Preliminary Results, Helicopter Electromagnetic And Magnetic Survey Of The Seco Creek Area, Medina And Uvalde Counties, Texas

Abstract

A helicopter electromagnetic and magnetic (HEM) survey was completed of a 209 square kilometer (81 square miles) area of the central Edwards aquifer. The survey area was centered on the Valdina Farms sinkhole along the Seco Creek drainage. The Edwards aquifer in this area consists of three hydrologic zones; catchment, recharge, and confined. The Glen Rose Formation is exposed in the catchment area. The recharge zone is situated in the Balcones fault zone where the Devils River Group of the Edwards aquifer has been exposed by normal faults. The magnetic data is not discussed in depth here, but does have high amplitude closed anomalies caused by shallow igneous intrusives. The Woodard Cave Fault which separates the recharge and catchment zones is in places associated with a weak linear magnetic low. The HEM data has been processed to produce apparent resistivities for each of the six EM coil pairs and frequencies. Maps of the apparent resistivity for the five horizontal coil pairs show that the catchment, recharge, and confined zones all have numerous linear features which are likely caused by structures, many of which have not been mapped. The distribution of high resistivity areas reflects the lithologic differences within the Trinity and Edwards aquifers. Three methods to compute electrical depth sections have been evaluated; a) tau space inversions, b) differential depth imaging, and c) Occam inversion. Of these the Occam inversion method was found to produce the smoothest depth sections with the needed detail and least amount of over and undershoot. Depth sections along two flight lines show; a) depth and approximate thickness of the Del Rio Clay, b) upper and lower units of the Devils River Formation, c) upper and lower units of the Glen Rose Formation, d) structures within each zone, and e) known karst features within the recharge zone correlates with a low resistivity on the edge of a high resistivity trend.