Understanding the Utility of Gravity and Gravity Gradiometry for Geothermal Exploration in the Southern Walker Lake Basin, Nevada

Abstract

Ground gravity has been instrumental in understanding structure at depth for many geothermal targets. An efficient approach to the interpretation of these data is to model a basin using a series of 2D sections. However, the validity of 2D modeling is questionable when highly 3D structures are present. 3D modeling is often needed when the complexity of 3D structure increases. Full 3D inversions require dense data coverage over the basin and beyond, but large-scale data collection can be time consuming and expensive. The difficulties associated with both the cost and coverage of ground gravity data may be overcome by utilizing the newly available airborne gravity gradiometry surveys. The southern Walker Lake Basin, Nevada, where the Navy Geothermal Program Office is actively exploring, is a complex basin bounded by N-NNW striking normal faults to the west and Walker Lane type dextral faults to the east. Given the structural complexity and rapid variations in both the basin depth and surface topography in this area, it is clear that 3D modeling is required to quantitatively utilize gravity data in the Southern Walker Lake Basin. We examine and compare 2D density sections to 3D surface inversion modeling of the basin. Preliminary results indicate that the basin constructed using a sequence of 2D sections cannot fully match the observed data and also introduces spurious features. We investigate the data density and distribution required to fully image the complex basin. Within this context, we also examine the feasibility of using airborne gravity gradiometry. This method allows efficient acquisition of gravity gradient data with dense data over a large area. We show through synthetic simulations that the improved data coverage and 3D modeling not only improve the characterization of local structures, but also provide an understanding of regional structure surrounding the target area.