Contribution of vertical electrical soundings, Landsat 8 and SRTM images to the comparative study of the hydrogeological characteristics of the Foto, Keleng and Foreké areas, West Cameroon

Abstract

The study area is located in the town of Dschang, West-Cameroon, on the southern flank of the Bambouto Mountains. The acute shortage of drinking water supply points due to lack of structural knowledge on existing aquifers. The present work allows the establishment of a hydrogeological map of the study area. The data from the vertical electrical soundings carried out on 64 points, show 12 electrical anomalies: A, H, K, Q, AK, HA, HK, HQ, KQ, KA, KH and Q; divided into two domains: a resistive domain with resistivities between 2892.63 and 27 323.56 Ω m and a conductive domain with resistivities between 0.43 and 2892.63 Ω m. The conductive domain is more represented in the basaltic zone while the most resistant domain is represented by the gneissic and the granitic zones. Hydraulic conductivity in the study area ranges from 0.004 to 44.852 m/day, porosity from 0.192% to 42.615%, longitudinal conductance from 0.001 to 1.101 Ω⁻¹ and transmissivity values from 0.019 to 504.023 m²/day. The area includes two types of aquifers; saprolite aquifer from 1.6 to 16.26 m with a roof varying between 1.5 and 16 m and aquifers on thick fractures from 16.26 to 38.34 m with a thickness varying between 16 and 36 m. The major and minor fracturing directions are respectively N 60° to N 70° E and N 0° to N 30° E. According to the hydrogeological potentials, Foto which is made up of basaltic formation is very favourable, Foreké on granitic formation is moderately favourable and Keleng on gneissic formation is not very favourable.

Hydrogeology; Aquifers; Structure; Dschang; Hydraulics parameters.

1. Introduction

Groundwater accounts for 98–99% of the earth's freshwater reserves (Margat, 2008). The lack nearby acess to drinking water means that people have to walk for hours to buy supplies instead of going about their normal businesses. Due to the accelerated and uncontrolled development of cities, almost 65% of the inhabitants in Africa are not connected to a drinking water distribution network (PGDDAU, 2003). This situation is fully verified in Cameroonian cities, particularly in the city of Dschang, where people are facing an increased lack of drinking water supply (Da Costa et al., 2005). The city of Dschang has already been the subject of several studies in the fields of geology. The works of Kenfack et al. (2011), show that there are three petrographic types in the locality of Ngoua: sedimentary rocks, magmatic and metamorphic. Bella et al. (2014), demonstrate that the tertiary trachytic dome of Foréké-Dschang consists of porphyric and microlitic lava containing sanidine, pyroxene and oxides. It is dated 16–8.8 Ma and was placed in gneisses belonging to the Pan-African basement. According to Kwekam et al. (2015), the Dschang biotite granites are believed to result from the partial fusion of metamorphic and igneous rocks under the action of CAFB (Central African Fold Belt) tectonics in Cameroon. According to Benammi et al. (2017) the Ngoua lignite is between 20 and 20.2 Ma in age. Fozing et al. (2019) presented by analysis that the Pan-African granitic pluton of Dschang is located near the Central Shear Zone of Cameroon. All these previous works are focused on petrography and geotechnics. Yet the drinking water sector in Cameroon lags behind many countries with comparable incomes (AMCOW, 2011). The supply of this resource is very limited in the city of Dschang. The urban perimeter benefits from a classic CDE AEP characterised by low extension, almost permanent cuts and dubious potability (Guemuh, 2003). A few insufficient and poorly maintained water supply sources enable the populations of the urban perimeter to reduce the shortage of drinking water. The most representative are the springs of Madagascar, Gendarmerie, Assentsa, Vallée, Lefock, Tchoualé, the fountain of Campus B, the borehole and spring of Campus C, the boreholes of the Foto chiefdom, of Zembing after the signal hill, the constructed wells of Tsinkop, Keleng, Lépia, Foréké, Tchoualé and Tapalé. Despite these numerous water points, the coverage rate for access to water is less than 50% (MBA et al., 2019). This situation puts certain sectors of the city in water shortage. In other places where efforts have been made by the municipality to install boreholes, they are non-functional. This is the case in Mak'a at the Fokoué crossroads, and at the entrance to the Foto chiefdom, where people are sometimes forced to resort to the spring at Campus C. The failure of these structures could be due to lack of control over the characteristics of groundwater. The aim of this work is to produce a map of the hydrogeological potential, in order to target the areas favourable for the installation of hydraulic works.

