Improving microseismic cluster localization using DAS and Distance Geometry Solvers: application to data from the FORGE geothermal experiment

Monitoring induced seismicity is vital in Enhanced Geothermal Systems (EGS) applications. Permanent seismic monitoring networks at EGS sites aim to continuously detect and locate new seismicity occurring in the vicinity of the stimulation zone. This work focuses on localization of clustered seismicity, measured with a vertically deployed fiber optic cable with Distributed Acoustic Sensing (DAS) measurements, and one or more stations of the surface network at the Utah FORGE geothermal site, United States. DAS has distinct advantages when deployed in downhole vertical settings: it is temperature robust, it can be deployed behind the borehole casing and remain there for its operational lifetime, and it can densely sample the vertical wavefield. Detection of clustered seismic events along the DAS cable and one or more surface stations allows us to use and adapt a relative inter-event distance algorithm (HADES1) to include DAS data. The relative distance between clustered earthquakes is computed from the S- and P-wave arrival time differences, leading to event locations in a relative frame. A minimum of four 'master events', earthquakes with known absolute location, are then needed to find the correct orientation and position of the cluster. This orientation is obtained by finding the minimal difference in observed and calculated S- and P-wave arrival times while rotating the seismic cluster. Initial synthetic tests show that with a good estimate of the local velocity, the location and orientation of the seismic cluster can be retrieved. Following the synthetic tests, the data from the installation at the Utah FORGE site is used to test and validate our method. This study is a contribution to the international DEEP Project aimed at establishing a full-scale real-time test bench for seismic monitoring and forecasting at the Utah FORGE EGS Site. 1 HADES: https://github.com/wulwife/HADES