Development of Geohydrologic Model of the Wildcat Fault Zone

Abstract

We have conducted field investigations of the Wildcat fault in Strawberry Canyon in the East Bay Hills of Berkeley, California, including a literature survey, aerial-photographic—based geomorphological study, geologic mapping, geophysical surveys, trenching, and borehole drilling and hydraulic testing. A geologic model was constructed, which became the basis of the hydrologic model. We outline the effort of constructing the geohydrologic model of the Strawberry Canyon area. We also created an East Canyon sub-model, which is a part of the Strawberry Canyon area. These models were constructed using Petrasim commercial software, which is a pre- and post- processor for TOUGH2, a non-isothermal multiphase flow and transport simulator. One of our goals is to understand the role of the Wildcat fault in controlling the natural-state groundwater flow. Another goal is that with limited data in numbers and areal extent, we evaluate the viability of modeling a relatively complex geologic area in hopes of to building a model that is valid for a scale larger than the observation. We performed both manual and automated inversion analyses and produced reasonable matches between the observed head data and model predictions. By varying the structure of the Wildcat fault, the base-case representation, which includes a high permeability damage zone and a low permeability fault core, best matches the observed head data. Using the sub-model, we conducted two-phase non-isothermal simulations utilizing the pressure and temperature data from the boreholes. We also used the information obtained from pump tests including permeability anisotropy of the fault plane. After parameter searches, we were able to match the head and temperature profiles along boreholes relatively well. We then used the best matching models to predict the observed rate of head decline during a dry period and found that anisotropic fault zone with 5 % porosity predicts the rate of decline reasonably well. There is a potential that the rate of decline may be useful to estimate the parameters downstream where there are no boreholes for observation/testing.