Hydrogeophysical and Structural Investigations Using VES: A Case Study of Dosso Region in the Southwestern Part of the Meso-Cenozoic Iullemmeden Basin (Southwestern Niger) ()
1. Introduction
Water supply demands increase in sub-Saharan African countries since these decades. In Niger republic and particularly in the study area namely Dosso region (Figure 1), the population’s drinking water supply is in more than 80% dependent on groundwater from the Iullemmeden basin [1] . The Iullemmeden sedimentary basin contains one of the most important groundwater resources of Niger Republic.
In the past two decades, many hydraulic programs were thus set up to meet these needs and to cover the ever-increasing water demands of the population. Unfortunately, their implementation faced a strong demand which impacts the efficient management of these resources.
It therefore appears important to improve knowledge and understanding of the geological but also hydrogeological characteristics of the aquifers of the Iullemmeden basin. Thus, several studies carried out have made it possible to improve knowledge of this basin [1] - [9] . However, none has addressed the question of the influence of tectonics in this basin, neither its impact in the architecture of the basin leading to its geometry model. The aim of this study is to improve the knowledge of the multilayer aquifer system of the Iullemmeden basin in the Dosso region.
Specifically, this involves to:
- establish a resistivity scale of the different geological formations of the study area;
- determine the role played by tectonics in the geometry of multilayer aquifers;
- propose a geometric model of the spatial arrangement of the multilayer aquifers in the study region.
To achieve these objectives, as part of this study, geophysical prospecting methods, mainly electrical methods, will be used. These geophysical methods constitute a powerful tool to explore the geology of the subsurface and gather more information about the layers and structures of the subsurface [10] .
The study area namely Dosso region is one of the eight (8) regions in the republic of Niger. It is located in the southwestern part of Niger (Figure 1(B)) in the Iullemmeden basin. It covers an area of 33,844 km2 so around 2.7% of the national territory. The study area is bounded to the north-east by the Tahoua region, to the north-west by the Tillabéry region and to the south-east by the Republic of Nigeria and to the South by the Republic of Benin (Figure 1(B)).
Reference [1] reveals that the Dosso region is an agricultural area with around 70% of farmland and the canopy represents about 20%. Housing areas are mostly located in the main towns like Dogondoutchi and Gaya in the north-east and south-east, respectively; in Loga, Birnin N’Gouaré and Dosso in the western part of the study area. The population is around 2,037,713 inhabitants [11] . The Digital
Elevation Model (DEM) ranges from 150 to 350 m. Rainfalls can reach 280 mm/year and the Niger River flows towards the South of the Region. Soil survey of the study area showed that the erodible sandy soils namely aeronosols are much represented (96%) while the hydromorphic and ferrallisol soils occupy substantially 3.43% and 0.57%, respectively [1] .
2. Geological Context
The Iullemmeden basin is a vast intracratonic basin (Figure 1(A), Figure 1(C), Figure 2), which belongs to several countries (Algeria, Benin, Mali, Niger and
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Figure 2. Geological map of the studied area. [18] modified.
Nigeria). It takes its name from the important Targui federation called Kel Iullemmeden, populating its central part [12] .
It extends from North to South for a length of approximately 1000 km and from East to West over a width of 800 km [3] . The Iullemmeden basin communicates to the West with that of Taoudenni via the Gao strait. To the East, it is continuous with the eastern Niger basin (or Chad basin) via the Damergou threshold [13] .
The Iullemmeden basin in its overall structure presents a syncline-shaped geometry whose southern axis is located approximately between the 3˚ and 4˚ East meridians [3] [13] [14] .
The sedimentary infilling is essentially made up of Paleozoic (to the north) and Meso-Cenozoic (to the south) deposits. The Iullemmeden basin was the site of sedimentation characterized by a migration of depositional areas from the north-east to the south-east. Paleozoic formations crop out in the northern part of the Iullemmeden basin (Tin Séririne sub-basin and Tim Mersoï sub-basin), as mentioned by [13] as well as at its southwestern end (Kandi Benin-Niger basin). Mesozoic sediments occupy most of the basin. The most recent Cenozoic seriesoverlay sometimes directly the Liptako and the North Benin basements.
Numerous fault systems have affected the basement and the sedimentary cover of the basin in its North to north-eastern and south-western parts, the orientations of which are NW-SE for the latter (Figure 1). The significant changes in thickness, facies and discontinuities, as well as in the nature of the beds, are the striking sign of the synsedimentary tectonics that affected the Iullemmeden basin [3] [13] [15] [16] .
The Iullemmeden basin, like many other basins, has undergone several deformations episodically. They have led to the tectonic readjustments of the basin according to the major lineaments of the African plate [13] [14] .
