Contribution of multi-method geophysics to assess the regional setting of Bouillante Geothermal Province (Guadeloupe, West French Indies)

Bouillante geothermal field (Basse-Terre, Guadeloupe, French West Indies) is located near the Western coast of Guadeloupe Island, about 15 km from the presently active Soufrière volcano. We have studied the subsurface structure of a zone encompassing the geothermal area using geophysical data. The need to understand the geological context of the Bouillante geothermal Province has led to numerous onshore and offshore geophysical investigations. This work presents a compilation of all available, subaerial and marine, gravity and magnetic, electric and magnetotelluric data acquired during the last 30 years. To the first order, this synthesis could act as a guide in the implantation of upcoming surveys in order to better constrain the Bouillante geothermal Province, and to improve the knowledge the island evolution as a whole. Oldest electric and magnetotelluric data were digitalized from manuscripts. Although we faced different problems related to missing acquisition parameters, we partially reconstruct MT tensors to provide a qualitative interpretation of resistivity distribution at depth. Geophysical models have been initiated based on geological constraints and previous geophysical interpretations. We are able to differentiate structures pre- and post-dating the different magnetic inversions using the magnetic measurements according to the polarity of the anomalies. The gravity measurements are used to detect and characterise the dense complexes from the low dense areas, allowing us to complete the magnetic measurements in regard of the nature of the Bouillante Chain formations. The electromagnetic surveys allow us to determine the distribution of electrical resistivities which we interpret in terms of water saturation of rocks, hydrothermal alteration and the presence of hydrated mineral. The integration of the geophysical results has allowed us to build up an updated model of the geothermal system based on geological constraints and previous geophysical interpretations. At the scale of the Province of geothermal Bouillante, the previously proposed conceptual model is clarified through a formations layering associated with petrophysical parameters (density, magnetization, resistivity), in agreement with the classical scheme of geothermal reservoirs. The distribution of resistivities in depth highlights the succession of resistive-conductive-resistive layers, classic in volcanic field. The shallow resistant is relatively well explain in terms of massive formations, dense (not affected by hydrothermal alteration) and recent, associated with the Axial Chain or Chain Bouillante. The conductive intermediate layer marks the intermediate clay cover (cap rock) of the geothermal system, associated with low dense and demagnetized rocks altered by hydrothermal action (at least in the productive areas recognized on the surface). The deep resistive layer is associated with dense formations, older according to depth (oldest formations of the Axial Chain or lava flows from the northern chain). This latter could correspond to the deep geothermal reservoir whose morphology and extension are discussed. Based on our geophysical data and modeling, the geothermal activity could be more developed toward the north compared to the previously proposed extension. Moreover, a major volcano-structural control is evident on various configurations of the geothermal field. The various faults recognized in this key sector seem to affect significantly the internal structure of the Province of Bouillante, and consequently the development of geothermal activity. The horst and graben structure in conditions undoubtedly the morphology of the clay cover and operation of the field. This post-doctoral study was carried out in the framework of a "Carnot institute" labeled project.