194 Geophysics in Geothermal Exploration fracturing the rock and stimulating fluid circulation within the formation to extract energy. • Sedimentary reservoirs are typical contexts where geothermal energy extraction is based on the exploitation of the natural thermal gradient, which can be locally accentuated by thermal conductivity contrasts, such as in a deep porous and permeable sedimentary layer. • “Mountain” reservoir: where the heat from the natural thermal gradient is advected upwards through fluid circulation in deep crustal faults linked to the formation of mountain ranges. 6.2.1 Seismological analysis The analysis of seismic events, whether they occur locally or have a more regional origin, can provide a range of information to characterize a geothermal area, understand the global geological configuration and locate the most suitable prospection zones. Several seismic attributes can be derived during those analysis, depending on the data available and the geophysical context. Overall, those methods have been used for the location and characterization of multiple geothermal targets such as heat sources, hydrothermal activity, faulted and permeable zones, of fluid migration pathways and to characterize reservoir properties. Pérez and Cuellar (2018) provide a synthetic summary of the different kinds of analysis that can be performed within a seismological analysis dedicated to geothermal exploration. The following paragraphs transcribe some of this summary, while adding more recent references and analysis. The most direct approach is to analyze the seismic activity itself. Locating events, evaluating magnitudes, identifying focal mechanisms and statistical distributions are different and complementary ways to relate the temporal and spatial distribution of seismic activity with geothermal key parameters. Another set of methods aims to understand and map the seismic properties of the subsurface by analyzing how the seismic events propagate within it. The aim is to evaluate seismic velocity models or seismic attenuation models and track the specific signature of geothermal targets. Seismic activity characteristics analysis Location and magnitude of events The intensity of seismic activity can be associated with tectonic processes but also with fluid dynamics related to geothermal heat sources. Overall, the characteristics of seismic activity provide a direct signature of the energy contained within the magmatic and hydrothermal system, i.e. the geothermal target potential (Pérez and Cuellar, 2018).
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