The study area is located in the southwestern part of the Iullemmeden Basin in Niger (Figure 2). A synthesis of geological and hydrogeological knowledge of the study area (South-West of the Iullemmeden basin) based on data from previous studies determined the reference lithological levels [3] . These latter retained as a basis for stratigraphic correlations presented from bottom to top as follows:
v The Precambrian basement made up of granites and gneiss, is subcrops only along the Niger River in the Dosso Region [3] [9] . In the southern border of the study area, the Continental “hamadien” formation rests unconformably on the Precambrian basement.
v The Continental “hamadien” (Ch) formation is composed of fine to coarse sandstones with kaolin cement and medium to coarse sands more or less consolidated in the Dallol Maouri, with a coarser and more homogeneous facies towards the South. Overall, the Continental “hamadien” is represented in the study area by Upper Cretaceous continental deposits.
v The Paleocene/Ypresian marine deposits are essentially made of limestones, marls, marl-limestones and attapulgites to the top. In the study area, this marl-limestone series separates the continental sediments of the Upper Cretaceous (Continental “hamadian”) and the Tertiary (Continental terminal).
v The Continental terminal 1 (Ct1) formation is also characterized by thickness and facies variations in the study area. It overlays the limestones and the marl-limestones, or the gray-blue clays (attapulgites). In the northern part of the Dallol Maouri and in the Dosso area, the medium to coarse sandy facies changes to alternations of fine clayey sands and sandy clays with concretions of ferruginous oolites and pyrite crystals. In the West of Dallol Foga, the Ct1 formation is made of soft oolitic sandstones. In the Dallol Bosso on the other hand, it is represented by gravels and oolites in a clayey matrix and slightly silty clays with detrital elements and carbonaceous pastes.
v The Continental terminal 2 (Ct2) formation is composed of two layers of gray clays with lignites and peats with a layer of sand and/or ferruginous oolites in between. The gray clays transition in places to more or less sandy or silty facies and show significant variations in thickness.
v The Continental terminal 3 (Ct3) formation is essentially composed of sands, heterogranular sandstones and more or less clayey silts. Globally, the Ct3 formation is made up of more or less clayey sands and silts in most of the study area.
3. Hydrogeological Context
Previous studies investigated on the hydrogeology in the South-West of Iullemmeden basin [1] [7] [9] [17] where the study area is located. They revealed that the hydrogeology of the basin is mainly controlled by geological parameters more specifically by lithology, topography and tectonics. In this part of the Dosso region, groundwater resources are mainly located in the sedimentary formations of the Cretaceous (Continental “intercalaire”/“hamadien”), the Tertiary (Continental terminal) and the Quaternary (alluvial valleys) (Figure 2).
4. Data Collection and Methodology
4.1. Data Collection
The geophysical data of the Dosso region were obtained following a geophysical survey campaign. The Electrical Survey (ES) sites were selected prioritising localities with a borehole with available lithostratigraphic descriptions allowing to make good correlations. The profiles were defined after a reconnaissance mission in order to obtain a good spatial distribution of the boreholes, enabling the morphology and the geometry of the aquifers to be reconstructed. 104 vertical electrical soundings with AB lengths between 200 and 2000 m were carried out throughout the Dosso region (Figure 3).
4.2. Material and Investigation Method Used
Electrical prospecting measuring electrical resistivity is the method implemented in the study area. Itis a preferable geophysical technique for groundwater exploration. As, the water content and its distribution change the electrical properties of the rocks. The resistivity is inversely proportional to the rock porosity and its water saturation. Water salinity also plays an important role as resistivity decreases when the salinity increases. If clays are present in the rock, the value of the resistivity decreases [19] [20] [21] .
The most important characteristics of this electrical survey method consist to determine: 1) the geometric characteristics of groundwater reservoirs in terms of thickness, depths of the water-bearing layers, from the earth’s surface and, 2) the lithological contrast in the horizontal and vertical direction and, 3) the expected water salinity.
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Figure 3. Vertical Electrical Surveys (VES) Spatial distribution.
The principle of electrical resistivity technique measuring electrical resistivity is based on the injection into the ground of an electric current of intensity I between two electrodes A and B, and the measurement of the potential difference ΔV induced between another pair of electrodes M and N. The positioning of these surface electrodes are variable and influence basically the current distribution. The value of the apparent electrical resistivity (ρapp) in the subsoil is deduced from the values of the current intensity I, the potential difference ∆V and the geometric factor K specific to the geometry of the device used (spacing between the different electrodes) based on Ohm’s law according to the following formula.
(1)
(2)
There are several configurations (Schlumberger, Wenner, Dipole-dipole, Gradient) to measure the electrical resistivity of the ground depending on the objective and the problem encountered.
