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 aquifers and exposes these aquifers to surface contaminants. In this study, refraction tomography surveys were used in conjunction with a generalized form of a seismic fracture density model to estimate the vertical and lateral extent of fracturing in a 12-m thick overburden of loess, clay, glacial till, and out-wash above a longwall coal mine at 90 m depth. This generalized model accurately predicted fracture trends and densities from azimuthal P-wave velocity variations over unsaturated single- and dual-parallel fractures exposed at the surface. These fractures extended at least 6 m and exhibited 10-15 cm apertures at the surface. The pre- and postsubsidence velocity ratios were converted into fracture densities that exhibited qualitative agreement with the observed 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, the seismically derived fracture density estimates were plotted as a function of subsidence-induced draw-down 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 (unfractured) specific storage for the same aquifer.