# Typology of Sounding Curves and Lithological 1D Models of Mineral Prospecting and Groundwater Survey within Crystalline Basement Rocks in the East of Cameroon (Central Africa) Using Electrical Resistivity Method and Koefoed Computation Method

Authors: Daniel Hervé Gouet; Arsène Meying; Harlin Leonid Ekoro Nkoungou; Stephane Patrick Assembe; Philippe Njandjock Nouck; Théophile Ndougsa Mbarga
Journal: International Journal of Geophysics
 (2020)

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## Abstract

Resistivity method using seventy-sixth (976) Schlumberger vertical electrical soundings along forty-one (41) profiles are conducted in the Batouri and Ngoura subdivisions, East region of Cameroon, to investigate the subsurface layering, mineral potential, and groundwater resource characteristics. Results of quantitative and qualitative interpretation of data using Koefoed computation method reveal two to five layers having geometrical and electrical characteristics of geological layer models: topsoil (0.5m<ep<3m; 600<ρ<3000Ω.m), lateritic soils (3<ep<20m; 1000<ρ<4000Ω.m); conductive layer (5<ep<110m; 100<ρ<1000Ω.m); fractured/weathered granite (20<ep<60m; 1000/5000<ρ<3000/9000Ω.m); clayey layer (5<ep<20m; 600/1500<ρ<2000/3500 (dry) Ω.m); and fresh granites (ρ>4000Ω.m). From the qualitative interpretation of VES curves, the subsurface layering is depicted by nine (09) types of sounding curves (G, H, A, QH, KH, HK, HA, HKH, and KHK) characterizing the vertical changes and the typology of sounding curves in the East Cameroon crystalline basements. The lithology of the subsurface is dominated (more than 80%) by geoelectrical and lithological 1D models derived by the H, QH, KH, HK, HA, HKH, and KHK curve types. These models are characterized by the presence of conductive layers and fractured/weathered granites derived from tectonic activities of the region. Also, the resistivity method (VES) applied in this study bring information about variation of the resistivity with depth, geological structures, fractures, and rupture zones in the underground until 120 m depth. These abovementioned information reveal proper hydrogeological and mining conditions for an efficient evaluation of the mineral potential and groundwater resources.


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## Body

## 1. Introduction

Groundwater and mineral resources are very important for a sustainable development of a locality. Groundwater is the only source of water in many areas where development of surface water is not economically viable. Its accumulation in alluviums and immature sedimentary rocks is in pore spaces, whereas in compact sedimentary, igneous, and metamorphic rocks, it is largely due to secondary porosity and permeability resulted from weathering, fracturing, jointing, and faulting activities [1]. Mineral deposits are the attractive sources of capital investments from foreign countries, and the discovery of minable substances can therefore create employment in a local and regional scale. Deposited grains are mainly scattered within alluvial, eluvial, and colluvial materials and lithified clasts [2–6], whereas ore bodies in igneous and metamorphic terrains are generally found in specific structures such as lineations, veins, veinlets, faults, and folds [7, 8]. Thus, prospecting for groundwater resources and minable substances, although of interest, is complicated.Small-scale gold mining activities are common and currently going on in Batouri and Ngoura subdivisions in the East Region of Cameroon. Miners mainly extract gold from superficial materials. Consumed surface water in these localities is not of quality. With the aim to discover viable groundwater source and primary gold occurrence “deposits” in those localities, geophysical survey was carried out in seven villages (Bougmama, Wallo, Mama I, Mama II and Garoua Sambe in Batouri subdivision; Tassongo and Colomine in Ngoura subdivision). This survey carried out within crystalline basement formations was done with the geoelectrical method. The electrical nature of 1D model layers helps to have an idea for the groundwater and economic mineral potential within the surveyed crystalline basement formations.


## 2. Geographic and Geologic Setting

The area of study (Figures1(a) and 1(b)) is made up of two sites: site 1 and site 2. Site 1 (Figure 1(a)) is located in the Ngoura subdivision in the heart of the Eastern region of Cameroon. Its boundaries stretches from 14.296° to 14.472° eastward and from 4.921° to 5.054° in the north direction (Figure 1(a)). Site 2 (Figure 1(b)) is located in the Batouri subdivision in the heart of the Eastern region of Cameroon. It stretches from 14.254° to 14.396° eastward and from 4.521° to 4.732° northward.Figure 1
(a) Localization map of electrical soundings and study area (area 1). (b) Localization maps of electrical soundings and study area (area 2).
(a)
(b)The surveyed localities are found in the transition zone between the Pan-African domain and the Congo Craton in the northern edge of the Cameroon faults’ zone [9, 10]. This transitional zone is essentially composed of partly outcropping Precambrian age metamorphic and magmatic basement rocks [11, 12]. These are mainly granites and migmatites rejuvenated during the Pan-African orogeny [10, 12]. Other rocks (Figure 2(a)) identified by Gazel and Giraudie [13], Regnoult [14] and Mvondo et al. [12] are as follows:
(i)
Biotite and muscovite quartzites, sericitic muscovite and conglomeratic quartzites, chloritic and sericitic schists, paraamphibolites, orthogneisses, biotite gneisses, pegmatites, and quartz veins (Figure2(b)) all of the Precambrian basal complex (ages from 2.5 and 1.8 billion years)
(ii)
Calcoalkaline granites (with porphyritic and alkali facies), granodiorites and syenites (containing heterogeneous and undifferentiated biotite facies), quartz-diorites, mica-schists, migmatites, and embrechite gneisses
(iii)
The main sedimentary rocks encountered are sandstones, sand, marl, limestones, and Paleozoic and Mesozoic conglomerates [13, 14].
(iv)
Soils are red ferrallitic or lateritic on the hills and plateaus and the geological cover of the study area can be seen in artisanal mining pits [15].Figure 2
(a) Geological map of the study area [13]. (b) Outcrops of quartz vein (A) and granite rock (B) observed in the study area.
(a)
(b)The tectonic of the area is characterized by four deformations phases (D1, D2, D3, and D4: [11, 16]).The tectonic setting of the areas similar to that of the Central Africa Pan-African chain [17] is characterized by four ductile and/or brittle-ductile deformation phases (D1, D2, D3, and D4: [11, 16]), buried faults, and nappe features [18–20]. For Olinga et al. [16], the Pan-African deformation which affected the study localities was controlled by tectonic thrusts and late strike-slip shear zones. The thrusting of the Pan-African Nappe over the Congo Craton (D2 deformation phase) is followed by a strike-slip shearing trending ENE-WSW (D3 deformation phase). During these two stages, deforming conditions were ductile to brittle-ductile. The dominant structural features for the D3 phase are penetrative foliation (steeply dipping N or S), an association ENE-WSW stretching lineations, and a N-S to NE-SW fold [11, 16]. A dextral transpressional model was assumed to explain the observed thrust and shear movements [16].Plans of movement of northern flake lean toward the south and the meridional flake toward the north was due to compressive tectonics [17]. For Njonfang et al. [17], the region was compressed in the N-S direction between two blocks and it resulted in shear movements which pushed the region outside the central part. During this process, channels, faults, fractures, and shear zones were created (Wippern and Seyferle, 1966; [17]). Magmatic activities are materialized by the infilling of fractures, faults, and shear zones by pegmatite and quartz-rich veins (Figure 2(b)).Groundwater is the water filling the pore spaces in rocks. Sedimentary materials and fractured crystalline rocks commonly form aquifers or reservoirs. Clays, shales, intrusive igneous bodies, and metamorphic rocks are common aquitards and are typically characterized by insignificant interconnected pore spaces. The nature and distribution of aquifers and aquitards are controlled by geological and geomorphological factors (Palacky et al., 1981; [21]). Typical geological environments for groundwater occurrence include alluvial fill and gravel beds overlying crystalline bedrock or other impervious material, sand and gravel, and fault and fracture zones in crystalline basement rocks (Palacky et al., 1981; [21]).


