Controlled Source Audio Magnetotelluric (CSAMT) Geophysical Investigation in the Upper San Pedro Basin, Southeastern Arizona

Ground water in the Upper San Pedro Basin in southeast Arizona is the main source of water for domestic, industrial, and agricultural use. As the population and demand on water resources increases, an improved understanding of the hydrogeologic framework is needed for effective water management. Geophysical surveys have proved useful for characterizing the subsurface and contribute to the understanding of the hydrogeologic framework. This study uses Controlled Source Audio Magnetotellurics (CSAMT) to characterize relations between alluvial aquifers of sand and gravel, confining layers of silt and clay, and bedrock using electrical resistivity. Seven stations along a five-kilometer transect were used to determine the resistivity of several aquifers. CSAMT data were processed and modeled using software from Zonge Engineering and Interpex Ltd. In the simplest representation of the data, four electrical layers were modeled. Resistivity values ranged from 15-30 ohm-m between 20 and 50 meters depth and 5-10 ohm-m between 50 and 200 meters depth. The lowest resistivity values may correspond to a previously identified silt and clay layer. Drillers' logs indicate depth to bedrock between 100 and 150 meters. The CSAMT data indicate that depth to bedrock having resistivity greater than 100 ohm-m is greater than the survey depth of 500 meters. This indicates that the sand and gravel aquifers are thicker than the drillers' logs indicate, and that the drillers' logs are unreliable.