An inversion operation involving rearrangement of seismic information elements so that reflections and diffractions are plotted at their true locations. The need for this arises since variable velocities and dipping horizons cause elements to be recorded at surface positions different from the subsurface positions. Time migration assumes that velocity varies only in the vertical direction whereas depth migration allows for horizontal variation of velocity also; both time and depth migration results can be displayed in either time or depth. Originally done by hand on interpreted seismic data, migration is now a computer operation on uninterpreted data using some form of, or approximation to, the wave equation. Also called imaging, the transformation of seismic data recorded as a function of arrival time into a scaled version of the true geometry of subsurface geologic features that produced the recorded seismic energy. Imaging involves focusing and positioning and depends on a specific earth model. Focusing involves collapse of diffractors, maximizing amplitude, reproducing wavelet character, etc; positioning involves locating events correctly, sharpening event terminations relative to faults, salt flanks, unconformities, etc. A type of inversion (q.v.). Hand migration was based on measurements of the arrival time and direction of the apparent dip (which defined the direction of the raypath). Because a common-midpoint stack does not correctly stack dipping events, poststack migration is cheaper than, but inferior to, prestack migration. DMO (q.v.) operation prior to stacking sometimes produces results equivalent to prestack migration (see Figure D-20e). Migration is often 2D where only the apparent dip component in the line direction is known. Conceptually, 3D migration is simply an extension of 2D methods but often 3D migration is done by first migrating in one direction and then migrating this intermediate result in the cross direction (double 2D migration). Migration can be accomplished by integration along diffraction surfaces/curves (Kirchhoff migration), by numerical finite-difference or phase-shift, downward-continuation of the wavefield, and by equivalent operations in frequency-wavenumber or other domains (frequency-domain migration). See Figures M-11 and 12 and also map migration (from unmigrated time maps), Kirchhoff (diffraction collapse) migration, downward continuation, Stolt (f-k) migration, τ-p migration, Gadzag (phase-shift) migration, imaging principle, pseudospectral migration, DMO poststack versus prestack migration, time versus depth migration, Sheriff and Geldart (1995, 326–33).