Detection of Three-Dimensional Voids in Karstic Ground

Even apparently simple problems in karst terrain can require a great deal more effort to identify correctly, than can be at first realized. As an example, a private property owner in Bexar County, Texas noticed a suspicious depression next to his driveway. This was eventually followed by the formation of a small hole in the ground, which he was able to fill temporarily, while a geotechnical investigation was carried out to determine the possibility of a threat to the house structure. Traditional drilling did not encounter any voids in the ground, so the use of non-invasive electrical tomography was then employed. Two-dimensional resistivity lines were conducted across the property. These were able to detect a higher-resistivity zone close to the surface, but only of the order of a few hundred ohm.meters, which would not normally correspond to a true void, and was later shown to correspond to an outcrop of dense limestone above the water table. Since there were prima facie indications of a karstic void, a three-dimensional survey was carried out at the site, in a snake-like pattern across the back yard. Special software was used to integrate the results into a three-dimensional spatial model, which then showed the existence of a very-high resistivity zone (in excess of a thousand ohm.meters) indicative of a true void. This was calculated to be about 4 cubic meters in volume, but sufficiently far below the surface (and in an off-center location) that it was determined not to be a threat to the house foundations. The void was sufficiently localized that the probability of detection by conventional drilling was very small, and even the probability of detection by two-dimensional surveys could be shown to be significantly less than 50%.