Abstract
Joel Starr, PGS Marine Geophysical, evaluates the increasing range of NMO options avail able for today’s seismic data processors. NMO, or normal moveout, is the time shift needed to correct for the effect of offset and velocity in a CMP gather. NMO equations approximate the time shift which would be computed by tracing a ray through a horizontally layered Earth. A few years ago the 2nd order NMO equation, or hyperbolic NMO, was considered adequate in most cases. Today there are many options in the industry to apply higher order NMO which reduces the error in the approximation to the ray traced solution for longer offsets. There are two characteristics which are important when considering the application of a given NMO curve: 1) accuracy; how well the NMO curve approximates the ray traced solution, and 2) stability; how well the curve can tolerate small errors in the estimated velocity field (one would not want small errors in the estimated velocity field to cause large errors in the move-out time calculated). If the data being processed is isotropic in nature, then the NMO equation will be dependent on velocity, v and offset x.
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