For this study, the Schlumberger configuration (in which the four electrodes are aligned) was used. As part of this study, only vertical electrical sounding was used to measure the electrical resistivities of rocks. It should be remembered here that vertical electrical sounding is made up of a series of apparent resistivity measurements carried out with progressively increasing lines whose center and direction of movement of the device remain fixed. The variation in the apparent resistivities obtained is essentially due to the increasing penetration of the electrical current.
The equipment used for the geophysical campaign is the Syscal R1 Plus resistivimeter which is fully automatic resistivity meters. It was designed for intensive exploration of the subsurface using direct current electrical methods.
The SYSCAL R1 PLUS save the measurements in a built-in internal memory (more than 800 recordings). Each record contains the current intensity, voltage, resistivity, the confiuration (Schlumberger, Wenner, Dipole-dipole, Gradient), line lengths, station’s number (VES’s number). It automatically calculates the apparent resistivity when the values of AB/2 and MN/2 are entered.
4.3. Method for Interpreting Electrical Survey Data
The IPI2W in software (2002 version) was used as the treatment data tool. It is powerful software for the 2D interpretation of electrical surveys. It was developed by the Geophysics Laboratory of Moscow University [22] . Its particularity is to consider the profile of electrical surveys to be treated as a unit. This approach allows for better consideration of information. Using this principle, an average curve of a vertical survey can be plotted. This principle allows to consider the curve as being atypical for a specific sector. On the other hand it allows to consider its solution as a model for the inversion of the rest of the curves of the vertical surveys. It should also be noted that this software allows for each geoelectric section, a statistical analysis in the form of a representation which gives the frequencies of the apparent resistivity values of the electrical soundings forming the profile.
5. Results
Only the representative profiles are exposed and discussed. The profiles depending on the investigation depths reached with geophysics (AB lengths varying from 200 to 2000 m) and lithological logs of the boreholes are used as parametric surveys.
According to the appearance of the survey curves types obtained, the subsoil of the Dosso region is composed of several layers of resistivity ρ1, ρ2, ρ3 and thickness h1, h2, which gives four terrain configurations:
• a conductor between two resistors, type H sounding or “boat bottom” (Figure 4(A));
• a resistor between two conductors, type K or “bell” sounding (Figure 4(B));
• a resistivity which increases in stages, type A sounding (Figure 4(C));
• a resistivity which decreases step by step, type Qsounding (Figure 4(D)).
5.1. Interpretation and Discussion of Results
5.1.1. Resistivity Scale of Geological Formations
Parametric surveys are essential and are permit to establish true local resistivities of different geological formations considering, a certain constancy of facies in the study area. In order to facilitate the interpretation of the electrical soundings, Nineteen (19) parametric soundings spatially distributed throughout the study area, were carried out in the vicinity of the boreholes whose lithology was well described. The boundaries of the Continental terminal and Continental hamadian aquifers were thus defined based on the geochronology and their depths, giving a certain constancy of the geological facies described. In addition, the limits of the Ct aquifers namely Ct3, Ct2 and Ct1 was not really established except in Bagagi.
For calibration, the lithology of some boreholes will be described with their survey curves. The figures presented below summarize the lithology of these parametric boreholes as well as their survey curves.
1) Bagagi parametric survey
The Bagagi borehole drilling carried out in 1969, describes a Ct3 formation consisting of fine sandstones, clayey sandstone, sandy sandstones and clayey sandstones to a depth of 86 m. The resistivities obtained from the standard survey (Figure 5) vary from 45 to 1599 Ω∙m and characterize these Ct3 sandstones. Ct3 formation is a low conductor resistant at certain levels. The resistant level (1599 Ω∙m) corresponds to indurated sandstone and/or dry sand. The relatively conductive level (435 Ω∙m) corresponds to a level of sandy sandstones which is probably the level where the Ct3 aquifer would be located.
The Ct2 formation, made of sandy clays, variegated clays and soft sandstones with sandy passage, is located between 86 m and 186.6 m. The resistivities of the standard survey corresponding to Ct2 vary from 19.4 to 435 Ω∙m. These reflect a conductive level due to the presence of clay (Figure 5).
The Ct1 formation, constituted of oolite clay, marl, soft sandstone extends from 186.6 m to 291 m. The lower limit of Ct1 probably corresponds to the Upper Paleocene limit. The survey was unable to reach the lower limit of CT1. The presence of clay in this aquifer makes it conductive, the resistivity corresponding to this horizon is 19.4 Ω∙m. That of marls is 0.12 Ω∙m (Figure 5).
2) “Dogondoutchi km 300” parametric survey
This borehole realized in 1955 for the reconnaissance of the Iullemmeden basin in Niger, has a depth of 750 m. However, it should be noted that the depth of
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Figure 4. Configuration of electrical sounding curves of the Dosso region.
