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SURFACE GEOPHYSICAL METHODS FOR DETECTION <br />OF UNDERGROUND MINE WORKINGS <br />By <br />William J. Johnson and Robert E. Snow - D'Appolonia, Monroeville, PA <br />John C. Clark - Bay Geophysical Associates, Traverse City, MI <br />Abstract <br />Efforts to delineate underground mine workings in the vicinityof tailings impoundments has <br />relied on available maps and confirmatory boreholes. Often the characterization of regions <br />between boreholes remains suspect or entirely unknown. Surface geophysical measure- <br />ments can be used to supplement the borehole data and better delineate these intermedi- <br />ate zones. Furthermore, geophysics can be used to optimize the number and locations of <br />the boreholes. <br />There are several surface geophysical methods that are applicable to detect subsurface <br />voids: <br />• Gravimetry measures variations in the acceleration of gravity. The strength of this ac- <br />celeration generally depends on the density of the underlying material. Less massive <br />zones, such as cavities, generally constitute relative gravitational lows. <br />• Seismic measurements delineate reflections and refractions of compressional or shear <br />waves off subsurface layers with differing densities and wave velocities. There is usu- <br />ally a strong reflection on a cavity boundary. <br />• Electromagnetic and Electrical methods measure changes in resistivity. Cavities alter <br />the electrical resistance. Mine voids in coal mines are usually encountered as resistiv- <br />ity lows because of the presence of acidic mine water. <br />• Other Methods, such as magnetics and ground penetrating radar have been used for <br />detecting subsurface cavities, but can be subject to precision, interference, or depth <br />limitations which restrict their use in mining applications. <br />Project experience with various surface geophysical methods demonstrates that commer- <br />cially available technology can effectively detect and delineate mine workings. Two tech- <br />niques have the greatest potential for application. The DC resistivity method offers the <br />best potential for the rapid mapping mine workings at a depth of 50 — 100 feet or less. For <br />workings at a depth of 50 — 100 feet or greater, the seismic reflection method, especially <br />with the use ofS-waves, has the greatest potential for success. <br />Introduction <br />Two recent events have focused national attention on the need for mapping underground <br />mine workings, the failure of the Martin County Coal Corporation tailings impoundment <br />near Inez, Kentucky on October 11, 2000 and the July24, 2002 Quecreek Mine inundation <br />that trapped nine miners for 77 hours in Somerset County, Pennsylvania. In both cases, <br />unexpected conditions related to abandoned mines were the sources of the accidents. In <br />