Application of electrical geophysics to the release of water resources, case of Ain Leuh (Morocco)

. Being seen needs in increasing waters in our contry for fine domestics, manufactures and agricultural, the prospecting of subterranean waters by geologic and hydrogeologic classic method remains inaplicable in the cases of the regions where one does not arrange drillings or polls (soundings) of gratitude (recongnition) in very sufficient (self-important) number. In that case of figure, the method of prospecting geophysics such as the method of nuclear magnetic resonance (NMR) and the method of the geophysics radar are usually used most usually because they showed, worldwide, results very desive in the projects of prospecting and evaluation of the resources in subterranean waters. In the present work, which concerns only the methodology of the electric resistivity, we treat the adopted methodological approach and the study of the case of application in the tray of Ajdir Ain Leuh.


Introduction
The mountains are generally called "water towers of the planet". This qualifier is based on the precipitation potential associated with terrain and snow-ice retention. The barrier effect (according to moist air masses) and the altitudinal effect result in more abundant precipitation and snow-ice storage (depending on altitude) of precipitation; storage returned to surface and underground flows during the summer seasons. The mountain water appears in this scheme as an abundant and inexhaustible resource renewing from one year to another [14].
However, access to drinking water, especially in mountainous areas, is a major factor in economic development, and in improving the standard of living of populations and their stabilization in rural areas.
This study is part of securing the drinking water supply for the mountainous area at the Ajdir Ain Leuh plateau.
It is in this context, we present in this work the contributions of the application of the method of electrical soundings to the recognition of different aquifer formations. This will make it possible to define underground water potential and quantify the groundwater table in order to decide on the most appropriate option for mobilizing groundwater resources, taking into account productivities, quality as well as the constraints linked to the influence of water mobilization on water resources already exploited, including sources [5].

Description of the study area 2.1 Geographical setting
The Ain Leuh and Mrirt zones are located southeast of the Sebou basin between Ifrane and Khénifra on the main road n° 24 (Fig. 1), they belong to the Middle Atlas tabular plateau, framed on the north by the South-West corridor-Rifain, to the south by the High Atlas and the valley of Upper Moulouya, to the east by the valley of the Middle Moulouya and to the west by the Moroccan Meseta.
Morphologically, the Ain Leuh-Mrirt area is characterized by a tabular structure, more faulted than folded, by a monotonous relief implying the Middle Atlas pleated with accentuated folds, high mountains and deep marly depressions. It is a monotonous lithology of weakly folded liasic limestones which is responsible for this platitude.
The Causse, south of Ain-Leuh presents a typical landscape poljés adapted to the network of faults, this karst plateau, like all those of the same kind, has a very underdeveloped hydrographic network.
Water resources in these areas are used as sources by local people for irrigation of small irrigated areas and water supply. Generalized groundwater is used mainly for intensive and modern irrigation.

Climate context
The climate prevailing in the Middle Atlas is Mediterranean Type Mountain; it is characterized by a wet and cold climate. This particular climate of the Middle Atlas is due mainly to its altitudinal position, its geographical situation and its exposure to marine influences [15]. The clouds coming from the west give abundant precipitations (rain, snow) in contact with the Middle Atlas. The natural barrier that forms the chain Atlasique creates a dissymmetry on the climatic plan: the Atlantic facade exposed to the winds coming from the NW is more watered; as for the SE facade which is subject to the influence of the Saharan climate [16].
In general, the climate of the causses is a Mediterranean climate of mountain, cool and wet; it snows in abundance on the heights and the quantities of rain exceed 1100mm / year in Ifrane, but as we decent towards the south, the rains become less abundant.

Hydrogeological context
The Moroccan average atlasic causse has been the subject of several geological studies. Il est constitué de plateaux d'altitude comprise entre 1000 et 2200 m, constitués par la prédominance de formations carbonatées du Lias inférieur et moyen, découpées en blocs t consists of plateaus of altitude between 1000 and 2200 m, consisting of the predominance of carbonate formations of the lower and middle Lias, cut into fault blocks along several northeast-trending faults (Fig. 4). The Causse is limited to the west by the primary lands of Wadi Beht (Moroccan primary Meseta) and those of Tazzeka to the northwest. To the north, the limit is determined by Tertiary and Quaternary overburden in the South Rifain Corridor [17].
The dominant facies consists of dolomite and limestone and liasic, locally covered quaternary basaltic flows, resulting from recent volcanic activities (Figure 4). The dolomitic layers of the lower Lias lie in probable concordance on the Triassic (Colo, 1961). The middle Lias is characterized by limestones of different colors, in fact, the northern part of the Causse is essentially Dolomitic, that of the southern section is essentially limestone.
The Atlas Mean Causse includes two main aquifers of unequal interest; these are the aquifer of the quaternary basalts (Dolerites) and that of the dolomitic limestones of the Lias.

