USING ERT AND LIDAR TO DETECT VOID SPACE IN CRETACEOUS FORMATIONS AT NATURAL BRIDGE CAVERNS IN COMAL COUNTY, TX

Cretaceous limestones found at Natural Bridge Caverns provide an accessible snapshot of the Western Interior Seaway carbonate paleo-environment. Natural Bridge Caverns is located south of the Balcones Fault Zone in central Texas' Comal County. Regional groundwater movement has created a karst landscape throughout the highly fractured Kainer and Glen Rose units, resulting in large chambers and breakout domes. These features along with the cave's dimensional mass provide an ideal setting for robust data collection. Electrical Resistivity Tomography (ERT) surveys have been unsuccessful on-site due to the high concentration of calcite in the local units. Variation in electrode array configurations and data acquisition methods have resulted in successful images of the Hall of the Mountain King, a corridor with direct access from the surface. Multiple ERT crosscut scans of the northernmost opening of the initial discovery corridor have revealed discernable resistivity readings between the void space and surrounding rock layers. Inversion results indicate the presence of the known void along with an unknown corridor along both transects, revealing the discovery of an unknown corridor. Natural Bridge Caverns is the second largest commercial cave system in the state and this study could validate unknown parts of the cave system. Spatial results from a LiDAR scan of the known cave interior have been referenced against the ERT inversion results to calculate depth of the cave. The deviation of the z-coordinates at the surface, ceiling, and floor of the known void space provide a relative thickness of the overbearing calcite heavy units. This analysis is used as a calibration method for the ERT survey and provides a framework for estimating the size of the new discovery. The impedance caused by the calcite overburden can then be considered in the inversion results, resulting in more accurate images.