Several recent studies have demonstrated the ability to measure the effects of shear-wave (S-wave) birefringence using mode-converted (PS) waves. Standard PS-wave processing relies on the assumption that the subsurface is horizontally isotropic. The two horizontal components are typically rotated (about the vertical axis) to a direction that is oriented radial and transverse to the source receiver geometry. In the presence of azimuthal anisotropy in the subsurface geology, the upcoming S-wave will split into two-one polarized in the fast direction (S1) and the other in the slow direction (S2). If S-wave birefringence is ignored, then the radial component will be comprised of a mixture of both fast and slow shearwave energy. This mixing will result in a degradation of resolution and S/N ratio by destructive interference between the fast and slow S-wave arrivals. Recent developments in PS-wave processing have made it possible to resolve these fast and slow directions and the traveltime lag between them. The measured traveltime differences provide useful information regarding the magnitude of anisotropy, and the measured azimuth provides further insight into the orientation of stress fields and/or fracture directions. In September 2002, a 2D/4C test line was acquired at Ekofisk Field in the southern North Sea. This test line consisted of 40 four-component (4C) receivers and a 3D grid of sources. Subsequent analysis of the PS-wave data, primarily in the near surface, indicated the presence of significant birefringence. The measured birefringence correlates well with the local seafloor subsidence that has been induced by compaction within the underlying chalk reservoir at a depth of approximately 3 km. These results provide new information useful for characterizing fracturing and stress fields attributed to seafloor subsidence at Ekofisk. Additionally, the magnitude of the S-wave birefringence (time shift between S1 and S2) correlates very closely with the relative change in the S-wave static corrections that were derived during 2D processing of the radial component data-indicating that an important link exists between them at Ekofisk. This correlation suggests that the calculated PS receiver statics are related to both azimuthal anisotropy and near surface S-wave velocity variations. Such observations provide further evidence that the effects of S-wave birefringence should be considered when imaging shear-wave seismic data, and that corrections begin with statics.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.