205 6. The use of passive seismic methods for Geothermal exploration and monitoring to the bottom of the steam cap and the 3 km/s iso-velocity curve as being the base of the stratification composing the Hengill volcanic system. Another illustration of 3D Vs tomography application for geothermal characterization of volcanic contexts can be found in Martins et al. (2020), where the authors identify a low-velocity cavity which is interpreted as an area of up-flowing fluids where temperature and permeability are enhanced. Cheng et al. (2021) propose an ANSI tomographic application in a crustal fault context where hydrothermal fluids are known to reach the surface. They use relative spatial velocity variations observed in the area to infer the presence of faults (Figure 6.14) where hydrothermal fluids are expected to circulate. Their seismic analysis also shows good correspondence with resistivity models obtained with controlled sources audio-magnetotellurics (CSAMT) measurements, where low velocity anomalies correspond to low resistivity anomalies. In a sedimentary context, Planès et al. (2020) used a 3D Vs tomography to improve their understanding of the geological context at the Greater Geneva basin. They manage to identify geological and topographical relationship and also interpret isovelocity curves depth variation as a mean to identify the thickness of the sedimentary cover. Figure 6.13 Modified after Toledo et al. (2022). Rayleigh wave group velocity maps at two different frequencies.
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