Geophysical Study by Electrical Soundings of the Tartar Aquifer Unit, Boujdour Region, Morocco

The Tartar aquifer unit, is located at the SSO of the city of Boujdour, at a distance of nearly 86 km, and crossed (in its western part) by the National Road N1 connecting the towns of Boujdour and Lagouira passing through the vicinity of the city of Dakhla (PK40). It is exploited by rural settlements for domestic use (especially the inhabitants of fishing villages) and livestock watering, only through wells named Khtout Hobia (IRE 126/124) and Hassi Tartar known as Khtout Trayh (IRE 104/124). These wells have been tracked by a piezometric groundwater table and from 2011 to the present day. The interpretation of the electrical soundings in AB ≤ 2000 m allowed to differentiate the presence of two families of electrical soundings A and B, to establish the resistivity maps in AB = 200, 300 and 400 ihm∙m with qualitative aspects, to draw up the map of the isohypses of the roof of the intermediate Dt1 representing the impermeable floor of the aquifer and to highlight two types of discontinuities; electrical discontinuities corresponding to lateral facies changes (limit of erosion surfaces) separating the families A and B of electrical soundings and those corresponding to syn-sedimentary faults which structured the formations into horsts and grabens. The lithological sections of the existing water points and that of oil well 43-1 allowed the geological identification of the geoelectric layers highlighted by the electrical soundings diagrams. As a result, the sandstone and lumachelic formations constituting the aquifer are of Moghrebian-Pleistocene age represented by the resistant R (Family A), sometimes grouping, in its basal part, sandstone levels of the MiHow to cite this paper: Chibout, M., Benslimane, A., El Mokhtar, M., El Kanti, S.M., Faqihi, F.Z. and Gourari, L. (2020) Geophysical Study by Electrical Soundings of the Tartar Aquifer Unit, Boujdour Region, Morocco. International Journal of Geosciences, 11, 58-83. https://doi.org/10.4236/ijg.2020.113005 Received: August 21, 2019 Accepted: March 3, 2020 Published: March 6, 2020 Copyright © 2020 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
The Tartar area and its regions (Boujdour province) have been the subject of some geophysical campaigns, which had as an objective oil exploration where their results led to the realization of drillings such as 43-1 with a total depth of 3972 m (1961)(1962), and 47-1, located south of Tartar In the light of the results of these studies, which concerned surface and deep groundwater, the Sahara Hydraulic Region Directorate (HRD) and subsequently the Sakia El Hamra and Oued Eddahab Hydraulic Basin Agency (ABHSHOD) carried out several boreholes and wells, controlled by logs. The results of these water points were used to identify the geological properties of the geoelectric layers identified by electrical geophysics.
In addition to the geophysical studies mentioned above, there is the current geophysical sounding by boreholes and electric tomography, carried out in the Tartar area around the two wells in the Tartar region (126/124 from Khtout Hobia to the SW and 104/124 from Khtout Trayh to the NE). The study area is part of the rural commune of Jraifia, Boujdour province, Laâyoune-Saguia El Hamra region and falls within the Sakia El Hamra and Oued Eddahab Hydraulic Basin (ABHSHOD) action area.
In order to ensure better management of the water resources of the Tartar surface water table, the most heavily used in the region, it is advisable to better define the origin of the supply of this table, to have knowledge of its geometry and extension, with the aim of establishing a more reliable hydrogeological model. To do this, geophysics is asked to map the geological levels that could constitute the potential surface aquifer, known through the two wells of Khtout Hobia and Khtout Trayh, and to have an idea of the appearance of the roof of its impermeable floor. This is in order to better define areas of high hydrogeological potential.
In the following, we will focus only on the results of the qualitative and quantitative interpretation of the 140 short line length electrical soundings AB ≤ 2000 m and the reinterpretation of some long line length electrical soundings AB = 10,000 m.
The maps used for the establishment of the figures are the topographic maps of the Assaq wadi at 1/100,000 and the Spanish geological map No. 10 at 1/200,000.
Access to the field was difficult due to the presence of cliffs located east of the study area, talwegs, wadis and sand dunes.
The topography of the study area is monotonous and is summarized in a series of plateaus slightly inclined towards the Atlantic Ocean ( Figure 1) except for the presence of the dune cordon oriented SSO-NNE sometimes separated by slope breaks and by small discontinuous rock cuestas.
The study area is characterized by the presence of many Grarets, of relatively small size, the most important being the one called Graret Swedi, of vast size, located around Khtout Hobia.
The study area is characterized by a coastal Saharan-type climate in warm winter. In the region of Laayoune, the average annual rainfall is 44.55 mm. The average monthly temperature varies between 17.5 (January) and 26.6˚C (August) [1] [2].

