29 1. Wave propagation Figure 1.6 3D shot points (cross-spread with cross-line offset ranging from 0 to 40 m). In marine surveys, raw data can be presented in shot point gathers as for land acquisition. Another method of viewing the data is the constant offset section. For each shot, a trace at a given offset is selected. The set of constant offset traces is called a constant offset section. Figure 1.7 is an example of a constant offset section. The first event, named Primary, is the sea bottom reflected wave. The water bottom generates a very characteristic multiple, called a water-bottom multiple (labeled ‘Multiple’ on the Figure). A multiple reflection is an arrival that has been reflected several times (generally an odd number of times) during its travel path in the subsurface.
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