202 Geophysics in Geothermal Exploration However, as mentioned previously, evaluating intrinsic attenuation properties of the subsurface through seismic events analysis is not an easy task, for it requires to discard or correct for other seismic attenuation mechanisms which are rather related to the propagation path of the seismic waves such as geometrical spreading and multipathing (Ko et al., 2012). Lin et al. (2024) present a recent study of attenuation structures in the northern Taïwan volcanic zone where they explore the attenuation features of the area and their relation to the local magmatic and tectonic mechanisms. The authors collected seismic waveforms from 43 earthquakes using a seismic array of 118 stations, for a period of about 1 year, and isolated the P-waves and S-waves arrivals. Their data analysis process begins with a 1D inversion to establish a baseline attenuation model, corrected using a frequency-dependent power law that adjusts for changes in attenuation across frequencies. This model initializes a 3D inversion, where amplitude data is standardized to a reference frequency (5 Hz) to ensure consistency across frequencies and Fréchet kernels were computed by 3D ray tracing through the tomographic velocity model. The inversion estimates differential inverse quality factors (Q−1) perturbations, with prior covariances and smoothing ensuring model stability and resolution robustness. Ultimately, the authors manage to estimate 3D models of Qp and Qs (quality factors) in the area. The results of this analysis for geothermal exploration are the potential identification of hydrothermal activities and magma reservoirs at varying depth, which are characterized by high attenuation values (low Q factors), as illustrated in Figure 6.11. Figure 6.11 Illustration of subsurface attenuation characterization based on seismological analysis (modified after Lin et al., 2024). The maps show lateral variation of attenuation values extracted from the global attenuation 3D model, focusing on a specific depth of 5 km.
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