Impact of modelling shallow channels on 3D prestack depth migration, Elgin-Franklin fields, UKCS

A case study from the Elgin-Franklin fields is used to illustrate the influence of seabed low velocity anomalies on imaging deep targets below 5000 m depth. In the uppermost 400 m of sediment, glacial channels are present and generate locally low velocity anomalies resulting in pull-up/push-down effects observed on seismic reflection data. The original 3D prestack depth migration did not take into account these shallow velocity anomalies, but used a smooth interval velocity in the uppermost layer of sediment. In the southern area, the results were locally better than with prestack time migration, but in the northern area they were worse, or at best comparable. A new velocity model with a channel layer was built using well velocities for the initial model. Tomographic and migration iterations were used to refine all velocities, including the channel velocities, and for introducing anisotropy. Imaging of the pre-Cretaceous section was clearly an improvement over prestack time migration. For correct imaging, local shallow velocity variations must be identified and taken into account in the velocity model. Even though the overburden appears to be rather flat, prestack depth migration imaging performs better than prestack time migration overall.