Résumé
Borehole electrical images were recorded in the upper crystalline basement of the Cajon Pass scientific drillhole as part of an extensive program of downhole experiments. In spite of the large borehole size and the high resistivity of basement rocks, excellent images were obtained from 850 m to 1820 m. Such oriented electrical images resolve the texture of igneous rocks, and provide a means of identifying and mapping open or mineralized fractures. In order to study more precisely the response of the images to basement features, they were compared to sections where cores had been recovered. The images are in good agreement with the cores, thereby permitting their orientation. In all, over 800 fractures were mapped from 850 to 1820 m on the images. Whereas the foliations have generally low dips, the fractures are often steep. Most of the fractures strike NE-SW to E-W, in a direction orthogonal to the trace of the San Andreas Fault. This is consistent with the results of Zoback et al. [1987], hydraulic fracturing experiments at Cajon Pass [Zoback and Healy, this issue], and the analysis of borehole breakouts at Cajon Pass [Shamir et al., this issue]. The analysis of the electrical images suggests that the observed present-day stress regime in the vicinity of the San Andreas Fault might have remained constant over the past few million years.