1

OFFIAH, SOLOMON U.

RESISTIVITY SURVEY FOR GROUNDWATER IN AND AROUND

EZZA NORTH LOCAL GOVERNMENT AREA, EBONYI STATE

Physics and Astronomy

A THESIS SUBMITTED TO THE DEPARTMENT OF PHYSICS AND ASTRONOMY,

FACULTY OF PHYSICAL SCIENCES, UNIVERSITY OF NIGERIA, NSUKKA

Webmaster

Digitally Signed by Webmaster’s Name

DN : CN = Webmaster’s name O= University of Nigeria, Nsukka

OU = Innovation Centre

2011

UNIVERSITY OF NIGERIA

2

RESISTIVITY SURVEY FOR GROUNDWATER IN

AND AROUND EZZA NORTH LOCAL

GOVERNMENT AREA, EBONYI STATE

3

CERTIFICATION

This is to certify that this project work was submitted and approved by the

Department of Physics and Astronomy in partial fulfilment for the requirements for

the award of Master of Science in Physics and Astronomy, University of Nigeria,

Nsukka.

____________________________ _____________________________

DR. J.U. CHUKUDEBELU DR. P. O. EZEMA (Project supervisor) (Project supervisor)

__________________________ ______________________________

PROF C. M. I. OKOYE (External examiner) (HOD, Department of Physics and Astronomy)

4

DEDICATION

This project is dedicated to late Engr. John I. Anokwulu.

5

AKNOWLEDGEMENT

This piece of work would not have been possible without the collective

contributions from many people. Some of these people are mentioned below.

First and foremost, I wish to express my profound gratitude to God Almighty for

His marvelous inspirations, guidance and protection, especially when I was passing

through difficulties.

I am highly indebted to my project supervisors, Dr. J. U. Chukudebelu and Dr. P.

O. Ezema, for their immeasurable contributions towards the successful completion

of this work. This project would have proved futile without their fatherly pieces of

advice, constructive criticisms and suggestions. I must commend them for making

out time to go through this work in spite of their tight time schedules. I appreciate

the effort of Dr. P. O. Ezema for providing me with the theoretical master curves

and the auxiliary diagrams which I used during the manual interpretation of the

field data. It is my wish to express my special gratitude to Prof (Mrs) F. N. Okeke,

the Dean, Faculty of Physical Sciences, who patiently devoted her precious time in

order to give this work an excellent finishing touch in spite of her very tight time

schedules.

I also acknowledge the effort of all the staff of the Department of Physics and

Astronomy who have either directly or indirectly contributed towards the

successful completion of my academic programme. Worthy of mention include the

Head of the Department, Prof C. M. I. Okoye and the immediate past acting HOD,

Prof R. U Osuji. Others are Prof P. N. Okeke, Prof A. A. Ubachukwu, Prof A. O.

Animalu, Prof S. Pal, Dr E. Chukwude, Dr. B. A. Ezekoye, Dan Obiora, A. B. C.

Ekwealor and host of others, too numerous to mention. I equally appreciate the

assistance of some staff of the Department of Geology such as Mr S. Nwosu and

Mr Ejike Ugboaja who provided some of the materials which I used in the course

of the project work.

I owe a lot of thanks to Mr. Emmanuel Igwebuike for his encouragement and

support, especially for providing me with the modern equipment for the resistivity

survey. Worthy of mention also are Mr. Emmanuel Enang and his co-workers of

Felgra Links Nigeria Limited, Enugu who are into hydrogeological investigation.

They really provided good assistance during the fieldwork.

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The staff of the Ebonyi State Rural Water Supply and Sanitation Agency

(RUWASSA), Abakaliki are not left out. Many thanks to Mr. Obini and Mr. A.

Opoke for their contributions during the project work.

To my beloved parents, Mr. Hyacinth and Mrs. Dinah Offiah, it is my desire to

appreciate their unalloyed moral and financial support and for making my dreams

come true.

I will not forget to acknowledge my colleagues; Ugwu Chiebonam, Nneji Gabriel,

Ugwuanyi Maximus, Mete Ngozi, Ugwuanyi Sabastine and other postgraduate

students, friends and well wishers whose pieces of advice have greatly enhanced

the quality of this project.

TO GOD B E THE GLORY

OFFIAH SOLOMON U.

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ABSTRACT

Vertical electrical resistivity soundings were conducted in order to delineate the

groundwater potentials at some locations in Ezza North Local Government of

Ebonyi State. Twelve vertical electrical soundings were obtained using the

Schlumberger configuration with the aid of the OHMEGA terramenter (SAS1000).

The field data were subjected to interpretation by employing the method of partial

curve matching techniques using the master curves and the corresponding auxiliary

curves. A computer programme (RESOUND) was used to interpret the resistivities

and the thicknesses of the subsurface. The parameters obtained were used to

determine the resistivities and thicknesses of the subsurface layers. Two profiles,

VES11 and VES 12 indicated eight geoelectric layers. Five geoelectric layers were

evident at five locations namely VES 3, 4, 7, 8 and 10. Data from the remaining

five locations (VES 1, 2, 5, 6 and 9) revealed six layers each. The major lithologic

units of the area are shales, sandstone and mudstone. The water bearing rocks were

interpreted to exist at depths between 20m and 130m in most of the VES locations.

The results fairly correlated with some logged boreholes close to the survey area.

The aquifers have resistivities ranging between 9Ωm and 110Ωm.The geophysical

search for groundwater has shown that the survey area has good groundwater

potentials which if exploited would go a long way in reducing the problems of

seasonal water shortage and possible health problems associated with the

consumption of unhygienic surface water in the area.

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TABLE OF CONTENT

Title page i

Approval page ii

Dedication iii

Acknowledgement iv

Abstract vi

Table of content vii

List of figures ix

List of tables x

CHAPTER ONE: INTRODUCTION

1.1 Background 1

1.2 Location of the study area 3

1.3 Geology of the area 3

1.4 Groundwater 9

1.5 Porosity and permeability 10

1.6 Aquifer and aquiclude 12

1.7 Aims of the present research work 14

CHAPTER TWO: LITERATURE REVIEW

2.1 Literature review 15

2.2 Review of the resistivity survey technique 16

CHAPTER THREE: DATA ACQUISITION

3.1 Equipment for the fieldwork 18

3.2 Survey procedure and data collection 21

3.2.1 The Schlumberger electrode configuration 21

3.2.2 Data collection 23

3.3 Practical limitations and precautions 23

CHAPTER FOUR: PROCESSING AND INTERPRETATION OF THE FIELD

DATA

4.1 Data processing 38

4.2 Interpretation of the field data 38

4.2.1 Partial curve matching 38

4.2.2 Computer-based interactive modeling 53

4.3 Subsurface geoelectric sections of the vertical electrical soundings 65

4.4 Discussions 70

4.5 Conclusion 74

4.6 Recommendations 74

References

9

LIST OF FIGURES

Fig 1.1: Map of Nigeria Showing the location of Ebonyi State 4

Fig 1.2: Map showing the location of the study area, Ezza North L.G.A.

Ebonyi State, and the VES positions 5

Fig 1.3: The section of a 200m borehole near VES 1 8

Fig 1.4: The subsurface distribution of water 11

Fig 1.5: Well roundedness, well sortedness and poor cementation

of rocks increase rock porosity and permeability 11

Fig 3.1: The OHMEGA terrameter SAS1000 and the wire reels 19

Fig 3.2: The Schlumberger array 22

Fig 3.3: Some of the crew members during the field work 24

Fig 4.1: Master curves of the Schlumberger apparent resistivity 40

Fig: 4.2: The auxiliary curves 41

Fig 4.3 – 4.14: The field curves used in the manual interpretations

of the sounding data 44-49

Fig 4.15 The field curves models for the computer based interpretations 54-59

Fig. 4.16: The lithological sections of the vertical electrical soundings 66-68

Fig. 4.17: Geoelectric section relating VES 9, 10, 4, 5, 3, 6 and 1 69

10

LIST OF TABLES Table 1.1: The global grid positions and elevations of the profile centres 6

Table 1.2: Porosities and permeabilities of some geologic materials 13

Table 3.1: Data sheet for recording the field data 20

Table 3.2 – 3.13: The field data for different VES locations 26-37

Table 4.1: Layer parameters obtained from the interpretations

using partial curve matching technique 50-52

Table 4.2: Geoelectric interpretations of VES data from various

profiles using computer interactive programme 60-63

Table 4.3 Estimated depths of the water bearing rocks at the VES points 73

11

CHAPTER ONE

INTRODUCTION

1.1 Background

Groundwater is one of the very important natural resources. Though it is true that

greater percentage of the earth’s surface is composed of water including seas,

oceans, rivers, streams, ponds and others, yet none of these surface sources is as

hygienic or as economical for exploitation as the groundwater (Singh, 2007). The

amount of fresh water available for human use is less than 0.08% of all the water

on the planet (BBC Sci/Tech News, 2000). Groundwater is recommended for its

natural microbiological quality and its general chemical quality for most uses

(McDonald et al., 2002). Due to its scarcity, water related diseases are found in

many parts of the world. In Nigeria, for example, Okoronkwo (2003) attributed the

guinea worm infestation in some parts of Ebonyi State to ignorance and lack of safe

drinking water. The people, according to him, lacked boreholes and depended only

on ponds and other existing contaminated sources.

Over the years, boreholes have usually been drilled with or without previous

knowledge of the subsurface stratification in search of water. As a result of multiple

failed boreholes, researches grew towards minimizing failed wells, thereby

reducing the risk as well as cost of drilling (Adetola, and Igbedi, 2000).

Tremendous breakthroughs have been recorded in the use of electrical methods

used in the exploration of the subsurface minerals (Selemo et al., 1995).

Geophysics involves the measurement of contrasts in the physical properties of

materials beneath the surface of the earth and the attempt to deduce the nature and

the distribution of the materials responsible for these observations at the surface. It

involves the application of the principles of physics to the study of the earth. The

geophysical methods used in the investigation of the shallow features of the earth’s

crust vary in accordance with the physical properties of rocks. In seismic method of

exploration, seismic waves travel with different speeds through different materials

due to variations in their elastic moduli and densities. Variation of densities in the

subsurface can as well lead to change in gravitational acceleration at the surface

(gravity method). Measurable differences in magnetic field can be obtained at field

sites due to variations in magnetic susceptibilities, referred to as magnetic method.

12

Similarly, variations in the electrical conductivities of rocks and sediments can

produce different values of apparent resistivities as the distances between

measuring probes are increased or as the position of the probe is changed on the

surface (electrical resistivity method).

Electrical resistivity is one of the physical properties which can be used to

distinguish among different rocks. This is because the resistivities of different rocks

and minerals vary widely. While igneous rocks containing no water have very high

resistivities, metallic ores have very low resistivties (Telford et al., 1990). The

apparent resistivity of the subsurface as measured on the surface is a function of the

current, the recorded potential difference and the geometry of the electrode array.

Presence of water substantially controls the variation of the conductivities in the

shallow subsurface. The measurements indicate water saturation and connectivity

of pore spaces because water-bearing rocks and minerals have lower resistivities

and electric current usually follows the path of least resistance (Ezema, 2005).

Resistivity methods have been found successful for locating and accessing

groundwater. It is cost effective and subject to careful study of the geology of the

survey area. Hence, the geology of the study area must be well known before

embarking on resistivity survey. In electrical resistivity survey, current is passed

into the ground through two current electrodes. Two other electrodes are used to

measure the resulting potential difference produced by this current. The

information is used to calculate the apparent resistivity of the rock.

All substances act to retard the flow of electric current so that energy must be

expended to move charged particles. The extent to which a substance restrains this

movement is described by its electrical resistivity. The principal goal of electrical

resistivity surveying is to measure this physical property as a basis for

distinguishing layering and structure of the earth.

