Formation Micro-Scanner (FMS) and Acoustic Borehole Televiewer (BHTV) image logs from borehole FOH-3D in the volcanic and metavolcanic rock of the Fallon Naval Air Station, NV were analyzed to identify natural fractures and constrain the stress field acting on the fractures proximal to the borehole and to the geothermal system. Abundant natural fractures are revealed in these logs, defining two conjugate fracture sets, which strikes ~010° and dips at ~58° to the E and W respectively. The average azimuth of Shmin ± one standard deviation revealed by extensive borehole wall breakouts and tensile fractures, as well as petal-centerline fractures, is 113±22° for the near vertical portion of the borehole. Spectral analysis of the depth variation of the horizontal principal stress azimuth revealed in the BHTV image indicates that rotations of these stress components about the vertical axis yield a linear spectral slope of -3.03±0.2 log (deg2*m)(m) in a frequency range of from 10-2.4 to 10-1.35 m-1. This spectral slope is consistent with a stable, fractal dependence of stress rotations on length-scale and characterizes the inherent heterogeneity of the principal stress directions in the volume traversed by the borehole. Together, the organization of the fracture population into conjugate sets and an azimuth of SHmax consistently parallel to the strike of the conjugate set indicate that these fractures are well oriented for normal slip. The natural fractures have similar orientations to the Stillwater Range Front Fault proximal to the borehole, as well as numerous seismogenic faults west of the Stillwater Range in the Carson Basin. This azimuth of Shmin is similar to the regional Shmin azimuths for the normal faulting in the Basin and Range province and to previous stress orientation analyses of borehole-induced structures at Dixie Valley, Desert Peak, and Brady’s geothermal systems.

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