294 Geophysics in Geothermal Exploration Geophysical methods contribute to the understanding of geothermal systems. Electrical and electromagnetic methods are one of the geophysical techniques potentially sensitive to water content and temperature. Surface-to-borehole Controlled Source Electromagnetic Method (CSEM) can be used to establish an initial geo-electric state of a geothermal doublet and determine whether a cold front could be detected. Active and passive seismic methods help to better understand the geological structure of the subsurface, locating fractured zones and geological formation interfaces and potentially identify hydrothermal fluids presence and circulation pathways. Passive methods, being less invasive and cost-effective, are valuable tools. When combined, passive seismic, MT, and gradiometry can yield a shear velocity model, resistivity distribution with depth, and insights into bedrock location and fault structures. Seismic inversion and characterization are disciplines that aim at converting seismic amplitude into key reservoir properties, leading to valuable information between wells to lower the risk while planning exploration or development of geothermal production, either with low or high depth objectives. Furthermore, anisotropy magnitude and orientation, extracted by both VVAZ (Velocity versus Azimuth) and AVAZ (Amplitude versus Azimuth) analysis, can be linked to fracture intensity and orientation. The fracture characterization plays a crucial role in identifying zones with secondary porosity and enhanced permeability, increasing the prospectivity. The fracture connectivity must be evaluated to derisk the development of a geothermal project. As an example for a geothermal volcanic system, various geophysical methods were used to confirm the existence and location of deeper geothermal resources in Mayotte’s Petite-Terre volcanic Island and to define the geothermal drilling targets. Geophysical data allowed for the placement of a reservoir, a heat source, the base of the volcanic substratum, as well as several areas with higher vertical permeability (chimneys) connecting the reservoir to surface material ejection zones. MT measurements enabled the construction of a well-constrained 3D image of a potential geothermal target, Electrical profiles crossing the island detected the presence of faults, Gravity measurements were inverted to obtain a density model and confirm the presence of faults. The inversion is performed jointly with the inversion of MT data, using a global correlation of resistivity and density structures as a constraint. As an example of a geothermal rift and fault zones system, a multi-physics image of deep fractured geothermal reservoirs is essential to reduce the risks of deep geothermal resource, as shown by the establishment of the geothermal model In the Upper Rhine Graben. The example shows how the occurrence of fractured reservoirs characterized by natural brine circulations with fractured zones obliged developers to adapt geophysical exploration methods, geophysical well logging strategies as well as technical well design for reaching geothermal targets. The objective is always to select and combine the most appropriate geophysical methods to build the most comprehensive geological models for the specific geothermal system.
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