This document provides an overview of various groundwater exploration methods, including surface and subsurface techniques. Surface methods involve minimal facilities and include geomorphological analysis of landforms, geological and structural mapping, soil and vegetation analysis, remote sensing, and surface geophysical methods like electrical resistivity and seismic surveys. Subsurface methods like borehole logging and test drilling provide direct observations but are more expensive. Together, a multi-method approach can be used to explore groundwater resources and locate potential zones for development.

1. 1
Groundwater Exploration Methods
BY
PROF. A. BALASUBRAMANIAN
Centre for Advanced Studies in Earth Science,
University of Mysore
Mysore

2. 2
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.

3. 3
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.

4. 4
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.

5. 5
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.

6. 6
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.

7. 7
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.

8. 8
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

9. 9
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.

10. 10
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.

11. 11
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

12. 12
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.

13. 13
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.

14. 14
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.

15. 15
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.

16. 16
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.

17. 17
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.

18. 18
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.

19. 19
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.

20. 20
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.

21. 21
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.

22. 22
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.

23. 23
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.

24. 24
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.

25. 25
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.

26. 26
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.

27. 27
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.

28. 28
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.

29. 29
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),

30. 30
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.

31. 31
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.

32. 32
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.

33. 33
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.

34. 34
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.

35. 35
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.

36. 36
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.

37. 37
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.

38. 38
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.

39. 39
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.

40. 40
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.

41. 41
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.

42. 42
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).

43. 43
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.

44. 44
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.

45. 45
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.

46. 46
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.

47. 47
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.

48. 48
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.

49. 49
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

50. 50
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)

51. 51
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.

52. 52
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.

53. 53
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.

54. 54
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.

55. 55
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.

56. 56
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.

57. 57
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

58. 58
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.

59. 59
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.

60. 60
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.

61. 61
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.

62. 62
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.

63. 63
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.

64. 64
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.

65. 65
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.

66. 66
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.

67. 67
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.

68. 68
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.

69. 69
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).

70. 70
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.

71. 71
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.

72. 72
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.

73. 73
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

74. 74
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.

75. 75
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.

76. 76
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.