## 3. Electrical Resistivity Method

### 3.1. Principle

Electrical resistivity method consists of injecting an electrical current in the ground between two electrodes A and B, and then measuring the induced potential drop between two other electrodes M and N called potential electrodes (Figure3).Figure 3
(a, b) Schlumberger electrical sounding method.
(a)
(b)If the injected current intensity is known and the potential drop measured, it is therefore possible to determine the apparent resistivityρa (Rho) of the ground following formulae below [22–24]:
(1)ρa=2πAM−1−AN−1−BM−1+BN−1VMNIAB,where ρa is the apparent resistivity in Ohm.m; AM and AN are the distance between current electrode A and potential electrodes M and N, respectively, in meter (m); BM and BN are distance between current electrode B and potential electrodes M and N, respectively, in meter (m); VMN is the potential drop between potential electrodes M and N, in mV; IAB is the electric current injected between current electrodes A and B, in mA.For the Schlumberger symmetrical configuration (Figure3), the apparent resistivity ρa is given by Equation (2) as follows [22–24]:
(2)ρa=π/4AB2VMNMNIAB.This resistivity value enables to characterize a formation at the point (O) or station (Figure3). The resistivity of an earth’s material depends essentially on the humidity and the clay proportion in a given volume of that material [22, 23, 25, 26]. While clay and water fill in any vacuum in a rock, one can assume that the resistivity is a function of parameters such as fracturing, fractures and fissures clayey filling, porosity, and the clayey clogging of alluvium [22, 23, 25, 26]. Thus, the resistivity of fractured granites and clayey clogging of alluvium are lower than fresh granite (basement) and the barren stratum in the study area.


### 3.2. Vertical Electrical Sounding Theory

The vertical electrical sounding (VES) theory is an investigation technique which enables the downward assessment of the successive layers by determining their apparent resistivity and thickness values [25, 27–29]. The value of the resistivity characterizes a lithology without providing information upon its water bearing or its mineralization [25, 27–29]. However, it aids in comparing two lithologies and locating them from one another. That is, for a given geological layer, water-bearing strata and sterile (dry) strata do not exhibit the same resistivity.In geophysical investigation, two types of soundings are used during a campaign [25]:
(i)
The parametric soundings: they are set at the beginning of the first phase of a geophysical survey or reconnaissance stage. Since the a priori distribution of the resistivity in a region is unknown, they are set beside wells, drillings, or on outcrops (Figure4). Therefore, they will help in assessing the resistivity framework of the area and in choosing the length of cables to be used for resistivity profiling
(ii)
The interpretative soundings: they are set to confirm anomalies or discontinuities highlighted by past investigations. They also provide information on the vertical distribution of layers characterized by their true resistivities and thicknesses [25, 27–30]. They enable to plot cross-sections or geological section of the subsurfaceFigure 4
Electrical sounding test near the granite outcrop (a) and the mining site (b).
(a)
(b)In the framework of this study, soundings (parametric and interpretative) were realised using the Schlumberger array with AB/2 varying from 1.5 m to 300 m and MN/2 from 0.5 m to 30 m (Table1).Table 1
Example of the VES paper.
Schlumberger vertical electrical sounding
Sounding: BK1
Operator: Dr. Gouet
Locality: Colomine
Date: 08/05/2013
Region: East Cameroon
Instrument: Syscal junior resistivimeter
Position:X=250m
Coordinate (33 N; 431752 mE 550183 m N;alt=755m)
N∗
AB/2 (m)
MN/2 (m)
I (mA)
V (mV)
ρ (Ω.m)
1
1.5
0.5
2.698
444.598
1035.41
2
2.5
0.5
2.8
159.842
1076.06
3
5
0.5
2.736
27.685
786.76
4
7.5
1.5
2.501
25.091
567.21
5
10
1.5
5.058
23.027
466
6
15
1.5
11.365
20.948
429.96
7
20
2.5
13.145
22.249
418.77
8
25
2.5
19.721
21.208
418.08
9
35
5
17.271
20.237
441.73
10
50
5
27.794
20.132
563.2
11
75
7.5
37.339
21.017
656.48
12
100
10
42.111
20.037
739.91
13
150
15
47.594
20.172
988.66
14
200
20
49.591
19.962
1251.97
15
250
25
50.234
19.951
1544.05
16
300
30
50.399
20.527
1900.11In the Schlumberger array, the distance between A and B varies as well as the distance between M and N (Figure3). The obtained resistivity values correspond to the measured apparent resistivity values following Equation (3), at the centre O of the ABMN quadrupole (Figure 3).The interpretation of data from interpretative sounding will enable to determine the thickness of the weathered zone, then that of the fractured zone or the discontinuity in the basement. It represents results of 1D inversion of the apparent resistivity as a function of electrode spacing (half current distance) [25, 27–31]. The 1D inversion enables to deduce the true resistivity of the ground and from the resistivity, the composition of the different layers. It requires a log/log diagram where half distance AB is the abscissa and the apparent resistivity, in Ohm.meter is the ordinate.The data is processed, modelled, and interpreted using 1D inversion software WinSev 6.4 from Geosoft [31]. With this software, using the equivalence and suppression principles, it shows that each interpretative sounding curve realised is not unique [25]. Thus, the parametrical soundings and field geological data help to constraint resistivity and thickness models for each interpretative sounding curve [25].


## 3.1. Principle

Electrical resistivity method consists of injecting an electrical current in the ground between two electrodes A and B, and then measuring the induced potential drop between two other electrodes M and N called potential electrodes (Figure3).Figure 3
(a, b) Schlumberger electrical sounding method.
(a)
(b)If the injected current intensity is known and the potential drop measured, it is therefore possible to determine the apparent resistivityρa (Rho) of the ground following formulae below [22–24]:
(1)ρa=2πAM−1−AN−1−BM−1+BN−1VMNIAB,where ρa is the apparent resistivity in Ohm.m; AM and AN are the distance between current electrode A and potential electrodes M and N, respectively, in meter (m); BM and BN are distance between current electrode B and potential electrodes M and N, respectively, in meter (m); VMN is the potential drop between potential electrodes M and N, in mV; IAB is the electric current injected between current electrodes A and B, in mA.For the Schlumberger symmetrical configuration (Figure3), the apparent resistivity ρa is given by Equation (2) as follows [22–24]:
(2)ρa=π/4AB2VMNMNIAB.This resistivity value enables to characterize a formation at the point (O) or station (Figure3). The resistivity of an earth’s material depends essentially on the humidity and the clay proportion in a given volume of that material [22, 23, 25, 26]. While clay and water fill in any vacuum in a rock, one can assume that the resistivity is a function of parameters such as fracturing, fractures and fissures clayey filling, porosity, and the clayey clogging of alluvium [22, 23, 25, 26]. Thus, the resistivity of fractured granites and clayey clogging of alluvium are lower than fresh granite (basement) and the barren stratum in the study area.


