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Title: Cavity detection and delineation research. Report 1, Microgravimetric and magnetic surveys, Medford Cave Site, Florida
Authors: Butler, Dwain K.
Keywords: Cavities (Underground)
Cavity detection
Medford Cave, Florida
Geophysical exploration
Subsurface exploration
Gravimetric surveys
Issue Date: Mar-1983
Publisher: Geotechnical Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: Technical report (U.S. Army Engineer Waterways Experiment Station) ; no. GL-83-1 rept. 1.
Description: Technical Report
Abstract: This report reviews the scope of a research effort initiated in 1974 at the U.S. Army Engineer Waterways Experiment Station with the objectives of (a) assessing the state of the art in geophysical cavity detection and delineation methodology and (b) developing new methods and improving or adapting old methods for application to cavity detection and delineation. Two field test sites were selected: (a) the Medford Cave site with a relatively shallow (10- to 50- ft- deep) air-filled cavity system and (b) the Manatee Springs site with a deeper (approximately 100-ft-deep) water-filled cavity system. Results of field studies at the Medford Cave site are presented in this report: (a) the site geology, (b) the site topographic survey, (c) the site drilling program (boreholes for geophysical tests , for determination of a detailed geological cross section, and for verification of geophysical anomalies), (d) details of magnetic and microgravimetric surveys, and (e) correlation of geophysical results with known site geology. Although some correlation was observed at the Medford Cave site between magnetic anomalies and other geophysical anomalies and with geological conditions revealed by drilling, the magnetic survey in general did not yield results which were of value in defining this site. The microgravimetric survey, however, was extremely successful. The known cavity system was delineated as to locations and trends of cavities by the gravity anomaly maps. Six selected negative gravity anomaly features, in areas of the site with previously unknown subsurface conditions, were drilled for verification purposes, and all borings intercepted air- or clay-filled cavities or clay- or sand-filled pockets in the top of the limestone. Of 11 borings in positive gravity anomaly areas, only three intercepted cavities. The cavity system was adequately detected and delineated using a 20-ft station spacing. A station spacing of 10 ft allowed detection of smaller scale cavities and other solution features in the top of the limestone. Using simple assumed models for the cavities, computed depths to tops of cavities agree to better than 25 percent, thicknesses to better than 40 percent, and areal extent of cavities to better than 15 percent of known cavity dimensions. These percentages are completely model-dependent and in no way reflect the accuracy of the survey or the microgravimetric technique. Qualitative interpretation guidelines using complementary geophysical techniques for site investigations in karst regions are presented. Including the results of electrical resistivity surveys conducted at the Medford Cave site, the qualitative guidelines are applied to four profile lines, and drilling locations are indicated on the profile plots of gravity, magnetic, and electrical resistivity data. Borehole logs are then presented for comparison with the predictions of the qualitative interpretation guidelines.
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