(vel oss’ ∂t ē)
1. A vector quantity that indicates time rate of change of displacement.
2. In seismic usage (see Figure V-3) velocity refers to the propagation rate of a seismic wave without implying any direction, i.e., velocity is a property of the medium.
Adjectives preceding "velocity" are required to produce specific meaning and much confusion results because of the multitude of modifying adjectives in use (Figures V-3 and V-4). P- and S-wave velocities are given in Figure E-5 in terms of elastic constants for isotropic media and in Figure T-13 for polar anisotropy (transversely isotropic) cases.
Velocity is measured (or inferred) from sonic logs (q.v.), normal moveout (see velocity analysis), image focusing in depth-migration focusing analysis, well surveys (q.v.), and refraction time-distance curves (q.v.). For a discussion of the factors affecting seismic velocity, see Sheriff and Geldart (1995, 113–122).
3. Usually the apparent speed of a phase (phase velocity) is intended but sometimes the speed of the center of a packet of wave energy (group velocity). See group velocity, phase velocity.