## 3.2. Vertical Electrical Sounding Theory

The vertical electrical sounding (VES) theory is an investigation technique which enables the downward assessment of the successive layers by determining their apparent resistivity and thickness values [25, 27–29]. The value of the resistivity characterizes a lithology without providing information upon its water bearing or its mineralization [25, 27–29]. However, it aids in comparing two lithologies and locating them from one another. That is, for a given geological layer, water-bearing strata and sterile (dry) strata do not exhibit the same resistivity.In geophysical investigation, two types of soundings are used during a campaign [25]:
(i)
The parametric soundings: they are set at the beginning of the first phase of a geophysical survey or reconnaissance stage. Since the a priori distribution of the resistivity in a region is unknown, they are set beside wells, drillings, or on outcrops (Figure4). Therefore, they will help in assessing the resistivity framework of the area and in choosing the length of cables to be used for resistivity profiling
(ii)
The interpretative soundings: they are set to confirm anomalies or discontinuities highlighted by past investigations. They also provide information on the vertical distribution of layers characterized by their true resistivities and thicknesses [25, 27–30]. They enable to plot cross-sections or geological section of the subsurfaceFigure 4
Electrical sounding test near the granite outcrop (a) and the mining site (b).
(a)
(b)In the framework of this study, soundings (parametric and interpretative) were realised using the Schlumberger array with AB/2 varying from 1.5 m to 300 m and MN/2 from 0.5 m to 30 m (Table1).Table 1
Example of the VES paper.
Schlumberger vertical electrical sounding
Sounding: BK1
Operator: Dr. Gouet
Locality: Colomine
Date: 08/05/2013
Region: East Cameroon
Instrument: Syscal junior resistivimeter
Position:X=250m
Coordinate (33 N; 431752 mE 550183 m N;alt=755m)
N∗
AB/2 (m)
MN/2 (m)
I (mA)
V (mV)
ρ (Ω.m)
1
1.5
0.5
2.698
444.598
1035.41
2
2.5
0.5
2.8
159.842
1076.06
3
5
0.5
2.736
27.685
786.76
4
7.5
1.5
2.501
25.091
567.21
5
10
1.5
5.058
23.027
466
6
15
1.5
11.365
20.948
429.96
7
20
2.5
13.145
22.249
418.77
8
25
2.5
19.721
21.208
418.08
9
35
5
17.271
20.237
441.73
10
50
5
27.794
20.132
563.2
11
75
7.5
37.339
21.017
656.48
12
100
10
42.111
20.037
739.91
13
150
15
47.594
20.172
988.66
14
200
20
49.591
19.962
1251.97
15
250
25
50.234
19.951
1544.05
16
300
30
50.399
20.527
1900.11In the Schlumberger array, the distance between A and B varies as well as the distance between M and N (Figure3). The obtained resistivity values correspond to the measured apparent resistivity values following Equation (3), at the centre O of the ABMN quadrupole (Figure 3).The interpretation of data from interpretative sounding will enable to determine the thickness of the weathered zone, then that of the fractured zone or the discontinuity in the basement. It represents results of 1D inversion of the apparent resistivity as a function of electrode spacing (half current distance) [25, 27–31]. The 1D inversion enables to deduce the true resistivity of the ground and from the resistivity, the composition of the different layers. It requires a log/log diagram where half distance AB is the abscissa and the apparent resistivity, in Ohm.meter is the ordinate.The data is processed, modelled, and interpreted using 1D inversion software WinSev 6.4 from Geosoft [31]. With this software, using the equivalence and suppression principles, it shows that each interpretative sounding curve realised is not unique [25]. Thus, the parametrical soundings and field geological data help to constraint resistivity and thickness models for each interpretative sounding curve [25].


## 4. Koefoed Computation Method

The computation of the theoretical curve uses the method described by Koefoed [32]. It can run on the W-Geosoft-Winsev program developed by Miguel Borreguero and Jacques Jenny [31].


### 4.1. Theoretical Principle of the Koefoed Method

The potential at a point due to a point source of current over a layered earth described by Das and Verma [33] is given by
(3)Us=I2π∫0∞TλJ0λsdλ,where J0λs is the zeroeth-order Bessel function of the first kind, Tλ is the electrical impedance at the surface, defined by Koefoed [34] as the resistivity transform function of layer resistivities and thicknesses, and λ is the integration variable.Koefoed [32] also disassociated the resistivity transform function to extract the resistivity kernel function K1λ representing the deviation in response to the layered earth from that of a homogeneous half-space. Accordingly, at the air-earth interface,
(4)T1λ=ρ11+2K1λ,while at the top of the ith layer,
(5)Tiλ=ρi1+2Kiλ.From these basic expressions, Das and Verma [33] obtained the following simple expression for the apparent resistivity ρas due to a two-electrode system and given by
(6)ρas=2πsUsI,where s=AM is the separation between the two active electrodes.From Equations (4) and (7), we have
(7)ρas=s∫0∞TλJ0λsdλ.The action of a linear filter can be defined by a convolution integral of the type (Das and Verma, 180):(8)F=∫0∞GλHx−ydy,where F is the output function, G is the input function, and H is the filter function.Equation (7) giving the apparent resistivity for the two-electrode array can be expressed as a convolution integral by putting variables on a logarithmic scale as x=lns and y=lnl/A. Thus, Equation (8) assumes the form [33]
(9)ρax=ex∫−∞∞TyJ0ex−y−e−ydyor
(10)ρax=∫−∞∞Ty−J0ex−y.ex−ydy.Comparing Equation (10) with Equation (8), we find that Equation (10) is a convolution integral, where Ty is the input function, ρax is the output function, and ex−yJ0ex−y is the filter function [33]. Thus, the convolution of the resistivity transform function with this filter yields the apparent resistivity values for the two-electrode system.From the expression of the apparent resistivityρas for the two-electrode configuration, Das and Verma [33] obtained the following expression for the Schlumberger apparent resistivity ρaSchs:
(11)ρaSchls=ρas−s∂ρas∂s.


