Do Air-Filled Caves Cause High Resistivity Anomalies? A Six-Case Study from the Edwards Aquifer Recharge Zone in San Antonio, Texas


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May 2012


Of course they do! But it is rarely that caves are purely filled with air. A variety of sediments accumulates in caves and can be preserved more or less intact for long periods of time (Palmer, 2007). Presence of sand and gravel and clay deposits, mineralization, faults and fractures, perched water in caves are the rules rather than the exception. The existence of caves represents a hazard for urban areas. Therefore it is important to know the size, position and depth of caves before building or reconstruction. Cavity imaging using geophysical surveys has become common in the San Antonio area since early 2000 although their use has been going on in other parts of country for the last 25 years. It appears from these studies that the resistivity imaging method has been the primary technique among others, such as gravity, ground penetrating radar, magnetic, conductivity, etc. Resistivity values, in theory, increase dramatically over air-filled cavities. So it is expected to find high resistivity anomalies over the air-filled cavities. This article describes only resistivity imaging data collected over six caves between the years of 2000 and 2005, which are air filled and are located in the northern part of Bexar County, San Antonio, Texas. All caves but one was encountered through drilling and/or excavation for building and utility lines or power pole reconstructions. The study area falls into the part of the Recharge Zone of the Edwards Aquifer region and it represents a well- developed karstified and faulted limestone (Figure 1). The purpose of the study is to show that air-filled cavities do not always cause high resistivity anomalies due to the complex subsurface conditions, and they are sometimes are not separable as a cave anomaly from the surrounding rocks.


Air-Filled Caves, High Resistivity Anomalies, Case Study, Edwards Aquifer Recharge Zone, San Antonio, Texas

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Houston Geological Society Bulletin, Vol. 54, no. 9 (2012-05-09).