Use of Resistivity and EM Techniques to Map Subsidence Fractures in Glacial Drift

Abstract

Fractures caused by ground subsidence disrupt surface structures, alter hydraulic properties of rock or soil and may expose shallow aquifers to contamination from the surface. Geophysical techniques have the potential to rapidly and noninvasively determine the depth and lateral extent of subsidence fractures, even if they are obscured at the surface. This study tested the ability of two geophysical methods to characterize fractures in an overburden consisting of loess, glaciolacustrine deposits and till. Resistivity soundings and frequency-domain electromagnetic (EM) surveys were made before, during and after subsidence of a 90-m deep, 280-m wide longwall coal mine panel in the southern Illinois basin. Increases in resistivity of as much as 84 ohmm were recorded after subsidence over unsaturated drift cut by fresh air-filled fractures. These resistivity increases far exceeded seasonal fluctuations and were largest over the static tension zone just inside the panel margin. Resistivity increased only slightly in saturated portions of the drift after subsidence. Saturated drift resistivities returned to presubsidence 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 the water table; the maximum depth of fracturing could not be determined from soundings since deeper drift layers did not produce a distinctive resistivity response at the surface. Subsidence fracturing reduced apparent earth conductivities in the uppermost drift over the panel several milliSiemens/meter (mS/m), as measured with a Geonics EM31 electromagnetic induction unit. Subtraction of presubsidence apparent conductivities from post-subsidence conductivities revealed a zone of anomalously low conductivity coincident with the static tension zone near the south margin of the panel. Significantly lower apparent earth conductivities were measured with transmitter and receiver coils oriented perpendicular to fractures than with coils oriented parallel to fractures. This difference in response was used to estimate the width of fracturing within the static tension zone from vertical dipole conductivity profiles.