Sinkhole Characterization And Karst Conduit Identification Beneath A Natural Prairie In Northeastern Illinois

Sinkholes are important recharge points for shallow karst aquifers. In many cases, however,<br>they are filled with sediments or debris and have no topographic expression. In this study,<br>geophysical methods are employed to noninvasively characterize buried sinkholes, caves,<br>conduits, and solutionally-widened fractures beneath a fragile natural prairie area near Kankakee,<br>Illinois. Seismic refraction and electrical resistivity soundings suggest Silurian dolomite bedrock<br>lies at a depth of approximately 4 m across most of the site, beneath alluvial sands, silts, and<br>clays. Bedrock subcrops deeper in, and near the sinkholes, where it lies at a depth of<br>approximately 6 m. Electrical resistivity pseudosections suggest an undulating bedrock surface.<br>Buried sinkholes appear in inverted pseudosections as thick accumulations of conductive soil.<br>Electromagnetic conductivity profiling also identified high conductivity anomalies over the<br>locations of buried sinkholes, possibly due to infilling by clayey sediments or concentrated soil<br>moisture. Apparent conductivities recorded with vertical and horizontal dipole orientations<br>exhibit the same anomaly pattern. Values as high as 33 mS/m were recorded over the sinkholes,<br>compared to an average background conductivity of 22 mS/m. A zone of elevated conductivity<br>trends WSW from the sinkholes, and may indicate a conduit linked to other sinkholes in a ravine<br>approximately 0.5 km away. Ground-penetrating radar profiles (50 MHz) showed strong<br>attenuation and lower velocities over conductive sinkhole sediments. Complex reflections<br>(cross-beds and onlap/offlap structures) were also visible in reflections from the reef-facies<br>bedrock and in the overlying sediments.