Dictionary:Velocity

(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.

FIG. V-3. Seismic velocity terminology

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.

FIG. V-4. Velocity terminology involves adjectives preceding the word velocity to give a specific meaning. Is it not easy to see how confusion can arise when ‘‘velocity’’ is used without a qualifying adjective?

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).

FIG. E-5. Elastic constants for isotropic media expressed in terms of each other and P- and S-wave velocities (α=V_P and β=V_S,) and density ρ.

FIG. T-13. Transverse isotropy. (a) Wavefront with vertical axis of symmetry (TIV); (b) with horizontal symmetry axis (TIH) leading to azimuthal anisotropy; (c) phase (wavefront) angle θ and group (ray) angle for transverse isotropy; (d) elliptical wavefront where ε=–δ; in this case V_NMO>V_vertical; (e) anisotropic wavefront where ε=δ; in this case V_NMO<V_vertical; (f) wavefront for tilted symmetry axis; (g) wave equations for transverse isotropy.

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.

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