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Groundwater Exploration Methods

BY

PROF. A. BALASUBRAMANIAN

Centre for Advanced Studies in Earth Science,

University of Mysore

Mysore

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Objectives :

After attending this module, the user would be

able to know about the most popular methods of

groundwater exploration.

The surface geomorphological, geological,

structural, hydrogeological, geophysical and

remote sensing methods will be known. In

addition, the user will also get some basic ideas

about the subsurface methods of groundwater

exploration like well-logging and test drilling.

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Introduction

Groundwater is an invisible natural resource. It is

available in different proportions, in various rock

types and at various depths, on the surface layer

of the earth. In the historical past, when there is

no visible flow of water along the rivers, people

used to dig small pits, in the river alluvium, wait

and collect the groundwater coming through

seepage and use it for their drinking purposes and

for meeting the domestic needs.

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Similarly, to the people of mountainous regions,

natural springs provided the sources of water

supply.

Springs are the outcome of seepage from any

groundwater system, in hilly terrains or in

limestone regions.

More than 60 percent of the global population

thrives by using only the groundwater resources.

The groundwater which was existing at shallow

depths in the open wells, has gone deep due to

over-exploitation.

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Exploring these water sources become a

challenging task to geo-scientists.

Renewable resource

Groundwater is a renewable source. Groundwater

gets replenished after every rainfall. This is called

as rainfall recharge. The level of water seen in an

open well denotes the uppermost surface of the

zone of saturation of the porous media. This is

called as the water table.

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After every recharge, the water table raises,

denoting that the porous media has saturated with

more water. When we pump out water, the water

level goes down.

Continuous pumping of water, beyond the

recharge, will make the wells go dry and force to

deepen the well.

The search of groundwater got increased, due to

the non-availability of sources and due to the

declining water tables.

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Heterogeneous distribution

Groundwater is not uniformly distributed

everywhere. The occurrence of groundwater

varies from formation to formation. In a typical

crystalline hard rock terrain, the quantitative

occurrence of groundwater depends on the

weathered and fractured zones. The occurrence of

groundwater in a sedimentary terrain will be more

promising.

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Groundwater prospecting is a very thought

provoking scientific exercise in most of the

places.

There is a need to understand the methods of

groundwater exploration, as it is a practical

decision-making approach.

This module highlights some of the general

methods of groundwater exploration.

Exploring groundwater

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Groundwater exploration is a typical task of a

hydrogeologist or an engineer.

Identifying the location of its availability is a

challenging task. Exploration of groundwater

requires a basic understanding of its position in

the subsurface geological setup.

Groundwater Exploration is attempted through

either by direct or indirect methods. Test drilling

is the direct approach to find out the resource.

This is an expensive affair.

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Every individual can not go for test drilling.

During the last two centuries, more and more

techniques have been developed to explore the

groundwater.

They are classified into surface and sub-surface

methods.

Surface methods

The surface methods are easy to operate and

implement.

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These require minimum facilities like topo-sheets,

maps, reports, some field measurements and

interpretations of data in the laboratories.

The surface methods of groundwater exploration

include the following:

– Esoteric Methods

– Geomorphologic methods

– Geological & structural Methods

– Soil and Micro-Biological Methods

– Remote Sensing Techniques

– Surface Geophysical Methods

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Subsurface methods

The subsurface methods of groundwater

exploration includes both Test Drilling &

Borehole Geophysical Logging techniques. When

compared to the surface methods, the subsurface

methods are very expensive.

These are done for government level projects

where large scale investigations are carried out to

ascertain the results of surface surveys.

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The subsurface methods are very accurate

methods as the help in direct observations of

features in the form of bore-hole lithologs as core

samples and also geophysical measurements of

formation properties.

Esoteric methods

The Esoteric methods are the ancient methods.

These are the oldest water divining methods

practiced by ancient people for several centuries.

They are also called as water-dowsing.

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People believed that the flow of groundwater can

induce some vital currents above the surface.

When a wet plant twig is moved above such

zones, it tends to rotate the twig as well. Wet

twigs of trees, husk-removed coconuts, watches

and other materials have been used as dowsing

materials.

