resistivity survey for groundwater in and around …
TRANSCRIPT
RESISTIVITY SURVEY FOR GROUNDWATER IN AND AROUND
OBUKPA IN NSUKKA LOCAL GOVERNMENT AREA
BY
UGWUANYI, MAXIMUS C.
PG/M.Sc./06/41371
DEPARTMENT OF PHYSICS AND ASTRONOMY
FACULTY OF PHYSICAL SCIENCES
UNIVERSITY OF NIGERIA, NSUKKA
SUPERVISORS:
DR. J.U. CHUKWUDEBELU
DR. P.O. EZEMA
NOVEMBER, 2010
i
TITLE PAGE
RESISTIVITY SURVEY FOR GROUNDWATER IN AND AROUND
OBUKPA IN NSUKKA LOCAL GOVERNMENT AREA
BY
UGWUANYI, MAXIMUS C.
PG/M.Sc./06/41371
A THESIS PRESENTED TO THE DEPARTMENT OF PHYSICS AND
ASTRONOMY, FACULTY OF PHYSICAL SCIENCES, UNIVERSITY OF
NIGERIA, NSUKKA, IN PARTIAL FULFILLMENT FOR THE
REQUIREMENT FOR THE AWARD OF MASTER OF SCIENCE IN
GEOPHYSICS
SUPERVISORS: DR. J.U. CHUKWUDEBELU
DR. P.O. EZEMA
NOVEMBER, 2010
ii
CERTIFICATION
This is to certify that this project work was submitted and approved by the Department
of Physics and Astronomy in partial fulfillment for the requirement for the award of
Master of Science in Physics and Astronomy, University of Nigeria, Nsukka.
___________________________ _________________________
Dr. J.U. Chukwudebelu Dr. P.O. Ezema
(Project Supervisor) (Project Supervisor)
___________________________ _________________________
External Supervisor HOD, Department of
Physics and Astronomy
iii
DEDICATION
This project is dedicated to master, Nweze Collins C. and Ezeoha B.C
iv
ACKNOWLEDGEMENT
To God Almighty, I would forever remain grateful for His inspiration, guidance
and protection during the course of this onerous assignment.
Definitely, this piece of work would not have been possible without the
collective assistance, support, contributions, suggestions and criticisms from many
people. These people I would always be in their debt and are mentioned below. First
and foremost, I am highly indebted to my project supervisor Dr. J.U. Chukwudebelu for
his immense contribution and devotions towards the successful completion of this
project work. For his tolerance, fatherly advice, constructive criticisms and suggestions,
I remain profoundly grateful.
My gratitude also goes to my second supervisor Dr. P.O. Ezema of Physics and
Astronomy Department for providing me with software (Resist) and the platform to
learn using it for interpretation of VES data. He also made available master curves and
auxiliary charts which was used for partial curve pre-interpretation of the field data. He
guided me fatherly through some courses on solid earth geophysics which inspired and
motivated me as an undergraduate student of Geology/Physics to further in the same
field.
My special appreciation goes to the staff of the Department of Physics and
Astronomy and mostly my lecturers who have either directly or indirectly equipped me
for this task. Worthy of mention include the Head of Department, Prof. C.M.I. Okoye,
Prof. and Prof. (Mrs.) P.N. Okeke, Prof. A.A. Ubachukwu, Prof. Animalu, Prof. S. Pal,
Dr. R.N.C. Eze, Dr. E. Chukwude, Dr. Asogwa, Dr. Ezema and host of other academic
giants of the department.
v
I am equally grateful for the assistance of Geology Department and staff such as
Mr. Ugboaja (the geophysics technician), Mr. Oliver Eze. They provided me with
terrameter and assisted also in the field VES work.
I cannot forget to mention the staff of the National Geologic Survey Enugu
State, staff of Nnamdi Azikiwe Library, Nsukka and the staff of Enugu State Water
Board Nsukka Quarter for their assistance.
I am also in the debt of my friends, colleagues and course mates; Igatta
Nnaemeka, Eze Kenneth, Ugwu Kenneth, Ezeugwu Sabastine, Nneji Gabriel, Abbah
Cosmas, Mete Ngozi, Onah Corstecia, Odoh Okechukwu J., the Ezeohas, Mr.
Chukwunonyerem George, Uchenna Kenneth and others innumerable to mention who
have contributed and helped me in one way or the other through this study.
Finally, I am grateful to my parents and relations for their financial, moral and
physical support. May the Almighty God in Jesus name bless you all, Amen.
Chukwuma Maximus.
vi
TABLE OF CONTENTS
Title Page i
Certification ii
Dedication iii
Acknowledgment iv
Table of Contents vi
List of Figures viii
List of Tables x
Abstract xi
CHAPTER ONE: INTRODUCTION 1
1.1 Groundwater 1
1.2 Statement of problems 2
1.3 Purpose of the study 3
1.4 Location of the study area/accessibility 3
CHAPTER TWO: LITERATURE REVIEW 5
2.1 Regional geologic setting of the study area 5
2.2 Local geology 6
2.3 Electrical properties of rock materials 8
2.4 Principle of resistivity surveying 9
2.5 Current flow line distribution and current density 12
2.6 Development of potential field within the subsurface 16
2.6.1 The potential of a point current source at depth and on the
surface of a homogenous medium 17
2.6.2 The potential of two current electrodes at a finite separation on the
surface 18
2.7 Current penetration in a homogenous isotropic earth 22
2.8 Electrode configurations 24
vii
2.8.1 Wenner array 26
2.8.2 Lee partitioning array 26
2.8.3 Schlumberger configuration 26
2.9.1 The single boundary problem-optical analogue treatment 27
2.9.2 Potential function-single overburden problem 31
2.10 Interpretation methods 44
2.10.1 Complete curve matching 47
2.10.2 Partial curve matching 48
2.10.3 Computer interactive modeling method 51
CHAPTER THREE: INSTRUMENTATION/FIELD WORK 52
3.1 Instruments used for the survey 52
3.2 Reconnaissance study 52
3.3 Field work 54
3.4 Practical limitations, sources of error and precautions 69
CHAPTER FOUR: DATA PROCESSING AND INTERPRETATION 70
4.1 Introduction 70
4.2 Data processing 71
4.3 Analysis of VES curves 71
4.4 Quantitative data interpretation 73
4.4.1 Results of quantitative interpretation 76
4.4.2 Deductions from the results 92
4.5 Discussion of the result in terms of subsurface conditions 94
4.6 Correlation with geology/borehole result 97
4.7 Conclusion 105
4.8 Recommendation 106
References 108
viii
LIST OF FIGURES
Fig. 1.1: Map showing the location of the study area and the major access roads 4
Fig. 2.1: Current flow lines at a boundary separating stratified layer of earth of
different resistivity 14
Fig.2.2: Current flow and current density distribution at a medium with two zones
of contrasting resistivity 15
Fig. 2.3a: Buried point source of current in a homogenous ground 19
Fig. 2.3b: Point source of current at the surface of a homogenous medium 19
Fig. 2.4: A generalized four electrode array 21
Fig. 2.5a: Geometry for determining the current distribution in a uniform ground
below two electrodes 25
Fig. 2.5b: Fraction of current flowing above a depth z across the mid-plane between
current electrode with spacing 25
Fig. 2.6a: Wenner electrode configuration 28
Fig. 2.6b: Lee partitioning electrode configuration 28
Fig. 2.6c: Schlumberger electrode configuration 28
Fig. 2.7: Single boundary model for the determination of reflection and transmission
coefficient 30
Fig. 2.8a: Single over burden case 32
Fig. 2.8b: Primary source reflected from upper and lower planes 32
Fig. 2.9: Even number of multiple reflections with first reflection off the lower plane 35
Fig. 2.10a: Odd number of multiple reflections (three reflections) with first reflection
off the lower plane 36
Fig. 2.10b: Odd number of multiple reflections (five reflections) with first reflection
off the lower plane
36
ix
Fig. 2.11: Master curve of Schlumberger apparent resistivity for two layer earth
models 43
Fig. 2.12: The four standard curve shapes in vertical electrical sounding 46
Fig. 2.13a: Auxiliary point chart for type A and type H VES curve 50
Fig. 2.13b: Auxiliary point chart for type K and type Q VES curve 50
Fig. 3.1: The ABEM terrameter (SAS300) used for the field VES measurement 53
Fig. 3.2: Map of the area of study with inset of VES position location of boreholes
and their depth 55
Fig. 3.3: The log of Obukpa borehole (No.32) showing the lithology and depth of
penetration 56
Fig. 3.4: Schematic of the startup electrode arrangement for each VES profile 58
Fig. 4.1: Plot of a
against AS/2 used for dynamic of VES IV 72
Fig. 4.2: The resulting earth model from computer interpretation of VES I 77
Fig. 4.3: The resulting models from computer interpretation of VES II, III, IV, V,
VI, VII, VIII and IX 88-89
Fig. 4.4: Correlation of Obukpa borehole log with geoelectric resistivity data of VES V 99
Fig. 4.5: The entire geoelectric sections 100
Fig. 4.6: Correlation of VES III and IV with B.H.I 102
Fig. 4.7: Correlation of the entire geoelectric section with B.H.I. 103
Fig. 4.8: Geoelectric section of Ibagwa Road 104
x
LIST OF TABLES
Table 2.1: Stratigraphic successions in Anambra basin showing the lithology of
Nsukka formation overlying the Ajali formation 7
Table 2.2a: Bulk resistivity of some rock types 10
Table 2.2b: Resistivity of rock as a function of percentage water content 10
Table 3.1: Some of the existing boreholes and their depth of penetration 54
Table 3.2: The table used in the field for recording measurement readings 59
Table 3.2 – 3.11: The field data for different VES Profiles 60-68
Table 4.1: Result of first stage of computer interpretation of VES profile I data
(unadjusted output) 75
Table 4.2: Adjusted output of computer interpretation of VES profile I data 77
Table 4.3: The result of interpretation of geoelectric VES data of profile I 77
Table 4.4 (a-h): Unadjusted output of the first stage of computer interpretation
of other VES data (VES II – VES IX) 78-85
Table 4.5: The adjusted output values of computer based interpretation of other
VES profile (VES II – VES IX) 86-87
Table 4.6: The result of computer interpretation of geoelectric VES data of other
profiles 9VES II – VES IX) 90-91
Table 4.7: Estimated depths of the water bearing rocks at the VES points 95
xi
ABSTRACT
Geoelectrical measurements using the vertical electrical sounding (VES) method were
conducted in Obukpa, parts of Alor-Uno, parts of Ibagwa-Aka and environs in order to
determine quantitatively the depth to static water table, stable aquifer thickness and the
lithology of ground water host rock/sediments within the area. A signal averaging
system resistivity instrument (SAS 300) was used in the field to carry out the
measurements. Twelve (VES) profiles were run during the survey out of which four
traverses were commuted to one traverse during preprocessing. The generated survey
data for nine traverses were then subjected to basic processing treatment, curve analysis
and finally quantitatively interpreted using a computer interactive program
(RESOUND). Analysis of the VES curves was in terms of multiple horizontal layers.
Profiles V, VI, VII, VIII and IX show an ascending A curve and their variants. Profiles
II III and IV presented 4 layer KA curves while profile I generated a H curve type. The
results of the interpretation indicate the static water table to occur at varying depths of
about 215m, 220m, 222m, 220m, 215m, 218m, 212, 218m and 155m for the entire
profiles (VES I - VES IX) respectively. The average depth to the water table is thus
approximately 211m. The existing borehole data show that the deepest borehole in the
area penetrated to a vertical depth of 190m which by inference from correlation with
the survey result has not penetrated well enough a stable aquifer in these areas hence
the reason for the failure of these boreholes to yield sufficiently all year round. A
maximum traverse length AB of one kilometer was reached during the survey but the
result of the interpretation shows that this was not enough to establish the thickness of
the stable aquiferous horizon; however, VES I, VES II, VES III VES IV and VES VIII
have regions with potential perched aquifer existence. In profile I, the thickness is
about 55m and about 102m thickness is suggested in VES II. In profile II, the perched
aquifer exists at a depth of about 135m from the surface. Correlation and comparative
analysis among established resistivity values (ranges) for sediment, resistivity values of
the different geoelectric layers in this work and of lithological log of borehole show
mainly sandstone units. Evidently clear from the interpretation result is that a thick
sandstone layer of thickness within the range of 85m to 192m of dry bed overlies the
zone of permanent water saturation (water table).
1
CHAPTER ONE
INTRODUCTION
1.1 Groundwater
Subsurface water is the fraction of total precipitation which infiltrates the
ground and fills the voids in the rock or unconsolidated materials. The origin of
groundwater was not clearly established until by the later part of the seventeenth
century. A French hydrologist, Pierre Perrault, from the result of hydrologic
investigations in the basins of Seine river was the first to prove that the water
contained within earth was not drawn up from the oceans but rather was provided by
rainfall and snowmelt. In his assertion, about 97% of all water is contained within the
ocean basins while of the 3% outside the basins, nearly 80% is contained within the
glacier and polar ice. About 0.7% is represented by the more visible surface
accumulations of water and 20% of all the water outside the ocean basins resides
underground. Hydrologists also estimate that more than about 8 million cubic
kilometres of water exist below the earth’s surface as groundwater (Renton, 1994).
Groundwater therefore represents part of the subsurface water occurring in the
zone of saturation (phreatic zone) below the water table. Groundwater is one of the
most important natural resources (Singh, 2007). According to Plummer (2001), the
source of groundwater is rainfall and snowmelt. In prospecting for groundwater or
looking for good site to drill water wells, a certain favorable geologic material called
aquifer is sought for. These favorable materials are sedimentary deposits or rocks that
are sufficiently permeable to transmit economically significant quantities of water.
