Airborne and Land-based Electromagnetic (CSEM and MT) Surveying for Geothermal Exploration in the Martinique Island

Abstract

Exploring for geothermal resources using land-based electromagnetic methods can be very challenging due to the presence of strong human-generated and “geological” noise. In such context, some passive EM techniques like the Magneto-Telluric method may not be applicable at all and a dedicated toolbox of EM techniques capable of dealing with these issues is required. In this paper, we will focus on the challenges encountered while exploring for geothermal resources in volcanic islands, namely a highly heterogeneous near-surface creating strong static-shifts, the presence of a sea/land transition zone and a high degree of urbanization over the area of interest. We will show that the combination of airborne with land-based controlled-source EM (CSEM) and MT surveying provide a robust approach and illustrate these aspects with two EM datasets acquired over the Martinique Island, in the French Lesser Antilles. The first study extends over the Lamentin bay area located near Fort de France, the main city of the Martinique Island. Here, exploratory drilling since the 1980’s highlighted a polyphased hydrothermal system with fossilized hydrothermal features and a recent hydrothermal activity. The geothermal system is poorly understood and has prevented any exploitation of the geothermal resource to date. In an attempt to reduce the geological risk for future geothermal development, a joint 3D airborne and CSEM survey was performed. Here, the presence of anthropic EM noise made MT data hardly usable. Processing and inversion of the EM dataset allowed to map the geometry of the hydrothermal alteration “caprock” as well as underlying volcanic substratum and potential geothermal reservoir. The second study extends over the Petite-Anse area located in the South-West of the Martinique island. Here, evidences for high-temperature geothermal system ( more than 200°C) are present and the design of exploratory boreholes requires to map accurately the geometry of the “caprock” and potential geothermal reservoir. To do so, a high-resolution 3D CSEM and MT survey was conducted in Spring 2019. Similarly to the Lamentin area, MT data suffered from strong cultural noise. CSEM data quality is sufficiently good to map the extent of the area of geothermal interest and areas to avoid drilling into (e.g. dykes).