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40_05 - 5.0 Some F rontiers I n this final chapter of the...

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5.0 Some Frontiers In this final chapter of the book are gathered together imaging concepts that have been published, but have not yet come into routine industrial use. The first part of this chapter develops the mathematical concept of linear moveout and how it relates to velocity analysis. Data can be focused so that the interval velocity can be read directly. The latter part of the chapter is about multiple reflections. Here too linear moveout helps to define the prob- lem. You will see basic mathematical tools that have the power to deal with multiple reflections and lateral velocity variations. This chapter has many data processing proposals. They are not descriptions of production processes! Interpreting Seismic Data Initially I regarded this chapter as one for specialists interested mainly in devising new processes. Then I realized that in dealing with things that don't seem to work as they are expected to, we are really, for the first time, strug- gling t o contend with reality, not with what theory predicts. This can hold much interest for skilled interpreters. The heart of petroleum prospecting is the interpretation of reflection seismic data. What is seismic interpretation? To be a "routine interpreter" you must know everything on which theory and practice generally agree. To be a good interpreter you must know the "noise level" of alternate phenomena with similar effects. Anomalies in seismic data can arise from the complexity of the earth itself, from seismic wave propagation in the earth (deep, near surface, or out of plane), or from imperfections in recording and imaging techniques. T o make realistic judgements in so wide a realm, you must be a seismologist who is part geologist, part engineer, and part mathematician. This chapter will not teach you to be a good interpreter, but it will offer you a chance to observe some critical thinking about the relation- ship of seismic theory to seismic data.

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FRONTIERS 5.0 Some Frontiers Leaning Echo delay is much like depth. We usually measure angles by their departure from the vertical ray, while in reality zero-offset data is rarely recorded. The best seismic data is usually far from vertical. In this chapter a pattern of thinking is developed that is oriented about a selected nonvertical ray. Rotation of coordinates does not solve the problem since after rotation, the plane on which measurements would be made would no longer be simply z = 0. Rotation would also make a mess of the simple seismic velocity func- tion v (z ) by making it a strongly two-dimensional function v '(x I, z I). The view of offset presented in Chapter 3 may have seemed rather complete, but in fact it was not very general because square roots were expanded about the vertical ray. The Stolt stretch development in Section 4.5 illustrated the advantage of leaving the hyperbola top and getting out on the flanks.
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