This aquifer includes sandstone, well-jointed limestone, conglomerates and some
well-fractured volcanic rocks (such as columnar basalt) as well as well-fractured
crystalline rocks.
Aquifer can be broadly categorized into two; confined and unconfined aquifers.
Confined aquifer is one which is completely filled with water under confining
pressure and which is overlain by a relatively impermeable layer called aquitard.
2
Water rises up a well drilled into confined aquifer owing to the fact that the water is
under pressure. The level at which the water stands in the well defines an imaginary
surface called the potentiometric surface whose height above the aquifer depends on
the confining pressure in the aquifer. An artesan well develops when the
potentiometric surface rises above the ground level.
An unconfined aquifer is one in which water table condition prevails owing to
the absence of layer of relatively impermeable material on top. The conditions for the
formation of unconfined aquifer are specific. First, there is no impermeable
confinining bed. Secondly the water in the aquifer is not under pressure. Thirdly, an
unconfined aquifer is rapidly recharged by precipitation hence, it has rising and
falling water table according to the seasons. Hanging or perched aquifer also exists.
This occurs when a relatively impermeable layer occurs above the water table and
holds up infiltrating water to form a saturated lens of limited extent above the
saturated zone of the aquifer.
1.2 Statement of the problem
The people of Obukpa and Alor-Uno in Nsukka L.G.A, and Ibagwa-Aka in
Igbo-Eze South L.G.A are always faced with the condition of acute water scarcity.
This condition can be sometimes so severe especially during the dry season after a
heavy drawdown that they resort to going kilometers to neighboring communities in
search of potable water. Usually, the University of Nigeria community is always their
final resort besides commercial water tanker services.
These areas mentioned above are devoid of surface water (stream, rivers,
ponds and lakes). There are however few scattered boreholes within these areas which
by the time of this research work were either not pumping or at some level of
malfunction. Few individual attempts were made to ameliorate this situation of water
3
scarcity by sinking boreholes but the success rate has been low as this problem still
rears its ugly head hence water scarcity still haunts the people of this area.
The reasons for the state of the numerous boreholes both privately owned and
government assisted boreholes are attributed to the lack of adequate hydro
geological/geophysical information of the groundwater potentials within these areas
prior to siting of the boreholes.
1.3 Purpose of the study
This research work attempts to investigate and furnish the hydrogeological
parameters of the research areas by vertical electrical sounding (VES) technique of
electrical method of geophysical exploration and targets the following.
Delineation of groundwater horizon at different location and estimation of the
aquifer thichness.
Stratigraphic delineation of lithology in terms of the different resistivity
values.
1.4 Location of the study area/accessibility
The study area is located within latitudes 6o52N and 6
o58N and longitudes
and 7o20E and 7
o27E and covers an areal extent of about 40.825 square kilometres in
Nsukka and Igbo-Eze South Local Government Areas of Enugu State. It covers
Obukpa and parts of Alor-uno and Isiuja in Nsukka L.G.A, and parts of Ibagwa-aka
in Igbo-Eze South L.G.A. The region has an undulating topography and the elevation
varies between 359m and 413m above sea level (Fig.1.1).
These areas of study are accessible through a network of major and minor
roads in addition to several foot paths. The major access roads are the Nsukka-Alor-
Uno, Nsukka Enugu-Ezike road and Nsukka-Obollo Afor road from Ugwuawalawa,
linked at Beach Junction by Ibagwa road which also lead to Obukpa through M.C.C
road (Fig.1.1).
4
To
Itc h
i
To Enug
u Ezik
e
* Erike
* Elu Agu Obukpa
* Ibagwa Ani
* Alor-Uno
* Isi Uja
* Nsukka
* Stadium
* UNN
* Ogige Mkt
Umanu * Obukpa
* Amugwu
* Amogbo
* Onuiyi
* Beach
* Amebo
* Ovoko
Sch
Junction
Mkt * Ibagwa Aka
Sch
Aka
Mkt
G R A
N
7º20´E 7º27´E
6º58´N
6º50´N
7º27´E7º20´E
0 1 2 3km
6º50´N
6º58´N
R o ad Fo o th p ath To wn s/C o m m u n ities
*
Fig. 1.1: Map showing the location of the study area and the major access roads.
(Federal surveys, sheet 287,1963).
Camelite
* Ugwuawarawa
4
5
CHAPTER TWO
LITERATURE REVIEW
2.1 Regional geologic setting of the study area
The area under study falls within the Anambra sedimentary basin which
formed during the folding, uplifting and rifting of Santonian sediment in the Benue
trough. Tectonism in the Southern Nigeria started in the early Cretaceous time with
the separation of the African plate from the South American plate resulting in the
opening up of Atlantic Ocean (Burke et al., 1972; Murat, 1972; Kogbe, 1972). During
this period, about 132 million years ago, there was rifting caused by magmatic
upwelling and this brought about the formation of graben-like structure called the
Benue trough: a basin which became the site of deposition (Olade, 1975).
The Benue trough is a linear Northeast –Southwest trending intracraton rift
system that is about 800km long and about 80 – 150km wide ( Agagu et al., 1985) and
extends from Niger Delta to the Southern margin of Chad basin. Sedimentation in the
basin (Benue trough) began in the Aptian and continued until the Santonian when the
sediments in the Southern part of the trough were folded and uplifted to form the
Abakaliki anticlinorium, a smaller Afikpo syncline to the East and the Anambra basin
to the West. The three geologic structures altogether became then the major
depocentre in the Southern Benue trough and the study area regionally is sited within
the Anambra basin.
The stratigraphy of Benue trough corresponds to three main depositional
sequences of which the Companion-Maastrichtian sequence represents the sequence
of formations within the Anambra basin. The sequence of formation is Mamu
formation (lowest), Ajali formation, Nsukka formation, Imo formation, Ameki
formation, Ogwashi-Asaba formation and the topmost Benin formation (Table 2.1).
6
Mamau formation represents the paralic sequence of the disposition, followed by the
continental sequence of Ajali sandstone (Reyment, 1964). Ajali formation comprises a
thick succession of sandstones with minor shale interbeds. The sandstone is distinctly
grayish to pinkish-white and generally friable with extensively developed cross-
stratification. Provenance interpretations have favored fluvial (Murat, 1972), fluvio-
deltaic (Reyment, 1965; Hoque and Ezepue, 1977), inter-bar channel (Banerjee, 1979;
Amajor, 1986) depositional setting in different areas. The Nsukka formation overlies
the Ajali sandstone and is lithologically similar to Mamu formation. This is the
formation in most parts of Nsukka, Udi and Awgu divisions and consists of an
alternating sequence of laminated very fine-grained sandstones, siltstones and
mudstone with numerous coal seams at various horizons. The depositional
environment of the Nsukka formation inferable from surface exposure appears similar
to that of Mamu formation which is of sand plain marsh origin with occasional fluvial
incursions.
2.2 Local geology
The study area is found within the Anambra sedimentary basin whose rocks
are upper Cretaceous in age. The geologic formations are the upper Nsukka formation
and the underlying Ajali sandstone. The major land forms typical of this area are the
residual hills and dry valleys. These two major geomorphic structures are the resultant
effect of weathering and differential erosion of clastic materials which are remnant of
Nsukka formation. Ofomata (1967) recognized five types of these residual hills
according to their shapes. These residual hills sometimes form outliers on the Ajali
sandstone and are capped by thick deposit of red earthy material and laterite. These
laterites are permeable particularly those of Ajali sandstone thereby allowing easy
water percolation into the groundwater table during the rainy season.
7
Table 2.1: Stratigraphic successions in the Anambra basin showing the lithology of
Nsukka formation overlying the Ajali formation (Extracted from Nwajide, 1980).
Age Formation Lithology
Tertiary
Miocene-
Recent
Benin
Formation
Medium-coarse grained, poorly
consolidated sand with clay lenses
Oligocene
-Miocene
Ogwashi-Asaba
formation
Unconsolidated sands with lignite seams
Eocene Ameki formation Gray clayey sandstone and sandy clay
stone
Paleocene
Imo formation
Grayish fossiliferous and laminated
clayey shale
Upper
Cretaceous
Maastrichtian Nsukka formation Sandstone intercalated with shale and
clay
Ajali sandstone Poorly consolidated sandstone, typically
cross bedded with minor clay layer
Lower
Maastrichtian
Mamu
formation
Shale, sandstone mudstone and coal
seam
8
The Ajali sandstone consists mainly of medium to coarse grained
characteristically white colored sandstone but may be occasionally iron stained. The
sandstone is very permeable and readily recharged in its outcrop belt around the Idah-
Nsukka- Enugu escarpment (Agagu et al., 1985). Nsukka formation has a significant
groundwater potential and hosts a number of low to moderate yield wells in Nsukka
areas. A number of perched aquifer emerges from it and quite a number of low yield
wells also tap the perched aquifer in Nsukka areas (Ezeigbo and Ozioko, 1987). The
laterite capping in the area is aquiferous due to their vesicular nature hence it is
porous and permeable. These lateritic caps may be underlain by less pervious clay
beds leading to the formation of perched aquifers in some areas. Perched aquifer
discharge is seen in Asho hill in Nsukka, Aku hill in Obukpa, Abile hill in Ibagwa-aka
and Awula in Ibagwa-ani. In many areas however, the laterite cap has been washed
out and the clay bed underlying it missing hence perched aquifer does not form and
rain continues its vertically downward motion to the regional water table. The
sandstone members have a permeability of 2.0 – 20.7 x 10-10
cm/s (Mamah and
Ekene, 1989).
2.3 Electrical properties of rock materials
The variation of resistivity of a particular rock or sediment is enormous and is
greatly controlled by the percentage of water content within the pore spaces and
layers of rocks (Telford et al., 1990). Indeed Zohdy et al. (1974) noted that: “No other
physical property of naturally occurring rocks or soil displays such a wide range of
values. The resistivity values of some common rocks, soils, and chemicals are given
in Table 2.2(a). Table 2.2(b) shows the resistivity as a function of the percentage
water content. Metamorphic and igneous rocks typically have higher resistivity
values. The resistivity of these rocks is greatly dependent on the degree of fracturing
9
and the percentage of fracture filled with groundwater. Sedimentary rocks which
usually are more porous and have higher water content normally have lower
resistivity values. Clayey soils normally have a lower resistivity value than sandy
soils while the resistivity of groundwater is even lower ranging between 10 -100 ohm-
meter depending on the concentration of the dissolved salt.
Porosity is also a major factor controlling the resistivity of rocks. Generally,
resistivity increases as porosity decreases. The shapes and arrangement of pores can
greatly influence resistivity. This can result in greater current flow in some directions
than in others (http://www.esus.edu/indiv/s/slaymaker/Archives/Geophysics.htm)
Considerable overlap exists among the resistivities of different rock types making the
identification of a rock type solely on the account of resistivity data impracticable.
Thus resistivity layers, therefore do not necessarily correspond separately with each of
the lithostratigraphic sections of the subsurface, but rather a lump sections of
approximately common resistivity values under one bed (Obiakor,1984). According
to Fetter (1980), the following factors reduce resistivity: increasing water content,
increasing salinity of water, increasing clay content and decreasing grain size.
Assuming that water is available to fill voids, resistivity is lowered by increasing
porosity, increasing number of fractures and increasing weathering. Conversely,
resistivities are raised by increasing compaction and lithification.
2.4 Principles of resistivity surveying
Electrical resistivity method (particularly vertical electrical method) of
geophysical investigation has been favourably applied in areas of groundwater
potential studies. It has also been applied in the determination of faults, depth to
bedrock and as well as in the search for geothermal reservoirs.
10
Table 2.2 (a): Bulk resistivity of some rock types (Loke, 1997).
Igneous
Metamorphic
Sedimentary
Table 2.2(b): Resistivity of rock as function of percentage water content
(Telford et al., 1990).
Rock/sediment Percentage(%) water Resistivity (Ωm)
Granite 0 1010
Granite 0.19 1.8106
Basalt 0 6.0106
Basalt 0.95 4104
Coarse grained sandstone 0.39 9.6105
Coarse grained sandstone 0.18 108
Medium grained sandstone 1.0 4.2103
Medium grained sandstone 0.1 1.4108
Greywacke sandstone 0.16 4.7103
Greywacke sandstone 0.45 5.8104
Arkosic sandstone 1.0 1.4103
Rock types Resistivity (ohm metre)
Basalt
Granite
5.0x103-1.0x10
7
4.5x103 -1.3x10
6
Slate
Quartzite
Marble
6.0x102- 4.0x10
7
1.0x102-2.0x10
8
1.0x102-2.5x10
8
Conglomerate
Sand stone
Shale
Lime stone
Clay
2.0x103-1.0x10
4
8.4x103-1.0x10
5
20-2.0x103
50-4.0x103
1.0-102
11
The application of resistivity method in routine groundwater survey stems
from the fact that electrical conductivity in shallow subsurface is controlled, to some
extent, by the fluid present. The properties that affect the resistivity of rocks and
sediments include porosity, water content, composition (clay mineral and metal
content), salinity of pore fluid and grain size distribution (Krumbein and Monk,
1942). This method of geophysical investigation is founded on the fact that any
subsurface variation in resisitivity (conductivity) affects the form of current flow
within the earth and then the electrical potential distribution. In region of interest,
measurements of the resistivity of the ground are usually carried out by transmitting
controlled current through two outer current electrodes and picking up the developed
electrical potential within two inner more closely spaced potential electrodes. The
choice of electrode configuration to be employed in the field is controlled by the
target of the investigation; whether the interest of the survey is in vertical penetrating
investigation or in lateral investigation of anomalous resistivity contrast.
Usually, direct current from batteries or low frequency alternating current is
used to supply the current into the ground and the resistance R (ohms) is either
calculated from Ohm’s law IRV or read off from the instrument (terrameter).
