USING BOREHOLE BREAKOUTS TO CONSTRAIN THE COMPLETE STRESS TENSOR - RESULTS FROM THE SIJAN DEEP DRILLING PROJECT AND OFFSHORE SANTA-MARIA BASIN, CALIFORNIA

We use borehole breakouts in nonvertical drill holes to constrain the directions of the three principal stresses and their relative magnitud es. In this paper we start by modifying previously published breakout selection criteria to work with highly deviated borehole data. We pres ent a forward modeling technique using genetic algorithms and a nongra dient N-dimensional optimizer to find the best fitting stress state fo r a set of breakout data. The stress state is parameterized by three E uler angles and the stress state ratio phi. A technique is developed t o determine the 95% confidence weighted misfit between a model and the data. We then map out the 95% misfit confidence limits on the best fi tting stress state. This technique is applied to data published by Qia n and Pedersen [1991]. Removing their constraint of a vertical princip al stress direction reduces the misfit between the stress state and th e breakout data. We find that the best fitting Stress state they repor t with a vertical principal stress direction lies outside our 95% conf idence limits. We also invert breakouts in the offshore Santa Maria Ba sin, California. These data show a ''thrust faulting'' stress state wi th the maximum principal stress, S-1, at N148.5 degrees E plunging 31. 5 degrees. The 95% confidence range for the azimuth of S-1 ranges from N143.0 degrees E to N198.1 degrees E. The stress ratio phi was found to be 0.8211(0.584)(1).