Twelve years of vertical birefringence in nine-component VSP data

Since 1986, when industry scientists first publicly showed data supporting the presence of azimuthal anisotropy in sedimentary rock, we have studied v ertical shear-wave (S-wave) birefringence in 23 different wells in western North America. The data were from nine-component vertical seismic profiles (VSPs) supplemented in recent years with data from wireline crossed-dipole logs. This paper summarizes our results, including birefringence results in tabular form for 54 depth intervals in 19 of those 23 wells, in the Append ix we present our conclusions about how to record VSP data optimally for st udy of vertical birefringence. We arrived at four principal conclusions about vertical S-wave birefringenc e. First, birefringence was common but not universal. Second, birefringence ranged from 0-21%,but values larger than 4% occurred only in shallow forma tions (<1200 m) within 40 km of California's San Andreas fault. Third, at l arge scales birefringence tended to be blocky. That is, both the birefringe nce magnitude and the S-wave polarization azimuth were often consistent ove r depth intervals of several tens to hundreds of meters but then changed ab ruptly, sometimes by large amounts. Birefringence in some instances diminis hed with depth and in others increased with depth, but in almost every case a layer near the surface was more birefringent than the layer immediately below it. Fourth, observed birefringence patterns generally do not encourag e use of multicomponent surface reflection seismic data for finding fractur ed hydrocarbon reservoirs, but they do encourage use of crossed-dipole logs to examine them. That is, most reservoirs were birefringent, but none we s tudied showed increased birefringence Confined to the reservoir.