What can the polarisation of shear waves tell us?

The direction of polarisation of seismic shear waves and the time shift between shear-wave arrivals of different polarisation contain important clues to the internal structure of the medium through which the waves have passed. For instance, the direction of polarisation can be used to infer the average orientation of cracks (and thus the direction of tectonic stresses), while the time shift depends not only on the orientation of the ray path with respect to the plane of the cracks, but also on the product of crack density and path length through the cracked medium. Before the direction of polarisation and the time shift can be used for this purpose, one must ascertain that these parameters are not contaminated by spurious effects. Disturbances that must be corrected for occur at the free surface, but even in vertical seismic profiles (VSPs) disturbances might occur due to transmission through interfaces: unless the shear waves are polarised parallel or perpendicular to the 'plane of propagation' that contains the ray and the normal to the interface (or unless the angle of incidence is zero), the direction of polarisation changes at transmission due to the different transmission coefficients of the two component waves. For incidence at and beyond the critical angle, the transmission coefficient of the component parallel to the plane of propagation is complex, resulting in a time shift. The effects at the free surface and at individual interfaces are relatively small (though not negligible). However, passage through several interfaces can lead to the accumulation of time shifts. In the limit this accumulation results in two shear waves with different veloeities, i.e. to anisotropy.