The two main types of procedures employed in resistivity surveys are vertical

electrical sounding VES, and constant separation traversing CST. In constant

separation traversing, which is used to determine lateral variation in resistivity, the

current and potential electrodes are maintained at a fixed separation and

progressively moved along a profile. In vertical electrical sounding, the current and

the potential electrodes are progressively expanded about a fixed central point. By

progressively expanding the current electrodes, readings of the potential difference

13

are taken as current reaches to greater depth. This gives the information on the

resistivities and thicknesses of the underlying horizontal strata.

The modern equipment for measuring the potential difference and the current is the

signal averaging system (SAS) terrameter. The resistivity of the subsurface material

is a function of the magnitude of the current, the recorded voltage and the geometry

of the electrode configuration. The electrical resistivity obtained is termed

“apparent” because it is not likely that the subsurface materials beneath the survey

area are homogeneous. The apparent resistivities are subject to interpretation

techniques including the curve matching and/or computer interpretation. Based on

the resistivities and the thicknesses of the underlying formations and the available

geology of the area, the depth to water bearing rocks (aquifer) may be estimated.

1.2 Location of the study area

The area under survey lies between latitudes 06008

1 and 06

017

1 north of the equator

and longitudes of 07052

1 and 08

000

1 east of the Greenwich Meridian. Figure 1.1 is

the map of Nigeria showing the location of Ebonyi State. The map of the area under

survey, Ezza North Local Government Area, is shown in figure 1.2. The area which

covers about 246 squared kilometres lies in the south eastern part of Abakaliki, off

Enugu-Ogoja highway. Abakaliki is about 62km South East of Enugu and about 22

kilometres West of Afikpo in Ebonyi State. The global positioning system (GPS)

receiver was used in the field to obtain the global grid positions of the vertical

electrical sounding points, including the longitudes, latitudes and the elevations.

This instrument receives its data from the GPS satellite. The GPS locations of field

stations are shown on table 1.1. In addition to Enugu-Ogoja Road, the survey

location can equally be accessed through the Onueke market along the Abakaliki –

Afikpo Expressway.

1.3 Geology of the area

The study area belongs to the Asu River group shales. According to Reyment

(1965), the sediments of the Asu River group which was formed during the Albian

times were folded into open North-East trend known as the Abakaliki

anticlinorium.

14

Figure 1.1: Map of Nigeria Showing the location of Ebonyi State

(http://commons.wikimedia.org/wiki/File:Ebonyi_State_Nigeria.png)

Ebonyi State

15

Figure 1.2: Map showing the location of the study area, Ezza North L.G.A.

Ebonyi State, and the VES positions

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Table 1.1: The global grid positions and elevations of the profile centres obtained

during the field work using the GPS receiver m.

PROFILE

NO.

LOCATION OF

PROFILE

LATITUDE LONGITUDE ELEVATION

(m)

VES – 1

Adiagu-Oguji Nwudor 06

0 16

1 57N 07

055

1 06

11 E 61.9

VES – 2

Ekka Town Hall,

Azugwu 06

0 10

1 48

11N 07

0 57

1 00

11 E 57.0

VES – 3 Nkomoro-Omuzor Ogbo

Ojiovu 06

0 14

1 12

11 N 07

0 55

1 34

11 E 86.0

VES – 4 Ndiegu-Ogboji-Ukwu

Akpara 06

0 13

1 20

11 N 07

0 54

1 04

11 E 81.4

VES – 5

Umundiegu-Ohaike 06

0 14

1 02

11 N 07

0 54

1 08

11 E 63.4

VES – 6

Udenyi-Azuakparata 06

0 15

1 23

11 N 07

055

1 34

11 E 89.3

VES – 7 Inyere-Ngangbo Nwakpa

Umobi 06

0 10

1 50

11 N 07

0 55

1 29

11 E 67.4

VES – 8

Ogboji-Eguo-Ugwu 06

0 11

1 00

11N 07

0 57

1 14

11 E 68.3

VES – 9 Ohaccara-Ndiegu-

Ohaccara 06

0 11

143

11 N 07

0 53

1 06

11 E 61.3

VES – 10 Ndiegu Ekka-Onunwode

Ndiegu 06

012

1 26

11 N 07

0 53

1 26

11 E 82.9

VES – 11 Ekka Integrated Pri. Sch.

Ekka 06

0 10

114

11 N

08

0 00

1 58

11 E 87.2

VES – 12

Ohaugo Pri. Sch. Ekka 06

010

1 26

11 N 07

0 59

1 05

11 E 58.8

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The Afikpo syncline lies along the eastern and western sides of the anticlinorium.

The Asu River group is overlain by succession of shales, siltstone and sandstone,

with a shallow marine fauna. There are some mineral intrusions which may have

contributed to its numerous fractures. The Asu River formation is estimated to have

a maximum thickness of about 200m. Lead – zinc mineralization and the associated

mineralization like pyrites, chalcopyrites, salt and so on, occur in sills and dikes

forming massive bodies. Exposures of the rocks occur mostly along stream

channels in some areas. Lead – zinc exploitation has been going on in parts of the

area (Orajaka, 1972). The geological survey around the area reveals that the

location is part of the Ebonyi formation. This formation overlies the Abakaliki

siltstone and sandstone previously referred to in literature as “Unknown Formation”

(Reyment, 1965). It is now however referred to as the Ebonyi Formation

(Agumanu, 1990). The formation underlies a gentle undulating terrain in Ntezi-

Ezamgbo area and southward to Amagu-Agba. The Ebonyi River (wrongly spelt

“Aboine” in most published maps) and its tribulataries, namely Akaduru, Nramura

and Isumutu Rivers form the major drainage system in the area. The arrangement of

the layers consists of a rapidly alternating horizontal sequence of mudstone, shale,

limestone, siltstone and sandstone.

The section of a 200 meter-deep borehole BH1 drilled in 1963 at Umuezeoke near

Ezzamgbo (060 20

1 N, 07

0 56

1 30

11 E), about 6 km from VES 1 is shown in figure

1.3. The formation is divided into three informal units from top to bottom.

1. The upper siltstone-shale sequence is exposed at Amagu-Agba village. It

consists essentially of rapidly alternating siltstone and silty shale with

occasional thin sandstone beds.

2. The middle limestone-siltstone sequence unit outcrops at a quarry, 2km from

Ekemoha-Agba road junction (60 14

1 30

11 N, 7

0 54

1 45

11 E). It consists of

minor sandstone, siltstone, limestone and shale.

3. The lower mudstone-shale sequence is exposed at Umuezeoke (60 16

1 30

11

N, 70 56

1 00

11 E) along a drainage cut by River Akaduru. The sequence is

greyish, occasionally flesh-coloured and bedded with dark micaceous

streaks. It contains mudstone concretions.

18

Figure 1.3: The section of a 200 meter-deep borehole (GSN BH) near VES-1, drilled by

the Geological Survey of Nigeria (GSN) in February, 1963. Typical section is

also exposed along a road-cut about 1km near VES-3 and VES-6 (Agumanu,

1990).

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According to Agumanu (1990), typical sections are exposed at a quarry 2km from

the Ekemoha-Agba Road Junction (60 14

1 30

11 N, 7

0 54

1 45

11 E) and along a road

cut at Ntezi (60 25

1N, 7

0 55

1 E¨).

The study area has elevation between 57m and 89m above sea level. Marshy

conditions of lower elevation that also exist within the area are noted for rice

production in the area. Most of the numerous streams existing in the area are

seasonal. These seasonal rivers which are active during the rainy seasons, have as

the major drainage, the Ebonyi River, which flows to the Cross River, some

distance to the south near Afikpo.

The area is predominately shales, the intrusions that gave rise to the existence of

rocks and minerals in the area during the santonian upliftment, account for several

fractures within the shale. These fractures contain water, serving as the aquifer.

Hand – dug wells drilled at the nearby communities give considerably low yield.

For this reason, large water projects are at times being sourced from well-

investigated boreholes to sustain motorized pumps of small and medium discharge

capacities. Majority of the groundwater come from fractures within the rocks.

Electrical resistivity is a cost effective method for locating shallow fractured zones

in the area. The mudstones are highly weathered on the top. Significant

groundwater is only found where the mudstone and the shale are highly fractured.

1.4 Groundwater

One of the most important natural resources is groundwater (Adetola and Igbedi,

2000; Singh, 2007). The liquid water may appears on the planet earth in three

forms. Very large, medium and small bodies of standing water which appear in the

forms of oceans, seas, and lakes. Bodies of flowing water appear as rivers, rivulets,

streams and springs. Finally the subsurface water includes all forms of water

existing below the ground surface such as water films around grains of rocks,

droplets in rock pore spaces and cavities in rocks filling them partly or completely

over variable areas and creating underground reservoirs (Singh, 2007).

Though greater percentage of the earth is composed of water, there is little fresh

water on the earth (Montgomery, 1990). If the soil on which precipitation falls is

sufficiently permeable, infiltration occurs. Gravity draws the water downward until

an impermeable rock or soil is reached. The water begins to accumulate above that

20

layer. Immediately above the impermeable material is a zone of rock or soil that is

water saturated. This region is known as zone of saturation or the phreatic zone

(Montgomery, 1990). Water fills all the accessible pore spaces here. Above the

phreatic zone are rocks in which the pore spaces are partially filled with water and

partly with air. This is known as the zone of aeration or the vadose zone. While

subsurface water refers to the water occupying pore spaces below the ground

surface, groundwater represents the water in the zone of saturation (phreatic zone)

and below the water table. Water table is defined as the top of the zone of

saturation, where the saturated zone is not confined by overlying impermeable

rocks. All forms of water, including bodies of standing water and flowing water are

collectively referred to as surface water. The water table is not always below the

ground surface. Whenever surface water persists, as in a lake or stream, the water

table is locally above the ground surface and the water surface is the water table.

1.5 Porosity and permeability

The two major determinants of the availability, quantity and exploitability of

groundwater are the porosity and permeability of the host rocks (Montgomery,

1990). Porosity is the proportion of void space in material within mineral grains. It

is the volume of pore spaces compared with the total volume of a soil, rock or

sediment (Chernicoff and Whitney, 2002). It may be expressed in percentage.

Porosity determines how much water a material can hold. The spaces between

particles in soil, sediments and sedimentary rocks determine the porosity. Factors

that determine the porosity of rock include cracks, fractures, faults and vesicles in

volcanic rocks (Moonrey and Wicander, 2005). Porosity also depends on the type,

shape, size and arrangement of rock materials. As can be seen from figures 1.4 and

1.5, well rounded grains tend to have larger pore spaces and therefore hold more

water. When sediment contains grains of various sizes, it is said to be poorly

sorted. The finer particles tend to fill the voids between the coarser particles,

clogging the pores and reducing porosity. When cementation converts loose

sediment to sedimentary rock, the cement fills the pore spaces and further

diminishes porosity.

21

Figure 1.4: The subsurface distribution of water. (Chernicoff and Whitney, 2002)

Figure 1.5: Well roundedness, well sortedness and poor cementation of rocks

increase rock porosity and permeability (Chernicoff and Whitney, 2002)

22

Fine clayey mud holds much more water when saturated than coarse sediments

because clay contains higher percentage of minute pores than the coarse sands.

Water is very difficult to extract from such rock because of the extremely small size

of the pore (Chernicoff, and Whitney, 2002). The tiny pores spaces retard the

movement of water.

As the resistivities of sediments and rocks are controlled by the amount of water

present and the salinity (electrolytic conduction), clay mineral, all fine-grained or

increasing silt or clay content in poorly sorted rocks or sediment will reduce

resistivities (Burger, 1992). Thus in saturated materials, increasing porosity will

reduce resistivities.

