In seismic refraction, the critical angle θc for total internal reflection/head-wave generation is defined by Snell's Law as θc = arcsin(V1/V2), where V1 is the overlying (slower) medium velocity and V2 is the underlying (faster) medium velocity. The critical distance (where the head wave first emerges at the surface) and the crossover distance (where the refracted arrival first overtakes the direct wave in travel time) are related by a fixed multiplicative factor for given V1, V2, making them directly (linearly) proportional.
Since θc = arcsin(V1/V2), as V2 decreases (approaching V1), the ratio V1/V2 increases toward 1, meaning arcsin(V1/V2) increases toward 90°. This means the critical angle...
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