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Figure 5. Bagagi borehole parametric survey.
investigation of vertical electrical surveys carried out in the Dogondoutchi area does not exceed 400 m. However, it is useful for describing lithofacies to a depth of 400 m corresponding to the upper limit of the Paleocene (Figure 6).
The Quaternary consists of sands with a thickness of 10 m with a resistivity of dry sand corresponding to 2524 Ω∙m.
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Figure 6. “Dogondoutchi km 300” borehole parametric survey.
The upper limit of the Ct is approximately at 10 m deep and the lower limit is reached round 400 m. The Continental terminal formation (Ct) is composed of an alternation of sandy clays, sands, clayey sandstones, sandy clays and clays at its base. The resistivities corresponding to the Ct obtained from the parametric sound vary from 7.26 to 645 Ω∙m reflecting a conductive to relatively resistant terrain. The lower limit of the Ct at this borehole is reached at 400 m and is made of marl-limestone, limestone, marl. They mark the upper limit of the Paleocene with a resistivity of 9.14 Ω∙m.
3) Kiéché Boreholeparametric survey
This borehole drilled in 1956 shows the Quaternary consisting mainly of sand with a level of laterite crust over a depth of 20 m. The resistivity of the parametric sound corresponding to the Quaternary varies from 1424 Ω∙m for the dry sand to 17,099 Ω∙m for the laterite crust (Figure 7).
The Ct formation is constitued of clayey sandstone, alternation of sand and clayey sandstone, blue or black clay at the base. The resistivities of the Ct vary from 2 Ω∙m (clay or possibly a brackish water level) to 206 Ω∙m.
4) Dioundiou parametric survey
The Dioundiou borehole drilled in 1969 indicates the extension of the Ct formation from 0 m to a depth of 116 m (Figure 8). It is essentially constituted of medium sand with a level of lateritic gravel (1 m thick) at the top. From 7 m to 116 m depth, it is constituted of coarse sandstones, clayey and sandy sandstones, and clays. The resistivities corresponding to the Ct formation are 2037 Ω∙m and 4910 Ω∙m (sand), 21797 Ω∙m (eolian sands and lateritic shells), 81 Ω∙m to 528 Ω∙m (for clays, siliceous clayey-sandstone, clayey and sandy sandstones). The Paleocene is reached between 116 m and 135 m depth. It is made up of marl-limestone and grayish clays with an average resistivity of 337 Ω∙m.
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Figure 7. Kiéché borehole parametric survey.
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Figure 8. Dioundiou borehole parametric survey.
The Continental “hamadien” begins at a depth of 135 m (Figure 8), it is made up of clayey sands, white and gray sand, quartz, hard sandstone from a depth of 187 m. The average resistivities obtained for the Ch formation from the parametric sound are 337 Ω∙m for clayey sands, medium gray and white quartz sands) and 12,598 Ω∙m for indurated sandstones.
5) “Bengou I” borehole parametric survey
With a depth of 346.5 m, the “Bengou I” reconnaissance borehole was drilled in 1969. The quaternary has a thickness of 15 m (from 0 m to 15 m depth). It is essentially made of fine silto-clay sands, medium sands and ferruginized quartz sandstones (Figure 9). The parametric sound resistivity corresponding to the Quaternary vary from 85 Ω∙m to 2741 Ω∙m. The Paleocene is made of peats and silty clays with a thickness of 4 m (15 to 19 m) with an average resistivity of 43 Ω∙m (Figure 9). The Continental “hamadien” (Ch) formation starts from a depth of 19 m and ended at 342 m. It is made of fine to coarse sands, more or less soft fine sandstones, ferruginous and clayey sandstones, compact gray-blue marls at its bottom (from 330 m to 342 m deep). The resistivities corresponding to Ch vary from 43 Ω∙m to 298 Ω∙m for sandstones and sands and 5.4 Ω∙m for marls.
6) Talambou borehole parametric survey
This borehole, realized in 1987, has a depth of 104 m. The description of its lithology highlights fine clayey sands (0 to 2 m depth), sandy clays from 2 m to 8 m depth, clayey alterites from depth 8 m to 32 m, granite arenas from 32 m to 44 m. The unaltered pink granite is reached at a depth of 44 m (Figure 10). The parametric sound resistivity obtained reflect this lithology. Thus, fine clayey sands have a resistivity of 1200 Ω∙m, clayey sands 598 Ω∙m, clayey alterites and granite arenas 125 Ω∙m and finally the unaltered pink granites 17,459 Ω∙m.
7) Goutoumbou borehole parametric survey
The lithological description of the Goutoumbou borehole successively reveals fine red sands from 0 to 2 m depth, silty-clayey and clayey-silt from 2 m to 48 m, sandy-clayey from 48 m to 55 m, clayey-silts and silty clays from 55 m to 93 m. From 93 m to 146 m depth, there is an alternation of fine silty-clayey sands clayey-sands with silty-clayey oolites (Figure 11).
