Shear velocity anisotropy in SAFOD Main Hole from wireline logs and laboratory

In the summer of 2005 Phase 2 of the San Andreas Fault Observatory at Depth (SAFOD) borehole was completed and logged with wireline tools including a dipole sonic tool to measure P- and S-wave velocities. A zone of anomalously low velocity was detected from 3150 to 3414 m measured depth (MD), corresponding with the subsurface location of the San Andreas Fault Zone. Within the fault zone several slip surfaces were identified as well as two actively deforming shear zones at 3192 and 3302 m MD. Using data from borehole dipole sonic logs and laboratory ultrasonic velocity measurements of core samples from SAFOD, we present an analysis of shear velocity anisotropy in and near the San Andreas Fault Zone where it crosses the SAFOD borehole. In the Phase 2 borehole logs, we observe three zones of anisotropy: 3079 - 3248 m MD, 3248 - 3770 m MD, and 3770 - 3962 m MD. These zones have an average of 10%, 13%, and 5% anisotropy, respectively. The active shear zones, as identified by casing deformation and interpretation of velocity and resistivity logs, coincide with local decreases in anisotropy while the damage zone is located within the zones of highest anisotropy. We compare the fast polarization direction to the direction of SHmax and the orientations of fractures and bedding planes, determined from image logs, to address the degree to which anisotropy at SAFOD is stress-induced vs. structural. We also compare anisotropy in the logging data to preliminary anisotropy measurements obtained from ultrasonic velocity experiments performed on SAFOD core samples. Three orthogonal mini-cores were taken from whole round core samples and placed into a pressure vessel that is capable of independent syringe-pump control of axial and confining stress, as well as pore fluid pressure. Samples were saturated and pressurized to approximate the conditions in the fault zone at depth. P- and S-wave velocities through the samples were measured as a function of stress state. From these measurements the fast and slow S-wave directions were determined and used to calculate the anisotropy.