APPLICATION OF GEOELECTRICAL METHOD TO STUDY GROUNDWATER POTENTIAL IN ISARA, REMO NORTH MUNICIPAL AREA OF OGUN STATE, NIGERIA.

ifteen (15) Vertical Electrical Sounding (VES) and three (3) Constant Separation Traversing (CST) data were acquired within the study area using Pasi Terrameter (model 16GL). The Schlumberger electrode array was deployed for the VES while Wenner array was used for the 2-D CST. Both qualitative and quantitative approach were used for the VES measurements. The raw data were Curve-matched and inversion of the data using WinResist (1.0) to create a model of perfect fit indicating layer thickness and resistivity values for individual layers while the 2D resistivity data were processed using Res2Dinv software. The 2D resistivity structures revealed the lateral and the vertical variations of the subsurface information having resistivity values ranging from 81.8 to 5250 Ωm. The geoelectric sections revealed five to six geoelectric layers, which correspond to the topsoil, clayey sand, lateritic clayey sand and sand. The topsoil is characterized by resistivity values ranging from 29.8 to 358.5 Ωm and layer thickness of 0.6 to 0.8 m. The clayey sand had resistivity and layer thickness values ranging from 81.4 to 278.0 Ωm and 2.4 to 7.1 m respectively. The lateritic clayey sand had resistivity values ranging from 782.0 to 2414.1 Ωm. and layer thickness of 5.4 to 61.2 m. The sand in the third layer in VES 1 to 7, 9 and 10 has resistivity values ranging from 398.6 to 600.7 Ωm and layer thickness of 2.3 to 25 3 m is characterized as seasonal aquifer. The result of this study has further highlighted the capabilities of the electrical resistivity techniques in groundwater investigation.


INTRODUCTION
The importance of water for the existence of life deserves attention because health and growth are closely related to it. Water plays a very important role in the survival of plants and animals, hence the common saying "water is life".
Groundwater sources are generally recognized as the best water sources for domestic and industrial use worldwide (Hoque et al., 2009). The rapid pace of urban development and the increasing need for private, public and industrial water supplies require a private, public and industrial water supply due to population growth (Adul et al., 2001).
Groundwater is the main source of water supply needed for industrial, agricultural and domestic use in many areas. In some cases, over-exploitation has caused groundwater table subsidence and consequently restricted groundwater flow to deeper and fractured areas Kumar et al., 2011). In view of the increasingly scarce water resources in Isara, Remo and the increasing demand for water to meet the needs of the rapidly growing population, a holistic and well-planned long-term strategy to access and manage groundwater resources in this area is necessary.
Different factors have been used as indices for groundwater resources occurrence in different study areas.
Some of which includes, subsurface layers, and structural features on fractures that cause 'stratigraphical disturbances' (Tizro et al., 2010) among other factors also includes geoelectric and geological parameters (such as aquifer resistivity, aquifer thickness, overburden resistivity and overburden thickness) derived from 2D resistivity imaging.
Particularly, the resistivity method has been effectively used by a number of researchers in various fields of application including groundwater investigations (Hossam et al., 2022;Devi et al., 2001;Lenkey et al., 2005;Hamzah et al., 2007;Gupta et al., 2012;Maiti et al., 2012Maiti et al., , 2013a, Geophysical evaluation of buried Septic Tank (Airen and Akeredolu, 2021), groundwater contamination studies (Karlik and Kaya, 2001;Frohlich et al., 2008;Kundu and Mandal 2009;Park et al., 2007;Mondal et al., 2013), saltwater intrusion problems (Edet and Okereke 2001;Hodlur et al., 2006;Song et al., 2007;Hermans et al., 2012;Maiti et al., 2013b), geothermal explorations (Kumar et al., 2011), exploration of limestone deposit in Nigeria (Airen et al., 2020). Additionally, the delineation of aquifers is the prerequisite for the assessment of regional/local groundwater potential. Several researchers have carried out systematic hydro-geological and geophysical investigations (Bidichael et al., 2018;Adeyinka et al., 2015;Bose and Ramkrishna, 1978;Deolankar, 1980;Singhal, 1997;Pawar et al., 2009;Rai et al., 2011Rai et al., , 2013Ratnakumari et al., 2012) to delineate fracture zones within the trap sequence and sedimentary formations below the traps, which are considered to be a potential resource of groundwater. This study has been able to delineate the aquifer units and their characteristics within the study area and so, boreholes should be sunk at depth intervals of between 60 to 130m. Also, electrical resistivity methods have been used for the first time to investigate the groundwater potential in Isara, Remo North Local Government Area of Ogun State, which are underlain by thick formation of lateritic clayey sand.

