Introduction

Groundwater forms the largest, easily accessible and an important natural resource of water. The intricate joints, fractures, and tiny pore spaces of rocks together hold immense volume of groundwater (Lutgens et al. 2014). The groundwater free from bacteriological pollution and hence safe for household utilization constitutes half of all the drinking water worldwide (Elizondo and Lofthouse 2010; Siebert et al. 2010; Smith et al. 2016; Okafor and Mamah 2012). The annual rate of worldwide groundwater extraction is about 982 km3 where 70% portion is utilized in agriculture sector (Margat and Van der Gun 2013). With the growth of population at an enormous rate, the groundwater extraction is expected to increase from 15,000 m3/day at present to 37,000 m3/day by 2025 (Maury and Balaji 2014). Hence, the demand of water can be overcome by the identification of potential aquifers during the demarcation of subsurface geology.

There are several methods worked out in practice for the assessment of hydrogeological conditions, and among these, the geoelectrical resistivity methods are well known for effectiveness, reliability, and feasibility (Martinelli 1978; Oseji et al. 2006; Khalil 2010; Sikandar et al. 2010; Joshua et al. 2011). These methods have been widely employed for the subsurface mapping, groundwater assessment, exploration, and salinity demarcation relying on resistivity variations (Barseem et al. 2013; Choudhury et al. 2001; Khaled and Galal 2012). The integration of various subsurface parameters taken out from resistivity measurements can be effective as electrical resistivity properties correlated with hydraulic properties based on the grain size distribution, fabric, structure, and heterogeneities (DeLima et al. 2005; Hubbard and Rubin 2005; Niwas et al. 2006). The vertical electrical sounding (VES) is one of the most versatile methods used in groundwater resource estimation and exploration (Balaji et al. 2010; Khalil 2010).

The study area district Rawlakot Azad Jammu and Kashmir is located at 250 km from Islamabad Pakistan, in the sub-Himalayas, a part of Hazara Kashmir syntaxes (Thakur et al. 2010). The area lies between longitudes 73°37′45″ to 73°49′01″ east and latitudes 33°47′52″ to 33°51′51″ north covering parts of Survey of Pakistan toposheet no. 43 G/9 (Fig. 1). The topography of the area is extremely rugged with steep hills and valleys having a diverse landscape with an elevation between 500 to 2256 m from the sea level. The study area (Fig. 1) possesses thick Neogene sedimentary rocks. These rocks include the lithologic assemblages of planar bedded sandstone, shale, claystone of Nagri Formation (Fig. 2) and conglomerates passim in the Murree, Nagri, Dhok Pathan formations and recent alluvium. A measured stratigraphic section of surface sediments is shown in Fig. 2. The base of the formation includes mudstone/claystone alternating with shale and sandstone (Fig. 2). The mudstone ranges in thickness from 8.0 to 23.5 m. The shale beds have average thickness of about 53.6 to 63 m and yield leaf imprints and carbonate concretions. The sandstone of the section varies in thickness from 20.5 to 58.5 m thickness. The wood fossils and exotic conglomerate clasts are tessellated in few beds (Fig. 2). The conglomerate clasts have owned few millimeters to several centimeters diameter with rounded to sub-rounded and disc-shaped outlines. Moreover, the fluvial structures formed by floundering flow velocities include planar bedding and ripple laminations (Fig. 2).

Fig. 1
figure 1

Showing the location of study area, VES points, and regional faults on digital elevation model (DEM)

Full size image
Fig. 2
figure 2

A typical stratigraphic section exposed at Medan area

Full size image

The sandstone is interbedded with shale (Fig. 2) and acts as confined aquifers in a few areas. The rocks of the region are faulted (Fig. 1) and fractured. The spurt of water through fractures and some spouting through poorly consolidated sediments has been noticed. In addition, the area is drained by few intermittent streams, inundated during summer monsoon rainy season. The average rainfall in the area is 476 mm due to monsoon in July and August (Climate-data.org). The streams of the area in some places have been harnessed to form small dams. The stored water in these small dams is consumed during the extremely parched summer. However, most of the population is facing water shortage. So, the water scarcity is a major problem in few regions.