2. Study area

The study area is located in the West region of Cameroon, more precisely in the town of Dschang (Figure 1a), which is centred at coordinate points 5° 27′ North latitude, 10°04′ East longitude. This study area includes the localities of Foto, Keleng, Foreké Lépia and is located between coordinates 5° 26′ and 5° 28′ North latitude and between 10° 02′ and 10° 06′ East longitude with an altitude varying between 1300 and 1500 m and a surface area of about 17.37 km² (Figure 1b). The relief of the study area is mostly made up of hills at low altitudes (Figure 1b), 1458–1510 m, dominating in the localities of Foto and Keleng. Medium-altitude areas (1300–1353 m) which dominates in the locality of Foto and Foréké. The hydrographic network is of the subdendritic to parallel type (Figure 1b). There are two contributory streams: the waterfall and the stream of Campus A. Streams draining flat-bottomed valleys, such as those of Ngui and Foreké, form a string of flat-bottomed valleys from Zendeng (on the road from Fongo Deng to the military camp).

Figure 1.

Location map of the study area; a) Topographical map of Dschang district, b) Topographical map of the study area.

2.1. Geology and tectonics of study site

The study area belongs to the Cameroon Volcanic Line (CVL) which is a tectonic and plutono-volcanic megastructure, stretching over 1 600 km, and 100 km wide. It extends from the Atlantic Ocean (Gulf of Guinea) to the interior of the African continent (Lake Tchad), following a N30°E direction. It has two segments: an oceanic segment, which includes the islands of Pagalu, Sao Tome Principe, and Bioko. A continental segment, underlined by a succession of stratovolcanoes, collapse ditches and plutono-volcanic massifs (Tchoua, 1974). It constitutes the major tectonic event in Central Africa. The town of Dschang is made up of a large ensemble of rocks (Figure 2a) of several natures (plutonic, volcanic and metamorphic). It is located on the southern slope of the Bambouto Mountains, characterised by various volcanic products covering the granito-gneissic basement (Kwekam et al., 2020). The town complex of Dschang is based on Pan-African granitoids (Benammi et al., 2017). The study area is covered by several petrographic granites, basalts and gneisses Fozing et al. (2019) (Figure 2b). The existence of columnar basalts and blocks in some quarries in the locality of Foto and Foreké testify to the volcanic dynamism affecting the CVL. The presence of granite balls and gneissic slabs in the localities of Lépia, Foréké and Keleng shows the nature of the granito-gneissic basement (Kwekam et al., 2015).

Figure 2.

Geology of the site, a) Geological map of Dschang (Kwekam et al. (2020)); b) Modified geological map of the study area from Fozing et al. (2019).

3. Méthodology

3.1. Vertical electrical soundings

Seventy-three (73) survey points were programmed and 64 were carried out (Figure 3). The other 9 points are those located at sacred sites, bus stations and on the hillsides. The resistivity meter used for reading the apparent resistivity is a 4-point Light10W: it displays the results directly as apparent resistivity. The electrical probing points are carried out at the various nodes of each mesh in a 200 m section, using the Schlumberger-Wenner quadrupole, where the measuring electrodes are stationary and the distance between the injection electrodes is varied. The 4 electrodes are disposed along the same line, with the M and N electrodes which represents the measuring electrodes located inbetween the electrodes A and B which represents the electrodes for injecting electric current. The assembly is symmetrical with respect to a central point O. The results obtained are used to draw a curve of apparent resistivity as a function of AB/2.

Figure 3.

Synthesis of the various vertical electrical sounding points carried out.

3.2. Iso-resistivity maps

Iso resistivity maps represent the lateral variation of apparent resistivity values in a given area and at a defined depth. The different maps are modelled using Excel 2016 and ArcGis 10.4.1 software. The data recorded in Excel with the csv extension are then exported to ArcGis for the creation of the various maps.

3.3. 1D and 2D inversion of vertical electrical soundings

Geophysical inversion consists of calculating the average between the apparent resistivities and returning the results as true resistivity. It allows the modelling of sounding curves, resistivity sections via the Res2Dinv software, logs and profiles of the different sounding points.

3.4. Aquifer hydraulic parameters

An indirect method using empirical formulas allows the calculation of hydraulic conductivity, porosity, transmissivity and longitudinal conductance of the formations. Hydraulic conductivity, (K) expressed in meter per day (m/day), is a quantity that expresses the ability of a porous medium to allow a fluid to pass through under the effect of a pressure gradient. It is determined using Eq. (1) (Heigold et al., 1979, Oguama et al. (2020). Porosity represents all the voids present in the rock. It was estimated using Eq. (2) (Oguama et al., 2020). Transmissivity (Tr) is a hydraulic parameter that estimates the flow rate of water per unit width of the aquifer under the effect of hydraulic gradient, including the thickness of the aquifer. For the determination of the transmissity of the study area, the analytical relationship between the transverse resistance (S) in Ω⁻¹ (equation 3) on one hand and between transmissivity and longitudinal conductance on the other hand (equation 4) (Niwas and Singhal, 1981; Oguama et al., 2020) was used, taking into account a material (aquifer) prism with a cross-sectional unit and a thickness h in metres.

