Shear-wave splitting in anisotropic media
In 4-C seismic method, we reviewed the 4-C seismic method based on the conversion of P-waves to S-waves at layer boundaries. The converted S-wave propagating in an anisotropic medium exhibits a unique behavior. Consider a rock layer that contains vertical fractures oriented in, say the north-south direction, as sketched in Figure 11.7-31 — a case of a horizontally transverse isotropy, or equivalently, a case of azimuthal anisotropy. The S-wave that enters this transversely isotropic layer from below emerges from above as split and polarized in two orthogonal directions — the shear-wave component that is polarized parallel to the fracture orientation and the shear-wave component that is polarized perpendicular to the fracture orientation. Shear-wave splitting in azimuthally anisotropic media is sometimes referred to as shear-wave birefringence.
The shear-wave component polarized parallel to the fracture orientation travels with a speed faster than the shear-wave component in the perpendicular direction. In practice, we have the radial and transverse components; but not the slow and fast shear-wave components. The fast and slow shear-wave components, however, can be extracted from the radial and transverse geophone components (4-C seismic method) by a special rotation developed by Alford . The Alford rotational analysis of the shear-wave data provides an estimate of fracture orientation. This is strategically important in developing reservoirs within fractured rocks  ; specifically, in planning trajectories for production wells.
The split shear waves interfere with one another in a way that depends on the source-receiver azimuth. Figure 11.7-32 shows radial and transverse components sorted with respect to source-receiver azimuth. Each trace in these gathers is associated with a radial or transverse component in a specific azimuthal direction. Note that the radial component is fairly consistent from one azimuthal direction to the next. The transverse component, on the other hand, shows polarity change every 90 degrees. From these gathers, the Alford rotation produces the fast and slow shear-wave components while minimizing the energy in the transverse component.
- Alford (1986), Alford, R. M., 1986, Shear data in the presence of azimuthal anisotropy: Dilly, Texas: 56th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 476–79.
- Martin and Davis, 1987, Martin, M. A. and Davis, T. L., 1987, Shear-wave birefringence: A new tool for evaluating fractured reservoirs: The Leading Edge, 22–28.
- Ata and Michelena, 1995, Ata, E. and Michelena, R., 1995, Mapping distribution of fractures in a reservoir with PS-converted waves: The Leading Edge, 664–673.
- Seismic anisotropy
- Anisotropic velocity analysis
- Anisotropic dip-moveout correction
- Anisotropic migration
- Effect of anisotropy on AVO