Previously, the area has not been investigated for groundwater exploration. Only a few springs and piped water supply are the reliable but insufficient source of fresh water for inhabitants. An attempt has been made to estimate groundwater potential and depth beside the thickness of subsurface layers and their water-yielding capabilities using the VES and petrographic analysis.

Materials and methods

The electrical resistivity method was embarked in the study area in accordance with the previously applied methods for the exploration of water resources (Dobrin 1988; Ozcep et al. 2009; Alile et al. 2011). In other words, a resistivity survey was accomplished in the near surface sediments and in milder climatic conditions to access the information regarding the presence of water in the rocks. The area was chartered with the Schlumberger configuration in lieu of other methods. For this purpose, vertical electrical soundings (VES) were used in 24 locations up to the total AB/2 depth of 150 m. The precise locations of these stations were positioned on the digital elevation model (DEM) using global positioning system (GPS). The paraphernalia used for VES includes ABEM Terrameter SAS 4000 (Sweden) and accessories. A symmetric increase in the spread distance of the deployed current and potential electrodes was paced out for deeper penetrations. Three and four layers of curve’s shapes were obtained in qualitative interpretations (Table 1). In addition, the IPI2win software has formulated the resistivity model of earth (Orellana and Mooney 1966). The thicknesses and true resistivities of the subsurface layers have been calculated and enumerated under Table 2. Furthermore, the computer software (Surfer V.10) was assiduously used to portray the isoresistivity maps. The low (depression mark) and high values (peak) have been illustrated on the 2D and 3D isoresistivity maps of apparent resistivity. Moreover, the thickness of rocks was gauged during field work from the well-exposed sections of Nagri Formation (Middle, Siwaliks) in the area, using Tape-compass Clinometer method (Compton 1985) in order to understand facies and depositional environment of rocks. However, the thickness, lithology, fossils, and sedimentary structures (Fig. 2) were plotted on the software Sed. Log 3.0 version. The thin sections of rock samples were prepared at Geoscience advance research laboratory, Islamabad, Pakistan to understand mineralogy of rocks. However, besides mineralogy, the deformation in mineral grains and yielding to secondary porosity was encountered during petrographic study. The area has been deformed by regional faults in the region known as Shaeed Gala Thrust and Riasi Thrust (Rustam et al. 2016) as shown on DEM.

Table 1 The interpreted curve types of the study area
Full size table
Table 2 The interpreted subsurface geology based on geoelectric data
Full size table

Results

Apparent resistivity

The apparent resistivity maps have been made at different electrode spacing to delineate the resistivity variation with depth.

2D isoresistivity map at 15 m spacing

The apparent resistivity for 15 m spacing falls between 13.16 and 68.99 Ωm on the corresponding isoresistivity map displayed in Fig. 3. The analysis reveals highest apparent resistivity values of 68.99, 66.671, 44.455, and 41.623 Ωm at VES 24, VES 15, VES 18, and VES1, respectively. On the other hand, the lowest apparent resistivity values of 13.16, 13.498, 18.511, 21.938, and 23.33 Ωm have been interpreted at VES 20, VES 3, VES 6, VES13, and VES9, respectively.

Fig. 3
figure 3

2D isoresistivity map at 15 m spacing

Full size image

The 3D map (Fig. 4) displaying peak at the northern portion of the project area has figured out the presence of sandstone as compared to the presence of clays and boulder clays in the remaining areas. The depression is also visible in this region. Furthermore, the southwestern portion also portrays low values corresponding to VES19, VES20, and VES 21.

Fig. 4
figure 4

3D isoresistivity map at 15 m spacing

Full size image

2D isoresistivity map of apparent resistivity at 45 m spacing

The apparent resistivity for 45 m spacing has values lying between 4.57 and 86.585 Ωm on the corresponding isoresistivity map (Fig. 5). The highest apparent resistivity values of 86.585, 84.675, 69.305, and 64.281 Ωm have been detected at VES 19, VES 10, VES12, and VES4, respectively. The lowest apparent resistivity values of 4.57, 14, 15.716, and 20.623 Ωm have been numerically calculated at VES 3, VES 6, VES13, and VES 8 respectively.