K = 386,40ρ^−0,9383 (1)

Where ρ is the resistivity of the aquifer in Ω.m and K is the hydraulic conductivity in m/d.

Ø = 25,5 + 4,5 ln K (2)

Where K is the hydraulic conductivity in m/day and Ø is the porosity in %.

$$\text{S}=\frac{\text{h}}{\text{ρa }}$$ (3)

Where h is the thickness of the aquifer in m deduced from the log in 1D inversions, ρ the resistivity of the aquifer in Ω.m and S the longitudinal conductance in Ω⁻¹.

$$\text{Tr}=KS\text{ρ }$$ (4)

Where K is the hydraulic conductivity in m/day, S the longitudinal conductance in Ω⁻¹, ρ the aquifer resistivity in Ω.m and Tr is the transmissivity in m²/day.

3.5. Lineament mapping

The aim of this mapping is to determine the different alignments of geological objects, vegetation and topographical discontinuities in order to deduce the lineaments characterising a given area.

The modelling of groundwater which flows in fractured networks is carried out using a theoretical approach based on the geometrical characteristics of the lineaments. The rocks of the crystalline and crystallophyllous basement are intrinsically very low in permeability. The lineaments that affect them are responsible for almost all of their permeability properties (fracture permeability). Depending on their density, size, arrangement in the environment and physical properties, fractures considerably increase the hydraulic properties of bedrock. Indeed, crystalline and crystallophyllous rocks are endowed with a very low interstitial porosity, therefore, the formation of aquifers is only possible through the action of fracturing-lineament coupled with alteration. The elaboration of a lineament map in this case study allows us to target fractured or fissured sectors of hydrogeological interest. Landsat 8 and STRM images of the study area were used. The processing of these images using Envi, ArcGis and Géomatica software enabled the extraction and distribution of the lengths, angles and densities of the various lineaments. Rock worksV16.2 software allowed the installation of a steering rosette.

4. Results and discussions

4.1. Visible iso-resistivity maps

The resulting (Figure 4) maps have apparent resistivity values ranging from 96 to 87 462 Ω m and are divided into three main areas. The conductive domain corresponds to low apparent resistivity (96–1000 Ω m), a low-conductive domain with medium apparent resistivity (1000–10 000 Ω m) and a resistive domain with high apparent resistivity (10 000 to 87 462 Ω m). The maps at depths of AB/2 = 4.4 m; 6.3 m; 9.1 m; 13.2 m; 19 m; 33 m; (for MN/2 = 0.5 m); 13.2 m; 19 m; 27.5 m; 33 m; 40 m; 50 m (for MN/2 = 5 m); 70 m; 83 m (for MN/2 = 25 m), show a similarity in the apparent resistivity values from 177 to 19 647 Ω m. Figure 4 shows 11 maps which better show the lateral variation of the apparent resistivity at different depths in the study area.

Figure 4.

Apparent iso-resistivity maps, A) for AB/2 = 1.5 m; B) for AB/2 = 2.1 m C) for AB/2 3 m; D) for AB/2 = 27.5 m; E) for AB/2 = 50 m; F) for AB/2 = 58 m; G) for AB/2 = 58_25 m; H) for AB/2 = 83 m; I) for AB/2 = 100 m; J) for AB/2 = 120 m; K) for AB/2 = 140 m.

At AB/2 = 1.5 m (Figure 4A) the SSW of the sector is located in the area of high apparent resistivity (20 522–13 185 Ω m). The entire NW and the centre is marked by medium resistivity (13 185–1 066 Ω m). At this depth, the SW point has low apparent resistivities (1 066–346 Ω m). This variation in range could indicate the presence of more or less altered granite balls in the locality of Lepia. They are responsible for the existence of this resistant zone. The conductive and not very resistant zones at this depth can correspond to the silty, sandy and clayey soil layers as shown in the abacus of Marescot (2006). They are more or less porous and favour water circulation.

From AB/2 = 2.1 m up to AB/2 = 27.5 m (Figures 4B-4D), the SE, SW, NNE, E and S of the sector shows a discontinuous distribution of apparent resistivities where they vary from 2 486 to 177 Ω m passing through 3 770 Ω m for the conductive and low-conductive areas; from 87 462 to 10 974 Ω m passing through 20 628 for the SSE resistant areas of the sector. This distribution may explain the current flow in alteration layers for the conductive and low-conductive areas; respectively for basalt and gneiss alterations. The concentration of apparent resistivity values at the SSE point (Figures 4C-4D) indicates the existence of large gneiss slab observed in the Keleng locality. This apparent resistivity value (10 974 to 87 462 Ω m) corresponds to an almost healthy level. This discontinuous distribution under more or less fresh basement rocks extends to AB/2 = 58_25 m where MN/2 = 25 m (Figures 4D-4G).