The person handling the twig has some role of

induction and hence it is not applicable to

everybody attempting to divine water. All these

methods have been practiced since 17th century.

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There is no scientific explanation available with

reference to these approaches. Probability of

success is a mere coin-tossing experiment. These

methods are called as water divining.

Water Witching

Water witching is a traditional method adopted by

people to detect bore-well locations. Using a

forked stick to locate water source is known as

water witching.

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Although this method is lacking any scientific

justification for the method, water witches

diligently practice the art wherever people can be

persuaded of its potential value.

Commonly, the method consists of holding a

forked stick in both hands and walking over the

local area until the butt end is attracted

downward-ostensibly by subsurface water.

It is amazing that the idea of supernatural powers

has such a continued fascination for people to use

despite its limitations.

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Geomorphological Methods

Surface drainage is the subdued replica of

topography. It is controlled by the basement

rocks. Mostly, groundwater flow coincides with

the surface drainages. The streams and water

courses may also be controlled by some

underlying structures. Junctions of streams at the

down slopes are promising zones for groundwater.

Landforms originate due to several geological

processes.

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Some of them are likely to contain relatively

permeable strata.

River-borne modern alluvial terraces, floodplains,

stratified valley-fill deposits in abandoned

channels, glacial outwash and moraine deposits

are good landforms for groundwater.

Alluvial fans, beach ridges, partly drift-filled

valleys, sand dunes, moist depressions, and

marshy environments are good localities.

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Study of Land forms

Landforms are the likely indicators to show the

relatively permeable strata.

The locations of modern alluvial terraces and

floodplains, stratified valley-fill deposits, glacial

outwash plains, glacial deltas, kames , moraine

complexes, eskers, alluvial fans and beach ridges

are good locations for groundwater occurrence.

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Partly drift-filled valleys marked by a chain of

elongate closed depressions,

largely masked bedrock valleys cutting across

modern valleys that are indicated by local non-

slumping of weak shale strata in valley sides,

sand dunes assumed to overlie sandy glacio-

fluvial sediments,

nearby locations of lakes and streams are very

good indicators for groundwater prospecting.

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Topography and Drainage

Physiographic methods analyse the surface

topography and drainages.

The locations of confluence and junctions of

surface streams at the downstream points of small

watersheds are good locations for groundwater for

confluence.

Hydraulic gradients of groundwater systems will

always follow the topographic gradients and

slopes.

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Such locations are also suitable for water

collection and storage for recharge.

Drainage density of stream network

Drainage density is the ratio between the total

length of all streams and the area of watershed or

river basin. The resultant drainage density is used

to indicate the potentiality of groundwater. If the

drainage density is low, groundwater potentiality

will be more.

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If it is high, due to more streams, runoff will be

more.

Geological Methods

A geologic investigation begins with the

collection, analysis, and hydrogeologic

interpretation of existing topographic maps, aerial

photographs, geologic maps and logs, and other

pertinent records.

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This should be supplemented, when possible, by

geologic field reconnaissance and by evaluation

of available hydrologic data on stream flow and

springs, well yields, groundwater recharge,

discharge, and levels and water quality.

In some places, the drainages may be fully

controlled by the presence of minor and major

structures like joints, faults and lineaments.

Such zones are good and potential zones for

groundwater exploration.

These are the conduits for groundwater flow.

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Structural methods

Contact points between permeable water-bearing

strata overlying relatively impermeable strata-

usually along the sides of valleys that cut across

the interface between different strata are suitable

locations for groundwater.

Springs occurring on or near the base of hillsides,

valley slopes, and local scarps are indicators of

groundwater occurrence over hilly terrain.

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Dykes are good barriers for arresting the flow of

groundwater.

Location of dykes and analyzing their dip and

strike help in selecting the groundwater potential

zones in the upstream side.

Well-inventory

Well-inventory is a method of analyzing the well-

cuttings and inner surfaces of open dug wells to

know about the subsurface geology, structures,

seepage zones, fluctuations of water levels, rate of

recovery after pumping and the geo-

environmental setting of the wells in a region.

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This method helps to analyse the data collected

from more number of the wells of a region and

come to a conclusion about the regional

groundwater potentialities.

