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

Publisher —

Authors —
J.C. Wynn, US Geological Survey, Vancouver, Washington;
Floyd Gray, US Geological Survey, Tucson, Arizona;
T.E. Nordstrom, Dexin Liu, E.V. Reed, Zonge Engineering, Tucson, Arizona;
F.A. Villaseñor, SEMARNAT, Cananea, Mexico;
Gerry Connard, NGA Inc., Corvallis, Oregon

Paper — [pdf] AMT_Inversion_with_AeroMAG

Airborne geophysical studies on the American side of the San Pedro Valley of Arizona and Mexico have allowed us to map depth to crystalline basement in this area where groundwater is critically important. This basin, whose head lies in northern Mexico, hosts a major US-Mexico migratory bird fly-way.  A desire to preserve the surface water in the San Pedro River led to the creation of the San Pedro National Riparian Conservation Area in 1988. To preserve the surface water, one must know something about the aquifer underlying it. It is impossible to understand the groundwater regime except in the context of the volcanic and sedimentary history of the region, and neither the geology nor the geophysics can be carried out independently of the other, but the whole together contribute substantially more than the parts.

On the American side of the San Pedro Valley basin, time-domain airborne geophysical methods were used to map the relatively conductive groundwater typical of an arid region to depths of 150 to 400 meters in the absence of human cultural interference. To better understand the hydrology of the basin as a whole, geophysical surveying has been extended southward into the Sonoran San Pedro Valley of northern Mexico.

An airborne magnetic survey in northern Mexico has been processed to depth-to-magnetic-source, and concatenated to a magnetic data set from southern Arizona to show depth to basement for the San Pedro Valley drainage.

We then conducted a scalar audio-magnetoelluric (AMT) survey over four different lines in the Sonoran San Pedro basin, and processed these data using a smooth-model inversion to conductivity-vs-depth profiles. As we view the conductivity inversion results, we are in fact visualizing the highly conductive water typical of an arid climate; in effect, we broadly image the saturated sediments. We then used an analytic signal depth-to-source algorithm on magnetic data along the same profiles to constrain the AMT inversion.

The result is a unique set of geophysical profiles that clearly show basement structure beneath the Sonoran San Pedro basin to depths of up to 800 meters. These constrained profiles help resolve basement controls on groundwater flow in northern Mexico leading to the US frontier.