ELECTRICAL RESISTIVITY CHARACTERIZATION OF BEDROCK FOR CONCEALED MINERAL DEPOSITS BENEATH BASIN FILL SOUTH OF THE BODIE-AURORA VOLCANIC FIELD, CALIFORNIA AND NEVADA
U.S. Geological Survey scientists are characterizing exposed and concealed bedrock by the use of textural classification of Earth’s magnetic field data. As an additional constraint to the delineation of concealed lithology, magnetotelluric data are being used to characterize the electrical properties of bedrock beneath basin fill in the Mono Basin area in Mono County, California, and Mineral County, Nevada. The magnetotelluric method is a passive surface geophysical technique that uses the Earth's natural electromagnetic fields to investigate the electrical resistivity structure of the subsurface from depths of tens of meters to tens of kilometers. In the upper crust, the resistivities of geologic units are largely dependent upon their fluid content, pore volume porosity, interconnected fracture porosity, and conductive mineral content. At greater depths, higher subsurface temperatures cause higher ionic mobility that reduces rock resistivities.
Regional magnetotelluric soundings were collected in June 2016 primarily along four northwest-southeast profiles ranging in length from 15 to 25 kilometers. Sounding locations were chosen to cross known mineral alteration zones and to characterize bedrock beneath covered potential extensions of deposits. The magnetotelluric profile soundings were first inverted with a 2-D resistivity inversion for the initial analysis and later by a 3-D magnetotelluric inversion because all of the observed data indicated a 3-D electromagnetic response. The resistivity profiles obtained from these inversions both contribute to and constrain lithologic characterization derived from Earth’s magnetic field data.