The groundwater flow paths could be easily

identified through well-inventory.

Promising zones could be identified for further

investigations though this method.

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Soil and Micro-Biological Methods

Geo-botanical indicators are valuable tools in

groundwater exploration.

The anomalous growth of vegetation and

alignment of big trees on a straight line, growth of

termite mounds and location of age old, deep

rooted heritage trees can indicate the occurrence

of groundwater at shallow depths.

Presence of Halophytes, plants with a high

tolerance for soluble salts, and white efflorescence

of salt at ground surface, indicate the presence of

shallow brackish or saline groundwater.

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Xerophytes, the well-known desert plants ,

subsisting on minimal water, suggest a

considerable depth to the water table.

All these are supplementary tools in detecting the

locations of groundwater zones.

Moist depressions and seepages

Moist depressions,

marshy environments, and seepages,

string of alkali flats or lakes (playas) along

inactive drainage systems, salt precipitates (e.g.,

salt crusts),

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localized anomalous-looking "burn out" patches

in the soil, and

vegetation associated with salt migration and

accumulation are good indicators for groundwater

availability.

Depression springs, where land surface locally

cuts the water table or the upper surface of the

zone of saturation, Contact springs containing a

permeable water-bearing strata overlying

relatively impermeable strata-usually along the

sides of valleys that cut across the interface

between different strata are good locations.

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The presence of artesian springs occurring on

undulating upland till plains, and artesian springs

occurring on or near the base of hillsides, valley

slopes, and local scarps are very good indicators.

Geophysical methods

Exploring the ground water by geophysical

method is termed Ground water geophysics.

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Geophysical investigations are conducted on the

surface of the earth to explore the ground water

resources by observing some physical parameters

like density, velocity, conductivity, resistivity,

magnetic, electromagnetic & radioactive

phenomena.

Geophysical methods comprise of measurement

of signals from natural or induced phenomena of

physical properties of sub surface formation.

Geophysical methods detect the differences, or

anomalies of physical properties within the earth's

crust.

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Density, magnetism, elasticity, and electrical

resistivity are properties that are most commonly

measured.

The purpose of exploration is to detect the

indirect indicators and locate the potential zones

for exploitation. The main geophysical methods

which are useful in solving some of the problems

of hydrogeology, are the Electrical, Seismic,

Gravity, and Magnetic methods.

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Gravity Method

The gravity method is a widely used geophysical

method for finding out mineral resources and

groundwater in sedimentary terrain.

Gravimeters are used in this method to measure

the differences in density on the earth's surface

that may indicate the underlying geologic

structures.

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Because the method is expensive and because

differences in water content in subsurface strata

seldom involve measurable differences in specific

gravity at the surface, the gravity method has little

application to groundwater prospecting.

Under special geologic conditions, such as a large

buried valley, the gross configuration of an

aquifer can be detected from gravity variations.

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Magnetic Method

The magnetic method enables detecting the

magnetic fields of the earth which can be

measured and mapped. Magnetometers are the

equipments used to measure the magnetic fields

and variations.

Because magnetic contrasts are seldom associated

with groundwater occurrence, the method has

little relevance for exploring groundwater.

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Seismic Method

Seismic methods are of two kinds as seismic

refraction and reflection methods.

The seismic refraction method involves the

creation of a small shock at the earth's surface

either by the impact of a heavy instrument or by a

small explosive charge and measuring the time

required for the resulting sound, or shock, wave to

travel known distances.

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Seismic waves follow the same laws of

propagation as light rays and may be reflected or

refracted at any interface where a velocity change

occurs. Seismic reflection methods provide

information on geologic structure thousands of

meters below the surface, whereas seismic

refraction methods-of interest in groundwater

studies-go only about 100 meters deep. The

travel time of a seismic wave depends on the

media through which it is passing through. The

velocities are greatest in solid igneous rocks and

least in unconsolidated materials.

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Based on these indications, it is possible to

delineate the subsurface zones of fractures,

fissures, faults and lineaments.

Analyzing Seismic velocities

A basic understanding of the characteristic

seismic velocities for a variety of geologic

materials is necessary. These velocities help to

identify the nature of alluvium or bedrock. In

coarse alluvial terrain, seismic velocity increases

markedly from unsaturated to saturated zones.