Permeability is the measure of how readily fluid passes through materials. It is

related to the extent to which the pores or cracks are interconnected (Moonrey and

Wicander, 2005). The crucial factor that determines the availability of groundwater

is not just how much water the ground can hold, but whether the water can flow

easily through the pore spaces. Water flows slowly through rocks when the pores

are very small as in clayey sediments. Some water molecules may stick as fine

films to adjacent particles, slowing the flow even further. Hence water flows more

easily only when the pores are relatively large. According to Chernicoff and

Whitney (2002), the pores between grains of sand are more than 1000 times greater

than the pores in clay, explaining why sand is much more permeable than clay.

Both porosity and permeability play important roles in groundwater movement and

recovery. Wet sand dries easily, but once clay absorbs water, it may take some days

to dry out because of its low permeability. Table 1.2 shows the porosities and

permeabilities of some rocks and sediments.

1.6 Aquifer and aquiclude

A permeable layer of rock responsible for transporting groundwater is called an

aquifer. It is an underground layer of water-bearing permeable rock or

unconsolidated materials like gravel, sand, silt or clay from which groundwater can

be usefully extracted. The most effective aquifer (water bearing rock) is a deposit

of well-sorted and well rounded sand and gravel.

23

Table 1.2: Porosities and permeabilities of some geologic materials. (After Montgomery, 1990)

POROSITY

(%)

PERMEABILITY

(m/day)

UNCONSOLIDATED MATERIALS

Clay 45 – 55 Less than 0.01

Fine Sand 30 – 52 0.01 – 10.

Gravel 25 – 40 1,000 – 10,000

Glacial tilt 25 – 45 0.001 – 10.

CONSOLIDATED ROCKS

Sandstone and Conglomerate 5 – 30 0.3 – 3

Limestone (crystalline and

unfractured)

1 – 10 0.00003 – 0.1

Granite (unweathered) Less than 1 – 5 0.0003 – 0.003

Lava 1 – 30 (mostly less than

10)

0.0003 – 3 Depending on the

presence of fractures or

interconnecting gas

bubbles

24

Limestone, in which fractures and bedding planes have been enlarged by solution

are also good aquifers. Shales and many igneous and metamorphic rocks make poor

aquifer because they are typically impermeable unless fractured. Aquicludes refer

to the rocks or materials that prevent easy movement of groundwater. They are the

impermeable rock materials. In all groundwater exploration programmes, the main

objective is to locate the zone of saturation and determine its geometry and

character (Singh, 2007).

1.7 Aims of the present research work

The present survey work is aimed at investigating the groundwater potentials of

some selected communities within Ezza North Local Government Area of Ebonyi

State by conducting vertical electrical soundings and interpreting the VES data

obtained at the various locations. The research work is also aimed at helping the

hydrogeologists to locate the promising sites for drilling of successful boreholes to

reduce the unnecessary expenses as a result of random and non scientific means of

search for groundwater. Above all, as groundwater is considered a better source of

economic and hygienic water (Mcdonald et al., 2002), its location and exploitation

will help reduce the existing seasonal water scarcity, long distance trekking and

overcrowding of the few streams, rivers and ponds (which are prone to

contaminations). The implementation of the results obtained from this work will go

a long way in improving the peoples health conditions (Malin, 1982), especially as

it concerns the guinea worm infestation (Okoronkwo, 2003)and other water borne

diseases which are common to the some part of Ebonyi Sate. This is because

groundwater has excellent natural microbiological quality and generally adequate

chemical quality for most uses (McDonald et al., 2002).

25

CHAPTER TWO

LITERATURE REVIEW

2.1 Previous works

Electrical resistivity survey is a very attractive tool for describing the subsurface

properties without digging. The idea of using electrical resistivity to study the

subsurface of rock bodies was introduced by Schlumberger in 1912 (Meyer de

Stadelhofen, 1991). The technique enables the improvement of our understanding

of soil structure and how it relates to various fields such as agronomy, archaeology,

geology and civil engineering (Samouelian et al., 2008). Resistivity measurements

do not give direct access to soil characteristics. It requires qualitative and

quantitative interpretations to link the electrical measurements with respect to soil

characteristics. Though this method was first adopted in geology by oil companies

searching for oil reservoirs and for mapping geological formations, the technique is

now extensively used in groundwater exploration. This is because soil materials

and properties can be quantified through the geoelectrical properties.

Several authors have successfully applied the resistivity method in groundwater

exploration. Alile et al. (2008) confirmed the suitability of the electrical resistivity

method in groundwater exploration, since there was a high correlation between the

VES results and the borehole values obtained from two sites in Edo State, Nigeria.

Many borehole sites have been surveyed across the different geological provinces

of Nigeria with the aid of VES by Selemo et al. (1995). Appropriate measures were

taken in order to accommodate the problems of equivalence and suppression. The

result of their findings revealed that there should be proper understanding of both

the general and the local geology in order to take the final decisions which are

based on the aquifer characteristics of the lithologic units.

Vertical electrical resistivity sounding method was successfully used in locating the

site for successful borehole drilling and for the confirmation of the Bende-Ameka

formation in Agbede south-western Nigeria (Adetola and Igbedi, 2000). The

method was also used in survey for groundwater in Idemili and Anambra LGAs of

Anambra State (Obiakor, 1984) and for locating a deep water-bearing fractured

zone in basement rock at Central Mining Research Institute (CMRI) in Dhanbad,

New Delhi (Singh et al., 2006). Mohammed and Lee (1985) located the proper

sites for borehole drilling in Perlis using the off-wenner electrical resistivity

26

procedures. Although the similarity of the electrical properties of the bedrocks

made the interpretations more difficult, McDougal et al (2003) employed the

vertical electrical sounding in the investigation of subsurface geologic conditions as

they relate to groundwater flow in Wyoming. Investigations carried out using nine

different sites along the Jhang Branch Canal revealed that resistivity survey is an

inexpensive method for characterizing groundwater conditions (Arshad et al.,

2007). Here, the interpretations of the resistivity data demonstrate that the sites

which has aquifer depth between 30m and 140m, indicated the existence of large

quantity of fresh groundwater. In the assessment of the groundwater of Yola –

Jimeta areas, Eduvie (2000) used the method to arrive at the conclusion that the

groundwater potentials of the Bima sandstone is very high and requires properly

designed and constructed boreholes for maximum yields. on the bases of resistivity

sounding data, it may be possible to demarcate the unproductive zones where

prevalence of clay is indicated (Bose et al., 1973). The productive zones may

subsequently be classified into subzones according to the order of their

groundwater potentials. The interpreted data of the groundwater exploration using

the vertical electrical sounding technique with Schlumberger configuration which

were conducted by Dhakate et al., (2008) in Wailpally watershed area of Nalgonda

district in India were used to develop maps of groundwater potentials. The maps

show the regions of good, moderate and poor aquifer zones.

2.2 Review of the resistivity survey technique

Propagation of electric current in rocks and minerals may be in three ways namely

electronic (ohmic) conduction, electrolytic conduction and dielectric conduction.

Electronic conduction is the normal type of current conduction in metallic materials

which contain free electrons. In electrolytic conduction, current is carried by ions at

comparatively slow rate. Dielectric conduction takes place in poor conductors or in

insulators, which have very few or no free charge carriers (Telford et al., 1990).

Electrical resistivity sounding is intended to detect changes in resistivities of the

earth with depth at locations, assuming horizontal layering. This is achieved by

successive increase in electrode spacing. A direct current or low frequency

alternating current signal is driven into the ground with the aid of two current

electrodes (Dobrin, 1976) and the resulting potential difference recorded by a

27

sensitive instrument at various locations on the surface of the earth. The

information from the data can be used to deduce the goelectric section of the earth.

In practice, two current electrodes are employed. The positive electrode (A), the

source, sends current into the ground while the negative electrode (B), the sink,

collects the returning current. The apparent resistivity obtained at the surface with

the aid of the electrodes is given by

2, 2.1

1 1 1 1

, 2.2

a

V

I

AM BM AN BN

VG

I

where G is the geometrical factor which depends only on the spatial arrangement

of both the current and potential electrodes. Equation 2.2 is of practical importance

in the determination of earth’s resistivity. The physical quantities measured in field

determination of resistivity are current, I, flowing between the two current

electrodes, the difference in potential, ,V between the two measuring potential

electrodes, M and N, and the distance between the various electrodes (Keller and

Frischknecht, 1966).

The field procedure for electrical resistivity surveying may involve either vertical

electrical sounding or constant separation traversing. The latter, which is also

known as electrical mapping, deals with lateral variations of resistivity along the

horizontal ground. It is primarily useful in mineral prospecting and for the location

of faults or shear zones (Keary and Brooks, 1984). Vertical electrical sounding

(VES) is particularly useful in the determination of electrical conductivity with

depth, with the assumption of horizontal profiling. VES has been the most

important geophysical method for groundwater prospecting in many areas

(Parasnis, 1986). The essential idea behind VES is the fact that as the distance

between the current electrodes is increased, the current passing across the potential

electrodes carries a current fraction that has returned to the surface after reaching

increasingly deeper levels. The technique is extensively used in geotechnical

surveys to determine overburden thickness and also in hydrogeology to define

horizontal zones of porous strata.

28

CHAPTER THREE

DATA ACQUISITION

3.1 Equipment for the fieldwork

The most important equipment required for field measurements in vertical electrical

sounding survey includes suitable source of d. c. or low frequency power supply;

highly sensitive meters for measuring current, I and potential difference, V;

electrodes for making electrical contact with the earth and insulated, low-resistance

wires. Modern equipment have been selected and used to suit the tasks at hand. The

major instrument used in the present electrical resistivity survey is the terrameter.

The equipment is designed to measure both current and potential simultaneously

and automatically display the resistance of the ground. There are many types of

terrameters but the one we employed in this work is the OHMEGA signal

averaging system (SAS 1000). Figure 3.1 shows the OHMEGA (SAS1000)

terrameter and the four wire-reels used during the data acquisition. Other

equipment that were employed during the field work include well insulated long

wires with low resistance. They were used to connect the four electrodes to the

terrameter. Four stainless steel electrodes (ABEM Instrument Manual 2009), each

measuring about 50cm long, measuring tapes, data sheets, hammers and cutlasses

were also used. Table 3.1 shows the data sheet used in recording the field data

during the survey work.

29

a

b

Fig. 3.1: (a) The OHMEGA terrameter SAS 1000 (b) The terrameter and the wire reels

30

SCHLUMBERGER VERTICAL ELECTRICAL SOUNDING

DATA SHEET

SURVEY LOCATION ____________________________________________ DATE ________________

STATION NO__________________________________________________________________________

LATITUDE__________________________LONGITUDE_________________ELEVATION_________

OPERATOR___________________________________________________________________________

S/N ( )2

ABm

2

M Nm G R

am

1 1.5

0.50

6.28

2 2.0 11.78

3 2.5 18.85

4 3.5 37.70

5 4.5 62.80

6 6.0 112.32

7 8.0 200.00

8 10.0 313.00

9 15.0 706.00

10 10.0

3.50

39.39

11 15.0 95.49

12 20.0 174.04

13 25.0 275.03

14 35.0 544.35

15 45.0 903.44

16 55.0 1352.29

17 45.0

14.0

205.24

18 55.0 317.45

19 75.0 609.21

20 95.0 990.74

21 125.0 1731.35

22 165.0 3033.04

23 215.0 5165.11

24 165.0

55.0

691.24

25 215.0 1233.95

26 280.0 2152.98

27 370.0 3823.96

28 500.0 7054.50

Table 3.1: Data sheet for recording the field data (Schlumberger array).