The parametric sound at this borehole gives resistivities reflecting this lithology. Thus for fine red sands, the resistivity varies from 102 Ω∙m to 1833 Ω∙m, clayey-silts and silty-clayey have a resistivity varying from 43 Ω∙m to 323 Ω∙m, clayey-sands indicates an average resistivity of 85 Ω∙m, clayey-silts 85 Ω∙m to 1131 Ω∙m. Fine clayey-sands and fine sands with oolites have an average resistivity of 1131 Ω∙m.
8) Wadata (Falwel) borehole parametric survey
To parameterize the resistivities of the layers in this part of the study region, the Wadata borehole (Falwel) was used. The observed layers define from top to bottom are: fine sands of 0 to 3 m, lateritic sands of 3 to 10 m, lateritic clays of 10 to 15 m, clayey-sands from 15 to 20 m, and an alternation of sandy-clayey and fine sands from 20 to 152 m (Figure 12). It is important to mention that water level in boreholes was measured during the survey where it is possible.
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Figure 9. “Bengou I” borehole parametric survey.
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Figure 10. Talambou borehole parametric survey.
The layers surveyed present an average of resistivities that are summarized as follows: fine sands 5909 Ω∙m, lateritic sands 306 Ω∙m, lateritic clays 9016 Ω∙m, layer made up of alternating clayey-sands, sandy clays, and sand 927 Ω∙m and for a layer of sandy clay from 201 m with an average resistivity of 214 Ω∙m.
9) “Birnin N’Gaouré II” borehole parametric survey
The Birnin N’Gaouré borehole drilled in 1954 was also used to parameterize the resistivities of the lithological layers. From top to bottom of this borehole,
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Figure 11. Goutoumou borehole parametric survey.
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Figure 12. Wadata borehole (Fawel) parametric survey.
there are very coarse white sands from 0 to 12 m, lateritic gravels from 12 to 13 m, soft less or more clayey sandstones from 13 to 50 m, compact clay from 50 to 80 m, clayey sandstone from 80 to 90 m, black marls from 90 to 95 m, and an alternation of clayey sands, compact clays, clayey sands and clays from 95 to 170 m (Figure 13). The resistivities corresponding to the different layers according to the parametric sound are presented as follows: coarse white sands, a layer of lateritic gravels with a resistivity of 4488 Ω∙m, soft sandstones from 15 to 50 m with a resistivity of 53 Ω∙m to 453 Ω∙m, a layer made up of alternating compact clays, clayey sandstones, clayey sands, sandy clays from 50 to 170 m, having an
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Figure 13. “Birnin N’Gaouré II” borehole parametric survey.
average resistivity of 453 Ω∙m. From a depth of 196 m, the survey is characterized by the presence of a very resistant level probably corresponding to very hardened compact sandstones. The latter have an average resistivity of 4137 Ω∙m.
In summary, we can conclude that from the correlations of the standard surveys with the lithological logs of the boreholes, typical resistivity markers were highlighted (Table 1). Thus, an average resistivity scale was established for the main formations of the studied region. The average resistivity values calibrated in the table below (Table 1) make it possible to interpret the resistivity values obtained with the other surveys, for the Quaternary, Continental Terminal (CT), Paleocene and Continental Hamadian formations (CH).
The surveys of the study area through the correlations of the parametric surveys coupled the lithological logs of the boreholes highlight typical resistivity markers (Table 1). An average resistivity scale was established for the main formations of Dosso region. These calibrated average resistivity values permit to interpret the resistivity values obtained.
5.1.2. Geoelectric Cross-Sections
The Iullemmeden basin undergone extensive tectonics which affected the Meso-Cenozoic formations (Ch and Ct) of the studied area.
Observation of the fracturing map of Dosso region [23] shows that the large sub-meridian fractures correspond to the replay in the sedimentary cover of the mega-shears which cut up the pan-African mobile zone. The geoelectric sections resulting from correlations of electrical soundings with lithological data from boreholes in the region make it possible to investigate the role of tectonics in sedimentation. A total of 21 profiles were produced (Figure 14(A), Figure 14(B)), among which the four (4) most representative will be interpreted and discussed.
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Table 1. Scale of resistivities in the Quaternary, Continental terminal (Ct), Paleocene and Continental “hamadien” (Ch) geological formations in Dosso region.
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(A) (B)
Figure 14. (A) Location of profiles (No. 1 to 10); (B) Location of profiles (No. 11 to 21).
They exhibit the following formations of: Quaternary, Continental terminal (Ct), Continental “hamadien” (Ch) and the Precambrian basement.