The resisivity for homogenous and isotropic medium or apparent resistivity a
for inhomogeneous medium is then calculated as a function of the metered or
calculated resistance R and the spatial electrode configuration. The resistivity
a
or is related to the resistance R via a constant known as the geometrical
factor (G) which is typical of the kind of electrode configuration used.
Vertical electrical sounding (VES) method of geophysical investigation has been
favourably applied in areas of groundwater potential studies. It has been applied in the
determination of faults, depth to bedrock and as well as in the search for geothermal
reservoirs. Obiakor (1984) used this method in establishing the best area to harvest
12
groundwater in Idemili and Anambra L.G.A of Anambra state. Adetola and Igbedi
(2000) used VES method to establish successfully the site for successful borehole
location and the confirmation of the Bende-Ameki formation in Agbede,
Southwestern Nigeria. VES method has also been successfully used to map the
subsurface formation on the Eastern red sea coast of Jordan (Awni, 2007) and in
quantitative assessment of groundwater reserve of unconfined aquifer in Burkit Jalil-
Serdang area, Malaysia (Hago, 2000). Okolie (2005) estimated the groundwater
potential in parts of Niger Delta using VES method. Basically, vertical electrical
sounding is one of the best methods of investigating the electrical properties of the
subsurface such as resistivity/conductivity variation with depth and is the best
geophysical method known and applied for groundwater prospecting in many areas
(Parasnis, 1986; Emenike, 2001).
2.5 Current distribution and current density within a homogenous subsurface
Burger (1992) demonstrated that the current flow lines follow a tangent
relation such that
1
2
2
1
tan
tan
, 2.1
where and are as defined in figure 2.1(a). These current flow lines are oriented
in direction determined by the resistivity of the medium in which the current flows.
Figs.2.1 (b) and (c) show the orientation of these flow lines with respect to the
resistivities of the subsurface layers.
Consider a current source and sink electrodes (C1 and C2) as shown in figure
2.2(a) on the surface of a stratified earth with resistivity 1
and 2 . Supposing the
earth is homogenous, 1
= 2 hence the current lines are radiated outward equally in
all directions when the source and sink are relatively far from each other. Noted is that
13
the current flows in direction perpendicular to the hemispherical equipotential
surfaces described. The equipotential surfaces have radius r and area A = 2r
The current density (J) is the ratio of the quantity of current (I) flowing per
unit cross-sectional area (A) of the media in which the current flows that is I/A. It
expresses the spacing of the current lines which consist of moving charges. This is an
important concept of electrical resistivity method which has been found to depend on
the resistivites of the subsurface. Closely spaced current lines indicate a high current
density while a low current density is indicated by more widely spaced current lines.
For a homogenous subsurface in which1
= 2 , the current flow lines as well as the
current density (J) is uniformly distributed. For a nonhomogenous subsurface in
which a layer of less resistivity material sits on top of a layer of greater resistivity,
that is 2
> 1 , more current flows above the interface. The current flow lines and the
equipotential surfaces are more closely spaced and the current density is greater in the
region above the interface relative to the case of a homogenous subsurface. The
converse effect is produced when a greater resistivity layer sits on top a less resistivity
layer. The current density is reduced above the interface as the current flow lines are
less and more widely spaced. The above investigated effect are illustrated in figure
2.2 and will be useful in developing quantitatively the relationship existing between
current electrode separation as used in vertical electrical sounding and current depth
penetration as would be shown later.
14
Fig. 2.1: Current flow lines at a boundary separating materials of contrasting resistivity.
(a) For generation of equation 2.1
(b) Refraction at boundary when 1 < 2
(c) Refraction of current line at boundary when 1>2
(Extracted from Burger, 1992)
1
2
ρ1
ρ2
N
(b)
1
2
ρ2
ρ1
N
(a)
ρ1
N
ρ2
(c)
15
2
1
2
1
(a) Homogenous layers; current density uniformity distributed
(b) Non-homogenous strata 21
; more current flow in the upper layer
(c) 21
; Current density J is higher in 2
layer as more current flow in the less resistivity
Fig.2.2: Current flow and current density distribution at interface separating strata of contrasting
resistivity (From Burger, 1992).
C1 C2
C2
C2 C1
C1
2
1
12
12
1
2
1
16
2.6 Development of potential field within the subsurface
For an isotropic and homogenous medium in which continuous current flows,
the electrical potential which develops within can be calculated theoretically by
solving Laplace’s equation resulting from two basic theorems.
1. Ohm’s law;
E = J , 2.2
where E is the electric field in (Vm-1
), is the resistivity m and J is the current
density in (Am-2
).
2. Divergence theorem;
. J = 0. 2.3
The implication of divergence theorem is that in a region of finite conductivity,
charges do not accumulate to any reasonable extent during current flow. From the fact
that the electric field E is the gradient of a scalar potential V , we obtain from
Ohm’s law that
J = - V , 2.4
where is the conductivity of the medium in ( 11 m ).
Combining equations 2.3 and 2.4, we have
0. V . 2.5
0.2
VV 2.6
Condition of isotropy and homogeneity implies that is constant through the
medium; hence V . vanishes and we have
002
asV . 2.7
Equation 2.7 is the Laplace’s equation which is then solved subject to boundary
conditions presented by the symmetry of the current flow within the media to derive
the potential V . In spherical polar coordinate, Laplace’s equation becomes
17
0sin
1sin
sin
1
2
2
222
22
V
r
V
rr
Vr
rV . 2.8
2.6.1 Potential of point current source at depth and on the surface of a
homogenous medium
From the symmetry of the cases of point current sources (Fig. 2.3), the
potential is only a function of the distance r from the current source hence is
independent and as stipulated by Laplace’s equation.
Therefore equation 2.8 becomes,
02
r
Vr
r. 2.9
Integrating equation 2.9 twice, we have
V = Br
A . 2.10
Defining the level of the potential at great distance from the current source as zero,
that is V = 0, as ,r A and B are constants of integration
,0
BA
0, B . 2.11
Substituting equation 2.11 into equation 2.10, we have
V = -Ar-1
. 2.12
The symmetry of the current flow assumes the current density vector J uniform
throughout the spherical and hemispherical equipotential surfaces described by
current source at depth and on the surface of a homogenous medium respectively.
These equipotential surfaces are orthogonal to the current flow lines (Figs. 2.3a and
b). Hence the total current crossing a spherical or hemispherical surface is given by
sdJsdJIs
. , 2.13
18
where S is the spherical or hemispherical surface area.
.2
4
2
2
surfacetheonJrI
depthatJrI
2.14
Recalling that 1,
IAJ
dr
dVVandVJ
Therefore
2
1,
r
A
dr
dVbut
dr
dVIA
2
124
r
ArI
24
Iand
IA respectively.
But r
AV
Therefore
.2
4
surfacetheonr
IV
depthatr
IV
2.15
From equation 2.15, it is clearly seen that for any number n of current sources
distributed at surface of a homogenous and isotropic medium, the potential at an
observation point is given by
n
n
r
I
r
I
r
IV ..................
22
2
1
1
. 2.16
2.6.2 The potential of two current electrodes at finite separation on
the surface of homogenous and isotropic medium
In the earlier point current source case, it is worthy of note that there were two
current electrodes but one of which is considered to be at an infinite distance hence
the influence in the developed potential is considered negligible.
19
Uniform medium of
resistivity ρ
Current flow
Equipotential
Equipotential surface
Current flow
C2
C1
C1
C2
Fig 2.3 (a): Buried point source of current in homogeneous ground
(Telford et al. 1990).
Fig. 2.3 (b): Point source of current at the surface of a homogeneous medium
(Telford et al. 1990).
20
In this two current electrodes consideration, the two current electrodes are at finite
distances with their associated potential electrodes. This arrangement corresponds to
the popular four electrodes configuration routinely used in resistivity survey (Fig.
2.4). The effects of the current sources to the developed potential at any nearby
surface point are pronounced. Since the currents supplied at both electrodes are equal
and in opposite direction, we obtain, following the same route as was used for point
source case, the change in potential V due to this configuration.
Thus,
The potential V1 due to C1 at P1 is given by
1
12 r
IV
. 2.17
The potential V2 due to C2 at P1 is given by
2
22 r
IV
. 2.18
The potential due to C1 and C2 at P1 is given by
21
21
11
2 rr
IVV
. 2.19
Similarly, the potential due to C1 and C2 at P2 is
43
43
11
2 rr
IVV
. 2.20
The difference in potential V between P1 and P2 is
4321
4321
1111
2 rrrr
IVVVVV
. 2.21
From equation 2.21, we have
.1111
24321
rrrrI
V
21
Fig. 2.4: A generalized four electrode array; two current electrodes (C1 and C2) and
two potential electrodes (P1 and P2) on the surface of homogenous earth.
LF ac = low frequency alternating current source and dc = direct current source.
(Modified from Telford et al., 1990)
C1 P1 P2 C2
r1
r3
r2
r4
V
I
Surface
Power
dc or LF ac
22
4321
1111
2
rrrr
I
V . 2.22
Equation 2.22 defines the resistivity , for the homogenous medium. For an
inhomogeneous earth, equation 2.22 defines the apparent resistivity a
.
The quantity measured in resistivity survey is the resistance
I
V. The
geometrical factor G is thus . This factor is typical of the type
of electrodes configuration applied in the field.
2.7 Current penetration in a homogenous isotropic earth
Consider the current flow in a homogenous medium between source and sink
current electrodes C1 and C2 respectively as in fig.2.5a. The horizontal current density
at point P is
21
112
1
rrI
xV
pJ
x
x
,
where r1 = [x2 + y
2 + z
2]
1/2, and r2= [(L-x)
2 + y
2 + z
2]1/2
. The potential at P is given by
equation 2.19.
23
.)()()()(2
)()(2
1
23
22223
222
21
22221
222
zyxlxLzyxxI
J
zyxLzyxI
J
x
x
x
If the point P is on the vertical plane midway between C1 and C2, then, x = L/2 = a.
Therefore,
.
2 23
22223
222
zya
a
zya
aIJ
x
Combining the two terms in the above equation since from geometry (Fig. 2.5a)
L = 2a
2
3222
)2
(2
zyL
LIJ
x
. 2.23
Equation 2.23 shows the variation in current density with depth z across the
plane when the electrode separation is maintained constant. If, on the other hand, the
electrode spacing is varied, Telford (1990) showed that Jx is a maximum when L=z .
Similarly, estimate of the fraction of current flowing through a strip of this
vertical plane, between depths z1 and z2 can be made using equation 2.23. The current
through an element dydz of the strip is
dIx = Jx dy dz =
.
)2
(2 2
3222
dydz
zyL
LI
Integrating between the limit z1 and z2 and rearranging, we obtain the fraction
of the total current through a long strip, thus
24
2
1
2
12
23
222
2
2
2
2
z
z
z
z
x
zL
dz
zyL
dydz
L
I
I
L
z
L
z1121 2
tan2
tan2
2.24a
The fraction of the total current flowing through the strip at z2 tends to an
infinite value is
L
z
I
Ix 11 2
tan2
1
. 2.24b
The electrode spacing necessary to force a given fraction of the current into
the ground below a depth z is given from equation 2.24b. It is seen from the plot of
L
zagainst
I
Ix (Fig. 2.5b) that fifty percent of the current injected into the ground is
confined above a horizontal plane with a depth of one-half of current electrode
separate (L = 2z). Seventy percent of the current is confined above a depth equal to
the electrode separation and almost 90% of the current crosses the depth when L = Z/3.
Clearly, the greater the electrode separation, the greater the depth to which a given
percentage of current penetrates. For a good current penetration therefore, we must
use large enough current electrode separation so that sufficient current reaches the
target depth (Telford, et al., 1990).
2.8 Electrode configurations
Varieties of electrode arrays are employed in the field during resistivity survey
based on the target of the survey. Each electrode array has its limitation as well as
advantages over others. The commonly used electrode configurations are due to Frank
Wenner and Conrad Schlumberger and their modifications.
25
0.2
0.8
0.6
0.4
0.2
0 0
Fig. 2.5 b: Fraction of current that flows above depth z across the mid plane
between current electrodes with spacing L (Modified from Telford et al., 1990).
C2 Ground surface
Fig. 2.5 a: Geometry for determining the current distribution in a uniform ground
(Lowrie, 1997).
1.0
L
a a
x L-x
V
r2 r1
z
C
1 0
P
J
x
ρ
4
3
2
1
5
L
z
I
Is
2tan
2 1
I
Is
L
z
26
2.8.1 Wenner array
The Wenner configuration is a special case where the four electrodes are
equally spaced along a straight line. This is described in Fig.2.6a. The distance
between any two adjacent electrodes is called the array spacing ‘a’. The geometric
factor obtain using Wenner electrode configuration is
a
aaaa
G
21
2
1
2
11
2
. 2.25
2.8.2 Lee partitioning array
This array is a modification of the Wenner configuration. Here, a third
potential measuring electrode is introduced at the mid point O of the ordinary Wenner
array (Fig.2.6b). A potential difference is then measured between both M and N and
the centre electrode at O. The geometric factor for each half of the array is
a
aaaa
G
4
2
3
2
3
2
11
2
. 2.26
2.8.3 Schlumberger configuration
The electrode arrangement used in this work is the Schlumberger
configuration. The choice of Schlumberger array for this work is due to the fact that it
has the highest resolution and involves minimal labour cost than other configurations,
like Wenner array, Lee partitioning method, pole-dipole method and others.
In this method, four collinear electrodes are used with the outer two being
current electrodes (C1 and C2) and the inner two (P1, P2) the potential electrodes (Fig.
2.6c). C1 and C2 are spaced far apart symmetrically about the centre O and are at
separation ‘a’ each from the centre. The inner potential electrodes are more closely
spaced and are equidistant about O at a separation of 2
b . The geometric factor is
27
4
2
1
2
1
2
1
2
1
22
b
b
a
ba
ba
ba
ba
G
.
