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CHAPTER 6

GROUNDWATER EXPLORATION 6.1

INTRODUCTION Water is one of the m

water is one of the prerequisites for development and industrial growth. In areas where surface water is not available, groundwater constitutes significant part of active fresh water resources of the world and is obviously dependable source for all the needs. The stress on water resources started due to exploding population, irrigation, domestic and industrial demands. The finite water resources are being explored to quench the thirst of millions of the populace. Although the groundwater resources are widely distributed, nature does not provide groundwater at the places of our choice. The occurrence and distribution of groundwater resources are confined to certain geological formations and structures. The groundwater at all locations may not be directly used if the quality of water is poor. All these problems can be solved using proper exploration techniques. The proper exploration of groundwater resources involves apart from source location, the well design and construction. These are all an integral part of the scheme of exploitation and management.

6.2

GROUNDWATER EXPLORATION Groundwater exploration in past years has reached a place of importance to the

world and supplying groundwater to the needy is most precious of all. Prospecting for water is essentially a geological problem and the geophysical approach is dependent on the mode of the geological occurrence of water. It needs a lot of information on various aspects such as geology, stratigraphy, geomorphology, geophysical techniques, etc. Geology is the most important consideration, as different rock types will generally have a distinctive porosity and permeability. Knowledge of stratigraphy is essential to know the position and thickness of water-bearing horizons and the continuity of confining beds are of particular importance in groundwater exploration. Structural geology is used in conjunction with stratigraphy to locate water-bearing horizons which have been displaced by earth movements. Structural studies are also used to locate weathered, fractured, faulted and jointed patterns in rock formations. Remote sensing techniques are particularly helpful in many geomorphic and structural studies related to hydrology. The remote sensing data are found extremely useful in identifying 115

the various geologic, geomorphic units, structures such as faults, lineaments, joints, fractures, folds and drainage which are important as they control the movement and occurrence of groundwater. Geomorphology is indispensable in studying the occurrence of subsurface water in areas of late Pleistocene to recent deposits. After a thorough study of the satellite imagery and geomorphology map, a field check is highly necessary to know the geomorphological features to assess the groundwater potential. The geomorphic units such as pediments, flood plains, drainage pattern, soil types and lineaments which primarily control the occurrence, movement and potential of groundwater have to be investigated in detail. The groundwater potential of an area mainly depends on the hydrogeological set up, for which a detailed and systematic hydrogeological survey is a prerequisite. Well inventory study is very important in any groundwater exploration programme. Especially in hard rock terrain groundwater confines to the weathered mantle, joints and fractures. The weathering thickness, joint and fracture system of the area ought to be studied in depth. Water level measurements and water level fluctuation studies are the important factors in the assessment of groundwater potential. Only by a systematic hydrogeological study, the groundwater abstracting structures such as open well, bore well, tube well have to be finalised. The recharge and discharge areas ought to be identified. The fluvial hydrological studies such as the river and stream flows, whether it is perennial and other details are important in quantifying the potential. Geophysical methods such as electrical, electromagnetic, seismic and gravity are used to explore the groundwater. Geophysically, the location of groundwater may be determined in three ways: direct, stratigraphic and structural (Bhattacharya and Patra, 1968; Elijah A. Ayolabi, 2005). The stratigraphic method which is relevant to this study implies locating water-bearing formations through distinguishing physical properties imparted by the presence of water, giving rise to electrical resistivity contrasts. The electrical resistivity methods give fairly accurate results in groundwater investigation. Electrical resistivity methods assumed considerable importance in the field of groundwater exploration (Pal and Majumdar, 2001; Majumdar and Pal, 2005; Narayanpethkar et al., 2006) because of its inexpensive, easy operation and its capacity to identify between fresh and saline water zones, the method is used worldwide. The resistivity methods are used successfully to estimate the thickness of the formation and also the electrical nature of the formation which provides useful information regarding

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the groundwater potentialities (Griffiths and King, 1965; Parasins, 1966; Balakrishna, 1980).

