Low-Magnitude Seismicity With a Downhole Distributed Acoustic Sensing Array-Examples From the FORGE Geothermal Experiment

We show the capabilities of a downhole distributed acoustic sensing (DAS) array in detecting, locating, and characterizing low-magnitude earthquakes occurring in the vicinity of the Frontier Observatory for Research in Geothermal Energy (FORGE) site in Utah. Continuous data for 10.5 days were acquired in a monitoring well at the FORGE geothermal site during the initial stimulation of an enhanced geothermal system in April-May 2019. Earthquake activity beneath Mineral Mountains, Utah also occurred within 10 km of the FORGE monitoring well. During the experiment, four earthquakes were cataloged by routine processing of the University of Utah Seismograph Stations. Our processing of DAS data finds 77 earthquakes during that period, of which 16 are visible on the regional network. Five additional events are found by template matching DAS data. The magnitude of completeness obtained by DAS processing is better by at least M = 0.5 than the dense surface array around the FORGE site. Depth estimation using DAS is more reliable than using a surface array. While a single vertical DAS array is limited in terms of event location due to its azimuthal ambiguity, multiple DAS wells or a combination of a downhole array with surface stations or near-surface horizontal DAS could jointly resolve locations. All detected events probably originated from the two active source areas, located 3-5 and 8-10 km away from the FORGE site, respectively. Recorded events can be reliably clustered into several distinct families thanks to the spatial density and large number of DAS channels.

Automated summary

The study demonstrates the effectiveness of downhole distributed acoustic sensing (DAS) arrays in detecting and locating low-magnitude earthquakes near the FORGE site in Utah. DAS processing shows higher completeness and reliability in depth estimation compared to surface arrays. The results suggest that multiple DAS wells or a combination of downhole and surface arrays could jointly improve location accuracy.