Contribution of geophysical methods in characterizing the structure of El Fahs plain: Hydrogeological implications

Abstract

The El Fahs plain aquifer belongs to Tunisian Northern Atlas and it is particularly important due to its exploitation, well yields and the water's chemical quality. In recent years, the increasing exploitation is becoming unsustainable and this warrants studies and policies to better manage the groundwater resources in the El Fahs plain. Therefore, it is crucial to understand the subsurface structure and to determine the zones that are favourable for the exploitation of potential units. For these purposes, a geophysical study by gravity and electrical resistivity methods has been conducted.The gravity method consisted in 1365 gravity data points collected from uniformly-distributed stations. The Bouguer anomaly map of El Fahs plain and its surrounding areas provided information about the subsurface density variation and showed positive anomalies indicating raised zones and negative anomalies characterizing the depression zones. Quantitative treatment applied to Bouguer anomaly map highlighted new deep structures with details on their depths (up to 2800 m) and dips. These results demonstrate that the plain corresponds to a collapsed area limited by three fault systems E-W, NW-SE and N–S. Some of these faults are conform to the existing ones. However, a new N–S fault system is highlighted, which produced a new structural map.A study by vertical electrical sounding (VES) survey was conducted in order to provide details on El Fahs plain. It consisted of 114 Schlumberger VES with lengths of current line AB ranging from 400 to 600 m. Interpretation results of the VES shows that El Fahs plain is covered by a very heterogeneous and thick Quaternary filling deposits (48 m by place). The Mio-Pliocene deposits have a thickness that occasionally exceeds 190 m, and it is composed of conductive (<15 Ω m) and resistant (>16 Ω m) units. The underlying Upper Cretaceous is composed of conductive (<17 Ω m) and resistant (>20 Ω m) units.From a hydrogeology point of view, the gravity results show that the groundwater flow is strongly influenced by the basin morphostructure and the existing faults in the area. Furthermore, the VES results reveal that three normal faults are responsible for collapsing the central part of the plain and favouring groundwater flow in this direction.