Geophysical Contribution to the Determination of the Limit between Laayoune and Foum El Oued Aquifers, Origin of Supply Sources of Oued Sakia El Hamra, Laayoune Province, Morocco

Laayoune and the Foum El Oued aquifers are in hydraulic communication only at the level of Oued Saguia El Hamra. The present study has accordingly made use of all the hydrogeological, hydrological, geological and geophysical data that preceded the watershed of Oued Saguia El Hamra in its downstream part. These data are by no means omplementary with the objective of having a better understanding of the boundary line between Laayoune and Foum El Oued aquifers and the origin of feeding the sources of Oued Saguia El Hamra. This study will focus only on the previous geophysical studies where a reinterpretation of electrical soundings has proved useful as a result of the recent well-logging results. It makes it possible to highlight the presence of a significant rise in the truncated marly substratum of Oued Saguia El Hamra and depressions (left and right banks) which could correspond to stream channels or depressed areas. At the level of the Wadi bed, there has been a regular immersion of the conductive level roofs from east to west towards Foum El Oued favoring the flow of wastewater from the zone and spraying the brackish water sources towards the groundwater of Foum El Oued. In the light of the reinterpretation of electric polls, plus as well as the geophysical surveys by electrical tomography and high definition made at the right and left banks of the Oueed Saguia El-Hamra, it was possible to verify the existence of dry ridge separating the two webs of Laayoune and Foum El Oued and stream channels or depressed areas of the left bank for drainage of brackish water to sources located along Oued Saguia El Hamra. The true resistivity models tomography profiles confirm the presence of the backbone at the left and right banks and the graben of the left bank for underground water drainage of the web Laayoune to sources welling the river Saguia El Hamra. They reveal the presence of a quaternary plio-cover (coastal platform Moghrebian) as being heterogeneous and affected by many electrical discontinuities, particularly in the level resistant R2 (coquina sandstone). These discontinuities could correspond to lateral changes in facies and/or synsedimental faults compartmentalizing the plio-quaternary formations into a system of Horsts and Grabens that relies on the whole (D1, Cs) attributed to formations from the Miocene to the Upper Cretaceous. The contours of the map show the distribution of the three families of electrical soundings A, B and C limited by two main electrical discontinuities D and M. East of discontinuity million, this map reflects the look of the wall of the Pliocene-Quaternary resting on the impermeable upper Cretaceous floor. To the west of this electrical discontinuity M, the map reflects the behavior of the roof of marl deposits of Miocene age.