### 4.2. Computation of Model Curves

To compute the two-electrode apparent resistivity for a specific model, the resistivity transformT1 for the model should be convolved with the filter set mentioned in the preceding section [33]. This function T1 is computed for equal increments of y=lnl/λ from the following iterative expression valid for any number of layers [35]:
(12)Ti−1=ρi−1Ti+ρi−1tanhλdi−1ρi−1+Titanhλdi−1,where T=TN=ρN is the number of layers in the subsurface and di is the thickness of the ith layer. The above equation represents a stacking process of layers, beginning from the bottom, which finally yields T1 at the surface. After computing the resistivity transform functions at equal intervals, they are convolved with the filter set (for horizontal coplanar loops, as already mentioned) to yield the apparent resistivity par ρax for the two-electrode system as [33]:
(13)ρas=∑k=0nCkTykρax=∑k=0nCkTyk,where Ck are the filter coefficients, yk=lns–n0+kln10/10, n0 is the abscissa of the first filter coefficient, and n is the suffix of the last filter point used.Using the above convolution expression, a theoretical two-electrode sounding curve was computed for a specific three-layer earth model; the result is very suitable [33]. We obtain also the similar result for a Wenner or Schumberger theoretical sounding curve [33]. Thus, Das and Verma [33] present a technique based on the concept of digital linear filtering to compute electrical sounding curves for electrode systems.The computation of the theoretical curve uses the method described by Koefoed [32]. It can run on the W-Geosoft-Winsev program developed by Miguel Borreguero and Jacques Jenny [31]. The program compute Schlumberger theoretical electrical sounding curves in the area study for earth models to three and five layers following the field apparent resistivity data. This computation permits to have thicknesses (eP) and true resistivity values (ρ) of geological layers of the inversion of the apparent resistivity as a function of depth [33].


## 4.1. Theoretical Principle of the Koefoed Method

The potential at a point due to a point source of current over a layered earth described by Das and Verma [33] is given by
(3)Us=I2π∫0∞TλJ0λsdλ,where J0λs is the zeroeth-order Bessel function of the first kind, Tλ is the electrical impedance at the surface, defined by Koefoed [34] as the resistivity transform function of layer resistivities and thicknesses, and λ is the integration variable.Koefoed [32] also disassociated the resistivity transform function to extract the resistivity kernel function K1λ representing the deviation in response to the layered earth from that of a homogeneous half-space. Accordingly, at the air-earth interface,
(4)T1λ=ρ11+2K1λ,while at the top of the ith layer,
(5)Tiλ=ρi1+2Kiλ.From these basic expressions, Das and Verma [33] obtained the following simple expression for the apparent resistivity ρas due to a two-electrode system and given by
(6)ρas=2πsUsI,where s=AM is the separation between the two active electrodes.From Equations (4) and (7), we have
(7)ρas=s∫0∞TλJ0λsdλ.The action of a linear filter can be defined by a convolution integral of the type (Das and Verma, 180):(8)F=∫0∞GλHx−ydy,where F is the output function, G is the input function, and H is the filter function.Equation (7) giving the apparent resistivity for the two-electrode array can be expressed as a convolution integral by putting variables on a logarithmic scale as x=lns and y=lnl/A. Thus, Equation (8) assumes the form [33]
(9)ρax=ex∫−∞∞TyJ0ex−y−e−ydyor
(10)ρax=∫−∞∞Ty−J0ex−y.ex−ydy.Comparing Equation (10) with Equation (8), we find that Equation (10) is a convolution integral, where Ty is the input function, ρax is the output function, and ex−yJ0ex−y is the filter function [33]. Thus, the convolution of the resistivity transform function with this filter yields the apparent resistivity values for the two-electrode system.From the expression of the apparent resistivityρas for the two-electrode configuration, Das and Verma [33] obtained the following expression for the Schlumberger apparent resistivity ρaSchs:
(11)ρaSchls=ρas−s∂ρas∂s.


## 4.2. Computation of Model Curves

To compute the two-electrode apparent resistivity for a specific model, the resistivity transformT1 for the model should be convolved with the filter set mentioned in the preceding section [33]. This function T1 is computed for equal increments of y=lnl/λ from the following iterative expression valid for any number of layers [35]:
(12)Ti−1=ρi−1Ti+ρi−1tanhλdi−1ρi−1+Titanhλdi−1,where T=TN=ρN is the number of layers in the subsurface and di is the thickness of the ith layer. The above equation represents a stacking process of layers, beginning from the bottom, which finally yields T1 at the surface. After computing the resistivity transform functions at equal intervals, they are convolved with the filter set (for horizontal coplanar loops, as already mentioned) to yield the apparent resistivity par ρax for the two-electrode system as [33]:
(13)ρas=∑k=0nCkTykρax=∑k=0nCkTyk,where Ck are the filter coefficients, yk=lns–n0+kln10/10, n0 is the abscissa of the first filter coefficient, and n is the suffix of the last filter point used.Using the above convolution expression, a theoretical two-electrode sounding curve was computed for a specific three-layer earth model; the result is very suitable [33]. We obtain also the similar result for a Wenner or Schumberger theoretical sounding curve [33]. Thus, Das and Verma [33] present a technique based on the concept of digital linear filtering to compute electrical sounding curves for electrode systems.The computation of the theoretical curve uses the method described by Koefoed [32]. It can run on the W-Geosoft-Winsev program developed by Miguel Borreguero and Jacques Jenny [31]. The program compute Schlumberger theoretical electrical sounding curves in the area study for earth models to three and five layers following the field apparent resistivity data. This computation permits to have thicknesses (eP) and true resistivity values (ρ) of geological layers of the inversion of the apparent resistivity as a function of depth [33].


## 5. Material and Data Acquisition

For a suitable coverage of the study localities, data have been collected through nine hundred and seventy-six (976) electrical soundings along forty-one (41) profiles. The data acquisition is done following the vertical electrical sounding method using the Schlumberger array. The interstation spacing along profiles is 100 meters. The AB maximum length was held at 600 m to detect desired lithological formations at an approximate depth between 114 and 130 meters as proposed by Loke [36]. In the study area, the main directions of strains are N-S and ENE-WSW [16]; thus, profiles of sounding surveys were mainly oriented in the E-W and NNW-SSE directions (Figures 1(a) and 1(b)) in order to reduce the anisotropic effects by considering the Su and Yue [37] studies.The direct current resistivity meter, Syscal Junior 48 (IRIS Instrument) system, was used. This unit runs under the Rho mode which enables one to measure the resistivity (Rho) of ground structures. Acquisition of electrical data were made using the Schlumberger electrical sounding method where the current was injected through A and B electrodes (Figure3) and a potential was measured through receiving electrodes M and N [24].The electrical soundings were interpreted by WinSev 6.4 software from Geosoft using the Borreguero and Jenny inversion program [31]. They allowed to obtain the ground distribution of layers at each station, hence, providing geoelectrical and geological 1D models of the study area.


## 6. Results

Results of the study were presented as typology of sounding curves, mapping of the curve types, pseudosections, geoelectrical sections or profiles, and lithological models.


