# Dictionary:Dispersion

**1**. Variation of velocity with frequency. Dispersion distorts the shape of a wavetrain; peaks and troughs advance toward (or recede from) the beginning of the wave as it travels. Leads to the concept of group velocity *U* distinct from phase velocity *V*. Where =wavelength, *f*=frequency, and *V*=phase velocity,

The dispersion of seismic body waves is very small under most circumstances, but surface waves may show appreciable dispersion in the presence of near-surface velocity layering. See Figure D-16. The dispersion of electromagnetic body waves is large in most earth materials. For ground roll and channel waves (see Figure C-2c, d), phase velocity decreases with frequency, sometimes called **normal dispersion**, and where velocity increases with frequency, **anomalous dispersion**.

**2**. A statistical term for the amount of deviation of a value from the norm. See *statistical measures*.

**3**. **Angular dispersion** is variation of velocity with direction, as in an anisotropic medium. Leads to the concept of ray velocity distinct from phase velocity; see Figure A-14a.