210 Geophysics in Geothermal Exploration Figure 6.16 Schematic illustration of different injection-induced processes occurring in geothermal reservoir and their associated seismic response in terms of P-wave seismic velocity and attenuation. Modified after Guo and Thurber (2022) – Supporting information. 6.3.2 Ambient noise seismic interferometry monitoring ANSI-based methods have been gaining momentum during the last two decades as relevant tools for seismic monitoring of the subsurface. ANSI-based monitoring approaches are often referred to as the dv/v method, since it consists of tracking velocity changes by comparing ANSI-based reconstruction waveforms in time through phase-shifts measurements. Such an approach has the potential to detect very subtle shear wave velocity changes (when dealing with surface wave), possibly on the order of 0.1% and lower depending on the site conditions. Geophysicists have demonstrated the sensitivity of the method to various phenomena such as precipitation, atmospheric pressure loading, thermoelastic stresses, ground water change, stress relaxation and others. Applications to geothermal monitoring have not yet been very numerous, but edifying studies yet exist that highlight the strong potential of ANSI for geothermal operations surveillance.
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