Page 519 - Acquisition and Processing of Marine Seismic Data

510 11. SEISMIC MIGRATION

FIG. 11.18 The effect of 10% lower and 10% higher velocity on the output of poststack Kirchhoff time migration. Over- and
undermigration occur when the migration velocity is faster or slower than the actual velocity of the medium, respectively.

FIG. 11.19 Collapse of the diffraction hyperbola by downward continuation of the zero-offset wave field. (A) A diffraction
hyperbola is recorded with the receivers at the surface. The thick blue line shows the total spread length. (B) If the receivers are
moved downwards and the spread length is shortened accordingly, the hyperbola gets narrower. (C) When the receiver is at
the apex, the hyperbola is completely collapsed. If the downward continuation proceeds, then a mirror image of the hyperbola
occurs, as in (D) and (E).