Geophysical Activities Over the Utah FORGE Site at the Outset of Project Phase 3

Abstract

The Utah Frontier Observatory for Research in Geothermal Energy (FORGE) enters Phase 3 activities with a suite of geophysical observations and models for characterizing the site and providing background properties for upcoming experiments. Reprocessed and migrated seismic reflection results expand the domain of good imagery and show a clear alluvial bedrock interface dipping ~25 degrees westward with a depth that agrees with the well 58-32 intercept at ~3200 feet (975 m). Two-dimensional regularized inversion of gravity data used the bedrock profile as a primary constraint and yielded a model with localized high density below the basement interface nearby to the south of the FORGE site that may represent local dioritic composition. Geodetic GNSS monuments anchored into alluvium over the FORGE site area with bedrock-anchored reference sites display cm-scale possible ground motions in initial processing which may include significant seasonal hydrological effects. Initial InSAR survey scenes suggest similar vertical motion. New tensor magnetotelluric surveying adds 122 sites for testing the overall structural model at several scales and abuts related surveying across the Mineral Mountains and Roosevelt Hot Springs system to the east. Inversion mages from the latter data set show low resistivity lineaments in the upper few km trending just west of north. A low resistivity body in the 4-8 km depth range below the Quaternary rhyolite flows and domes is offered as a possible residual source zone for the eruptives.