Faulting and Groundwater in Arid Environments: Airborne Electromagnetics in Support of Framework Hydrology in the northern Mojave Desert, southern California

Abstract

Groundwater basins within the Mojave Desert of the south-western United States are often compartmentalized by faults. As part of an effort to understand and manage groundwater resources in arid environments, the U.S. Geological Survey is investigating a number of basins within the Fort Irwin National Training Center (NTC) using multiple geophysical methods. Gravity, aeromagnetic, ground-based and airborne time-domain electromagnetic (TEM) data were collected over Leach Basin, a geologically complex, internally-drained basin bisected and flanked by Quaternary faults including the Garlock and Death Valley fault zones. The airborne TEM data show abrupt changes in earth response across faulted boundaries, reflecting the strong resistivity contrast between igneous rocks and basin sediments. The distribution of faults throughout the basin can be directly obtained from the airborne data. A resistivity stratigraphy has been developed by integrating borehole geophysical logs, lab resistivity measurements, and ground-based gravity and TEM soundings. The results are applied to the airborne resistivity models and are used to trace aquifer hydrostratigraphy throughout the basin. Interpreted parameters include the depth to basement, the depth to water, and the thickness of the primary aquifer. Together with hydrologic investigations, these results are being used to estimate groundwater storage within the basin