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The study area, located in the Liptako basement of Niger, faces a serious problem of drinking water supply in recent decades. This is linked to exponential population growth and reduced rainfall. The purpose of this study is to map fracture networks from Landsat 7 satellite imagery to identify major subterranean flow corridors in the area. The methodological approach based on the collection of data (Landsat 7 images, DEM/SRTM, flow, transmissivity, static level) and the geological and hydrogeological field reconnaissance and the processing of these data with the following software (ArcGis, Envi, Surfer and RStudio) The results of this study yielded that the number of satellite lineaments is 995, the preferred directions of these satellite lineament are: N0°-10° (13%) and N90°-100° (12%). On the hydrographic network, in situ, the most frequent fracture, vein and dike directions are: N90°-100° (12%), N130°-150° (11%), and N50°-60° (10%). The structural domains identified probably correspond to the main corridors and reservoirs of underground flows. The values of the induced permeability are between 0.05 × 10
^{-8} and 3.4 × 10
^{-7} m/s. The permeability induced by satellite lineaments, which is strongly related to high fracturing densities and major water stream is involved in the hydrodynamic functioning of discontinuous aquifers in the study area.

In basement areas, groundwater reservoirs are found mainly in fracture networks and alterations [

The study area consists of basement formations, is facing a significant migration of populations, from the northern part of the Tillaberi region (Ouallam), in search of a favorable climate. However, the infrastructures built by the Niger authorities during the 1000 boreholes campaign since the end of the 1970s, remain insufficient in face of strong population growth: 36,679 inhabitants in 1988 and 104,070 inhabitants in 2016 (according to the projections of INS, 2016). All this has led the rural population to use surface water during winter (marigots, rivers, koris), which are sensitive to possible anthropogenic pollution, cause of various waterborne diseases (cholera, dysentery, typhoid fever, poliomyelitis). During the dry season, these populations rely on groundwater using traditional wells [^{3}/h. Therefore, it seems urgent to undertake a thorough study for better exploitation of the water potential. This study aims specifically to: establish a map of linearity and induced permeability of the zone, make correlations between the induced permeability and the hydrogeological and hydrodynamic properties.

Located in the department of Say, the study area is limited to the north by the urban community of Say, south-west by the borders of Burkina Faso and Benin, to the south by the Benin border, to the east by the Niger River and the rural commune of Kirtachi, and to the West by the rural commune of Gueladio (^{2}, or 36.24% of the Say Department. The Niger River is the main permanent flowing stream in the area while Goroubi,

Diamangou and Tapoa are its tributaries. These tributaries also play an important role; they cover the eastern part of a sub-catchment basin of the Niger watershed, which has an outlet for the hydrometric station at Tondikwaré.

This zone straddles the Sahelian zone in the north where the rainfall varies from 400 to 600 mm per year and the Sahelo-Sudanian zone whose annual rainfall is greater than 600 mm. The rainy season runs from mid-May to October and the dry season prevails throughout the rest of the year. The average rainfall module calculated over the 1981-2016 period is 698 mm (National Meteorological Direction, 2017). Annual average temperature values range from 28˚C to 30.4˚C for the period 1961-2016.

The vegetal covers in the form of a mosaic of facies, is of tiger bush type on the battleship plateau, while on the glacis, it is rather organized agroforestry park largely dominated by Combretaceae [

The low relief is weakly wavy with altitudes between 170 m and 260 m. The highest elevations are located at the top of control hills and mounds in the study area.

Soils are ferruginous tropical on the sandy plateau and litho-regosols type on ferruginous cuirasses [

The main geological formations of the study area are divided into three large groups (

These formations are affected by more or less intense accidents which principal directions are: NW-SE, NE-SW, E-W and N-S [

The second subassembly consists of Infracambrian formations consist of volcanic rocks, consisting of arkosic sandstones, quartzitic sandstones and conglomerates.

Finally, the last set includes the Continental Terminal (CT3) and Quaternary formations: the sedimentary rocks of the CT3 consist mainly of fine to coarse clayey sandstones, sandy clay levels with several levels of ferruginous oolites which are discrepancies on the more or less kaolinized Birimian basement [

The hydrogeological context is characterized by the presence of two types of aquifers: discontinuous aquifers (altered, fissured and fractured zones) and continuous aquifers encountered in sedimentary formations. Basement rocks are aquifers only at the level of the first 100 meters [

The material used in this study is composed of the following data and tools:

- An extract of the GeoCover Circa 2000 mosaics (N30-10 and N30-15) of the October 2000 Landsat 7 satellite ETM + image downloaded from http://www.zulu.nasa for the realization of the map of lineaments.

