FIG. B-4. Birefringence involves splitting an S-wave into two waves that travel at different velocities and hence have different traveltimes. Such a situation might occur in travel through a vertically fractured medium (transversely isotropic). The first wave is polarized perpendicular to the fractures and it travels at the lower speed S2; the third wave polarized parallel to the fractures travels with the higher speed S1. The second and fourth waves polarized at other angles split into two waves, one traveling perpendicular and the other parallel to the fractures. (From Sheriff and Geldart, 1995, 57).