215 6. The use of passive seismic methods for Geothermal exploration and monitoring • Low frequency sensitivity: to allow for detection and analysis of low frequency waves, a key parameter for instance to analyze interface waves while requiring a high penetration depth (in ANSI-based approaches for instance). • Cost: to allow for dense networks deployment, hence improving the accuracy of earthquake-based seismological analysis and enhancing the resolution of the seismic models derived either from ANSI methods or earthquake tomography approaches. All those improvements will participate in further highlighting the very strong potential of passive seismic methods to serve as key methods in geothermal exploration and monitoring geophysical strategies. Passive seismic methods are the most cost-effective way to provide information about the subsurface seismic properties. In geothermal contexts, geophysicists typically track velocity and attenuation anomalies to infer the presence of faults, heat sources, hydrothermal fluid circulation patterns, and address the general geological and tectonic context. In addition to the intrinsic value of such passive seismic characterization strategies, they can also be used to improve the accuracy of the global geophysical approach by completing other geophysical datasets and enhancing their interpretation. Further developments and applications of joint or constrained inversion schemes will likely be a cornerstone of passive seismic methods integration in geophysical assessments in geothermal contexts, as in many other geo-resource explorations (e.g. natural H2, helium). Monitoring strategies for geothermal operations surveillance have naturally benefitted from passive seismic methods thanks to the intrinsic continuity of the data acquisition. The most known application is microseismic monitoring, which aims to track in time and space the seismicity potentially induced by geothermal operations. In addition, methods based on monitoring the subsurface seismic properties – not only the seismic activity – have been gaining momentum in the last few years to better understand the geothermal target behavior in production phase. Methods such as time-lapse earthquake tomography and ambient seismic noise interferometry (ANSI) provide such capacities, yet developments and applications to multiple different contexts are required to improve their sensitivity to operations-induced processes and hence propagate their use as common tools in geophysical geothermal monitoring strategies. The strength of passive seismic methods relies on the fact that they have been built while searching for the tiniest, hidden bit of information within a seismic signal that is initially not well understood, globally uncontrolled, yet continuously produced by our environment. The dedication of scientists for exploiting and enhancing the information contained in what originally appeared to be a disturbance for active seismic studies is remarkable. As of this day, and as is illustrated within this chapter, geophysicists have now the possibility to extract spatial and temporal seismic information from the whole ambient seismic signal, including both the coherent part of the signals (through earthquake and microseismic analysis) and the incoherent part of the signal (through ANSI approaches in particular). They have turned the ambient seismic signal into a highly valuable source of information from which every
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