Technical Papers - Airborne Electromagnetics

Airborne EM as a 3-D Aquifer-mapping tool

Abstract

The San Pedro River in southeastern Arizona hosts a major migratory bird flyway, and was declared a Riparian Conservation Area by Congress in 1988. Recharge of the adjacent Upper San Pedro Valley aquifer was thought to come primarily from the Huachuca Mountains, but the US Army Garrison of Fort Huachuca and neighboring city of Sierra Vista have been tapping this aquifer for many decades, giving rise to claims that they jointly threatened the integrity of the Riparian Conservation Area. For this reason, the US Army funded two airborne geophysical surveys over the Upper San Pedro Valley, and these have provided us valuable information on the aquifer and the complex basement structure underlying the modern San Pedro Valley. Euler deconvolution performed on the airborne magnetic data has provided a depth-to-basement map that is substantially more complex than a map obtained earlier from gravity data, as would be expected from the higher-resolution magnetic data. However, we found the output of the Euler deconvolution to have "geologic noise" in certain areas, interpreted to be post-Basin-and-Range Tertiary volcanic flows in the sedimentary column above the basement but below the ground surface.

The EM component of the airborne geophysical survey used the Geoterrex-Dighem 20-channel multicoil GEOTEM system¹ configured in a CASA aircraft. We inverted these airborne EM data using the proprietary Geoterrex-Dighem Conductivity-Depth-Transform (CDT) algorithm and Encon's EM-Flow¹ inversion software, and compared the resulting resistivity-vs-depth sections to the limited E-Logs available in the survey area. We find that the CDTs map the conductive aquifers to deeper depths, but the EM-Flow Conductivity-Depth-Inversions (CDI's) provide somewhat better vertical resolution at shallower depths. The CDTs generally map the water table down to about 150 meters depth except in areas with high resistivity, low-porosity overlying rocks (a confined aquifer situation). Here the CDTs apparently map where water exists below a silt-clay, mid-basin unit that because of lower porosity keeps the bulk of the water below the equipotential line. The CDTs show an increasingly higher resistivity above the E-Log values for depths below 150 meters; we believe, however, that we still have qualitative information down to about 400 meters depth in areas where there is no cultural interference such as power lines and pipelines. Experiments suggest that it may be possible to "tune" the inversion process to provide more accurate resistivity data at these greater depths

Proceedings Volume, SAGEEP-2000 Conference, 20-24 Feb 2000, p. 93-100. For a full copy version of this paper, visit Jeff Wynn's, USGS Website.