DETECTION OF BEDROCK CAVES, REGOLITH VOIDS AND CLANDESTINE TUNNELS: MICROGRAVITY AND ELECTRICAL RESISTIVITY TECHNIQUES

The Center for Cave and Karst Studies, CCKS, has been using microgravity to locate caves from the ground surface since 1985. The geophysical subsurface investigations began during a period when explosive and toxic vapors were rising from the karst aquifer under Bowling Green into homes, businesses, and schools. The USEPA provided the funding for this Superfund Emergency, and the CCKS was able to drill numerous wells into low-gravity anomalies to confirm and even map the route of caves in the underlying limestone bedrock. Microgravity has also been used on many occasions to investigate sinkhole collapses. It identified potential collapse areas by detecting voids in the unconsolidated material above bedrock.

In the late 1990s the CCKS started using the Swift/Sting resistivity meter to perform karst geophysical subsurface investigations. The system provides good depth to bedrock data, but it is often difficult to interpret bedrock caves from the modeled data. Some areas of high resistivity indicate caves, but others simply indicate pockets of dry limestone, and the signatures looks virtually identical. Therefore, the CCKS performs microgravity over all suspect areas along the resistivity traverses. A low-gravity anomaly that corresponds with a high-resistivity anomaly indicates a cave location. A high-resistivity anomaly that does not also have a low-gravity anomaly indicates a pocket of dry limestone. Numerous cored wells have been drilled both into the anomalies and on both sides to confirm the cave locations and to establish the accuracy of the technique.

The September 11, 2001 World Trade Center catastrophe was the catalyst for the formation of a program within the CCKS to use the techniques for locating bedrock caves and voids in unconsolidated materials for search and rescue and for locating clandestine tunnels. We are now into our third year of a grant from the Kentucky Science and Technology Center to develop a robot that will measure microgravity and other geophysical techniques. The system was recently tested over known tunnels during a blind test along a section of the U.S. border at Calexico, California.