Introduction

Estimation of groundwater potential through geophysical prospecting has become significant due to unscientific management and the use of resources for various purposes in recent years. Sustenance of this resource requires proper targeting, assessment and management of groundwater sources. The electrical resistivity methods are widely used in groundwater exploration studies (Todd 1959; Alile et al. 2011) because it is least expensive and requires no special training. Water in barren formations can be determined based on the contrast in electrical resistivity (Zohdy et al. 1974). The earlier phases of geophysical investigations for groundwater were attributed to unconsolidated alluvial and semi-consolidated sedimentary tracts. In recent years, greater importance is given to the exploration of groundwater in hard rock areas, as of the present study area.

In hard rock terrain with in-situ weathering and freshwater beneath, this method is used to find the thickness of the weathered layer (Chidambaram 2000; Tahmasbi et al. 2012; Gautam and Biswas 2016). The resistivity surveys were used to define groundwater contamination (Lawrence and Balasubramanian 1994; Ezeh 2011; Hussain et al. 2017; Kumar et al. 2002) and also to carried out the characterization of groundwater flow regime through fracture network with the help of geophysical method. Well documented studies on electrical resistivity were also carried out by Srinivasa et al. (2000), Israil et al. (2006), Srivastava and Bhattacharya (2006), Kukillaya (2007), Semere and Woldai (2007), Mondal et al. (2008), Oh et al. (2011), Venkateswaran et al. (2014) and Sahebrao et al. (2014). Electrical resistivity method is particularly well suited for groundwater studies in hard rock terrains (Balakrishnan et al. 1984; Bhimasankaran and Gaur 1977; Srivastava and Bhattacharya 2006; Srinivasamoorthy et al. 2013; Prasanna et al. 2009; Seyedmohammadi et al. 2016; Panda et al. 2017; Roy and Elliot 1981) to made a significant observation regarding the depth and exploration using DC electrical method within a specified domain of the resistivity, layer thickness, and electrode spacing. Electrical resistivity surveys were also conducted in shales for the estimation of resistivity and depth to the basement by Balakrishnan et al. (1979) and Balasubramanian et al. (1985) worked on resistivity method by a combination of iso-resistivity and isopach map to classify freshwater and saltwater horizons. Later the use of the computer in the analysis of resistivity data for direct interpretation was carried out to attain significant results (Basokur 1990; Joshua Emmanuel et al. 2011).

2-D resistivity methods were successfully used for detection in the shallow subsurface to obtain information by deeper penetration for shallow subsurface study, Nordiana et al. (2013) has identified the structural, geological contact of sedimentary lithologies by using the 2-D resistivity imaging. Fractures are the important conduits for groundwater flow in hard rocks. The flow of groundwater through a fracture network is largely influenced by hydraulic anisotropy resulting from the geometry and orientation of the fractures. A similar study for potential groundwater evaluation using electrical resistivity method in a typical basement complex area of Nigeria has been carried out by Ayolabi et al. (2009), who have demonstrated the significance of coefficient of anisotropy in groundwater potential evaluation. Dewashish et al. (2014) have evaluated the heterogeneity in hard rock aquifers due to structural disturbances from resistivity sounding data. Stephan et al. (2017) has attempted to identify the freshwater lens on the island of Langeoog, in Germany by adopting geophysical techniques. This study aims to understand the groundwater quality variation along the KT (Cretaceous-Tertiary) boundary along the contact of hard rock and sedimentary formation. It also tries to differentiate the lithological contact with the aid of 2D resistivity imaging. Subsurface model has also been attempted in three dimensions to understand the vertical and lateral resistivity profile variations in different lithologies.

Study area

The area proposed for the study is Ariyalur region which is positioned in Ariyalur district of Tamil Nadu (Fig. 1) with an aerial extent of about 1774 km2. It falls between latitudes 11°449′ and 10°974′N and longitudes 78°808′ and 79°275′E. Four formations namely Quaternary, Tertiary, Cretaceous, and Achaean, are represented in Ariyalur region. The northwestern part of the study region is elevated and sloping towards the south-east.

