Harvey Henson

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

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 seismo...

Age estimation of the Cottage Grove Fault System (Illinois, U.S.A.) from high-resolution seismic reflection studies of coal measures

Abstract Geologic and high-resolution seismic reflection studies of Pennsylvanian coal deposits have provided new insights concerning the Cottage Grove Fault System (CGFS), a major east-west oriented wrench fault system in southern Illinois. The age of faulting has previously been estimated to be late Pennsylvanian to early Permian, based upon KAr dating of igneous intrusive rocks associated with subsidiary faults. Although these dates are accurate for intrusions and faulting at certain locations, the duration of Cottage Grove master fault zone or subsidiary faulting is still unclear. High-resolution seismic reflection profiles were collected approximately 6.4 km north of the master fault zone in the vicinity of mapped subsidiary faulting to study coal deposits. Several small-offset, high-angle normal and reverse faults were interpreted from the seismic records. Two small-offset fault zones observed on the seismic records, interpreted to be subsidiary faults of the CGFS, bound a gentle asymmetric anticline. Several small sandstone channels located on the down-thrown sides of these faults are interpreted from the seismic data. Vertical stacking of these sandstone channels and other sands within the coal measures is also observable in the correlated well data. Location of channel deposits suggests depositional control by these faults, indicating that subsidiary faulting was contemporaneous with sand deposition. Thus, subsidiary faulting of the CGFS occurred as early as middle Pennsylvanian time.

Georadar Investigation of Karst in a Limestone Quarry near Anna, Illinois

SUMMARY Clay-filled karst was encountered during normal operations of a limestone quarry near Anna, Illinois. Attempts to determine karst extent with a drill rig became too dangerous. Consequently, a ground-penetrating radar (GPR) system was used to map karst extent and to determine any possible relationship to fractures observable along quarry walls. Georadar profiles were located adjacent to major fractures to intersect exposed portions of the karst feature. Reflector velocity information was obtained using the commonmidpoint (CMP) method at selected locations along the radar profiles. Radar profiles were collected using 50 and 100 MHz antennae. Karst features filled with sediment and air were imaged remarkably well on georadar data using both antennae. Stratigraphy associated with the karst includes 4 m of finegrained limestone overlain by 5-6 m of interbedded chert and limestone. This vertical transition in stratigraphy is observable on the processed georadar data, as are several fractures which intersect the main cavity. Fractures served as conduits for local groundwater, which resulted in carbonate dissolution, and for sediment which later filled the void. Karst extent was determined quickly and efficiently using simple ground-penetrating radar methods, and quarry operators were provided with information necessary to continue production of limestone materials.