193 6. The use of passive seismic methods for Geothermal exploration and monitoring seismic velocity within the Earth’s crust can be influenced by environmental disturbances, such as precipitation (Sens-Schönfelder and Wegler, 2006; Tsai, 2011), atmospheric pressure loading (Olivier and Brenguier, 2016), thermoelastic stresses (Hillers et al., 2015; Meier et al., 2010), ground water change (Voisin et al., 2016; Gaubert-Bastide et al., 2022). Noise-based velocity monitoring has also improved understanding of tectonic and volcanic processes, allowing for the detection of long-term post-seismic relaxation in fault zones (Brenguier et al., 2008; Hobiger et al., 2012), velocity decreases as precursors to volcanic eruptions (Brenguier et al., 2011; Wegler and Sens-Schönfelder, 2007), and interactions between seismic and volcanic systems (Brenguier et al., 2014). Application to geothermal monitoring contexts is addressed in the following paragraphs. 6.2 Passive seismic methods for geothermal exploration Geothermal exploration aims at detecting subsurface areas that hold favorable conditions for geothermal exploitation. Depending on the geological context, and particularly on the type of geothermal field being considered, the targeted geological configuration can vary, as will the associated geophysical signature. Moeck et al. (2014) propose a classification of geothermal contexts in two main categories: Convective systems: which are characterized by the presence of fluid in a reservoir that is set in motion within a convective loop induced by the presence of a heat source. The heat is transported to the surface by the fluid as a function of the permeability of the medium. Convective systems can take place in various geological contexts: • Volcanic reservoirs: where the convection is controlled by the magma chamber. The productive zone of the reservoir is the up-flow zone, which concentrates the hottest fluids and the top of the reservoir is enclosed by conductive clay-caps. • Magmatic reservoirs: where fluids circulate in a network of permeable faults near a recent hot magmatic body acting as the heat source. • Non-magmatic reservoirs: where fluids are circulating in a permeable fault network set up during extensive crustal dynamics. As the crust gets thinner, the upwelling of the Moho increases heat flow, creating local thermal anomalies. Conductive systems: In these geothermal systems, the heat comes from the natural thermal gradient, to which may be added heat flows from granites. As the heat sources are too weak to allow convection, the temperature field is distributed by conduction through the material. Again, one can distinguish different geological contexts leading to conductive geothermal systems: • Igneous reservoirs: which are not reservoirs per se, but thermal anomalies linked to radioactive disintegration. Exploiting the geothermal type of resource implies
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