A large-scale electromagnetic geophysics operation around the geothermal reservoir in Bouillante (Guadeloupe) was conducted between 5 and 17 April 2021. It took more than 20 people to install the gauging instruments, both at sea and on land, across an area of approximately 40 km². This involved 10 BRGM employees, 4 experts from the University of Brest and the companies IMAGIR and MAPPEM Geophysics, as well as a vessel and a team of divers from the company ATSM, and a Guadeloupean technical support team.
The deep electromagnetic (EM) techniques used here, such as the Magnetotelluric (MT) and Controlled-Source EM (CSEM) methods, are used for electrical conductivity imaging of the deep subsoil to obtain information that enables the characterisation of geothermal reservoirs at depths reaching several kilometres. In particular, these methods make it possible to distinguish between the aquifer itself and the clay layer above it, where conductivity is greater. For these techniques, the typically-Caribbean volcanic island terrain presents particularities arising partly from the complex geological environment, the aquifers’ proximity to the sea, the mountainous relief and the high population density, a source of noise that can hinder geophysical operations.
The operation conducted in Bouillante has made it possible for the very first time to test a combination of very deep imagery obtained via the historical MT method (500m-10km), with a more lightweight and less costly alternative (CSEM), thus increasing both output and resolution at mid-depth range (0-1.5km). In addition, the partial roll-out of the system at sea will enable us for the first time to extend the imaging of the reservoir to the part that lies beneath the sea. The use of these combined land-and-sea geophysical methods is a world first in this context.
The MT and CSEM data are currently being processed in the aim of building a 3D model of the subsoil’s electrical conductivity and hence mapping the geothermal reservoir of Bouillante. This work is conducted jointly by BRGM, the University of Western Brittany and the company IMAGIR. In his PhD dissertation, Simon Védrine supports these innovative processing techniques, endeavouring to assess the value and the limitations of different methods in the volcanic-island context, to enhance processing and digital modelling tools in order to accommodate the complexities of the terrain (mountains, land-and-sea contrasts) and include other existing geophysical data (from airborne EM systems, electric streamers at sea, historical MT surveys).