seismic velocity: The speed with which an elastic wave propagates through a medium. For non-dispersive body waves, the seismic velocity is equal to both the phase and group velocities; for dispersive surface waves, the seismic velocity is usually taken to be the phase velocity. Seismic velocity is assumed usually to increase with increasing depth and when measured in a vertical direction it may be 10–15% lower than when measured parallel to strata.
The velocities of various rock types vary rather widely so it is usually difficult to determine rock type based only upon velocities. The table to the right shows rough ranges of velocities in units of kilometers per second for several types of earth materials. Therefore seismic surveys are most effective at delineating structure, .i.e boundaries where rock type changes.
The relations between elastic properties and velocity, introduced under "Fundamentals", are given again here.