Basaltic aquifer
The basaltic aquifer of the Middle Atlas Causa can be subdivided into two hydrogeological sub-basins: Tigrigra (area of approximately 546 km²) and Timahdite (area of approximately 435 km²). These two compartments are separated by a watershed SSW-NNE, passing between the centers of Ain Leuh and Timahdite.
The approximate balance of the Quaternary basalt water table of the Middle Atlas is summarized in the table below. Water circulates in cracks, fractures and karst channels. These discontinuities favor the infiltration of rainwater, which is the main source of food for the Liasique aquifer.
The only feeds in the basin are meteoric; the emissions are constituted by the drainage of the waters of Ain Aicha-Hammad (n ° IRE 49/30), the Wadi Ain-Leuh and the sources Aioun-Akadous and toufstelt.
In the absence of piezometric monitoring of the Aïn Leuh-Azrou basin and large withdrawals from wells and boreholes, the water balance (of this basin) is assumed to be balanced. The drainage flow of Ain Leuh wadi and quaternary basalts (Tioumliline and Tagounite) is estimated (from this assessment) at about 41 Mm³ / year (1.3 m³ / s), which is of the same order of magnitude than that evaluated by the Sebou Basin Hydraulic Agency (ABHS) in 2003 (Table 2).

Principle of the ES method (fig. 4)
The electrical survey allows to study the variation of the resistivity of the soil with the depth [1-9-10-11]. Data acquisition in the field is as follows: A current of intensity (I) is sent through two electrodes A and B by means of batteries or a generator. Using a potentiometer (direct reading) or a recorder, the potential difference (V) between two measurement electrodes M and N is measured by increasing the length of line AB each time. The values of apparent resistivities obtained represent deeper and deeper depths [1-3-5-6-7-9].
The Schlumberger quadrupole, which has been applied in the context of the present study, is characterized by a small distance MN in front of AB in order to introduce the notion of electric field thus facilitating the theoretical calculations. The application of the ohm law makes it possible to calculate for each line lenner AB an apparent resistivity value (R) defined by the following formula [1-9-13]: R = K. V/I K being a coefficient that depends on the AMNB geometry.

Processing of ES diagrams
The electrical sampling diagram (ES) is obtained by plotting on a grid with a bilogarithmic scale the values of AB / 2 (in m) on the abscissa and the apparent resistivities (in ohm.m) on the ordinate [1-9-12-13].
The study of these diagrams, their comparison with each other and with abacuses as well as their analysis using computer programs make it possible to determine, in the majority of the cases, the vertical succession of "electric layers" as well as the true resistivity of each. This requires, of course, that the contrasts of resistivities between the different formations are sufficient [1- [9][10][11][12].
The quantitative interpretation of the electrical soundings conducted in the study area was done on a microcomputer using specific interpretation and processing software. These allow an analysis of smoothed curves with theoretical charts to eliminate the effects of possible taps that could harm the computer analysis [1-5-9-13].

Number of surveys
The geophysical measurements collected in the field were measured by a GRM 3000 resistivity meter. A total of thirty-nine vertical electrical soundings (39 ES) which shows the following distribution (Fig. 5) The 35 coverings were distributed over 7 profiles of 5 electrical soundings according to a 4Kmx4Km mesh ( Table  3).

AB line length
The totality of these electrical soundings was carried out with variable current transmission line lengths AB ranging up to 3 km depending on the depth and thickness of the geological formations traversed.

Electrical soundings diagrams
The analysis of the diagrams of the electrical soundings made it possible to highlight the existence of a deep resistant substratum noted Rp and a heterogeneous cover formed of alternation of conductive and resistant grounds.
The comparative study of the lithological layers revealed by the rigging drilling with the horizons detected on the diagrams of the electrical soundings, made it possible to establish geoelectric correlations. The results obtained led to distinguish three main families of electrical soundings ( a) Family 1: Table 5 shows the geoelectric characteristics of electrical soundings belonging to this family: These electrical soundings were carried out on the outcrops of the lower Jurassic carbonate formations (lower Lias). The analysis of the diagram of these electrical soundings made it possible to highlight two deep resistant levels having resistivities of the order of 1420 Ohm.m and 5000 Ohm.m (figure 6).

