Technical Papers - Airborne Electromagnetics

Assessing Groundwater Perching Horizons using Synthetic, Ground, and Airborne TDEM Data at the Pantex Plant, Texas

Jeffrey G. Paine, Bureau of Economic Geology, Austin, TXS. Todd Harris, BWXT Pantex, Amarillo, TX, James M. Phelan, Sandia National Laboratories, Albuquerque, NM

Abstract

At the U.S. Department of Energy's Pantex Plant near Amarillo, Texas, recharging groundwater encounters a perching fine-grained zone (FGZ) above the main Ogallala (High Plains) aquifer, a critical agricultural, municipal, and industrial water supply. We conducted modeling and ground-based time-domain electromagnetic induction (TDEM) measurements to examine whether TDEM might help assess the integrity of the FGZ and its ability to retard the flow of groundwater to the main aquifer. Conductivity models based on well logs and representative TDEM soundings demonstrated that changes in thickness of the FGZ and perched aquifer are detectable if changes in other stratigraphic horizons are minimal. Based on these results, we conducted an airborne geophysical survey using Fugro's GEOTEM system.

Conductivity-depth transforms (CDTs) from airborne measurements matched the nearest ground-based profiles reasonably well. Pseudo-depth slices constructed from CDTs depicted apparent conductivity at critical depths, enabling refinement of borehole-based stratigraphic and hydrologic models. Elevated apparent conductivities were observed where interpreted FGZ integrity was good and the saturated zone is relatively thick; low apparent conductivities are interpreted to indicate areas where the FGZ and associated saturated zone are thin or absent. Survey results are being used to support groundwater investigations, development of fate and transport models, and upcoming corrective action decisions.

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