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The estimate of propagation velocities in the reflection or refraction seismic method is essential to the effective imaging of the subsurface. Wave propagation in a fully elastic medium gives rise to several propagation modes, among them are the longitudinal and transverse compressional waves (P-waves and S-waves), which are commonly used in the reflection and refraction seismic methods. With appropriately sorted data, the arrival time of a wave returning from a particular reflecting interface in the subsurface will vary as a function of source-receiver offset. This variation in arrival time with offset is called "moveout" and is controlled by the propagation velocity. The velocity of propagation enters the processing of reflection seismic data first as an essential parameter of a time coordinate transformation required before data of varying source-receiver offsets can be stacked to enhance signal-to-noise ratio. Velocities estimated in this manner are called "moveout velocities." Velocities also enter the processing sequence as a parameter of an imaging operation called "migration." Early efforts at velocity estimation were only accurate enough to provide parameters to process data, but high-quality data collected using present-day technology allow us to make accurate enough estimates of propagation velocity to infer subsurface geology. Complications arise, however, due to the effects of reflector structure and lateral velocity gradients. Current developments in seismic velocity estimation include measurement of shear wave (S-wave) velocities, use of wide-angle arrivals for more accurate P-wave velocity estimates, and methods requiring areal coverage (three-dimensional seismic).