Conventional two-pass 3-D time migration is exactly equivalent to full 3-D time migration in a homogeneous medium. For vertically inhomogeneous media representing typical earth velocities, however, conventional two-pass 3-D migration fails to correctly image dips beyond about 45 degrees. This failure is the result of an inherent velocity error incurred during the first pass of a two-pass 3-D migration.For a vertically inhomogeneous medium, the theory of residual migration can be combined with the results for homogeneous media to derive a series of successive two-pass migration stages which are equivalent to a full 3-D migration. Each stage of this generalized two-pass 3-D migration is implemented using an appropriate constant migration velocity. In practice, the required number of two-pass stages can be reduced to a computationally manageable few; and the I/O can be reduced by one-third to one-half of that required using a straightforward application of repeated two-pass migrations.This procedure allows existing 2-D migration programs to be upgraded to steep-dip 3-D migration programs by use of a simple I/O structure. Any of the basic 2-D migration algorithms can be used, but we have employed a 50-degree finite-difference algorithm. In addition, generalized two-pass 3-D migration overcomes the dip limitations of the underlying 2-D finite-difference migration algorithm for the same reasons that cascaded 2-D migration extends the dip range of 2-D migration algorithms. Synthetic data examples clearly show the success of this method in imaging steep dips in vertically inhomogeneous media.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.