201 6. The use of passive seismic methods for Geothermal exploration and monitoring Figure 6.10 Modified after Simiyu (2009). Vp/Vs ratio map extracted from a 3D tomography at the Menengai geothermal area. The low value anomaly correlated with the expected location of the heat source at this geothermal site. Attenuation models In the context of geothermal exploration, the two key resources to identify are heat and fluid. Attenuation anomalies have been shown to highlight such favorable targets. Sato et al. (1989) and Mavko (1980) propose that an increase of seismic attenuation properties can be related to an increasing temperature. Hough et al. (1999) interpret a pseudo mapping of the thermal distribution by imaging the variability of attenuation structure in Coso geothermal reservoir. This behavior of dependence between attenuation and temperature of geothermal rocks at reservoir conditions as been highlighted by Jaya et al. (2010) and Poletto et al. (2018) with a petrophysical approach using Biot-Gassmann relation. On the other hand, Haberland et al. (2009) associate attenuation increasing to an augmentation of the fluid content. For Grab et al. (2017) velocity structures reflect lithology while attenuation is a better indicator for reservoir permeability and fluid saturation in magmatic geothermal reservoir. Hudson et al. (2023) use attenuation tomography to map crustal fluid pathways and hydrothermal/geothermal systems in volcanic context. Attenuation imaging method can also provide complementary information to the traditional approaches used in geothermal exploration. In volcanic context, Muskin et al. (2013) demonstrate that regions of high attenuation and high conductivity are related to high fluid content. Multiple studies have also proven anelastic properties to be a good indicator of the presence of magma or melting materials, which tend to significantly dissipate seismic energy during wave propagation. This correlation has typically been documented in studies investigating mantle properties and characteristics (e.g. Karato, 2004; Nakajima, et al. 2013).
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