Geophysical data combined with geological and hydrogeological data were analyzed to characterize the geometry of Oued El Hajel and Ouled Asker deep water tables (Sidi Bouzid). The obtained results allowed refining the geostructural schema by highlighting the individualization of the NE-SW underground convexity of Ouled Asker and the anticline of axis Es Souda-Hmaeima and Ezaouia on either sides of two hydrogeological thresholds. The geometrical analysis determined the spatial extension of Ouled Asker and Oued El Hajel subbasins. The seismic cartography of semideep and deep reservoirs (Oligo-Miocene; Eocene and upper Cretaceous) associated with the main subbasins contributed to proposing hydrogeological prospect zones for a rationalized groundwater exploitation.
Agricultural development and rapid population growth contributed to an increasing demand for water resources. Consequently groundwater became a vital necessity in the Sidi Bouzid governorate in central Tunisia. This area is characterized by a semiarid to arid Mediterranean climate with irregular annual rainfall that does not exceed 350 mm/year and long periods of drought [
The Sidi Bouzid water table is one of the most important aquifers in Tunisia. However, the overexploitation of this aquifer requires a reassessment of deep water resources. Among these deep aquifers those of Oued El Hajel and Ouled Asker are of an ultimate interest. They are lodged into narrow NE-SW Atlasic synclines [
Location of the study area and the used geophysical data on the geological and structure map of central Tunisia [
In the present study, the characterization of Oued El Hajel and Ouled Asker water tables was developed and applied to deeper aquifers through the combination of geophysical, tectonic, and hydrodynamic data.
A detailed geometrical characterization of these subbasins by geophysical methods will limit each deep water table and help image the geometry of these aquifers such as the gravity survey method. In this study, gravity was selected in order to give a regional picture of the subsurface geology before making extensive surveys by the seismic reflection method.
For a better use of these deep aquifers and a rationalization of the future groundwater exploration in the arid area of central Tunisia, prognostic wells were realized based on seismic mapping and the estimation of water reservoirs depths.
Geophysical and hydrogeological studies consist of an integration between modern geophysics and conventional methods (rainfall, piezometric, lithology, hydrodynamics…). To define the hydrogeological setting of the study area, a topographic map covering the Jebel Es Souda and its surroundings has been presented (Figure
Topographic map of the study area.
Oued El Hajel and Ouled asker deep water tables belong to a zone characterized by a multiphase tectonic regime. The most important movements that affected this area were mainly compressive and extensional events [
The Ouled Asker water table is a part of Ouled asker syncline which is bordered by the anticlines of Jebel Mghila to the North-West, Jebel Labaiedh to the North, and Jebel Ezaouia to the North-East and to the south by Jebel Hamra. It is a multilayer aquifer system containing Pliocene-Quaternary shallow aquifers that reach 40 m of thickness. Semideep aquifers are also present in this area and they are made of Oligo-Miocene sand and sandstone. The deepest aquifers of this water table are mainly made of Upper Cretaceous-lower to middle Eocene carbonate.
The aquifers of Oued El Hajel and Ouled Asker deep water tables are made of Beglia formation (Langhian-Serravallian), Ain Grab formation (Langhian), Fortuna formation (Oligocene), Metlaoui (lower Middle Eocene), and Abiod formations (Campanian- Maastrichtian) (Figure
Lithostratigraphic column of Jebel Es Souda modified from [
The gravity data that has been used for the current study were obtained from the Office National des Mines (ONM). The free-air and Bouguer corrections were made using the sea level as reference and the average density was fixed to 2.4 g/cm3. In order to elaborate the complete Bouguer map of the zone, a grid was performed using the gravity data with a spacing of 1 point per Km2.
The seismic acquisition in the study area was conducted by the General Geophysical Company (GGC) for the benefit of the “Union Texas Tunisia” company in the region of Kasserine from March to August 1981. Four seismic lines L1, L2, L3, and L4 were analyzed and interpreted in this work. The latter crosses the main structures of the study area such as Oued El Hajel and Ouled Asker synclines which contain the target deep water tables (Figure
Bouguer gravity map of the study zone showing the positive and the negative trends of the anomaly.
These anomalies are separated by accentuated gravity gradients that indicate the presence of several discontinuities in the subsurface. These discontinuities are controlling the geometries of the different structures in this study area. The Bouguer gravity map illustrated in Figure The first NE-SW positive anomaly axis goes to −33 mGal and it corresponds to Jebel Hamra and Jebel Ezaouia. The second N-S to NNE-SSW positive axis reaches −18 mGal and it corresponds to Jebel Es Souda-Hmaeima. The third N-S positive axis corresponds to the N-S axis lineaments and it reaches −13 mGal.
The principal negative gravity axes are from the west to the east as follows (Figure A first NE-SW negative gravity axis delineates the Ouled Askar syncline and it reaches −46 mGal. The second negative axis reaches −31 mGal with a N-S to NNE-SSW major direction and it corresponds to the Oued El Hajel syncline.
Several upward continuations were performed based on the Bouguer gravity grid of the studied area. At a depth of 8000 m we notice the persistence of particular anomaly sources which are considered as the deepest and most rooted. Also a gravity gradient was identified separating both the negative and the positive anomaly axes which are almost vertical, and isogal curves were well drawn. Thus, the regional gravity maps were chosen (Figure
Regional gravity anomaly map of the study area.
Therefore the regional gravity anomaly will be subtracted from the Bouguer gravity anomaly in order to obtain the residual gravity map (Figure
Residual gravity map of the study zone.
