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This chapter describes the methods by which residual moveout observations made in the depth gather, the product of prestack depth migration, may be used to refine a velocity model. It will be shown that when an earth model is correct, reflections in the depth gather will be flat. We will show how the non-flatness in the depth gather, termed ìdelaysî, can be used to infer how to refine the model so that flatter depth gathers are produced with the next migration. Two techniques are described: the one-dimensional method and the ray-based global tomographic method.

Two dimensional seismic data can be thought of as a three dimensional data entity. The third dimension is offset. Figure 7.1 shows a schematic of an unmigrated 2D line with offsets ranging from 1 to 40.

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