Abstract

The advent of velocity-sensitive depth imaging techniques such as pre-stack depth migration has created a demand for more accuracy in velocity model building. Traditional methods that use seismic processing velocities are limited by the many assumptions inherent in the Dix formula and the distortion caused by complex ray paths. Techniques based on model ray-tracing allow more accurate estimates of interval velocity to be made, which, when constrained by an interpreter's geological knowledge of an area, have the ability to produce superior velocity models for depth conversion or prestack depth migration.

The role of the interpreter in any depth imaging project is often understated because of a lack of understanding of velocity modelling and depth imaging techniques. The purpose of this paper is to introduce some of these concepts to interpreters in an attempt to promote more active involvement in velocity modelling projects and for them to not simply rely on the processing experts to build geologically consistent velocity models.

Velocity estimation techniques such as stacking velocity inversion and coherency inversion are equally applicable to depth conversion projects as they are to pre-stack depth migration. Coupled with map migration techniques, these methods allow the interpreter to reduce the uncertainty of conventional depth conversion and ultimately reduce the geological risk of prospects.

Pre-stack depth migration and tomographic model refinements are an essential and powerful tool to produce the optimum seismic data quality. These methods are described to give the explorationist a practical understanding of the workflows involved in depth imaging.