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Symposium on Geotechnical Methods for Mine Alapping 6'erificationS Charleston, West Virginia Oclober 29, 2002 <br />Case History No. 8 — S -Wave Seismic Survey over Abandoned Coal Mine Workings <br />at a Power Plant Site, Danville, Illinois <br />In an effort to identify potential collapse hazards near a settling pond, several high- <br />resolution shear wave (HRSW) reflection seismic profiles were acquired on and around the <br />berm of a settling pond. The attached figure shows one of the profiles acquired over an <br />area outside the settling pond with borehole control. The reflection from the coal seam is <br />shaded in yellow. Three borings were located on or adjacent to the profile. One boring <br />encountered a complete coal section, a second boring encountered an abandoned mine <br />drift (missing coal/void), and third boring encountered a section where but coal had been <br />mined and had subsequently collapsed. The upper display shows the amplitudes of the <br />reflected seismic energy and the lower display shows the instantaneous phase attribute of <br />the seismic signal. Continuous sections of coal are indicated by the continuity of the high- <br />lighted reflection bands on both displays. The mine workings are indicated by attenuated <br />reflections (amplitude display) and the occurrence of diffractions at and above the coal <br />seam (instantaneous phase display). The area of collapse is indicated by a disruption of <br />reflections and the chaotic response of instantaneous phase. A feature that is probably <br />anothermine void is present at the right side of the profile, but has not yet been confirmed <br />by drilling. <br />Conclusions <br />Geophysics has the potential to be a useful tool in delineating coal mine workings. For <br />shallow workings (50 —100 feet or less), the most promising technique is DC resistivity as <br />this method is rapid and relatively inexpensive. For deeper targets, the high resolution <br />seismic reflection technique has a demonstrated capability, especiallywhen S -waves are <br />recorded. The techniques have the potential to complement one another in the 50 — 100 <br />foot depth range. None of the geophysical methods represent stand-alone techniques that <br />will replace the need for confirmatory borings. The utility of the geophysics is that it offers <br />a continuity of data that allows for the targeting of boreholes in areas where voids will most <br />likely be encountered. Perhaps more significantly, geophysics offers the possibility of <br />identifying the places where mine voids will not be encountered, greatly enhancing the effi- <br />ciency of a drilling program. <br />Reference <br />National Academy of Sciences, 2002, Coal Waste Impoundments, Risk, Responses, and <br />Alternatives, National Academy Press, Washington, D.C. <br />10 <br />