We consider two approaches: one based on rays, the other based on plane-waves. In the ray approach, we split our analysis between the use of an correct and an incorrect migration velocity. When the migration velocity is correct, the source and receiver rays focus at the same correct image point where they are coplanar. This property of the rays makes it simple to transform the 3-D geometry into 2-D for which the theory is available Biondi and Symes (2003). We then analyze the problem when an incorrect migration velocity is used. Incorrect migration velocity yields non focused ray resulting in an apparent image point, and generally not coplanar rays. We introduce an apparent propagation plane in which the 2-D theory can be applied. We will then discuss a plane-wave approach of the problem. Finally a synthetic example is used to verify our theoretical results.

10/14/2003