TITLE:
Application of Time Domain Electromagnetics (TDEM) for Refining the Geometry of a Coastal Aquifer
AUTHORS:
Tchantipé N’tcha, Bernadin Elegbede Manou, Messan Fabrice Lawson, Mory Kourouma, Nicaise Yalo, Bio Guidah Chabi, Valérie Kotchoni, Moussa Boukari, Mellone Glessougbe, Tokpo Ninamou
KEYWORDS:
TDEM, Aquifer Geometry, Continental Terminal (CT), Hydrogeological Modeling
JOURNAL NAME:
Open Journal of Modern Hydrology,
Vol.15 No.4,
October
29,
2025
ABSTRACT: The Miocene-Pliocene aquifer of Benin’s Coastal Sedimentary Basin (BSC) is the most heavily exploited aquifer for supplying water to the cities of Cotonou, Porto-Novo, and their surrounding areas, thanks to two catchment fields located on the Sakété and Allada plateaus. This study, which focuses on the Sakété plateau, aims to improve knowledge of the geometry of this part of the aquifer shared with Nigeria. Inhabited by a high population density with varied economic activities, this aquifer is subject to excessive withdrawal and deterioration in water quality. Furthermore, in this part of Benin, there has been a spectacular proliferation of private wells and boreholes, often referred to as private autonomous water stations. This situation of unregulated exploitation puts further pressure on the aquifer and thus compromises the sustainable exploitation of water resources already subject to the effects of climate change. It is therefore necessary to investigate and deepen our knowledge of its geometry in order to better understand the system. In order to refine the wall and edges of the aquifer’s geometry, 32 geophysical surveys were carried out along three well-defined transects, using the Time Domain Electromagnetic (TDEM) method. This method, known for its flexibility in identifying conductive soils such as clays, confirmed that the aquifer is multi-layered, interspersed with clay lenses of varying thickness in places. The aquifer wall, consisting of clay, is estimated to be between 50 and 100 m deep at the northern edge of the plateau near the Lama depression, with an outcrop of the clay layer towards Nigeria on the same cross-section. On the western profile, the wall is located between 90 and 120 m depending on the site, while on the southern profile of the plateau, the depth of the wall is almost constant at 80 m and deepens (110 m) towards the Nigerian border in the east. The Feflow numerical code was used for a finite element mesh of the aquifer. The continuous super mesh is characterized by 193,864 nodes, 272,892 prism-triangle elements with 06 nodes per element. 527 borehole logs and TDEM surveys were used in conjunction to establish a conceptual model of the three-dimensional geometry.