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|Title:||Evaluation of geophysical methods for cavity detection at the WES Cavity Test Facility|
|Authors:||Butler, Dwain K.|
|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-80-4.|
Abstract: The study reported herein was undertaken to plan and construct a controlled Cavity Test Facility for use in preliminary evaluation of geophysical methods as cavity location or delineation tools and to evaluate several geophysical techniques to determine whether signatures could be obtained that would help to either locate a cavity or, once located, to determine its size, depth, or shape in plan view. Seismic studies conducted at the Cavity Test Site consisted of surface compression- (P-) wave refraction surveys, crosshole shear- (S-) wave surveys, shallow seismic reflection surveys, "Meissner wave-front" surveys, and a sonar investigation. Results for each of these studies are given. Surface resistivity surveys were also made, including the following methods : Modified Bristow or Bristow-Bates, Wenner profiling, Schlumberger sounding, and dipole-dipole. Subsurface resistivity tests were also accomplished. Radar tests using three different radar systems - a 4.2-GHz continuous wave-frequency modulated (CW-FM) system, a 100-Hz pulse radar system, and a 300-MHz pulse radar system - were also performed at the Cavity Test Facility. Results of the attempt to detect cavities at the WES Cavity Test Facility were mostly negative. It is recommended that an alternate test site be found where conditions are not as extreme, that cavity detection research continue, and that the following geophysical methods be investigated : (a) microgravimetric techniques including gravity-gradient measurements, (b) high-resolution seismic profiling, (c) expanding spread seismic fan shooting, (d) cavity diffraction signatures with crosshole geometry, and (e) continued study of subsurface probing radar to determine the lithology dependence of its applicability. Appendix A presents seismic reflection records, and Appendix B contains data sheets for Bristow-Bates surveys.
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