USE OF INTEGRATED GEOPHYSICAL TECHNIQUES TO LOCATE A KARST CONDUIT IN THE INNER BLUEGRASS REGION, KENTUCKY

In karst terrains, mapping groundwater flow can be challenging because it is focused through conduits that do not necessarily coincide with 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. Survey sites were selected based on sinkhole locations and results of dye tracing. ER and SP profiles were perpendicular to the inferred trunk conduit orientation.

Eight ER profiles (1252 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 using one stationary reference electrode and another roving electrode at a fixed interval. To check SP data reproducibility, the test profile was repeatedly measured on different days, while several other profiles were measured more than once.

The SP technique has been used by many researchers to detect the electrokinetic potential generated by groundwater flow. 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. Although SP data collected over multiple days along the test profile differ significantly, they show similar trends. Field drift in SP data is highly sensitive to temperature changes during the time of measurement. The data were reduced to overcome drift by reoccupying the base station at a fixed time interval. The integrated use of ER and SP method led to the identification of a conduit at each site, although mud-filled voids encountered during drilling suggest that these may be tributary conduits rather than the trunk conduit.