### 6.1. Typology of Sounding Curves

A total of nine hundred and seventy-six (976) electrical soundings were performed, including sixteen (16) parametric soundings. The study area is dominated by granitic formations and the tectonic setting shows that the area had been compressed between two blocks. After, this compressive tectonism created fractures and shear zones capable of modifying electrical conductivity of geologic formations [38]. In the study area, the main directions of strains are N-S and ENE-WSW [16]. Thus, in order to have the best knowledge of the resistivity values following a particular layer and to reduce anisotropic effects [37], profiles of sounding surveys were E-W and NNW-SSE oriented mainly (Figures 1(a) and 1(b)). In reality, geological formations are far from being isotropic. This is especially true of shale and clay formations. In clay, for example, the resistivity is the same in all directions along a layer but has a different value perpendicularly to the stratification. It therefore shows that, if resistivity is measured in the field with an array oriented characteristic parallel to the bedding planes, the measured resistivity is higher than the longitudinal resistivity by a ratio α [39].The processing, modelling, and interpretation of electrical data were performed with Geosoft’s WinSev 6.4 software. This software does not show the scale of residual adjustment errors. They are not valid for superposed curves of different electrical soundings [31]. The scale begins always at 1% and has the same progression as the resistivity scale. Focusing only on interpretative soundings, eight (08) types of sounding curves (Figures 5–14) corresponding to particular geological arrangements have been highlighted. Based on previous works [1, 25, 27–30, 40, 41] and field geological data, interpretative soundings enabled to characterize the typology of sounding curves of the study area and then to deduce a synthesis of the distribution of thicknesses (ep) and true resistivity values (ρ) of geological layers from different sounding curves types. The red curves in the Schlumberger electrical sounding curves (Figures 5–14) characterize the distribution of thicknesses (ep) and true resistivity values (ρ) of geological layers in the study area. They represent the results of 1D inversions of the apparent resistivity as a function of depth through Schlumberger electrical soundings [31]. The 1D inversion using Winsev software is used for the calculation of the theoretical curve [31] following the method described by Koefoed [32]. It permits to have the blue curves (Figures 5–14) which correlate with the experimental points perfectly (Figures 5–14). This correlation minimizes the errors of 1D models given by red curves.Figure 5
Sounding curve (BE10) of type G.Figure 6
Sounding curve (BK3) of type H.Figure 7
Sounding curves (AJ27) of type A.Figure 8
Sounding curve (BQ29) of type HK.Figure 9
(a, b) Sounding curves (BQ7 and BK22) of type QH.
(a)
(b)Figure 10
(a, b) Sounding curves (BN6 and BP17) of type KH.
(a)
(b)Figure 11
(a, b) Sounding curves (AF4 and AP8) of type KH.
(a)
(b)Figure 12
Sounding curve (AF20) of type HA.Figure 13
Sounding curve (BM2) of type HKH.Figure 14
Sounding curve (BM37) of type KHK.


### 6.2. Mapping of the Curve Types

The mapping of the curve types shows the characteristics of the horizontal variety of layers in the studied areas. Figure15 shows the mapping of the electrical sounding curve types in the Colomine area (Figure 1(a)). This mapping presents along the horizontal line the curve types identified which ranges from G, H, A, QH, KH, HK, HA, HKH, to KHK and shows the variety of structures along the studied profiles.Figure 15
Map of curve types of the Colomine area.


### 6.3. Characteristic of the Curve Types

Curves types identified ranges from G, H, A, QH, KH, HK, HA, HKH, to KHK varying between two to five geoelectric layers. In the Colomine locality (Figure1(a)), the H and KH curve types dominate, constituting 45.5% and 20.9% of the total, while the QH, HKH, G, HA, A, HK, and KHK types constitute 8.9%, 7%, 6.6%, 5%, 3.3%, 1.8 and 1%, respectively (Figure 16).Figure 16
Histogram of the curve types in the Colomine area.


### 6.4. Pseudosections

The pseudosection is the inversion of apparent resistivity following horizontal and vertical lines. It reflects the resistivity distribution versus electrode spacing values (AB/2) and shows the electrical characteristic of layers following horizontal and vertical lines. Along pseudosections (Figures17(a) and 17(b)), the resistivity varies in depth and following the horizontal line, and then we note the vertical and horizontal changes of layers. These changes characterize a diversity of geological structures in depth and along the horizontal line and therefore attest the diversity of observed curve types. Generally, in the pseudosections of the study area, maximum resistivity values appeared in the upper parts because of the dry alluvium or of the presence of basement in the subsurface. They decreased toward the middle parts because of the influence of a conductive discontinuity or of an aquifer then increased with depth because of the resistant basement. In Figure 12, the pseudosection of L11 is shown as an example. In Figure 14, flanks of the pseudosection were lower than the central part because of the influence of a conductive discontinuity or of an aquifer and the coarse grain size dry alluvium or basement structures along the central part.Figure 17
(a) Geolectrical pseudosection of profile L11 (Colomine area 1). (b) Geolectrical pseudosection of profile L17 (Colomine area 1).
(a)
(b)


