AZIMUTH MOVEOUT FOR 3-D PRESTACK IMAGING

We introduce a new partial prestack-migration operator called ''azimut h moveout'' (AMO) that rotates the azimuth and modifies the offset of 3-D prestack data. Followed by partial stacking, AMO can reduce the co mputational cost of 3-D prestack imaging. We have successfully applied AMO to the partial stacking of a 3-D marine data set over a range of offsets and azimuths. When AMO is included in the partial-stacking pro cedure, high-frequency steeply dipping energy is better preserved than when conventional partial-stacking methodologies are used. Because th e test data set requires 3-D prestack depth migration to handle strong lateral variations in velocity, the results of our tests support the applicability of BMO to prestack depth-imaging problems. AMO is a part ial prestack-migration operator defined by chaining a 3-D prestack ima ging operator with a 3-D prestack modeling operator. The analytical ex pression for the AMO impulse response is derived by chaining constant- velocity DMO with its inverse. Equivalently, it can be derived by chai ning constant-velocity prestack migration and modeling. Because 3-D pr estack data are typically irregularly sampled in the surface coordinat es, AMO is naturally applied as an integral operator in the time-space domain. The AMO impulse response is a skewed saddle surface in the ti me-midpoint space. Its shape depends on the amount of azimuth rotation and offset continuation to be applied to the: data. The shape of the AMO saddle is velocity independent, whereas its spatial aperture is de pendent on the minimum velocity. When the azimuth rotation is small (l ess than or equal to 20 degrees), the AMO impulse response is compact, and its application as an integral operator is inexpensive. Implement ing AMO as an integral operator is not straightforward because the AMO saddle may have a strong curvature when it is expressed in the midpoi nt coordinates. An appropriate transformation of the midpoint axes to regularize the AMO saddle leads to an effective implementation.