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In seismic method, the depth to water table can

be mapped, with an accuracy of 10 percent,

where the geologic conditions are relatively

uniform. The changes in seismic velocities are

governed by changes in the elastic properties of

the formations. The greater the contrast of these

properties, the more clearly the formations and

their boundaries can be identified.

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Electrical resistivity method

The purpose of electrical surveys is to determine

the subsurface resistivity distribution by making

measurements on the ground surface. From these

measurements, the true resistivity of the

subsurface can be estimated. The ground

resistivity is related to various geological

parameters such as the mineral and fluid content,

porosity and degree of water saturation in the

rock.

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Electrical resistivity surveys have been used for

many decades in hydrogeological, mining and

geotechnical investigations. More recently, it has

been used for environmental surveys. Each

electrical property is the basis for a geophysical

method.

The resistivity measurements are normally made

by injecting current into the ground through two

current electrodes (C1 and C2 in Figure 1), and

measuring the resulting voltage difference at two

potential electrodes (P1 and P2).

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From the current (I) and voltage (V) values, an

apparent resistivity (pa) value is calculated, using

pa = k V / I, where k is the geometric factor

which depends on the arrangement of the four

electrodes. The electrode arrangement in these

investigations are called as arrays. Some of the

most common electrode arrays are Wenner,

Schlumberger, pole-pole, pole-dipole and dipole-

dipole array.

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Vertical electrical sounding

Vertical electrical sounding, VES, is used to

determine the resistivity variation with depth.

Single VES should only be applied in areas,

where the ground is assumed to be horizontal

layered with very little lateral variation, since the

sounding curves only can be interpreted using a

horizontally layered earth (1D) model. To

measure the apparent resistivity values a

resistivity meter is used.

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Resistivity meters normally give a resistance

value, R = V/I, so in practice the apparent

resistivity value is calculated by pa = k R.

The calculated resistivity value is not the true

resistivity of the subsurface, but an “apparent”

value which is the resistivity of a homogeneous

ground which will give the same resistance value

for the same electrode arrangement. The

relationship between the “apparent” resistivity and

the “true” resistivity is a complex relationship.

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To determine the true subsurface resistivity, an

inversion of the measured apparent resistivity

values using a computer program must be carried

out. The measured apparent resistivity values are

normally plotted on a log-log graph paper. To

interpret the data from such a survey, it is

normally assumed that the subsurface consists of

horizontal layers.

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Profiling

Another classical survey technique is the profiling

method. In this case, the spacing between the

electrodes remains fixed, but the entire array is

moved along a straight line. This gives some

information about lateral changes in the

subsurface resistivity, but it cannot detect vertical

changes in the resistivity. Interpretation of data

from profiling surveys is mainly qualitative.

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The most severe limitation of the resistivity

sounding method is that horizontal (or lateral)

changes in the subsurface resistivity are

commonly found. In many engineering and

environmental studies, the subsurface geology is

very complex where the resistivity can change

rapidly over short distances. The resistivity

sounding method might not be sufficiently

accurate for such situations.

Resistivity surveys give a picture of the

subsurface resistivity distribution.

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To convert the resistivity picture into a geological

picture, some knowledge of typical resistivity

values for different types of subsurface materials

and the geology of the area surveyed, is

important.

The resistivity values of common rocks and soil

materials are given below: table Sl.No. RESISTIVITY

-m

AQUIFER CHARACTERISTICS

1. < 20 Indicates a chloride ion concentration of 250

ppm (Aquifer may be fine sand & Limestone)

2. 50 – 70 Porosity is the principal determinant of

resistivity

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3. 20 – 30 Pore fluid conductivity dominates / affected by

both water quality and lithology

4. 30 – 70 Affected by both water quality and lithology

5. < 10 Delineate sediments enriched with salt water

6. < 1 Clay / sand saturated with salt water

7. 15 – 600 Sand and Gravel saturated with fresh water

8. 5 Saltwater or Clay with saltwater

9. < 10 Brackish aquifer

10. 10 – 20 Moderately fresh

11. 20 – 160 Freshwater

12. 0.2 – 0.8 Clay

13. 0.6 – 5 Dry sand contaminated

14. 0.3 – 0.8 Brine bearing sand

15. 3 – 6 Red clay

16. < 19 Clay / clay mixed with kankar

17. 64 – 81 Weathered sandstone

18. 57 – 111 Weathered granite and other crystalline rocks

19. < 10 Saline coastal zone sand (Sedimentary)

