The case for applying wave equation depth migration in the North Sea

Richard Leggott, John Cowley and R. Gareth Williams of Veritas DGC explain how the company has found that wave equation depth migration - contrary to some expectations - can be applied to North Sea data with successful imaging results. Pre-stack depth migration (PSDM) is now an established tool for imaging seismic data in areas with significant lateral velocity variations. PSDM has proved to be of benefit to interpreters in various parts of the North Sea including areas with salt features as well as areas with, for example, gas clouds and shallow channels. To date, pre-stack depth migration has been performed using a Kirchhoff imaging algorithm. Although this provides a good improvement over time migration algorithms, it is still an approximation to the full solution; in its normal implementation it can only image a single arrival from a shot to any given sub-surface location. Newer methods called Wave Equation migration that overcome some of the assumptions associated with Kirchhoff methods are now becoming available. These new methods are computationally intensive, but they can provide better results in some circumstances. Wave equation migration is being used on large volumes of seismic data in the deep water Gulf of Mexico where rugose, tabular salt features cause complex wave fronts to reach the exploration targets. These can only be imaged partially with normal Kirchhoff methods, whereas the Wave Equation algorithm allows much better imaging and amplitude control. So far, these methods have not been used in the North Sea where the geology is often thought of as being structurally simpler than the sub-salt plays of the Gulf of Mexico. Nevertheless, we find that Wave Equation Migration (WEM) does give better results than those obtained with Kirchhoff methods in the North Sea, especially in areas of the central and southern North Sea characterized by Mesozoic and Tertiary salt tectonics.