The major lineaments and principle accidents affecting the Jebel Es Souda-Hmaeima and its neighboring structures will be highlighted using the HGGM map. These accidents affect the different hydrogeological reservoirs and water tables of Ouled Asker and Oued El Hajal according to two main directions: a first NE-SW to NNE-SSW direction associated with Jebel Hamra-Jebel Ezaouia, to Jebel Es Souda-Hmaeima, and to the underground bulge of Ouled Asker. A second N-S main direction associated with the N-S axis and the southern part of Jebel Es Souda. Other minor E-W directions were also recorded (Figure
The structural lineaments after the horizontal gradient gravity magnitude map (HGGM).
For a better subsurface architecture visualization of this area, gravimetric profiles such as the HGGM and the residual gravity anomaly were performed and coupled along with a hydrostratigraphic section. The latter was obtained by correlating the hydrologic wells W1, W2, and W3 (Figure
The hydrostratigraphic section (Figure
Subsurface structure of the study area using a hydrostratigraphic cross sections linking hydraulic well (W1, W2, and W3), horizontal gradient gravity magnitude, and residual gravity anomaly profiles.
Euler solution to detect contact zone (structural index 0, window 10
The different seismic profiles were calibrated and interpreted using previous studies [ the carbonate horizons of the Abiod formation (Campanian-Maastrichtian) and the Metlaoui formation (Ypresian); the sandstone horizon of the Oligocene Fortuna formation; the sandy and conglomeratic Middle Miocene (Langhian-Serravallian) horizons of the Beglia formation.
In this seismic section, box folds are identified as well as tilted blocks due to a set of major listric faults. Significant negative flower structures were recognized in the area resulting from grafting of second order faults on the major listric faults mentioned previously. Also horst and graben system were associated with this tectonic set. In the South eastern seismic line part, a decrease in the intensity of fracturing and thickening of geological layers is noted. Also an important gutter structure covering the Oued El Hajel syncline which is a basin marked by an intensive subsidence was identified (Figure
Interpreted seismic lines across the study area (L1 and L2).
Interpreted seismic lines across the study area (L3 and L4).
Hydrogeological sketch of Ouled Asker basin with W400 and W460 fictive wells.
Interval velocities calculated using the Dix formula (1955) were adopted, and profiles of average velocities were established for both lines 3 and 4 to estimate the depths of target aquifers and to provide several fictitious well (L3: W380; W480 and L4: W400; W460) in the center and edges of Oued El Hajel and Ouled Asker basins facilitating the exploitation of the main deep aquifers in this study (Figures
(a) W380 and W480 fictive wells associated with the L3 seismic line and (b) average velocity variation associated with L3 (black dashed line frame). (+) Depths estimated from the interval.
(a) W400 and W460 fictive wells associated with the L4 seismic line and (b) average velocity variation associated with L4 (black dashed line frame). (+) Depths estimated from the interval velocities.
Concerning the third seismic line L3 and after the distribution of depths relative to the average velocities, at a double time from 0.8 s to 1 s, the Upper Cretaceous is characterized by a velocity variation from 2700 (m/s) to 3030 (m/s) for depths up to 1215 m and at a less important double time from 0.3 s to 0.5 s, the velocities variations extend from 1600 (m/s) to 2000 (m/s) with depths ranging from 387 m to 520 m. It is also noted that the change in average velocities is proportional to the lateral variation of sedimentary sequences being structured in moderately folded to the SW higher areas, against sunken, and subsidence areas to the NE corresponding to the Oued El Hajel gutter (Figure
The first aquifer consists of a siliciclastic reservoir (Langhian-Serravallian): Ain Grab and Beglia formations (Figure The second aquifer is a sandstone reservoir (Oligocene-Lower Miocene (Aquitanian)): Fortuna formation (Figure The third aquifer consists of a carbonate reservoir (Upper Cretaceous: Campanian- Maastrichtian): “Abiod formation,” and Eocene: Ypresian “Metlaoui formation” (Figure
3D visualization of the Langhian-Serravallian (Middle Miocene) reservoir.
3D visualization of the Oligocene-Aquitanian (Lower Miocene) reservoir.
3D visualization of the Upper Cretaceous reservoir.
The integration of the geophysical, geologic, stratigraphic, and well data leads to the following.
(i) The geometrical characterization of the Oued El Hajel and Ouled Asker water table with NE-SW, N-S, and E-W lineaments.
(ii) The identification of target aquifers for the hydrogeological prospection which was based on the seismic analysis and interpretation. The obtained results helped in identifying the different units forming the aquifers: limestone levels (Upper Cretaceous to Lower Eocene)—sandstones and sand levels from the Oligo-Miocene to the Mio-Plio-Quaternary. In fact, the study area is mainly formed of large gutter structures characterized by a differential encasement from Oued El Hajel to Ouled Asker, strait anticlines and an overlapping tendency along the NE-SW à NNE-SSW fault corridors; based on the integration of different geophysical methods, it was possible to conclude that the study area is mainly formed by horst and graben structures, box folds and, also several flower structures with fan-shaped configuration. The latter present an important hydrogeological interest. The study area is also affected by an important faulting system with deep rooted faults that implicated the individualization of depressed area presenting potential zone in which it was possible to suggest a number of fictive wells.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The research was supported by Sahel-Kairouanais project (CERTE 2010–2013, resp., Pr. M. Bedir). The authors are very grateful to ONM and ETAP for the scientific supports.