(di frak’ sh∂n or dif frak’ sh∂n) A redistribution in space of the intensity of waves resulting from the presence of an object. 1. Penetration of wave energy into areas forbidden by geometrical optics, e.g., the bending of wave energy around obstacles without obeying Snell’s law, as explained by Huygens’ principle. The phenomenon by which energy is transmitted laterally along a wave crest. When a portion of a wavetrain is interrupted by a barrier, diffraction allows waves to propagate into the region of the barrier’s geometric shadow. See Figure D-11. 2. An event observed on seismic data produced by diffracted energy; see Figure D-12. Such events result at the termination of reflectors (as at faults) and are characterized on seismic records and sections by a distinctive curved alignment. A simple diffraction lies along a diffraction curve (whose curvature depends on the velocity distribution above the diffracting point). Phantom diffractions involve energy that reaches the diffracting point by a longer route than the direct one (as with a diffracted reflection); they have more curvature than appropriate for their arrival time. Diffractions generated by a line source that is not at right angles to the line appear to have less curvature, becoming flatter as the line generating the diffraction becomes parallel to the line of observation. A reflection can be thought of as the interference result of diffractions from points lying on the reflector. When correctly migrated, a simple diffraction collapses at the location of the diffracting point. See Sheriff and Geldart (1995, 63-68 and 159-161).