6.2.1 Electrical Resistivity Method Many methods have been adopted for the exploration of groundwater among them Electrical Resistivity Method is used at most throughout the world. The basic principle of the electrical methods of exploration for groundwater is based on the concept of resistivity. All the geological formation posses a property called resistivity which determines the ease in which the electrical current flows through them. Resistivity may be defined as the resistance offered by a unit cube of the material, when a unit current passes through it in a direction perpendicular to two of its opposite faces. The terms resistivity and resistance are related by the equation =R(A/L), where, R is the resistance, L is the length of the block and A is the cross-sectional area of the block. The resistivity m ( -m), The unit of resistance

Thus the resistivity ( ) of a regular block can

be determined by measuring its resistance (R), which is given by V/I, where, V is the potential difference or voltage between the two ends of the medium and I is the current (Kaul et al., 1990).

6.2.2 Application of Resistivity Methods in Hard Rock Terrains Different types of geological materials have different resistivity. The resistivity of the geological formation depends mainly on its porosity, moisture content, quantity of water, salinity of water and electrical property of the rock itself governed by the preferred orientation of constituent minerals. Therefore, the measured resistivity will facilitate in the estimate of weathered zone thickness, extent of weathering, depth of the massive rock, quality of water and delineate the sheared and fractured zones, structures such as dykes, faults and lateral extent of aquifers.

6.2.3 Field Methods In resistivity surveys, electrical current is sent into the ground through two electrodes known as current electrodes and the resulting potentials are measured with the help of two other electrodes known as potential electrodes. The measured apparent resistivity (

) for each half-

a

lowing

formula:

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where,

is apparent resistivity, K is the factor depending on the

geometry of electrode configuration, V is the voltage measured across potential electrodes and I is the current sent into the ground. The apparent resistivity for a given electrode separation may vary within wide limits depending upon the nature of surface material. The direction of electrode spread in relation to anisotropic characters and lateral inhomogeneities etc. The apparent resistivity map for a given electrode spacing indicates the variation of resistivity in the subsurface layer with a thickness approximately equal to electrode spacing (Zohdy et al., 1974). There are two main variations of resistivity surveys, namely profiling and vertical Electrical Sounding (VES). Profiling is used to determine the lateral variation of resistivity from area to area, whereas VES is used to investigate the vertical variations of rock strata in a given location.

6.2.4 Resistivity Profiling Electrical profiling investigations are conducted in order to trace lateral boundaries of lithological units having different electrical properties. In this method the electrode separation is kept constant and the setup is moved from point to point and apparent resistivities are determined for each station. In practice, uniformly distributed locations will serve the purpose. For a given spacing the depth of penetration is propositional to the spacing. For registering lateral discontinuities at depth, it is need to use large electrode spacing. In practice, a minimum of two spacing are used for profiling, one for shallow and other for deeper exploration. The apparent resistivity values for a given electrode spacing are plotted and contoured to prepare Iso-resistivity maps. The zones of high and low apparent resistivity areas are marked to give an idea about the epicentral location of the target and its lateral extent. Profiling is useful in areas where the subsurface formations are horizontal or nearly horizontal and posses sufficient resistivity contrast (Mooney et al., 1966). In groundwater prospecting, the resistivity profiling is useful in the following situations: To identify and mapping conductive zones in high resistivity hard rock areas To identify gravel formation which acts as good alluvial aquifer tracks To identify and mapping structures such as faults, joints, shear zones, fractures, dykes and lineaments in hard rock areas To demarcate salt/fresh water boundaries (sea water intrusions in coastal areas) To detect favourable hydrological horizons, palaeo-river channels, buried river valleys, groundwater pollution zones, etc. 118

Determining the direction and intensity of joints and fractures (Murali Sabnavis and Patangay, 1998) In general, the low resistivity zones in hard rock areas and a higher resistivity zone in sedimentary formations help to distinguish clay beds from sand beds. Fault zones and structurally disturbed zones also show relatively low resistivity.

