A cursory look at a continuous velocity log (CVL) provides sufficient evidence that hydrocarbons and other natural resources are certainly not deposited in simple geologic environments consisting of only a few homogeneous sedimentary layers bounded by first-order interfaces. Rather, resources typically are found within highly stratified layers which can be folded or faulted and can wedge out in various directions or be in unconformable contact with other subsurface masses. In addition, the various properties affecting seismic wave propagation within the earth may often change laterally within layers.
The true geology in all details can never be completely recovered from seismic reflections. Rather, one ought to aim for the recovery of a suitable subsurface model that provides all desired information related to expected resources and is reasonably consistent with other available surface or borehole measurements. In the past, models were greatly simplified. In the future they will become more complex, and one can expect them to provide a more equitable balance between theory and observation.
Deriving an accurate subsurface model from surface measurements, i.e., solving the inverse problem, is certainly no simple task. The process is particularly difficult where the geology (and, hence, the seismic trace) is complicated. Therefore, it is not surprising that many complementary methods have evolved which all aim for inverse solutions: estimates of images of the subsurface.
Various seismic waves are generated and recorded in response to a near-surface explosion. Some arise in close sequences of layers (short peg-leg multiples) or are generated at faults, discontinuities, strong
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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.”