High Resolution Seismic Reflection for Characterizing Longwall Coal Mine Subsidence

As part of an Illinois mine subsidence initiative, the U. S. Bureau of Mines has conducted high resolution surface seismic surveys at two longwall coal mine sites in southern Illinois to help characterize subsurface subsidence. Premine and postmine surveys conducted above 183 meter (600 foot) deep longwall panels consisted of 12-fold common depth point (CDP) data collection, using a 24-channel engineering seismograph, 60 Hz land geophones and a nondestructive surface energy source. Source offset ranged from 91.4 meters (300 feet) to 152.4 meters (500 feet) and a station interval of 3.1 meters (10 feet) were determined from noise test data at each site. Data from these surveys were processed into seismic sections using commercially available microcomputer-based software. Sonic logs from a nearby borehole and mine maps showing mined and unmined areas were used in the processing and interpretation of the data. The sonic logs were used to generate a plot of the two-way traveltime through each formation and a cumulative plot of traveltimes through progressive layers to the coal seam. A coaxial cable was grouted into the borehole and Time Domain Reflectometry (TDR) was used to monitor overburden movement during mining. This data was compared with the seismic sections and cumulative traveltimes plots to identify bridging within the overburden. Bieniawski's Rock Mass Rating system (RMR) was used to characterize the overburden. The RMR was computed for each formation and is used to determine the deformation modulus, then the modulus is combined with parameters based on mine geometry and overburden properties to compute a bending stiffness and bridging potential for each bed. Processed seismic sections, combined with two-way traveltime plots interpreted fracture zones, and plots of bridging potential, show a number of features which are useful in characterizing subsurface subsidence. The mined and unmined areas at these sites are clearly discernable on the records. Moreover, seismic signatures can be interpreted to indicate fractures, bed terminations, and void or rubblization zones. The seismic sections over mined areas show zones within the overburden where bridging has probably occurred. In addition, one of the premine sections contains evidence of one or more shallow buried stream channels, which are common geologic structures in this region. This report concludes that high resolution seismic reflection methods combined with borehole logs and mechanical properties of each formation are a potentially effective research tool for studying subsurface subsidence characteristics.