6.2.5 Vertical Electrical Sounding VES method is used to investigate the vertical variation in electrical property of the formation. In resistivity sounding method the centre of the electrode configuration is kept fixed and the distance between current electrodes called the electrode separation is increased in steps and measurements are made for each electrode separation. The depth of penetration increases with the increase of electrode separation. The apparent resistivity values obtained with increasing values of electrode separation are used to estimate the thickness and resistivities of the subsurface formations. The measured resistivity values can be correlated with vertical geological sections.

6.2.6 Electrode Configuration In the exploration of groundwater by resistivity methods, there are number of ways of setting up of current and potential electrodes. The choice of an array and the distance between the electrodes is very important for obtaining the best possible information on the subsurface geology of a given area. Keller and Frischnecht (1966) have described different configurations viz, Schlumberger, Wenner, Dipole Dipole, Trielectrode, Lee-partitioning, etc. The Schlumberger method in particular has practical, operational and interpretational advantages over the rest of the methods (Bhimasankaram, 1977). The maximum current electrode spacing depends on the depth to be investigated in a given situation.

6.2.7 Schlumberger Electrode Configuration The Schlumberger configuration is most widely used for quantitative interpretation in VES. Schlumberger originally proposed this electrode arrangement. Four electrodes are placed symmetrically along a common line with the outer two serving as current electrodes and the inner two as potential electrodes. The inner pair of potential electrodes (MN) is located at the centre of the array and the separation between them is small compared to the current electrode distance (AB), usually less 119

than one-fifth of the current electrode distance (Fig. 6.1). The apparent resistivity values obtained with this array are attributed to the midpoint of configuration, which is called as completely uniform earth is given by the formula (Keller and Frischnecht, 1966)

where,

= apparent resistivity, V = potential difference between potential

electrodes, I = current flowing, AB = current electrodes, MN = potential electrodes.

6.2.8 Wenner Configuration In this configuration, four equally spaced and collinear electrodes are used. The outer current electrodes (A and B) provide current to the ground, whereas the inner two potential electrodes (M and N) are used to measure the voltage drop due to earth held fixed and all four electrodes being separated by equal distances at all times (Fig. 6.2). The apparent resistivity for this type of electrode arrangement is given by (Keller and Frischnecht, 1966) the formula:

where,

a

= Apparent resistivity, a = Distance between two electrodes,

V = Potential difference between potential electrode and I = Current sent into the ground.

6.2.9 Resistivity in Hard Rock Terrains The typical hydrogeological section of a hard rock terrain consists of a soil zone followed by a weathered zone are overlying bedrock, which is fractured to varying degrees. Weathered zone is more permeable than bedrock and an appreciable portion of the available groundwater is stored in that zone. The fractures, joints and other openings present in the rock act as conduits for circulation of groundwater rather than for accumulation. However, these structures under favourable situations act as potential zones of groundwater accumulation. According to Narasimhan (1972), the occurrence of groundwater beyond a depth of about 70 m below ground level is not significant due to the tendency of joints, fissures and other such openings to tightly close down at that depth.

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However, there are several instances of potential groundwater accumulation at much deeper levels. The thickness of water-bearing zones of weathered and semiweathered layer is variable. Their resistivities vary from 30 to 200

-m. Such low resistivity regions indicate

potential groundwater zones. Depending on the degree of jointing, granite with full of joints and cracks filled with water may show resistivities of the order of 50 to 250

-m

(Ramachandra Rao, 1975). Ramanujacharya (1974) and Balakrishna (1980) have given the following general resistivity range for granitic terrains: Formation Highly weathered layer Semiweathered layer Fractured and jointed granites

Representative Resistivity Range ( -m) 20 50 50 120 120 200

Hard granites

6.3

>200

RESISTIVITY SURVEYS IN THE STUDY AREA Fifty-one VES have been carried out at different parts of the study area using