Introduction
Many geophysical (seismic, gravimetric, aeromagnetic and especially geoelectric), hydrological and hydrogeological studies were performed at the basin of Sakia El Hamra and Oued Eddahab (South of Morocco) [1] [2] [3] [4]. Each of these studies was developed to solve a specific problem in a well-defined area.
The majority of geophysical studies, including particularly the geoelectric (50 studies) and seismic (about 400 Km) aimed to map geological levels that could be potential aquifers.
In light of the results of these studies, the Regional Direction of the Hydraulic of Sahara (DRH) and subsequently the Basin Agency of Sakia El Hamra and Oued Hydraulic Eddahab (ABHSHOD), performed several drill holes, controlled by logs for the release of water resources for potable water supply, irrigation, industrial and for watering livestock [5] [6] [7].
In these previous studies, a geophysical electrical tomography was performed at the level of the left and right banks of the Oued Saguia El-Hamra with the aim of verifying the existence of the dry dorsals between Foum El Oued and the two plies Laayoune and the fluvial channels of the left and the right sides for drainage of brackish water to the sources of the river Saguia El-Hamra.
In addition to the drop in water levels, this overuse of the groundwater is ac- ter/salt-water (Upconing) interface of the deep salt water [8].
For a sustainable management of the underground water resources of these aquifers, namely that of Foum El Oued as the most requested in the region, this aquifer requires the update of its geometry and the determination of its limits in order to establish more reliable mathematical models. Hence, we will focus mainly on the eastern boundary between the layers of Foum El Oued and Laayoune. The understanding of this limit will allow to better define the origin of the groundwater recharge of Foum El Oued and the possible contaminations by the sewage of Laayoune city and the brackish waters of Laayoune aquifers.
The area under study, which spans the bed of Oued Saguia el Hamra and its left and right banks, is located between the spreading zone and the dunes located east of Foum El Oued ( Figure 1). It is a part of the urban district of Laayoune, Laayoune province, Laayoune-Saguia El Hamra region and falls within the action area of the Hydraulic Basin of Saguia El Hamra and Oued Eddahab (ABHSHOD).
The topography of the study area is monotonous and can be summed up as a series of gently sloping plateau towards the Atlantic Ocean (Figure 1), except for the presence of the SSW-NNE-oriented dune cordon that is sometimes separated by slope failures and by small discontinuous rocky cuestas. The only reliefs observed in the area are the followings:  NNE-SSW oriontation dune cords located west of the study area;  Flanks of the left and right banks of the Oued Saguia El Hamra, where the reliefs reach more than 60 m at the level of Laayoune and disappear in the area of Foum El Oued. From a hydrological background, the main hydrographic element of the Laayoune region is represented by Oued Saguia El Hamra, and it is characterized by discontinuous flows depending on the intensity of rainfall and floods.
The study area is characterized by a warm coastal Saharan-type climate. In Laayoune region, the annual average precipitation is 44.55 mm. The monthly average temperature varies between 17.5 (January) and 26.6˚C (August) [3] [8].
Geological and Hydrogeological Framework a) Regional geology The basin under study is surrounded by four structural geological units in the Sahara:  The Anti-Atlas massif;  The Reguibat Ridge;  Mauritanides;  The Tindouf basin.
The syncline of NW Africa has developed at the boundary between the Atlantic Ocean and the stable African Shield since the recent Precambrian. To the west of the Hercynian folded central chain, a thick series of Mesozoic and Cenozoic sediments were deposited in the Sahara basin and rest on the Precambrian or the Paleozoic age bedrock. The maximum onshore deposit is located in the Daoura region with more than 10 km thick. Laayoune basin is a monoclonal shallowly dipping towards the west [9]- [13].
The highly tectonized Hercynian folded central zone is highly compressed and is overlapped to the east with anticlinal and synclinal structures elongated in the N-S direction and clamped in the transverse direction. This is the area that separates the Tindouf basin to the east of the Laayoune basin to the west. These two basins are weakly tectonized.
From a structural background the tow major tectonic directions that control the structuring of the Laayoune basin [14] are as follows:  Direction ENE-WSW of the Anti-Atlas and the Reguibate chain.
 Direction NNE-SSW of the North West Africa geosyncline.   The left and right banks consist of three main series:  A limestone series starting from top to bottom with calcarenites which then tunrs bioclastic and lumachellic.  A micro-conglomeratic sandstone series with sand passages in the center that are well developed north of the right bank and become more clayey.  A limestone lumachellic series.
These formations are based directly on marly bedrock of Miocene to Late Cretaceous ages [19] [20] [21]. In the bed of Oued Saguia El Hamra, the formations mentioned above are eroded, up to the marl bedrock and replaced by recent alluvial deposits consisting of sands, clays, flint marls resting on gray marls plastic (cuts lithological drilling 962/120 and 963/120).

c) Hydrogeology
The main superficial aquifers formed by the Plio-Quaternary formations con-   aquifer is of the order of 5 m with a water level located around 40 m deep. The water released from the aquifer is brackish with salinity close to 7 g/l [23].
The aquifer of the left bank circulates in lumachellic limestones rests on the marly substratum. Water output is brackish with salinity ranging from 2.5 g/l in the east to 9 g/l in the west.

