Joint 3D inversion of nearshore and land MT and CSEM data in coastal areas of volcanic islands: application to the Bouillante geothermal field

Simon Védrine (1) , P Tarits (2) , François Bretaudeau (1) , S Hautot (3) , Mathieu Darnet (1)

1 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)
2 GEO-OCEAN - Geo-Ocean
3 IMAGIR

A large-scale magnetotelluric (MT) and controlled-source electromagnetic (CSEM) acquisition campaign was conducted around the exploited Bouillante geothermal field in Guadeloupe in the Lesser Antilles, supplemented by a regional airborne transient electromagnetic survey (AEM). The target is the characterization of the geothermal reservoir and the associated clay cap, generally identified by its highly conductive electrical signature, typical of a conventional volcanic geothermal system in an andesite-type geological environment. MT allows targeting the deepest parts of the subsurface, but the natural signal on which it relies is on average weak at low latitudes and can be much weaker than anthropogenic noise, making its use with dense acquisition challenging close to inhabited areas. CSEM allows completing and densifying the data coverage in the most urbanized areas thanks to the use of high power active current sources and less demanding measurement systems. Moreover, MT and CSEM 3D modeling, as well as inversion, in the coastal areas of volcanic islands can be challenging due to numerical errors induced by the presence of the sea/land interface and large variations in nearshore bathymetry and topography. These errors must be estimated and corrected to best invert the field data. Here we present the workflow and results of the 3D CSEM and MT modeling and inversion independently. The resistivity model obtained with CSEM is consistent with the known geology and the 3D MT model, but provides improved resolution as well as new information on the extension of the reservoir nearshore. We also discuss the complementarity between MT and CSEM in this context, and how to perform their joint inversion in order to benefit from the high resolution and sensitivity of CSEM on the first 2 km and the ability of MT to provide deeper information.

Mots clés: Joint 3D inversion; Volcanic islands; Geothermal exploration; MT; CSEM

Domaines: Géophysique [physics.geo-ph]; Electromagnétisme

Simon Védrine, P Tarits, François Bretaudeau, S Hautot, Mathieu Darnet. Joint 3D inversion of nearshore and land MT and CSEM data in coastal areas of volcanic islands: application to the Bouillante geothermal field. EMIW2022, Sep 2022, Çeşme, Turkey. ⟨hal-03703135⟩