1) Geoelectric cross-section No. 1 (North of Dosso region)
The geoelectric cross-section (Figure 15(B)) of Dogon Kirya area is composed of seven (7) electrical soundings (VES1 to VES7). From top to bottom for the VES1, there is a succession of four alternating types of terrain: resistant-conductive-resistant-conductive. These terrains correspond respectively to dry sands, sandy clays, clayey sandstones, clayey sands, and marls. The VES5 shows three types of terrain: resistant-conductive-resistant. The surface terrain corresponding to Quaternary sands is widespread throughout the sector (VES7 to VES3) as seen in the pseudo section (Figure 15(A)). The underlying layers of sandy clays (approximately 90 Ω∙m) and clayey sandstones/clayey sands rest on a very conductive layer of loamy clays (17 - 36 Ω∙m) and marl (2 Ω∙m) respectively. Between the surveys (VES1, VES2, VES3, VES5, VES4) of the geoelectric section (Figure 15(B)), the offsets of the different layers occur. These shifts were attributed to sets of normal faults (F) leading a structuring intouplifted zones and collapsed zones. This is clearly highlighted in the pseudo-section (Figure 15(A)) which indicates sudden variations attributed to probable accident replays between VES6, VES1, VES2, VES3, VES5 and VES4.
The accentuation of the slopes between the VES is caused by the effects of tectonics. Then they can retained:
• the steep slope between VES7 and VES6 would be caused by the morphology of the terrain. This morphology probably comes from fault movements which would have lowered the VES7 zone and raised that of VES6;
• the steeper slope between VES1, VES2 on the one hand and VES3 in the other hand would have been caused by the conductive layer below. In fact, this layer was lowered to the level of VES3 due to a probable accident. These effects are observed from a 100 m depth;
• the slope between VES3-VES5 and VES5-VES4 clearly shows a subsidence of the second compartment VES4 compared to the compartments of VES5 and VES3. The resistant layer between the two boreholes (VES3 and VES4) rises under the effect of a replay of faults would have created these two compartments.
2) Geoelectric cross-section of profile No. 3 (Bagagi to Kiéché)
The North-South geoelectric cross-section (Bagagi to Kiéché) (Figure 16(A) and Figure 16(B)), exhibits the effects of the fault played in this sector. The structuring of the studied area is organized into uplifted (horst) and collapsed (graben) compartments.
VES12 and VES13 at Bagagi are not more than a kilometer apart, but the disappearance of the conductive horizon at VES12 in the underlying layer can be attributable to the effect of tectonics.
Between VES13 on the one hand and VES14 on the other hand, the presence of a probable normal fault (F4) which would have favored the collapse of a compartment between Bagagi (VES13) and Matankari (VES 14). Also in the
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Figure 15. Geoelectric cross-section and pseudo cross-section of transec No. 1.
Matankari area (between VES14, VES17 and VES18), the presence of several faults (F) favours the creation of at least two compartments, one of which is uplifted and the other collapsed. Around Dogondoutchi (VES16, VES10, VES11), the disappearance of the very resistant horizon occurs. The presence of faults on this profile would have led to the creation of collapsed and uplifted compartments.
Up to a depth of 50 meters, the effect of the topography of the area influences the organization of the surface layers. Between 100 and 250 meters depth, the variation is minimal, this is consistent with the continuity of the formations on the geoelectric section (Figure 16). Beyond 250 m, there is a clear variation (at the level of the different surveys) that linked to the effects of tectonics.
3) Cross-section No. 15 (Daniakou to Sabon Birni towards the Niger River)
This geoelectric section (Figure 17(B)) shows the uplifting of the Precambrian basement from 123 m depth at Sabon Birni (VES1) and 110 m after Sabon Birni towards the Niger River. It thus shows the limit of the sedimentary infilling of the Iullemmeden basin in Dosso region. On the pseudo-section (Figure 17(A)), the limits of the resistant horizons correspond to the basement.
The analysis of the values (Figure 17) shows a certain consistency between VES45 and VES47, exhibiting the continuity of the formations in this area (Dallol Maouri). The values of the following surveys VES47, VES48, VES92 and VES50 correspond to a significant deepening of the layers, while the decrease in the values at the level of VES51 and VES52 towards the Niger River reflects the uplifting of the bedrock.
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Figure 16. Geoelectric cross-section and pseudo section of profile No. 3.
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Figure 17. Geoelectric cross-section and pseudo cross-section of profile No. 15 (North-South from Daniakou to Sabon Birni).
4) Cross-section No. 16 (South-West between Tounga. M. Maichanou and Talambou)
This geoelectric cross-section (Figure 18(A) and Figure 18(B)) shows the same configuration as the previous one. At Talambou (VES54), the basement consisting of unaltered granite is encountered in the borehole which is used as a reference from a depth of 44 m. While at Sia village (VES53) which is located less than 10 km away, the resistant layer is more than 200 m away. The effects of tectonicsoccurred in this sector by uplifting or lowering the layers.