Methodology
Geophysical methods employed in this work are mainly DC geoelectrical methods based primarily on vertical electrical soundings (SEV) and electrical tomography profiles (ERT). These last two methods, known as non-destructive subsurface exploration methods, are widely used in hydrogeology and engineering geology. The aim behind adopting these geoelectric methods is to obtain the electrical resistivity expressed in ohm•m, the layers basement from the measurement of the potential difference between two reception electrodes that is generated by the circulation of the current injected at the surface by two emission electrodes. The value of the resistivity, according to [24] [25], depends mainly on the water content, the mineralization of the water, the clay content as well as the granulometry.
These geoelectric methods have been extensively described in many theses, articles and books. Among these, we cite those of [26] [27] [28].  significantly lower than that between the electrodes A and B, with the aim of minimizing the potentials derived from telluric and vagrant currents.
In electrical tomography (ERT), we employ a large number of electrodes (72 or 96) at fixed distance (10 m for example) connected to a multicore cable 710 to 950 m in length and placed in a pattern ( Figure 5(b)). A laptop, in which the measurement sequence is programmed, is connected to a communication box and automatically selects the electrodes used for the current injection and the potential measurement according to the Wenner-Schlumberger device with the aim of having a better resolution of horizontal and vertical structures. The current is injected via a resistivity meter and the Switches allow executing a previously chosen measurement sequence.
While the electrical soundings allow to obtain the vertical succession of resistivities at the center of the device at different depths, the electrical tomography allows to obtain a continuous geoelectric imaging of the subsoil, i.e. a pseudo-section in resistivity as a function of the depth where the distribution of the resistivity varies vertically and horizontally along the profile [29] [30]. This electrical imagery would represent the combination of soundings and electric tows.
In the electrical tomography method, in which the multi-electrode cable and the number of electrodes are limited, it is necessary to perform the acquisition in "Roll-Along" mode [31] [32] [33]. Put differently, this performance aims to extend the tomography profiles by adjusting their length depending on the desired objective. This technique consists of covering a previous profile with a new profile ( Figures 6-8). In this case, it was possible to apply the covers 2/3, 5/8 and 3/4, where the depth of investigation obtained is significantly greater than the depth of the roofs of the Miocene marl to Upper Cretaceous. The covers used in the framework of this work are illustrated in Figures 6-8 below.   It is important to note that geoelectric methods (sounding and tomography) are affected by the principles of equivalence and delete principles.
In the electrical survey, all the measurements obtained for each chosen length will be plotted on charts to bilogarithmic scales. The GeoStudi's SEV along with the software will allow, after the smoothing of the curve and the data inversion, to obtain a mathematical model that gives, in turn, the depths of the roofs and walls of the different layers directly above the center of the electrical sounding. This model will be readjusted by the geophysicist in order to have a better estimate of the true resistivity for each layer while remaining within the equivalence limits.
In electrical tomography, after the transfer of apparent resistivity measurements acquired, the results obtained will be represented along a pseudo-section in apparent resistivity as a function of the depth. Data inversion using Res2dinv software will provide a mathematical model of true resistivity [34].
The lithological sections with the logs were used for calibration and geological identification of the geoelectric layers put by the diagrams of the electrical soundings.
In order to confirm the results of the interpretation of the electrical soundings, it was possible to carry out five (05) electrical tomography profiles ( Figure  9) with a total length of 7870 m to the right of the significant recovery zones. The marly substratum roof is located on the right and left banks of Oued Saguia El Hamra and on the right of the depressions of the left bank of Oued Saguia El International Journal of Geosciences   time by 03 bobines of the 360 m. This is in order to obtain the continuous geoelectrical imaging of the under-ground with an investigation depth of 110 m, along a profile of 1670 m (Named LAA3 and LAA4 profiles).
3) Roll along Cover of the 3/4 (6/8): Pseudo-section of electrical tomography of 08 bobines of the 120 m each with a length of 950 m (96 electrodes). Extended each time by 02 bobines of the 240 m. This is in order to obtain the continuous geoelectrical imaging of the under-ground with an investigation depth of 132 m, along a profile of 1670 m (Named LAA5 profile).
Figures 6-8 and Table 1 illustrate the different roll-along covers used for this work.
The GF-Instruments ARES II multi-channel resistivity meters and licensed software for data transfer and processing (GEOSTUDI 2D mathematical inversion, Res2dinv, SEV plus ) were used for the acquisition of new electrical soundings and electrical tomography profiles; this hardware and software is owned by AFRICA GEO-SERVICES.
The situation of the electrical soundings, the profiles of electrical tomography, the interesting water points of the study area (wells and boreholes) and the geoelectric sections are represented by Figure 9.