Schlumberger method of electrode configuration by using Aqua Meter to determine the groundwater potential zones of the study area. The resistivity meter is placed at an observation station, which is suitable for spreading the cable on either direction. The electrodes for measuring on the potential difference are placed on either side of the chosen point. Two current electrodes are driven into the ground to 10 to 15 cm deep on either side of the centre. These current electrodes are connected to instrument. The electrical connections and separations are checked before each measurement. The apparent resistivity value is determined by sending current (I) into the ground and measuring the potential drop (V/I) ratio multiplied by configuration current (K). The electrode spacing (AB) is then increased and the corresponding apparent resistivity value is measured. The operation is repeated again and again whereas the current electrodes are extended further away from the centre, keeping the potential electrodes (porous pots) stationary. When the potential difference value becomes very small, the distance between the potential electrodes are increased. After this increase, the apparent resistivity can be measured for increasing current electrode separations using the larger potential electrode separation. The apparent resistivity with Schlumberger electrode configuration is computed by using the formula: 121

2

where, AB = Distance between current electrodes, MN = Distance between potential electrodes, V = Voltage measured across potential electrodes and I = Current flowing. The VES data have been used to determine the thickness and resistivity values of weathered, semiweathered and fractured layers.

6.3.1 Interpretation of Field Curves The interpretation of resistivity sounding data in terms of thickness and resistivity of the underlying beds with reasonable limit of accuracy is difficult problem. For a correct interpretation of geoelectrical sounding curves, a sound knowledge in geology of the area, some borehole data and considerable practical experience are essential. Geoelectrical sounding curves are interpreted qualitatively in terms of geology of the area. The qualitative methods of interpretation can be used only for preliminary interpretation as explained by Mooney et al. (1966), Bhattacharya and Patra (1968). There are several methods of interpretation of VES data, in the present study curve matching technique is used.

6.3.2 Curve Matching Method The curve matching technique is used to determine the layer parameters from the VES curves. The field curves are matched with standard curves to get the layer parameters. Several such standard curves are available viz, Mooney et al. (1966), Zohdy (1968) and Rajkswterstaat (1975). In this study layer parameters have been obtained by using IPI2WIN

ware. The layer parameters obtained by the analysis

are presented in the Table 6.1. The VES curves obtained are of three layered, the top most layers have the resistivity values ranging from 12 to 65

-m and its thickness

ranges 1.2 to 1.9 m. This variation in the resistivity values due to the local conditions such as, soil moisture content. The resistivity of the second layer varies between 12 to 156.2

-m and the thickness ranges between 4.5 to 15.5 m, this layer corresponds to

weathered layer. The third layer generally represents the semiweathered/fractured rocks exhibiting resistivity values ranges from 36 to 385

-m. In some cases, the third

layer represents the compact hard bed rock.

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6.3.3 Iso-Resisitivity Maps Iso-resistivity maps of the study area prepared by contouring the apparent resistivity values, corresponding to the electrode spacing (AB/2) of 30 and 60 m by using ArcGIS 9.3 software. These maps are helpful in delineating low apparent resistivity zones and are favourable locations for groundwater storage, provided the weathered layer is sufficiently thick and permeable. The apparent resistivity values ranges from 14 to 65

-m and 45 to110

-m (Table 6.1) at 30 and 60m electrode

spacing, respectively. The Iso-resistivity maps for both 30 and 60 m electrode spacing indicates (Maps 6.1 and 6.2) that the high resistivity values have been found in northern (Nandi), northeastern (Jangamakote), western (Devanahalli), southern (Sarjapura and Dommasandra) and southeastern (Sulibele) parts of the study area. The high resistivity values in these areas are due to the presence of massive bedrock at shallow depth and absence of water-bearing zones. The remaining part of the study area have moderate to low apparent resistivity and these regions are promising zones for groundwa...