31

3.2 Survey procedure and data collection

The survey was conducted in order to delineate the groundwater potentials of some

communities within Ezza North Local Government Area of Ebonyi State and to

ascertain the possibility of drilling successful water boreholes in the area. There are

many different types of electrode arrays that can be used in resistivity survey

(Samouelian et al.,2008). The most commonly employed geometrical

configurations are the Wenner array and the Schlumberger array. Others include the

Lee partitioning method (Brown and Respher, 1972), dipole-dipole array and pole

dipole array. Each of the electrode arrangements has its own advantages and

disadvantages depending on the aim of the survey. The geometrical factor G,

associated with each array type can be evaluated from equation 2.2. In this work,

we employed the vertical electrical sounding technique using the Schlumberger

array.

3.2.1 The Schlumberger electrode configuration

Unlike the Wenner electrode configuration (Robinson and Coruh, 1988; Lowrie,

1997), the current electrodes are spaced much further apart than the potential

electrodes in the Schlumberger array as shown in fig 3.2, such that

and 2 2

b bAM BN a BM AN a .

This gives

2

or . 3 .14

a a

a b VG R

b I

The notations a = AB/2 and b = MN are frequently used in Schlumberger array.

The potential electrodes are kept fixed while the current electrodes are expanded

symmetrically about a central point. To maintain measurable potentials, it is

however necessary to increase the potential electrode spacing (MN = b) at very

large current electrode spacing. In addition to overcoming the large amount of work

required to move the electrodes, the Schlumberger configuration has other

advantages. The lateral resolution is better, because resistivity is sampled between

the relatively small spacing M and N. Also the potential and current electrodes can

be changed independently.

32

b

V

A

a a

L

2

b

2

b BA

M N

Fig. 2.10b: The Schlumberger array

b

V

A

a a

L

2

b

2

b BA

M N

Fig. 2.10b: The Schlumberger arrayFigure3.2:

33

Moreover, in Schlumberger arrangement, the speed of operation is increased and

errors due to surface inhomogeneity between the potential electrodes are easily

detected.

3.2.2 Data collection

The period for the investigation was chosen between May and July when the

ground was considerably moist. This ensured good current conduction between the

earth and the electrodes. Most of the measurements were taken along

approximately straight roads, where the electrodes were moved in a straight line.

The data collection was performed by a four-man crew, which includes the research

reporter. Two persons were positioned at the centre of the spread. One of them, (the

instrument controller), is responsible for adjusting, reading and recording the data

from the terrameter. He also communicated to the other men when to take the

necessary steps of the observational procedures. He monitors, through the

terrameter display, when electrical contacts were poorly established. The

OHMEGA terrameter usually displays negative resistance when signal current to

the ground is insufficient. At such stages (i.e. at very long electrode spread length),

the instrument observer adjusts to higher current signals.

The second person at the centre of the spread adjusts the potential electrodes when

necessary. This was usually done during looping. At looping stage different

resistance readings were taken at the same current electrode separation. The

essence of looping is to permit the detection of near surface inhomogeneities. The

second person at the spread centre also establishes communication contact between

the instrument controller and the two rear-men especially when they are very far

from spread centre. The rear-men are responsible for measuring the current

electrode spacing, moving and driving the current electrodes into the ground.

Mobile phones were used for communication at very large electrode spacings.

Figure 3.3 shows some of the crew members during data collection at one of the

VES sounding points.

3.3 Practical limitations and precautions

In order to obtain good results, accounts were taken of some practical limitations to

the geoelectric surveys.

34

Fig. 3.3: Some of the crew members during the field work

35

One of the major problems encountered during the field work was limited space for

electrodes layout. In many of the sounding locations, traverses were carried out

along approximately straight roads to have more access to enough electrode length

space. This notwithstanding, in some areas we were unable to reach up to our

desired spread length of about 1000m. We tried as much as possible to avoid

locating the centre of spread at positions where buildings, farmland and other

structures could limit the space for the field work.

As the presence of buried pipelines, cables and other metallic conductors could

constitute electrical noise to the field data, none of the sounding points was located

in the vicinity of such conductors. Although it is generally necessary to carry out

the electrical resistivity work when the ground is relatively moist, the survey work

was not carried out on the days when there were heavy down pour. Water logged

soil may result to enormously high conduction near the ground surface.

To reduce the effect of topography on the resistivity work, the investigations were

done in areas where there are slight or no undulations. Rugged topographies were

avoided. This is because there is no topography correction in resistivity surveys as

we have in seismic exploration (Burger, 1992). Well-insulated and light weighted

wires of very low resistance were used. Such wires ensure high quality insulation

since leakage between the current circuit and the measuring circuit is one of the

primary sources of errors in resistivity measurements (Keller and Frischknecht,

1966). Low resistant wires are used because high resistance, especially in the wires

connecting the potential electrodes may significantly affect the measured

resistance. Finally, a modern version of the resistivity survey equipment, the signal

averaging system (SAS 1000), was used. This instrument can discriminate against

low frequency electrical noise due to natural origin. The system also makes series

of automatically repeated measurements and displays the average value.

A total of twelve sounding profiles were executed within the selected communities.

The data from these profiles are shown in tables 3.2 to 3.13. In each case, the

values of the apparent resistivity were computed using the pre-calculated values of

the geometrical factor, G. The observed resistance, R and the calculated values of

the apparent resistivities are shown on the fifth and the sixth columns respectively

as shown in table 3.1. The approximate standard error in the calculated apparent

resistivities was within the limit of 22.6Ωm.

36

Table 3.2 Data for VES 1 (Adiagu-Oguji Nwudor)

S/N ( )2

ABm

2

M Nm G R

am

1 1.5

0.50

6.28 185.2000 1164

2 2.0 11.78 44.4000 523

3 2.5 18.85 22.6000 425

4 3.5 37.70 7.6900 290

5 4.5 62.80 3.3200 209

6 6.0 112.32 1.2290 138

7 8.0 200.00 0.5190 103

8 10.0 313.00 0.2380 75

9 15.0 706.00 0.0817 58

10 10.0

3.50

39.39 2.7800 110

11 15.0 95.49 0.7380 70

12 20.0 174.04 0.3660 64

13 25.0 275.03 0.2230 61

14 35.0 544.35 0.1110 60

15 45.0 903.44 0.0702 63

16 55.0 1352.29 0.0476 64

17 45.0

14.0

205.24 0.2180 45

18 55.0 317.45 0.0542 17

19 75.0 609.21 0.1282 78

20 95.0 990.74 0.1328 131

21 125.0 1731.35 0.0709 123

22 165.0 3033.04 0.0467 142

23 215.0 5165.11 0.0946 65

24 165.0

55.0

691.24 0.0945 117

25 215.0 1233.95 0.0283 61

26 280.0 2152.98 0.01128 43

27 370.0 3823.96 0.00371 26

37

Table 3.3 Data for VES 2 (Ekka Town Hall, Azugwu)

S/N ( )2

ABm

2

M Nm G R

am

1 1.5

0.50

6.28 102.9000 647

2 2.0 11.78 48.8000 575

3 2.5 18.85 26.3000 495

4 3.5 37.70 10.1000 381

5 4.5 62.80 4.9300 310

6 6.0 112.32 2.1900 246

7 8.0 200.00 0.9860 198

8 10.0 313.00 0.5090 160

9 15.0 706.00 1.2480 881

10 10.0

3.50

39.39 3.7700 148

11 15.0 95.49 1.5350 147

12 20.0 174.04 0.3580 62

13 25.0 275.03 0.3190 88

14 35.0 544.35 0.1296 71

15 45.0 903.44 0.1288 116

16 55.0 1352.29 0.3190 43

17 45.0

14.0

205.24 0.2230 46

18 55.0 317.45 0.1307 42

19 75.0 609.21 0.0701 43

20 95.0 990.74 0.3880 38

21 125.0 1731.35 0.0142 25

22 165.0 3033.04 0.0010 3

23 215.0 5165.11 0.0020 1049

24 165.0

55.0

691.24 0.0452 31

25 215.0 1233.95 0.0227 28

26 280.0 2152.98 0.0128 28

27 370.0 3823.96 0.0055 21

28 500.0 7054.50 0.0085 60

38

Table 3.4 Data for VES 3 (Nkomoro-Omuzor Ogbo-Ojiovu)

S/N ( )2

ABm

2

M Nm G R

am

1 1.5

0.50

6.28 101.3000 636

2 2.0 11.78 44.5000 524

3 2.5 18.85 23.0000 434

4 3.5 37.70 7.7700 293

5 4.5 62.80 2.8500 179

6 6.0 112.32 1.0060 113

7 8.0 200.00 0.3220 65

8 10.0 313.00 0.1215 38

9 15.0 706.00 0.0754 53

10 10.0

3.50

39.39 1.4940 59

11 15.0 95.49 0.4620 44

12 20.0 174.04 0.2380 42

13 25.0 275.03 0.1484 41

14 35.0 544.35 0.0819 45

15 45.0 903.44 1.7060 1613

16 55.0 1352.29 0.5340 722

17 45.0

14.0

205.24 0.2110 43

18 55.0 317.45 0.1398 44

19 75.0 609.21 0.0523 32

20 95.0 990.74 0.1466 145

21 125.0 1731.35 0.0263 46

22 165.0 3033.04 0.0029 9

23 215.0 5165.11 0.0383 198

24 165.0

55.0

691.24 0.6760 467

25 215.0 1233.95 1.8310 2260

26 280.0 2152.98 1.8250 3930

27 370.0 3823.96 0.3930 1503

28 500.0 7054.50 0.6450 4550

39

Table 3.5 Data for VES 4 (Ndiegu-Ogboji-Ukwu Akpara)

S/N ( )2

ABm

2

M Nm G R

am

1 1.5

0.50

6.28 13.4000 84

2 2.0 11.78 7.2100 85

3 2.5 18.85 4.0300 76

4 3.5 37.70 1.5730 59

5 4.5 62.80 0.8120 51

6 6.0 112.32 0.3950 44

7 8.0 200.00 0.2260 45

8 10.0 313.00 0.1249 39

9 15.0 706.00 0.0589 41

10 10.0

3.50

39.39 1.2190 48

11 15.0 95.49 0.5400 52

12 20.0 174.04 0.3140 55

13 25.0 275.03 0.2150 59

14 35.0 544.35 0.1295 71

15 45.0 903.44 0.0060 78

16 55.0 1352.29 0.0619 84

17 45.0

14.0

205.24 0.0511 11

18 55.0 317.45 0.0880 28

19 75.0 609.21 0.6610 403

20 95.0 990.74 0.2140 212

21 125.0 1731.35 0.0240 35

22 165.0 3033.04 0.0136 41

23 215.0 5165.11 0.0631 326

24 165.0

55.0

691.24 0.1252 87

25 215.0 1233.95 0.6350 78

26 280.0 2152.98 0.0382 82

27 370.0 3823.96 0.0157 60

40

Table 3.6 Data for VES 5 (Umudiegu-Ohaike)

S/N ( )2

ABm

2

M Nm G R

am

1 1.5

0.50

6.28 92.3000 580

2 2.0 11.78 41.0000 483

3 2.5 18.85 22.7000 427

4 3.5 37.70 8.2400 311

5 4.5 62.80 3.5800 225

6 6.0 112.32 1.3110 147

7 8.0 200.00 0.4590 92

8 10.0 313.00 0.2360 74

9 15.0 706.00 0.0941 66

10 10.0

3.50

39.39 1.9380 76

11 15.0 95.49 0.6380 61

12 20.0 174.04 0.3110 58

13 25.0 275.03 0.2010 55

14 35.0 544.35 0.0994 54

15 45.0 903.44 0.0601 54

16 55.0 1352.29 0.0416 56

17 45.0

14.0

205.24 0.2300 47

18 55.0 317.45 0.0603 51

19 75.0 609.21 0.0925 56

20 95.0 990.74 0.0611 61

21 125.0 1731.35 0.0370 64

22 165.0 3033.04 0.0192 58

23 215.0 5165.11 0.0097 50

24 165.0

55.0

691.24 0.0796 55

25 280.0 2150.00 0.0129 28

41

Table 3.7 Data for VES 6 (Udenyi-Azuakparata)

