundary with different electrical properties, the receiving antenna records variations in the reflected return signal. The principles involved are similar to reflection seismology, except that electromagnetic energy is used instead of acoustic energy, and the resulting image is relatively easy to interpret. Integration of GPR data with other surface geophysical methods reduces uncertainty in site characterization. GPR provides the highest lateral and vertical resolution of any surface geophysical method.
Best penetration is achieved in dry sandy soils or massive dry materials such as granite, limestone, and concrete. GPR provides the greatest resolution of currently available surface geophysical methods. It is the only reliable method for detecting buried plastic containers.
Limitations and Concerns
Depth of penetration (typically 1 to 15 meters) is less than direct current (DC) resistivity and electromagnetic (EM) methods, and is further reduced in moist and/or clayey soils and soils with high electrical conductivity. Penetration in clays and in materials having high moisture is sometimes less than 1 meter.
The GPR method is sensitive to noise—i.e., interference caused by various geologic and cultural factors. For example, boulders, animal burrows, tree roots, and other phenomena can cause unwanted reflections or scattering. Cultural sources of noise can include reflections from nearby vehicles, buildings, fences, power lines, and trees. Electromagnetic transmissions from cellular telephones, two-way radios, television, and radio and microwave transmitters may also cause noise on GPR records. Shielded antennae are used to limit these types of reflections.
The bulkiness of equipment can limit use in rough terrain.
Unprocessed images require processing, as they provide only approximate shapes and depths (continuous microwave methods are still in developmental stages).
Ground penetrating radar is a widely accepted field screening technology for characterizing and imaging subsurface conditions. Ground penetration radar (GPR) is most commonly used for locating buried objects such as tanks, pipes, and drums, as well as shallow unexploded ordnance (UXO). It can also be used to map the depth of a shallow water table, identify soil horizons and the bedrock subsurface, and demarcate trench boundaries. It is also used to delineate the physical integrity of manmade structures, such as slurry walls and permeable reactive barrier walls. GPR can also detect plastic, glass, concrete, or wood.
Technology Development Status
GPR is commercially available.
Other Resources and Demonstrations
The American Society for Testing and Materials (ASTM) has an approved “Standard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation.”
Also see http://www.serdp-estcp.org/Program-Areas/Munitions-Response/Land/Sensors/MR-200126/MR-200126 for description of airborne radar demonstration to detect UXO on the surface, with a possibility to delineate some shallow subsurface features.
See http://costperformance.org/monitoring/pdf/5_crystal.pdf and http://costperformance.org/monitoring/pdf/13_wurtsmi.pdf for case studies.