2.27
The apparent resistivity calculated using this arrangement is
Rb
b
a
I
VG
sa
4
2
, . 2.28
In routine resistivity survey, numerous electrode arrays are in use but the
choice of array to be used is controlled by the target objective of the survey. The
Schlumberger array can be used in vertical sounding for depth probe as well as in
electrical mapping for lateral resistivity variations diagnosis. Fig.(2.6c) shows the
Schlumberger expanding current electrode array centred on the origin O used in this
work which effectively measures the variation of resistivity with depth below the
origin. The fixed separation of the inner potential electrodes (P1 and P2) helps to
minimize the effect of local shallow inhomogeneities in the vicinity of all
observations.
2.9.1 The single boundary problem-optical analogue treatment
The earth appears to be in layers having different resistivities. The optical
analogue works on the simple assumption that electric current behaves in many
respects similar to light rays hence can be transmitted and reflected within these
different resistivity boundaries. The single-boundary treatment thus serves solely as a
means of identifying the meaning of a reflection or transmission coefficient in terms
of resistivity contrast.
28
Fig. 2.6a: Wenner electrode configuration
Fig. 2.6b: Lee partitioning electrode configuration.
Fig. 2.6c: Schlumberger electrode configuration.
A.B represents the position of the current electrode C1 and C2 respectively.
M.N represents the position of the potential electrodes P1 and P2 respectively.
I
a a
A M N B Surface
V
a
I
a
A M N B Surface
a
VOM VON
O
a/2 a/2
I
∆V
b
b/2 b/2
N B M A Surface
a
O
29
The first step in the optical analogue approach is the assumption that the
interface of the earth model separating two homogeneous layers of different
resistivities 21
and behaves as a semitransparent mirror and is planar (Fig. 2.7).
Suppose a source located at A is viewed from observation positions P1 and P2 above
and below the transparent boundary respectively. The light rays from the source will
travel directly to P1 and part will be reflected from the mirror. The amount reflected
depends on a property of the mirror referred to as the reflection coefficient ‘K’ and it
is equal to the light intensity times the reflection coefficient. The transmitted light will
also get to an observer at P2 and the amount will be the light intensity times one minus
the reflection coefficient assuming zero absorption occurs. In the optical analogue, the
same reflection and transmission are obtained if current source C is used instead of
light source at A.
Placing a current source C with intensity I at position A as in figure 2.7, the
potential developed due to direct, reflected and transmitted current at P1 and P2
respectively would be determined as follows.
The potential V at P1 due to direct and reflected component of the current is
2
1
1
1
441
r
IK
r
IV
P
. 2.29
The potential V at P2 due to transmitted component is
3
2
4
1
2r
KIV
P
. 2.30
These potential functions V must be continuous across the boundary between the two
media. The normal component of current flow through the boundary must also be
continuous. These continuity conditions of potential entails that on the boundary plane
where r1 = r2 = r3 = r, 21 pp
VV hence
30
Fig. 2.7: Single boundary model for the derivation of reflection and transmission
coefficient.
I A1
Boundary layer
Source
Observer
r1
r2
r3
ρ0
ρ1
I (I-K) P2 IK
P1
31
r
KI
r
K
r
I 1
4
1
4
21
r
K
r
K 1121
Therefore, 12
12
K . 2.31
Similarly 12
12
1
K . 2.32
Equation 2.31 defines the reflection coefficient (K) whereas equation 2.32 defines the
transmission coefficient (1-K) both with respect to the resistivities 21
and above
and below the interface.
2.9.2 Potential function for single overburden problem
The single overburden earth model (Fig.2.8a) is used to develop the electrical
potential function on the surface of an inhomogeneous earth layer. Using the optical
analogue, a current source C of intensity I is placed within an earth layer of thickness
t and resistivity1
. This 1
-layer is bounded by two parallel planes separating it from
a semi-infinite space above with resistivity 0
and a semi-infinite space below with
resistivity 2
. The current source C and observation point P are located at a depth h,
beneath the earth’s surface but separated by a horizontal distance r. The reflection
coefficients in the upper and lower interfaces are designated K1,0 and K1,2
respectively.
Using the method of images (optical analogue), we generate the potential due
the primary current source as well as the potentials due to numerous reflections from
both the upper and lower boundaries following the steps illustrated in figs. 2.8. The
treatment given below follows Keller and Frischknecht, (1966) and Telford, (1990).
The diagrams (Figs. 2.8-2.10) and equations 2.33-2.51 are modified from the same
sources.
32
Fig. 2.8 (a): Single overburden problem.
•
•
•
• •
•
I
P
K1,2
K1,0 h
C r I
K1,2
K1,0
t
P
r
h
h A
C1(0)
t-h
t-h
I1=IK1,2
IK1,0
C1(2)
t
Fig. 2.8 (b): Primary source reflected once from upper and lower plane.
33
Step 1:
Following the derivation of equation 2.16 for a current at depth, the potential
due to direct current source(C) at P is
r
IV
4
1
0 . 2.33
Step 2:
An observer at P sees images and of the primary source C reflected from
both the upper and lower boundaries as in fig.2.8b. The apparent intensities of the
upper and lower image sources are and respectively, where and
are the reflectivity of the upper plane when viewed from underneath and that of
the lower plane when viewed from above respectively. The contributions to the
potential function at P from these images are:
21
0,1
22
10
1
24 hr
IK
V
2.34
21
2,1
22
12
1
24 htr
IK
V
, 2.35
where the superscript in equations 2.34 and 2.35 refers to the medium in which the
image source appears to be and the subscript indicates that this is in the first series in
a series of images.
Steps 3: Consider multiple reflections and only paths in which the first bounce is off
the lower plane
(a) For paths with even numbers of reflections
a(i) Two reflections, one from each plane and first bounce off the
lower plane (Fig. 2.9a). Contribution to the potential at P is
21
22
0,1,2,110
2
24 tr
KIK
V
. 2.36
34
a (ii) The contribution to potential due to two pairs of reflections each from upper and
lower plane (Fig. 2.9b). The potential at P is
21
22
0,12
,2,12
10
3
44 tr
KIK
V
. 2.37
From contributions given by the above equations, it is obvious that increased
reflections from the boundaries reduces the image strength by a factor 2
0,1
2
2,1KIK and
the image is located a distance 2t further above the top plane than the preceding
image. The potential due to an infinite series of such images is given by
122
0,12,11
21
24n
nn
I
ntr
KIKV
. 2.38
(b) For paths with odd number of reflections, first reflections off the lower plane
(Fig.2.10).
b(i) Two reflections from the lower plane and one from the upper plane( Fig. 2.10(a)
The contribution to the potential function derived from is given by
21
22
0,1,2,12
12
2
224 httr
KIk
V
2.39
b(ii) Three reflections from lower and two from upper plane (Fig. 2.10b).
Contribution to the potential at P is given by
21
22
0,12
,2,13
12
3
244 httr
KIK
V
. 2.40
The effect of such increased number of reflection is to reduce the strength by2,10,1
KK .
The image is located a distance 2t further below the lower plane. The potential due to
an infinite series of such image is:
122
0,1
1
2,11
21
224n
nn
II
htntr
KIKV
. 2.41
35
t
2t-h
C I
r P
h
t-h
t-h
4t-h
3t-h
t-h
C3(0)
h
t-h
I
C P
•
•
•
•
•
•
•
• •
•
Fig. 2.9 (a): Even number of reflections (two
reflections).
Fig. 2.9 (b): Even number of reflections (four
reflections).
t
2t-h
C2(0)
C2(0)
C1(2)
C2(2)
C1(2)
r
36
Fig. 2.10 (a): Odd number of
reflections (three reflections).
Fig. 2.10 (b): Odd number of reflections (five reflections).
3t-h
5t-h
t-h
t-h
h
C
2t-h
4t-h
I
r
C I
h
t-h
3t-h
t
t
r
P
P
•
•
• •
•
•
•
•
•
•
• •
C2(0)
C3(0)
C1(2)
C2(2)
C3(2)
C2(2)
C1(2)
C2(0)
t-h
37
There would also be ray paths with even or odd number of reflection with first bounce
off the upper surface. Following the same route as for the paths with first bounce off
the lower plane, we may construct a series of images. For paths with even number of
reflections, first reflections off the upper plane. The potential due to an infinite series
of such image is:
122
2,10,11
21
24n
nn
III
ntr
KIKV
. 2.42
(ii) For the paths with odd number of reflections, first reflection off the lower plane.
The potential due to an infinite series of such images is given by
21
22
2,1
1
0,11
224 hntr
KIKV
nn
IV
. 2.43
The total potential at P, Vp is the sum of the potential contributions of
equations 2.33, 2.34, 2.35, 2.38, 2.31, 2.42 and 2.43.
2
12
12
1
2
0,12,1
12
2,1
2
0,11
21212
1
14
r
tn
KK
r
ht
K
r
h
K
r
IV
nn
n
P
221
K
21
K
221
1n2
2,1
1
1,0
1n2
2,1
n
1,0
2
0,1
1
2,1
12
12
12
1
r
h
r
tn
K
r
tn
K
r
ht
r
tn
KKnnnnn
n
2.44
Equation 2.44 is considerably simplified by stating that C and P are on the upper
boundary (h = 0), and that the upper half-space is air ( .0
). The reflection
coefficient then approaches
38
1
1
1
1
1
00
10
0,1
K
Set =K.
Equation 2.44 therefore reduces to:
1 1222 2
12
12
1
1212121
114 n n
nn
I
P
r
tn
K
r
tn
K
r
t
K
rV
.
21211
21
2 21
21
n
n
n
n
r
tn
K
r
tn
K 2.45
Three of the four series terms in equation 2.45 are identical with each other, and the
fourth term is made identical also by reducing the counter by one and including the
lone term into the series:
1 122
1
2 21
21
21
2112121n n
nn
r
tn
K
r
tn
K
r
t
K. 2.46
The total potential function (VT) becomes
12
1
21
21
212 n
n
T
r
tn
K
r
IV
. 2.47
Recognizably, the total potential function (Eqn.2.47) consists of two parts. The part
due to homogeneous and isotropic half-space called the normal potential and given as
r
I
2
1
39
and the part introduced by the condition of inhomogeneity called the disturbing
potential given as
12
1
21
21
22 n
n
r
nt
K
r
I
.
The series equation 2.47 does not converge when the resistivity contrast between the
overburden and the lower half-space is very large but converges for special conditions
in which the contrast ‘K’ is small. That is K<1 while the denominator decrease
indefinitely.
In the Schlumberger electrode arrangement used in this work, by virtue of the
small potential electrode separation, what is therefore measured in the single
overburden problem is the potential gradient, that isr
V
. By differentiating equation
2.47 with respect to the electrode separation ‘r’ and multiplying by 2 to take into
account the two current electrodes involved, we obtain the apparent resistivity
measured using the Schlumberger electrode array. Thus
12
2
1
23
21
212 n
n
T
r
tn
K
r
I
r
V
. 2.48
Therefore,
12
1
2
,2
3
21
212
n
n
sa
r
tn
K
r
V
I
r
. 2.49
40
The implication of equation 2.49 is obtained by assessing the relationship
existing between electrode spacing ‘r’ and the apparent resistivity sa ,
in the equation
with respect to different K values.
At the limit of a very small value of electrode separation that is when r<<t, the
series term in equation 2.49 tends to zero as the summation contain a factor of r
tin
the denominator which is large if r is small so that1
a
. This implies that what is
measured in the field when the electrode separation is small approximates the
resistivity of the upper layer1
.
At large electrode separations, that is as r tends to an infinite value
( )r however, equation 2.49 reduces to
1
121
n
n
aK
But
1
11
1
n
n
KK
Ksa
1
221
1,
But 12
12
K
Therefore,
12
12
,
1
221
sa
2 .
This implies that at large electrode spacing ‘r’ the resistivity measured approximates
the resistivity of the lower layer. The above result and implication form the basis of
operations of VES method of geophysical survey used in this work. The result of the
assessment of equation 2.49, that is that 1,
sa
for r<<t and 2,
sa
for r>>t
41
work on the condition that -1<K<1. For 1K , the series contained in equation 2.49
does not converge but if K = 1, sa ,
increase indefinitely implying that resistant or
insulating layer underlies a more conductive layer.
Considering the current density ‘J’ uniform at a great distance r compared to
the overburden thickness t, that is J is uniform at r>>t. The current flowing within a
half-space occupied by insulator can be given from Ohm’s law. Thus,
JE1
s
rtJdsJI 2.
rt
IJ
2
rt
IE
2
1 . 2.50
The above equation 2.50 is multiplied by 2 since two current source electrodes are
used in the Schlumberger array. Therefore
I
Er
r
V
I
rsa
2
2
,
22
1
2
,X
rt
IX
I
rsa
t
rsa
1
,
= constant. 2.51
The implication of equation 2.51 is that a linear relationship exists between
apparent resistivity and electrode spacing. A plot of 1
,
sa
against t
r will generate a
straight line with a slope S = 1. The ratio sa
r
,
= 1
t is a constant called the
longitudinal conductance.
The above relation shows that with the basic electrode array used in the
Schlumberger method, the conductance of all rocks lying above an insulating
42
underlying rock can be read directly from the data. The sounding curves rises at 45O
when apparent resistivity is plotted as a function of the electrode spacing on
logarithmic coordinates.
For K = -1, implying that a resistant layer overlies a conductive half-space
however, there is no such simple interpretation and treatment as above for K = 1.
Using therefore equation 2.49, curves of apparent resistivity as a function of electrode
spacing can be constructed for different values of K, within the limits of -1<K<1. The
curves are constructed on a double logarithmic scale graph and are called master
curves. The relationship between the apparent resistivities (that would be measured
using Schlumberger array over a single overburden case) and the electrode spacing for
resistivity contrast ρ2/ρ1 between 0 and ∞ is shown in fig.2.11 as master curves.