- A DEM 30 m resolution SRT image downloaded from http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/Africa/

- The Liptako geological map (1/200,000) provided by the Ministry of Mine, the Kirtachi topographic map at a scale of 1/200,000, and the data sheets concerning 199 hydraulic boreholes are acquired at the Ministry of the hydraulic and sanitation and their partner.

- The documents used are mainly composed of thesis reports and reports on tectonic studies.

They are essentially composed of softwares:

- Envi 4.4 for the processing of satellite images;

- ArcGIS10.2.2 and Surfer for mapping watershed delineation, administrative boundaries, tracing of lineaments, mesh of study area, determination of lineament angles and lengths;

- Linwin and Oriana, for the directional rosette of Landsat images and the directional rosette for field structure measurements;

- RStudio for programming the computation of induced permeability and statistical calculations of hydrogeological and hydrodynamic parameters.

The methodological approach is as follow:

The preliminary phase of satellite image processing consists of eliminating radiometric noise in the ETM + bands and correcting the geometric distortions in order to make them perfectly superimposable to the existing thematic maps (topographic, geological and photogeological maps), [

The processed image was introduced into the ArcMap system where the lineaments were manually extracted at a scale of 1:50,000. The analog plot was based on the natural signatures of visual fractures to the naked eye such as: geological contact, plant alignment; the rectilinear borders of the cornices of the lateritic cuirasses, the contrasts of tone and texture.

The validation phase of the linear map is essential to judge the relevance of the applied method [

tracks, high voltages, etc.); 2) the second is based on the comparison between the major directions of the lineaments recorded on the Landsat image, the lineaments resulting from the hydrographic network and the microstructural field measurements. The lineaments of the hydrographic network come from an automatic extraction with the support DEM; 3) and finally the third is to verify the existence in the field of these lineaments from a geological and hydrogeological study.

The study of the relationships between the variables: transmissivities (T), specific flow rates (Qsp), and thicknesses of clay alterations (EA) required their transformation into bi-logarithmic. The correlation between T and Qsp highlights the geometry of fractures and their connectivity with drilling, while that between T and EA provides information on the productivity of aquifers.

According to the literature, there are two methods for modeling the underground flow in a fractured medium, namely the continuous model and the discrete model. The continuous model generates in the laboratory all the parameters coming into it while the discrete model uses data determined in the field, making it possible to find values similar to the reality of the field. Thus, the discrete model of Franciss (1970) was chosen as part of this work. For the calculation of induced permeability, several authors in West Africa including [

The application of the method of Franciss is based essentially on two assumptions defined as follows:

Fractures are assumed to be more or less vertical to simplify calculations. Thus, the study of the fracture density of the ETM + image by the directional rosette shows that these appear in their great majority vertical and/or subvertical;

- The thickness of the crushed area or the average gap between the two compartments of a surface fracture can be considered as an empirical linear function of the apparent length (L) of the megafracture. Thus, the application of this method requires the estimation of these two parameters C and Kf:

e = C ⋅ L → C = e i L i (1)

With,

e: thickness of the crushed area (m); corresponds to the difference between the depths of the first and last water inflow, or the length of the strainer in the case where the borehole has a single water inlet;

Li: length of the lineament (m);

C: empirical coefficient of proportionality. This coefficient was determined in the field by comparing the length and the opening of the hundreds of fractures measured in the field.

K f = T i e i ( m / s ) (2)

With,

Ti: Transmissivity of the sector (m^{2}/s), the Transmissivity values data, from the drilling datasheets, were determined by the methods of Cooper-Jacob and Theis while using the method of the ascent. Indeed, the study area is very contrasted from the point of view of transmissivity. Thus, to standardize the medium, all the exceptional transmissivity values were suppressed; therefore 61 values were retained;

Kf: hydraulic conductivity of the sector (m/s), comparable to the apparent average permeability characterizing the entire studied surface. According to Franciss, the coefficient of permeability of a fracture is expressed by a symmetrical tensor of the second order and the tensor K which expresses the cumulative effect of several fractures is obtained by the sum of the tensors of each fracture contained in the space system considered. The modules of the vectors K are the maximum permeabilities (K_{max}) and minimum permeabilities (K_{min}) along the major axis and the short axis of the ellipse and form between them, an angle of 90˚, and are expressed according to the following formula:

K ( max min ) = 1 2 ( K M N + K ∞ ) ± 1 2 [ ( K M N − K ∞ ) 2 + 4 K N O 2 ] (3)

With,

K_{NN} and K_{∞}: permeabilities with respect to a North-South and East-West axis system. The average induced permeability of the base (K_{moy}) is determined by the following relation:

K m o y = 1 / 2 ⋅ ( K max + K min ) (4)

K max = 0.5 ⋅ K f D ⋅ C L ( 1 + cos 2 λ − sin λ ⋅ cos λ ) (5)

K min = 0.5 ⋅ K f D ⋅ C L ( 1 − cos 2 λ + sin λ ⋅ cos λ ) (6)

With,

K_{max} and K_{min} respectively represent the maximum and minimum principal permeabilities;

λ: indicates the direction of the permeability.

