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.
Seismic measurements of velocity are averaged over the horizontal distance through which the seismic energy travels. Sediment velocities generally increase with depth due to increased pressure of the overburden. Fluids within pores tend to make the rocks less compressible and lead to higher interval velocities for P-waves. The adjacent figure summarizes typical velocities for differing lithologies and porosities. Carbonates in particular show a large range in velocities depending on porosity. Generally is its correct to stack the data with seismic velocity but little else. Nevertheless seismic velocity is often used for depth conversion and migration purposes and can be calibrated to well information or used where well information is particularly sparse.