The behaviour of electrical potential in a single overburden model is used as
the basis for interpretation of measurement made over an earth consisting of any
number of horizontal layers as the result obtained in this single overburden problem
can be extended to a more complex layering situation of the earth. These theoretically
computed master curves with their auxiliary curves help in the interpretation of field
resistivity data by curve matching (partial and complete curve matching) and are also
the basis of computer interactive methods of resistivity data interpretation. The
method of images becomes unworkable for the treatment of more than two horizontal
layers. The potential distribution at the surface of a horizontally stratified earth is then
better obtained by solving Laplace’s equation for the potential under appropriate
boundary conditions. This facilitates the computation of master curves for any small
number of horizontal layers.
43
Fig. 2.11: The master curve used for interpretation of VES field data obtained by
Schlumberger array (Keller and Frischknecht, 1966)
44
2.10 Interpretation methods
The mathematical analysis for quantitative interpretation developed via
method of images is most highly developed for electrical sounding technique and
dealing with single overburden problems. With the analysis of equation 2.49 in
section 2.9 for different values of resistivity contrast within the limit of ρ2/ρ1 or k
within the limit –1≤ k ≤ 1, typical set of curves are developed which are used in the
interpretation processes of the VES field profiles. This process involves the
comparison of field profiles with characteristic curves.
The first step in interpretation of VES measurement is to plot the field data on
a double logarithmic coordinate graph. For Schlumberger VES data (considered in
this work), the apparent resistivity )(a
is plotted as the ordinate while the electrode
spacing 2
ABa is plotted as the abscissa of the logarithmic graph. There are
several methods employed in the interpretation of VES data. These include both
qualitative analysis as well as quantitative interpretation.
Qualitative analysis of the field profile is geared towards understanding the
characters of the beds between surface and the underlying beds (Telford et al., 1990).
For convenience in selecting the method of interpretation of resistivity sounding data,
the curves are classified into four basic shapes for three horizontally layered earth
with resistivities .,321
and Figure 2.12 illustrates these shapes. A curve which
has a definite minimum at the intermediate depth is classified as a type H curve while
that with a definite maximum at the intermediate depth is classified as a type K curve.
These indicate the presence of a three-layer bed sequence with the resistivity ratios
varying as 321321
and respectively. Curves which show uniform
45
increase or decrease in resistivity value with depth are classified as type –A or type Q
respectively. The resistivity ratios are therefore .321321
and
The classification above is made on the assumption that each of the curve
types in their crudest form is for two beds over a basement although in general these
characteristics sounding curves (Fig. 2.12) represent multiple layers. Similarly, some
ideas also of the relative bed thicknesses may be obtained from the horizontal extent
of the maxima and minima as well as the flanking portions in all cases (Telford et.al.,
1990). When there are more than three layers with different resistivities apparent on a
field curve, several letters are used to classify the curve. A type-HK curve indicates a
sequence of resistivites 4321
(Keller and Frischkneit, 1966).
Similarly, use can also be made of the maximum and minimum point to
estimate the resistivity and layer thickness. The coordinates of the extreme points in
curve types H and K (i.e. maximum or minimum a
and electrode separation) may be
used with certain characteristic curves for three layer employing a particular electrode
spread. For Schlumberger array in which :(a)1
(max)
a is plotted against
1
2
for
various values of z2/z1 and (b) the ratio 1
(max)
z
L is plotted against z2/z1 for various
values of 1
2
, L(max) being the electrode spacing at which
a is maximum or
minimum. Thus owing to the fact that the value of 11
(max(max)
z
Land
a
is known
(presumably 1
and z1 can be found from a two-layer curve match on the left of the
profile); horizontal lines drawn across the characteristic curve gives two sets of
possible values of 1
2
and z2/z1 , corresponding to the intersection. If values of z2/z1
are plotted against 1
2
, we obtain two curves which intersect at one point. The point
of intersection therefore represents the correct values of z2 and 2
(Telford et al,
1990).
46
Fig.2.12: The various types of sounding curves over multilayer structure
(Telford et al., 1990).
47
Quantitative interpretation of single overburden VES profile can be achieved through
curve matching (complete and partial curve matching) and computer iterative
approach. In most cases, the field data may be directly compared with a set of
theoretical curves by superposition. The field data plotted on a transparent sheet on
logarithmic scale is laid over the theoretical curves keeping both axes parallel. The
field curve is made to slide over the theoretical curve until the field curve matches
with one of the set of the theoretical curves. The coordinate of the point where
1
1
h
aa
on the theoretical curve determines the value of the resistivity )(
1 of
the top layer and the thickness (h1) of the same layer on the field curve axis while the
actual curve fit gives the value of K and hence 2
(Keller and Frischknecht, 1966).
2.10.1 Complete curve matching
This entails the use of a set of theoretically computed curves called master
curves generated for mathematical models with two or three layers covering an
infinite uniform substratum. As the number of layers increases, say from three 3-
layers to 4-layers the parameters required to specify completely combination of
resistivity and thickness increases hence the compilation of the set of curve becomes
too cumbersome. Eric and Joachim (1979) published sets of three-layer curve models
for VES measurement with Schlumberger array.
In complete curve matching, the field curve is plotted on a transparent double
logarithmic graph of same scale as the master curves. The field curve is then
superimposed on a similar shaped set of master curves and moved around until a best
fit is obtained with the axes of the graphs parallel. The value of 2
ABcoinciding with
the coordinate of the theoretical cross (that is the point where a-axis and axisa is
(1, 1 on the master curves) represents the thickness of the first layer h1 and resistivity
48
1 of the first layer respectively. The thicknesses h2, h3 and resistivities
3,2 of the
other layers are obtained from the appropriate parameters belonging to the matching
master curve (Parasnis, 1986).
2.10.2 Partial curve matching
In order that sounding may be located in the best areas to obtain good results,
and so that poor results may be recognized before a great deal of field effort has been
expended, preliminary interpretation must be done in the field. Partial curve matching
is the procedure most commonly used for preliminary interpretation (Keller and
Frischknecht, 1966).
This technique requires matching of small segment of the field profile with
theoretical curves computed for single overburden. Starting from left (small electrode
spacing), matching progresses towards the right (longer spacing). When a reasonable
portion of the curve is interpreted, layers comprising the interpreted portion of the
sounding curve are lumped together to form a fictitious uniform layer with effective
resistivity ρ* and thickness H such that (H = h1 + h2 +………hn). This fictitious layer
is then used in place of the surface layer when next portion of the curve is analyzed.
Considering a three-layer case for which h2 ≥ h1, it has been established that
the thickness of the fictitious layer and resistivity are related by the expression
2
2
1
1
hhH
, 2.52
where the ratio
His known as the longitudinal conductance of layers above a
resistive substratum.
From equation 2.52 the ratio of the resistivity
of the lumped layer to that of the
first layer 1
is obtained as,
49
1
1
12
12
1/
/
h
H
h
H
. 2.53
Carrying out matching of the segment of the field curve with the two-layer
master curves at small electrode spacing, the coordinates of the origin of the master
curve called the theoretical cross give the value of11
hand . 2
is generated from
the value of resistivity contrast(12
/ ) for the curve of best fit. By similar matching
at large electrode separation, we obtain ρ*, H and the resistivity contrast*
3/ . The
resistivity ρ3 of the third layer is thus determined. The thickness h2 of the second layer
can be calculated using equation 2.52. Since 112
// hHand can now be
determined,
can then be calculated. Equation 2.53 can be used to generate a set of
curves called the auxiliary point curves also used during partial curve matching
method of interpretation of the field data.
Plotting a graph of 11
*// hHagainst on a log-log scale for different values
of resistivity contrast 12
/ we are able to prepare auxiliary charts to complement the
master curves for easy determination of *
which is read as the ordinate of the
appropriate curve. The different auxiliary point charts used in partial curve matching
of different VES profile curves are shown in figure 2.13.
This method of interpretation, described for three-layer case can in principle
be extended to any number of layers by the alternate use of the two layer master
curves and their corresponding auxiliary curves. In employing this approach, it is
assumed that each successive layer is much greater that the lumped or combined
thickness. This constitutes a major limitation to the approach as the assumption poses
a serious geologic restriction to its application. Therefore the result of these methods
of interpretation is largely dependent on the proficiency, shrewd judgment and
mastery of the interpreter.
50
Type A
Type H
Type K
Type Q
a
b
=z2/z1
μ=12
/
- =z2/z1
-μ=12
/
Fig.2.13: The auxiliary point chart for the (a) type A and H (b) type K and Q
(Telford et al., 1990).
51
2.10.3 Computer interactive modeling method
As number of horizontal layer increases beyond three, the number of
parameter required to specify completely the earth model also increases and it
becomes very difficult, almost impossible to interpret VES data using curve matching.
However, computer programs such as Resound, Resists, Applet, Ermodel, Ersound
and so on which can interpret multiple layer problem with much ease have been
designed.
In computer based interpretation approach, an initial earth model (ideas
obtained from pre-processing) which is believed to be responsible for the observed
values is assumed and the resistivity and thickness parameters estimated. These
parameters are substituted into the computer program and are modified by trial and
error until a close match is established between the calculated and the observed
resistivity curves (Koefoed, 1979). From the computer model, which was modified to
best approximate the field observations, the resistivities and layer thicknesses are
generated and displayed by the computer.
One major limitation of this approa`ch is over interpretation. This is because
the programs are usually sensitive to slight changes in resistivity which it could
regard as due to new layer. Hence, an ordinary four-layer curve might be interpreted
as more than 7-layer by the computer. To overcome this requires therefore and
experienced geophysicist (who also knows the geology of the study area) in the
interpretation so that the result of the computer interactive interpretation could be
averaged to represent to a fair approximation, the true earth condition.
52
CHAPTER THREE
INSTRUMENTATION/FIELD WORK
3.1 Instruments for the survey
The instrument used for the field work was provided by the geology
department and it consisted of Abem digital terrameter and accessories. This
instrument is a signal averaging system branded SAS300. It measures in different
modes and run in four cycles (Fig 3.1).
In the resistivity mode, it comprises a battery-powered deep penetrating
resistivity meter with an output sufficient for current electrode separation of up to
2km under good surveying conditions. The ratio of the developed potential (V) to the
current (I) supplied IV / is automatically calculated and averaged over the selected
number of cycles and the value is digitally displayed in milliohms, ohms or kilohms.
The overall ranges extend from 0.5 milliohm to 1999 kilohms.
SAS300 is a three-unit compact piece of measuring instrument housed in a
single casing. These units are the transmitter, the receiver and the microprocessor.
These units work together as a single unit to produce the reading which is displayed
on the screen. The voltage signal created by the transmitted current signal is received
by the receiver after discrimination between noise and the signal. The microprocessor
controls and monitors all measurements to ensure optimal accuracy. It runs a one
second thorough check on the circuit and switch position.
3.2 Reconnaissance study
The field work proceeded in two stages. From the first stage which was the
reconnaissance studies, the work progressed into second phase being the fieldwork
proper. The reconnaissance studies were undertaken between 24th
April and 26th
April, 2008. It entailed a surveillance trip to the locations chosen for the VES
profiling.
53
Fig. 3.1: The instrument (Abem Terrameter SAS 300) used in the field work.
Potential electrode terminals
RANGE selector CURRENT selector
Current electrode terminals
CYCLES selector Resistivity range Desiccator cartridge
54
During this period, possible VES points to be used were located and marked. Also
located during this part of the fieldwork were the locations of existing boreholes
besides the observation of the geomorphic structures of the areas. Possible access
routes were identified. The above as well as other information got apriori about the
geologic and hydrogeologic nature of the area formed the basis of the assertions made
earlier and which were then investigated in this research work. The map of the area of
study (Fig 3.2) as well as the borehole logs (Fig3.3) were procured. Table 3.1 shows
the boreholes within the area of study and their depth of penetration.
3.3 Field work
The actual field work was more involving and entailed running VES traverses
in locations selected within the area of study. It started on 14th
May and ended on the
27th
July, 2008 during which time the ground was adequately wetted through rain fall
and was in the best conductive condition for vertical electrical sounding (VES).
Seven people were involved in the traversing work. These included two
geophysics technicians who brought the terramter and operated it, four hired staff
(two operated on either flank of the profile centre) and the researcher. The researcher
worked at the centre of the spread with the operator and they manipulated the
equipment and read out the value of the resistance displayed. The writer also worked
with the technician from the centre to expand the potential electrode separation during
looping. He also carried out checks on the continuity of the traverse line on either side
from the centre.
55
VES
215m
Erike
I
X
*
*Ibagwa Ani
*Umanu
BHA
X 190m
*Ibagwa Aka
Sch. *Aka
Mkt
*Amebo
Sch
VES VI
X 218m
BHF
X 216m
*Ovoko
*Obollo Roa d
*Obukpa*Amugw u
VES VIII
X 212m
VES VIII
X 222m
VES IV
X 220m
*Amogbo
*Onuiyi
Isi Uja*
BHB
X 110m
VES II
X 220m
*Alor-Uno
BH C
X 159m
*Ugwuawarawa
E 199m
D 193m
*Beach*Nsukka
B.H.I
X 236m
*Stadium
*Ca melite
*Ogige Mkt
*UNN
Junction
VES IX
X 155m
Erike*
VES III
X 218m To E
nugu E
zike
To
tc
hi
6º58´N
7º20´E 7º27´E
6º58´N
6º50´N
7º27´E7º20´E
6º50´N
0 1 2 3km
Road
X Depth Towns/Communities*
Foothpath VES Centre BH Borehole
Borehole
B. H. A Eluagu 190.0
Isiuja-Amagbo 110.0(hand dug)
Ibagwa Road 183.0
Onuiyi 159.0
Ibagwa Road 189.0
Amaebo Obukpa 186.0
Ibagwa Not available
Obollo Road 236.0
B. H. B
B. H. C
B. H. D
B. H. E
B. H. F
B. H. I
B. H. G
Location Depth of penetration (m)
Table 3.1: Some of the existing boreholes and their depth of penetration
Fig. 3.2 Map of the area of study with inset of VES positions, locations of Borehole and their depth of penetration.
