The zero-velocity layer: migration from irregular surfaces

Abstract

Seismic data acquired in areas with irregular topography are usually corrected to a flat datum before migration. A time-honored technique for handling elevation changes is to time shift the data before application of migration. This simple time shift, or elevation-static correction, cannot properly represent wide-angle or dipping reflections as they would have been recorded at the datum. As a result, when elevation varies significantly, accuracy in event positioning may be compromised for migration and other wave-equation processes, such as DMO. Although computationally intensive, wave-equation datuming can be used to accurately extrapolate the data recorded over an irregular surface to a flat datum. Here, we propose an efficient technique for doing migration from irregular surfaces using almost any migration algorithm. As in elevation-static corrections, surface-recorded data are time-shifted to a horizontal datum; for our process, however, we choose that datum elevation to be at or above the highest elevation in the survey. The choice of this datum elevation does not compromise final results because the datum elevation can always be adjusted to any other level after migration. In the migration step, the velocity is set to zero in the layer between the surface and the datum; below the original surface, the interval velocity represents the best estimate of the subsurface geology. By adding a zero-velocity layer, the migration algorithm is applied to the data from the flat datum and no lateral propagation is allowed until nonzero velocity is encountered at the recording surface. Synthetic field data examples demonstrate that use of the 'zero-velocity layer' significantly improves imaging accuracy relative to conventional migration from a flat datum. Moreover, the geologically derived migration-velocity field need not be adjusted to compensate for shortcomings in the datum-static procedure. Furthermore, the scheme can employ the same efficient finite-difference migration algorithms used in conventional processing. The technique can be extended to prestack processes such as DMO, shot- and receiver-gather downward extrapolation, and migration and thus suggests a unified approach to processing data from irregular surfaces.