Study Area
The study area (Hassi Tartar) is located in the central part of the sedimentary basin of Laayoune-Dakhla. This vast marginal sedimentary basin was formed on the northwestern edge of the African craton when the Atlantic Ocean began to open in the Triassic period. It is part of a coastal geological context formed during the geological events responsible for the stratification of the Tarfaya-Boujdour-Dakhla sedimentary basin, it was the regression and transgression of the Atlantic Ocean. It consists of the outcrops of lithological formations from the Tertiary to the Quaternary age described under a simplified nomenclature International Journal of Geosciences "Meso-cenozoic Tarfaya-Laayoune-Boujdour-Dakhla Basin" [3] [4]. Several authors have worked on various aspects of the regional and structural geology of this sedimentary basin [5]- [13] based on data from oil wells and seismic soundings carried out by the Compagnie Générale de Géophysique [14]. These different authors were able to describe the sedimentary basin since the Triassic and recent Quaternary periods by defining the structural geology and litho-stratigraphy of the various geological formations identified by hydraulic and oil drilling. According to the technical note SEGM/n˚ 107 (Stratigraphic correlations of drilling in Western Sahara) and the Spanish maps at a scale of 1:200,000 and 1/1,000,000 ( Figure 2 and Figure 3), regional geology is characterized by two sedimentary cycles: the Precambrian and Primary (from Precambrian to Carboniferous) and Meso-Cenozoic (from Triassic to Quaternary).
Locally the Tartar region ( Figure 3), part of the coastal zone, is covered mainly by tertiary age Quaternary lithological formations that begin at the top with Holocene formations (Quaternary Roof) formed of dunes, beach sands, evaporated silts and carbonated sandstones that rest on Mogrebian-Pleistocene formations (Quaternary base) formed of sandstone and lumachelles (Marine Pliocene). These formations are directly based on Neogene formations known as Ugranat formations (sandstone, lumachelles and carbonate cement microconglomerates) of Miocene age formed which overcome the sandy marl marls. International Journal of Geosciences   Figure 8). In addition, the quality of water flowing in sandy and sandstone formations is classified according to Table 1 below as good to average.

Methods and Materials
The geophysical method used in this work is mainly the direct current geoelectric method, essentially vertical electrical soundings (VES). This is the non-destructive subsurface exploration method, is widely used in hydrogeology and engineering geology. The principle of this geoelectrical method consists in measuring the apparent electrical resistivity, expressed in ohm•m, of the subsoil layers from the measurement of the difference in potential between two reception electrodes generated by the circulation of the current injected at the surface through two emission electrodes. The resistivity value, according to [16] [17], depends essentially on the water content, water mineralization, clay content and granulometry.
Geoelectrical methods have generally been described in several theses, articles and books. Among the latter are those of [18] [19] [20]. For vertical electrical sounding (VES), the emission (A, B) and reception (M, N) electrodes are distributed in the field according to the Schlumberger device ( Figure 9), it is the most suitable device for geoelectrical methods applied to hydrogeology, where the distance between the M and N electrodes is significantly smaller than that between the A and B electrodes, in order to minimize the potential deduced from telluric and vagrant currents. Vertical electrical soundings make it possible to obtain the vertical succession of resistivities in the center of the device at different depths.
It is recalled that the geoelectric method is affected by the principles of equivalence and deletion. All measurements obtained from the electrical soundings, in each selected length, are plotted on bilogarithmic scale diagrams. GeoStudi's SEVplus software will then, after smoothing the curve and inverting the data, obtain a mathematical model giving the depths of the roofs and walls of the different layers plumbing the centre of the electric borehole. This model will be readjusted by the geophysicist in order to have a better estimate of the true resistivity and thickness for each layer while remaining within the limits of equivalence based on the information gathered from the interpretation of the parametric soundings carried out on the boreholes and on the outcrops.
The hardware used in this work, is GF-Instruments' multichannel ARES II resistivimeter and GEOSTUDI's licensed data transfer and processing software (SEV plus ), was used to acquire vertical electrical boreholes, this apparent resistivity meter and processing software, are available from AFRICA GEO-SERVICES.
The situation of the electrical soundings drilled in the study area, wedging boreholes, the two named groundwater wells (Khtout Hobia and Khtout Trayh) of interest to this area and also the geo-electric sections are shown in the following Figure 10.

