Technical Papers - Airborne Electromagnetics

Application of time-domain airborne electromagnetic induction to hydrogeologic investigations on the Pajarito Plateau, New Mexico, US

W. Scott Baldridge, Gregory L. Cole, Bruce A. Robinson, Los Alamos National Laboratory, Earth and Environmental Sciences Division, D462, Los Alamos, New Mexico, and George R. Jiracek, San Diego State University, Department of Geological Sciences, San Diego, California.

Abstract

We conducted a time-domain airborne electromagnetic (AEM) survey of part of the semiarid Pajarito Plateau of northern New Mexico to determine depths and lateral extent of perched aquifers in the vadose zone and depths and pathways of infiltration to the regional aquifer. The electrical resistivity of the plateau ranged over three orders of magnitude (<20 to>2000 Ωm) to a depth of at least 300 m. Borehole and surface-derived data allow the correlation of resistivity images with the hydrogeology of the plateau. As expected, water exerts a significant control on resistivity. However, the presence of large amounts (up to 90%) of clay in some units, in conjunction with water, also has a significant effect, lowering resistivity (to ≤30 Ωm) more than the presence of clay-free saturated zones alone. Because of the resulting low resistivity, we are able to better delineate a large, known volume of clay-altered volcaniclastic rock and postulate the presence of another. Resistivity values of 100–300 Ωm correlate with depths to saturated zones where no clay is present, but they do not allow us to distinguish between one large or several smaller perched groundwater zones and the underlying regional zone of saturation. We imaged a region of significant infiltration related to a sewage treatment plant and to near-surface hydrogeologic conditions conducive to infiltration and correlated with a region of preferential transport of anthropogenic chemicals through the vadose zone. AEM data provide an important synoptic view of the shallow (few hundred meters) resistivity structure of the plateau. Although interpretation of the data is not unique, when combined with borehole geologic, hydrologic, and geochemical data, it can provide relative depths to saturated zones, delineate regions of high clay content (zones of alteration), and image regions of recharge to the regional aquifer.

Published in GEOPHYSICS, VOL. 72, NO. 2 (MARCH-APRIL 2007); P. B31–B45.

PDF copies of this paper can be obtained by email from Richard Smith