The evolution of the resistivity values on this profile shows the influence of the very resistant layer of 2666 Ω∙m located between 16 and 37 m depth at VES53. Indeed, the decrease in conductance at VES53 up to a depth of 50 m is due to the presence of this very resistant layer at this depth. Beyond the depth of 50 m, the configuration of the layers as shown on the geoelectric section (Figure 18) with the granitic basement close to the surface observed at Talambou. The slope of the conductance on this profile demonstrates that beyond a depth of 50 m the bedrock rises as a result of normal faulting. This type of configuration was recorded in Algeria in the Mekam Sidi Khalifa sector by [24] .
5.1.3. Synthetic Geological Sections
Synthetic geological sections were carried out based on altitude values taken from the DEM, the lithostratigraphical logs and the electrical survey data. The sections show the variability of the geometry of Ct and Ch formations in the studied areas where the lower and upper limits of the formations have been reached. There is an irregular geometry of these formations. On profile n˚ 2 (Figure 19) which represents the section between Kolifo and Bagagi, the limits of the different horizons forming the Ct3 formation are well marked. The thickness of Ct3 formation, composed of more of less clayey sandstones, between Kolifo (VES9) and Bagagi (VES12) is 110 m in Kolifo and approximately 85 m in Bagagi. The thickness of the clay level constituting Ct2 in this sector varies from 160 m at Kolifo (VES9) to around 100 m towards Bagagi (VES12). Finally, the medium sands of Ct1 recognized in this sector (Kolifo-Bagagi) have a thickness of 110 m. At the bottom, the presence of resistivities of around 3 Ω∙m indicates probably the presence of a marl or marl-limestone layer which represents the Paleocene upper limit.
On the profile No. 3 (Figure 20) between Bagagi and Kiéché, the limits of the Ct are well marked. At Bagagi and Matankari, the Quaternary is absent. At Dogondoutchi, the Quaternary consists of dry sand about 9 m thick, while at Kièché its thickness is approximately 5 m.
In the vicinity of Bagagi, the Ct has a thickness of 295 m, and 300 m around Matankari. At Dogondoutchi, the lower limit of Ct is reached at approximately 400 m, corresponding to the upper limit of the Paleocene, consisting of marllimestone, limestone and marl.
Profile No. 15 (Figure 21) gives the upper and lower limits of the Ct, the Ch and the basement in the area between Daniakou and Sabon Birni towards the Niger River and the Nigeriaboundary. Around Daniakou (VES45), the thickness of the medium sands and soft sandstones of the Ct is reached at approximately the
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Figure 18. Geoelectric cross-section and pseudo cross-section of profile no. 16 South-West of Dosso (Tounga M. Maichanou in Talambou).
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Figure 19. Synthetic geological section from Kolifo to Bagagi.
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Figure 20. Synthetic geological section from Bagagi to Kiéché.
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Figure 21. Synthetic geological section from Daniakou to Sabon Birni.
depth of 80 m. At Bana (VES48), the Ct is absent (0 m). At Daniakou (VES45), the upper limit of medium to coarse sands and fine to coarse sandstones of the Ch is reached at around 80 m of depth. At Bana, the Continental “hamadien” is sub-outcropping and its lower limit is reached at Sabon Birni (VES51) at a depth of 180 m.
The contact with the base is located near Sabon Birni (VES51) at a depth of 180 m and reached 110 m towards the river Niger.
The profiles No. 16 (Figure 22), and No. 17 (Figure 23), show the contact with the base (granites and gneiss) at Talambou village (VES 54) and Boumba village (VES55). Low resistivities observed at VES72 (Tounga Mallan Maichanou’s village), VES87 and VES100 (village of Fono Birgui) of approximately 4 - 17 Ω∙m correspond either to highly mineralized clays or marl-limestones, or to trapped marine brines.
5.1.4. Synthetic Hydrogeological Sections
The synthetics hydrogeological sections were established based on some piezometric datas and the Ct aquifers boundaries according to the lithology.
The morphology of the land and the positioning of certain layers constituting the different aquifers result from movements of the base under the effect of tectonics. The level of the water table was drawn from Bagagi to Kiéché on profile No. 3 (Figure 24), the boundaries of the Ct aquifers were marked. In addition the upper and lower limit of the sandy levels of Ct3 are clearly visible. At Bagagi, the Ct3 has a thickness of 85 m while that of the Ct2 is 110 m, and that of the Ct1 is around 100 m.
At Matankari, the lower limit of the Ct3 is reached at 70 m, that of the Ct2 is reached at 230 m and the Ct1 at approximately 300 m. This latter corresponds to the upper limit of Paleocene made up of marl-limestone, limestone and marl.
