geophysical exploration

GEOPHYSICAL EXPLORATION
Department of Geological Engineering
Submitted To:
ENGR. AHSAN MEHMOOD
Submitted by:
2017-GE-43
MISHKAT SAKHI
UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

Abstract
I am highly indebted to Engineer Ahsan Mehmood for his guidance and constant supervision as
well as for providing necessary information regarding the Geophysical Exploration Techniques. I
would like to express my gratitude towards my parents & teachers of Geological Engineering
department for their kind co-operation and encouragement which help me in completion of these
lab reports. I would like to extend my sincere thanks to all of them.

Table of Contents
Introduction to various Geophysical exploration equipment.......................................................... 1
Determination of Layer thickness by using Seismic Survey .......................................................... 8
Introduction to Shielded Antenna Ground Penetrating Radar (GPR)........................................... 16
Resistivity survey and the interpretation of data for depth and resistivity calculation................. 20
Introduction to the gravity survey................................................................................................. 27
Introduction to Magnetic Survey .................................................................................................. 30

1
Lab#1
Introduction to various Geophysical exploration equipment
1.1 Abstract
This lab mainly consists of introduction to all the geophysical exploration instruments that are
present in Geophysics lab of Geological Engineering Department. The lab focuses on the working
principles and details of accessories of these instruments. Main purpose of geophysical instruments
is in petroleum industry for the exploration of hydrocarbon.
1.2 Objective:
• To get familiar with use of geophysical instruments
• To be able to interpret the data from these instruments
1.3 Related Theory:
List of Equipment:
Terraloc MK6
Terameter SAS 4000
Gravimeter
Magnetometer
Ground Penetration Radar (GPR)
Borehole Well Logger
Miniseis
• Terraloc MK-6:
It is used for seismic refraction survey. It can give good results up to 100 m for shallow depth.
This instrument can find depth of soil layer, water table, and shallow construction. Terraloc is
housed in a rugged diecast Aluminum casing to ensure highest performance and reliability. It is a
self-contained multi-channel instrument with internal compatible PC, a hard disk, a storage disk
and a standard VGA graphic display.
Figure: 1.01 Terraloc MK-6

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Accessories:
• Geophones
• Seismic cables
• Hammer
• Battery
• Shock plate/Hammering plate/Rubber pad
Geophones:
It is a device used in surface seismic acquisition, both onshore and on the seabed offshore. It detects
ground velocity produced by seismic waves and transforms the motion into electrical impulses.
Geophones detect motion in only one direction. Hydrophones, unlike geophones, detect changes
in pressure rather than motion as they are used in marine data acquisition.
Seismic Cable:
They are used to connect all the geophones which record the seismic waves resulting from seismic
vibrations. A single cable usually has 12 or 24 slots of geophones to be connected. A great number
of these "seismic events" can be used to map the strata and structures deep underground to detect
faults, oil and gas accumulations, salt domes, etc.
Figure: 1.02 Geophones
Figure: 1.03 Seismic Cables

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Hammer:
It is used to produce vibrations on the rubber pad which is eventually recorded by the geophones.
The weight of hammer is approximately 7 kg.
Battery:
A portable battery is carried along with terraloc to provide it power so that it can record the seismic
data in its drive and display the windows
Rubber Pad:
This pad is used along with hammer, when hammer is used to produce vibrations, it is stroked on
this circular rubber pad, using this rubber pad the noise is reduced and waves propagate properly
towards the geophones.
Figure: 1.04 Hammer
Figure: 1.05 Battery
Figure: 1.06 Rubber Pad

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• Terrameter SAS 4000:
Terrameter is used for resistivity survey. It is efficient up to 100m depth. It can be used for water
table detection, water quality. It consists of a basic unit called terrameter SAS 1000 or SAS 4000.
SAS stands for Signal Averaging System- a method where by consecutive readings are taken
automatically and the results are averaged continuously. SAS results are more reliable than those
obtained using single shot system.
Accessories:
• Electrodes
• Electric cables
• Crocodile connector
Electrodes:
Instead of geophones in terrameter, these electrodes are used for catching the signals and then
sending them towards the terrameter.
Electric Cables:
Just like the sonic cables present in seismic survey, the electric cables are used in the resistivity
survey. These cables are attached with electrodes and eventually they get connected with
terrameter.
Figure: 1.07 Terrameter
Figure: 1.08 Electrodes
Figure: 1.09 Electric Cables

