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cross-over distance (xc) The distance on a seismic refraction survey time-distance chart at which the travel times of the direct and refracted waves are the same. This distance also marks the point when the refracted wave overtakes, and thus arrives before, the direct wave. The cross-over distance is related to the refractor depth, h, and the velocities of the overlying medium and the refractor, V1 and V2 respectively, such that xc = 2h[(V2 + V1)/(V2 − V1)]1/2. The value of xc will always be greater than twice the refractor depth.