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Symposium on Geotechnical Methods for Mine Mapping Verifications Charleston, !Vest Virginia October 29, 2002 The survey was conducted along seven parallel profiles with the DC resistivity technique using the pole -dipole configuration and placing the electrode in moist soil at the base of a distant sinkhole. The data were then processed as a single block of data from which it was possible to extract both horizontal and vertical slices of the data set. The slice of resistivity data through the level of the Pittsburgh Coal seam depicts a pattern that appears to relate to rooms and pillars. Further analysis of the electrical cross sections through the coal pro- vide additional evidence of the relationship of the electrical measurements to the old mine. When depicted as a 3D block, the interpreted results show where barrier coal is present. The data do not define what the mine is filled with. Similar to the Regency Park example, the mined -out area is a marked resistivity low. Subsidence craters at the ground surface indicate that the edge of the block of barrier coal in the eastern part of the survey area is an open entry. Although the remainder of the mine could also be a void, it could also be filled with gob that would also be expected to exhibit a low resistivity. This survey is depicts another example of how the results of a geophysical survey could help define the scope of a drilling program. Borings could easily be targeted using the geophysical results to increase the probabilityof encountering mine workings. Conversely, the results also serve to identify the continuity of zones where mine workings are not ex- pected to be present. Case History No. 5 — Locating Mine Workings with the GPR Technique at a Gas Sta- tion in Imperial, Pennsylvania The gas station surveyed in Imperial, Pennsylvania was originally constructed in the 1960s and reconstructed in 1996. In the process of creating land suitable for development, a bench was cut in a hillside, which inadvertently placed the structures only a few feet above the top of the Pittsburgh Coal seam. In the summer of 2001, approximately 30 years after the station began operation, a small opening appeared in the asphalt pavement through which a mine tunnel could be observed and mapped to a limited degree. D'Appolonia was contracted to determine the extent of the known mine opening and to determine if any other openings could be present. This case history represents the unusual situation where the coal workings were in the range of penetration for the GPR system. Although the roof material was a weathered claystone, normally a difficult medium for the GPR method, the top of the visible mine tun- nel was onlyfour feet below ground surface. The survey was conducted with both 50 and 200 MHz antennas with a RAMAC GPR sys- tem. The 200 MHzantenna did not have a sufticientdepth of penetration so a 50 MHzan- tenna was employed. The 50 MHz antenna provided an adequate depth of penetration (to approximately 10 feet) but was also susceptible to reflections from surface objects. Spe- cifically, the five gas pump islands represented a significant interference. Nevertheless, it did prove possible to image the known mine tunnel, as well as other suspected tunnels. The centerpoints of the known and suspected tunnels were plotted on a map of the service station and were found to define a consistent N -S alignment. The service station has been subsequently remediated by D'Appolonia. The reflections identified from the GPR survey did prove to be tunnels, although the GPR method did not distinguish if the tunnels were open or filled with gob.