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Crocodile Clamp:
It is just a normal clamp that opens in the same way as crocodile opens its mouth, this clamp is
used in connecting the cables and electrodes with terrameter.
• Gravimeter:
Gravimeter, also called gravity meter is a sensitive device for measuring variations in the Earth’s
gravitational field, useful in prospecting for oil and minerals. It is use for gravity survey and it is
portable instrument. The less the gravity of an area related to earth’s gravitational acceleration, the
more it is preferable for oil exploration.
• Magnetometer:
A magnetometer is an instrument that measures magnetism either the magnetization of a magnetic
material like a ferromagnesian minerals present in rocks, or the direction, strength and relative
change of a magnetic field at a particular location. Magnetometers are widely used for measuring
the Earth's magnetic field and in geophysical surveys to detect magnetic anomalies of various
types. It is use in magnetic survey and it is portable instrument. This is efficient up to 1000m.
Figure: 1.10 Crocodile Clamp
Figure: 1.11 Gravity Meter
Figure: 1.12 Magnetometer

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• Ground Penetration Radar (GPR):
This instrument can be used for road checking, metal, chemical cavity or buried drum detection. It
is efficient up to 100-200ft. Electromagnetic waves are transmitted into the ground and as this
energy propagates to greater depth, a series of reflections are directed back to the surface where
they are detected. The method is suitable for only shallow investigation (up to tens of meters in
the best condition. It is used to detect utility lines, cable lines, gas pipelines etc.
• Borehole Well Logger:
Borehole logging is a time and money-saving approach of gaining detailed information of the
subsurface, which is otherwise only obtainable from performing and analyzing numerous cores.
Borehole logging data is typically used to characterize geology, fracture patterns, fluid flow, and
geologic structural properties. Its application is commonly used to maximize the information
obtained from geotechnical borings. It can determine lithology from gamma ray log and resistivity
log readings.
Figure: 1.13 GPR
Figure: 1.14 Borehole Well Logger

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• Minimate/ Miniseis:
It is a portable device used to evaluate ground on a shallow depth. It records s and p waves and a
printer is attached with it that prints out the whole trend of waves. This trend is also saved in the
memory of mini-mate so that it can be used later.
1.4 Applications:
All these instruments have a wide range of applications, mainly they are used by Oil and Gas
Industry as a pre-confirming procedure for hydrocarbon potential in subsurface. Once geophysical
survey is done, then for its confirmation, a wild cat well is drilled out and it confirms about the
presence of hydrocarbons. The geophysical exploration is also used for geotechnical investigation
i.e. to know about the underground utility lines present on shallow depth prior to any construction,
so that these lines would not be damaged. There are numerous other applications of these
techniques currently in the world.
1.5 Conclusion:
The geophysical instruments are very expensive and useful. The main usage of these instruments
started in the late 90’s and these instruments are getting advanced and getting up to date with time.
1.6 References:
• https://www.mining-technology.com/contractors/controls/abem
• https://en.wikipedia.org/wiki/Gravimeter
• https://archive.epa.gov/esd/archive-geophysics/web/html/magnetic_methods.html
Figure: 1.15 Mini-mate

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Lab#2
Determination of Layer thickness by using Seismic Survey
2.1. Abstract:
In this lab we practice with Terraloc MK6 in field. This equipment is used for seismic
refraction survey. Seismic Refraction depend on the fact that seismic waves have differing
velocities in different types of soil or rock. It is used for all shallow investigations.
2.2. Objective:
• To get familiar with seismic refraction survey
• To get familiar with terraloc MK6, its different parts and accessories.
2.3. Apparatus:
Taraloc MK6.
Geophones (24 (setup) +1 (triggering)).
Seismic cables.
2.4. Accessories:
12volt battery.
Electric cables.
Hammer and rubber pad.
Triggering coil.
2.5. Related Theory:
2.5.1. Seismic Refraction:
Seismic refraction is a geophysical principle (see refraction) governed by Snell's Law Used in the
fields of engineering geology, geotechnical engineering and exploration geophysics, seismic
refraction traverses (seismic lines) are performed using a seismograph(s) and/or geophone(s), in
an array and an energy source.
Note: The methods depend on the fact that seismic waves have differing velocities in different
types of soil (or rock): in addition, the waves are refracted when they cross the boundary between
different types (or conditions) of soil or rock. The methods enable the general soil types and the
approximate depth to strata boundaries, or to bedrock, to be determined.
2.5.2. Seismic Refraction Survey:
The velocity of sound travelling through the sub-surface varies with material composition and
compaction. Seismic energy transmitted from a source at the surface will undergo refraction at
boundaries between different media and eventually return to the surface. Seismic refraction