Fig. 1
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Geology map of the study area and VES location point

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The geological formation of the study area consists of Archean to quaternary (Table 1; Fig. 2). The Archean is the hard-rock formation comprised of Charnockites, Peninsular Granites, and Gneisses which traversed by Pegmatite and Quartz veins. The study area forms a part of the Cauvery basin, which is in the SE part of India. The formation is represented by 5–6 km thick pile of syn-rift (> 1000 m) and post-rift (~ 5000 m) sedimentary rocks resting over the Archaean basement (Rangaraju et al. 1993). The sediments of Cauvery Basin are well exposed in Tamil Nadu, in a linear patch along the Ariyalur, Virdhachalam, and Pondicherry districts. The stratigraphic succession of the study area is categorized based on lithology, as Uttatur, Trichinopoly, and Ariyalur Formations (Blankford 1862). The Ariyalur formation is exposed in the eastern and northeastern part of the Ariyalur district, and it is focused in the present study. Recent lithostratigraphic categorization of the Cretaceous sequence in the Cauvery Basin is separated into the Ariyalur formation as Sillakkudi, Kallankurichchi, Ottakovil, and Kallamedu. The lower division of the Sillakkudi formation contains un-fossiliferous calcareous sandstones in the northern region and fine sandstone in the central and southern region. Most of the part of the Sillakkudi formation contains Sandstone. The Kallankurichchi formation overlies by the Sillakkudi formation.

Table 1 Lithostratigraphy of Ariyalur Group (Modified after Sastry et al. 1972)
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Fig. 2
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Representing the elevation, lithology, drainage, and lineaments of the study area

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Materials and methods

The resistivity survey was carried out by using resistivity meter (IGIS, Hyderabad, India), adopting Schlumberger electrode configuration and VES method at 15 selected locations out of which 3 falls in Archean, 7 in Cretaceous, 2 in Tertiary and 3 in Quaternary Formation. The electrode configuration for Schlumberger consists of four electrodes. The apparent resistivity for the four electrodes configuration depends upon the distance between the sources, (M and N) and V/I ratios. Out of four electrodes, two are potential, and two are current electrodes. The configuration for Schlumberger is two inner Potential (M and N) electrodes, are spaced at closer intervals, and two outer current electrodes (A and B) are placed apart from the potential at longer distances, (MN << 1/5 AB). M and N are kept fixed, and Aand B are moved outwards in steps. At some stage MN voltage will in general fall below the reading, the accuracy of the voltmeter in which the distance MN is increased (five or ten-fold) maintaining the condition MN << AB (Ballukraya and Sakthivadivel 1983). ROCKWORKS 16 has been used to model the subsurface using the resisitivity depth profile representing thehard rock and the sedimentary formations.

The geophysical survey was conducted, and the curves were manually plotted and then checked with IPI2 Win software, and the RMS error was less than ± 2%. Most of the values of manuallyplotted curves were matching with the curves plotted in IPI2 Win V.2.1 software (Geoscan-M Ltd 2002). The layer parameters like apparent resistivity (ρa) and thickness (h) of different layers were calculated manually by curve matching technique and cross-checked with IPI2Win (v.2.0) software (Geoscan-M Ltd 2002). Minor deviations due to subsurface discrepancies and error in field data were corrected by smoothening the field curve to have natural interpretation. The values obtained by the curve matching method were used for further studies. For three-layer cases, the apparent resistivity curve takes four basic shapes known as Q (or DH, descending Hummel), A (ascending), K (or DA displaced Anisotropic), and H (Hummel type with minimum) depending upon the relative magnitudes of ρ1, ρ2 and ρ3.

