Study of the Underflow and the Deep Water Table by Geophysical Methods (Vertical Electrical Soundings and Electrical Tomography) in the Oum Lagsab Area (Central Tunisia)

The region of Oum Lagsab, part of the North Gafsa basin, is characterized by a semi-arid climate where water resources are strongly influenced by irregular rainfall and a relatively dense hydrographic system. This work is a synthesis based essentially on the contribution of geophysics to specify specific results by supplementing and reinterpreting the data of previous work. Thus, this work constitutes a kind of comparison of the data of several geophysical campaigns to arrive at an updated vision of the region of Oum Lagsab. This work is only an update of the study of aquifers in the area (the underflow aquifer and the deep aquifer in the northern Gafsa). This update should make it possible to understand better the architecture of the aquifers and the mechanisms of their discharges and to evaluate the impact of the underground dam on the recharge of the water table of Oum Lagsab. The hydrochemical properties show that the waters are fresh near the wadi bed while they are more or less polluted at the peripheries of the water table. The six profiles of the electric tomography made in the bed of the Oum Lagsab show that the underflow layer is housed in clay and clay sand levels. The two geoelectric sections made it possible to identify coarse levels attributed to the Mio-Pliocene. They are likely to present aquifer potentialities, and the relatively resistant level attributed to the limestones of the Maastrichtian (Berda Formation) is likely to present an aquifer unit, especially when sufficiently karsted. From a structural point of view, two syncline structures are present in which the Mio-Plio-Quaternary filling undergoes considerable thickening.