Crossed Square Array Azimuthal Resistivity as a Method to Characterize Karst Enhanced Jointing Trends in the Lower Glen Rose Limestone, Kendall County, Texas

This paper investigates the karst related anisotropy in the subsurface at the Cibolo Preserve in Boerne, Texas in relation to the dominant local fracture trends and geologic framework. Three azimuthal crossed square-array (CSA) resistivity surveys were conducted at locations over known karst affected carbonate bedrock at the Cibolo Preserve. Data from the CSA surveys was plotted in polar charts and analyzed for apparent anisotropy and mean apparent resistivity. A total of six direct current (DC) dipole-dipole array electrical resistivity profiles were measured at two of the survey sites and two capacitance-coupled (OhmMapper) resistivity profiles were measured at one. Additionally, ground penetrating radar data was collected to cross reference with the OhmMapper resistivity profiles, which agreed favorably with locations of anomalies along the transect lines. Results from the crossed square-array resistivity surveys showed two dominant strike directions of 032̊ and 144̊. These azimuths related well to measured primary and secondary fracture strike directions measured in rock outcrops. High resistivity anomalies from the profiles were plotted on their corresponding transect lines on a map, and line segments were drawn to connect anomalies that were interpreted to be related to each other. The resulting linear trends from this interpolation matched the primary and secondary strike directions determined from the azimuthal surveys. The computed mean apparent resistivity was interpreted as a resistivity sounding curve, which compared well to the corresponding measured resistivity profile lines. High and low resistivity portions of the sounding curve closely matched the resistivity observed in the DC resistivity profiles, and thus provided an additional mechanism to assess karst development at depth. Finally, published methods for estimation of secondary porosity from azimuthal surveys were applied to estimate the porosity at the CSA survey locations. The computed values compared well to published values given the environmental setting and meteorological effects that may have affected the datasets. Overall, this study showed the effectiveness of the crossed square-array resistivity survey method in assessing anisotropy in a fractured karst affected environment. This survey technique shows promise in characterizing karst related hydrologic or geologic proclivity for local scale investigations.