| Abstract: |
Image logs from boreholes drilled by the Navy Geothermal Program Office (GPO) at five geothermal areas in the western United State (Fallon Naval Air Station, NV, Hawthorne Army Depot, NV, Coso Geothermal Field, Naval Air Weapons Station China Lake, CA, Chocolate Mountains Aerial Gunnery Range, CA (Marine Corps Air Station, Yuma, AZ) and Naval Facility El Centro, CA) were analyzed to identify natural fractures intersecting the borehole, to determine the stress field acting on fractures proximal to the borehole and to quantify the amount of stress heterogeneity with depth. Horizontal principal stress orientations vary on the multiple kilometer scale largely due to the structural setting, but seem to vary at the meter and centimeter scale in the studied boreholes due to slip on fractures proximal to the hole, intersected lithology and, in highly stressed environments, slip on large regional faults. The average maximum horizontal principal stress orientation, revealed by induced structures in the studied boreholes, range from an azimuth of 081° to 133° and the standard deviations in stress orientation has a range of 11° to 46°. Spectral analysis was used to determine the variation of principal stress orientation from a calculated vertical axis. This analysis yields a linear spectral slope for each borehole, which was then compared to earthquake frequency and magnitude data and lithologic data from Gamma Ray logs to determine the control on stress variation. The amount of standard deviation and the linear spectral slope determines the stress heterogeneity for each of the studied boreholes. A high variation of principal stress orientation suggests a greater number of optimally oriented fractures to transmit fluid and thus a greater opportunity to intersect fluid pathways. |