S/N ( )2

ABm

2

M Nm G R

am

1 1.5

0.50

6.28 161.900 1017

2 2.0 11.78 82.0000 966

3 2.5 18.85 46.0000 883

4 3.5 37.70 25.1000 944

5 4.5 62.80 10.3300 649

6 6.0 112.32 4.1500 466

7 8.0 200.00 1.3200 264

8 10.0 313.00 0.5090 160

9 15.0 706.00 0.1092 77

10 10.0

3.50

39.39 4.5400 179

11 15.0 95.49 0.7280 70

12 20.0 174.04 0.3270 57

13 25.0 275.03 0.2040 56

14 35.0 544.35 0.0995 54

15 45.0 903.44 0.5940 54

16 55.0 1352.29 0.0428 58

17 45.0

14.0

205.24 0.2980 61

18 55.0 317.45 0.2070 66

19 75.0 609.21 0.1201 73

20 95.0 990.74 0.0778 77

21 125.0 1731.35 0.0469 81

22 165.0 3033.04 0.0285 86

23 215.0 5165.11 0.0158 82

24 165.0

55.0

691.24 0.1150 80

25 215.0 1233.95 0.0683 84

26 280.0 2152.98 0.0441 95

27 370.0 3823.96 0.2270 87

42

Table 3.8 Data for VES 7 (Inyere-Ngangbo Nwakpa Umobi)

S/N ( )2

ABm

2

M Nm G R

am

1 1.5

0.50

6.28 27.2000 171

2 2.0 11.78 14.7100 173

3 2.5 18.85 8.6400 163

4 3.5 37.70 4.0700 153

5 4.5 62.80 2.1500 135

6 6.0 112.32 1.0250 115

7 8.0 200.00 0.4530 91

8 10.0 313.00 0.2520 79

9 15.0 706.00 0.1090 77

10 10.0

3.50

39.39 1.3150 52

11 15.0 95.49 1.9160 183

12 20.0 174.04 2.5100 437

13 25.0 275.03 3.3200 914

14 35.0 544.35 1.5330 834

15 45.0 903.44 1.4260 1288

16 55.0 1352.29 0.0638 86

17 45.0

14.0

205.24 0.3770 77

18 55.0 317.45 0.2430 77

19 75.0 609.21 0.1274 78

20 95.0 990.74 0.0760 75

21 125.0 1731.35 0.0420 73

22 165.0 3033.04 0.0231 70

23 215.0 5165.11 0.0085 44

24 165.0

55.0

691.24 0.1129 78

25 215.0 1233.95 0.0513 63

26 280.0 2152.98 0.0275 59

27 370.0 3823.96 0.0139 53

43

Table 3.9 Data for VES 8 (Ogbuji – Eguo-Ugwu)

S/N ( )2

ABm

2

M Nm G R

am

1 1.5

0.50

6.28 152.3000 957

2 2.0 11.78 97.4000 1147

3 2.5 18.85 66.2000 1247

4 3.5 37.70 37.3000 1048

5 4.5 62.80 24.8000 1560

6 6.0 112.32 11.4000 1281

7 8.0 200.00 4.5300 908

8 10.0 313.00 1.7680 554

9 15.0 706.00 0.1791 127

10 10.0

3.50

39.39 1.3040 51

11 15.0 95.49 0.8890 85

12 20.0 174.04 0.6750 117

13 25.0 275.03 1.8890 520

14 35.0 544.35 0.3960 215

15 45.0 903.44 0.8940 808

16 55.0 1352.29 0.2780 375

17 45.0

14.0

205.24 0.2670 55

18 55.0 317.45 0.1618 51

19 75.0 609.21 0.0798 49

20 95.0 990.74 0.0577 57

21 125.0 1731.35 0.0267 46

22 165.0 3033.04 0.0158 48

23 215.0 5165.11 0.0075 39

24 165.0

55.0

691.24 0.0859 60

25 215.0 1233.95 0.0470 58

26 280.0 2152.98 0.0203 44

27 370.0 3823.96 0.0094 36

44

Table 3.10 Data for VES 9 (Ohaccara-Ndiegu Ohaccara)

S/N ( )2

ABm

2

M Nm G R

am

1 1.5

0.50

6.28 60.8000 382

2 2.0 11.78 26.9000 317

3 2.5 18.85 12.4500 235

4 3.5 37.70 4.5500 171

5 4.5 62.80 2.3300 146

6 6.0 112.32 1.1240 126

7 8.0 200.00 0.5790 116

8 10.0 313.00 0.3150 99

9 15.0 706.00 0.1178 83

10 10.0

3.50

39.39 2.3700 93

11 15.0 95.49 0.7690 74

12 20.0 174.04 0.3630 63

13 25.0 275.03 0.2120 58

14 35.0 544.35 0.0962 52

15 45.0 903.44 0.0574 52

16 55.0 1352.29 0.0391 53

17 45.0

14.0

205.24 0.2490 51

18 55.0 317.45 0.1634 52

19 75.0 609.21 0.1028 63

20 95.0 990.74 0.0645 64

21 125.0 1731.35 0.0327 57

22 165.0 3033.04 0.0163 49

23 215.0 5165.11 0.0099 51

24 165.0

55.0

691.24 0.3900 270

25 215.0 1233.95 1.3870 1711

26 280.0 2152.98 0.3530 759

27 370.0 3823.96 0.0973 372

45

Table 3.11 Data for VES 10 (Ndieagu Ekka-Onu Nwode Ndiegu Ekka)

S/N ( )2

ABm

2

M Nm G R

am

1 1.5

0.50

6.28 148.2000 931

2 2.0 11.78 76.5000 901

3 2.5 18.85 42.5000 801

4 3.5 37.70 16.6800 629

5 4.5 62.80 7.9600 500

6 6.0 112.32 3.0100 338

7 8.0 200.00 1.1820 237

8 10.0 313.00 0.4760 149

9 15.0 706.00 0.1308 92

10 10.0

3.50

39.39 4.1000 162

11 15.0 95.49 0.9250 88

12 20.0 174.04 0.4060 71

13 25.0 275.03 0.2430 67

14 35.0 544.35 0.1204 66

15 45.0 903.44 0.0703 63

16 55.0 1352.29 0.0467 64

17 45.0

14.0

205.24 0.3400 70

18 55.0 317.45 0.2210 70

19 75.0 609.21 0.1086 66

20 95.0 990.74 0.0621 62

21 125.0 1731.35 0.0341 59

22 165.0 3033.04 0.0196 60

23 215.0 5165.11 0.0100 52

24 165.0

55.0

691.24 0.0818 57

25 215.0 1233.95 0.0413 51

26 280.0 2152.98 0.0233 50

27 370.0 3823.96 0.0149 57

46

Table 3.12 Data for VES 11 (Ekka Integrated School, Ekka)

S/N ( )2

ABm

2

M Nm G R

am

1 1.5

0.50

6.28 101.9000 640

2 2.0 11.78 51.8000 610

3 2.5 18.85 33.1000 624

4 3.5 37.70 13.9300 525

5 4.5 62.80 6.9100 777

6 6.0 112.32 2.6600 534

7 8.0 200.00 0.9530 299

8 10.0 313.00 0.4210 297

9 15.0 706.00 4.7800 188

10 10.0

3.50

39.39 1.0900 104

11 15.0 95.49 0.4680 82

12 20.0 174.04 0.2710 75

13 25.0 275.03 0.1403 76

14 35.0 544.35 0.0984 89

15 45.0 903.44 0.5620 76

16 55.0 1352.29 0.3790 78

17 45.0

14.0

205.24 0.2490 79

18 55.0 317.45 0.1443 88

19 75.0 609.21 0.1029 102

20 95.0 990.74 0.0698 121

21 125.0 1731.35 0.0506 153

22 165.0 3033.04 0.0334 173

23 215.0 5165.11 0.2010 139

24 165.0

55.0

691.24 0.1080 133

25 215.0 1233.95 0.0637 137

26 280.0 2152.98 0.0368 141

27 370.0 3823.96 0.0127 90

47

Table 3.13 Data for VES 12 (Ohaugo Primary School, Ekka)

S/N ( )

2

ABm

2

M Nm G R

am

1 1.5 0.50

6.28 207.0000 1300

2 2.0 11.78 118.1000 1391

3 2.5 18.85 76.4000 1440

4 3.5 37.70 32.9000 1240

5 4.5 62.80 16.7500 1052

6 6.0 112.32 7.3100 821

7 8.0 200.00 3.0200 604

8 10.0 313.00 1.0018 319

9 15.0 706.00 0.3900 275

10 10.0 3.50

39.39 12.6100 497

11 15.0 95.49 3.2500 310

12 20.0 174.04 1.2920 225

13 25.0 275.03 0.6440 177

14 35.0 544.35 0.2770 151

15 45.0 903.44 0.1534 139

16 55.0 1352.29 0.1110 150

17 45.0 14.0 205.24 0.8400 172

18 55.0 317.45 0.4440 141

19 75.0 609.21 0.3380 206

20 95.0 990.74 0.1039 103

21 125.0 1731.35 0.1110 192

22 165.0 3033.04 0.4660 141

23 215.0 5165.11 0.0260 134

24 165.0 55.0 691.24 0.1827 126

25 215.0 1233.95 0.0976 120

26 280.0 2152.98 0.0912 196

27 370.0 3823.96 0.0692 264

48

CHAPTER FOUR

PROCESSING AND INTERPRETATION OF THE FIELD DATA

4.1 Data processing

Processing of the field data started in the field while the field work was still in

progress. The geometrical factors, G, for the electrode spacings were pre-calculated

and recorded on the data sheet. These were obtained using the expression

1 1 1 12 / .G

AM BM AN BN

For the Schlumberger array 2

and , hence .42

AB a ba b M N Gb

The

apparent resistivity for each electrode spacing was calculated by multiplying the

geometrical factor G by the resistance .V

RI

That is 2

or .4

a a

a b VG R

b I

The apparent resistivity was later plotted against half the electrode spacing, on log-

log graph sheet which has the same scale as the theoretical master curves used

during the interpretation by the method of partial curve matching.

4.2 Interpretation of the field data

A total of twelve vertical electrical sounding profiles were conducted. The

interpretations were done quantitatively and qualitatively. The interpretation of the

actual resistivities in terms of subsurface geology and groundwater conditions of

the study area, were carried out on the basis of supplementary geological

information from the area.

4.2.1 Partial curve matching

Although the recent development in computer technology had made the

interpretation of resistivity sounding data less cumbersome, it is advisable that

preliminary interpretations be made in the field so that sounding may be located in

the best areas to obtain good results, and so that poor results may be recognised

before much work has been done. This is usually achieved by partial (auxiliary)

curve matching.

49

In partial curve matching technique, the field data for the observed apparent

resistivity is plotted against the electrode spacing on a transparent double

logarithmic graph with same modulus as the master curves. The field curve is slid

on the master curves, with respective axes been kept parallel, until a match is

obtained with one of the theoretical curves. The value of 2

AB and a

coinciding

with the theoretical cross (that is the point where x – and y – axes is (1, 1) on the

master diagrams) represents the layer’s thickness h1 and resistivity 1

respectively.

The thicknesses h2, h3… and resistivities 2 3, , ... of the other layers are obtained

from the appropriate parameter belonging to the matching master curves (Parasnis,

1966). We note that master curves are computed assuming 1 1

1 and 1 .m h m

The theoretical master diagrams (Eric and Joachim, 1979 ) used in this work is

shown in figure 4.1.

50

Figure 4.1: Master curves of Schlumberger apparent resistivity for a two-layer-earth (Eric and

Joachim, 1979).

51

Figure 4.2: Auxiliary curves for (a) type A and type H (b) type K and type Q (Telford et al., 1990).

