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

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