For many years, seismic anisotropy was often neglected, mostly because of the inherent heavy mathematical and computational tools needed to describe and model its effects on seismic waves. The usual basic knowledge about propagation in isotropic media cannot easily apply to anisotropic media, where new phenomena come up, such as birefringence (or shear-wave splitting), or difference between directions of propagation of phase velocity and of group velocity. Consequently, geophysicists often claimed that it was a second-order effect, and considered the Earth as isotropic.

This hypothesis was assumed to be a good approximation, because of the random orientation of crystals in...

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