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surveying makes use of this phenomenon to determine ground structure by observing the time
taken for energy to travel through the subsurface.
2.5.3. Requirement of Survey:
Seismic refraction surveying requires three components: a seismic source to generate the signal, a
signal-enhancement seismograph to control the survey and record the data, and a series of
geophones to detect the arrival of seismic waves at multiple points on the ground surface. Seismic
refraction mechanism is shown in figure 2.1.
Figure 2.1: Seismic Refraction mechanism
2.5.4. Equipments for Survey:
Terraloc MK-6:
ABEM's Terraloc Mark 6 seismograph has been especially designed for the full spectrum of
shallow seismic applications. Terraloc is a compact, and complete seismograph with all we need
to collect our seismic data in single casing. We are to add just geophones, cable and power.
Terraloc is housed in a rugged die-cast Aluminum casing to ensure highest performance and
reliability wherever it is a self-contained multi-channel seismograph with internal PC compatible
computer, a hard disk, a storage disk and a standard VGA graphic display.
Figure 2.2: Terraloc MK 6

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Applications:
• Terraloc MK 6 is the best one instrument to find out the depth of bed rock.
• It helps to locate the water table.
• It also helps to determine the velocities of waves through different layers.
• We can also compute depth of layer boundaries and so their thicknesses too.
• Real time noise monitoring
• Frequency spectrum analysis
• Refractor velocity indication
Specifications:
• On-site geophone and cable testing
• Real time noise monitoring, display as bar graphs or waterfall
• Fast and detailed analysis
• Frequency spectrum analysis
• Allows you to check where your filters should be set
• Refractor velocity indication
• Quick check of targets found and much more
2.5.5. Geophones:
A device used in surface seismic acquisition, both onshore and on the seabed offshore, that detects
ground velocity produced by seismic waves and transforms the motion into electrical impulses.
Geophones detect motion in only one direction. Conventional seismic surveys on land use one
geophone per receiver location to detect motion in the vertical direction. Three mutually
orthogonal geophones are typically used in combination to collect 3C seismic data. Hydrophones,
unlike geophones, detect changes in pressure rather than motion.
Figure 2.3: Geophones

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2.5.6. Seismic Cables:
It is used to connect all the geophones which record the seismic waves resulting from either.
Vibration or dynamite when recording seismic data. This is basically setting off a sound wave and
then recording the time and path it takes to bounce off the rock layers (strata) below. A great
number of these "seismic events" can be used to map the strata and structures deep underground
to detect faults, oil and gas accumulations, salt domes, etc.
2.5.7. Specifications of Terraloc MK6:
Table 1: Specifications of Terraloc MK6
Up hole channel Channel 12 or 24, redirectable to a separate connector
Power 10 to 30V DC External battery or internal battery
Frequency Range 2-4000 Hz
Sampling Rate 25, 50, 100, 250, 500 µs, 1, 2 ms
Maximum Input Signal ±250 mV
Minimum Input Signal ±0.24 µV
Processor 386/486 (Depending on model)
Memory 4 to 32 MB RAM (Depending on model)
Energy source:
1-Impulsive Energy Source:
Energy that is produced as a result of explosion of various explosives is termed as impulsive energy
Figure 2.4: Seismic Cables used to connect geophones

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2-Non-Impulsive Energy Source:
Energy that is produced as a result of hitting a plate or hammering action is termed as non-
impulsive energy.
2.6. TYPES OF SPREAD:
Following are the types of spread in which geophones are placed on ground.
1-Spilt Spread:
Here source is placed in center or middle of line spread. Means half of the geophones are at one
side of source and half are on the other.
2-End Spread:
Source is placed at one end but kept at a same distance that is present between geophones.
3-Cross Spread:
Source is placed on meeting point of two straight line spreads
4-In Line Offset Spread:
Here source distance is not same to that of the distance kept between geophones
5-L- Spread:
Source is placed at the corner of two spread lines.