The SE point of Figure 4G in the locality of Foréké shows an extension of the granitic balls previously observed at 27.5 m in the locality of Lépia where they become less resistant (5 459- 4 4433 Ω m). At the same depth, the centre of the sector reveals a concentric anomaly of average apparent resistivity (2 384–1400 Ω m) which correspond to eruptive rocks in the abacus Palacky (1987) and Marescot (2006). These eruptive rocks may correspond to a large fractured basaltic block that contains an aquifer. Beyond AB/2 = 58 m to AB/2 = 140 m passing through 83 m, 100 m and 120 m (Figures 4H-4K) the SE, SW and NE tip, show an increasing variation of conductive (from 554 to 722 Ω m) and resistant (from 10 893 to 87 961 Ω m) domains. This variation could indicate a continuity of fractured and almost healthy formations. It is more pronounced at 140 m, where the values are within those observed in the abacuses of Marescot (2006) and Tesis (2010); they vary from 1000 to 100 000 Ω m for eruptive and fresh metamorphic rocks that may correspond to the basement rocks of the study area. All the interpretations of these maps at different depths (from AB/2 = 1.5 m to AB/2 = 140 m) highlight the altered layers. They would vary from 1.5 m to 3 m for soil levels developed on gneiss and granite and over 3 m for levels developed on basalt. These layers are porous and correspond to the conductive areas, thus facilitating the infiltration of water towards the fractured levels. The low-conductive and resistant areas are predominantly granitic and gneissic zones and present rocks with horizontal cracks in the first few metres, which correspond to the limits between the different light and dark bands in the gneissic sector, and vertical in depth corresponding to the spaces between the basalt prisms and fractures in the granites. The density of the fractures decreases with depth. These levels fractured at depth allow the fracture aquifers to be set up.

4.2. Sounding curves

The inversion of the data of the different vertical electrical sounding points has revealed 12 main types of curves: A (12.5%), H (3.12%), K (20.31%), Q (4.68%), AK (4.68%), HA (4.68%), HK (9.37%), HQ (01.56%), KQ (09.37%), KA (01.56%), KH (20.31%) and QH (7.81%) (Figure 5). The true resistivities in the whole sector vary from 0.43 to 27 323.5 Ω m with an average of 1 067.14 Ω m; for thicknesses ranging from 0.35 to 38.34 m with an average of 05.42 m. These values are higher than those found by Obiora et al. (2015) north of Ezza, by Njueya et al. (2016) in Ebone and its environs, by Falade et al. (2019) in Akure in south-west Nigeria and by Oguama et al. (2020) in Enugu. These authors find respectively, the curves of type QQH; QHK; QHA; QQQ; HAK; KHK; QQ for resistivities ranging from 03 to 5 800 Ω m with soil layers ranging from 0.2 to 214.7 m. HK, K, Q and HKH anomalies with resistivities ranging from 15 to 300 Ω m. H, A, HA, AK type curves with resistivities ranging from 43 to 6 277 Ω m for 0.5–21.1 m and resistivities ranging from 06.2 to 204.2 Ω m for thick soil layers from 18.2 to 56.4 m. This difference is linked firstly to the geological context (petrographic type in the basement zone and level of alteration in the sedimentary zone), and secondly to the more technical equipment technology in the recent case studies. This multitude of geometrical characterisation of the curves obtained in this case study confirms the basis of the three-layer models (A, H, K and Q) stated by Chapellier (2000). More than three layers for types HK, KA, KQ and others (Koussoubé et al., 2003) in crystalline basement zones. The diversity of the curves observed with respect to the position of the sounding points is explained by the topographical position and variation of relief in the study area (Penz, 2012). This diversity of curves obtained by petrographic type for a different type of lithology highlights the idea that the shape of the sounding curves does not depend on the lithological nature but mainly on the structure of the terrain (Njueya et al., 2016), or even on its mineralogical composition.

Figure 5.

Histogram showing proportions of the different VES curves.

Figure 5 shows the proportions of the different types of curves. The whole sector is dominated by the K (20.31%) and KH (20.31%) type curves (Figure 5); with a significant percentage on basaltic and gneissic formations (7.81% for K curves and 10.93% for KH types). The HQ and KA type curves show a small percentage (1.56%) and are present on basaltic and granitic formations. The Q and HA type curves are absent on gneissic, granitic and basaltic formations respectively. The KA and QH type curves are absent on basaltic and gneissic formations (Figure 6). Figure 6 shows the distribution of these 12 types of curves at each VES point in the study area.

Figure 6.

Distribution of the different VES curves types.

