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moveout (stepout)
1. The difference between the two-way travel times of reflected energy detected at two geophone offset distances.

2. (normal moveout, NMO, ΔT) At an offset distance x, the difference in two-way travel time between the detected reflection event at x (tx) and at zero offset (t0), such that ΔT = txt0 ≈ x2/(2V2t0), where V is the seismic velocity of the medium above the reflector.

3. (dip moveout, DMO, ΔTd) In the case of a planar dipping reflector, the difference between the moveout up-dip and down-dip, proportional to the angle of dip ɵ, such that: ΔTd = 2χsin ɵ/V, where χ is the offset distance from the mid-point.