Fig. 5
figure 5

2D isoresistivity map at 45 m spacing

Full size image

2D isoresistivity map of apparent resistivity at 130 m spacing

The apparent resistivity for 130 m spacing falls at 8.93 to 100.1 Ωm on the corresponding isoresistivity map (Fig. 6). The highest values of 100.1, 94.228, 76.258, and 73.493 Ωm have been numerated at VES 4, VES11, VES22, and VES 12, respectively, contrary to the lowest apparent resistivity values of 8.93, 12.67, 21.874, and 25.71 Ωm at VES 3, VES13, VES 8, and VES 7, respectively.

Fig. 6
figure 6

2D isoresistivity map at 130 m spacing

Full size image

The 3D map (Fig. 7) displaying three low peaks at the north and north eastern portion of the study area has figured out clays with maximum apparent resistivity of almost 100.1 Ωm. However, the rest of the area depicts low resistivity values.

Fig. 7
figure 7

3D isoresistivity map at 130 m spacing

Full size image

3D isoresistivity map of transverse resistance

The properties of the conducting layers are determined in terms of longitudinal conductance and those of the resistive layer by transverse resistance (Yungul 1996). This is the total resistance through a l-m column cut perpendicular to the plane (Parasnis 1979; Nwanko et al. 2011). The decrease in resistivity pointing towards groundwater potential in the area (Gowd 2004; Joseph 2012; Niaz et al. 2016).

The 3D map (Fig. 8) displaying three peaks has figured out the highest transverse resistance values at VES 5, VES12, and VES 19 in central portion of study area. The rest of the areas have revealed lowest resistivity values with good groundwater potential.

Fig. 8
figure 8

3D isoresistivity map of transverse resistance

Full size image

Anisotropy

When resistivity of the rock varies with direction of current flow, the rock is said to be anisotropic. A co-efficient of anisotropy is found by taking the square root of the ratio of resistivities (Olasehinde and Bayewu 2011) measured in two principle directions, mathematically:

$$ \uplambda ={\left(\uprho t/\uprho l\right)}^{1/2} $$
(1)

where

λ:

coefficient of anisotropy

ρt = T:

where T is transverse resistance and t is the total thickness of rock units

ρl = H/S:

where H is the total thickness and T is the longitudinal conductance

The parameter of anisotropy is the basic agents which controls the effective thickness and effective resistivity.

Anisotropic variations

The coefficient of anisotropy values (λ) lies between − 0.4 and 4.2 (Fig. 9). The 3D maps displaying two peaks have figured out the highest anisotropy values above 3 at VES 10, 11, 17, 18, 19 and VES 20 in the northeastern and southwestern portion of study area. This relatively high values of coefficient of anisotropy in northwestern and southeastern portion suggests that these are due to the near surface inhomogeneity and also due to the variation in structural features like faults, joints, and fractures and is relevant to the groundwater development in the area (Olasehinde and Bayewu 2011; Bayewu et al. 2014). The area is traversed by two faults (Fig. 1) Shaeed Gala Thrust and Riasi Thrust (Rustam et al. 2016). The high anisotropy values observed in this area are due to these faults. The other areas on map display lowest anisotropy values and hence show homogeneity and marked as alluvium.

Fig. 9
figure 9

3D isoresistivity map of anisotropy

Full size image

Interpretation of VES curves

The VES curves in Fig. 10a, b were interpreted to find confined- and semi-confined-type aquifers in the area. The low resistivities in the third and fourth layers indicate the saturated sandstone bounded by shales (Fig. 9) form the confined aquifers. These low values of resistivity in VES 17, 11, 15 are due to presence of Fault Shaheed Gala Thrust, while in VES 10 and 23, low values are indicating good potential of groundwater due to another fault known as Riasi thrust. The faults are acting as a barrier for groundwater development (Niaz et al. 2016; Niaz et al. 2017). The interpretation reveals that area consist of confined and semi-confined aquifers with the considerable thickness. The tube wells can be installed in this area.

Fig. 10
figure 10

a Interpreted of VES curve types. b Interpreted of VES curve types

Full size image

Petrography

The petrographic results indicate that the essential minerals such as quartz, feldspar, and rock fragments (igneous sedimentary and metamorphic origin) (Fig. 11) coexist in the rock.

