a Study of Fault Zone Hydrology

The Nuclear Waste Management Organization of Japan and Lawrence Berkeley National Laboratory are presently collaborating at a dedicated field site to further understand, and to develop the characterization technology for, fault zone hydrology. To this end, several deep trenches were cut, and a number of geophysical surveys were conducted across the Wildcat Fault in the hills east of Berkeley, California. The Wildcat Fault is believed to be a strike-slip fault and a member of the Hayward Fault System, with over 10 km of displacement. So far, three boreholes of ~ 150 m have been core-drilled; one on the east side and two on the west side of the suspected fault trace. The lithology at Wildcat Fault mainly consists of chert, shale and sandstone, extensively sheared and fractured; with gouges observed at several depths and a thick cataclasite zone. After conducting hydraulic tests, the boreholes were instrumented with temperature and pressure sensors at multiple levels. Preliminary results from these holes indicated that the geology was not what was expected: while confirming some earlier, published conclusions about Wildcat, they have also led to some unexpected findings. The pressure and temperature distributions indicate a downward hydraulic gradient and a relatively large geothermal gradient. Wildcat near the field site appear to consist of multiple faults. The hydraulic test data suggest the dual properties of the hydrologic structure of the fault zone. At this writing an inclined fourth borehole is being drilled to penetrate the main Wildcat. Using the existing three boreholes as observation wells, we plan to conduct hydrologic cross-hole tests in this fourth borehole. The main philosophy behind our approach for the hydrologic characterization of such a complex fractured system is to let the system take its own average and monitor long term behavior, instead of collecting a multitude of data at small length and time scales, or at a discrete fracture scale, and then to “up-scale,” which is extremely tenuous.