f INVESTIGATION FOR HYDRAULIC CONDUITS INVOLVED IN SINKHOLE FORMATION THROUGH THE USE OF INTEGRATED SURFACE AND BOREHOLE GEOPHYSICS

The Nachusa Grasslands Conservancy is located near Franklin Grove in northern Illinois. It was used previously for agriculture and is currently undergoing restoration into a natural prairie. A sinkhole, ~40m in diameter, exists in the northern portion of this area. At this location Ordovician age dolomite subcrops beneath St. Peter sandstone and several meters of unconsolidated sand, gravel and clay. Geophysical surveys were deployed to determine the location and nature of hydraulic conduits related to the sinkhole’s formation. The area surrounding the sinkhole was investigated using EM conductivity profiles, resistivity soundings, ground- penetrating radar (GPR) profiles, supplemented by Geoprobe® conductivity logs and soil cores. Resistivity soundings and the Geoprobe® logs suggest bedrock averages about ~5.0 m deep near the sinkhole. Core samples were also collected and are presently being examined. The EM conductivity profiles showed lateral variations of 1-2 mS/m, but lack the resolution needed to definitively locate conduits. GPR using 50 and 100 MHz antennas proved the best tool for examining the subsurface. These high resolution images show possible conduits as diffraction hyperbolas below the bedrock surface. The water table is not visible, suggesting it lies at least several meters below the bedrock surface. An area of suspected multiple, small conduits has been detected on the north side of the sinkhole trending northeast to southwest. Future work will include expanding the GPR survey grid for better coverage, followed by additional borings to confirm the existence of hydraulically active conduits.