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Symposium on Geotechnical A4ethods for Mine A4apping Ierifications, Charleston, West Virginia October 29, 2002 <br />contrasts. Potentially applicable techniques include: <br />Gravity — The detection of mine workings with the gravity method is based on the meas- <br />urement of minute changes in the earth's gravity field caused by the lack of near -surface <br />mass associated with the mine. Basically, you weigh less when you are standing over a <br />mine. The measurement of the gravity field for this application is referred to as microgra- <br />vimetryand requires the use ofspedalized gravimeters that with a sensitivity of a microgal, <br />approximately a billionth of the <br />earth's gravity field. An air-filled <br />void represents the maximum den- <br />sity contrast that could be caused <br />by a mine opening and a mine at a <br />depth of about 30 feet would in <br />theory be detectable with commer- <br />cial equipment. In practice, the <br />gravity method is time-consuming <br />to acquire and elevation control re- <br />quirements are such that it is pref- <br />erable to have a topographic sur- <br />vey crew accompany the geo- <br />physicist to measure the precise <br />elevation of the instrument at each <br />reading. For a target as shallow as <br />30 feet, the width of the gravity <br />anomaly is about 100 feet, which <br />implies that the survey requires a <br />significant amount of accessible space, which is not often present. Furthermore, it is often <br />difficult to correct the gravity data for variations caused by tides, surrounding topography, <br />instrumental drift, and elevation. In particular, the authors have found that microtopog- <br />raphic changes can significantly affect gravity readings. Unless the target is in a flat, open <br />area and the depth does not exceed about 40-50 feet, the gravity method will probably not <br />be practical. <br />Seismic Methods - The seismic <br />techniques consist of measuring <br />the travel time required for a seis- <br />mic wave generated at or near the <br />surface to return to surface or <br />near -surface detectors (geo- <br />phones) after reflection or refrac- <br />tion from acoustic interfaces be- <br />tween subsurface horizons. The <br />seismic reflection method is the <br />most powerful of all geophysical <br />techniques in mapping subsurface <br />layering and is by far the most <br />commonly applied method to ex - <br />3 <br />