191 6. The use of passive seismic methods for Geothermal exploration and monitoring Local dispersion curves sampled from each point of the maps at every available frequency are inverted independently into 1D shear wave velocity models constrained with depth. The final pseudo-3D shear wave velocity model is built by stitching 1D models side by side. Note that recent advances in seismic ambient noise tomography lead to a 1 step full 3D imaging procedure (Zhang et al., 2018) where 3D Vs model is resolved using a probabilistic inverse approach. Figure 6.3 Illustration of a raw FTAN diagram, measured group speed curve as the solid line, from Bensen et al. (2007). Note also that recent advances have started to pave the way for ambient noise tomography of attenuation properties, which can bring valuable insights for geothermal characterization and surveillance (Pérez and Cuellar, 2018), as will be discussed in the following paragraphs. Attenuation analyses through ANSI methods is still in relatively early developments, hence in this paper we do not provide an overview of the technical description of the process or different possible approaches, but we refer the reader to the work of Boschi et al. (2019), Magrini and Boschi (2021) for – non exhaustive – examples of how the seismic attenuation properties of the subsurface can be derived from the ambient seismic signal using ANSI processes. 6.1.4 Monitoring The cross-correlation operation described in the previous section, which leads to an estimate of the EGF and the emergence of coherent seismic waves can be done sequentially, using any temporal resolution (minutes, hours, days, month, year), depending
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