Result and discussion

VES curves

A maximum of five layers were identified in few regions, but the study area is dominated by three layers. The interpreted master curves were mostly three-layer curves, namely A followed by H and K type. VES location number 7, 15 are of A type, whereas the VES No. 2, 3 and 12 are of H type. 4, 6, 8, 10 and 11 are K type. Four other curve locations show KK type (VES 1); HHK type (VES 5); KH (VES 9, 14) and KA type (VES 13). Table 2 indicates maximum, minimum and average values of resistivity and thickness. It is inferred that the maximum resistivity of the first layer is of 66.1 Ω m, the second layer is 2373 Ω m, and the third layer is 45,428 Ω m. The maximum resistivity of ρ1 (resistivity of the first layer) was observed in Nannai of Achaean formation and minimum is observed at Ariyalur which lies in sedimentary formation. The maximum thickness of the first layer is 2.74 m and the second layer is 86.1 m. The higher thickness is observed in the first two layers of Ariyalur (VES 11). The weathered zone was found to be more significantalong the hard rock and sedimentary contact. The resistivity of highly weathered saturated gneisses of Archean age ranges from 27 to 120 Ω m. The thickness of the first layer shows an increasing trend of Tertiary > Alluvium > Achaean, however the second layer, has a increasing trend from Achaean > Tertiary > Alluvium. The area of low apparent resistivity value indicates the occurrence of relatively good conductors while those with high values indicate poor conductors (Prasanna et al. 2008). In general, three significant zones were delineated as the weathered, fractured and massive layer in hard rock region and the sedimentary regionas topsoil, sandy layer and clayey layer in this region.

Table 2 Resistivity (in ohm meter) and thickness (in meter) of the survey location with maximum, minimum and average
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The resistivity of the formation depends on the property of fluid and medium. The variation of resistivity is attributed to three main reasons, the presence of clay and content in fractures, the massiveness of rock and conductivity of fluid medium (Chidambaram 2000). Lesser resistivity was noted in weathered regions; this may be due to higher conductivity in waters or presence of clay content in fractures. As per the classifications of Verma et al. (1980), Type I formation have intergranular porosity without clay content likenon-shaly sandstone which is rarely found in hard rock terrain, but when it gets saturated with fresh water, it shows intermediate resistivity. Type II formation with intergranular porosity and clay content like shaly sandstone and weathered rock with clay content,when it gets saturated fresh water it shows lower resistivity because the clays conduct electricity by electronic as well as the electrolytic process. In Type III formation comprises of the fractures, jointed and un-weathered formations with no clay content,when gets saturated with fresh water they have relatively high resistivity becausethe rock has a lower active area of a cross-section which current must travel as compared to Type I rock. In type IV formation, a decrease of resistivity values will reflect higher conductivity values. However, there are combinations of II and III type of formations indicating the semi weathered nature. The weathered layer corresponds to Type II inmost of the Gneiss region; owing to this nature the ρa values are lesser in these regions. The Type III nature is found to be dominant in the Charnockite regions and the massive layer of Gneiss with higher resistivity. The semi-weathered layer is also noted in the four-layer cases where the combination of II and III type is observed.

Spatial variation

The apparent resistivity values and the thickness have been distributed spatially and overlaid on, lithology, drainage and lineaments map of the study area to get an overall perspective of the variations concerning the thematic layers. The first layer resistivity map indicatesthat thenorthwest and southwest region show higher resistivity values along the lithological contact (Fig. 3). The hard rock aquifer can be considered primarily as horizontal discontinuous in local scale concerning with fractures and joints along with appreciable clay content. The higher 1st layer thickness is noted in southeast direction represented by the Quaternary formation; this may be due to the persistence of deeper water level favored by overexploitation of groundwater in this region. The second layerresistivity (Fig. 3) indicates that major part of the study area is covered with resistivity ranging from 1.98 to 95.84 Ω m. The resistivity values are increasing towards the Archean formation at Oganur, Nannai, and Kallagam. The higher IInd layer thickness (39.8 m) is observed in the central part and southeastern region.

Fig. 3
figure 3

Spatial distribution of both first and second layer resistivity and thickness of the study area

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Histogram

The resistivity and thickness of the layers are represented in Figs. 4, 5, and it indicates that the maximum and average resistivity of the formations reflect the increase in resistivity with depth, i.e., the first layer with lesser resistivity and the third layer with higher resistivity. Higher resistivity is observed in the 3rd and 2nd layer, i.e., an average of 250.71 and 3072.07 Ω m respectively which is represented by charnockite and gneiss (Fig. 3). The massiveness of this rock type is well represented by higher resistivity values, which are more prominent in charnockite and gneiss of the study area. In general, thedifference in resistivity with depth indicates the variation of weathering intensity with depth or variations may also be attributed to the presence of weaker zones. It is inferred from the above data, charnockite and gneiss show higher resistivity and higher thickness in fractured layers. However, in sandstone, it shows low to intermediate resistivity values. It is observed that higher resistivity values indirectly reflect less weathering or absence of clay material in fracture or water with lesser conductivity.