b) Family 2:
This family is characterized by the thickening of the level coverage. It consists essentially of alternating conductive and resistant levels of the lower and middle Jurassic whose total thickness is of the order of 150m ( Table 6). The analysis of the diagram of this electrical survey allowed to individualize a succession of electric grounds (RT, Cp and Rp). The deep resistive RT substrate with an electrical resistivity of 2000 Ohm.m corresponds according to the geology of the region and the lithological cut of the rigging drilling to shale formations of the Primary. The latter is based on a conductive level Cp (50 ohm.m) whose thickness is 22m.
The set of Rp and Cp lands is surmounted by a resistant level noted RT, the value of the electrical resistivity is of the order of 1000 Ohm.m. This resistant level is attributed to Permo-Trias basalts.   The interpretation of these electrical soundings and their correlations with the lithological succession given by the corresponding calibration holes made it possible to distinguish ( figure 8): -A deep resistant substrate Rp whose resistivity is equal to 550 ohm.m. Its roof is located at a depth of 157m; and which, according to the description of the drilling, corresponds to oil shale of primary age.

Geoelectric cuts
On the one hand, the geoelectric sections make it possible to visualize in a clear manner the structure in depth of the study area and to follow the lateral and vertical variation of the geological formations, and on the other hand to determine the roof and the wall of all formations likely to constitute aquifers.
Two types of geoelectric cutting were carried out as part of this work, the North-South longitudinal crosssections and the West-East cross-sections. In order to better follow in depth the variation of the different electrical levels detected, two longitudinal NE-SW geoelectric cuts and two NW-SE cross sections were made. (figure 9): the longitudinal geoelectric section B comprises the electrical soundings B1, B2, B3, B4, B5, B6 and B7; -the longitudinal geoelectric section D comprises the electrical soundings D1, D2, D3, D4, D5, D6 and D7; -the transverse geoelectric section 2 comprises the electrical soundings A2, B2, C2, D2 and E2;     Examination of this map shows that the ranges of the thickness values of this aquifer level RT oscillate between 6m and more than 56m. The maximum thickness of this reservoir is observed at the southwestern end of the study area, while the low thicknesses (thickness ≤ 22m) are distributed north and west of the study area.( figure  14).
Overall, aquifer level R1 shows a thick zone located in the southwest and a thin zone located northeast of the study area.
The isohypse maps were developed from the roof elevation values of the aquifer levels found on the geoelectric sections (R0, R1, RT and Rp).
This map is based on the values of the aquiferlevel wall coast recorded on geoelectric sections (Figure 15).
Examination of this map allowed showing: that the roof moves progressively from the northwest to the southeast of the study area; -that the high altitude values of the roof recorded in the south-east of the study area reflect the raising of the roof of this level towards the surface at this location; -a rise of the roof of this aquifer R0 towards the South-East in the direction of the outcrops of dolomitic-limestone formations of Lias medium-superior. The following map shows that the roof of this aquifer level varies from 1350 m and more than 2050m. This resistant level corresponds according to the geoelectric correlation with limestone and dolomitic formations of the lower Lias. The analysis of this map ( figure 16) shows: -That the high altitude values are recorded in the north-west of the study area while the low values are observed in the southeast of the study area ; -The roof of this aquifer level shows a rise from north-west to south-east of the study area ; -The high values recorded in the South-East reflect the rising of the roof of this aquifer level in these places. This map (Figure 17) is based on the values of the roof coast of the aquifer level RT recorded on the geoelectric sections. Examination of the map of the map made it possible to highlight: -A low elevation area (≤1600), located in the northwestern part of the study area ; -An area located in the south-east of the study area, where the roof of the aquifer reaches more than 1950 m altitude; -The roof of this aquifer level RT is gradually rising towards the south-east of the study area. of local prospecting, it allows at least to define the vertical succession of existing aquifers in addition to the local petrophysical characteristics that can be compared with those of the closest rigging holes.
The comparison of drilling data with the different geophysical surveys carried out on our study area makes it possible to draw the following conclusions: -Reveal the existence of four resistant electric horizons R0; R1; R2; RT and Rp.
-Define the geometry of these aquifer levels by drawing the isopach maps ; -Follow laterally and vertically the structure and extension of these aquifer levels.
-To monitor the importance and hydrogeological productivity of these aquifer levels by mapping their transverse resistance RT. Thus, we confirm by our results that the electrical resistivity method is an important guide for the recognition of underground aquifers and for the implantation of mechanical soundings of recognition by increasing their chance of success and thus minimizing the cost of recognition.

acknowledgments
We thank the management of the Oum Er-Rbia Hydraulic Basin Agency for providing us with the data used in this article as well as for their help and constructive suggestions. We also thank all those who contributed directly or indirectly to the development of this work.