MATERIALS AND METHOD
The Schlumberger array current electrode separation (AB) varied from a minimum of 2 to 620 m while measurements were made at sequences of electrodes at 10, 20, 30, 40, 50 and 60 m at a maximum length of 200 m for the Wenner array.
The geoelectric sections were generated using the results of the iterations of the VES data. Four to six geologic layers, including; topsoil, clayey sand, sand and lateritic clayey sand were inferred from the interpretation of the VES data.
The acquired vertical electrical sounding (VES) data were processed both quantitatively and qualitatively. The quantitative interpretation of the depth sounding curves was carried out using the partial curve matching technique (Bhattacharya and Patra, 1968). In order to do this, the VES data were plotted on a transparent paper. The partial curve matching technique involved the use of a standard two (2) layer master curves and four (4) auxiliary type curves (H, K, A, and Q). This procedure required segment-by-segment curve matching starting from the position with shorter electrode spacing and moving towards those with longer spacing. The results of the VES curves obtained from the partial curve matching were then used to constrain the interpretation by the computer using inversion software known as WinResist Software. This invariably reduces overestimation of depths in the curve matching. The result of the computer iteration shows the quantitative analysis to know the resistivity, thickness and depth. The qualitative interpretation of the depth sounding curves was carried out based on individual geo-electric characteristics on the number of layers represented by the four types of the auxiliary curves (A, H, K, and Q) and also from the profiles and maps involves inspection for patterns/anomaly signatures that are diagnostic of the target.
The acquired Wenner apparent resistivity datasets were tomographically inverted to obtain true electrical resistivity distribution of the study area using the ''RES2DINV'' finite-difference software, based on the smoothness-constrained least squares inversion by a quasi-Newton optimization method (Loke and Barker 1996). An initial 2D electrical resistivity model is generated, from which a response is calculated and compared to the measured apparent resistivity values of the field data. The optimization method then attunes the resistivity value of the model block iteratively until the calculated apparent resistivity values of the model are in close agreement with the measured values of the field data. The absolute error provides a measure of the differences between the model response and the measured data which is an indication of the quality of the model obtained. Using this scheme, 2D inverted models of true resistivity variation of sub-surface geological formations for the study area have been computed. The RES2DINV software offers two inversion options robust inversion (Loke 2000) and smoothness-constrained least squares inversion (Loke 2014). It has been reported by Dahlin and Zhou (2004) that the robust inversion is better than the smoothness-constrained least squares inversion. In situations where the subsurface geology comprises a number of almost homogeneous regions but with sharp boundaries between different regions, the robust inversion scheme attempts to find a model that minimizes absolute changes in the model resistivity values (also known as L1 norm or blocky inversion method), thereby giving appreciably superior results. The smoothness-constrained optimization method (also known as L2 norm) on the other hand tries to minimize the squares of the spatial changes (or roughness) of the model resistivity values and tends to construct a model with a smooth variation of resistivity values. This approach is used only if the subsurface resistivity varies in a smooth or gradational manner.
The electrical resistivity method consists of measuring the potential at the surface, which results from a known current flowing into the ground (Marianna et al., 2022;Oladunjoye et al., 2020;VanNorstrand and Cook 1966;Ritz et al., 1999). A pair of current electrodes. The apparent resistivity ρa is expressed in equation 1.

