Lawrence Berkeley National Laboratory

In two separate shallow field experiments, at two distance scales, we have used continuous monitoring to estimate the effect of barometric pressure on crosswell travel time and thereby calibrated the stress sensitivity of the rock volume between the wells. In a 3 m experiment we found a stress sensitivity of 10-6/Pa while in a 30 m experiment the sensitivity was 5 x 10-8 /Pa. Results from a deeper (1 km), 2 month experiment at the San Andreas fault observation boreholes will be presented if analysis is completed.