At Dogondoutchi, the Ct3 has a thickness of approximately 170 m, those of the Ct2 and the Ct1 are respectively 130 m and 100 m. The Paleocene marl and marl-limestone begins from400 m in this sector.
At Kiéché, the Ct3 lower limit is reached at a depth of 190 m, that of the Ct2 at a depth of 340 m and for the Ct1 at approximately 390 m.
Brackish ground waters were also identified at different levels in the study area through local resistivity values.
Profile No. 14 shows a synthetic hydrogeological profile going from “Bella 2” to Yelou, where the local resistivities values obtained highlight brackish groundwater at a depth of 71 m at Kawara N’Debé (VES44). At Yelou (VES46) between 88 m and 116 m of depth, a highly mineralized clay level was located. This level could originate from contamination by trapped brines (Figure 25).
On profile No. 7 between the villages of Salkam and Kadidi a brackish groundwater was identified considering VES27 survey at a depth of 143 m (Figure 26). In the same order at the level of profile n˚ 8 (Figure 27), which goes from the village of Kalla Walailawa (VES96) to Tounga Tchiadi (VES81), a brackish water table was located at a depth of 102 m precisely in the locality of Tounga Zamnaou
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Figure 22. Synthetic geological section of Tounga Mallan Maichanou to Talambou.
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Figure 23. Synthetic geological section from Boulangueye to Boumba.
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Figure 24. Synthetic hydrogeological section from Bagagi to Kièché.
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Figure 25. Synthetic hydrogeological section of Bella 2 to Yelou.
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Figure 26. Synthetic hydrogeological section from Salkam to Binguel (Tibiri Doutchi).
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Figure 27. Synthetic hydrogeological section of Kalla Wailalawa to Tounga Tchadi (Dallol).
Zouma (VES 102). The profile No. 20 going from Kohinza Rizi (VES 91) to Bonbayin Gonda(VES 85), indicates the presence of two brackish ground waters (Figure 28). The brackish water or highly mineralized clay levels were located by electrical soundings in the village of Kiarangui (VES95) at a depth of 90 m. In the village of Kohinza Maraiché (VES104), the brackish water was also identified at a depth of 89 m.
Based on these different maps established from geophysics and drilling logs, we can say that the nature of the lithological facies, their variations and their distribution in the study area can have a certain influence on the hydraulic characteristics of the aquifers.
6. Conclusions
The hydrogeophysical study of the Dosso region, in the southwestern part of the Iullemmeden basin enabled an atypical characterization of the study area. About 104 wells, spatialed vertical electrical soundings, with AB lengths ranging between 200 and 2000 m, were carried out throughout the Dosso region. These investigations carried out as part of this study yielded the following results:
1) The resistivity scale obtained made it possible not only to delineate the various aquifers in the study area, but also to highlight the presence of brackish water aquifers;
2) The normal play of faults has strongly influenced the geometry of multi-layer aquifers in the Dosso region;
3) A model for the spatial organisation of multi-layer aquifers in the Dosso region has been proposed.
Thus the study has shown the different hydrogeological characteristics of the Quaternary, Continental terminal (Ct) and Continental “hamadien” (Ch) formations in the study area. The electrical soundings exhibited the lateral and vertical thicknesses variations of the geological formations across the basin in the study area. It has also shown that the basin has undergone intense tectonics activities. As part of the thickness variations within the basin we can mention that of the Continental terminal, ranging from 0 m in Bana to 400 m around Dogondoutchi (VES10 and VES11). A similar observation can be made for the Continental “hamadien”, whose thickness varies from 0 m (north of Sabon Birni) to 180 m (south of Sabon Birni).
Geophysical investigations have also shown the existence of salt water (or perched brackish groundwater) in the study area, hence the low resistivities recorded in certain areas, notably in the Dallols (Maouri and Bosso) and in the Ct3 formation. These perched layers of marine brine have been identified in the vicinity of the following villages: Kawara N’Debé, Bonbayi Gonda, Fono Birgui, Tounga Zamnaou Zombo, Kohinza Maraiché, Kiarangui.
These marine brines may have been trapped after the retreat of the Paleocene sea, which would have left salts dissolved by the water of the different aquifers, hence the presence of the local brackish water aquifers in the southwestern part of the Iullemmeden basin.
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Figure 28. Synthetic hydrogeological section from Kohimza Rizi to Bonbayi Gonda (Dallol Bosso).
As a perspective, it is envisaged to correlate the geophysical information’s of the study area with the physico-chemical datas of the water to determine the impact that the electrical resistivities of the rocks could have on the quality of water intended for human consumption but also for agriculture.
Acknowledgements
The authors are deeply grateful to the anonymous reviewers for their constructive comments. The first author thanks the “PhD Geology UAM” and the “SF Team” for the collaboration.