To complete this procedure, a laboratory study was done. This consists of meshing the sector’s linear map to determine the fracturing variables. The mesh of the study area corresponds to 39 circles of 10,000 m of diameter of which each georeferenced circle is inscribed in a square of 10,000 m of side representing the mesh. Thus, within each circle, the total number of lineaments, their length and their orientation relative to the North were determined. After determining the hydraulic conductivity (Kf) and the empirical proportionality coefficient (C) of the region, the values of the induced permeabilities are calculated. The application of the Franciss model makes it possible to integrate Kf and C, as well as the geometrical parameters of the lineaments to determine the values of the induced permeabilities of each mesh. The program established under the software R, by integrating the model of Franciss, makes it possible to calculate the induced permeabilities of the base.

The result of the directional rosette (

Indeed, the statistical analysis of cumulative lengths shows that no class reaches a frequency of 10%, and the two main directions are: N0˚-10˚ (9%) and N160˚-170˚ (8%). The secondary directions are N10˚-20˚, N20˚-30˚, N30˚-40˚ and N140˚-150˚ with aoccurrence frequency of 7%. However, the directional class N0˚-10˚ remains the most dominant in fracture density.

The rosette on the structural data (fractures, quartz veins and pegmatite) found in the field (

The hydrogeological verification phase was conducted in several villages spread over the area of the study area. Various species of vegetation: tree, shrub and sub-shrub were observed. Among the plant species, there are Lannea microcarpum and Parkia biglobosa (Nere) to Alambare, Moli Haoussa, and Pamboua. As shrubby and sub-shrubby plant species, the type of Diospyros mespiliformis and Piliostigma reticulatum has been identified. This category of plants is widespread throughout the area namely to Diango, Kotaki, Dar Salam, Kobouri. All these plants are hydrophilic and are good indicators of the presence of groundwater. The termite mounds (Macrotermes bellicossus and Macrotermes subhalinus) have also been observed in many villages, including BabobaKoira and Tondobanda. These results are consistent with those found by [

The synthesis of the results obtained from the various supports (

The values of Transmissivities vary from 1.30 × 10^{−05} to 2.20 × 10^{−03} m^{2}/s, with an average of 2.28 × 10^{−04} m^{2}/s and a standard deviation of 3.36 × 10^{−04} m^{2}/s.

The correlation between transmissivities and specific flows provides information on the hydraulic properties of reservoirs [^{2} = 0.81; thus the relation linking these two parameters is linear and can be written in the following way:

ln ( Q s p ) = 0.8316 ln ( T ) − 8.8096 et R 2 = 0.81 (7)

The thicknesses of saprolitein the study area (

Classes | Very low | Low | Middle | Strong | Very strong |
---|---|---|---|---|---|

Thicknesses of saprolite | <10 m | 10 - 15 m | 15 - 25 m | 25 - 40 m | 40 - 70 m |

Number (n = 117) | 5 | 16 | 45 | 40 | 11 |

Percentage | 4.27 | 13.68 | 38.46 | 34.19 | 9.40 |

For this model, the hydraulic conductivity and the coefficient of proportionality were obtained as follows:

ü Hydraulic conductivity

K f ( m / s ) = ∑ T i e i = 3.14 E − 01 33881.9 = 9.25 × 10 − 6 (8)

The value of Kf is acceptable because it belongs to the range of known values which vary between 10^{−8} and 10^{−4} m/s for the crystalline formations of West Africa [

ü Coefficient of proportionality (see Equation (1), in the Methods Section)

e = 28 cm

L = 5000 cm

C = e/L = 5.6 × 10^{−3}

This value is of the same order of magnitude as those found by [

The average permeability (k-avg) was chosen for this work. It ranges from 0.05 × 10^{−8} to 3.4 × 10^{−7} m^{2}/s with an average of 1.46 × 10^{−7} m^{2}/s and a standard deviation of 1.12 × 10^{−7} m^{2}/s. The analysis in

ü A large axis with very high N-S to subvertical permeability that goes from SidiKoira, BaleyKoira to Boki areas;

ü Another major axis oriented W-E;

ü An axis oriented NE-SW that goes from BahobaKoira via TankoundéSékouKabé to Sénokokondjé.

Nevertheless, we can notice that the permeability is very low in the extreme northwestern and southwestern parts of the study area.

Relationship between induced permeability and hydrogeological properties

A correlation matrix (

The correlation of these parameters highlights their intervention in the operation and productivity of the boreholes in the study area.

