Earlier treatment of zero-offset VSP data arrival times in terms of perfectly inelastic particle interactions is extended to include walkaway modes. The space-time tracks of particles moving under the influence of a uniform force field accelerate in such a way that they capture approximately the moveout in the VSP gather. In zero offset data and walkaway modes NMO correction is realized through transformations from the laboratory reference frame of the collisions to rest frames of individual particles before and after colliding. Galilean transformations suffice for zero-offset data, whereas something akin to relativistic transformations are needed for walkaway modes.
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