INVESTIGATION OF SINKHOLE FORMATION USING INTEGRATED NEAR-SURFACE GEOPHYSICS, NACHUSA GRASSLANDS, NORTHERN ILLINOIS

Abstract

The Nachusa Grasslands, owned and operated by the nonprofit The Nature Conservancy, is located in Franklin Grove, IL. It was previously used for agriculture and is currently undergoing restoration into a natural prairie. A sinkhole, ∼40m in diameter, exists within the Grasslands. In this area, Ordovician age dolomite subcrops beneath St. Peter sandstone and several meters of unconsolidated sand, gravel and clay. Geophysical surveys were conducted to investigate for the location and nature of hydraulic conduits related to the sinkhole's formation. The area surrounding the sinkhole has been surveyed using EM conductivity profiles, 2D resistivity profiles and 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 in depth near the sinkhole. Soil core samples have been analyzed for dominant grain size and intervals of increased clay or sand content. The EM conductivity profiles showed lateral variations of 1-2 mS/m, but lack the vertical resolution needed to clearly define subsurface features. GPR using 50 and 100 MHz antennas provided the best resolution of 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, small conduits has been detected on the north side of the sinkhole trending northeast to southwest. Future work will expand the 2D resistivity surveys, continue GPR surveys over suspected conduits, followed by additional borings to confirm the existence of hydraulically-active conduits.