### 6.5. Geoelectrical and Lithological Sections of the Sounding Curves

The vertical electrical sounding interpretation results in the study areas were used to prepare 2D geoelectric sections or geoelectrical profiles. The 2D geoelectric sections show the vertical and horizontal changes of layers along profiles in the underground until 120 m depth. The 2D geoelectric sections along profiles L10 and L11 (Figures18(a) and 18(b)) are shown as examples. We note a diversity of 1D geoelectric models following the horizontal line and the vertical changes of layers in depth. In the studied areas, the 2D geoelectric sections identified vary between two to five geoelectric layers.Figure 18
(a) Geoelectrical section of profile L10 (Colomine, area 1). b. Geoelectrical section of profile L11 (Colomine, area 1).
(a)
(b)Results from the vertical electrical soundings using the Borreguero and Jenny program show evidences of several electrical sounding curve types (H, A, QH, KH, HK, HA, HKH, and KHK). Each curve type defines a geological bedding model of the surveyed subsurface. The electrical characteristics of geological layers (resistivityρ and thickness ep) obtained from the 1D inversion of measured geoelectrical data, the geological exploration on outcrops, and in gold mining pits (Figure 2(b)), and from previous studies of Palacky [42], Akpabio and Ekpo [43], Oladapo et al. [44], Alile et al. [45], Raimi et al. [29], and Coker [27], help to plot geoelectrical models for the subsurface derived from the different types of electrical sounding curves produced (Figures 5–14). The geoelectrical models provide lithological 1D models for the crystalline basements in the Batouri and Ngoura subdivisions. It permits to characterize normal (the resistivity increases in depth (e.g., ρ1<ρ2<ρ3)) or abnormal (we note a decrease of the resistivity in depth (e.g., ρ1<ρ2>ρ3<ρ4)) distribution of the electric properties of layers [24, 30, 38]. The lithological 1D model of an abnormal distribution of the electric properties of layers is well in the mineral and groundwater prospecting within crystalline basement rocks [24, 30, 38].The G-type (ρ1<ρ2), H-type (ρ1>ρ2<ρ3), and A-type (ρ1<ρ2<ρ3) sounding curves were illustrated by the BE10, BK3, and AJ27 sounding curves (Figures 5–7), respectively. They characterize lateritic levels (G and H types) and a weathered or fractured bedrock overlain by a thin overburden (A type). The general interpretation states two- and three-layered earth models. Lateritic recovering represents the most superficial and first layer (1<ep1<20m; 600<ρ1<3000Ω.m). The second layer for H and A types represents the conductive layer and the fractured/weathered granite, respectively. It is characterized by a lower resistivity (100<ρ2<1000Ω.m and 600<ρ2<2000Ω.m) with thickness varying from 10 m to 110 m. The last layer (G, H, and A types) corresponds to the resistive basement (ρ2 or ρ3>2500Ω.m). The above interpretation enables the realisation of geoelectrical loggings and to define lithological models of the investigated subsurface (Figures 19–21). The lithological models of sounding curves G and A types present normal distributions in depth of the electric properties of layers, while the lithological model of sounding curves of H type present an abnormal distribution in depth of the electric properties of layers.Figure 19
Lithological model of sounding curves of type G.Figure 20
Lithological model of sounding curves of type H.Figure 21
Lithological models of the sounding curves of type A.The HK-type (ρ1>ρ2<ρ3>ρ4), QH-type (ρ1>ρ2>ρ3<ρ4), KH-type (ρ1<ρ2>ρ3<ρ4), and HA-type (ρ1>ρ2<ρ3<ρ4) sounding curves were illustrated by BQ7 and BK22; BN6, BP17, AF4, and AP8; BQ29; and AF20 sounding curves (Figures 8–12), respectively. They characterize conductive layer intrusion (Figure 8), lateritic hardpan in high altitude (Figure 9(a)) and low altitude (Figure 9(b)) areas, megafractures occurrences (Figures 10 and 12), and the involvement of a weathered quartz vein (Figure 11(a)) within basement formations and fractured basement (Figure 11(b)). The first layer corresponds to a thin laterite or a mixture of sandy and lateritic soil (1<ep1<2m; 4000<ρ1<10000Ω.m). The second layer corresponds to a relatively thick lateritic layer, a regolith granitic or a conductive level (10<ep2<20m; 1500<ρ2<4000Ω.m). The third layer characterizes a conductive layer or a thin or thick highly fractured and/or weathered granite (50<ep3<100m; 100<ρ3<1000Ω.m) and the last corresponds to a less or highly fractured granitic basement (ρ4>4000Ω.m). The above interpretation enables the realisation of geoelectrical loggings and to define lithological models of the investigated subsurface (Figures 22–25). The lithological models of sounding curves of QH, HK, and HA types present abnormal distributions in depth of the electric properties of layers.Figure 22
Lithological model of the sounding curves of type HK.Figure 23
(a, b) Lithological models of sounding curves of type QH.
(a)
(b)Figure 24
(a, b) Lithological models of the sounding curves of type KH.
(a)
(b)Figure 25
Lithological model of the sounding curves of type HA.The BM2 and BM37 sounding curves (Figures 13 and 14) representing the HKH-type (ρ1>ρ2<ρ3>ρ4<ρ5) and KHK-type (ρ1<ρ2>ρ3<ρ4>ρ5) sounding curves, respectively, characterize two consecutive groundwater levels (two adjacent conductive layers or clayey sandy layers) in the hydrogeological survey (HKH-type) and a lateritic layer on thalwegs (KHK type). The HKH-type and KHK-type sounding curves suggest five-layered earth models with: the first being an overburden lateritic soil; the second and fourth, two clayey sandy conductive layers (2<ep2<20m; 100<ρ2<1000Ω.m) for HKH-type and dry clayey laterites (1<ep2<10m; 2000<ρ2<4000Ω.m) and the weakly weathered and fractured granitic rocks (50<ep4<110m; 5000<ρ4<10000Ω.m); the third, dry clayey laterites (5<ep3<20m; 2000<ρ3<3500Ω.m) linking the two conductive layers for HKH type, while for KHK type, it represents the conductive level which correspond to the HA type in Figure 12. The last layer depicts the less fissured or fresh granitic basement for HKH type, while for KHK type, it represents the highly altered and fractured granitic basement (ρ5<2000Ω.m). The geoelectrical loggings were realised and lithological models were defined for the investigated subsurface (Figures 26 and 27). The lithological models of sounding curves of types HKH and KHK present abnormal distributions in depth of the electric properties of layers.Figure 26
Lithological model of the sounding curves of type HKH.Figure 27
Lithological model of the sounding curves of type KHK.


## 6.1. Typology of Sounding Curves

A total of nine hundred and seventy-six (976) electrical soundings were performed, including sixteen (16) parametric soundings. The study area is dominated by granitic formations and the tectonic setting shows that the area had been compressed between two blocks. After, this compressive tectonism created fractures and shear zones capable of modifying electrical conductivity of geologic formations [38]. In the study area, the main directions of strains are N-S and ENE-WSW [16]. Thus, in order to have the best knowledge of the resistivity values following a particular layer and to reduce anisotropic effects [37], profiles of sounding surveys were E-W and NNW-SSE oriented mainly (Figures 1(a) and 1(b)). In reality, geological formations are far from being isotropic. This is especially true of shale and clay formations. In clay, for example, the resistivity is the same in all directions along a layer but has a different value perpendicularly to the stratification. It therefore shows that, if resistivity is measured in the field with an array oriented characteristic parallel to the bedding planes, the measured resistivity is higher than the longitudinal resistivity by a ratio α [39].The processing, modelling, and interpretation of electrical data were performed with Geosoft’s WinSev 6.4 software. This software does not show the scale of residual adjustment errors. They are not valid for superposed curves of different electrical soundings [31]. The scale begins always at 1% and has the same progression as the resistivity scale. Focusing only on interpretative soundings, eight (08) types of sounding curves (Figures 5–14) corresponding to particular geological arrangements have been highlighted. Based on previous works [1, 25, 27–30, 40, 41] and field geological data, interpretative soundings enabled to characterize the typology of sounding curves of the study area and then to deduce a synthesis of the distribution of thicknesses (ep) and true resistivity values (ρ) of geological layers from different sounding curves types. The red curves in the Schlumberger electrical sounding curves (Figures 5–14) characterize the distribution of thicknesses (ep) and true resistivity values (ρ) of geological layers in the study area. They represent the results of 1D inversions of the apparent resistivity as a function of depth through Schlumberger electrical soundings [31]. The 1D inversion using Winsev software is used for the calculation of the theoretical curve [31] following the method described by Koefoed [32]. It permits to have the blue curves (Figures 5–14) which correlate with the experimental points perfectly (Figures 5–14). This correlation minimizes the errors of 1D models given by red curves.Figure 5
Sounding curve (BE10) of type G.Figure 6
Sounding curve (BK3) of type H.Figure 7
Sounding curves (AJ27) of type A.Figure 8
Sounding curve (BQ29) of type HK.Figure 9
(a, b) Sounding curves (BQ7 and BK22) of type QH.
(a)
(b)Figure 10
(a, b) Sounding curves (BN6 and BP17) of type KH.
(a)
(b)Figure 11
(a, b) Sounding curves (AF4 and AP8) of type KH.
(a)
(b)Figure 12
Sounding curve (AF20) of type HA.Figure 13
Sounding curve (BM2) of type HKH.Figure 14
Sounding curve (BM37) of type KHK.


## 6.2. Mapping of the Curve Types

The mapping of the curve types shows the characteristics of the horizontal variety of layers in the studied areas. Figure15 shows the mapping of the electrical sounding curve types in the Colomine area (Figure 1(a)). This mapping presents along the horizontal line the curve types identified which ranges from G, H, A, QH, KH, HK, HA, HKH, to KHK and shows the variety of structures along the studied profiles.Figure 15
Map of curve types of the Colomine area.


## 6.3. Characteristic of the Curve Types

Curves types identified ranges from G, H, A, QH, KH, HK, HA, HKH, to KHK varying between two to five geoelectric layers. In the Colomine locality (Figure1(a)), the H and KH curve types dominate, constituting 45.5% and 20.9% of the total, while the QH, HKH, G, HA, A, HK, and KHK types constitute 8.9%, 7%, 6.6%, 5%, 3.3%, 1.8 and 1%, respectively (Figure 16).Figure 16
Histogram of the curve types in the Colomine area.


