Aeromagnetic and Surface Seismic Surveys for Dike Detection in Illinois Coal Seams, Saline County, Illinois

Location of the three magnetic survey blocks and two seismic profiles in Saline and Gallatin Counties.

For over a century, coal mines in northern Saline County, Illinois have been plagued by ultrabasic igneous dikes that intrude into the coal seams, locally altering the composition of the coal and causing operational problems with the mine equipment and mine planning. We conducted tests integrating two high-resolution geophysical techniques in an effort to detect the dikes and map their locations. First, we acquired high-resolution magnetic data in the area where we suspected the dikes to be present. Several very distinct, large magnitude linear anomalies could be directly associated with known dikes. These and other similar anomalies were mapped in the study area. Second, we acquired high-resolution seismic reflection data in a profile that crossed three of these linear magnetic anomalies. The seismic data provides a cross-sectional view of the shallow subsurface, including the igneous dikes.

We also acquired a seismic test profile in an area that both bordered an abandoned longwall mine and crossed the projected path of a known dike. This seismic test was not in the area covered by the magnetic survey, but the dike and mine boundaries were known from mine records. The seismic test successfully identified both the dike and the boundary of the longwall mine.

High-resolution aeromagnetic surveys successfully locate and map shallow magnetic dikes in Saline and Gallatin Counties. Seismic reflection profiles provide additional detail on the depth and orientation of the dikes and can be used to map the edges of abandoned longwall mines. Further testing of the seismic method could demonstrate its utility in mapping the edges of other types of abandoned mines.