(Modified from Federal surveys base map, sheet 287, 1963).
VES V
215m
Elu Agu Obukpa
X
*
N
GR A
55
56
a
b
End of well
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
240
250
BH.I
Brownish clay
a quote from t
document or
the summary
of an
interesting
point. You
can position
the text box
anywhere in
the document.
Use the Text
Box Tools tab
to change the
formatting of
the pull quote
text box.]
Light brownish
medium-coarse grain
sand sand e or the
summary of an
interesting point. You
can position the text
box anywhere in the
document. Use the
Text Box Tools tab to
change the formatting
of the pull quote text
box.]
Light brown-white
medium grain sand
quote from the
document or the
summary of an
interesting point.
You can position
the text box
anywhere in the
document. Use the
Text Box Tools tab
to change the
formatting of the
pull quote text box.]
Light brown coarse sand
sandsandTypea quote
from the document or the
summary of an interesting
point. You can position
the text box anywhere in
the document. Use the
Text Box Tools tab to
change the formatting of
the pull quote text box.]
Gravelly brown coarse sand
the summary of an interesting
point. You can position the
text box anywhere in the
document. Use the Text Box
Tools tab to change the
formatting of the pull quote
text box.]
Dark brownish coarse sand
quote from the document or
the summary of an
interesting point. You can
position the text box
anywhere in the document.
Use the Text Box Tools tab
to change the formatting of
the pull quote text box.]
Reddish lateritic sand
document. Use the Text
Box Tools tab to
change the formatting
of the pull quote text
box.]
Extremely coarse sand soil
Medium-coarse grained sand stone
with clay intercalation
Extremely coarse sandstone
Clay intercalations
Extremely coarse sandstone with clay
Medium grained sandstone
Medium-coarse grained sandstone
Well sorted med.-grained sand stone
Coarse grained sandstone
End of bore hole.
Depth (m)
Top lateritic sand
Bore hole data
Fig. 3.3: Bore hole log
(a) Obukpa borehole( Enugu State Water Board record)
(b) Obollo road borehole (Bemzal well completion report)
57
Two people manned each current electrode; they marked out the required
spread length with the tapes and hammered the electrodes connected to the cables into
the ground and rewound them when each traverse was completed. Communication
between the field crew (especially current electrode handlers) and the centre operator
were made possible with the aid of GSM phone. He would always call especially
when the current electrodes handlers were at distances of about 150m from the spread
centre.
The electrode configuration used was the Schlumberger array where the
potential electrode separation was kept constant and the current electrodes were
moved outwards symmetrically about the centre of the spread. In this work,
measurement of each profile started with a potential electrode offset of 1meter and
current electrodes separation of 3.0m as in fig. 3.4.
AB is the current electrodes separation. M and N are the potential electrode
positions and X marks the centre or the origin of the profile. Table (3.2) shows the
chart (for the current and potential electrodes separations) that was used in the field
work. Normally, this chart was prepared before going to the field. AB/2 and MN/2 are
the half current and potential electrode separations respectively.
Looping was necessary at positions AB/2 = 15m, 50, and 200m. At these
distances of the current electrode from the origin, the potential electrode positions
were increased from 0.5m to 3.5m, 14.0m and 42m respectively. This was necessary
because at these distances, the displayed meter resistance was so small and sometimes
a negative value was obtained. With the increase in MN, appreciable readings were
obtained. The effect of the change is completely diminished when looping (repeating
readings for two already occupied current electrode separations) with the new
potential electrode spread.
58
Fig. 3.4: Schematic of the startup electrode arrangement for each VES profile.
AB = current electrode separation.
MN = potential electrodes separation.
X= center of the electrode spread.
(Modified from Telford, et al., 1990)
A maximum separation of 42m for the potential electrodes and 1 kilometer for
the current electrode separation were achieved in this work. The traverse length was
limited to 1000m by the total length of the current cable which was just 1000m.
Topography and thick hedges which needed to be hewed also slowed down the pace
of the field work. A major hindrance was incessant disruption by rainfall. By the end
of the field work, a total number of twelve VES traverses were run. Owing to the fact
that the first four traverses ran were too close to each other, they were however
commuted to one traverse hence the field data for nine profiles are shown in tables
3.3-3.11.
A 1.5m
X N B
3.0m
1.0m
0.5m
M
59
SCHLUMBERGER VERTICAL ELECTRICAL SOUNDING
DATA SHEET
SURVEY LOCATION________________________________________DATE___________________
STATION NO_______________________________________________________________________
LATITUDE_____________________________LONGITUDE________ELEVATION_____________
OPERATOR ________________________________________________________________________
Tab.3.2: The table used in the field for recording measurement readings.
S/N
Electrode spacing
G (m)
R( )
)( ma
)(2
mAB
)(2
mMN
1 1.50
0.50
6.28
2 2.00 11.78
3 3.00 27.48
4 5.00 77.75
5 8.00 200.28
6 10.00 313.35
7 15.00
3.50
95.49
8 20.00 174.04
9 30.00 398.42
10 40.00 712.58
11 50.00
14.00
258.51
12 75.00 609.13
13 100.00 1100.01
14 150.00 2502.50
15 200.00
42.00
1430.02
16 300.00 3300.02
17 400.00 5918.01
18 500.00 9284.01
60
Table 3.3: Data from VES profile I Erike, Obukpa
VESI
S/N
Electrode Spacing
G (m)
R( ) )(
2m
AB )(
2m
MN
1 1.50
0.50
6.28 407.00
2 2.00 11.78 188.20
3 3.00 27.48 81.20
4 5.00 77.75 25.70
5 8.00 200.28 12.75
6 10.00 313.35 5.50
7 15.00 706.07 3.75
8 15.00
3.50
95.49 40.70
9 20.00 174.04 23.00
10 30.00 398.42 9.50
11 40.00 712.58 5.00
12 50.00 1116.50 0.50
13 50.00
14.00
258.51 21.20
14 75.00 609.13 20.00
15 100.00 1100.01 10.50
16 150.00 2502.50 5.25
17 200.00 4465.99 4.40
18 200.00
42.00
1430.02 20.00
19 300.00 3300.02 10.50
20 400.00 5918.01 8.01
21 500.00 9284.01 5.50
61
Table 3.4: Data from VES profile II Alor-Uno
VESII
S/N
Electrode Spacing
G (m)
R( ) )(
2m
AB )(
2m
MN
1 1.50
0.50
6.28 352.00
2 2.00 11.78 167.00
3 3.00 27.48 80.10
4 5.00 77.75 38.30
5 8.00 200.28 19.40
6 10.00 313.35 15.00
7 15.00 706.07 7.70
8 15.00
3.50
95.49 63.00
9 20.00 174.04 38.00
10 30.00 398.42 13.00
11 40.00 712.58 10.80
12 50.00 1116.50 6.01
13 50.00
14.00
258.51 29.10
14 75.00 609.13 8.80
15 100.00 1100.01 3.61
16 150.00 2502.50 1.20
17 200.00 4465.99 0.66
18 200.00
42.00
1430.02 1.70
19 300.00 3300.02 0.90
20 400.00 5918.01 0.54
21 500.00 9284.01 0.33
62
Table 3.5: Data from VES profile III Amaogbo, Nsukka
VESIII
S/N
Electrode Spacing
G (m)
R( ) )(
2m
AB )(
2m
MN
1 1.50
0.50
6.28 339.00
2 2.00 11.78 188.70
3 3.00 27.48 106.50
4 5.00 77.75 50.20
5 8.00 200.28 27.00
6 10.00 313.35 17.01
7 15.00 706.07 7.75
8 15.00
3.50
95.49 39.01
9 20.00 174.04 18.20
10 30.00 398.42 6.70
11 40.00 712.58 3.62
12 50.00 1116.50 2.06
13 50.00
14.00
258.51 6.50
14 75.00 609.13 4.40
15 100.00 1100.01 3.25
16 150.00 2502.50 1.75
17 200.00 4465.99 1.50
18 200.00
42.00
1430.02 4.00
19 300.00 3300.02 2.20
20 400.00 5918.01 1.40
21 500.00 9284.01 0.86
63
Table 3.6: Data from VES profile IV Isiuja, Nsukka
VESIV
S/N
Electrode Spacing
G (m)
R( ) )(
2m
AB )(
2m
MN
1 1.50
0.50
6.28 290.00
2 2.00 11.78 178.70
3 3.00 27.48 35.22
4 5.00 77.75 19.20
5 8.00 200.28 14.30
6 10.00 313.35 5.71
7 15.00 706.07 3.50
8 15.00
3.50
95.49 36.10
9 20.00 174.04 15.50
10 30.00 398.42 5.82
11 40.00 712.58 1.52
12 50.00 1116.50 0.82
13 50.00
14.00
258.51 3.90
14 75.00 609.13 2.20
15 100.00 1100.01 1.71
16 150.00 2502.50 1.20
17 200.00 4465.99 0.45
18 200.00
42.00
1430.02 2.00
19 300.00 3300.02 1.31
20 400.00 5918.01 0.87
21 500.00 9284.01 0.56
64
Table 3.7: Data from VES profile V Eluagu, Obukpa
VESV
S/N
Electrode Spacing
G (m)
R( ) )(
2m
AB )(
2m
MN
1 1.50
0.50
6.28 32.00
2 2.00 11.78 23.70
3 3.00 27.48 23.00
4 5.00 77.75 11.25
5 8.00 200.28 4.50
6 10.00 313.35 2.25
7 15.00 706.07 0.87
8 15.00
3.50
95.49 5.12
9 20.00 174.04 2.10
10 30.00 398.42 1.81
11 40.00 712.58 1.37
12 50.00 1116.50 1.18
13 50.00
14.00
258.51 3.75
14 75.00 609.13 2.81
15 100.00 1100.01 2.12
16 150.00 2502.50 1.37
17 200.00 4465.99 1.00
18 200.00
42.00
1430.02 2.93
19 300.00 3300.02 1.56
20 400.00 5918.01 0.87
21 500.00 9284.01 0.50
65
Table 3.8: Data from VES profile VI Ochikum CSS, Obukpa.
VESVI
S/N
Electrode Spacing
G (m)
R( ) )(
2m
AB )(
2m
MN
1 1.50
0.50
6.28 40.40
2 2.00 11.78 27.80
3 3.00 27.48 17.43
4 5.00 77.75 9.37
5 8.00 200.28 4.62
6 10.00 313.35 3.37
7 15.00 706.07 1.68
8 15.00
3.50
95.49 12.31
9 20.00 174.04 7.87
10 30.00 398.42 3.87
11 40.00 712.58 2.43
12 50.00 1116.50 1.81
13 50.00
14.00
258.51 14.43
14 75.00 609.13 5.00
15 100.00 1100.01 3.37
16 150.00 2502.50 1.93
17 200.00 4465.99 1.18
18 200.00
42.00
1430.02 3.62
19 300.00 3300.02 2.50
20 400.00 5918.01 2.12
21 500.00 9284.01 1.10
66
Table 3.9: Data from VES profile VII Amaugwu, Obukpa
VESVII
S/N
Electrode Spacing
G (m)
R( ) )(
2m
AB )(
2m
MN
1 1.50
0.50
6.28 18.00
2 2.00 11.78 10.00
3 3.00 27.48 4.87
4 5.00 77.75 2.12
5 8.00 200.28 1.42
6 10.00 313.35 1.18
7 15.00 706.07 0.81
8 15.00
3.50
95.49 4.43
9 20.00 174.04 3.43
10 30.00 398.42 2.31
11 40.00 712.58 1.50
12 50.00 1116.50 0.60
13 50.00
14.00
258.51 5.06
14 75.00 609.13 2.81
15 100.00 1100.01 1.82
16 150.00 2502.50 1.31
17 200.00 4465.99 0.87
18 200.00
42.00
1430.02 4.50
19 300.00 3300.02 1.93
20 400.00 5918.01 1.43
21 500.00 9284.01 0.81
67
Table 3.10: Data from VES profile VIII Ishiagu, Ibagwa Road.
VESVIII
S/N
Electrode Spacing
G (m)
R( ) )(
2m
AB )(
2m
MN
1 1.50
0.50
6.28 81.40
2 2.00 11.78 53.10
3 3.00 27.48 30.30
4 5.00 77.75 13.06
5 8.00 200.28 5.50
6 10.00 313.35 3.75
7 15.00 706.07 1.81
8 15.00
3.50
95.49 11.68
9 20.00 174.04 7.06
10 30.00 398.42 4.04
11 40.00 712.58 2.87
12 50.00 1116.50 1.75
13 50.00
14.00
258.51 9.18
14 75.00 609.13 4.31
15 100.00 1100.01 2.93
16 150.00 2502.50 1.72
17 200.00 4465.99 0.74
18 200.00
42.00
1430.02 3.05
19 300.00 3300.02 2.00
20 400.00 5918.01 1.31
21 500.00 9284.01 0.87
68
Table 3.11: Data from VES profile IX Itchi-Unadu Road.