Results and Interpretation
The interpretation of the electrical soundings and the correlation between them The diagrams of these electrical soundings ( Figure 10) are representative of these electrical sounding families.

Correlation of Electrical Soundings at the Level of Discontinuities D and M
The electrical discontinuity D deduced from the correlation between the electrical soundings corresponds to a change in the geoelectric behavior of the conducting substratum Cm and the lower part of its cover ( Figure 10)

Examination of Electrical Soundings
The western zone of the D discontinuity, characterizing the eastern part of the    Basically, a conductive substratum Cs is attributed to the gray plastic marls whose roof is located at 63 m depth.
On the standard drilling pattern 6LS30 at hole 963/120 (Figure 12

Examination of Electrical Tomography Profiles
Following the results obtained from the quantitative interpretation of the electrical soundings the five electrical tomography profiles LAA1, LAA2, LAA3, LAA4 and LAA5 were made (Figure 9) to confirm these results especially at the level of the ridge (high zone) and the brackish water drainage zones of Laayoune aquifer towards the sources (low zones), located north and south of the town of Laayoune, east of the electrical discontinuity M,The contribution of this geophysical method by electrical tomography has been widely proven in several cases from geophysical survey to the structural mapping of aquifers that are superficial or semi-deep.
The first three profiles LAA1, LAA2 and LAA3 concern the ridge of the left bank (h1), while the LAA 5 addresses the dorsal of the right bank (h3). As for the profile LAA4, it concerns the low zone d1 (graben) belonging to the left bank.  East of the electrical discontinuity M, the examination of the pseudo-section in true resistivity obtained along the LAA4 profile that was carried out on the graben d1 (geoelectric section RG, Figure 17) shows the presence of plio-Quaternary cover (coastal marine deposits) as very heterogeneous and are affected by many electrical discontinuities that correspond to synsedimentary faults. The latter compartmentalized the conducting substratum Cs (black marl of the Upper Cretaceous) and its plio-quaternary cover into a system of horst and grabens.
In particular, we will consider graben d1, located between stations 52 and 102, as

Analysis of Geoelectric Sections
Three geoelectric sections have been established (Figures 18-20) from the interpretation of the electrical soundings. The sections RD and RG concern the right and left bank, while the SH concerns the bed of Oued Saguia El Hamra. These

Examination of the Isohypse Map of the Wall of the Plio-Quaternary Formations
The quantitative interpretation of the electrical soundings allows to draw the map of the isohypses of the wall of the plio-quaternary formations that represents the moghrebian slab (Coastal marine deposit) and gives an idea on the pace and the electrical behavior of the roofs of the conducting levels Cm, Cs,   East of the electrical discontinuity M, the map reflects the shape of the roof of the conducting substratum Cs attributed to the marls of the Upper Cretaceous.
The latter represents the impermeable floor of Laayoune aquifer in this sector.
The latter is marked by the presence of two low zones d1 and d2. Their axes are oriented substantially SW-NE towards the sources of Oued Saguia El Hamra.
These low areas correspond to grabens that will serve as drainage channels for underground water for the supply of springs that drip at the left and right banks of the wadi.
Other low areas (grabens) such as d3 and d4 are observed respectively east of the lower zone d1 and at the NW foot of the ridge of the left bank the discontinuity M and west of the electrical discontinuity D. These low areas serve as draining water to the sources of the wadi and to the water table of Foum El Oued.

