Characterization of fractures in shallow granite from the modeling of the anisotropy and attenuation of seismic waves

We try to characterize the fracturing of the anisotropic shallow granite (< 250 m) present at Garner Valley, southern California. The shear-wave splitt ing observed on a downhole seismic array installed there (Archuleta et al., 1992) is modeled by performing numerical simulations of propagation of P a nd SV waves in 2D fractured media. The calculations are performed using a b oundary integral equation method, which takes into account multiple scatter ing and full crack diffraction. The value of the time delay measured at Gar ner Valley between the fast S wave and the slow S wave is well reproduced f or different models of fractured media. In order to differentiate between t hese models, we also measure and calculate the attenuation of the transmitt ed S waves. From the comparison between the observations and the numerical simulations, we conclude and infer the following: (1) The presence of fract ures in granite explains the seismic anisotropy and attenuation observed at Garner Valley. (2) The cracks and fractures are nearly vertical and are or iented in a north-south direction. (3) There is no characteristic crack len gth, but rather, the crack length distribution seems to be fractal. (4) The density of fractures present in the shallow granite at Garner Valley is ab out 10(-4) representing the total volume of the fractures over the total vo lume of the fractured zone.