USE OF SURFACE GEOPHYSICAL TECHNIQUES TO LOCATE A KARST CONDUIT IN THE CANE RUN - ROYAL SPRING BASIN, KENTUCKY

Groundwater flow in karst terrains is difficult to map because it can be concentrated through conduits that do not necessarily coincide with the surface features. We applied electrical resistivity (ER) and self-potential (SP) techniques at three sites to locate an inferred trunk conduit feeding a major spring in the Inner Bluegrass region of Kentucky. Royal Spring is the primary water supply for the city of Georgetown; the upper part of its basin coincides with the Cane Run watershed. ER and SP profiles were perpendicular to the inferred trunk conduit orientation. ER profiles (972 m total length) were measured using a dipole–dipole electrode configuration with 2- to 3-m spacing. SP measurements were taken along those ER lines and an additional test profile (230 m) using one stationary reference electrode and another roving electrode at a fixed interval. The low resistivity of water in the conduit, as compared to the high background resistivity of limestone bedrock, is the ER exploration target. A negative SP anomaly corresponds to a low ER anomaly for most of the profiles, but a few are not comparable. Five of seven SP profiles measured over a period of several months were found to be reproducible. Although the overall trends of the final SP profiles for different dates were similar, the SP magnitudes varied with the amount of precipitation and the average soil temperature. The low-resistivity anomalies in the 2D inverted sections and corresponding negative SP anomalies could be water-filled conduits, although mud-filled voids encountered during drilling suggest that these may be tributary conduits rather than the trunk conduit