Using a dense linear array for passive seismic converted wave imaging at a geothermal site: The FOAL 1 experiment at Utah FORGE

ABSTRACT Recent advances in enhanced geothermal system (EGS) development have opened new frontiers for geothermal energy production. However, static geophysical imaging and time-lapse monitoring of these systems can be challenging, with recent pilots located in areas with attenuating near-surface sediments and generally low-quality surface seismic data. These same problems are relevant to achieving accurate subsurface characterization, which is essential for optimizing drilling and reservoir development and enhancing the economic viability of EGS by ensuring sustainable energy extraction. The applicability of source-independent converted wave imaging was demonstrated using microseismic energy to passively image key geologic structures at an active EGS pilot site. To test this imaging concept, a dense linear nodal (3C) data set was acquired at the Frontier Observatory for Research in Geothermal Energy (FORGE) facility located in Milford, UT. This acquisition campaign, FORGE Observation Array Linear (FOAL) 1, was conducted during the April 2022 stimulation of a deep EGS injection well (FORGE well 16A). Despite a short observation period (approximately 1 month) and a limited number of seismic events, the granite–alluvium interface — the major velocity contrast at the FORGE site — was successfully imaged. The results suggest the potential of our approach for site characterization and real-time monitoring of subsurface changes. Moreover, this illuminates the prospects of using related large-N technologies, including distributed acoustic sensing for source-independent imaging.