## 6.4. Pseudosections

The pseudosection is the inversion of apparent resistivity following horizontal and vertical lines. It reflects the resistivity distribution versus electrode spacing values (AB/2) and shows the electrical characteristic of layers following horizontal and vertical lines. Along pseudosections (Figures17(a) and 17(b)), the resistivity varies in depth and following the horizontal line, and then we note the vertical and horizontal changes of layers. These changes characterize a diversity of geological structures in depth and along the horizontal line and therefore attest the diversity of observed curve types. Generally, in the pseudosections of the study area, maximum resistivity values appeared in the upper parts because of the dry alluvium or of the presence of basement in the subsurface. They decreased toward the middle parts because of the influence of a conductive discontinuity or of an aquifer then increased with depth because of the resistant basement. In Figure 12, the pseudosection of L11 is shown as an example. In Figure 14, flanks of the pseudosection were lower than the central part because of the influence of a conductive discontinuity or of an aquifer and the coarse grain size dry alluvium or basement structures along the central part.Figure 17
(a) Geolectrical pseudosection of profile L11 (Colomine area 1). (b) Geolectrical pseudosection of profile L17 (Colomine area 1).
(a)
(b)


## 6.5. Geoelectrical and Lithological Sections of the Sounding Curves

The vertical electrical sounding interpretation results in the study areas were used to prepare 2D geoelectric sections or geoelectrical profiles. The 2D geoelectric sections show the vertical and horizontal changes of layers along profiles in the underground until 120 m depth. The 2D geoelectric sections along profiles L10 and L11 (Figures18(a) and 18(b)) are shown as examples. We note a diversity of 1D geoelectric models following the horizontal line and the vertical changes of layers in depth. In the studied areas, the 2D geoelectric sections identified vary between two to five geoelectric layers.Figure 18
(a) Geoelectrical section of profile L10 (Colomine, area 1). b. Geoelectrical section of profile L11 (Colomine, area 1).
(a)
(b)Results from the vertical electrical soundings using the Borreguero and Jenny program show evidences of several electrical sounding curve types (H, A, QH, KH, HK, HA, HKH, and KHK). Each curve type defines a geological bedding model of the surveyed subsurface. The electrical characteristics of geological layers (resistivityρ and thickness ep) obtained from the 1D inversion of measured geoelectrical data, the geological exploration on outcrops, and in gold mining pits (Figure 2(b)), and from previous studies of Palacky [42], Akpabio and Ekpo [43], Oladapo et al. [44], Alile et al. [45], Raimi et al. [29], and Coker [27], help to plot geoelectrical models for the subsurface derived from the different types of electrical sounding curves produced (Figures 5–14). The geoelectrical models provide lithological 1D models for the crystalline basements in the Batouri and Ngoura subdivisions. It permits to characterize normal (the resistivity increases in depth (e.g., ρ1<ρ2<ρ3)) or abnormal (we note a decrease of the resistivity in depth (e.g., ρ1<ρ2>ρ3<ρ4)) distribution of the electric properties of layers [24, 30, 38]. The lithological 1D model of an abnormal distribution of the electric properties of layers is well in the mineral and groundwater prospecting within crystalline basement rocks [24, 30, 38].The G-type (ρ1<ρ2), H-type (ρ1>ρ2<ρ3), and A-type (ρ1<ρ2<ρ3) sounding curves were illustrated by the BE10, BK3, and AJ27 sounding curves (Figures 5–7), respectively. They characterize lateritic levels (G and H types) and a weathered or fractured bedrock overlain by a thin overburden (A type). The general interpretation states two- and three-layered earth models. Lateritic recovering represents the most superficial and first layer (1<ep1<20m; 600<ρ1<3000Ω.m). The second layer for H and A types represents the conductive layer and the fractured/weathered granite, respectively. It is characterized by a lower resistivity (100<ρ2<1000Ω.m and 600<ρ2<2000Ω.m) with thickness varying from 10 m to 110 m. The last layer (G, H, and A types) corresponds to the resistive basement (ρ2 or ρ3>2500Ω.m). The above interpretation enables the realisation of geoelectrical loggings and to define lithological models of the investigated subsurface (Figures 19–21). The lithological models of sounding curves G and A types present normal distributions in depth of the electric properties of layers, while the lithological model of sounding curves of H type present an abnormal distribution in depth of the electric properties of layers.Figure 19
Lithological model of sounding curves of type G.Figure 20
Lithological model of sounding curves of type H.Figure 21
Lithological models of the sounding curves of type A.The HK-type (ρ1>ρ2<ρ3>ρ4), QH-type (ρ1>ρ2>ρ3<ρ4), KH-type (ρ1<ρ2>ρ3<ρ4), and HA-type (ρ1>ρ2<ρ3<ρ4) sounding curves were illustrated by BQ7 and BK22; BN6, BP17, AF4, and AP8; BQ29; and AF20 sounding curves (Figures 8–12), respectively. They characterize conductive layer intrusion (Figure 8), lateritic hardpan in high altitude (Figure 9(a)) and low altitude (Figure 9(b)) areas, megafractures occurrences (Figures 10 and 12), and the involvement of a weathered quartz vein (Figure 11(a)) within basement formations and fractured basement (Figure 11(b)). The first layer corresponds to a thin laterite or a mixture of sandy and lateritic soil (1<ep1<2m; 4000<ρ1<10000Ω.m). The second layer corresponds to a relatively thick lateritic layer, a regolith granitic or a conductive level (10<ep2<20m; 1500<ρ2<4000Ω.m). The third layer characterizes a conductive layer or a thin or thick highly fractured and/or weathered granite (50<ep3<100m; 100<ρ3<1000Ω.m) and the last corresponds to a less or highly fractured granitic basement (ρ4>4000Ω.m). The above interpretation enables the realisation of geoelectrical loggings and to define lithological models of the investigated subsurface (Figures 22–25). The lithological models of sounding curves of QH, HK, and HA types present abnormal distributions in depth of the electric properties of layers.Figure 22
Lithological model of the sounding curves of type HK.Figure 23
(a, b) Lithological models of sounding curves of type QH.
(a)
(b)Figure 24
(a, b) Lithological models of the sounding curves of type KH.
(a)
(b)Figure 25
Lithological model of the sounding curves of type HA.The BM2 and BM37 sounding curves (Figures 13 and 14) representing the HKH-type (ρ1>ρ2<ρ3>ρ4<ρ5) and KHK-type (ρ1<ρ2>ρ3<ρ4>ρ5) sounding curves, respectively, characterize two consecutive groundwater levels (two adjacent conductive layers or clayey sandy layers) in the hydrogeological survey (HKH-type) and a lateritic layer on thalwegs (KHK type). The HKH-type and KHK-type sounding curves suggest five-layered earth models with: the first being an overburden lateritic soil; the second and fourth, two clayey sandy conductive layers (2<ep2<20m; 100<ρ2<1000Ω.m) for HKH-type and dry clayey laterites (1<ep2<10m; 2000<ρ2<4000Ω.m) and the weakly weathered and fractured granitic rocks (50<ep4<110m; 5000<ρ4<10000Ω.m); the third, dry clayey laterites (5<ep3<20m; 2000<ρ3<3500Ω.m) linking the two conductive layers for HKH type, while for KHK type, it represents the conductive level which correspond to the HA type in Figure 12. The last layer depicts the less fissured or fresh granitic basement for HKH type, while for KHK type, it represents the highly altered and fractured granitic basement (ρ5<2000Ω.m). The geoelectrical loggings were realised and lithological models were defined for the investigated subsurface (Figures 26 and 27). The lithological models of sounding curves of types HKH and KHK present abnormal distributions in depth of the electric properties of layers.Figure 26
Lithological model of the sounding curves of type HKH.Figure 27
Lithological model of the sounding curves of type KHK.