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2.7. Data:
Following are the data of geophones placed on the ground.
Table 2: Data of distance and time to compute thickness
Geophone No. Distance
(ft)
Time (ms)
1 5 65
2 10 70
3 15 75
4 20 80
5 25 87
6 30 93
7 35 100
8 40 104
9 45 108
10 50 115
11 55 120
12 60 123
13 65 128
14 70 130
15 75 140
16 80 142
17 85 148
18 90 150
19 95 158

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20 100 161
21 105 167
22 110 170
23 115 174
24 120 180
Table 3: Data of forward and reverse shot
Distance
(ft)
Forward shot
(ms)
Reverse shot
(ms)
1 12 44
2 26 42
3 34 40
4 32 38
5 34 35
6 36 30
7 37 30
8 37 28
9 40 24
10 41 22
11 43 13
12 45 8
2.8. Comments:
• It can be used in measure in thickness of top and loose soil
• It is used for all shallow investigations
• Depth of bed rock can be determined

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• A very useful lab it is in petroleum industry
2.9. References:
geophysics.stanford.edu/
http://www.reynolds-international.co.uk/uploads/files/10tssseismicrefraction.pdf

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Lab#3
Introduction to Shielded Antenna Ground Penetrating Radar (GPR)
3.1 Abstract:
The lab is about GPR, it uses electromagnetic wave propagation and scattering to image and
identify changes in the electrical and magnetic properties in the ground. The details of the
apparatus and performance will be discussed further.
3.2 Objective:
• To get familiar with procedure of GPR survey
• To study the major parts of GPR and terms used in GPR survey.
3.3 Apparatus:
• Shielded antenna GPR
• Distance measuring wheel
• Monitor
• Battery
3.4 Related Theory
The theory related to GPR is given below.
3.4.1 Ground Penetration Radar:
Ground Penetrating Radar (GPR) system is used for electromagnetic surveys in geophysics. GPR
system is primarily used for medium to high resolution radar surveys. The shielded antenna
construction makes the antennas especially suitable for urban investigations or at sites with a lot
of background noise. Like all parts of the GPR system, the shielded antennas are modular i.e.
Transmitter and receiver.
Figure: 3.1: Ground Penetrating Radar

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3.4.2 Working principle of Ground Penetrating Radar:
The antenna electronics, the pulling handle and the measuring wheels are interchangeable between
the different antennas. This reduces cost for owning and upgrading a system. These shielded
antennas offer modularity at its best and are very efficient tools for subsurface mapping. GPR uses
high-frequency (usually polarized) radio waves and transmits into the ground. When the wave hits
a buried object or a boundary with different dielectric constants, the receiving antenna records
variations in the reflected return signal.
The fixed antenna has a constant frequency. With the increase in frequency, the depth of GPR
decreases. It shows that for the shallower depth, the frequency is more.
3.4.3 Main components and working of GPR:
A GPR system is made up of three main components:
• Control unit 2
• Measuring wheel
• Shielded antenna
• GPR monitor for display purpose
GPR waves travel through many different materials. Different types of soil, concrete, fill material,
debris, and varying amounts of water saturation all have different dielectric and conductive
properties that affect the GPR waves, and thus GPR data interpretation.
3.4.4 Terminologies used in GPR survey:
The terminologies used in GPR survey are given below.
Frequency
Depth
Resolution
We infer that for the shallower depth, the frequency is more and for more frequency, the resolution
of image will be good. Therefore, to get the image of high resolution, it must be used for shallower
depth and not for the exploration of hydrocarbons.
𝐟𝐫𝐞𝐪𝐮𝐞𝐧𝐜𝐲 ∝
𝟏
𝐝𝐞𝐩𝐭𝐡 𝐨𝐟 𝐆𝐏𝐑
∝ 𝐫𝐞𝐬𝐨𝐥𝐮𝐭𝐢𝐨𝐧 𝐨𝐟 𝐢𝐦𝐚𝐠𝐞
3.5 Procedure:
• Set up the GPR instrument according to the manufacturer’s instructions before use.
• The people operating the instrument should minimize the use of metallic objects such as
jewelry and belts with metal parts. Steel support shoes/boots cannot be worn. Be aware of

