When a sound wave crosses an interface between layers of two different velocities, the wave is refracted.  That is, the angle of the wave leaving the interface will be altered from the incident angle, depending on the relative velocities.  Going from a low-velocity layer to a high-velocity layer, a wave at a particular incident angle (the "critical angle") will be refracted along the upper surface of the lower layer.  As it travels, the refracted wave spawns upgoing waves in the upper layer, which impinge on the surface geophones.

Sound moves faster in the lower layer than the upper, so at some point, the wave refracted along that surface will overtake the direct wave.  This refracted wave is then the first arrival at all subsequent geophones, at least until it is in turn overtaken by a deeper, faster refraction.  The difference in travel time of this wave arrival between geophones depends on the velocity of the lower layer.  If that layer is plane and level, the refraction arrivals form a straight line whose slope corresponds directly to that velocity.  The point at which the refraction overtakes the direct arrival is known as the "crossover distance", and can be used to estimate the depth to the refracting surface.

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