## 7. Discussion

Geophysical resistivity techniques are based on the response of the earth to the flow of electrical current. In the shallow subsurface, the presence of water controls much of the conductivity variation. Measurement of the resistivity is a measure of the amount of water saturation and connectivity of pore space. Increasing water content and increasing salinity of the underground water will decrease the measured resistivity. So, increasing porosity of rock and increasing number of fractures will tend to decrease measured resistivity if the voids are water filled [46]. The geophysical methods (VES) were applied in this study, not only to bring information about fresh water bodies in the underground but also information about the geological structure, fractures, and rupture zones in the underground until 120 m depth. By this VES method (Schlumberger sounding), the variation of the resistivity with depth is measured, depending on the electric properties of the geologic sequences in the subsurface [46]. The electric properties are influenced by the lithology, the water saturation degree of pore spaces, the salinity of the mineralogical fluids involved, and the mineralized bodies along the tectonic lines (fractures and rupture zones) [15, 46]. The electric properties of the geologic sequences in the studied subsurface characterize the horizontal and vertical resistivity distributions in the underground and shapes of the vertical electrical sounding curve types.Measures of electric properties of the geologic sequences of the crystalline basements in Batouri and Ngoura subdivisions of nine hundred and seventy-six (976) Schlumberger electrical surveys and their 1D inversion enabled to propose a shape set of sounding curves. Nine (09) types of sounding curves (Figures5–14) materializing the arrangement of geological layers are defined, as well as their electrical variation, based on Telford et al. [30], Keary and Brooks [38], Parasnis [24], Singh and Stephen [47] and similar studies. These sounding curve types characterize the electric typology of the geologic sequences of the study areas and highlights by pseudosections and geoelectric sections (Figures 17(a), 17(b), 18(a), and 18(b)). The acknowledgement of the type of sounding curves for the Batouri’s and Ngoura’s crystalline basements help to define Schlumberger vertical electrical sounding curves (G, H, A, QH, KH, HK, HA, HKH, and KHK) which correspond to their subsurface structures. More than 80% of the Schlumberger electrical sounding curves are dominated by the H, QH, KH, HK, HA, HKH, and KHK types in the Colomine, Tassongo, and Mama II localities and the histogram of Colomine area (Figure 16) confirms this recovery of the sounding curve types referred above. These sounding curves are characterized by conductive and fractured/weathered levels inside granitic structures and can be interpreted as the electrical discontinuities or abnormal distributions of the subsurface resulting from weathering, fracturing, jointing, and faulting activities (Palacky et al., 1981; [21, 24, 30, 38]). The electrical discontinuities suggest an important heterogeneity of subsurface structures and put in case favored areas of mineral and groundwater targets in the crystalline basements (Palacky et al., 1981; [21, 24, 30, 38]).The electrical characteristics (resistivityρ and thickness ep) of each geological layer, resulting from the 1D inversions of the apparent resistivity as a function of depth through Schlumberger electrical soundings using the method described by Koefoed [32], suggest that the 1D geoelectrical models from various sounding curve types correspond to crystalline basements. Meanwhile, Chapellier [25], Oladapo et al. [44], and Kumar et al. [1] correlate the 1D geoelectrical models with the 1D geological models proposed for the same subsurface sections. In addition, the geological reconnaissance (observations on outcrops and goldmine pits) and data from previous studies (e.g., [27, 29, 42–45, 48]) help to design lithological 1D models related to each type of sounding curves through 1D geoelectrical models obtained. Correlation between geoelectrical and geological models in the same region help to define and propose geoelectrical and lithological 1D models for the crystalline basements through the geometrical and electrical characteristics of geological layer models: topsoil (0.5m<ep<3m; 600<ρ<3000Ω.m), lateritic soils (3<ep<20m; 1000<ρ<4000Ω.m), conductive layer (5<ep<110m; 100<ρ<1000Ω.m), fractured/weathered granite (20<ep<60m; 1000/5000<ρ<3000/9000Ω.m), clayey layer (5<ep<20m; 600/1500<ρ<2000/3500 (dry) Ω.m), and fresh granites (ρ>4000Ω.m). It is therefore acknowledged that the Batouri-Ngoura’s crystalline basements exhibit various lithological models which are particularly characterized by conductive layers and fractured/weathered granites. The conductive layers and fractured/weathered granites (Figures 20–27) confirm the discontinuity of subsurface structure proof by the Schlumberger electrical sounding curves. They depict proper hydrogeological and ore bodies occurring conditions for an efficient evaluation of the groundwater resources and mineral potential within the Batouri-Ngoura’s basements [1, 22, 24, 30, 38, 49]. The lithological dominant 1D models from the H-, QH-, KH-, HK-, HA-, HKH-, and KHK-type curves for Colomine, Tassongo, and Mama II (Figures 20 and 22–27), show that the crystalline basements in those localities have favorable conditions for mineral and groundwater prospecting. Thus, the 1D inversion of Schlumberger vertical electrical soundings using the method described by Koefoed [32] is the efficient background in the characterization of the typology of sounding curves and the location of favored areas of mineral and groundwater targets in the crystalline basements.


## 8. Conclusion

The vertical electrical sounding data acquisition in the Batouri and Ngoura subdivisions (East-Cameroon) along forty-one (41) profiles using the resistivity method has permitted to carry out an investigation of the subsurface layering, mineral potential, and groundwater resource characteristics until 120 m depth. Results of this investigation were presented as typology of sounding curves, mapping of the curve types, pseudosection,s and geoelectric and lithological sections. Pseudosection and geoelectric section characteristics reveal two to five layers underground and present the vertical changes of layers. The mapping of the curve types presents the horizontal variety of layers. The sounding curves show the variation of the resistivity with depth depending on the electric properties and tectonic activities of the geologic sequences in the subsurface. From the qualitative interpretation of VES curves, the subsurface layering is depicted by nine (09) types of sounding curves (G, H, A, QH, KH, HK, HA, HKH, and KHK) characterizing the vertical changes and the typology of sounding curves in the East Cameroon crystalline basements. The lithology of subsurface in the study area is dominated (more than 80%) by geoelectrical and lithological 1D models derived by the H-, QH-, KH-, HK-, HA-, HKH-, and KHK-type curves characterized by an abnormal distribution of resistivity in depth. The abnormal distribution put in case conductive layers and fractured/weathered granites, which depict proper hydrogeological and mining conditions. Thus, the resistivity method applied in this study bring information of target zones for the mining and groundwater investigations in the regions crossed by crystalline basements.



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*Source: 8630406-2020-02-28.xml*