4.3. 1D and 2D inversion and associated geological model

Three profiles (Figure 7) were made taking into account the petrographic types, topography and VES alignment. Profile P1 is oriented SE-NW profile P2 NW-SE and, profile P3 is SW-NE. The construction and interpretation of the geological models of the 2D inversion are mainly based on geology, structure, lithological nature, different variations of resistivity ranges and field observations. In addition, the blocky arrangement of the different resistivity ranges here translates into the level of 2D inversions, fractures presented in Figure 12, notably those not perceptible by remote sensing and those resulting from the different lithological contacts (Figure 3, Figure 9B-9C-9E-9F–9H–9I); these fractures are essential for characterising the hydrological characteristics of the rock mass hosting water, and are, in addition to the hydraulic parameters, one of the principal parameters. Hence the need to take them into account when building geological models.

Figure 7.

Distribution of resistivity profile cutting lines.

Figure 12.

Linear map and rosette of directions.

Figure 9.

Representation of anomalies related to the resistivity sections; A) iso-resistivity map at AB/2 = 70 m and MN/2 = 25 m; B) resistivity section of profile P1; C) geological model of the resistivity section of profile P1; D) iso-resistivity map at AB/2 = 758 m and MN/2 = 25 m; E) resistivity section of the P2 profile; F) geological model of the resistivity section of the P2 profile; G) iso-resistivity map at AB/2 = 140 m and MN/2 = 25 m; H) resistivity section of the P3 profile; I) geological model of the resistivity section of the P3 profile.

The lithological sections of the vertical electrical sounding points of profiles P1, P2 and P3 (Figure 8) from the 1D inversion shows the variation of the different soil layers and fractured rock levels taking into account the morphology of the field. The different aquifer zones are located around 1.94 and 20.72 m, in contrast to those observed in 2D (Figure 9) where they vary from 1.94 to 70 m.

Figure 8.

Lithological sections from 1D inversion; a) profile P1, b) profile P2, c) profile P3.

The data from the iso-resistivity maps at AB/2 = 58, 70 and 140 m superimposed on the geological map of the sector, allows a geological model of the resistivity sections P1, P2 and P3 to be drawn up thanks to field observations (Figure 9). From the surface to the base, in the first sections of land, they present weathered product which can be a sandy, clayey-silt, clayey-sand and clayey soil levels with resistivities varying between 265 and 873 Ω m. The base of the sections shows a more or less healthy and fractured base with resistivities between 46 and 9 306 Ω m. This arrangement is almost similar to that found by Wendgouda (2012) in Kourwéogo. It is different from that of Abdulrazzaq et al. (2020) in Iraq. From the surface to the base in the basement zone, the first author presents laterite armouring, clay soil levels, cracked and sound basement for resistivities ranging from 22.7 to 21 792 Ω m. The second, in the Karstic area, shows arable, marly, clay and dolomitic (weathered product) soil levels, with deep fractured basement for resistivities ranging from 1.34 to 54 Ω m. These observed differences in the variation of resistivity values and the associated geological patterns would lie within the geological and tectonic context of each study area.

The lateral and vertical variations of the resistivity ranges at the 2D section of the P1 profile (Figures 9A-9C), in addition to fractures related to lineaments from remote sensing, allow the detection of existing geophysical fractures in the more or less fresh and wealthered rocks located beyond 25 m of the section (Figure 9C) for resistivities ranging from 18.3 to 44.6 Ω m. Given the geology and topography of the section, the various alteration fronts are located upstream of the fractures and the fluctuation of resistivity in these fractured zones may show a potential fractured aquifer at depth and an alteration aquifer at the surface (Figure 9C).

The lateral variation of the resistivities arranged as a block at the resistivity section of the P2 profile (Figure 9E), makes it possible to give a geological sketch of this resistivity section (Figure 9F) thanks to the different soil profiles and auger wells studied. At about 4 m, the section shows average resistivity levels (120–873 Ω m), corresponding to clay, sand and gravel soils (Figure 9F). These gravels are dominant in the basaltic zone and could therefore correspond to the lateritic gravels observed upstream of the zone. At depths greater than 20 m, the block arrangement of resistivity ranges (120–2 356 Ω m) indicates, in addition to the fractures from remote sensing, existing fractures on the iso-resistivity map at AB/2 = 58 m, and those from inversions (Figures 9D-9E). The existence of these fractures in a less resistant environment show the presence of an aquifer over fractured at depth, overcomed by an aquifer over weathered at the surface (Figure 9F).

