## 2 Materials and Methodology
This method is quick and cost-effective and has proven more successful for describing subsurface lithological and hydrogeological features [34; 35] and groundwater conditions and subsurface layers [36; 37]. This method is also used to estimate the hydrological parameters of the aquifer. Many researchers have evaluated aquifer parameters using the resistivity method [38; 39; 40]. This method has been utilized for indirect estimation of K and T, reducing the need to drill boreholes that were used for pumping tests [41; 42; 43]. Electrical resistivity survey is a practical application of [PERSON]'s law, as shown in Equation (1), to map the subsurface changes in earth resistivity and correlate them with the hidden geological formations. \[\mathrm{R}\propto\Delta\mathrm{V}/\mathrm{I}\cdots\mathrm{or}\cdots\mathrm{R }=\mathrm{K}\times(\Delta\mathrm{V}/\mathrm{I}) \tag{1}\]  
where R is apparent resistivity (ohmmeter); \(\Delta\mathrm{V}=\mathrm{Voltage}\) (potential drop) measured in milli-volt; I = current (milli-ampere); and K = Schlumberger constant of proportionality. The apparent resistivity depends on the injected current I, measured potential V, and the geometric factor K [33; 44]. Current injected into the subsurface is affected by many features of the strata such as priority organic components, minerals such as silt, clay, packing of void spaces, fluid presence, etc. in subsurface strata [45]. This method is used profitably to determine qualitatively the type of water-bearing formation, e.g., sand, sandstone, gravel, boulder provided conditions are favorable and not complicated by abrupt lateral changes in lithology. In resistivity surveys, a commuted direct current is introduced into the ground via two electrodes (A & B). The potential difference is measured between the second pair of electrodes (M & N). The four electrodes are arranged in any of several possible patterns. The current and potential measurements are then used to calculate apparent resistivity values. The layout of current and potential electrodes is shown in Figure 2. For data acquisition, Schlumberger electrode configuration is applied which is the most widely used in the world for conducting electrical resistivity surveys. The electrodes are placed along a straight line on the earth's surface by satisfying the condition given in Equation (2). In this study, a total number of 50 VES probes were conducted to map the subsurface lithologies. Some of the points near the water wells were used for calibration purposes. The geoelectric parameters of VES-30 were correlated with lithologic data from 1 of 17 water wells and interpreted the lithologies of VES-30 were shown in Figure 3. IXID software is used to calculate the true resistivity together with the thickness of all possible geoelectric layers encountered in the depth of investigation. The modeled (true) resistivities of VES points reflect the subsurface lithologies up to a depth of penetration and hence provide information on the subsurface geology of the area. The resistivity of the rock is controlled by various factors such as water and clay content, porosity, temperature, and fault [46]. The subsurface lithologies are interpreted on a local basis relating to the nature and characteristics of the rocks existing in that area [46; 47]. The average resistivity of all materials calculated in the field known as the apparent resistivity of 50 probes was analyzed for the estimation of aquifer quality. The inverse iterative technique was applied to obtain true resistivity and layer thickness for different sub-surface layers using IXID software. Factors, such as porosity, temperature, clay content, and water content, affect the actual resistivity of the material [46]. The nature and characteristics of the rock mass existing in the area play an important role to interpret the sub-surface lithologies [48].