185 6. The use of passive seismic methods for Geothermal exploration and monitoring data continuously and at high resolution. This approach allows seismologists to capture even faint shifts in wave properties, which would be missed with a triggered or intermittent recording system. These subtle changes in the Earth’s wave velocities provide crucial data for understanding not just earthquake-prone areas but also regions experiencing slower processes, like crustal deformation or fluid shifts in fault zones. Through these advances, passive seismic methods now serve as a powerful lens into the Earth’s structure and its subtle movements and transformations, further bridging seismology with physics to deepen our understanding of the dynamic planet we inhabit. A cost-effective, high-value tool in the geothermal geophysical toolbox Passive seismic methods present two main interests in the context of geothermal industry development. First, their cost-effectiveness makes it an economically competitive tool for exploring geothermal subsurface context, at different scales, in complementarity to electrical, electromagnetic and other geophysical methods. Second, their input data – the ambient seismic signal – being continuous, free of charge, those methods can also be implemented as a continuous measurement allowing for quasi-real time monitoring of the subsurface. The problematics surrounding geothermal exploration and the associated geophysical targets are as diverse as the variety of existing geothermal contexts. In this paper we will first present the practical and theoretical basis of the passive seismic approaches that are being used for geothermal exploration and monitoring purposes. Then we will focus on exposing how the diversity of geothermal geological contexts calls for different seismic responses and hence different geophysical objectives and how the global exploration and monitoring strategies can be improved thanks to the integration of passive seismic measurements. 6.1 Methods 6.1.1 Seismological analysis The analysis of seismic events and microseismic events can provide significant amount of information about the subsurface, either by better identifying and locating the structural mechanisms that induce the seismicity, or by analyzing the seismic wavefield properties associated with those events, i.e. seismic waves velocity and attenuation properties in the vicinity of the seismic array. The following paragraph proposes a summary of the most used and emerging techniques to achieve both objectives.
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