The individualisation in several separate blocks of the resistivity ranges observed at the resistivity section of the P3 profile highlights all the fractures obtained by remote sensing and geophysical surveys (Figures 9G-9H). This arrangement is similar to that of section P2 profile. In this section (profile P3), the resistivities of the different blocks varies from 97.8 to 2007 Ω m; with a high concentration of medium resistivity in the lowlands. This section is marked by a thicker alteration layer which penetrates deepe about 55 m and is thought to correspond to highly fissured and weathered granite (Figures 9G-9I). These different blocks present in highly conductive places with resistivities between 10.6 and 21.6 Ω m (Figures 9H-9I) which could constitute potential aquifer levels on fracture (Figure 9I).

These profiles give a sketch of the subsoil in the sector via its geology, they show the electrical behaviours along the vertical and horizontal lines of the different sections and indicates the different types of aquifers. In fact, the studies links these sections to the different iso-resistivity maps, allowing to highlight the geophysical lineaments not perceptible by remote sensing.

4.4. Hydraulic parameters

The maps of hydraulic conductivity, porosity, longitudinal conductance and transmissivity (Figure 10) were constructed using the analysis of data from the various vertical electrical sounding points. The hydraulic conductivity in the study area varies from 0.004 to 44.852 m/d, the porosity Ø from 0.192% to 42.615%, the longitudinal conductance S from 0.001 to 1.101 Ω⁻¹ and the transmissivity values from 0.019 to 504.023 m²/day (Table 1). These values are roughly equal to those found by Falowo et al. (2020) in the central part of Ondo State in Nigeria where hydraulic conductivity K varies from 0.1382 to 48.1210 m/day and Tr from 2.4705 to 221.3568 m²/day. They are much lower than those found by Oguama et al. (2020) in North Enugu where hydraulic conductivity K varies from 2.71 to 70.45 m/s, Ø from 33.71 to 49.44% and Tr from 49.2288 to 1127.944 m²/day. The range of porosities up to 43% here reflects the influence of weathered layers, notably the superficial deposits coresponding to the weathering products (Figure 9) far from the bedrock and the granitic arenas made of unconsolidated sands closer to the bedrock (Figure 9I).

Figure 10.

Spatial variation of hydraulic parameters; A) hydraulic conductivity K, B) porosity Ø, C) longitudinal conductance S, D) transmissivity Tr.

Table 1.

Variation of hydraulic parameters at VES points.

Station	Number of layers	Aquifer resistivity (ρin Ω.m)	Layer thickness (h in m)	Hydraulic conductivity (K in m/day)	Longitudinal conductance (S in Ω−1)	Transmissivity (Tr in m2/day)	Porosity (Ø in ​%)
VES1	3	66.34	9.71	7.720	0.146	74.963	34.697
VES2	4	231	3.17	2.411	0.014	7.643	29.460
VES3	5	2347.68	5.44	0.277	0.002	1.508	19.727
VES4	4	699.2	9.57	0.858	0.014	8.211	24.811
VES5	5	1179.32	3.32	0.527	0.003	1.749	22.617
VES6	5	126.74	6.09	4.221	0.048	25.704	31.980
VES7	5	392.31	27.5	1.471	0.070	40.453	27.237
VES8	3	1613.46	2.71	0.393	0.002	1.066	21.301
VES9	5	106.04	12.49	4.984	0.118	62.256	32.728
VES10	5	328.1	7.8	1.738	0.024	13.556	27.987
VES11	3	3748.12	14.94	0.179	0.004	2.677	17.763
VES12	3	2105.32	4.34	0.307	0.002	1.332	20.184
VES13	4	2802.98	1.94	0.235	0.001	0.456	18.982
VES14	3	620.32	7.78	0.959	0.013	7.464	25.314
VES15	3	845.15	20.72	0.719	0.025	14.897	24.015
VES16	3	1179.02	6.49	0.527	0.006	3.420	22.618
VES17	3	203.94	7.81	2.708	0.038	21.150	29.983
VES18	3	256.19	31.57	2.189	0.123	69.109	29.026
VES19	5	12.33	12.48	37.098	1.012	462.987	41.761
VES20	5	205.71	4.02	2.686	0.020	10.799	29.947
VES21	3	465.3	3.25	1.255	0.007	4.077	26.521
VES22	5	106.42	13.13	4.968	0.123	65.228	32.713
VES23	3	788.38	19.69	0.767	0.025	15.105	24.307
VES24	3	236.24	13.66	2.361	0.058	32.252	29.366
VES25	4	202.92	3.94	2.721	0.019	10.720	30.004
VES26	5	147.54	4.03	3.663	0.027	14.761	31.342
VES27	5	47.27	11.73	10.591	0.248	124.231	36.120
VES28	3	2325.05	3.79	0.280	0.002	1.060	19.767
VES29	5	945.46	9.67	0.648	0.010	6.262	23.544
VES30	5	135.91	7.5	3.954	0.055	29.658	31.687
VES31	3	212.85	18.44	2.602	0.087	47.985	29.804
VES32	3	634.23	15.49	0.940	0.024	14.557	25.220
VES33	5	92.08	7.73	5.686	0.084	43.953	33.321
VES34	3	762.84	3.57	0.791	0.005	2.824	24.445
VES35	3	283.97	38.34	1.989	0.135	76.244	28.593
VES36	3	139.08	13.47	3.870	0.097	52.132	31.590
VES37	5	10.06	5.75	44.852	0.572	257.900	42.615
VES38	5	789.45	16.81	0.766	0.021	12.879	24.301
VES39	4	785.86	4.47	0.769	0.006	3.439	24.321
VES40	3	1666.67	10.66	0.382	0.006	4.068	21.165
VES41	5	511.89	3.47	1.148	0.007	3.983	26.120
VES42	3	2464.29	5.37	0.004	0.000	0.019	0.192
VES43	3	323.64	3.1	1.760	0.010	5.457	28.045
VES44	4	1015.39	2.97	0.606	0.003	1.799	23.245
VES45	3	264.31	6.28	2.126	0.024	13.353	28.895
VES46	5	563.78	7.38	1.049	0.013	7.741	25.715
VES47	5	596.59	3.3	0.995	0.006	3.283	25.477
VES48	5	128.14	6.38	4.178	0.050	26.653	31.934
VES49	5	1988.35	6.56	0.324	0.003	2.123	20.424
VES50	5	278.62	6.15	2.024	0.022	12.449	28.673
VES51	3	2940.12	11.12	0.225	0.004	2.499	18.782
VES52	3	414.52	6.87	1.397	0.017	9.600	27.006
VES53	3	305.05	11.71	1.860	0.038	21.782	28.293
VES54	5	1862.73	7.09	0.344	0.004	2.439	20.698
VES55	4	721.04	14.14	0.834	0.020	11.790	24.682
VES56	3	752.02	3.12	0.802	0.004	2.501	24.505
VES57	5	32.79	1.6	14.897	0.049	23.836	37.655
VES58	5	62.24	3.51	8.194	0.056	28.760	34.965
VES59	4	87.99	4.78	5.932	0.054	28.356	33.512
VES60	5	56.11	2.89	9.026	0.052	26.084	35.400
VES61	5	1008.67	7.28	0.610	0.007	4.438	23.273
VES62	3	347.92	6.2	1.645	0.018	10.201	27.741
VES63	5	12.47	13.73	36.710	1.101	504.023	41.714
VES64	3	390.81	4.25	1.476	0.011	6.274	27.253