TYPE - A

TYPE - H

TYPE - K

TYPE - Q

52

The observed apparent resistivity of the field data was plotted against half the

electrode spacing, on a log-log graph of the same modules as the master diagrams.

The field curve was then transferred to a transparency. These curves are shown in

figures 4.3 to 4.14. Each was placed on the master diagrams and slid around it, with

respective axes being parallel, until one of the theoretical curves coincides with (or

is interpolated between two adjacent) master curves. The parameter, ρ1/ρ2 for this

curve is read (or estimated in most cases). The coordinates of the point where ρ1/ρ2

=1 and a/h1 =1 on the master sheet determined the values of resistivity ρ1 of the

first layer, and its thickness h1. The parameter, ρ1/ρ2, was then used to determine

the resistivity of the second layer 2

.

The data for VES 8 will now be used to explain how partial curve matching

technique was used in the manual interpretations. By rough inspection, the curve

must have at least five geoelectric layers. The second layer is more resistive than

the surface layer; the third is less resistive than the second. The resistivity of the

fourth layer is greater than that of the third but less than that of the fifth layer. The

procedure involved in this interpretation is described as follows.

Step 1: The apparent resistivity curve was plotted on a log-log graph of the same

modulus as the theoretical curves, and then transferred to a transparency.

Step 2: This curve was superimposed on the master curves and moved around, with

respective axes being parallel, until a reasonably long portion of the field

curve at shorter electrode spacing coincided with one of the master curves.

The resistivity parameter for this curve k1 was copied. In VES 8 k1 = ρ1/ρ2

= 4.

Step 3: The origin of the theoretical curves (i.e. the “theoretical cross”), T1 was

located and marked on the field curve. The coordinates of T1 on the field

curve gave the values of1 1 and h . In VES 8, ρ1 = 90Ωm and h1 = 1.0m;

ρ2 = k1ρ1 =360Ωm.

Step 4: The field curve was then placed on the auxiliary curves belonging to the

curve family under interpretation. In VES 8, this portion is type-K curve.

Keeping the theoretical cross T1 at the origin, the auxiliary curve for ρ1/ρ2

= 4 was traced (dashed line) on the field curve.

Step 5: The curve was then transferred to the master diagrams again and slid over it

as before, but this time, the origin (T1) was moved along the copied

53

auxiliary curve in step 4, until another reasonable long portion of the field

curve matches one of the master curves. The origin of the master curves

was now located and marked on the field curve as T2. The coordinates of T2

gave the lumped resistivity 2

and the lumped thickness 2

h

corresponding

to the first two layers and regarded as a single overburden fictitious upper

layer. The resistivity parameter ρ3/ρ*2 was recorded. In VES 8,

3*2 2 2

2

0.1, 210 , 4.1 .k m h m

The resistivity of the third

layer is calculated. 3 2 2

0.1 210 21 .k m

Step 6(a): The field curve was again transferred to the auxiliary curves. T2 is

placed at the origin of the auxiliary curve. The curve corresponding to K2

= ρ3/*ρ2 is copied (dashed line) on the field curve.

Step 6(b): Still on the auxiliary curves, the previous theoretical cross T1 was placed

at the origin. The relatively vertical (dashed) line coinciding with the last

theoretical cross (T2) was noted and recorded. This gave the thickness

ratio ν1 = h2/h1. In VES 8, ν1 = h2/h1 1.5, so h2 = ν1xh1 = 1.5x1.0 = 1.5m

Step 7: For the remaining segments of the field curve, step 5 to step 6b were

repeated to obtain subsequent values of i

and hi . Ti, i

and hi were

successively replaced by *

1 1,

i iT

and

*

1ih

respectively to obtain the

subsequent values of T, and .h The partial curve matching process

shows that VES 8 has five geoelectric layers with resistivities, 90, 360, 21,

504 and 47m from the surface layer and respective thicknesses of 1.0,

1.5, 7.8 and 77.0m The fifth is the infinite depth layer.

The same procedure was used in interpreting the other sounding data. The field

curves for the quantitative interpretations of sounding data are shown in figures 4.3

to 4.14. The various resistivities and thicknesses obtained during the interpretations

with partial curve matching techniques are shown in table 4.1.

54

Figure 4.3: The field curve used in the manual interpretation of VES 1 with the aid of the

master and the auxiliary diagrams

Figure 4.4: The field curve used in the manual interpretation of VES 2 with the aid of the

master and the auxiliary diagrams

55

Figure 4.5: The field curve used in the manual interpretation of VES 3 with the aid of the

master and the auxiliary diagrams

Figure 4.6: The field curve used in the manual interpretation of VES 4 with the aid

of the master and the auxiliary diagrams

56

Figure 4.7: The field curve used in the manual interpretation of VES 5 with the aid of the

master and the auxiliary diagrams

Figure 4.8: The field curve used in the manual interpretation of VES 6 with the aid of the

master and the auxiliary diagrams

57

Figure 4.9: The field curve used in the manual interpretation of VES 7 with the aid of the

master and the auxiliary diagrams

Figure 4.10: The field curve used in the manual interpretation of VES 8 with the aid of

the master and the auxiliary diagrams

T1

T2

T3

T4

58

Figure 4.11: The field curve used in the manual interpretation of VES 9 with the aid of

the master and the auxiliary diagrams

Figure 4.12: The field curve used in the manual interpretation of VES 10 with the aid of

the master and the auxiliary diagrams

59

Figure 4.13: The field curve used in the manual interpretation of VES 11 with the

aid of the master and the auxiliary diagrams

Figure 4.14: The field curve used in the manual interpretation of VES 12 with the

aid of the master and the auxiliary diagrams

60

Table 4.1: Layer parameters obtained from the interpretations using partial curve

matching technique.

NO Geoelectric

layer

Resistivity

m Thickness (m)

Cumulative depth

(m)

VES 1

1 710 13 1.3

2 107 3.0 4.3

3 51 34.1 38.4

4 378 34.0 72.4

5 20

VES 2

1 750 1.2 1.2

2 225 5.4 6.6

3 48 30.1 36.7

4 24 46.4 83.0

5 3

VES 3

1 545 1.7 1.7

2 38 8.5 10.2

3 19 49.6 59.8

4 229 25.0 84.8

5 720

VES 4

1 100 1.4 1.4

2 30 1.8 3.2

3 33 11.5 14.7

4 95 214.7 229.4

5 9

61

Table 4.1: Layer parameters obtained from the interpretations using partial curve

matching technique (continued)

NO Geoelectric

layer

Resistivity

m

Thickness h

(m)

Cumulative

depth Z (m)

VES 5

1 680 1.2 1.2

2 340 1.7 2.9

3 162 3.0 5.9

4 23 71.3 77.2

5 156 35.5 112.7

6 7

VES 6

1 1020 1.3 1.3

2 714 3.1 4.4

3 56 4.3 8.7

4 50 48.4 57.1

5 128

VES 7

1 195 1.2 1.2

2 117 4.1 5.3

3 41 8.8 14.1

4 93 63.9 78.0

5 51

VES 8

1 90 1.0 1.0

2 360 1.5 2.5

3 21 7.8 103

4 504. 77.0 87.5

5 47

62

Table 4.1: Layer parameters obtained from the interpretations using partial curve

matching technique (continued)

S/NO Geoelectric

layer

Resistivity

m Thickness (m) Cumulative

depth (m)

VES 9

1 480 1.2 1.2

2 48 0.2 1.4

3 104 7.8 9.3

4 35 35.2 14.5

5 120 52.5 97.0

6 8

VES 10

1 1000 1.1 1.1

2 100 2.2 3.3

3 64 84.7 84.7

4 26

VES 11

1 640 1.1 1.1

2 512 5.5 6.6

3 3 6.2 12.8

4 12 55.2 68.0

5 576 5.4 73.4

6 33

VES 12

1 5800 1.0 1.0

2 1740 2.0 3.0

3 285 9.8 12.8

4 90 66.0 78.8

5 200

63

4.2.2 Computer-based interactive modelling

Considering the large number of parameters required in the interpretation of field

data with several horizontal layers, computer programmes have been designed for

easier and more efficient results. In the computer based interacting modelling, the

field data is input into the computer. The computer-theoretically-calculated curves

are modified by trial and error until a very close match is attained between the

calculated and the observed resistivity curves (Koefoed, 1979). The computer

displays the resistivities and layer thicknesses of the model, which was adjusted to

approximate or fit the field observations.

There are several types of computer software applications that can be used in the

interpretation of VES data. These include DCSCHLUM, RESOUND, OFFIX,

IP12WIN, APPLET and so on. The programme used in the interpretation of the

present work is RESOUND. In RESOUND, the raw data, namely the current and

the potential electrode spacing respectively and the observed resistance are input

into the system. The software does automatic computations for the values of the

geometrical factor (G) and the apparent resistivity a for each electrode spacing.

The resistivity curve is automatically displayed. Then series of iteration adjustment

processes are made. The computer theoretical model is automatically modified to

improve the fit with the field data.

One of the major problems associated with computer interpretation is that it

produces a result of unmanageable number of multiple layers where a few number

of layers are expected. This is because any slight change in resistivity is regarded

by the computer as due to additional geologic layer. An ordinary four-layer curve

might be interpreted as more than eight-layers in computer interpretation. To obtain

a reasonable practicable number of layers, the resistivities and thicknesses are

averaged. This constitutes another drawback since it requires an expert in this field

who also knows the geology of the area under study. Therefore, the results obtained

by different interpreters are bound to differ to some extent.

The curves obtained with the computer interpretations for the various vertical

electrical sounding points are as shown in figures 4.15. Table 4.2 shows the

corresponding results of these interpretations.

64

VES 1

VES 2

Figure 4.15: The field curves models for the computer-based interpretations. The

crosses represent the field data points.

65

VES 3

VES4

Figure 4.15: The field curves models for the computer based interpretations

(continued).

66

VES 5

VES 6

Figure 4.15: The field curves models for the computer based interpretations

(continued).

67

VES 7

VES 8

Figure 4.15: The field curves models for the computer based interpretations

(continued).

68

VES 9

VES 10

Figure 4.15: The field curves models for the computer based interpretations

(continued).

69

VES 11

VES 12

Figure 4.15: The field curves models for the computer based interpretations

(continued).

70

Table 4.2: Geoelectric interpretations of VES data from various profiles using

computer interactive programme.

Geoelectric layer

(GL)

Resistivity

(Ωm)

Thickness

(m)

Cumulative

thickness (m)

VES 1 (Adiagu Oguji)

1 985 0.8 0.8

2 220 2.7 3.5

3 36 30.5 34.0

4 350 21.0 55.0

5 480 50.0 105.0

6 15 - -

VES 2 (Ekka Town Hall)

1 430 1.0 1.0

2 288 1.0 2.0

3 88 14.0 16.0

4 14.5 44.0 60.0

5 4 48.0 108.0

6 9 - -

VES 3 (Nkomoro – Omuzor)

1 385 0.8 0.8

2 250 1.2 2.0

3 18 18.0 20.0

4 10 12.0 32.0

5 458 - -

6

71

Table 4.2: Geoelectric interpretations of VES data from various profiles using computer

interactive programme (continued).

Geoelectric layer

(GL)

Resistivity

(Ωm)

Thickness

(m)

Cumulative

thickness (m)

VES 4 (Ndiegu-Ogboji)

1 55 0.8 0.8

2 30 1.2 2.0

3 19 22.0 24.0

4 325 41.0 65.0

5 285 - -

VES 5 (Umundiegu Ohaike)

1 702 0.8 0.8

2 360 1.7 2.5

3 49 12.5 15.0

4 51 31.0 46.0

5 122 64.0 110.0

6 8 - -

VES 6 (Udenyi Azuakparata)

1 1008 0.8 0.8

2 1385 1.7 2.5

3 54 17.5 20.0

4 65 40.0 60.0

5 92 49.0 109.0

6 98 - -

72

Table 4.2: Geoelectric interpretations of VES data from various profiles using computer

interactive programme (continued).