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20. 10 – 20 Clay with or without diffused water

21. 20 – 60 Freshwater zone

22. 200-10000 Crystalline rocks: Granite and other igneous

rocks and crystalline schist of normal physical

character, compact sand stones, quartzite,

marbles

23 100-1000 Consolidated sedimentary rocks:Slates, shale,

sand stone, limestone

24 0.5-100 Unconsolidated sedimentary rocks: Marls,

clays, sands, alluvium and surface soils

25 4-800 Oil bearing sands:

Igneous and metamorphic rocks typically have

high resistivity values.

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The resistivity of these rocks is greatly dependent

on the degree of fracturing, and the percentage of

the fractures filled with ground water.

Sedimentary rocks, which usually are more

porous and have a higher water content, normally

have lower resistivity values. Wet soils and fresh

ground water have even lower resistivity values.

Clayey soil normally has a lower resistivity value

than sandy soil. However, note the overlap in the

resistivity values of the different classes of rocks

and soils.

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This is because the resistivity of a particular rock

or soil sample depends on a number of factors

such as the porosity, the degree of water

saturation and the concentration of dissolved salts.

The resistivity of ground water varies from 10 to

100 ohm•m. depending on the concentration of

dissolved salts. Note the low resistivity (about 0.2

ohm•m) of sea water due to the relatively high salt

content. This makes the resistivity method an

ideal technique for mapping the saline and fresh

water interface in coastal areas.

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Wenner array

This is a robust array which was popularized by

the pioneering work. The Wenner array is

relatively sensitive to vertical changes in the

subsurface resistivity below the centre of the

array. However, it is less sensitive to horizontal

changes in the subsurface resistivity. The Wenner

array has a moderate depth of investigation.

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For the Wenner array, the geometric factor is

2(22/7)a, which is smaller than the geometric

factor for other arrays. Among the common

arrays, the Wenner array has the strongest signal

strength.

This can be an important factor if the survey is

carried in areas with high background noise.

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Schlumberger array

In the Schlumberger array, A and B are current

electrodes, and M and N are potential electrodes.

Let the current I enter the ground at A and return

at B. Assuming the medium below the surface of

the earth to be homogeneous and isotropic of

resistivity p, the potentials V M and V N as

measured at M and N, respectively.

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The calculations are done using these two

equations:

VM = pl/27r 1/(a - b/2) - 1/(a + b/2)

VN =pl/27r 1/(a + b/2) - 1/(a - b/2)

from which p = 7r(a 2/b-b/4) (V M -VN /I).

Denoting (VM -VN ) by AV, and acknowledging

the fact that, in reality, the medium is anisotropic,

the apparent resistivity pa as measured by the

Schlumberger array is given by:

Pa = 7r(a 2 /b - b/4) AV/I

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If a and b are measured in meters, and oV and I in

millivolts and milliamperes respectively, pa

would be in ohm-meters (Slur).

Equation (1) may be written as:

Pa =K/I AV

where K = (a2 /b - b/4) is the geometric factor for

the Schlumberger array.

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Dipole-dipole array

This array has been, and is still, widely used in

resistivity/I.P. surveys because of the low E.M.

coupling between the current and potential

circuits. The spacing between the current

electrodes pair, C2-C1, is given as “a” which is

the same as the distance between the potential

electrodes pair P1-P2. Thus the dipole-dipole

array is very sensitive to horizontal changes in

resistivity, but relatively insensitive to vertical

changes in the resistivity.

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That means that it is good in mapping vertical

structures, such as dykes and cavities, but

relatively poor in mapping horizontal structures

such as sills or sedimentary layers.

Interpretation of data

The interpretation of each VES curve is carried

out in two steps.

First, an approximate interpretation is obtained by

the curve-matching methods, and another

interpretation is based on the results obtained

through the automatic interpretation using a

computer program.