Discussions and Conclusions
In the province of Laayoune, the majority of geophysical studies in particular, geoelectric and seismic aimed to map the geological levels likely to constitute potential aquifers. According to previous studies conducted by the DHR and ABHSHOD, the geophysical study by electrical tomography profiles was carried out on the left and right banks of Oued Saguia El Hamra to verify the existence of the dry dorsal that separates the two plains of Foum El Oued and Laayoune and the lower zones of the left and right banks which serve the drainage of brackish water towards the sources of Oued Saguia El Hamra.
Overall, this shows, in particular, the presence of the following results arising from the interpretation, reinterpretation of the soundings and the analysis of the electrical tomography profiles confronted with the lithological sections of the boreholes and the logs. These results are represented by the map of the isohypses of the wall of the Plio-Quaternary formations ( Figure 21).  The appearance of the roof of the impermeable floor of Foum El Oued aquifers, in its eastern part, and Laâyoune corresponding to the Miocene marls at the level of the Foum El Oued aquifer and Miocene marls at the Upper Cretaceous level. Layers of Laayoune;  The geometry of the Plio-Quaternary aquifers of Foum El Oued (eastern part) and Laayoune (western part);  The electrical discontinuity denoted by D which corresponds to a change of facies of the roof of the conducting substratum Cm and of the lower part of its cover limiting the extension of the Foum El Oued aquifer to the east;  The electrical discontinuity marked M which would represent the eastern limit of the Oligo-Miocene tectonic pit limiting the Laayoune brackish groundwater to the west;  Other electrical discontinuities affecting the Plio-Quaternary formations International Journal of Geosciences corresponding to lateral facies variations;  At the left and right banks of Oued Saguia El Hamra, rising marly conductive substratum corresponding to the dry dorsal axis substantially SW-NE. These lifts, bordered on the west by the discontinuity D and on the east by the discontinuity M, could represent the boundary between Laayoune and Foum El Oued aquifers, with the exception of Oued Saguia El Hamra and its banks or its ascents are absent;  At the level of the Oued Saguia El Hamra bed, a monoclinale drawn particularly by the roofs of the drivers C1 and Cs plunging regularly towards the catchment field of Foum El Oued. At this level, we are witnessing the absence of the ridge favoring the feeding of the Foum El Oued groundwater by the flood waters of Oued Saguia El Hamra and the contamination by sewage and brackish waters of Laayoune groundwater. This contamination has contributed to a considerable reduction in the resistivity values of the sandy past (particularly the r2 level) allowed aquifers;  Two low areas located to the south and north of Laayoune d1 and d2 and elongated along the SW-NE direction which correspond either to grabens which seem to feed respectively the sources of the left and right banks of Oued Saguia El Hamra. At the ridges h1 and h2, the resistant level R2 corresponding to the lumachellic sandstones of the plio-Quaternary base are affected by synsedimentary faults that contributed to the fracturing and alteration of the lumachellic sandstones and subsequently to their karstification. This favored the birth of the paleodrainages within the ridge, which served to circulate water from the Laayoune groundwater to the Foum El Oued groundwater across the ridge, thus justifying the presence of the brackish groundwater of Foum El Oued. At present, the communication between the two layers is only through Oued Saguia El Hamra and when the ridge is dry.
In perspective, it is suggested to update the Foum El Oued groundwater model and establish the Laâyoune groundwater knowing that the eastern limit of the Foum El Oued groundwater, which is the subject of this study, has been well defined and the limit West is imposed by the Atlantic Ocean. The North and South remain to be determined by the realization of other geophysical studies at the Oligo-Miocene pit. This is for the sake of better optimization and management of water resources.