Hydrogeological settings of a volcanic island (San Cristóbal, Galapagos) from joint interpretation of airborne electromagnetics and geomorphological observations

摘要

Many volcanic islands face freshwater stress and the situation may worsenwith climate change and sea level rise. In this context, an optimummanagement of freshwater resources becomes crucial, but is often impeded bythe lack of data. With the aim of investigating the hydrogeological settingsof southern San Cristóbal Island (Galapagos), we conducted ahelicopter-borne, transient electromagnetic survey with the SkyTEMsystem. It provided unprecedented insights into the 3-D resistivity structureof this extinct basaltic shield. Combined with remote sensing and fieldwork,it allowed the definition of the first hydrogeological conceptual model ofthe island. Springs are fed by a series of perched aquifers overlying aregional basal aquifer subject to seawater intrusion. Dykes, evidenced byalignments of eruptive cones at the surface, correspond to sharp sub-verticalcontrasts in resistivity in the subsurface, and impound groundwater in asummit channel. Combined with geomorphological observations, airborne electromagnetics are shown tobe a useful for hydrogeological exploratory studies in complex, poorlyknown environments. They allow optimal development of land-based geophysicalsurveys and drilling campaigns.