Snells Law and the Critical Angle

Reference

Snells law governs how seismic energy bends at velocity contrasts, and the critical angle it defines is the basis of refraction surveying.

The equation

sin i / V1 = sin r / V2, sin ic = V1 / V2
  • i and r are incidence and refraction angles from the normal
  • V1, V2 are velocities above and below the interface
  • ic is the critical angle where refracted energy travels along the interface

Worked example

For soil at 1800 meters per second over rock at 3600, the critical angle is 30 degrees. Beyond that incidence, energy runs along the rock head as a refracted wave, and its arrivals at the surface map rockhead depth.

When to use it

The engine behind first arrival refraction and the reason velocity inversions are invisible to it: a slow layer under a fast one never produces a critical refraction, and the method silently skips it.

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