# Dictionary:Coherence

(kō hēr’ ∂ns) **1**. The property of two wavetrains having a well-defined phase relationship, i.e., being in-phase. **2**. Measures of the similarity among more than two functions. For example, seismic reflection events are coherent in a linear way with respect to dip, in a hyperbolic way with respect to normal moveout, and in a nonanalytic though systematic way with respect to geophone locations. The principal evidence for a separate seismic event is coherence among the members of a set of seismic traces over a short time interval (of the order of 1½ or so cycles of the dominant frequency) compared with less coherence elsewhere. Coherence is used qualitatively in record picking and quantitative measures of coherence are used in automatic picking schemes. Time slices and horizon slices often display a coherence measure (see Figure A-24d). See *semblance* and Sheriff and Geldart (1995, 145–146; 288–289). **3**. A measure of the similarity of two functions or portions of functions. If the functions have power spectra *P*_{ii} and *P*_{jj} and cross-power spectra *P*_{ij} (which may be complex), their coherence is *P*_{ij}/(*P*_{ii}*P*_{jj})^{1/2}. Also called **coefficient of coherence**. Coherence is a frequency-domain concept analogous to correlation in the time domain.