Fig. 4
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Resistivity values of the layers

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Fig. 5
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Thickness of the layers

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Pseudo section

An attempt was made to generate a pseudo section of apparent resistivity values for selected VES locations of the study area. The pseudo sections help to understand the subsurface lithology variations and facilitate to delineate potential aquifer zones in the hard rock and sedimentary terrain. Pseudo sections were made with; three profiles which were constructed from the north-west and south-east direction as shown in Fig. 6 and named as A–A′, B–B′ and C–C′. In the constructed profile, a maximum number of electrical resistivity locations has been considered for the generation of the possible better pseudo section. In the first profile (A–A′) three vertical electrical sounding locations of (VES-2, 1 and 14) were considered for construction of pseudo section as shown in (Fig. 7). This pseudo section shows that the overall resistivity varied from less than 1.97–258 Ω m. It is observed that apparent resistivity gradually increases towards the depth and resistivity of the upper layer varies from < 1.97 to 2.96 Ω m. The top layer is inferred up to 1.4 m from the ground level on the western side (in the hard rock region) and increased to above 50 m in VES 1. Whereas, on the eastern side lower resistivity values were observed up to 2 m then it starts increasing up to 100 m depth. Below the top layer, resistivity range of 2.96–9 Ω m, which categorizes the second layer and the third layer is found with resistivity ranging between 10 and 30 Ω m. Similarly, high resistivity zone is found at shallow depth in the western part of the study area. This fact depicts the contact between the hard rock and the sedimentary formation reflect low resistivity values.

Fig. 6
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Profile direction of pseudo section

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Fig. 7
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Pseudo section resistivity of profile 1

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In the second profile (B–B′) three VES locations such as (VES-6, 11 and 15) were considered, and the constructed pseudo section is shown in (Fig. 8). The resistivity varies from < 1.49 to nearly 40 Ω m, where low resistivity varies from 1.49 to 1.90 Ω m and is found in middle part of the profile. The thickness of low resistivity zone is high, and it varied from 1 to 2 m from the surface at the western margin. Below this zone, resistivity varies from 3 to 12.8 Ω m, in the west part of the study area with a lesser thickness. Further, below resistivity zone of 7–15 Ω m is observed with a higher thickness on the south-eastern side, but it decreases in VES 11. The resistivity of 17–40 Ω m is observed with a more significant thickness on the eastern side. The high resistivity could be due to a massive rock, and low resistivity could be due to highly weathered rock formation or the contact as represented in Fig. 8. In the third profile (C–C′) three vertical electrical sounding locations of (VES-7, 10 and 9) are considered for the pseudo section as shown in (Fig. 9). The overall resistivity varies from less than 1.20 Ω m to nearly 36 Ω m. It is observed that the resistivity gradually varies with depth and resistivity of the top layer varies from, less than 1.27–2.5 Ω m. The upper layer is noted up to 7 m from the ground level on the western side and increases to above 14 m in the mid of the profile and along the eastern part. Below the upper layer, resistivity varies from 3.60 to 7.80 Ω m representing the second layer, and further, the deeper layers have a resistivity of 8–15 Ω m. The pseudo section shows that low resistivity is found at a deeper level in the eastern side. High resistivity zone is found at shallow depth in the western part and eastern part of the study area indicating the variation in lithology. The observation of the profiles signifies that the apparent resistivity values indicate the variation in lithology. Further, it is also clear from the cross section that there exists a clay layer along the contact between the hard rock and the sedimentary formation. The vertical variation of thickness and resistivity also reflects the difference in lithologies.

Fig. 8
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Pseudo section resistivity of profile 2

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Fig. 9
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Pseudo section resistivity of profile 3

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The apparent resistivity values of these nine locations have been modelled vertically using ROCKWORKS 16 (Fig. 10). Consisting of three locations in hard rock (6, 7 and 9) and six in the sedimentary formation (1, 2, 10, 11, 15 and 14). The hard rock locations were towards the western part of the study area. The vertical resistivity model shows that there is a clear demarcation of the hard rock formation and the sedimentary formations as depicted in the two-dimensional profiles. Higher resistivity values were noted in the hard rock regions and the lesser resistivity in the sedimentary terrain. Model depicts that in the western part of the study area the deeper depths are more massive and the low resistivity values are reflected in the shallow layers indicating the presence of weaker zones (Srivastava and Bhattacharya 2006; Kukillaya 2007). But, still the lower resistivity values are noted even in deeper depths at one location in the hard rock aquifer in location 9 indicating the possibility of vertical infiltration from the surface. Further in most of the sedimentary locations the shallow depths reflect less resistivity indicating more contamination/high conductivity in the shallow aquifers.

