TDEM prospections for inland groundwater exploration in semiarid climate, Island of Fogo, Cape Verde

Abstract

Groundwater resource assessment is fundamental for agriculture in arid or semiarid conditions, where precipitations are irregular and scarce. In volcanic islands, groundwater resources are often located at significative depths, thus hindering direct exploration. The application of geophysical techniques is therefore necessary for a preliminary evaluation of water availability and to identify potential drilling points. In this study, Time Domain Electromagnetic Method (TDEM) soundings were used for groundwater prospection in the surroundings of a vineyard on the island of Fogo, Cape Verde. A pseudo-2D profile of ten sounding points was located within the vineyard, and other localized prospections were performed close to known boreholes, for attempting correlations of TDEM evidence with water depth and stratigraphy. The data acquired were interpreted by means of three inversion algorithms to obtain an uncertainty evaluation of the results: the particle-swarm-optimization algorithm, the linearized 1D inversion and the spatially constrained inversion were applied. These methods provided a set of equivalent solutions of the TDEM inverse problem to be evaluated and compared. The inverted models are highly consistent and show little mismatch at greater depths. The main outcome regarding the resistivity distribution in the vineyard subsurface is the evidence of a nearly 150 m-thick conductive region (1–10 Ωm). Information from existent water wells enhanced the hydrogeological interpretation of the profiles and the detection of potential water-saturated formations.