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possible interference from other metal objects such as vehicles, fences and power lines in
the immediate area. Cell phones should not be used.
• Lay out straight transects using tape measures. Use a GPS to determine the exact locations
of all start and end points of all the measurement lines.
• Walk along with the GPR on the field, the screen would show clear bands but as some
underground structure will come, the bands will get disturbed.
• Walk along with a slow constant speed, mark the areas where the apparatus has been used
for latter interpretation
• Use the saved data from monitor for further interpretations about the field with the help of
an expert and suggest the subsurface structure.
Figure 3.2: Working of GPR in site
3.6 Interpretation of data:
There are some variations in the wiggles when moving the GPR to some other place which means
there are some structure variations under the surface. The above variations in the wiggle show that
there must be some water mains or gas pipelines beneath the surface.
3.7 Applications:
• Detection of natural cavity and fissure
• Anamolies in the ground shows the presence of organic matter
• Subsidence mapping
• Mineral exploration and resource evolution
• Location of faults, dykes, coal seams
• Contaminant plume mapping
• Mapping and monitoring pollutants within ground water
• Location of buried fuel tanks and oil drums
• Location of gas leaks

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• Location of sewerage line and utility line
• Engineering and construction
• Road pavement analysis
• Location of reinforcement in concrete
• Location of public utilities (pipes, cable)
• Location of buried structure
3.8 Conclusion:
It can be concluded that GPR is a useful instrument but it can only be used for shallow depths so
for hydrocarbon exploration it cannot be used, it is used for other purposes like finding utility lines
or cavities etc.
3.9 References:
http://www.gp-radar.com/GPR.htm
http://www.global-gpr.com/gpr-technology/how-gpr-works.html
http://wwwrohan.sdsu.edu/~geology/jiracek/sage/documents/Sensors%20and%20Software%20G
PR%20Manual.pdf
http://faculty.kfupm.edu.sa/es/ashuhail/undergraduate/GEOP402/2010/GPR-Intro.pdf
http://www.eltesta.com/Downloads/files/AN-1.pdf
http://www.geophysical.biz/gpr1.htm
johnrleeman.com/documents/GPR_survey_design.pdf
https://www.indiafilings.com/learn/fdi-reporting-to-rbi-using-form-fc-gpr/
https://gpg.geosci.xyz/content/GPR/GPR_interpretation.html

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Lab#4
Resistivity survey and the interpretation of data for depth and resistivity
calculation.
4.1 Abstract:
This lab is about the resistivity survey. Resistivity geophysical surveys measure variations in the
electrical resistivity of the ground, by applying small electric currents across arrays of ground
electrodes. This technique is very useful in petroleum industry as well as in other earth related
engineering works like in mine openings etc.
4.2 Objective:
• To determine the relative estimation of the layers resistivity.
• To find the depth of layer boundaries.
4.3 Apparatus:
• Terrameter SAS 4000.
• Electrodes.
• External battery.
• Electric cables.
• Measuring tape.
• Crocodile connectors.
• Connecting cables.
4.4 Related theory:
4.4.1 Resistivity survey:
Electrical resistance surveys (also called earth resistance or resistivity survey) are one of a number
of methods used in archaeological geophysics, as well as in engineering geological investigations.
In this type of survey electrical resistance meters are used to detect and map subsurface
archaeological features and patterning.
Figure 4.1 Terrameter

