Fracturing Of Glacial Drift And Bedrock Over Longwall Mine Panels: Integrated Geophysical And Hydrological Measurements

Abstract

P- and S-wave refraction profiles were combined with resistivity soundings, resistivity and electromagnetic profiles, azimuthal surveys and hydrological data to characterize fracturing and associated hydrogeological changes over three subsided longwall coal mine panels in the southern Illinois basin. Approximately 6 ft of Herrin #6 coal was mined from the base of a 260-320 ft section of Pennsylvanian rocks capped by 40-90 ft of unconsolidated glaciolacustrine deposits. Subsidence-induced fractures significantly altered the mechanical and hydrogeological properties of the overburden. Fracturing of the drift over one panel to at least the depth of the water table (lo-20 ft) was marked by decreases in shallow SH-wave velocity and a non-uniform pre- to post-subsidence drop in the water table. Apparent resistivity also increased in the dynamic tension zone where surface fractures opened just behind the mine face. Inversion of resistivity soundings could not be used to constrain the depth of fracturing in this area, however. Increases in earth conductivity, decreases in apparent resistivity, and the development of azimuthal resistivity variations suggest long-term fracturing of drift along panel margins, and, in some cases over barrier pillars. Fracturing of the upper bedrock (to 80 ft below the bedrock surface) was indicated by reduced P- and SH-wave velocities, sharp potentiometric declines, and post-subsidence increases in hydraulic conductivity. SV- head waves from the bedrock surface, however, showed no such reduction in velocity, suggesting bedrock Vsv is controlled primarily by bedding anisotropy.