213 6. The use of passive seismic methods for Geothermal exploration and monitoring procedure inside the geothermal reservoir that led to the gas kick (Figure 6.19a). Figure 6.19b shows that for the pairs not crossing the reservoir this decoherence drop is not observed. The authors argue that the decoherence drop can also be the result of changes in the scattering properties of the subsurface (e.g. Larose et al., 2010; Obermann et al., 2013, 2014; Planès et al., 2014), which in the St Gallen case may be associated to geothermal induced processes such as pore pressure changes related to gas release, critical prestressing of a fault, or changes in attenuation properties also due to the presence of gas. Although the physical interpretation of the decoherence variation is not clear, this study highlights the fact that other seismic attributes possibly more sensitive to some physical phenomena can also be derived from ANSI based monitoring analysis and that a strong potential also exists in the development of such novel approaches. In particular, the focus on attenuation properties monitoring is a promising research lead since attenuation is particularly sensitive to the temperature field and the nature and distribution of fluids within the subsurface, which are both key aspects of geothermal surveillance strategies. In the end, deriving attenuation properties will allow to provide the full seismic response of the geothermal system, in space, and in time. Figure 6.18 Modified after Sanchez-Pastor et al. (2021). Each line highlights a different seismic station. (a) Modelisation of the temperature variation in the medium, (b) Estimated steam evolution (c) Vp and Vs computed from rock physics modelisation (d) Monitored dv/v and monitored subsidence.
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