Offshore imaging with complex overburden: Understanding gather complexity and resulting attribute accuracy through synthetics

Abstract

The objectives of this study are to determine the effect of complex overburden in offshore continental settings on the seismic wavefield and on the resultant seismic images and attributes. Seismic waves exhibit complex wave paths when propagating through heterogeneous geologic structures. A key step in understanding the consequences for imaging and, more importantly, amplitude versus angle behavior is to model this complexity in a known earth-velocity model. The earth model used, represented by the seismic velocities, should exhibit similar behavior to the real data but still remain as simple as possible. This is also the case in velocity-model building. However, the inability to include subtle geologic features leads to inaccuracy and uncertainty in the velocity model. Understanding how this inaccuracy manifests itself is vital in any predictive exercise using seismic attributes. To improve this understanding, several synthetic data sets were simulated using full-wavefield synthetics. It is evident from the results that subtle velocity features are required in the imaging velocity field to ensure accurate angle gather behavior whether output is from a ray-based Kirchhoff migration or full-wave equation algorithm. If this requirement is met, seismic attributes from an amplitude-preserving migration can be used as a facies prediction in conjunction with the correct imaging algorithm.