In an effort to better understand imaging challenges in the deep water Gulf of Mexico, in 2011 we constructed a large 3D model loosely based on the complex salt geology of the Garden Banks protraction area of the deep water GoM. We then simulated a regional WAZ (wide azimuth towed streamer) seismic survey over this model, producing data that on casual inspection could be mistaken for real. We then performed velocity-model building and imaging on this dataset as if it were real. In particular, the team performing the analysis never saw the correct model.
For much of the model, especially where the salt was relatively simple, we found that the resulting velocity model was quite accurate, even if lacking in fine detail. Reverse-time migration of the seismic data through these parts of the velocity model produced an imperfect but usable image. In other places the salt structures were misinterpreted, causing large-scale errors in the migration velocity model, which resulted in an unusable, shattered image below the salt.
We conclude that traditional velocity-model-building techniques can miss features that occur at too large a scale. Reliably imaging under complex salt in the Gulf of Mexico may require new velocity-model-building methodologies to be developed that are specifically designed to deal with the problem of large velocity heterogeneities.
A previous study (Etgen et al., 2014) showed that fine-scale features associated with abrupt velocity heterogeneities (in particular, top of salt) can damage the seismic image. In this study our goal was to examine the other end of the velocity heterogeneity size spectrum, by simulating acquisition and processing of a large 3D WAZ dataset in an area of complex salt containing large structural features.
We found that existing models available to us were unsuitable for this purpose. We concluded that:
1) The model needed to be 3D, because complex salt geology is intrinsically 3D. A 2D model cannot adequately represent the challenge.
2) The model needed to be very large, big enough to contain modern wide-azimuth long-offset acquisition geometries with plenty of room to spare, to avoid the results being contaminated by edge effects and to allow us to test multiple novel acquisition strategies using the same synthetic dataset. It also needed to be deep enough to contain diving waves out to wide offsets. The final model design was 18 km deep by 85.5 km by 106 km wide.