Fig. 11
figure 11

The photomicrographs showing the deformations facilitating the secondary porosity in the rocks (a). b, c Fractures in quartz grains. d, e Feldspar decompositions. f Chemical compaction or pressure solution in quartz (as indicated by arrows on the confronting sides of light and dark band). g Ductile deformation of metamorphic rock fragment. h Porosity in sandstone

Full size image

The fracturing and chemical compaction of quartz (Fig. 11a, b, c, f), the decomposition of feldspar (Fig. 11d, e) and the ductile deformations of rock fragments (Fig. 11g) has developed secondary porosity (Bloch 1994) and affect the migration of fluids through the rocks. However, the pores formed by the dissolution are too small to develop effective porosity. The porosity in rock has been confirmed through the existence of fluids (Fig. 11h) during petrography.

Discussions

The rippled laminations and sheet-like sandstone interbedded with mudstone reveals crevasse splay deposits (Ulak 2009). The Laminated sandy mudstone and fine-grained sandstone are deposited by the floundering flow velocities of channel. The planar bedding (Fig. 2) has favored a sandy meandering and flood flow river system deposits (Ulak 2009). The planar stratified sandstone (Fig. 2) implies a shallower fluvial system and suggests upper flow regime origin for sandstone (Ulak 2009). The thick bedded mudstone and carbonate concretions in shale (Fig. 2) beds indicate a meandering stream pattern in a sinuous water and flood flow. The calcareous concretions were originated under hydromorphic conditions in warm, arid to semiarid climes (Gile et al. 1965; Kraus 1999). The high bed and suspended loads have deposited the matrix-supported conglomerates (Harms et al. 1982). The mudstone was deposited in the flood-dominated alluvial fan environment (Searle et al. 1990; Blair and McPherson 1994).

The fluvial system of Siwaliks evolved through meandering to braided and finally formed flood plain system (Khan et al. 1997; Ulak and Nakayama 2001; Nakajima 1982; Hisatomi and Tanaka 1994; Zaleha 1997; Willis 1993). The upliftment of Himalayas towards the north is the main contributor of deposition in the Foreland basin (Ulak 2009). The flow direction as deduced though the ripple laminations and planner bedding (Fig. 2) indicates southward migration of channel.

The diversion of river pattern within the region has formed confined aquifers of the regions (Sapkota 2003). The aquifers confined between subsurface lithologies were interpreted by means of resistivity surveys. The medium and high-resistivity zones in the area were marked by vertical electrical sounding (VES) curves. The resistivity data is further interpreted to find the lithologies (boulders, sand, and clays overlying the area). The resistivity values are low in fine sediments and high in coarse sediments (Table 3). The sediments at shallow depth like those of near surface rocks are dominated by clays and sandy material as well as alluvium. The resistivity values in a few regions are also affected by the moisture content and fluids trapped in pore spaces of rocks (Fig. 11h). The fracturing, dissolution, and decomposition of minerals have developed secondary porosity in the rock to facilitate the movement of water though pores (Fig. 11).

Table 3 The resistivity ranges of subsurface lithologies
Full size table

Conclusion

The conjugate techniques, geoelectrical method, and petrographic analysis are very helpful for the identification of subsurface groundwater reserves. The lithofacies deposited by meandering, braided, and flood plain system during stream diversions in the area have formed aquifers of the region. There is strong correlation between the exposed litho-log and the calculated thickness of lithologies on the bases of geoelectrical data. The qualitative and quantitative interpretations have revealed the presence of groundwater within formations and have figured out the confined and semi-confined-type aquifers in the sedimentary rocks. The apparent resistivity maps indicate the decrease in resistivity with the increase of depth pointing towards good groundwater potential in the area. The thick sandstone (10–42 m) at different locations indicates the promising groundwater potential in the area. The highest anisotropy values at the VES 10, 11, 17, 18, 19, 20, and 21 due to the regional faults in the area indicate the good groundwater potential. The petrography has provided the clues of the secondary porosity and fluid migrations through the rocks. So, the dug wells and tube wells can be installed in the area with promising endurance.