Seismic monitoring at the Utah Frontier Observatory for Research in Geothermal Energy (FORGE) is required for: (1) seismic hazard and risk assessment and mitigation; and (2) monitoring of fracture growth and reservoir development. Using the University of Utah regional earthquake catalog combined with more detailed seismic analyses compiled as part of the FORGE project, seismicity in the region can be characterized by low rates and low magnitudes. There is a region of small magnitude (M less than 2.5) natural tectonic activity and possibly seismicity related to injection at the Roosevelt Hot Springs geothermal system to the east of the FORGE site and a tectonic source zone to the south of the FORGE site near the town of Milford, Utah. Pre-development seismic monitoring has found that the immediate footprint of the FORGE facility is aseismic. To inform future seismic monitoring programs, seismic monitoring was performed during a stimulation test of well 58-32, a 2297 m vertical well, in the spring of 2019. Two boreholes were drilled to depths of ~300m and ~1000m. In the shallower hole, two sensors were tested to evaluate comparative sensitivity. These sensors were: (1) a three-component high-frequency geophone with four sensors per component; and (2) a three-component accelerograph. In the deeper well (~1,000 m TVD), stimulation was monitored with an industry-proven 12 sensor, three-component string of sondes, and a Distributed Acoustic Sensing (DAS) fiber optic cable. The seismic signals from these tests were analyzed to measure detection levels. In addition, a dense array of three-component geophones was installed on the surface to assess the detection threshold and to characterize the local velocity structure. The data collected during this experiment will be used to inform the seismic monitoring program for future drilling and the stimulation of deep, highly deviated production and injection wells. In this paper, we discuss the results of seismic monitoring at the Utah FORGE site and plans for future monitoring.

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