The initial hydraulic-fracture operations at Utah FORGE involved three injection stimulation stages conducted at or near the toe section of the first completion well. Various seismic receiver configurations were used to monitor and map the induced microseismicity including deep borehole receiver strings deployed to detect and accurately map the low-magnitude (Mw ≤ -1.0) microseismicity associated with fracture creation, stimulation and growth. Microseismic data for the 3rd stage had the best downhole coverage. This included three vertical, multi-level downhole geophone strings providing a total of eighteen 3-component (3C) geophone receivers deployed within or just above the target reservoir. Using this three-well downhole array, we have conducted a preliminary study using waveforms of 30 high quality events to evaluate the determination of their source mechanisms. Determining seismic source mechanisms essentially involves resolving the source seismic radiation patterns. With extensive receiver configurations, this can often be done using the P-wave polarities and amplitudes alone. Sparse receiver coverage, such as provided by the Utah FORGE downhole network, requires resolving the relative radiation patterns of the P- and S-wave phase amplitudes and their first motions (polarities). We used a combination of P, SH, and SV phase first motions and amplitude ratios of SH/P, SV/P and SH/SV to obtain stable solutions from the sparse array. The 3C geophone orientations were determined using downhole string shots and the wellbore perforation shots. We also used the event source locations, determined using the travel times from the 3-well array, to improve the relative receiver orientations. The larger azimuthal spread of the event locations also helped to identify, and remove from consideration, poor receiver response caused by some horizontal channels being under amplified. The receiver waveform data were rotated to P, SH, and SV phases and the three phase polarities were precisely picked. Coherency across the oriented vertical strings allowed SV and SH phases to be identified and picked consistently with confidence. The combination SH/P and SV/P amplitude ratios were especially helpful in constraining the solutions using the sparse receiver coverage. SH/P helps constrain fault strike but is insensitive to fault dip. Conversely, SV/P helps constrain fault dip but is insensitive to fault strike. For the 30 events examined, we obtained 18 double-couple solutions (shear displacement on a fracture plane). Most solutions (12) are dominantly strike-slip with one failure plane oriented subparallel to the plane outlined by the hypocenters. The remaining solutions indicate predominantly normal-fault displacements along similarly oriented fractures.

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