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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

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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.

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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

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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:

a 2V

aI

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 Representative Resistivity Range ( -m) Highly weathered layer 20 50

Semiweathered layer 50 120

Fractured and jointed granites 120 200

Hard granites >200

6.3 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:

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2 2

a

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 groundwater development. The north and eastern part of the study area found

patches of lateritic outcrops. In these areas a sudden fall in apparent resistivity,

indicates the contact zone of lateritic and basement massive weathered rock found

gravel layer, it shows a moderate to good amount of groundwater potential and are

prospective zones for further development in the study area.

6.3.4 Iso-Thickness Maps

The thickness of soil and weathered layers are also very important from the

point of groundwater potential zones, as the percolation of rainwater is mainly

controlled by these layers. The thickness of the first (h1) and second (h2) layers are

varies from 1.2 to 1.9 m with an average of 1.65 m. The thickness of second layer

varies from 4.5 to 15.5 m and an average of 8.23 m. The variation in the thickness

mainly due to the variation in lithology and landforms. The Iso-thickness map of h1

and h2 (Maps 6.3 and 6.4) shows the anomalous zones in northern, western, central,

southern and southeastern parts of the study area. The lithologies of these anomalous

zones are weathered gneisses and granites. They have more chance for infiltration of

rainwater and are the potential zones of groundwater.

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6.3.5 Total Longitudinal Conductance Map

Determination of total longitudinal conductance (S) provides an important

check on the curve interpretation and can be used for the preparation of S-map. The

total longitudinal conductance has been determined from the slope of the terminal

branch of the sounding curve called S-line, rising at an angle of 45 to the AB/2 axis

(Zohdy et al., 1974). If the S-line is extended back to intersect the a = 1 -m line,

then the intersect point on the x-axis is equal to S, the total longitudinal conductance

above the final layer S in mohs or Siemens (Murali

Sabnavis and S

the slope of this line (Keller and Frischnecht, 1966). When a number of layers are

involved in a geo S

S = S1 + S2 + S3 + S4 + S5 + , where, S1 = h1/ 1

In this S IPI2WIN software and values

are presented in the Table 6.1. The longitudinal conductance in the study area varies

from 0.35 to 0.96 mhos. The spatial variation of S-map indicates that the high values

are noticed in northern, western, central, southern and southeastern parts of the study

area indicating good aquifer conditions (Map 6.5). The increase in the S values

indicates the decrease in the overall resistivity of the formations and increase in the

thickness of overburden. High values of S can be considered as an important index of

groundwater potential.

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Table 6.1 Layer Parameters for the VES Data

Sl. No.

Location

Resistivity of Different Layers ( -m)

Thickness of Layers (m)

Total Thickness

(m)

S (Mhos)

