Contribution of multi-method geophysics to the understanding of a high-temperature geothermal province: The Bouillante area (Guadeloupe, Lesser Antilles)

Abstract

The Bouillante high-temperature geothermal field (Guadeloupe, French Lesser Antilles) is located on the west coast of Basse-Terre Island, about 15km from the currently active Soufrière volcano. We have studied the shallow structure of the geothermal zone using a compilation of all available gravity, magnetic and resistivity data, both subaerial and marine, acquired during the last 30years. Geophysical models have been developed based on geological constraints and previous geophysical interpretations. The resistivity distribution from electromagnetic surveys is interpreted in terms of water saturation of rocks, hydrothermal alteration and presence of hydrated minerals. Gravity is used to distinguish and characterize the denser formations from the low density areas. Finally, we are able to differentiate structures which pre- and post-date various recent magnetic inversions using the polarity of the magnetic anomalies. By integrating these results the previous conceptual model for the geothermal province has been improved, with the layering of the geological formations as defined by geophysical parameters now being taken into account, in agreement with the classical scheme of geothermal reservoirs. The shallow resistive layer is explained in terms of recent massive formations. The conductive intermediate layer marks the low density, demagnetized clay cover of the altered geothermal system. The deep resistive layer is associated with dense formations increasing in age with depth, and corresponds to the deep geothermal reservoir. Based on our geophysical data, the geothermal activity could be more developed towards the east and north compared to the previously proposed area. A volcano-structural control seems to affect the internal structure of the province, and consequently the development of geothermal activity. This current work could be used as a basis for future surveys of the Bouillante province in order to improve constraints, and to increase our knowledge of the island's overall evolution.