Using Analytic Signal Analysis On Aeromagnetic Data To Constrain Amt Inversions, Sonora San Pedro Basin, Mexico

Airborne geophysical studies on the American side of the San Pedro Valley of Arizona<br>and Mexico have allowed us to map depth to crystalline basement in this area where<br>groundwater is critically important (Alley and others, 1999; Leake and others, 2000; Wynn and<br>others, 2000; Wynn, 2000/2003). This basin, whose head lies in northern Mexico, hosts a major<br>US-Mexico migratory bird fly-way. A desire to preserve the surface water in the San Pedro<br>River led to the creation of the San Pedro National Riparian Conservation Area in 1988. To<br>preserve the surface water, one must know something about the aquifer underlying it. On the<br>American side of the basin, time-domain airborne geophysical methods were used to map the<br>relatively conductive groundwater typical of an arid region to depths of 150 - 400 meters in the<br>absence of human cultural interference. In order to better understand the hydrology of the basin<br>as a whole, geophysical surveying has been extended southward into the Sonoran San Pedro<br>Valley of northern Mexico. An airborne magnetic survey in northern Mexico has been processed<br>to depth-to-magnetic-source, and concatenated to a magnetic data set from southern Arizona to<br>show depth to basement for the San Pedro Valley drainage. We then conducted a scalar Audio-<br>MagneTotelluric (AMT) survey over four different lines in the Sonoran San Pedro basin, and<br>processed these data using a smooth-model inversion to conductivity-vs-depth profiles. As we<br>view the conductivity inversion results, we are in fact visualizing the highly conductive water<br>typical of an arid climate - in effect, we broadly image the saturated sediments. We then used an<br>analytic signal depth-to-source algorithm on magnetic data along the same profiles to constrain<br>the AMT inversion. The result is a unique set of geophysical profiles that clearly show basement<br>structure beneath the Sonoran San Pedro basin to depths of up to 800 meters. These constrained<br>profiles help resolve basement controls on groundwater flow in northern Mexico leading to the<br>US frontier. It is impossible to understand the groundwater regime except in the context of the<br>volcanic and sedimentary history of the region, and neither the geology nor the geophysics can<br>be carried out independently of the other, but the whole together contribute substantially more<br>than the parts.