Detection and mapping of mine subsidence fractures in glacial drift using surface electrical methods
- Northern Illinois Univ., DeKalb, IL (United States)
Fracturing caused by coal mine subsidence disrupts surface structures, alters drainage patterns, changes the hydraulic properties of overburden rocks and soil and may expose shallow aquifers to contamination from surface sources. In this study electrical resistivity soundings and frequency-domain electromagnetic (EM) surveys were made before, during and after subsidence of a 40-70 ft section composed of loess, till and glaciolacustrine deposits in the southern Illinois coal basin. During subsidence, resistivity increased from approximately 60 to 140 ohm-m in the upper 5 ft along the south margin of the panel. This {open_quotes}layer{close_quote} of elevated resistivity may reflect air-filled fractures penetrating the soil to the water table and shallow drift aquifer. Sounding lines oriented parallel to fractures measured resistivity increases averaging 90 ohm-m, whereas those oriented perpendicular to fractures exhibited resistivity increases averaging 30 ohm-m. After subsidence, resistivity of near-surface materials declined dramatically (100 ohm-m in some areas), reflecting closure of shallow fractures and seasonal increases in soil moisture. Postsubsidence EM measurements recorded a low-conductivity zone over the south panel margin 5 months after subsidence, probably as a result of fractured soil between depths of 6 to 20 ft. These zones may represent areas of locally dewatered drift which had not yet received recharge from winter precipitation due to low permeabilities in the overlying clay-rich sections of the overburden. This experiment illustrates how electrical geophysical methods provide a viable and cost-effective means to assess fracturing and the hydrogeologic impact of subsidence on shallow aquifers overlying longwall coal mines.
- OSTI ID:
- 381531
- Report Number(s):
- CONF-960477--
- Country of Publication:
- United States
- Language:
- English
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