CONTROLLED SOURCE AUDIO MAGNETO- TELLURIC (CSAMT) GEOPHYSICAL INVESTIGATION OF THE MIDDLE SAN PEDRO RIVER BASIN, SOUTHEASTERN ARIZONA

Abstract

Groundwater resources are essential to support the growing population of Benson as well as the agriculture, and wildlife throughout the Middle San Pedro Basin. A refined model of the hydrogeologic framework within the region is necessary to allow for the most efficient allocation of the area’s ground water resources by city planners and water managers in charge of future development. New data were collected by the University of Arizona’s Geophysics Field Camp to update and improve this representation. This survey utilizes Controlled Source Audio Magnetotellurics (CSAMT) to characterize previously unstudied locations in the San Pedro Basin. CSAMT data were processed and interpreted using software from Zonge Engineering and Interpex Ltd. Seven receiver stations along a five-kilometer Middle San Pedro transect were used to determine the resistivities of several basin-fill units. Results show some resistivity variability with respect to depth among the sampled regions. In the simplest representation of the data, four layers were modeled. Resistivities ranged from 15-30 ohm-m in the near-surface units. Deeper units showed resistivities of 5-10 ohm-m. These results were plotted to help identify conductive aquitard (clay) and potentially more resistive aquifer units. This clay unit could correspond to a thin unit of the St. David Formation, which has previously been identified as a confining unit between two separate basin aquifers. CSAMT data indicate that the local bedrock is deeper than 500 m.