This document describes various geological and geophysical methods used for groundwater exploration, including remote sensing, surface geophysical methods, and sub-surface geophysical well logging techniques. It discusses geomorphological mapping of surface features indicative of groundwater such as stream junctions. Electromagnetic, electrical resistivity, seismic, and gravity methods are described. The principles, equipment, and applications of each method are explained in detail. Well logging techniques including electric, radioactive, induction, sonic, and fluid logs are also summarized.

2. Geological methods
Remote sensing
Geophysical exploration and logging
Test drilling
Water level measurements
Hydrogeological mapping

5. • Geological:
• Geomorphologic methods
• structural Methods
• Remote Sensing
• Surface Geophysical Methods
surface
methods
•Test drilling
Sub-surface
methods

7. 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.
• 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 also
good localities.

8. 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.

9. Topography and Drainage indicating groundwater occurrence
• 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 alwaysfollow the
topographic gradients and slopes.
Drainage density concept for groundwater potential
• 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.

15. The gravity method is a widely used geophysical method for finding
out mineral resources and groundwater in sedimentary terrain.
Gravimeters(accelerometer) are used in this method to measure the
differences in density on the earth's surface that may indicate the
underlying geologic structures.

16. 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.

17. GRAVITY MAP

18. Magnetic Method
The magnetic method
magnetic fields of the
enables detecting 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.

19. Magnetic method
AERIAL INSTRUMENT
RECORDINGS
GROUND BASED
RECORDINGS

20. 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.

21. • 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.

22. 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.

23. 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.

24. VIBROSEIS EXPLOSIVE DETONATION
SEISMIC METHOD

25. GEOPHONES pick up the reflected sound waves
and record that data to digital medium

26. 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.

27. • Electrical resistivity measurements are normally made by
injecting current into the ground through two current electrodes
and measuring the resulting voltage difference at two potential
electrodes.
ELECTRICAL
RESISTIVITY METHOD

28. 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.

29. 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.
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.

30. 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.
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

31. 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.

32. 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.

33. 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

34. 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

35. 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.

36. • The resistivity of these rocks is greatly dependent on the degree of
fracturing, and the percentage of the fractures filled with ground water.
• Sedimentaryrocks, 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.

37. 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.

38. Electromagnetic Method
The term electromagnetism is defined as the production of a magnetic field by current
flowing in a conductor.
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.

39. • 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:-
1. Passive and
2. Active methods.
The Passive method uses the natural ground signals (e.g., magnetotellurics),
natural sources like lightning, magnetosphere activities, etc.
The Active method uses a transmitter to induce ground current, using an
artificial source.

40. 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.
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.

41. 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.

42. General Principles of EM Operation
There are two methods of EM surveys.
1. 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.
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.
2. The second method is the FDEM –Frequency- domain (FDEM) EM surveys.

43. 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).

44. 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.

45. 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.

46. 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.

47. 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

48. 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.

49. 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.

50. The SP logs are highly useful in deciphering
saline water & clay predominant zones. The
Resistivity logs are used for ground water &
mineral explorations.

66. Scheme for the treatment of hydrological data