Airborne and land-based controlled-source electromagnetic surveying in challenging electromagnetic environments – application to geothermal exploration in a volcanic island

International audienceExploring for underground 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. We focus here 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 a combined airborne and land-based controlled-source EM approach provide a robust approach and illustrate these aspects with an EM dataset acquired over the Martinique Island, in the French Lesser Antilles. We believe this work sets the foundations for more robust geophysical exploration workflows in such challenging EM environments. Introduction Land-based electromagnetic (EM) methods, attempting to detect contrasts in electrical resistivity between target resources and their surroundings, have been developed and utilized for exploring buried resources such as minerals, hydrocarbons, geothermal energy, or groundwater for more than a century (Streich, 2016 and reference therein). Noise has complicated EM recordings ever since the first EM measurements were made. Various types of noise have been recognized (Szarka, 1988): human-generated noise emitted, as examples, by the power grid, power plants, railways, pipelines, industrial and agricultural facilities. Large metallic bodies, such as well casings, can also strongly alter EM field behavior locally. Magnetotelluric signal is regarded as noise in the context of active EM surveying. Instrument noise limits measurable EM field levels. Finally, subsurface heterogeneity like bodies at depth, small-scale structure near the sources or receivers not resolvable by the measurement technique used, can mask target responses and hence be considered as " geological noise " (Streich, 2016). In order to explore for underground resources in a large variety types of environments (e.g. urbanized areas, mountainous terrains, volcanic areas, sedimentary basins), we have developed a comprehensive toolbox of EM techniques capable of dealing with such challenging EM environments. In this paper, we will focus on the challenges encountered while exploring for geothermal resources in volcanic islands, namely a highly heterogeneous near-surface, the presence of seawater and a high urbanization over the areas of interest. We will show that combined airborne and land-based controlled-source EM methods provide a robust approach and illustrate these aspects with an EM dataset acquired over the Martinique Island, in the French Lesser Antilles