A three-dimensional interpretation of an aem survey in the upper San Pedro Basin, Arizona

In March 1997, a time-domain Airborne ElectroMagnetic (AEM) survey was flown over the upper San Pedro basin, Arizona, by Geoterrex-Dighem Ltd. of Ottawa, Canada. This survey was contracted and analyzed by the U.S. Geological Survey Geologic Division. A mathematical transformation of the AEM data, referred to as a conductivity depth transform or CDT, was used to build a three-dimensional model of resistivity in the basin. Comparisons of the CDTs and resistivity measured in electric well logs from nine Fort Huachuca test wells show that the CDTs are a good representation of the electrical conductivity of basin sediments for the upper 150 m. Below 150 m trends in conductivity systematically decrease with depth, and therefore conductors deeper than 150 m represented on the CDTs are underestimated. The depth to the water table generally corresponds to the uppermost maximum of conductivity, but there are many exceptions. It is possible that the relationship is coincidental with those areas where the water table happens to fall in a zone of (more conductive) silts and clays. The eastern half of the basin near the Tombstone Hills contains an increasing proportion of silts and clays with depth. We infer that this is due to weathering of the volcanic rocks off of the Tombstone Kills into a low energy environment during the filling of the basin. This resulted in a southwestward extending wedge of silt and clay which is most extensive at depth and least at the surface. The lower part of this wedge may include outflow tuff from the Tombstone caldera. This material limits the permeability of much of the aquifer and thus is hydrologically significant.