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Layer stripping methods establish first the velocity and then the structure of each layer sequentially from top down. Chapter 4 described the procedures for defining layer structure. In this chapter we describe the techniques for defining layer velocity from moveout in the time gather. Chapter 7 describes the techniques for deriving layer velocity from the depth gather.

Moveout forms the basis for deriving velocities from the time gather. Two families of techniques are used: Dix-based and model-based. Dix-based methods assume an unstructured subsurface of flat layers. Because of this Dix-based interval velocities can be calculated globally rather than in a layer stripping manner. By global I mean that the entire stack of layers may be analyzed at once in a vertical column. In contrast, the model-based techniques presented in this section derive interval velocities in the context of a structured subsurface and work in a layer-stripping mode. In Chapter 7 global model-based techniques are presented which make use of the initial model as a starting point for creating a refined model.

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