VESIX
S/N
Electrode Spacing
G (m)
R( ) )(
2m
AB )(
2m
MN
1 1.50
0.50
6.28 126.00
2 2.00 11.78 63.50
3 3.00 27.48 30.91
4 5.00 77.75 14.12
5 8.00 200.28 6.75
6 10.00 313.35 3.93
7 15.00 706.07 2.25
8 15.00
3.50
95.49 14.30
9 20.00 174.04 8.50
10 30.00 398.42 4.05
11 40.00 712.58 2.62
12 50.00 1116.50 1.87
13 50.00
14.00
258.51 5.81
14 75.00 609.13 3.62
15 100.00 1100.01 2.12
16 150.00 2502.50 1.18
17 200.00 4465.99 4.10
18 200.00
42.00
1430.02 1.50
19 300.00 3300.02 0.70
20 400.00 5918.01 0.35
21 500.00 9284.01 0.06
69
3.4 Practical limitation, error sources and precaution
To generate accurate results in the field, accounts were taken of some limitation to VES
survey. Limited space for the current electrode spread length of 1000m was a great concern as
farmlands, buildings and other structures often come in the way of traverse line. Another often
encountered problem was the presence of buried pipelines, cables and other metallic conductor in
the vicinity of the traverse line which could constitute spurious electrical signal (noise). Rugged
topography, flooded terrains and winding paths were also of great practical concern while running
the traverse.
To obtain maximum spread length of 1000m, the researcher tried as much as possible to
avoid locating the centre of spread at positions where obstacles would be encountered. More so,
none of the sounding points was located within the vicinity of source of spurious electrical signals.
To reduce the effect of topography, the traversing were run where there are slight or no undulation.
For the fact that there is no topography correction in resistivity survey as in seismic survey
(Burger, 1992), rugged topographies were avoided. Well insulated and light weighted wire of very
low resistance were used to ensure high quality insulation since leakage between the current circuit
and measuring circuit is one of the primary error source in resistivity survey (Keller and
Frishknecht, 1966). Finally, the survey work was not carried out on the days when there were
heavy downpour as water logged soil may result to enormously high conduction near the ground.
The errors in this experimental survey arise from the following factors: The precision of the
instrument used in the measurements and interpretation. The accuracy with which each of the
physical measurements were made and uncertainties could also arise from improper alignment of
electrodes besides the mastery and proficiency of the data analyst in data handling during the
interpretation.
A total of nine sounding profiles were executed within the area of survey (Table 3.3-3.11). In
each case, the values of the apparent resistivity were computed using the precalculated geometrical
factor G and the measured resistance R with the aid of equation 2.28. The approximate maximum
error in the calculated apparent resistivities was obtained within the limit of ±28 Ω m. The error in
measured electrode separation was determined by the precision of the measuring tape which is
with ±0.2m while the resistance R was determined within limit of ±2 percent fractional error.
70
CHAPTER FOUR
DATA PROCESSING AND INTERPRETATION
4. 1 Introduction
The basis for carrying out vertical electrical sounding irrespective of the
electrode array used is that the farther away from the current source the measurement
of the potential, the deeper the probing will be. This has been stated in many
references on geophysical prospecting that the depth of the probing depends on how
far apart the two current electrodes are placed.
In this present work, vertical electrical sounding (VES) was carried out using
Schlumberger expanding electrode array method hence the respective current
electrode spacing AB/2 is increased at successive intervals. The processing and
interpretation of the measured data were accomplished through a three- stage
treatment of the data as discussed below.
The first stage involves pre-processing of the raw data, calculation of the
apparent resistivity and the plotting of the vertical electrical sounding curves for each
of the survey location. In the second stage, VES profiles were analyzed in terms of
their various layer of actual resistivity following the treatment in section 2.8. The
third stage involves the use of resistivity interpretation software called the RESOUND
to quantitatively interpret the data. This is the direct interpretation approach which
generates quantitatively both the resistivity and thickness parameters of the actual
subsurface condition.
4.2 Data processing
Processing of the data generated during the course of VES profiling in the
different selected locations began right in the field while the work progressed. It
started with correct recording of half the current electrode separation a=AB/2, the
potential electrode spacing b=MN and the values of the measured resistance R on the
71
data sheet. Consequently, the geometrical factor G for Schlumberger array was pre-
calculated using the relation
4
2b
b
aG and from the data on tables 3.3 to 3.11,
the apparent resistivity values were calculated from the relation
,4
2
,R
b
b
asa
where R is the measured resistance in those tables.
Then, the apparent resistivity ρa was later plotted against half the current
electrode spacing on a log-log graph generating an electrical sounding curve. The
apparent resistivity (ρa) was plotted on the ordinate while half the current electrode
spacing (AB/2) plotted on the abscissa.
4.3 Analysis of VES curves
The raw field data were plotted on log-log graph. The apparent resistivity ρa
was plotted on the ordinate while half the current electrode spacing AB/2 plotted on
the abscissa of the double log coordinate graph. Usually, VES curves may have subtle
inflections due to the presence of noise hence the interpreter is required to make
decision as to how real or significant such features are. Often, field curves with such
subtle inflections are smoothed to produce curves of best fit: a curve that would
represent to a fair approximation the variation of resistivity with depth. The different
portions of the electrical sounding curve are then analyzed with reference to the
standard VES curves shown in figure 2.12. Fig.4.1 illustrates the process of curve
analysis using VES profile IV. The first portion of the curve from left labeled as type-
K has a distinctive maximum point showing that the resistivity relation of the
different parts are such that ρ1< ρ2<ρ3. The later portion shows a continuous rise
72
103
Type KA curve (4321
)
AB/2(m)
Fig. 4.1: Plot of a
against AB/2(m) used for analysis of VES IV (dot represents field data points)
)( ma
1
2
3
4
K A
73
in the value of resistivity with increase in electrode spacing hence it is labeled as type-
A. Apparently, VES profile IV has more than three layers and is characterized as a
type-KA curve with resistivity ratio ρ1<ρ2>ρ3<ρ4. Analysis of all the VES curves on
the account of their distinctive characteristic features in the field of apparent
resistivity, the VES stations show different types of curve: type-A curves are
presented in stations V, VI, VII, VIII and IX. Type-KA is obtained in stations II, III
and IV while in VES station I, a type-HH curve is obtained. These curve types
undoubtedly represent a minimum of four horizontal earth layers hence the need to
use a computer based method of interpretation solely.
Suffice it to say at this point that this analysis based on the simple curve
shapes and nature does not provide much information with regard to the layer
parameter of the subsurface. The knowledge of the curve shapes however is important
in that it gives the interpreter the idea of the model to be expected and used during
preliminary interpretation as well as computer based interpretation.
4.4 Quantitative interpretation
Having established from pre-processing of the VES field data that multiple
horizontal layers exist and considering therefore the large number of parameters
required in the interpretation of the VES field data with several horizontal layers,
computer-based interpretation was employed using a computer interactive program
called the RESOUND. This interpretation tool has been used and has been found
adequate for quantitative layer parameter determination in relation to resistivity
sounding data interpretation. Interpretation using the RESOUND program was
achieved through a two stage interactive approach. First, the raw data which are the
metered resistance (R) values and the half the potential and current electrode
separations (MN/2 and AB/2 respectively) at each profile point are fed into the
system. The system automatically computes the geometric factor G at each survey
74
point as well as the corresponding apparent resistivity values. A correcting factor
which is used to adjust the raw data is also generated automatically for each traverse
point. Secondly, the smoothening factor is used to adjust the apparent resistivity after
which the electrical sounding curves (plot of ρa against a) is automatically plotted and
displayed.
The RESOUND processing and interpretation sequence follow the procedure
below using the data of profile I. First, the raw field data (Tab. 3.3) were fed into the
system containing the REOUND software and the output (Tab. 4.1) was generated
and displayed as shown in the chart. The input data include half the current electrode
separation (AB/2), half the potential electrode separation (MN/2) and the resistance
(R) at each point of the traverse line. The displayed output data includes the geometric
factor, apparent resistivity and the ratio factor columns (Tab. 4.1).
During the second stage of` the processing and interpretation, the generated
smoothening factor (ratio factor) was used to multiply the apparent resistivity values
Within its domain and the final result (output) were displayed as the adjusted values
(Tab.4.2). During this stage, the mean values of the looped measurement were
adjusted hence the counter number reduces to 18 in the output.
These adjusted apparent resistivity outputs were then plotted against half the
current electrode spacing for the profile producing the electrical sounding curve and
the interpreted layer model (Fig.4.2). On account of the interpretation using
RESOUND program, profile I was modeled an eight-horizontally layered earth model
(Tab.4.3). The above sequences of operations were also applied to the other VES field
data.
75
Table 4.1: Result of first stage of computer interpretation of VES profile I data.
Unadjusted Out put
S/N
Electrode spacing
G (m)
R( )
)( ma
Ratio factor )(
2m
AB )(
2m
MN
1 1.50 6.28 407.00 2556
2 2.00 11.78 188.20 2217
3 3.00 27.48 81.70 2245
4 5.00 77.75 25.70 1998
5 8.00 200.28 12.70 2544
6 10.00 313.35 5.50 1723
7 15.00 706.07 3.75 2648
8 15.00
3.50
95.49 40.70 3886
3.14
9 20.00 174.04 23.00 4003
10 30.00 398.42 9.50 3785
11 40.00 712.58 5.00 3563
12 50.00 1116.50 0.50 558
13 50.00
14.00
258.51 21.20 5480
0.32
14 75.00 609.13 20.00 12183
15 100.00 1100.01 10.50 11550
16 150.00 2502.50 5.25 131838
17 200.00 4465.99 4.40 19650
18 200.00
42.00
1430.02 20.20 28886
0.22 19 300.00 3300.02 10.00 33000
20 400.00 5918.01 8.00 47344
21 500.00 9284.01 5.50 51062
4.60
0.50
76
4.4.1 Result of quantitative interpretation
The application of RESOUND computer interactive software in the
processing and interpretation of the remaining field data from the other VES station
generated the unadjusted outputs shown in table 4.4 and the adjusted outputs in table
4.5.The plot of the adjusted apparent resistivity values against the current electrode
spacing AB/2 for the remaining VES stations gave rise to the electrical sounding
curves and the inserted earth model (Fig.4.3). The interpreted results of resistivity and
the layer parameters of the models were also generated and shown in tab.4.6.
Table 4.2 Result of the second stage of computer interpretation of VES profile I data
showing the adjusted output values of the apparent resistivity, the electrical sounding
curves with the interpreted layer model and the layer parameter.
77
VES I
Tab. 4.2: Adjusted output
AB/2 (m) Apparent
resistivity(Ωm)
1.50 11758
2.00 10198
3.00 10327
5.00 9191
8.00 11702
10.00 7922
15.00 12192
20.00 12509
30.00 11885
40.00 11188
50.00 1753
75.00 3899
100.00 3696
150.00 4204
200.00 6322
300.00 7260
400.00 10416
500.00 11234
Fig.4.2: The resulting model.
Tab.4.3: The result of interpretation of geoelectric VES data of profile I.
