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