Multiples have traditionally been considered coherent noise, and so much effort has been put into developing dereverberation algorithms. However, our point of view is that multiple reflection energy can often provide better illumination of the subsurface than primary reflection imaging. As an example, Figure 1 depicts the subsurface illumination provided by primary reflections recorded with VSP receivers in a well. Even though the frequency content in VSP data can be at least twice that of CDP data, a narrow cone of coverage means that the expense of a VSP survey is not always justified. However, if the reflections from free-surface multiples are used, then the coverage is almost as wide as that in a 3D CDP survey.

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