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|Authors:||Ballard, Robert F.|
|Publisher:||Geotechnical Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Technical report (U.S. Army Engineer Waterways Experiment Station) ; GL-82-9.|
Abstract: This study evaluated numerous geophysical techniques to determine their applicability to the detection of clandestine tunneling activity, either in progress or completed, which is directed against field fortifications. The first priority was to develop a rapid and reliable approach for detecting tunneling at shallow depths (less than 50 m) in rock. The course of this investigation operated under the premise that a rapid reconnaissance survey using only surface geophysical methods would first be performed followed by a detailed or high-resolution survey in which strategically placed boreholes would be included. Two well-documented test sites, both located in the State of Florida, were chosen for evaluation of the methods. The Medford Cave test site, near Ocala, Fla ., was an air-filled cave system located about 20 to 30 ft below the ground surface. The second test site near Chiefland, Fla., was a state park called Manatee Springs . This site differed from Medford Cave in that the cavities were located approximately 100 ft below the ground surface, were water-filled, and were mapped by cave divers. In addition, two existing seismic triangulation systems developed by the U. S. Bureau of Mines were also evaluated for application to the tunnel detection problem. One permanently installed system is located at the CONOCO-owned Loveridge Mine in West Virginia. The second system is portable and was observed in operation at a mine site in Kentucky. Both systems concepts were considered to be well suited (with minor modifications) for the detection of clandestine tunneling. Those geophysical methods determined to be best suited for a tunnel detection reconnaissance survey were: (a) ground-probing radar, (b) electrical resistivity (Wenner array), (c) conventional seismic refraction, (d) seismic refracted wave form, (c) seismic refraction fan-shooting, and (f) microgravity. The methods considered best for a detailed high-resolution survey were: (a) crosshole radar, (b) crosshole seismic, (c) borehole microgravity, and (d) surface electrical resistivity (pole-dipole).
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