A modeling and migration study of fault shadows: A synthetic example
Sitamai W. Ajiduah, Gary F. Margrave
This report presents a study that uses 2D finite difference modeling and a one-way wave equation depth migration method to investigate weak illuminations in footwall reflectors. I examined the quality of footwall imaging from poststack and prestack migrations with exact model and created approximate models using an iterative method from an initial unfaulted (flat) model. Appearances of fault shadow effect were seen in the geologic model as time pull and sags, in poststack time migrations as distorted reflections which appeared as sub-seismic fault. In the depth migrated images, they occurred as anticlines and synclines which may be false or real. We observed a confined section of the footwall was poorly illuminated. Results from the prestack depth migrations was quite significant with improved imaging. The velocity model time anomalies are the results of the truncation of the overlying stratigraphy by the fault throw causing an abrupt velocity contrast across the fault. Seismic rays undergo ray bending and experience traveltime distortions as it propagates across fault truncations. In addition, the presence of the dipping faults in the model will create some non-hyperbolic reflections which will frustrate poststack migration efforts. The poor images in poststack time migration implies that events are migrated to their incorrect positions in vertical time with a Dix-based RMS velocity transformation that is suboptimal. The inadequacies of poststack depth migrations in perfectly imaging footwall reflections can also be attributed to the dip-dependence effect of normal moveout. NMO and stacking of events along hyperbolas without prior dip moveout correction will cause apparent disruptions and smeared reflections. A better imaging solution is obtained from the prestack depth migrations which showed improved and continuous footwall reflections without seismic artifacts. In conclusion, fault shadow is a velocity and wave propagation problem and requires good understanding of the faulted environment and velocities.