Apparent Resistivity at

1 2 3 h1 h2 30 m 60 m

1 Nandhi 36.23 64.32 125 1.7 8.2 9.9 0.85 54 64

2 Bendiganahalli 54.2 75 120 1.9 7.35 9.25 0.85 45 65

3 Siddlaghatta 64 35 120 1.9 15.5 17.4 0.96 35 70

4 Gottigere 45 23 225 1.6 12.5 14.1 0.86 45 54

5 Meluru 55 65 145 1.7 9.6 11.3 0.75 54 65

6 Vijayapura 52 15.2 135 1.7 8.2 9.9 0.84 36 65

7 Chikkaballapura 27.12 45.32 365 1.8 7.2 9 0.8 45 65

8 Avathi 65 84 385 1.8 11.2 13 0.54 23 100

9 Jangamakote 65 156.2 78 1.9 9.3 11.2 0.45 56 58

10 Venkatapura 25 23 112 1.75 8.5 10.25 0.56 14 45

11 Devanahalli 35 24 36 1.2 8.5 9.7 0.65 28 65

12 Keshavara 21 45.23 128 1.7 8.36 10.06 0.45 42 75

13 Mallur 54.2 75 120 1.9 7.35 9.25 0.85 45 65

14 Anneswara 21 45 184 1.5 6.84 8.34 0.75 45 55

15 Sugatta 45 84 165 1.6 7.24 8.84 0.84 54 65

16 Hosahudya 36 54 84 1.2 6.84 8.04 0.42 45 74

17 Kogilu 26 12 225 1.9 9.5 11.4 0.65 36 75

18 Sulibele 40 20 132 1.3 10.4 11.7 0.84 45 110

19 Bagalur 24 45 112 1.2 8.6 9.8 0.96 42 96

20 Kodigehalli 25 42 84 1.3 7.5 8.8 0.35 24 65

21 Hunsamaranahalli 24 36 41 1.4 7.5 8.9 0.54 21 65

22 Patrenahalli 22 25 36 1.5 7.2 8.7 0.52 32 75

23 Keshavapura 26 12 45 1.8 8.2 10 0.62 36 56

24 Dommasandra 22 45 52 1.9 7.3 9.2 0.85 45 75

25 Mugulur 28 65 45 1.7 8.2 9.9 0.95 36 84

26 Keshavapura 23 45 65 1.6 7.2 8.8 0.85 45 75

27 Mangammanapalya 18 23 45 1.6 7.5 9.1 0.75 36 84

28 K. Narayanapura 20 33 65 1.7 8.2 9.9 0.45 45 95

29 Muddenahalli 24 45 85 1.9 8.5 10.4 0.63 25 58

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Sl. No.

Location

Resistivity of Different Layers ( -m)

Thickness of Layers (m)

Total Thickness

(m)

S (Mhos)

Apparent Resistivity at

1 2 3 h1 h2 30 m 60 m

30 Kanithahalli 28 52 75 1.8 7.5 9.3 0.85 45 85

31 Begur 22 45 65 1.5 7.2 8.7 0.85 45 75

32 Sarjapura 15 25 45 1.4 6.8 8.2 0.65 54 85

33 Mandur 23 45 78 1.9 7.5 9.4 0.75 45 96

34 Nagavara 22 36 75 1.8 8.5 10.3 0.65 36 85

35 Nagenahalli 22 36 85 1.7 7.5 9.2 0.58 45 95

36 Gantiganahalli 16 29 75 1.6 8.5 10.1 0.45 36 85

37 Jakkur 22 36 45 1.7 9.2 10.9 0.65 22 85

38 Budigere 22 36 75 1.8 8.5 10.3 0.65 45 75

39 Devaganahalli 14 18 45 1.8 7.5 9.3 0.65 25 75

40 Doddagubbi 12 22 45 1.5 7.3 8.8 0.45 22 45

41 Hoshigal 25 65 56 1.6 4.5 6.1 0.45 23 75

42 Yelahanka 18 24 56 1.7 7.5 9.2 0.65 28 48

43 Singasandra 18 35 78 1.8 9.6 11.4 0.65 25 65

44 Bilekahalli 22 27 84 1.7 6.5 8.2 0.65 35 75

45 Bidarahalli 16 42 75 1.8 7.8 9.6 0.85 45 84

46 Navarthna Agrahara 22 28 85 1.6 7.4 9 0.75 65 78

47 Hosakote 28 45 85 1.8 8.5 10.3 0.75 65 85

48 Chikkajala 34 64 124 1.6 7.5 9.1 0.85 36 45

49 Laksandra 21 36 75 1.7 7.8 9.5 0.65 45 55

50 Karahalli 35 22 45 1.5 8.5 10 0.54 35 84

51 Jakkur 18 24 65 1.7 9.8 11.5 0.96 45 72

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Map 6.5 Iso-Longitudinal Conductance Map the Study Area

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Figure 6.1 Schlumberger Electrode Configuration

A, B: Current electrodes; M, N: Potential electrodes;

O: Midpoint; V: Voltmeter; A: Ammeter Figure 6.2 Wenner Electrode Configuration