Premine study of shallow coal seams using high‐resolution seismic reflection methods

Abstract

Geological investigations in the Illinois Basin coalfields have shown that significant differences in safe and economical exploitation of coal depends directly on accurate mapping of the roof rock overlying the seam, as well as on geological structures in the coal measures. In roof rock transition zones above the Herrin (No. 6) coal where the nonmarine Energy shale changes to the Anna shale, a change often occurs from low to high sulfur coal and from low to high stability roof rocks. In many instances, use of borehole data alone is inadequate to locate these features in advance of mining. High‐resolution seismic reflection data collected near Harco, Illinois were used as part of premine planning to help predict roof instability, areas of low sulfur coal, and geologic disturbances. Several faults, channels, and facies changes affecting the Herrin (No. 6) and the Springfield (No. 5) coal seams at depths of 137 m and 167 m, respectively, were interpreted and modeled. One‐ and two‐dimensional synthetic seismograms calculated from geological data from drill holes along the seismic line were used to aid in the interpretion of the seismic reflection data. Results obtained from the high‐resolution reflection survey combined with drill hole information clearly show that use of borehole data alone is inadequate to locate geological features that might affect coal mine operations, even if the boreholes were spaced 25 m apart. Thus, high‐resolution reflection surveying should be employed whenever feasible for the safe and economical exploitation of coal deposits.