Conceptual Model of Geothermal Area “X” Based on Integration of Magnetotellurics (MT) and Gravity Method

Abstract

The geothermal prospect area in Pasaman Regency, West Sumatra, is located within a structurally active zone influenced by the Sumatra Fault System. This study aims to construct a comprehensive subsurface model through the integration of magnetotellurics (MT) and gravity data to understand the conceptual model of the geothermal system. A total of 26 MT measurement points were analyzed using 2D inversion to map the resistivity structure, combined with 278 gravity measurement points modeled in 3D using the conjugate gradient inversion method. MT results reveal the presence of a low-resistivity clay cap (2.8–11.2 ohm.m) between +500 m and –500 m above sea level, underlain by a medium-resistivity zone (15.8–89.1 ohm.m) interpreted as a geothermal reservoir at a depth of approximately 2.5 km. This zone is bounded by two major fault structures, which are inferred to act as conduits and traps for geothermal fluids. Gravity data were reduced using a density of 2.65 g/cm³. Separation of regional and residual gravity anomalies using the Trend Surface Analysis (TSA) method yielded residual anomalies ranging from –10.9 to 6.7 mGal. The 3D gravity modeling of residual anomalies identified zones with relatively high density (0.3 g/cm³) associated with lava rocks and relatively low density (–0.4 g/cm³) associated with sedimentary rocks. Integration of both methods indicates that this geothermal system is of the fault-hosted type, with faults serving as the primary pathways for fluid migration and as key controls on geothermal accumulation.