2D and 3D Interpretation of Magnetic Survey Data in the Presence of Reversely Magnetized Rocks

Abstract

Magnetic survey data is often used as part of a geothermal exploration program to better understand geologic structure, hydrothermal alteration, and depth-to-bedrock. However, geologic interpretation of magnetic survey data can be complicated by the presence of reversely magnetized rocks. Tertiary volcanic rocks (which are generally magnetic) are abundant in Nevada and the rest of the Great Basin where geothermal exploration takes place. In addition, many magnetic reversals have occurred during the Tertiary which means that reversely magnetized volcanic rocks are not rare and their presence likely obscures the geologic interpretation of magnetic survey data at many geothermal prospects, possibly leading to erroneous interpretation. Paleomagnetic analysis is the best tool to determine if a rock outcrop is reversely magnetized or not; however, paleomagnetic data is not commonly available because it requires specialized expertise to acquire. Several other tools and techniques are presented here to help discern whether reversely magnetized rocks may be present in a study area to aid geologic interpretation of magnetic survey data. These include: comparison of magnetic survey data with geologic maps, magnetic susceptibility measurements, radiometric age dating, and analytic signal mapping. Examples of 3D geophysical inversion modelling of magnetic survey data are also presented to outline the challenges and opportunities of modelling reversely magnetized rock bodies in 3D to better characterize their geometry and structure in the subsurface. Specifically, a 3D magnetic inversion modelling technique called Magnetic Vector Inversion is shown to successfully model reversely magnetized rock units at the Argenta Rise study area which is part of the U.S. Department of Energy funded INGENIOUS project.