Dip‐moveout by Fourier transform

Abstract

The conventional normal‐moveout (NMO) and common‐midpoint (CMP) stacking process enhances reflections having a particular moveout velocity, while attenuating events (such as multiple reflections) having different moveout velocities. Unfortunately, this process also acts as a dip filter applied to the CMP stack. In other words, NMO and stacking enhances reflections having a particular slope in the CMP stack, while attenuating reflections having different slopes. NMO and stacking, like any dip filter, degrades lateral resolution. Fortunately, this dip‐filtering action can be suppressed by applying, in addition to NMO, a prestack process known variously as DEVILISH, prestack partial migration, and dip‐moveout. As the latter term implies, this process is a dip‐dependent moveout correction that enables reflections from both horizontal and dipping reflectors to be stacked with the same NMO velocity. Stated another way, NMO velocities estimated from dip‐moveout‐corrected seismograms are independent of the dips of subsurface reflectors. Dip‐moveout by Fourier transform is a method for performing dip‐moveout (DMO) correction in the frequency‐wavenumber domain. The implementation of this method, which resembles the implementation of a discrete Fourier transform, is quite different from and compares favorably with previously published finite‐difference DMO algorithms. DMO by Fourier transform, unlike DMO by finite‐differences, is accurate for all offsets and all dips, provided that velocity is constant. Because velocity is never constant, some accuracy is inevitably lost; but the application of DMO by Fourier transform to recorded seismograms demonstrates the ability of this process to enhance (1) the dip bandwidth of CMP stacks and (2) the accuracy of velocity estimates.