3D Gravity Inversion for Identifying a Subsurface Pluton and Estimating Geothermal Heat Potential: A Case Study at Newberry Volcano, Oregon

Abstract

: Newberry Volcano, located in Oregon, USA, is an active silicic caldera and one of the primary targets for the development of Enhanced Geothermal System (EGS) in Oregon, United States. The geothermal potential in this region is believed to originate from a subsurface pluton acting as a heat source. This study utilizes 911 gravity measurement points covering an area of 35 × 37 km2, acquired from the Geothermal Data Repository, to identify subsurface structures based on the distribution of rock density, including the depth, extent, and geometry of the pluton. Spatial corrections were applied to the gravity data, and the average rock density was estimated using the Parasnis and Nettleton methods, yielding a value of 2500 kg/m³. The analysis involved gravity data correction, separation of regional and residual components using first-order polynomial trend surface analysis, and 3D inversion modeling using the conjugate gradient method. The model was constructed over six depth layers, reaching a maximum depth of 5.75 km. The residual anomaly map revealed a circular positive anomaly on the western flank of the caldera, interpreted as the response of a high-density plutonic body. The 3D inversion identified shallow body at a depth of 3 km with a dimension of 9 × 6 km2, and another deeper body at 4.25 km depth with an extent of 14 × 13 km2. The density range obtained in the inversion is (-39) – 76 kg/m³ relative to 2.500 kg/m³. A root mean square (RMS) value of 0.7 shows that the modeled and observed gravity data are very similar. From inversion result, potential heat stored within the pluton can be estimate. A 3 km deep pluton holds 30.834 EJ of total heat that can produce 1,172 MW from the average annual potential electric capacity over 30 years of operation.