Splitting of an incident S-wave into two (or more) waves with different polarizations, also called birefringence, S-wave splitting, mode splitting, double refraction c.f. many works by Stuart_Crampin, e.g. . In a polar anisotropic (transversely isotropic) medium, for each travel direction only two orthogonal polarizations of plane shear waves are allowed (although they are not necessarily orthogonal to the propagation direction). An S-wave of arbitrary polarization entering such a region in a direction other than along the symmetry axis splits into two S-waves (one of which is quasi-shear) that propagate at different velocities S1 for the faster, S2 for the slower (often written S1, S2). It is the elastic analog of optical birefringence.
For polar anisotropy (aka VTI anisotropy), due to e.g. unfractured shales or horizontal thin-layers, the two waves are the SH- and qSV-waves, the qSV not necessarily being polarized perpendicular to the propagation direction. In these cases, the two shear modes degenerate to one (no shear-wave splitting), for propagation in the vertical direction.
For azimuthal anisotropy, due to e.g. fractures with preferred orientation (non-horizontal), the two waves are the qS1- and qS2-waves, neither necessarily being polarized perpendicular to the propagation direction. In this case, there will be shear-wave splitting, even for propagation in the vertical direction. For parallel fracturing the S1 mode that is polarized parallel to the fractures travels faster than the S2 mode that is polarized perpendicular to the fracture direction (see Figure B-4), See also polar anisotropy, Thomsen anisotropic parameters, and anisotropy (seismic).
- Crampin, S., 1985. Evaluation of anisotropy by shear-wave splitting: GEOPHYSICS, 50(1), pp. 142-152.