204 Geophysics in Geothermal Exploration As for Vp/Vs analysis, examining the quality factor ratio of P and S waves (Qp/ Qs Ratio) can also inform on the specific features of the geothermal field. For instance, low Qp/Qs ratio (P-waves more attenuated than S-waves may indicate a medium partially saturated with fluids (Pérez and Cuellar, 2018). The presence magmatic bodies, on the contrary, would result in almost complete attenuation of the S-wave, hence high Qp/Qs ratio (Georgsson, 2009). Another interesting example is found in the work by Yeh et al. (2021), who used Qp/Qs analysis to investigate the subsurface context of the Taipei area in Taiwan. They observe low Qp/Qs values that they interpret a due to a dominance of scattering attenuation due to a highly fractured medium but denying the existence of a magmatic chamber, as was pointed out by an earlier study based on velocity analysis only (Lin, 2016), hence illustrating the interest of analyzing jointly both velocity and attenuation features to obtain a geological and geothermal assessment as complete as possible. 6.2.2 Ambient noise seismic interferometry (ANSI) ANSI methods are another possibility to derive 3D models of the subsurface seismic properties. By opposition to earthquake-based velocity tomography, ANSI tools present the advantage of allowing data acquisition and producing results even in areas where earthquake activity is low, where tomography analysis based solely on detection of seismic events would not be possible. The geophysical value brought by ANSI studies (3D models of seismic properties), their flexibility for implementation in various contexts (passive sensors deployment over a few days or weeks), along with the economic advantage inherent to passive seismic studies has brought ANSI methods and particularly 3D Vs tomography technology as a nearly common geophysical tool in today’s geothermal exploration strategies. Hence a several applications of ANSI-based tomography models are presented in the literature, addressing various geothermal contexts. Toledo et al. (2022) investigated the potential of the method in the volcanic context. The authors derived a 3D Vs model at the Theistareykir geothermal field in Iceland, where a clear separation between a high Vs anomaly and a low Vs anomaly is observed, which follows a N/NW orientation (Figure 6.13). Such observation agrees with the trend and pattern observed through a Magnetotelluric investigation of the site, hence confirming the interest of ANSI based Vs models for geothermal characterization. Overall, low velocity anomalies coincide with bodies of low electrical resistivity, which would consistently suggest the presence of rocks saturated with hydrothermal fluids (Toledo et al., 2022) Also in the volcanic context, Sánchez-Pastor et al. (2021) also observe a consistent correspondence between low electrical resistivity anomalies and low Vs anomalies at the Hengill geothermal field in Iceland. The authors also pushed their interpretation to identify iso-velocity curves as transition zones within the geothermal system, proposing the 2 km/s iso-velocity curve as being a proxy
RkJQdWJsaXNoZXIy NjA3NzQ=