Using dual-azimuth data to image below salt domes

Abstract

In complex geological areas, subsurface illumination is largely determined by the acquisition geometry. More recently, multi-azimuth and wide-azimuth surveys are increasingly being used to maximize subsurface coverage, but these can present new challenges; for example, how to combine the images from different azimuths into a single migrated image for use by the interpreter. Traditionally, the common practice is to ignore potential azimuthal velocity variations and build a single vertical velocity model, in some cases incorporating VTI anisotropy. Data from different azimuths are pre-stack migrated separately, but with a single velocity model usually derived from one azimuth, and then combined into a single image after application of residual move-out corrections. We present a case history of building a single velocity model for pre-stack depth migration of dual-azimuth data in the southern North Sea. The dual-azimuth data gave us enhanced illumination of the target zone, but somewhat to our surprise, it also showed strong azimuthal velocity variations in the overburden. We devised a methodology and built a single velocity model that incorporated these azimuthal velocity variations so that all events are migrated to their correct position. When the data sets are combined, primary events are reinforced while noise cancels out to give one optimal data set.