A curve of maximum convexity, the relation between the arrival time and observer position for primary energy that has been diffracted from a point. See Figure D-13. [One should speak of a diffraction ‘‘surface’’ to emphasize the 3D aspect.] The curvature of reflected energy cannot exceed this curvature (except for reverse branches and certain situations such as diffracted reflections). Diffraction curves are specific for a particular velocity function, like the wavefront chart to which they are related and from which they can be constructed. Diffraction curves are used in identifying simple diffractions, locating the diffracting points (see Figure M-11), in determining velocity from the diffraction curvature, and as migration operators in Kirchhoff migration (q.v.). Errors in interpreting diffractions can result if the diffracting point lies to the side of the seismic line, if the diffraction event results from a line diffractor that is not normal to the seismic line, or if the diffraction is not simple. See Hagedoorn (1954).