Applications of Pre-Stack Depth Migration

Complex shallow faulting is a common problem which can lead to poor results when using the CDP assumption. Wave-field distortion makes it impossible to select a stacking velocity that does not deteriorate stack response. The only way to achieve a high-quality stack is to predict and remove the wave-field distortion before stack, that is, by correctly migrating the pre-stack gather.

Predicting the distortion is difficult, as it requires the estimation of the actual interval velocity field. Many methods have been proposed for estimating velocities, three of which are iterative pre-stack depth migration, ray-trace modelling, and interactive interval velocity analysis. Each of these has advantages and disadvantages, but iterative pre-stack depth migration is generally the most useful. Reflection tomography can also be useful.

Once accurate velocities have been obtained, many algorithms are available to do the actual migration. Two methods are compared; a shot-record finite-difference algorithm and a common-offset, Kirchhoff algorithm. The latter algorithm has practical advantages that lends itself more to iterative pre-stack depth-migration velocity analysis and generally makes it more attractive.

Pre-stack depth migration, with both algorithms, is demonstrated on a real data set from offshore north-western Australia. The examples show that iterative pre-stack depth migration can successfully improve seismic images in difficult areas.