Review of Electrical Soundings
The preliminary interpretation of the electrical soundings is made as the electrical soundings are acquired in order to ensure the correct unwinding of the device and to have a better quality of the measurements. At this stage, it was possible to implement a reconnaissance borehole with a total depth of 67 m at the plumb of the 6TA5 electric borehole (X = 525,673, Y = 2,809,176, Z = 439 m).
The sections of interesting boreholes, both hydraulic and petroleum, were used to identify the geological geological layers of the geoelectric layers highlighted by the diagrams of the electrical boreholes. To this end, it was possible to carry out some electrical boreholes located in the vicinity of boreholes 126/124 (132/124) of Khtout Hobia, 104/124 (179/124) of Khtout Trayh, 265/124, 68/124 and the oil well 43-1.
The interpretation of the diagrams of the all electrical boreholes, carried out with a line length less than or equal to 2000 m, revealed the presence of an alternation of resistant and conductive layers R0, C0 (A0), R (Bc0, Dt0), Dt1, Dt2 directly supporting the conductive deposit noted C1. This alternation represents the sandy-sandstone formations in which are interspersed clays, conglomeratic levels and limestone banks of Quaternary, Neogene and Eocene age in its highest part. As for conductor C1, surmounted by this alternation, it corresponds to the clay and silty formations with the presence of flint belonging to the median Eocene starting point. This conductor C1, well-marked on the diagram, is well developed and reaches a thickness of 250 m in line with the 20TJ23 standard borehole diagram plumbing oil drilling 43-1 ( Figure 11) with a line length AB = 10,000 m. This conductor surmounts the R2 + R3 resistant assembly responsible for the final ascent of the curve whose roof reaches a depth of 910 m coinciding, in its highest part with the carbonate formations of the Eocene base and the silty one with the presence of Paleocene flint. In its basal part, it is attributed to sandy-sandstone formations with upper Cretaceous clay inter-leavings and dolomitic levels and sandy-sandstone formations with lower Cretaceous conglomeratic levels interleaved ( Figure 11).
The interpretation of the electric holes made it possible to establish 09 geoelectric sections and 03 interpretive maps (map of the families of the electric International Journal of Geosciences For the following, we will only be interested in the covering of the conductor C1 formed by the alternation of resistant and conductive layers where the resistant level R (sandstone and sandstone), well-marked on the diagrams of the electric boreholes, is of interest since it is admitted aquifer in the area of the Khtout Hobia and Khtout Trayh wells.
The correlation between the different electrical soundings made it possible to differentiate between two families of electrical soundings A and B (Figure 12), each characterized by a well-defined electrical response. The difference between these two families lies in the geoelectric behaviour of the resistant level R and its sandy-sandstone covering. The eastern part of the study area is dominated by International Journal of Geosciences    Examination of the diagram of the 13TA6 electrical sounding (Figure 13) shows the presence, in its lower part from the resistor R, of the same succession of resistive and conductive layers R, Dt1, Dt2 and C1 as that of the previous 9TA6 diagram (Figure 13). However, in particular, there is a lateral change of facies within the overlap of the resistant level R, especially the layer above it where the intermediate level Ao highlighted by the previous diagram has manifested itself electrically as a conductor and is none other than the C0 conductor with a resistivity of 18 ohm•m. The presence of marl past and fine sands instead of coarse ones at the Holocene base could contribute to a decrease in resistivity from 55 ohm•m (level A0 -SEV 9TA6, Figure 13) to 18 ohm•m (level C0-SEV 13TA6, Figure 13) in this case.
In the case of the 3TA4 electric borehole diagram ( Figure 14) adjacent to Khtout Hobia wells, the conductor C0 overlying the resistance level R has a resistivity value of 55 ohm•m close to that obtained from the intermediate level A0 in way of the 9TA6 electric borehole diagram ( Figure 14).
Comparing the three diagrams of the electrical boreholes mentioned above, especially at the resistant level R which attracts attention, we observe a remarkable evolution of the resistivity which decreases considerably from South to North. It goes from 262 (SEV 3TA4) to 91 (SEV 13TA6) passing through the value of 221 ohm•m obtained at the right of the 9TA6 electric borehole.
This decrease in the resistivity of the resistant R from south to north could be related either to one or to the combined effect of the following factors:  The change in the facies of resistant R from a karstified sandstone, lumachelles and carbonate sands facies in the South to sandy facies with marly sands;  Increasingly high grade from south to north;  The plunging of the roof and wall of the resistance fighter with an increasingly important development of its roofing. This dive is illustrated in Figure  14.
2) Electrical sounding 6TA5 (Figure 15) Following the preliminary interpretation of the electrical holes made just at the end of the acquisition of the field measurements, it was possible to choose the reconnaissance drill site 265/124 directly above the 6TA5 electrical hole, belonging to family A of the electrical holes, where its diagram (Figure 15) shows the same sequence of electrical layers as the 9TA6 electrical hole diagram ( Figure 13). This electrical borehole is located in a high zone where the roof and wall of the resistant R, admitted aquifer, are located respectively 15 and 37 m deep. Its true resistivity obtained is 112 ohm•m with a thickness of 22 m. This borehole is dry.
3) Electrical sounding 3TA6 (Figure 16) This electrical borehole is located approximately 2 km SE of the 126/124 Khtout Hobia well. Its diagram (Figure 16) is part of family A of electrical boreholes with the same succession of resistant and conductive layers as the above-mentioned electrical borehole diagrams (9TA6 and 6TA5). International Journal of Geosciences

