Abstract

We try to characterize the fracturing of the anisotropic shallow granite (<250 m) present at Garner Valley, southern California. The shear-wave splitting observed on a downhole seismic array installed there (Archuleta et al., 1992) is modeled by performing numerical simulations of propagation of P and SV waves in 2D fractured media. The calculations are performed using a boundary integral equation method, which takes into account multiple scattering and full crack diffraction. The value of the time delay measured at Garner Valley between the fast S wave and the slow S wave is well reproduced for different models of fractured media. In order to differentiate between these models, we also measure and calculate the attenuation of the transmitted S waves. From the comparison between the observations and the numerical simulations, we conclude and infer the following: (1) The presence of fractures in granite explains the seismic anisotropy and attenuation observed at Garner Valley. (2) The cracks and fractures are nearly vertical and are oriented in a north-south direction. (3) There is no characteristic crack length, but rather, the crack length distribution seems to be fractal. (4) The density of fractures present in the shallow granite at Garner Valley is about 10−4 representing the total volume of the fractures over the total volume of the fractured zone.

First Page Preview

First page PDF preview
You do not currently have access to this article.