Near-surface shear-wave birefringence in the North Sea: Ekofisk 2D/4C test

Abstract

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 shear-wave 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.