FRACTURE DENSITY ESTIMATES IN GLACIOGENIC DEPOSITS FROM P-WAVE VELOCITY REDUCTIONS

Subsidence-induced fracturing of glaciogenic deposits over coal mines in the southern Illinois basin alters hydraulic properties of drift aq uifers and exposes these aquifers to surface contaminants. In this stu dy, refraction tomography surveys were used in conjunction with a gene ralized form of a seismic fracture density model to estimate the verti cal and lateral extent of fracturing in a 12-m thick overburden of loe ss, clay, glacial till, and outwash above a longwall coal mine at 90 m depth. This generalized model accurately predicted fracture trends an d densities from azimuthal P-wave velocity variations over unsaturated single- and dual-parallel fractures exposed at the surface. These fra ctures extended at least 6 m and exhibited 10-15 cm apertures at the s urface. The pre- and postsubsidence velocity ratios were converted int o fracture densities that exhibited qualitative agreement with the obs erved surface and inferred subsurface fracture distribution. Velocity reductions as large as 25% were imaged over the static tension zone of the mine where fracturing may extend to depths of 10-15 m. Finally, t he seismically derived fracture density estimates were plotted as a fu nction of subsidence-induced drawdown across the panel to estimate the average specific storage of the sand and gravel lower drift aquifer. This value was at least 20 times higher than the presubsidence (unfrac tured) specific storage for the same aquifer.