Geoelectric layer

(GL)

Resistivity

(Ωm)

Thickness

(m)

Cumulative

thickness (m)

VES 7 (Inyere-Ngangbo)

1 110 1.0 1.0

2 110 1.0 2.0

3 50 6.0 8.0

4 250 42.0 50.0

5 70 - -

VES 8 (Ogbuji-Eguo-Ugwu)

1 90 1.0 1.0

2 400 1.5 2.5

3 60 12.5 15.0

4 750 45.0 60.0

5 50 - -

VES 9 (Ohaccara-Ndiegu)

1 900 1.0 1.0

2 240 3.0 4.0

3 120 21.0 25.0

4 50 15.0 40.0

5 180 40.0 80.0

6 90 - -

73

Table 4.2: Geoelectric interpretations of VES data from various profiles using computer

interactive programme (continued).

Geoelectric layer

(GL)

Resistivity

(Ωm)

Thickness

(m)

Cumulative

thickness (m)

VES 10 (Ndiegu Ekka)

1 1100 2.0 2.0

2 170 6.0 8.0

3 50 17.0 25.0

4 65 70.0 95.0

5 40 - -

VES 11 (Ekka Integrated School)

1 360 0.9 0.9

2 185 0.6 1.5

3 950 1.5 3.0

4 42 2.0 5.0

5 65 15.0 20.0

6 120 55.0 75.0

7 652 57.0 132.0

8 150 - -

VES 12 (Ohaugo Primary School)

1 1452 0.9 0.9

2 3854 0.6 1.5

3 1350 1.5 3.0

4 225 7.0 10.0

5 150 10.0 20.0

6 115 55.0 75.0

7 110 55.0 130.0

8 458 - -

74

It can be observed that the computer interpretations show similar trends of

resistivity variation with depth as obtained with the partial curve matching. Most of

the curve-matched interpretations indicated five geoelectric layers. However, it

should not surprising to notice that the computer programme gave greater number

of geoelectric layers. Although five VES interpretations (VES 3, 4, 7, 8 and 10)

revealed five layers, six to eight geoelectric layers are evident in some of the VES

results. This may be attributed to the fact that the computer programme considers

slight variation in resistivity within the subsurface as due to change in different

geologic layers. Consequently, there may be an unusual large number of layers

which may not be practically obtainable. This can be clearly observed by

comparing the results of VES 11 and VES 12 which revealed eight layers each in

computer based interpretations but showing the presence of only six and five layers

respectively in curve matching method.

The results of the computer interactive modelling were referred to in the final

analysis of the sounding data. This is because the computer programme is believed

to be more efficient than the partial curve matching techniques. In the process of

the manual interpretations, several approximations were made while matching the

field curves with both the main and the auxiliary diagrams. In the first place, the

resistivity ratios, 2 1

on the master curves are approximated values. Similarly,

the thickness ratios, 2 1

h h on the auxiliary diagrams are also estimated values.

Some of the field data curves at times fall between two master curves. In such

cases, the interpreter further approximates the values of these ratios. Thus there are

no absolute values on the choice of the resistivity and the thickness ratios during

curve matching procedures. The overall consequence of these approximations, as

one expects, are results with low degree of accuracy as when compared with the

computer model interpretations. For these reasons, the manual partial curve

matching technique is usually employed as trial solutions that are optimized by the

computer interpretations.

75

4.3 Subsurface geoelectric sections of the vertical electrical soundings

The results of the twelve vertical electrical soundings of the selected locations

indicated between five to eight geoelectric layers. The thicknesses of the top layers

vary between 0.8m to 1.0m. These are usually the top lateritic overburden soil

observed during the field work. Most of the field curves show trend of initial

decrease and later increase in resistivities with depth to the sounding probes. The

initial decrease in resistivities could be as a result of increase in water saturation.

The lower values encountered before the rise in resistivities could be attributed to

the water saturated fractured shale and mudstone aquifers which are the major

water bearing rocks in the survey area. The geoelectric sections of the vertical

electrical soundings are shown in figure 4.16. The vertical electrical sounding

positions for VES 1, 3, 4, 5, 6, 9 and 10 were obtained at approximately along the

North-South direction of the Western region of the study area as can be seen in the

map (figure 1.2). The geologic section relating the inferred formations of VES 1,

VES 3, VES 4, VES 5, VES 6, VES 9 and VES 10 is depicted in figure 4.17. These

sections are based on the number of layers interpreted by the computer. The

borehole drilled by the Geological Survey of Nigeria (GSN BH), near Amuda and

about 6km from VES 1, is also shown in this cross section (beside the VES points).

76

m

Clay/shale

Dry

Shale

Top Sand Depth (m)

0.8m

Sand 220 3.5

Shale 36

34

55

350

985

480

Wet

Shale 15

105

VES1

1.0m

108

VES2

Top Sand

Z0

Sand 288

2.0

Shale 88

16

60

145

430

4

Wet

Shale 9

m

Clay/shale

Shale

Sand

Clay Shale

VES3

m

Clay/shale

Top Sand 0

0.8m

250 2.0

18

20

32

10

385m

458

Z(m)

Clay/shale

VES4

Top Sand

0

0.8m Sand 30

2

19

24

65

325

55m

285

m

Clay/shale

Clay/shale

Clay/shale

Figure 4.26 The lithological section of the vertical electrical soundings

Fig. 4.16: The lithological sections of the vertical electrical soundings deduced from the computer interactive interpretation.

77

VES5

0.8m

0 Top Sand

Clay/Shale 350 2.5

Clay/Shale 49 15

51

702m

122

46

Clay/Shale

Clay/Shale

110

8 Clay/Shale

Top Sand 0

0.8m

1385

2.5

54

20

65

1008m

92

60

109

98m

VES6

Sand

Clay/Shale

Clay/Shale

Clay/Shale

Clay/Shale

1.0m

Clay/Shale

Clay/Shale

Clay/Shale

Top Sand 0

110m 2.0

50

8

250

110m

70

50

Sand

VES7

Top Sand 0

400m 2.5

60m

15m

750m

90m

50m

60

Sand

Clay/Shale

Clay/Shale

Clay/Shale

VES8

1.0m

Fig. 4.16: The lithological section of the vertical electrical soundings deduced from the computer interactive interpretation.(continued)

78

Fig. 4.2 continued

2.0

25

Top Sand 0

240m 4.0

120

50

900m

180

40

Sand

Clay/Shale

Clay/Shale

Clay/Shale

1.0m

VES9

90

25

80

Clay/Shale

Top Sand 0

170

50

1100m

65

Clay/Shale

VES10

Shale

Shale

40 Shale

8

95

132

Top Sand 0

0.9

65

120

652

75

VES11

150

360m 185 950

42

20

1.5

3.0

5

Top Sand 0

0.9m

225

20

115

VES12

150

1452m 3854 1350

42

10

130

1.5

3

115

110

458

75

Fig. 4.16: The lithological section of the vertical electrical soundings deduced from the computer interactive interpretation.(continued)

79

surface level

Pepply laterite lithology

Shale with occasional sandy

streaks

Clayey shale with

sandstone band

Shale/sandstone

Alternation of shale and

sandstone

Fine sandstone with black

shale and limestone

Appreciable depth of shale

with thin alternating layers

of sandstone

Fig. 4.17: Geoelectric section relating VES 9, 10, 4, 5, 3, 6 and 1, obtained

along the North-South direction of the western region of the study area.

Depths are shown in m and resistivities in Ωm

80

4.4 Discussions

Although few hand dug wells were seen in the vicinity of some data collection points,

logged borehole data very close to the study area were not accessible during the field

work. This not withstanding, the results of the interpreted geophysical survey in most of

the investigated locations fairly correlated with the logged boreholes data from the

neighbouring communities. These boreholes were drilled by Andiki Construction

Company Ltd, under the Ebonyi State HDF/UNICEF Assisted Rural Borehole

Construction Project in 2005. The correlation is not surprising as both locations share

common geologic settings. The water bearing rocks in the survey area are predominantly

fractured shales located at depths between about 45m and 70m in agreement with some of

the logged boreholes in the neighbouring communities. However, few locations indicated

aquifers as deep as between 90m and 130m.

As explained earlier, the results of most of the geophysical survey show between five and

six geoelectric layers. The first layers are the top lateritic sand with average thickness of

about 1m. The intermediate layers are suggested to comprise of shales, sandstones and

mainly shales as reported by the literature on geology of the area (Agumanu, 1990; Umeji,

1985). It is suggested that the layers with relatively lower resistivity could be water

bearing formation. This is because the conductivity of the rocks increases with increasing

water saturation in porous rock strata.

At Adiagu-Oguji Nwudor, VES 1, the sixth layer with resisitivity of 15m at the depth of

about 105m could possibly be the water bearing rock. Although borehole about 34m deep

could yield a reasonable quantity of water, this may become unproductive at onset of dry

seasons. The water bearing rock observed at this depth could possibly be a perched

aquifer. Following a similar judgement the depth of the aquifer must be about 108m at

Ekka Town Hall (VES 2). At Nkomoro-Omuzor Ogbo-Ojiovu (VES 3), the decrease in

resistivity from 385m at the first layer to 10m at a depth of 32m before the fifth (the

infinite depth) layer with resistivity of 458m is an indication that the fourth layer which

has a thickness of about 12m must be water saturated. Hence a productive borehole is

recommended to be drilled to at least a depth of about 32m.

In VES 4 (Ndiegu Ogboji Ukwu Akpara), it could also be observed that there is an

increase in water saturation resulting to decrease in resistivity with depth just before the

fourth layer where the value sharply rose from 19m to 325m. The recommended

81

borehole depth at this location is about 24m. The thickness of the aquifer here is about

22m.

Following the results of geophysical sounding at Umundiegu Ohaike (VES 5), it can be

inferred that the water bearing rocks must be within the third and the fourth layers where

there are relatively lower resistivities of about 49m and 51m at the depths of 15m and

46m respectively. These might possibly be perched aquifers. At least, a borehole of about

46m deep is recommended here.

There was a sharp drop of resistivity values of VES 6 obtained at Udenyi Azuakparata

from 1008m to 54m at a depth of 20m before a gradual rise to 98m at the infinite

depth layer (sixth layer). The third and the fourth layers appear to be more saturated with

water with regard to their lower resistivities. Hence a borehole of about 60m is expected to

be drilled at this site for successful high quantity of water yield.

From the five geoelectric layers encountered at Inyere-Ngangbo Nwakpa Umuobi (VES

7), the last (infinite depth) layer with a lower resistivity of 70m below 50m is suggested

to be the water bearing formation at this location. The maximum electrode spacing at this

location may not have been enough to obtain the information at the base of the water

bearing formation. A bore hole of about 60m from the surface may give a reasonable

yield.

Similar judgement is also applied to VES 8 (Ogbuji-Eguo-Ugwu). Here the fifth (infinite

depth) layer with lower resistivity of 50Ωm below the depth of 60m appears to be more

saturated with water. It also appears that the depth of the sounding was not enough to

reach the aquifer. Consequently, it is suggested that a bore hole of not less than 70m deep

may give a reasonable yield.

In the result of the interpretation of the data obtained at Ohaccara-Ndiegu Ohaccara (VES

9), the resistivity decreased from 900m at the first layer to 50m at the fourth layer with

a depth of 40m from the surface before a slight rise in resistivity. Although borehole

drilled to this depth may produce a reasonable yield, there may be seasonal drop in water

yield as the main aquifer appears to be within the infinite depth layer.

The geophysical survey at Ndiegu Ekka (VES 10) revealed a decrease in resistivity with

depth from 1100m (at the first layer) to 40m (at the infinite depth layer). The depth to

the water bearing rock occurs at the fifth layer with a depth of about 95m. A successful

borehole is expected to be drilled to a depth of about 95m or more from the surface.

