A TEST OF COMBINING TIME-LAPSE ELECTRICAL RESISTIVITY IMAGING AND SALT INJECTION FOR LOCATING KARST CONDUITS

The use of electrical resistivity imaging to locate karst conduits has traditionally shown mixed success. However, time-lapse electrical resistivity imaging combined with a salt injection may improve conduit detection by elucidating regions where electrical conductivity changes due to salt passage. We tested the approach above a known karst conduit in the Kentucky Horse Park (Lexington, Kentucky). A salt tracer solution was injected into a karst window over a 45-minute interval, and repeat resistivity surveys were collected approximately one kilometer away along a 125-meter transect every 20 minutes. The resistivity transect was located near a well that penetrates the conduit, where a conductivity sensor was deployed. The in-situ conductivity data are essential for verifying the results of the electrical resistivity method. Conductivity in the well peaked at approximately 125% of the initial value ~3 hours after salt injection. Preliminary pseudo-sections show areas of increasing conductance that correlate to conductivity changes recorded in the well. If this imaging method proves viable after inversion of the resistivity data, time-lapse electrical resistivity imaging could allow for more reliable identification of karst conduits in the subsurface.