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Geo-electric resistivity is a geophysical method where two electrodes, known as current electrodes,
are used to inject an electric current intro the ground and the potential difference is measured
between two distant electrodes, known as potential electrodes.
Figure 4.2 Electrical resistivity survey
4.4.2 Apparent Resistivity:
Wherever these measurements are made over a real heterogeneous earth, as distinguished from the
fictitious homogeneous half-space, the symbol ρ is replaced by ρa for apparent resistivity. The
resistivity surveying problem is, reduced to its essence, the use of apparent resistivity values from
field observations at various locations and with various electrode configurations to estimate the
true resistivity of the several earth materials present at a site and to locate their boundaries spatially
below the surface of the site.
An electrode array with constant spacing is used to investigate lateral changes in apparent resistivity
reflecting lateral geologic variability or localized anomalous features. To investigate changes in
resistivity with depth, the size of the electrode array is varied. The apparent resistivity is affected by
material at increasingly greater depths (hence larger volume) as the electrode spacing is
increased. Because of this effect, a plot of apparent resistivity against electrode spacing can be used
to indicate vertical variations in resistivity. In Wenner array,
Ra = (V/I) ×G
Here,
G = 2πa
G= Geometrical correction factor
V= Potential difference
I= Current
a= Electrode spacing

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4.4.3 Cumulative Resistivity:
Cumulative resistivity is calculated by adding or integrating the apparent resistivity functions for
intervals that are a fraction of the thickness of the upper layer.
Table 4-Cumulative resistivity
Apparent Resistivity
(Ohm-m)
Cumulative Resistivity
(Ohm-m)
20.1 20.1
21.11 41.21
28.14 69.35
23.373 92.723
25.13 117.853
30.16 148.013
4.4.4 Depth model:
If there are more than two layers then the thicknesses and resistivities of each layer are modeled
using computer programs. The program guesses at the number of layers and makes a theoretical
plot. Parameters are changed until a satisfactory fit is achieved. Forward modeling can be used to
create resistivity models of the subsurface that would simulate apparent resistivities that correlate
with the measured data. This procedure is iterative. A starting resistivity model is chosen based on
a priori information (from ground truth or averaged geophysical measurements), and apparent
resistivity data are modeled for the type of field survey geometry used. These calculated data are
compared with the actual data and the resistivity model is updated based on the difference between
observed and calculated data. This procedure is continued until the calculated data match the actual
measurements to within an interpreter-defined level of error. One of the most important results of
inversion is better estimates of depth for cross-section plots, turning pseudo-sections into better
approximations of the subsurface variation.
Figure 4.3 Depth model

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4.5 Procedure:
• Take four electrodes and insert them in to the ground according to the spacing.
• Attach the electrodes with the instrument. And connect wires with the electrodes.
• Attach battery to the instrument and turn on the instrument.
• Record the readings until desired depth of investigation is achieved.
• Then we calculated the depth manually through time intercept method and cross over
distance method on graphs.
4.6 Observations and Calculations:
Table 2-Calculations of Apparent resistivity and Cumulative resistivity
Electrode
SpacingS
Potential
difference
Geometrical
correction
factor
Apparent
Resistivity
Cumulative
Resistivity(
(a) (V) (G) (Ra) (∑Ra)
(m) (Volt) (m) (Ω-m) (Ω-m)
1 0.8 6.285714 20.11429 20.11429
2 0.42 12.57143 21.12 41.23429
4 0.28 25.14286 28.16 69.39429
6 0.155 37.71429 23.38286 92.77715
8 0.125 50.28571 25.14286 117.92
10 0.12 62.85714 30.17143 148.0914
15 0.105 94.28571 39.6 187.6914
20 0.1 125.7143 50.28571 237.9771
25 0.098 157.1429 61.6 299.5771
30 0.086 188.5714 64.86857 364.4457
40 0.86 251.4286 864.9143 1229.36
50 0.065 314.2857 81.71429 1311.074

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4.7 Graphical representation:
The graphical representation is given below.
Graph 1:
Graph 1: Graph between cumulative resistivity and electrode spacing
Graph 2:
Graph 2: Graph between Apparent resistivity and electrode spacing
0
200
400
600
800
1000
1200
1400
1600
0 10 20 30 40 50 60
CUMULATIVERESISTIVITY(∑Ra)
(Ω-m)
ELECTRODE SPACING (a)
(m)
Graph between Cumulative resistivity and electrode spacing
1
10
100
1000
1 10 100
APPARENTRESISTIVITY(Ra)
(Ω-m)
ELECTRODE SPACING
(m)
Graph between Apparent resistivity and electrode spacing
(log-log graph)

