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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
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)
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.
7
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.
8
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
16
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
17
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).
19
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.
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.
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.