96 Geophysics in Geothermal Exploration Figure 2.31 Migrated section superimposed on the FWI velocity model, in a land example (Baeten et al., 2013). Full Waveform Inversion (FWI; Chauris, 2019) is a processing technique to derive quantitative images of the subsurface from seismic measurements. By quantitative, we mean for example P-wave velocity models expressed in m/s, and not only a structural image of the Earth as a classical stack section would provide. The principle is simple: the optimal model is the one for which the computed shot gathers are reproducing the observed shot data. More details on the formulation are provided in the literature (Louboutin et al., 2017, 2018). Beyond the apparent simplicity, the practical applicability of FWI is a difficult task. This is a non-linear process; the user should provide an initial model; the quality of the final inverted model depends on the reliability of the low frequency content of the observed data. A proper strategy should be established to iteratively determine the model (typically by successively introducing higher frequencies). One also needs to use the adequate wave equation to generate synthetic wave fields and associated shot gathers to mimic the physics of wave propagation. Finally, due to the limited data frequency band and limited data acquisition from the surface only, FWI does not necessarily lead to a unique solution. For example, if the user is interested in determining P-velocity and density models, there is an intrinsic trade-off between the two quantities, especially for short offset data. This is not specific to FWI: other imaging techniques suffer from the same effect, but this is visible in the FWI context as FWI is expected to provide quantitative results. Many FWI results have been published on real data in seismology, as well as at the exploration scales, at least in the marine case. The use of the technique on land with onshore data, however, has only been proven for a limited number of applications due to the presence of strongly energetic surface waves. The example shown in Figure 2.31, in a land acquisition context (Inner Mongolia, China), is challenging due to the presence of highly energetic surface waves (Baeten et al., 2013; Brossier et al., 2009). Here, surface waves are filtered out in
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