(n.) Geophysics. The rate at which a sound wave travels through a medium. Unlike the physicist's definition of velocity as a vector, its usage ingeophysics is as a property of a medium: distance divided by traveltime. Velocity can be determined from laboratory measurements, acoustic logs, vertical seismic profiles or from velocity analysis of seismic data. It can vary vertically, laterally and azimuthally in anisotropic media such as rocks, and tends to increase with depth in the Earth because compactionreduces porosity. Velocity also varies as a function of how it is derived from the data. For example, the stacking velocity derived from normal moveout measurements of common depth point gathers differs from the average velocity measured vertically from a check-shot or vertical seismic profile (VSP). Velocity would be the same only in a constant-velocity (homogeneous) medium. See: Acoustic, Acoustic Impedance, Angular Dispersion, Anisotropy, Apparent Velocity, Attribute, Average Velocity, Base Of Weathering, Birefringence, Channel Wave, Check-Shot Survey, Depth Conversion, Depth Migration, Discontinuity, Dispersion, Extensive Dilatancy Anisotropy, Gas Chimney, Horizon, Hydrocarbon Indicator, Interval Velocity, Processing, Pull-Up, Push-Down, Ray Tracing, Reflection Coefficient, Reflection Tomography, Refraction, Refractor, Root-Mean-Square Velocity, Seismic Trace, Sonic Log, Stacking Velocity, Static Correction, Synthetic Seismogram, Time Migration, Tomography, Velocity, Velocity Analysis, Velocity Anomaly, Velocity Correction, Velocity Layering, Velocity Survey, Vertical Seismic Profile, Wave, Wave Equation, Wavelength, Weathering Correction