Family B of Electrical Soundings (Figure 17)
The diagrams of the 5TA7 and 7TA3 electrical boreholes ( Figure 17  The sandy formations of the Moghreb-Pleistocene represented by the Resistance R would seem to have suffered, at first, a significant continental erosion where the top part of the sandy sand formations was stripped and then covered by the sandy marls constituting the conductor of the bilayer Bc0 during a marine advance (Regression). This conductor has a resistivity of 13 ohm•m and a thickness of 18 m, while the resistance of the base of the bilayer Bc0 is 62 ohm•m with a thickness of 18 m.

Analysis of the Apparent Resistivity Maps in AB = 200, 300
and 400 m (Figures 18-20) They are based on the gross apparent resistivity values (ρa) recorded at each electrical borehole measuring station for line lengths AB = 200 m, AB = 300 and AB = 400 m. They have a qualitative aspect taking into account the factors that influence these measurements such as the variation in altitude between electrical boreholes, the resistivities of surface soils and the lateral variations in facies.
The 03 maps (Figures 18-20) reflect the electrical behaviour of the resistant level R and, in places, that of the underlying intermediate level Dt1, whose roof would sometimes correspond to sandy-gravel formations, especially in subsided areas, and sometimes to sandy marls outside these subsidised areas.
Simultaneous analysis of these three maps shows, overall, the presence of the following three apparent resistivity ranges:

Analysis of Geoelectric Sections
The electric boreholes were carried out along the parallel profiles of direction substantially NE-SW more or less parallel to the Atlantic Ocean.  Analysis of this map shows the following:  Three collapsed zones marked G1, G2 and G3 limited by the 15 m isohypse curve. These areas are characterized by the presence of a low to medium gradient of isohypses. The first G1 is centered on the 12TA1 electric borehole (−19 m), the second is centered on the 5TA2 electric borehole (−20 m) and the third is centered on the 2TA4 electric borehole (−22 m).  Two high zones noted H1 and H2 where the first, of vast extent, is located between the isohypsis curves 15 and 30 m, the second, of rather reduced extension, is limited between the isohypsis curves 30 and 60 m. They are characterized by the presence of a low to medium gradient.
The strong gradient of the isohypses, reflecting the sudden change in the rating of the intermediate level Dt1, is observed between the collapsed zones G1, G2 and G3 and those high H1 and H2 showing the presence of electrical discon-tinuities affecting the coverage of the conductive level C1 (Eocene formations).
These discontinuities, already put in place by the geoelectric sections and the correlation between the various electrical holes, would correspond to synedimentary faults that have compartmentalized the structure into a system of horsts H1 and H2 and grabens G1, G2 and G3.

Conclusions
The The main results obtained as mentioned above are illustrated by the synthesis map shown in Figure 23.
At this stage, it could be the presence of two aquifers; one aquifer enclosed by the calcareous sandstones and the lumachellic with a salinity close to 0.8 g/l located at the level of Khtout Hobia, another aquifer enclosed by the sands with a salinity close to 0.4 g/l located at the level of Khtout Trayh. These two aquifers are located respectively inside the G3 and G1 gravels.
In terms of perspectives, it is suggested the establishment of other water points within the G1, G2 and G3 gravels. Other complementary geophysical studies should also be carried out in the North and South to delineate the two aquifers in order to develop a conceptual model of these aquifers.