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Finally, the layer-wise resistivities and

thicknesses are obtained from these

interpretations. Using them spatial variation maps

depicting the low resistivity contours and good

thickness aquifer horizons can be delineated.

Electromagnetic Method

The term electromagnetism is defined as the

production of a magnetic field by current flowing

in a conductor.

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Coiling a current-carrying conductor around a

core material that can be easily magnetized, such

as iron, can form an electromagnet. The magnetic

field will be concentrated in the core. This

arrangement is called a solenoid. The more turns

we wrap on this core, the stronger the

electromagnet and the stronger the magnetic lines

of force become. The magnetic field that

surrounds a current-carrying conductor is made up

of concentric lines of force. The strength of these

circular lines of force gets progressively smaller

the further away from the conductor.

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If a stronger current is made to flow through the

conductor, the magnetic lines of force become

stronger.

The strength of the magnetic field is directly

proportional to the current that flows through the

conductor. There are two methods as Passive and

Active methods. The Passive method uses the

natural ground signals (e.g., magnetotellurics),

natural sources like lightning, magnetosphere

activities, etc.

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The Active method uses a transmitter to induce

ground current, using an artificial source.

Principles of EM Surveying

The first step is to generate EM field by passing

an AC through a wire coil ( transmitter). The EM

field propagates above and below ground. If there

is conductive material in ground, magnetic

component of the EM wave induces eddy currents

(AC) in conductor.

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The eddy currents produce a secondary EM field

which is detected by the receiver. The receiver

also detects the primary field (the resultant field is

a combination of primary and secondary which

differs from the primary field in phase and

amplitude). After compensating for the primary

field (which can be computed from the relative

positions and orientations of the coils), both the

magnitude and relative phase of the secondary

field can be measured.

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The difference in the resultant field from the

primary provides information about the geometry,

size and electrical properties of the subsurface

conductor.

The apparent conductivity measured is the

average conductivity of one or more layers in the

ground in the proximity of the instrument, to a

depth of investigation. The depth of investigation

is dependent on the coil spacing, orientation,

operating frequency of the instrument, and the

individual conductivity of each ground layer.

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General Principles of EM Operation

There are two methods of EM surveys. One is the

TDEM which means Time-domain (TDEM) EM

surveys. The measurements are done as a

function of time. the Time-Domain

Electromagnetic (TDEM) methods are based on

the principle of using electromagnetic induction to

generate measurable responses from sub-surface

features. When a steady current in a cable loop is

terminated a time varying magnetic field is

generated.

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As a result of this magnetic field, eddy currents

are induced in underground conductive materials.

The decay of the eddy currents in these materials

is directly related to their conductive properties,

and may be measured by a suitable receiver coil

on the surface.

The second method is the FDEM –Frequency-

domain (FDEM) EM surveys.

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It is related to the measurements at one or more

frequencies. The FDEM Transmitter produces

continuous EM field. The secondary field is

determined by nulling the primary field ( need

two coils). The TDEM-Primary field is

applied in pulses ( 20-40 ms) then switched off

and the secondary field measured ( same coil

can be transmitter and receiver, more often

large coil on ground and move small coil

around).

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Geophysical Logging Techniques

The term “logging” refers to making records of

some measurements or observations.

Borehole geophysical logging is a procedure to

collect and transmit specific information about the

geologic formations penetrated by a well by

raising and lowering a set of probes or sondes that

contain water-tight instruments in the well.

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The data obtained is normally used to determine

the general lithology of formations, distribution

of structures, vertical flow of fluids, and the

water-yielding capabilities of the formations. The

geophysical logging of boreholes came a long

way since 1927, when

Schlumberger brothers ran the first electric log.

In India the geophysical logging of water well

was carried out for the first time in 1953 by GSI.

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Basically, there are two types of logging

techniques- first utilizing the natural source &

second utilizing stimulated controlled source.

Geophysical logging technique utilizes the

measurement of certain physical parameters

across different subsurface formations with the

help of sensing probe inside the bore hole

providing a continuous record of these parameters

versus depth.

