In the preceding chapters we largely made use of ray-theoretical concepts and emphasized various algorithms for computing interval velocities from traveltime measurements. Now we will consider some more practical aspects of computing interval velocities from CDP reflection time measurements. We will outline some procedures that should be adhered to in order successfully to apply the generalized "Dix-type" formulas established above to measurements obtained in a real (geo-) physical world. Foremost, our traveltime inversion algorithms require that the real earth be closely approximated by the models considered above.
As in many other inversion algorithms, "idealized" surface measurements are required; such measurements are derived from "real" surface measurements only after applying certain corrections. Geophysicists who have previously computed interval velocities from either stacking- or migration velocities know that, typically, a host of often maddening problems is encountered when working with real seismic data. Some of these problems result from applying invalid corrections to the surface measurements.
The most critical parameter that must be obtained from CDP reflections is the NMO velocity VNMO. At best, it equals the stacking velocity Vs for infinitesimal small offset only. A number of factors influence and bias VNMO and Vs. These quantities are commonly computed in the digital computer by a procedure known as a stacking-velocity analysis (SVA).
A successful SVA requires adequately preprocessing CDP gathers and choosing some coherency measure for the computation of velocity spectra. Picking, validation, and smoothing schemes may follow after the spectra are computed. In addition to providing estimates of Vs', these
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“Over the years, ray theory has furnished the exploration geophysicist with most of the working tools for understanding and interpreting events observed on reflection seismic sections. Even today, notwithstanding the pace at which the more powerful acoustic wave theory is introducing its new tools, ray theory, in the hands of the authors, retains its preeminence for providing insights into fundamental problems in reflection seismology. Professor Krey's earlier contributions are part of ray theory's rich heritage. Alongside C. Hewitt Dix and Hans Durbaum, he elucidated relationships between interval velocity and observed reflection moveout.”