Subsalt Imaging Via Target-Oriented 3-D Prestack Depth Migration
Authors: Jacewitz C.A. , Ratcliff D.W. , Gray S.H. 
Published: October 2008

INTRODUCTION
Subsalt imaging and subsalt trap identification have been recognized for years as important but difficult exploration objectives. By using subsalt imaging technology the explorationist hopes to determine the depth of the subsalt structures and the presence or absence of structural closure associated with the subsalt prospect, thereby obtaining a realistic assessment of the risks involved with drilling a subsalt well. Target-oriented 3-D prestack depth migration is the most powerful tool currently available to assist the explorationist in the quest for hydrocarbons below salt. In this paper we present a geophysical case study describing the steps taken to produce a useful subsalt image.

POSTSTACK DEPTH MIGRATION, PRESTACK PROCESSING, AND 3-D PRESTACK IMAGING
The first step of the 3-D prestack depth migration sequence involved several iterations of 3-D poststack depth migration. 3-D poststack migration allowed us to image sediments and faulting above salt, top of salt, and base of salt (assuming the base of salt has survived the stacking process). Iterative 3-D poststack depth migration was also essential in deriving the 3-D velocity field required in the 3-D prestack migration, but it was considerably less expensive than 3-D prestack depth migration. Imaging the subsalt target, which was considerably smaller in base area than the original survey, was accomplished by 3-D prestack depth migration.
In brief, the process of iterative 3-D poststack depth migration is as follows: Before the first migration, we generated a 3-D sediment velocity field using a number of standard velocity analysis techniques. Poststack depth migration, using this velocity field, produced a correct image of the sediments above salt (including faults), and the top of salt, but it incorrectly imaged the base of salt and any subsalt reflectors. Interpreting the top of salt in 3-D from the first iteration yielded an updated velocity field, with salt velocity inserted everywhere below the top of salt. The second poststack migration, using this velocity field, now showed a correct image of the base of salt, but not below the base of salt. Interpreting the base of salt in 3-D from the second iteration yielded a new update for the velocity field, which should allow correct imaging of any subsalt reflectors which survived the stacking process. Poststack migration using this velocity field, by imaging very few subsalt reflectors, confirmed the need for 3-D prestack migration, with the final velocity field from poststack migration providing the initial field for prestack migration.
Some important processing of the unstacked data preceded the 3-D prestack depth migration, including editing of noisy data, deconvolution, and sorting to common offset 3-D data volumes. The sort to common offset volumes allowed us to migrate manageable data sets on a Cray-2 computer in a relatively short time. Separate migration of the common offset volumes also allowed us to see how each volume contributes to the subsalt image (optionally editing poor images from the final stack), and to perform a migration velocity analysis which validates or invalidates our choice of 3-D velocity field. The prestack imaging was done by Kirchhoff migration, with traveltimes obtained by raytracing.