= ∆ (1)
Where K denotes a geometric coefficient dependent upon the electrode array, ∆V denotes the measured potential difference and I denote the current intensity. The vertical electrical sounding technique consists of using a multi-core cable with as many electrodes plugged into the ground at specific spacing, according to a sequence of readings predefined and stored in the internal memory of the equipment. The various combinations of transmitting (A, B) and receiving (M, N) pairs of electrodes construct the mixed sounding/profiling section, with a maximum investigation depth that mainly depends on the total length of the cable (Figure 1 and 2). In electrical methods, the spatial resolution and depth of investigation is linked to the distance between electrodes. In a first approximation, for Schlumberger and Wenner arrays, the maximum depth of investigation is of the order of 20 % of the total length of the cable and the total length of the resistivity profile.  (Milson, 1996)

RESULTS
The results of this investigation are presented as VES curves, tables, geoelectric sections, 2D resistivity structures and maps. Figures 3 and 4 show the characteristics curves obtained within the site. The results of the inversion of the VES results are summarized in Table 1.  The lowest thickness of the layer is 28.5 m, while the highest thickness falls at 61.2 m. Its resistivity ranges from 842.1 -1199.1 ohm-m. The last geoelectric layer delineated beneath this section is the sand whose resistivity value falls between 167.1 -313.3 ohm-m, but the thickness could not be determined.
The geoelectric section connecting VES 6 -10 ( Figure 4) delineated five inferred geoelectric layers. These are; topsoil, clayey sand, lateritic clayey sand and sand. The first horizon represents the topsoil whose resistivity and thickness ranges from 29.8 -358.3 ohm-m and 0.6 -0.8 m, respectively. Underlying this layer is the clayey sand column with a resistivity value of 89.0 -145.0 ohm-m and the thickness of this layer falls between 3.7 -7.1 m. Figure 9 is the geoelectric section comprising of VES 11 -15. This section shows that four inferred geologic layers were present along this section. The topsoil is characterized by resistivity and thickness ranging from 114.2 -308.0 ohm-m and 0.7 -0.8 m, respectively.

2D Electrical Resistivity Models
The 2D Electrical Resistivity models derived from the inversion of the acquired resistivity data along transects 1, 2 and 3 are displayed in the following figure (Fig. 8ac). The models show the subsurface resistivity distribution within the study area ranges from less than 26.3 ohm-m on traverse 3 to more than  Isoresistivity Map for Sand 1 and 2 Figure 10 shows the isoresistivity map of sand 1 and sand 2. Figure 10a is the isoresistivity map of sand 1. The resistivity of this sand ranges from 370 ohm-m to more than 670 ohm-m. The map shows that at the northern part, labeled A, of the study area, the resistivity is lowest while the sand is thickest at the central portion, From the foregoing, it can be inferred that the sand 2 is a preferred aquifer with relatively high thickness compared to sand 1 and better protection from contamination by the lateritic clayey sand. The isoresistivity map for sand two also shows that the western and part of the central area of the site has higher saturation and higher groundwater potential.

CONCLUSION
The current study employs the electrical resistivity techniques (2D resistivity imaging and VES via Schlumberger array) to investigate the presence of a prolific aquifer within the study area and determine the geoelectric parameter of the same. A total of three 2D profiles were occupied; these were able to penetrate a total depth of 33.1 m. Also, fifteen VES, five on each traverse, were carried out. Five to six subsurface strata were delineated. This includes; topsoil, clayey sand lateritic clayey sand and sand. The study showed that two sand layers which could represent the aquiferous layer within the area were present. The resistivity value of the first sand layer (sand 1) ranges from 383.1-681.5 ohm-m with a thickness range of between 2.5 -25.3 m and a resistivity value ranging from 383.1 -681.5 ohmm. This is directly overlain by very thin topsoil (0.6 -0.8 m). This is deemed not a good aquifer from which potable and sustainable groundwater development could be carried out. Sand 2, with a resistivity value of between 167.1 -608.1 ohm-m is overlain with overburdened materials with a thickness range of 39.8 -99.6 m. This sand represents the best aquifer within the area. It is recommended that groundwater development at a depth of not less than 60 m to about 130 m is feasible within the study area. This is expected to give a very good groundwater yield. From the isoresistivity map, it could also be inferred that the sand two (2) is a preferred aquifer with a relatively high thickness compared to sand 1 and better protection from contamination by the lateritic clayey sand. The isoresistivity map for sand two (2) also shows that the western and part of the central area of the site has higher saturation and, as such higher groundwater potential. This study has been able to delineate the aquifer units and their characteristics within the study area.