David Nichols

Depth imaging examples and methodology in the Gulf of Mexico

Advances in seismic imaging over the past several years have revolutionized interpretation of geologic structures in complex areas. Nowhere has the impact of this technology been greater than the Gulf of Mexico, where the ability to interpret structures in subsalt areas has led to very large oil discoveries. Interestingly, although many techniques used in imaging these complex areas are new, the basis for many may be found in the classic methods of time imaging, many of which are still used in large-scale production depth imaging. Inherent in these techniques has been the assumption of a locally flat-layered earth consistent with hyperbolic moveout, which although incorrect for complex areas such as the subsalt imaging in the Gulf of Mexico, carries with it a robustness and determinism. Gradually, depth-imaging techniques are evolving to overcome the limitations of a local flat-layer earth assumption but at the cost of this determinism. For a true depth model, velocities, densities, and other parameters must be determined in a spatially varying way, so there is an explosion in the number of parameters to be determined. A consistent earth model from the depth-imaging perspective relies on measurement redundancy—i.e., reflections from the subsurface appear on traces from numerous source-receiver pairs in the acquisition geometry. The imaging process is then done in a way that produces not one but many images, each created using energy that has traveled through different parts of the earth. Consistency requires that these images agree. Hence, difficulties have shifted from the imaging algorithms themselves, as with the local flat-layer assumption in time imaging, to those associated with the data, their redundancy, and the uncertainty in determining a consistent model. In this article we examine a number of examples of depth imaging in the Gulf of Mexico. In the process we chronicle depth-imaging methods as they …