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Graph 3:
Graph 3: Depth profile model
4.8 Interpretation:
Resistivity of first layer:
R1 = (20.11429+21.12+28.16+23.38286+25.14286+30.17143)/6
R1 = 24.68190667 Ω-m
Resistivity of second layer:
R2 = (39.6+50.28571+61.6+64.86857+864.9143+81.71429)/6
R2 = 193.8305 Ω-m
Depth of boundaries between layers:
Di = 22 m
There is misbehavior in the second layer which shows that there might be a cavity or a dead body
in the subsurface which shows the irregular behavior in the graphs. Also, there might be the dry
area in which the resistivity survey does not work well.
0
5
10
15
20
25
1 10 100 1000
DEPTH(m)
APPARENT RESISTIVITY (Ra )
(Ω-m)
Depth Profile Model
(semi log graph)

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4.9 Limitations:
• Limitations of using the electrical resistivity method in ground water pollution
investigations are largely due to site characteristic, rather than in any inherent limitations
of the method.
• Typically sites are located in industrial areas that contain an abundance of broad-spectrum
electrical noise.
4.10 Conclusion:
We can conclude from the above discussion that the resistivity survey is a fast method of
determining the subsurface properties, but it cannot work properly if the land is too dry, in that
case, land needs to be moistened and then readings should be taken. The resistivity survey is
best for ground water table determination.
4.11 References:
https://archive.epa.gov/esd/archive-geophysics/web/html/resistivity_methods.html
Cardimona, Steve. "Electrical Resistivity Techniques for Subsurface Investigation"
https://www.slideshare.net/AmitMishra387/resistivity-survey.

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Lab#5
Introduction to the gravity survey
5.1 Abstract:
Gravity surveying is an important part of geophysical exploration techniques that are non-
destructive in nature, this lab aims to provide the details of gravity surveying in the following
content.
5.2 Objective:
• Understanding of gravity surveying
• Familiarize with its apparatus and performing procedure.
5.3 Related Theory:
5.3.1 Gravity surveying:
Investigation on the basis of relative variations in the Earth ‘gravitational field arising from
difference of density between subsurface rocks
5.3.2 Gravimeter:
A gravimeter is an instrument used in gravimetry for measuring the local gravitational field of the
Earth. A gravimeter is a type of accelerometer, specialized for measuring the constant
downward acceleration of gravity, which varies by about 0.5% over the surface of the Earth.
5.3.3 Types of Gravimeters:
• LaCoste-Romberg G-509:
A closed‐loop LaCoste and Romberg gravimeter servo system is described. It has an ac capacitor
bridge for sensing and phase sensitive detection. The output voltage of the detector restores the
mass to equilibrium by electrostatic force. The observed earth noise power at 12 cycles per hour
during quiet periods is roughly an order of magnitude greater than the thermal fluctuation limits
of the gravimeter.
Figure: 5.1 LaCoste-Romberg G-509

28
• CG5-Autgrav:
CG-5 gravimeter uses a mass supported by a spring. The position of the mass is kept fixed using
two capacitors. The dV used to keep the mass fixed is proportional to the gravity. It offers all of
the features of the low noise industry standard CG-3M micro-gravity, but is lighter and smaller,
has a larger screen which gives a superior user interface. The CG-5 can be operated with minimal
operator training, and automated features significantly reduce the possibility of reading errors.
5.3.4 Specifications:
Self levelling
Rapid measurement rate (6 m/sec)
Filtering
Data storage
5.4 Procedure:
Following are the steps that must be monitor for gravity survey.
• Collect geological report o concerned area on regional level study of minerals, anticline
structures, metamorphic rocks, coal and oil & gas exploration.
• Construct proper grids and w.r.t these grids acquire data from field. The grids are easy to read
and give accurate readings.
• Point of known coordinates and from IGSN71 taking a base station and define gr value. The
values of the base stations are already known for entire earth. GSN71 is International gravity
standardization network.
• Time is very important (day/noon/night) for gravity values because they effect the equipment
directly. For that purpose, corrections are applied.
• Tide correction is applied for time. The apparatus has its own tide correction in it.
• It is also called mechanical correction. As the equipment is used for a long time so the apparatus
starts to show error. This error can be removed by applying the drift correction.
• On the basis of experience and geological knowledge of surveyor and the coordinates of that
area where survey has to be done should be known for manual use while in auto mode the
equipment can read coordinates itself.
• The values are feed in the instrument country wise.
• It can be calculated by subtracting the normal gravity values from the observed gravity values
5.5 Data Precision:
The gravity survey gives values in 3 types of units that are:
• m/sec2
(SI Unit)
• Gals (from scientist Galileo)
• g.u (gravity unit)

