Residual Potential Mapping of Contaminant Transport Pathways in Karst Formations of Southern Texas

An electrical geophysical survey, utilizing residual potential mapping (RPM) and high resolution resistivity (HRR), was completed at a TCE contaminated site near San Antonio, TX. RPM is a method based on the "mise-a-la-masse" technique, where the subsurface target is imaged using a direct-current transmission source placed at depth, and receivers placed on a regular grid on the surface. The RPM method records the disturbance in the potential field creating a two-dimensional plan view image of the changes in electrical properties. RPM is sensitive to lateral variations in these properties, making it an excellent tool for mapping facture zones and subsurface voids. Conversely, HRR is a direct-current method conducted strictly on the surface along linear transects. A two-dimensional profile of electrical properties is created. Closely spaced and parallel HRR profiles allow a three-dimensional reconstruction of the subsurface. Several transmitting locations, located between 60 and 150 ft below ground surface, were used for the RPM survey. Receivers were placed on the surface within a regularized grid of 310ft by 300ft. The results of the RPM survey showed distinct areas of high and low RPM response, relative to a modeled background potential that assumes homogeneity. A total of 16 parallel HRR profiles were collected over the same RPM grid with a depth of investigation of approximately 60 ft. The electrical resistivity collected with HRR showed a general pattern of high values near the surface and decreasing values with depth. Background removal was conducted on the profiles to enhance subtle features. The background-removed HRR profiles were used to create a three-dimensional plot showing possible interconnected pathways of karst features.