Contribution of an heliborne EM-magnetic survey to the understanding of the hydrogeological functioning of a volcanic island (Martinique – Caribbean island)

Abstract:

Water resources exploration on volcanic islands is challenging as these territories frequently face high population densities with increasing water demands. Improving the hydrogeological knowledge of these islands is thus a major objective in order to achieve a sustainable management of their water resources.

Taking advantage of a SkyTEM heliborne geophysical survey over Martinique Island (Lesser Antilles), which provides subsurface information despite dense vegetation and steep slopes, we conducted multidisciplinary studies on the island, at different spatial scales, in order to relate resistivity data with geological and hydrogeological data from boreholes and springs. The objective is to interpret resistivity data in terms of hydrogeological structures and properties, in order to constrain hydrogeological conceptual models adapted to the island.

We firstly demonstrate, at the aquifer scale, that heterogeneous hydrodynamic properties and channelized flows result from tectonically controlled aquifer compartmentalization along structural directions highlighted using resistivity and magnetic data. Furthermore, we show that the most fractured compartments have lower resistivity and higher permeability. Compartmentalization and transmissivity contrasts thus protect the studied coastal aquifer from seawater intrusion.

At the watershed scale, we put in evidence that the main geological structures lead to preferential flow circulations and that hydrogeological and topographical watersheds can differ, influencing river flowrates. Correlation between resistivity, geology and transmissivity data of four aquifers also reveals that the older the formation, the lower its resistivity, and the higher its transmissivity. This trend is interpreted as the consequence of tectonic fracturing produced by earthquakes in this subduction zone.

Finally, our approach allows characterizing the properties of aquifer and aquitard units of Martinique, leading to the proposition of hydrogeological conceptual models suitable to the complexity of the island, with heterogeneous geological formations presenting high lateral and vertical variability. Moreover, our study offers new guidelines for addressing relations between resistivity, geology and hydraulic conductivity for volcanic islands.