# Dictionary:Fig S-12

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FIG. S-12.

Snell's law relations when a wave crosses a boundary between two isotropic solid media; the wave changes direction such that for either incident P-wave or S-wave,

${\frac {\sin i}{V_{1}}}={\frac {\sin \theta _{p1}}{V_{p1}}}={\frac {\sin \theta _{s1}}{V_{s1}}}={\frac {\sin \theta _{p2}}{V_{p2}}}={\frac {\sin \theta _{s2}}{V_{s2}}}=p$ ,

where i is the angle of the incident wave with a velocity Vi=VP1 if a P-wave or Vi=VS1 if an S-wave; $\theta _{P1}$ and $\theta _{S1}$ are the angles of reflection of the P- or S-waves in medium 1, which have velocities VP1 and VS1, respectively; $\theta _{P2}$ and $\theta _{S2}$ are the angles of refraction of the P- and S-waves in medium 2 which have velocities VP2 and VS2, respectively; p is the raypath parameter (which will be a constant along a raypath through parallel layering). If $\sin _{P2}$ or $\sin _{S2}$ exceeds 1 as given by this equation, a head wave will be generated.