29
The precision of data depends upon how carefully the data has been acquired, it is recommended
to avoid any kind of noise while acquiring the data and a density contrast among subsurface
materials is necessary for gaining a precise data for further analysis. Normally, the igneous material
bodies give more value of “g” as they are denser when compared to metamorphic and sedimentary
rocks.
5.6 Applications:
The gravity surveying has applications in the following operations:
• Tectonic setting determination
• Structure and stratigraphy.
• Oil and Gas Exploration
• Mineral Exploration
• Geological Mapping
• Civil Engineering Works
• Geo-Technical Investigations
• Regional Gravity Studies.
5.7 References:
http://www.geophysical.biz/magnetic.htm
https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/JZ071i024p06005

30
Lab#6
Introduction to Magnetic Survey
6.1 Abstract:
Magnetometers are used to measure the magnetization and the direction of the magnetic field. This
following lab report focuses on the introduction and familiarization of magnetic survey, which is
one of the methods / techniques of geophysical exploration. Following content would include the
further details of this survey type.
6.2 Objectives:
• To familiarize with concept of magnetic survey
• To analyze the working principle of magnetometer
6.3 Related Theory:
6.3.1 Magnetometer:
Magnetometers are measurement instruments used for two general purposes:
• To measure the magnetization of a magnetic material like a ferromagnetic
• To measure the strength and, in some cases, the direction of the magnetic field at a point
in space.
The first magnetometer was invented by Carl Friedrich Gauss in 1833 and notable developments
in the 19th century included the Hall Effect which is still widely used.
Magnetometers are widely used for measuring the Earth's magnetic field and in geophysical
surveys to detect magnetic anomalies of various types. They are also used militarily to detect
submarines. Consequently some countries, such as the USA, Canada and Australia classify the
more sensitive magnetometers as military technology, and control their distribution.
They can be used as metal detectors: they can detect only magnetic (ferrous) metals, but can detect
such metals at a much larger depth than conventional metal detectors; they are capable of detecting
Figure: 6.1 Magnetometer

31
large objects, such as cars, at tens of meters, while a metal detector's range is rarely more than 2
meters.
6.3.2 Data Acquisition & Processing:
Data acquisition for magnetic surveys involves taking a series of point readings at regular intervals
on a survey grid. The spacing between grid lines and reading stations is dependent upon the
application. Generally smaller targets require higher resolution surveys and denser survey grids.
Modern cesium vapor magnetometers and gradiometers are more sophisticated, allowing data to
be collected either in continuous mode or as a set of point readings.
Data is stored digitally on site, and later downloaded on to a PC for post-survey processing and
interpretation. Various interpretation techniques are applied to the data using specialist interactive
software to identify the targeted anomalies. A combination of contouring and color shading is used
to highlight anomaly patterns. Survey results are presented as plans tied in to site co-ordinates, in
an engineering compatible format readily understandable by the client
6.4 Applications:
• Archaeology
To detect the archaeological sites, buried and submerged objects
• Coal exploration
Used to locate the sills and other obstacles which results in explosion
• Military applications
Used in defense and navy to perform the submarine activities.
• Defense and aerospace
Used on land, in the air, at and under sea, and in space applications
• Oil and gas exploration
Used while drilling the discovered wells
• Drilling sensors
Used to detect the direction or path for the drilling processes
• Plasma flows
Used while studying about solar wind and planetary body
• Health care monitoring
Used to perform cardiac applications like diagnostic system capable of non-invasively measure of
heart function
• Pipe line monitoring
Inspecting corrosion of the pipeline in the underground systems and also for monitoring purposes
these are used
• Surveyors
➢ Used in the geophysics applications
➢ Compasses

32
➢ Space applications
➢ Image processing of the magnetic data
6.5 References:
http://www.geophysical.biz/magnetic.htm
https://en.wikipedia.org/wiki/Magnetic_survey_(archaeology)
https://pangea.stanford.edu/research/groups/sfmf/docs/Magnetic.pdf

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