The values of K, Ø, S and Tr are very high in the basalt formation, and can be explained by the fact that the minerals present in basalts are very vulnerable to weathering, also due to the existence of a small proportion of the elements. The gneissic formation has very low hydraulic parameter values compared to the granite formation. This difference is related to the banded arrangements of sheet minerals constituting gneiss, in contrast to granite where quartz-feldspathic minerals scattered in the rock are abundant. These zones of high and medium hydraulic conductivity, porosity, longitudinal conductance and transmissivity indicate the permeable, conductive zones where the layers of soil are weak in impermeable clay elements and rich in coarse elements. For this purpose, they show areas with high hydraulic potentials.

Figure 11 shows the distribution of aquifer thicknesses. The NE, NW and SW shows a significant variation in thickness ranging from 1.6 to 16.25 m. This distribution is superimposed on the zones of high conductivity and porosity observed in Figure.10A-10B; thus showing the capacity of these aquifers to store water in porous media. The centre, east and tip of the SSW in Figure 11 are located in the zone where the aquifers vary in thickness from 16.23 to 38.34 m. This variation is almost similar to that observed in Figure 10, where the different values of water fluctuates considerably, ranging from environments with low hydraulic conductivity (0.532–13.824 m/day) and porosity (22.624–30.619%) to environments with high conductivity (13.824–44.885 m/day) and porosity (30.619–42.616%). The variations in transmissivity and longitudinal conductance remains practically similar with different aquifer thickness.

Figure 11.

Aquifer thickness distribution map.

