USE OF RESISTIVITY AND EM TECHNIQUES TO MAP SUBSIDENCE FRACTURES IN GLACIAL DRIFT

Fractures caused by ground subsidence disrupt surface structures, alte r hydraulic properties of rock or soil and may expose shallow aquifers to contamination from the surface. Geophysical techniques have the po tential to rapidly and noninvasively determine the depth and lateral e xtent of subsidence fractures, even if they are obscured at the surfac e, This study tested the ability of two geophysical methods to charact erize fractures in an overburden consisting of loess, glaciolacustrine deposits and till. Resistivity soundings and frequency-domain electro magnetic (EM) surveys were made before, during and after subsidence of a 90-m deep, 280-m wide longwall coal mine panel in the southern Illi nois basin. Increases in resistivity of as much as 84 ohm-m were recor ded after subsidence over unsaturated drift cut by fresh air-filled fr actures. These resistivity increases far exceeded seasonal fluctuation s and were largest over the static tension zone just inside the panel margin. Resistivity increased only slightly in saturated portions of t he drift after subsidence, Saturated drift resistivities returned to p resubsidence levels within a few weeks as newly-formed fractures below the water table filled with ground water, Models based on resistivity soundings suggest fracturing extended from the surface to at least th e water table; the maximum depth of fracturing could not be determined from soundings since deeper drift layers did not produce a distinctiv e resistivity response at the surface, Subsidence fracturing reduced a pparent earth conductivities in the uppermost drift over the panel sev eral milliSiemens/meter (mS/m), as measured with a Geonics EM31 electr omagnetic induction unit. Subtraction of presubsidence apparent conduc tivities from post-subsidence conductivities revealed a zone of anomal ously low conductivity coincident with the static tension zone near th e south margin of the panel. Significantly lower apparent earth conduc tivities were measured with transmitter and receiver coils oriented pe rpendicular to fractures than with coils oriented parallel to fracture s. This difference in response was used to estimate the width of fract uring within the static tension zone from vertical dipole conductivity profiles.