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Symposium on Geotechnical Methods for Mine Mapping r-erifrcation$ Charleston, West Virginia October 29, 2002 As a supplement to conventional drilling and sampling, a DC resistivity technique was em- ployed to address its capabilities in determining the location of abandoned mine workings. The resistivity testing was conducted along a 700 -foot -long test profile that traversed a main in the deeper seam, as well as a buried highwall 7� r� associated with the shallow seam. In terms of resistiv- ity target, the mine workings at a depth of 90 feet repre- C, sent a much more complicated target than the previous two case histories, considering its depth and complex conditions above the seam, including strip mining and m�'Por,dmm"""" �`m"� augermining of shallowerseam. a Setup for unconventionalpole- The survey was initiated using two configurations: con - dipole survey ventional dipole -dipole and an unconventional pole - dipole. With the conventional dipole -dipole configuration two electrodes separated by a constant spacing called the "a" spacing are used to inject current into the ground. With this configuration, two additional electrodes also separated by the "a" spacing are moved along the survey line at distances from the current elec- trodes that are multiples of the "a" spacing. The pole -dipole survey applied for this survey is considered "unconventional' because the "infinite" electrode was not placed in a remote position on the ground, butwas located within the deep coal seam. The results of the single test line provided two useful pieces of information. The existing mine map does not depict the true extent of shallow augering of coal from the former strip mine highwall. The resistivity results indicate that the augering extends about 150-200 feet from the highwall along the test line, farther than was indicated by the test borings. Also, the existing mine map appears to be a good representation of the deep mine workings. The dipole -dipole survey results depict the deep workings in a manner consistent with the mine map and the results of the boreholes. In terms of the effectiveness of the conducted resistivity surveys, conventional dipole - dipole surveying appears to provide the best resolution of the shallow subsurface. The unconventional pole -dipole survey provided deeper penetration and could resolve the loca- tion of deep mine workings. The overall conclusion of this geophysical application was that deep coal workings can be effectively imaged, even with complex conditions above the seam. Nevertheless, we do expect that if mine workings were located deeper than this case history, the DC resistivity technique would be stretched to provide conclusive results. Case History No. 4 — Locating Mine Workings at the Oak Hill No. 4 Mine, Monroeville, Pennsylvania The Oak Hill No. 4 Mine operated throughout most of the latter half of the 19th century up to the beginning of the 20"' century. Mine maps could not be reliably related to surface features in this area and the geophysical surveywas conducted to determine the extent of the coal barrier left over after the mining. This area was surveyed with a DC resistivity technique different from the other case histories in that a 3D technique was applied. The other case histories provide the results of 2D electrical profiling, but recent developments of software and computer technology now allow for the processing of electrical data in three dimensions. 7