Fig. 10
figure 10

Three-dimensional subsurface model to understand the vertical and lateral resistivity profile variations in different lithologies

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The relationship between resistivity and electrical conductivity

The relationship between the resistivity within-situ electrical conductivity can help to identify shallow contaminated zones (Sonkamble et al. 2014). The literature reveals the possibilities of the application on the relation between earth resistivity and groundwater electrical conductivity (Cartwright and Sherman 1972; Peinado-Guevara 2012; Sainato and Losinno 2006). Empirical relations between the earth resistivityof three layers and the measured electrical conductivity of groundwater is given in (Fig. 11). It is inferred that the locations with high resistivity show low conductivity and locations that have low resistivity reflects high conductivity. The apparent resistivity values of three layers for all locations were plotted against the conductivities of groundwater to understand the correlation between these two parameters (Fig. 11). The apparent resistivity of each layer was compared to the electrical conductivity of the groundwater collected from the location, and it was found to vary from resistive to conductive irrespective of location. The conductive locations (1, 4, 9, 11, 13 and 14) mainly associated with infiltration of leachates from the agricultural lands. It explains the effect of massive percolation of contaminants (leachate) into the subsurface comes from the agrochemicals applied to agricultural lands. Thus, the distribution of conductivities changes from place to place which governs the variation in resistivity of the first layer. The resistivity increases with depth at various VES points, except in locations 2, 3 5 and 12 for II layer and in 1, 4, 6, 11 and 14 for III layers, where inversion is noticed. The increase in resistivity with depth is assumed to be due to standard compaction or lithification of sediments at a deeper depth of burial. It is to be noted that the apparent resistivity values obtained indicate the resistivity value of the matrix (rock) and that of the medium (groundwater). It is to be noted that if the groundwater is more conductive and the matrix is more massive, the apparent resistivity value will be higher than that of samegroundwater with less massive rock matrix. It is also observed in general that when there is an increase in conductivity of groundwater, there is a decrease in the resistivity value of the first layer in the sedimentary formations and both first and third layer in the hard rock aquifers.

Fig. 11
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Variation of apparent resistivity of different layers with respect to the electrical conductivity of the groundwater

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Conclusion

The study infers the three significant zones as weathered, fractured and massive zone in hard rock aquifers and topsoil, sandy layer and clayey layer in the sedimentary region. A, H and K are the major master curves identified. Resistivity of the first layer varied from 1.9 Ω m (VES-11) to 66.1 Ω m (VES-3) and thickness varies from 0.28 m (VES3) to 2.74 m (VES-11). Resistivityof the second layer varies from 1.98 Ω m (VES-12) to 2373 Ω m (VES-14) and thickness varies from 0.15 m (VES-14) to 86.1 m (VES-11). The thickness of the first layer shows increasing trend from Tertiary > Alluvium > Achaean. However, the second layer, shows an increasing trend from Achaean > Tertiary > Alluvium. Northwest and southwest region show higher first layer resistivity values reflecting the hard rock aquifer. Southeastern part shows higher thickness along the Quaternary formation. The higher thickness is inferred due to the deeper water level in this region. Second layer resistivity increases towards Archeanformation. However, higher thickness noted in the central region along the contact and in the southeast part of the region that is considered to have higher 1st layer thickness. There is an increase in resistivity with depth indicating variation of weathering intensity with depth and due to the presence conductive groundwater. In general, all three pseudo sections show that low resistivity is found at deeper level in the eastern side. High resistivity zone is found at shallow depth in the western part of the study area indicating the variation in lithology. The locations 1, 4, 9, 11, 13 and 14aremore conductive and mainly associated with leachate of contaminants from agricultural lands. The higher conductivities are inferred to be present in the deeper layers of the hard rock and in shallow aquifers in most of the sedimentary formation, except for regions with deeper water level in the southeast.