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. Within south-central Texas, a karst landscape was created by groundwater movement in the Edwards Group and adjacent units, locally within the Kainer and Glen Rose Formations, resulting in large chambers and break-out domes. Whereas these features, along with the dimensional mass of the caves, provide an ideal setting for robust data collection, there is paucity in data. The Natural Bridge Caverns provide easy access into the strata and allows an opportunity to integrate Light Detection and Ranging (LiDAR) and Drip Count Data from the interior to validate surface Electrical Resistance Tomography (ERT) from above. This study lays a foundation for calculating the total volume of recharge, estimate cavern morphology, infiltration water movement and discover areas of cavern development. It has been speculated that the hydraulic gradient of the regional aquifer (the Edwards Balcones Fault Zone) system has had an upward steady flow state. The study is critical in understanding the preferential water movement through the system and for assessing cavern dimensions and discovering new caves. Our results from the northernmost opening of the initial discovery corridor reveal a very high resistivity contrast between the void space and the formations.