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These parameters are interpreted in terms of

lithology, porosity, moisture content & quality of

formation fluids. Different physical properties

like electrical conductivity, magnetic

susceptibility, radioactivity & velocity etc are

utilized.

The primary purpose of well logging is the

identification of formations traversed by a bore

hole & salinity of fluids. Well logging is used

a) for stratigraphic correlation, detection of bed

boundaries, porous & permeable zones

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b) for the water well design & construction and

c) for sea water intrusion studies of coastal

aquifers.

Logging methods

The different types of well-logging methods are:

a) Electric logging – electrical resistivity &

Self-Potential(SP).

b) Radioactive logging – gamma ray & neutron

logs.

c) Induction logging.

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d) Sonic logging.

e) Fluid logging – temperature, fluid

resistivity, flow meter & tracer logging.

f) Caliper logging.

Electric well logging involves the continuous

recording of electrical resistance / resistivity & SP

of the formations by a drill bore hole. In the SP

log, the potential drop between bore hole

electrode & a reference electrode @ the surface is

recorded.

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The SP logs are highly useful in deciphering

saline water & clay predominant zones. The

Resistivity logs are used for ground water &

mineral explorations.

Photogeology

Photogeology is the art of making aerial

photographs that are suitable for analyzing the

earth’s physiographic features, rack types,

structures, mineralized zones, water resources,

types of vegetation, zones of cultivation and

urbanization.

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The Photographs of the earth taken from the

aircraft or satellite can provide useful information

regarding groundwater conditions. The

technology of remote sensing has developed

rapidly in recent years. Stereoscopic examination

of black-and-white aerial photographs has gained

steadily in importance.

Observable patterns, colors, and relief make it

possible to distinguish differences in geology,

soils, soil moisture vegetation, and land use.

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Thus, photogeology can differentiate between

rock and soil types and indicate their permeability

and areal distribution-and hence areas of

groundwater recharge and discharge. Maps

classifying an area into good, fair, and poor

groundwater yields can be prepared. Aerial

photographs also reveal the fracture patterns in

rocks, which can be further related to the porosity,

permeability, and ultimately the well yields.

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They are suitable for identifying the formations

that are potential zones for the occurrence of

groundwater.

Remote Sensing techniques

Remote sensing is the science (and to some

extent, art) of acquiring information about the

Earth's surface without actually being in contact

with it. This is done by sensing and recording

reflected or emitted energy and processing,

analyzing, and applying that information.

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In much of remote sensing, the process involves

an interaction between incident radiation and the

targets of interest. Remote sensing shows an

increasing role in the field of hydrology and

water resources development. Remote sensing

provides multi-spectral, multi-temporal and multi-

sensor data of the earth’s surface which are

suitable for mineral explorations, water resources

evaluation, environmental monitoring and

groundwater targeting.

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Remote sensing techniques help in the

demarcation of groundwater potential zones,

identification of groundwater recharge sites and,

to analysis the future artificial recharge sites.

Applications of remote sensing

Satellite data products are much varied depending

upon the spectra considered.

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The high resolution satellite images are

interpreted (visually or digitally) to identify the

groundwater potential zones. Thematic layers are

prepared based on hydrogeomorphic units, land-

use/ land-cover/ lineaments, rock types, structures

and many other features.

The methodology involves the delineation of

hydrogeomorphic units which are influenced by

the hydro geological conditions of the area.

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The hydrogeological conditions are controlled by

the lithology, geomorphology, structures like

lineaments, faults and fractures.

The visual interpretation of satellite data in

conjunction with limited field verification of these

features will focus on the priority zones.

Most of them are reflected as hydrogeomorphic

units.

Remote sensing provides the distribution of these

units.

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Conclusion

Several geological, hydrogeological and

geophysical methods are employed to target the

groundwater potential zones. The interpretation

of satellite images and aerial photographs also

help more in this process.

Groundwater exploration is a very unique

exercise.

As it is a hidden resource, various indirect

methods are attempted to identify the points.

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The success in the groundwater targeting lies in

experience of understanding the geological

conditions, structural conditions and

hydrogeological conditions which favour the

occurrence of groundwater.

The modern tools like remote sensing and aerial

photography also provide a lot of spatial data for a

quick understanding of the domain for a better

decision-making.