Variables | NS | Q | S | Qsp | T | K | Ki | TD |
---|---|---|---|---|---|---|---|---|

NS | 1 | |||||||

Q | −0.09 | 1 | ||||||

S | −0.19 | −0.25 | 1 | |||||

Qsp | −0.03 | 0.85 | −0.31 | 1 | ||||

T | −0.08 | 0.82 | −0.32 | 0.65 | 1 | |||

K | −0.08 | 0.71 | −0.21 | 0.51 | 0.97 | 1 | ||

Ki | −0.06 | −0.03 | 0.2 | 0.02 | −0.03 | −0.01 | 1 | |

TD | 0.19 | −0.19 | 0.57 | −0.2 | −0.28 | −0.25 | −0.05 | 1 |

NS: static level; Q: flow rate; S: storage coefficient; Qsp: specific flow; K: permeability; Ki: induced permeability; TD: total depth.

The modeling of the preferential axes of underground flow by the method of Franciss is based on the linear network satellite with a circle diameter of 10 km. Thus, in the study area, the directional distribution of lineaments revealed four principal directions: N0˚-10˚, N90˚-100˚, N130˚-150˚ and N50˚-60˚. To these main directions are added the secondary directions which are: N60˚-70˚ (7%), N110-120˚ and N160˚-170˚ (6%). Nevertheless, the direction N0-10˚ is the most dominant with 13% frequency. Indeed, this family has been found by several authors who have worked in similar areas: in Cote d’Ivoire [

The direction N90˚-100˚ is the second direction in terms of frequency after that of N0˚-10˚ in the zone of Tamou. This direction is much more remarkable on the lineaments of the hydrographic network characterizing the major direction of the rivers. It is superimposed perfectly with the satellite lineaments, so these lineaments correspond to fractures that follow the surface waters. This result is similar to the work of [

The N50˚-60˚ directional class, which is one of the dominant directions observed in the field, is mainly found in the Goroubi sector where the rugged substratum is abundantly exposed. The accident mainly affected the outcrop of granodiorite formations. This direction belongs to the NE-SW orientation which also corresponds to the Eburnean direction.

The presence of active vegetation (tree species, shrubs and sub-shrubs) at the lineaments, confirm that the latter correspond to fractures. Consequently, these fractures represent corridors where the water reserve allows the good growth of plant species [

These results probably show that the dominant directional classes are the longest on the scale of 1/50,000 with the WGS 84 Repository/UTM, zone N31 for the ETM + image of LANDSAT 7. As the class N0˚-10˚ is the more important cumulative length. Therefore, there is good agreement in number and length in the preferential classes listed above; it is to be supposed that these directions are the most productive in this region. Thus, the model uses these lineaments to characterize the underground flow, while considering them as vertical and permeable fractures.

The values of the induced permeability of the study area vary between 0.05 × 10^{−8} and 3.4 × 10^{−7} m/s for squared meshes of 10 km. These values are higher than those found by [

^{3}/h, 5.14 m^{3}/h and 15 m^{3}/h. This result is similar to the work of [^{3}/h, 1.07 m^{3}/h and 1.2 m^{3}/h. This low permeability could be due to a low fracturing density and where the fractures are relatively less open and are not interconnected.

Finally, from the results in

Remote sensing combined with GIS is an essential technique for processing satellite images in order to map the fracture network. The latter made it possible to identify the preferential classes of major fracture and mega-fracture directions. Linear support served as databases to identify major underground flow axes: N-S, E-W and NE-SW, through the method of Franciss. This model yielded reasonable induced permeability results in the basement area. In addition, the groundwater circulation corridors coincide with the hydrographic network, which proves that parts of the aquifers are fed by surface water through drainage. However, this study confirms that the induced permeability does not directly affect the productivity of an aquifer in a fractured environment but is related to the density of the fractures (open and interconnected), to the flow rate and water table.

The authors thank very warmly and express their gratitude to the Non Governmental Organization: International Aid Service (IAS) Niger for the financial, material and human support it provided for the smooth running of this study. We also thank the Ministry of Hydraulics and Sanitation of Niger, Abdou Moumouni University (Niger) and Joseph Ki-Zerbo University (Burkina Faso) for their various contributions.

The authors declare no conflicts of interest regarding the publication of this paper.

Saidou, G.I., Koussoube, Y., Saley, A.K.H., Sandao, I., Hayes, P. and Ousmane, B. (2019) Groundwater Flow Modeling by Using the Permeability Induced by Satellite Lineaments in Discontinuous Aquifers and Semi-Arid Context: A Case Study of the Liptako Region (South-West of Niger). Journal of Water Resource and Protection, 11, 1090-1109. https://doi.org/10.4236/jwarp.2019.118064