141 5. Full waveform inversion classically used in FWI. In practice, a window is applied around the transmitted waves (direct and diving/refracted) so that only these waves are considered in the FWI, while removing the surface waves and reflected/diffracted waves. This means that large offsets should be recorded. The other reason to consider transmitted waves is that the objective function is more convex than for the reflected waves. This avoids the problem of falling into a local minimum. Figure 5.15 Depth sections in 3D velocity models on the marine Valhall OBC data set, obtained by ray-based reflection tomography (left), followed by FWI (right), for depths z = 150 m (top) and 1050 m (bottom). In the FWI images, channels (top) and the presence of gas (bottom) are clearly visible (Sirgue et al., 2009). Figure 5.16 Migrated section superimposed on the FWI velocity model, in a land example (Baeten et al., 2008).
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