82

Borehole is not strongly recommended here. It is likely that the depth to the aquifer at this

location was not reached.

In VES 11, obtained at Ekka Integrated Primary School, Ekka, the lower resistivities of

the fourth and the fifth layers probably indicate the presence of water saturated rocks. The

depth of the aquifer recommended to be drilled here is estimated to about 20m. However,

this might be a perched aquifer which may not yield considerably at the onset of dry

seasons.

The resistivities of VES 12, obtained at Ohaugo Primary School, Ekka sharply dropped

from 1452m (at the first layer) to 110m (at the seventh layer) at a depth of 130m. The

water bearing formation here is possibly the seventh layer with the least resistivity. This is

a relatively deep water bearing rock.

Table 4.3 is a summary of the estimated depths of the water bearing rocks.

83

Table 4.3: Estimated depths of the water bearing rocks at the VES points

VES LOCATION G.L. (m) THICKNESS

(m)

DEPTH

FROM

SURFACE

(m)

REMARKS

1. Adiagu Oguji 3

6

36

15

31

infinity

34

>105

Perched aquifer: about 34m deep.

Main aquifer: from depth of about 105m.

2.

Ekka Town

Hall

5

6

4

9

48

infinity

108

>108 Main aquifer : from the depth of about 108m

3.

Nkomoro –

Omuzor 4 10 12 32

A shallow thin aquifer: about 32m deep.

(Borehole is not strongly recommended here)

4.

Ndiegu-Ogboji

Ukwu Akpara 3 19 22 24

A shallow aquifer.

(Borehole is not strongly recommended here)

5.

Umundiegu

Ohaike

3/4

6

49/51

8

44

infinity

46

>110

Perched aquifer: about 46m deep.

Main aquifer: from depth of about 110m.

6.

Udenyi

Azuakparata 3/4 54/65 58 60

Depth to aquifer: about 60m

7. Inyere-Ngangbo 5 70 infinity >50 Main aquifer: from depth of about 50m.

8.

Ogbuji-Eguo-

Ugwu 5 50 infinity >60 Main aquifer: from depth of about 60m.

9.

Ohaccara-

Ndiegu

4

6

50

90

15

infinity

40

>80

Perched aquifer: about 40m deep.

Main aquifer: from depth of 80m.

10. Ndiegu Ekka 5 40 infinity >95 Main aquifer: from depth of about 95m.

11.

Ekka Integrated

School 4/5 42/65 17 20

Shallow aquifer.

(Borehole is not strongly recommended here)

12.

Ohaugo

Primary School 7 110 55 >130 Relatively deep aquifer.

84

4.5 Conclusion

The resistivity survey for groundwater in some selected communities in Ezza North local

government Area of Ebonyi State has confirmed that the survey locations have good

groundwater potentials. The results from this work have contributed greatly in an

improvement of the existing knowledge on resistivity survey for groundwater not only that

hydrogeologists could locate definite sites for drilling boreholes, but also at a cheap or

reduced cost. In addition, the exploitation of groundwater at the proposed locations will

help reduce the existing seasonal water scarcity, long distance trekking in search of water

and overcrowding of few streams, rivers and ponds (which are prone to contaminations).

Hence if the results are fully utilized and employed, groundwater so drilled will help to

improve the health condition of the people in the area, as unclean water that causes some

diseases like guinea worm and other water borne diseases would be avoided.

4.6 Recommendations

In addition to drilling boreholes at the recommended sites, it is suggested that subsequent

drilling of boreholes should not be embarked upon without geophysical investigations.

Going by the scientific search for groundwater, the number of unproductive and

abandoned boreholes will be reduced to the barest minimum.

Furthermore, we recommend the use of the frequency domain electromagnetic method

(FEM) for groundwater survey. This method measures the apparent conductivity of the

subsurface from the ratio of the secondary to the primary electromagnetic fields. Unlike

the popular electrical resistivity survey method, it is a quick and easy method for

determining changes in thickness of weathered zones or alluvium. The method can as well

be used in basement rocks to help identify fractured zones (McDonald et al., 2002),

though it requires a very careful geological control.

Finally, precautionary measures should be taken in order to minimize the errors introduced

in the vertical resistivity work as a result of non-straight line spread, poor electrical

contact, erratic conductivities due to buried metallic objects and fences, and the errors due

to rugged topography. In addition, the interpretation of the field profiles should be done

with the assistance of a very experienced geologist.

85

REFERENCES

ABEM Instruments (2009). ABEM instructional manual for terrameter SAS 4000/SAS

1000. http://www.abem.se/files/upload/manual_terrameter.pdf

Adetola, B. A. and Igbedi, A. O. (2000). The use of electrical resistivity survey in

location of aquifers: A case study in Agbede South Western Nigeria. Journal of Nigerian

Association of Hydrogeologists, Vol. 11, Pp. 7 – 13.

Agumanu, A. E. (1990). The Abakiliki and the Ebonyi formations: Sub-divisions of the

Albian Asu River Group in the Southern Benue trough, Nigeria. Bulletin of the Geological

survey of Nigeria Pp. 195 – 206.

Alile, M. O, Jegede, S. I. and Ehigiator, O. M. (2008). Underground water exploration

using electrical resistivity method in Edo State, Nigeria. Asian Journal of Earth Sciences.

Vol 1 Pp38 -43.

Arshad, M, Cheema, J.M. and Ahmed S., (2007). Determination of Lithology and

Groundwater Quality Using Electrical Resistivity Survey. Int. J Agric. Biol. Vol. 9, No.1,

pp 143 - 146. http://www.fspublishers.org

BBC Sci/Tech News, (2000). Water arithmetic “doesn’t add up”- Report of the World

Commission on Water for 21st Century.

<www.news.bbc.co.uk/2/hi/science/nature/671800.stm>

Bose, R. N, Chattarjee, D. and Sen A. K. (1972). Electrical resistivity surveys for

groundwater in the Auragabad Subdivision, Gaya District, Bihir, India. Geoexploration,

Vol. 11, Issues 1-3, 1973, pp 171-181.

Brown, J. W. and Respher, R. C. (1972). Detection of rubble zones in oil shale by

electrical resistivity technique. U.S. Department of Interior, Bureau of Mine, Washington.

Burger, H.R. (1992). Exploration Geophysics of the shallow subsurface. Prentice Hall,

Inc, Eaglewood Chaff, New Jersey 07632.

Chernicoff and Whitney, (2002). An introduction to physical Geology 3rd

ed Houghton

Mifflin Company New York

Dhakate, R., Negi, B. C. and Singh, V. S. (2008). Electrical resistivity survey to

delineate groundwater potential zones in Granite Terrain, Nalgonda District, India. Asian

Journal of Water, Environmental and Pollution, Vol. 5, No. 1, 2008

Dobrin, M. B, (1976). Introduction to geophysical prospecting (3rd

edition). Mc.Graw

Hill Inc. New York, USA.

86

Eduvie, M. O. (2000). Groundwater assessment and development in Bima sandstone:

case study of Yola – Jimeta Area. Journal of Nigerian Association of Hydrogeologists,

Water Resource Vol 11, Pp 33-38

Emenike, E. A. (2001). Geophysical exploration for groundwater in sedimentary

environment: A case study from Nanka over Nanka Formation in Anambra Basin, South

Eastern Nigeria. Global Journal of Pure and Applied Science Vol. 7, No. 1. pp 1-12.

Eric, M. and Joachim, H. (1979). Three layer model curves for geoelectrical resistivity

measurements using Schlumnerger array. Herauigeben Von Bundesanstalt fiir

Geowissenchaften und Ruhstoffe und den Geologischen landesamtern in der Bundes

Republic Deutschland.

Ezema, P. O. (2005). Physics of the earth and atmosphere. Rejoint communication

services, Enugu.

Federal Survey Nigeria, (1960). Federal survey of Nigeria.

Keary, P. and Brooks, M. (1984). An introduction to geophysical exploration. Blackwell

Scientific Publication, England.

Keller, G. V. and Frischknecht, F. C. (1966). Electrical methods in geophysical

prospecting. Pergamon Press, Oxford England.

Koefoed, O. (1979). Methods in geosounding principles. Elsevier Scientific Publishing

Company, Netherlands.

Lowrie, W. (1997). Fundamentals of Geophysics. Cambridge University New York.

Malin, F. M. (1982). Rural water supply and health: The need for a new strategy.

Scandinavian Institute of African Studies Uppsala, Sweden.

McDonald, A. M., Davies, J. and Dochartagh, B. E. O., (2002). Simple methods for

assessing groundwater resources in low permeability areas of Africa. British Geological

Survey Commissioned Report, CR/01/168N.

McDougal, R. R, Abraham, J. D. and Bisdorf, R. J., (2003). Results of electrical

resistivity data collected near the town of Guernesey, Platte County, Wyoming. U. S.

Geological Survey Open File 2004-1095

Meyer de Stadelhofen, C., (1991). Application de la géophysique aux recherches d’eau.

Ed.Lavoisier, Paris.

Mohammed, S. A. and Lee, C. Y., (1985). A resistivity survey for groundwater in Perlis

using offset Wenner technique. Karst Water Resources. IAHS Publ. no. 161, 221-232.

87

Montgomery, C. W. (1990). Physical Geology second edition. Wm. C. Brown Publishers

United States of America.

Moonrey, J. and Wicander, R. (2005). Physical geology eploring the Earth. 5th ed.

Thomson Learning, Belmon U.S.A.

Obiakor, I. P. (1984) Resistivity survey for groundwater in Idemili and Anambra Local

Government Areas, Anambra State. An M.Sc thesis presented to the Department of

Physics and Astronomy, University of Nigeria Nsukka.

Okoronkwo, I. L. (2003). Guinea worm infestation: A case of Ezzagu community in

Ebonyi State. West African Journal of Nursing. University of Nigeria Nsukka Virtual

Library.

Orajaka, S. (1972). Saltwater resources of East Central State of Nigeria. Nigerian

Mining, Geological Metallurgical Society Vol. 7, Pp 35-41.

Parasnis, D. S. (1986), Principles of applied Geophysics 4th edition. Chapman and Hall,

New York.

Reyment, R.A. (1965) Aspect of Geology of Nigeria. University of Ibadan Press, Ibadan.

Robinson, E. S. and Coruh, C. (1988). Basic exploration geophysics. John-Wiley and

sons, New York.

Samouelian, A., Cousin, I., Tabbagh, A., Bruand, A. and Richard, G. (2005).

Electrical resistivity survey in soil science: a review. Soil and Tillage Research, Vol.83,

Issue 2, pp173-193

Selemo, A. O. I., Okeke, P.O. and Nwankwor, G.I., (1995). An appraisal of the

usefulness of vertical electrical sounding (VES) in groundwater exploration in Nigeria.

Water Resources, Vol. 6 No 1 and 2, Pp. 61 – 67.

Singh, K. K. K., Singh, K. A., Singh, K. B., and Sinha, A. (2006). 2D resistivity

imaging survey for siting water-supply tube wells in metamorphic terrains: A case study of

CMRI campus, Dhanbad, India. The Leading Edge, 25, 1458 - 1460

Singh, P. (2007). Engineering and general Geology for B.E. (Civil Mining, Metallurgy

Engineering), B.Sc. and A.M.I.E courses. S.K. Katara and Sons, Delhi.

Telford, W. M., Geldart, L. P. and Sherif R. E. (1990). Applied Geophysics. Cambridge

University Press, Cambridge.

Umeji, O.P., (1985) Subtidal shelf sedimentation. An example from Turonian Eze Aku

Formation in Nkalagu Area, South Eastern Nigeria. Nigeria Journal of Mining, Geology

and Metallurgical Society Vol. 22 Pp. 119 – 124.