4.5. Linear analysis

The method of automatic extraction of lineaments from satellite images of the study area made it possible to highlight 15 lineaments with lengths between 0.3 and 2 km, with an average of 0.4 km. The rosette resulting from the synthesis of the lengths and directions of the lineaments (Figure 12) shows a major direction from N 70° to N 80°E, two secondary directions which are N 0° to N 10°E and N 60° to N 70°E and three minor directions N 10° to N 20°E, N 20° to N 30°E and N 30° to N 40°E. The direction N 0° to N 10°E observed on the gneissic formation of the sector is identical to that found by Wendgoudo (2012) which would correspond to the major direction of fractures put in place by the Burkina tectonics. The N 30° direction confirms the geological context of the Cameroon Volcanic Linge (Tchoua, 1974). in which the study area is located. These lineaments are more widespread on the basaltic, granitic formation and less on the gneissic formation. The abundance of these block lineaments at the level of these formations reflects the alignment of the watercourses and a level of advanced fracturing of the rocks observed respectively in the locality of Foto and Foréké. Throughout the sector, they reflect the transition zone between the different geological formations in the study area. The communication between these blocks of lineaments near watercourses at the level of basaltic and granitic formations may represent flow corridors between the aquifers of these sectors. The existing major directions may be the observed boundary between the basaltic granitic and gneissic formations (Figure 3), it may also represent the existence of a continuous fracture likely to contain an aquifer.

The arrangement in separate blocks and the orientation of the lineaments suggests that they constitute fractures crossing the study area. The drop of resistivity in these zones indicates the existence of a discontinuous aquifer on fractured basalt, gneiss and granite.

4.6. Hydrogeological map of the study area

The superimposition of morpho-structural data, electrical characteristics, hydraulic parameters and the lineament network made it possible to propose a map of the hydrogeological potential map of the study area (Figure 13). The distribution of this map into favourable, average favourable and unfavourable zones are based on several hypotheses.

Figure 13.

Hydrogeological potential map of the study area.

The favourable zones are environments where perennial watercourses abound. Fractures are abundant and communicates with each other. These assumptions are based on field observations, the lineament map and the hydrographic network (Figures 1b and 12). The hydraulic conductivity in these zones are between 0.279 and 44.852 m/day, the porosity varies from 19 to 42.852 %, the longitudinal conductance between 0.001 and 1.101 Ω⁻¹ and the transmissivity from 0.456 to 504.023 m²/day. They are highly conductive with true resistivities between 0.43 and 128.14 Ω m for aquifers thicker than 20 m; with geophysical anomalies of type H, KH, Q and HQ.

The moderately favourable areas are less conductive areas. They present individualised fractures parallel to the watercourses. The hydraulic conductivity at this level is between 0.179 and 37.098 m/day, the porosity varies from 17.763 to 41.761 %, the longitudinal conductance varies 0.001 to 0.571 Ω⁻¹ and the transmissivity between 1.06 and 257.9 m²/day. They are less conductive with resistivities varying from 128.14 to 2,325.05 Ω m; with aquifers 10–20 m thick for curves of type HA, HK, KQ, K, KA and A.

The unfavourable areas are marked by the absence of fractures and watercourses. Hydraulic conductivity at this level varies between 0.004 and 4.177 m/day, porosity from 0.192 to 31.933%, longitudinal conductance from 0.021 to 0.049 Ω⁻¹ and transmissivity from 0.019 to 23.653 m²/day. They are highly resistant with true resistivities ranging from 2,357.9 to 27,323.5 Ω m with surface aquifers less than 10 m thick for type A and K curves.

5. Conclusion

The present work proposed to carry out a hydrogeological mapping of certain districts of the city of Dschang. The hydrogeological potential map of the study area shows the areas with favourable water resources for the population, while the geophysical studies show twelve types of anomalies with resistivity values allowing to distinguish two areas in the field. A resistant domain corresponding to more or less fresh basement rocks and very clayey and compact soils; mostly represented upstream of the gneissic zone. A conductive domain marked on the surface by porous soils with a predominance of sand, setting up aquifers on weathered rocks; and at depth aquifers on fractured rocks. These conductive domains are mostly represented on basaltic and granitic formations and little represented on gneissic formations. These different fractures and aquifers are highlighted using 2D resistivity sections associated with the geological model and iso-resistivity maps. The hydraulic parameters highlight the different parameters of the aquifers via empirical formulas. The high values of hydraulic conductivity, transmissivity, longitudinal conductance, and porosity are allocated to the conductive domains. They are more represented in the localities of Foto and Foreké. They are less present in the Keleng locality with a dominance of alteration aquifers on the surface. The mapping of the lineament network shows a blocky repair of fractures, oriented in a major direction N 60° to N 70° E.

A hydrogeological potential map of the study area is proposed which shows the favourable, medium and unfavourable sectors for the implementations of a hydraulic structure.

The most favourable areas are located in the localities of Foto, Foreké. The moderately favourable sectors in the localities of Foreké and Lépia. The unfavourable areas cover a large part of the locality of Keleng.

Declarations

Author contribution statement

Talla Toteu Rodrigue & Kenfack Jean Victor: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Kengni Lucas & Njueya Kopa Adoua: Analyzed and interpreted the data.

Kenzo-Tongnang Merlot Le-sage & Tchomtchoua Tagne Stéphane: Performed the experiments.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement

The authors do not have permission to share data.

Declaration of interests statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.