Layer Resistivity(Ωm) Thickness(m) Depth
(m)
1 13520 0.8
0.8
1.5
2.5
10. 0
30.0
85.0
215.0
2 7520 0.7
3 10025 1.0
4 12550 7.5
5 13520 20.0
6 2255 55.0
7 17815 130.0
8 13325
Electrode spacing AB/2 (m) or depth (m)
78
Unadjusted Out put
S/N
Electrode spacing
G (m)
R( )
)( ma
Ratio factor )(
2m
AB )(
2m
MN
1 1.50 6.28 352.00 2210
2 2.00 11.78 167.00 1967
3 3.00 27.48 80.10 2200
4 5.00 77.75 38.30 2980
5 8.00 200.28 19.40 3890
6 10.00 313.35 15.00 4700
7 15.00 706.07 7.70 5400
8 15.00
3.50
95.49 63.00 6020
0.98
9 20.00 174.04 38.00 6610
10 30.00 398.42 13.00 5180
11 40.00 712.58 10.80 7700
12 50.00 1116.50 6.01 6700
13 50.00
14.00
258.51 29.10 7520
0.88
14 75.00 609.13 8.80 5360
15 100.00 1100.01 3.61 3960
16 150.00 2502.50 1.20 3000
17 200.00 4465.99 0.66 2950
18 200.00
42.00
1430.02 1.70 2430
1.06 19 300.00 3300.02 0.90 3970
20 400.00 5918.01 0.54 3200
21 500.00 9284.01 0.33 3060
1.09
0.50
Table 4.4a: Unadjusted output of the first stage of computer interpretation of VES II
79
Unadjusted Out put
S/N
Electrode spacing
G (m)
R( )
)( ma
Ratio factor )(
2m
AB )(
2m
MN
1 1.50 6.28 339.00 2130
2 2.00 11.78 188.70 2220
3 3.00 27.48 106.50 2930
4 5.00 77.75 50.20 3900
5 8.00 200.28 27.00 5410
6 10.00 313.35 17.01 5330
7 15.00 706.07 7.75 5470
8 15.00
3.50
95.49 39.01 3720
0.90
9 20.00 174.04 18.20 3170
10 30.00 398.42 6.70 2670
11 40.00 712.58 3.62 2570
12 50.00 1116.50 2.06 2300
13 50.00
14.00
258.51 6.50 1680
1.24 14 75.00 609.13 4.40 2680
15 100.00 1100.01 3.25 3580
16 150.00 2502.50 1.75 4380
17 200.00 4465.99 1.50 6700
18 200.00
42.00
1430.02 4.00 5720
1.45 19 300.00 3300.02 2.20 7260
20 400.00 5918.01 1.40 8209
21 500.00 9284.01 0.86 7980
0.62
0.50
Table 4.4b: Unadjusted output of the first stage of computer interpretation of VES III data
80
Unadjusted Out put
S/N
Electrode spacing
G (m)
R( )
)( ma
Ratio factor )(
2m
AB )(
2m
MN
1 1.50 6.28 290.00 1821
2 2.00 11.78 178.70 2110
3 3.00 27.48 35.22 968
4 5.00 77.75 19.20 1493
5 8.00 200.28 14.30 2860
6 10.00 313.35 5.71 1793
7 15.00 706.07 3.50 2470
8 15.00
3.50
95.49 36.10 3450
1.08
9 20.00 174.04 15.50 2700
10 30.00 398.42 5.82 2310
11 40.00 712.58 1.52 1995
12 50.00 1116.50 0.82 1675
13 50.00
14.00
258.51 3.90 2120
0.86 14 75.00 609.13 2.20 2380
15 100.00 1100.01 1.71 2420
16 150.00 2502.50 1.20 4250
17 200.00 4465.99 0.45 5360
18 200.00
42.00
1430.02 2.00 6440
0.71 19 300.00 3300.02 1.31 6600
20 400.00 5918.01 0.87 7750
21 500.00 9284.01 0.56 80801
1.51
0.50
Table 4.4c: Unadjusted output of the first stage of computer interpretation of VES IV data
81
Unadjusted Out put
S/N
Electrode spacing
G (m)
R( )
)( ma
Ratio factor )(
2m
AB )(
2m
MN
1 1.50 6.28 32.00 201
2 2.00 11.78 23.70 297
3 3.00 27.48 23.00 232
4 5.00 77.75 11.25 875
5 8.00 200.28 4.50 901
6 10.00 313.35 2.25 705
7 15.00 706.07 0.87 614
8 15.00
3.50
95.49 5.12 489
0.89
9 20.00 174.04 2.10 489
10 30.00 398.42 1.81 721
11 40.00 712.58 1.37 976
12 50.00 1116.50 1.18 1317
13 50.00
14.00
258.51 3.75 969
1.21 14 75.00 609.13 2.81 1712
15 100.00 1100.01 2.12 2330
16 150.00 2502.50 1.37 3430
17 200.00 4465.99 1.00 4470
18 200.00
42.00
1430.02 2.93 4190
1.29 19 300.00 3300.02 1.56 5150
20 400.00 5918.01 0.87 5150
21 500.00 9284.01 0.50 4640
0.71
0.50
Table 4.4d: Unadjusted output of the first stage of computer interpretation of VES V data
82
Unadjusted Out put
S/N
Electrode spacing
G (m)
R( )
)( ma
Ratio factor )(
2m
AB )(
2m
MN
1 1.50 6.28 40.40 254
2 2.00 11.78 27.80 328
3 3.00 27.48 17.43 479
4 5.00 77.75 9.37 729
5 8.00 200.28 4.62 925
6 10.00 313.35 3.37 1056
7 15.00 706.07 1.68 1186
8 15.00
3.50
95.49 12.31 1175
1.36
9 20.00 174.04 7.87 1370
10 30.00 398.42 3.87 1542
11 40.00 712.58 2.43 1732
12 50.00 1116.50 1.81 2020
13 50.00
14.00
258.51 14.43 3730
0.73 14 75.00 609.13 5.00 3050
15 100.00 1100.01 3.37 3710
16 150.00 2502.50 1.93 4830
17 200.00 4465.99 1.18 5270
18 200.00
42.00
1430.02 3.62 5180
0.75 19 300.00 3300.02 2.50 8250
20 400.00 5918.01 2.12 12550
21 500.00 9284.01 1.10 13090
1.34
0.50
Table 4.4e: Unadjusted output of the first stage of computer interpretation of VES VI data
83
Unadjusted Out put
S/N
Electrode spacing
G (m)
R( )
)( ma
Ratio factor )(
2m
AB )(
2m
MN
1 1.50 6.28 18.00 113
2 2.00 11.78 10.00 118
3 3.00 27.48 4.87 133
4 5.00 77.75 2.12 164
5 8.00 200.28 1.42 286
6 10.00 313.35 1.18 370
7 15.00 706.07 0.81 572
8 15.00
3.50
95.49 4.43 423
1.29
9 20.00 174.04 3.43 597
10 30.00 398.42 2.31 920
11 40.00 712.58 1.50 1069
12 50.00 1116.50 0.60 1183
13 50.00
14.00
258.51 5.06 1308
1.16 14 75.00 609.13 2.81 1712
15 100.00 1100.01 1.82 2200
16 150.00 2502.50 1.31 3280
17 200.00 4465.99 0.87 3890
18 200.00
42.00
1430.02 4.50 6440
0.70 19 300.00 3300.02 1.93 6370
20 400.00 5918.01 1.43 8460
21 500.00 9284.01 0.81 7520
0.95
0.50
Table 4.4f: Unadjusted output of the first stage of computer interpretation of VES VII data
84
Unadjusted Out put
S/N
Electrode spacing
G (m)
R( )
)( ma
Ratio factor )(
2m
AB )(
2m
MN
1 1.50 6.28 81.40 511
2 2.00 11.78 53.10 626
3 3.00 27.48 30.30 833
4 5.00 77.75 13.06 1015
5 8.00 200.28 5.50 1102
6 10.00 313.35 3.75 1175
7 15.00 706.07 1.81 1278
8 15.00
3.50
95.49 11.68 1115
1.22
9 20.00 174.04 7.06 1229
10 30.00 398.42 4.04 1618
11 40.00 712.58 2.87 2050
12 50.00 1116.50 1.75 1954
13 50.00
14.00
258.51 9.18 2370
1.01 14 75.00 609.13 4.31 2630
15 100.00 1100.01 2.93 3220
16 150.00 2502.50 1.72 4300
17 200.00 4465.99 0.74 3300
18 200.00
42.00
1430.02 3.05 4360
0.76 19 300.00 3300.02 2.00 6600
20 400.00 5918.01 1.31 7750
21 500.00 9284.01 0.87 8080
1.07
0.50
Table 4.4g: Unadjusted output of the first stage of computer interpretation of VES VIII
data
85
Unadjusted Out put
S/N
Electrode spacing
G (m)
R( )
)( ma
Ratio factor )(
2m
AB )(
2m
MN
1 1.50 6.28 126.00 792
2 2.00 11.78 63.50 749
3 3.00 27.48 30.91 849
4 5.00 77.75 14.12 1098
5 8.00 200.28 6.75 1352
6 10.00 313.35 3.93 1232
7 15.00 706.07 2.25 1589
8 15.00
3.50
95.49 14.30 1365
1.11
9 20.00 174.04 8.50 1479
10 30.00 398.42 4.05 1793
11 40.00 712.58 2.62 1867
12 50.00 1116.50 1.87 2090
13 50.00
14.00
258.51 5.81 2250
1.03 14 75.00 609.13 3.62 3540
15 100.00 1100.01 2.12 3980
16 150.00 2502.50 1.18 5310
17 200.00 4465.99 4.10 5270
18 200.00
42.00
1430.02 1.50 5860
0.92 19 300.00 3300.02 0.70 4950
20 400.00 5918.01 0.35 4140
21 500.00 9284.01 0.06 3250
0.95
0.50
Table 4.4h: Unadjusted output of the first stage of computer interpretation of VES IX data
86
Table 4.5: The adjusted output values of computer base interpretation of VES profile
II, III, IV and V.
AB/2(m) Apparent
resistivity(Ωm)
1.50 2410
2.00 2140
3.00 2400
5.00 3240
8.00 4230
10.00 5120
15.00 5920
20.00 N6510
30.00 5100
40.00 7580
50.00 6600
75.00 4700
100.00 3470
150.00 2630
200.00 2580
300.00 3160
400.00 3400
500.00 3260
AB/2 (m) Apparent
resistivity(Ωm)
1.50 1311
2.00 1369
3.00 1802
5.00 2400
8.00 3330
10.00 3280
15.00 3370
20.00 2870
30.00 2410
40.00 2320
50.00 2080
75.00 3320
100.00 4430
150.00 5420
200.00 8300
300.00 10530
400.00 12010
500.00 11580
AB/2(m) Apparent
resistivity(Ωm)
1.50 143
2.00 199
3.00 450
5.00 622
8.00 641
10.00 502
15.00 473
20.00 473
30.00 645
40.00 873
50.00 1178
75.00 2080
100.00 2830
150.00 4160
200.00 5430
300.00 6670
400.00 6670
500.00 6010
AB/2 (m) Apparent
resistivity(Ωm)
1.50 2750
n2.00 3180
3.00 1463
5.00 2260
8.00 4330
10.00 2700
15.00 3740
20.00 2920
30.00 2500
40.00 2160
50.00 1815
75.00 2030
100.00 2070
150.00 3640
200.00 4590
300.00 4710
400.00 5530
500.00 5760
VES V
VES II VES III
VES IV
87
AB/2(m) Apparent
resistivity(Ωm)
1.50 341
2.00 440
3.00 644
5.00 979
8.00 1243
10.00 1419
15.00 1593
20.00 1857
30.00 2090
40.00 2350
50.00 2740
75.00 2240
100.00 2720
150.00 3550
200.00 3870
300.00 6170
400.00 9380
500.00 9790
AB/2(m) Apparent
resistivity(Ωm)
1.50 108
2.00 112
3.00 127
5.00 157
8.00 272
10.00 352
15.00 544
20.00 768
30.00 1184
40.00 1375
50.00 1522
75.00 1992
100.00 2560
150.00 3810
200.00 4520
300.00 4470
400.00 5950
500.00 5280
AB/2(m) Apparent
resistivity(Ωm) 1.50 753 2.00 712 3.00 808 5.00 1045 8.00 1286 10.00 1172 15.00 1512 20.00 1638 30.00 1985 40.00 2070 50.00 2310 75.00 3640 100.00 4090 150.00 5450 200.00 5420 300.00 4570 400.00 3830 500.00 3000
AB/2(m) Apparent
resistivity(Ωm) 1.50 545 2.00 667 3.00 888 5.00 1083 8.00 1175 10.00 1253 15.00 1363 20.00 1502 30.00 1977 40.00 2500 50.00 2390 75.00 2640 100.00 3240 150.00 4330 200.00 3330 300.00 5030 400.00 5910 500.00 6160
VES VII
VES IX VES VIII
VES VI
Table 4.5: The adjusted output result of computer base interpretation of VES profiles VI,
VII, VIII and IX
88
Fig. 4.3: The models resulting from computer based interpretation of sounding data of
VES profile II, III, IV and V. The crosses represent the field points.
VE
S IV
VE
S V
VE
S I
I
VE
S I
II
89
Fig. 4.3: The models resulting from the computer based interpretation of sounding
data of VES profile VI, VII, VIII and IX. The crosses represent the field points.
VE
S V
III
VE
S V
I
VE
S V
II
VE
S V
III
2.5
90
Table 4.6: The result of computer interpretation of the geoelectric VES data of profile
II,III,IV and V
VES II VES III
VES IV VES V
Layer Resistivity(Ωm) Thickness(m) Depth
(m) 1 3125 0.8
0.8
1.5
2.0
14.0
33.0
135.0
220.0
2 850 0.7
3 7245 0.5
4 9550 12.0
5 7542 19.0
6 1587 102.0
7 12345 85.0
8 2250
Layer Resistivity(Ωm) Thickness(m) Depth
(m) 1 812 0.8
0.8
1.5
2.5
10. 0
30.0
222.0
2 2215 0.7
3 7854 1.0
4 4525 7.5
5 1258 20.0
6 20158 192.0
7 8520
Layer Resistivity(
Ωm) Thickness
(m) Depth
(m) 1 1025 0.7
0.7
1.5
2.5
14. 0
35.0
220.0
2 6752 0.8
3 3865 1.0
4 4135 11.5
5 680 21.0
6 12500 185.0
7 3542
Layer Resistivity(Ωm) Thickness(m) Depth
(m) 1 75 0.7
0.8
1.5
2.5
15. 0
45.0
215.0
2 3520 0.8
3 1250 1.0
4 650 12.5
5 6852 30.0
6 13520 170.0
7 3895
91
Table 4.6: The result of the computer interpretation of the geoelectric VES data of
profile VI, VII, VIII and IX.
VES VI VES VII
VES VIII VES IX
Layer Resistivity(Ωm) Thickness(m) Depth
(m) 1 215 0.8
0.8
1.5
2.5
15. 0
36.0
218.0
2 3520 0.7
3 1250 1.0
4 2785 12.5
5 1052 21.0
6 14528 182.0
7 5574
Layer Resistivity(Ωm) Thickness(m) Depth
(m) 1 95 0.7
0.7
1.5
2.5
7. 0
45.0
212.0
2 110 0.8
3 125 1.0
4 750 4.5
5 8572 38.0
6 11005 167.0
7 2875
Layer Resistivity(Ωm) Thickness(m) Depth
(m) 1 890 0.8
0.8
1.5
3.0
15. 5
35.5
155.5
2 450 0.7
3 1102 1.5
4 1899 12.5
5 2580 20.0
6 9758 120.0
7 1025
Layer Resistivity(Ωm) Thickness(m) Depth
(m) 1 530 0.8
0.8
1.5
2.5
14. 0
65.0
218.0
2 1328 0.7
3 2580 1.0
4 1145 11.5
5 4578 51.0
6 7852 153.0
7 5879
92
4.4.2 Deductions from the result.
Based on the result of data processing and interpretation, profile I generated an
eight- layer geoelectric section (Tab.4.3). The first layer of loose surficial material has
a high resistivity value of 13520 Ω-m stemming from the dry condition of the ground
during the survey. The resistivity values however increased from the second layer at a
depth of about 0.8m to the fifth layer at a depth of 30m. The sixth layer shows a
drastic decrease in resistivity value from 13520 Ω-m in the fifth layer to 2255 Ω-m in
the sixth layer. This reduction is probably as a result of the presence of water-bearing
sixth layer covering a range of depths from about 30m to about 85m.This layer
represents a perched aquifer horizon and has a thickness of about 55m. The seventh
layer witnessed a rise in resistivity which is attributed to the presence of a dry rock
horizon. The resistivity value is 17815Ω-m and extends from a depth of about 85m to
215m.This depth of about 215m represents the top of the last layer that could be
probed by the VES current electrode array range employed within this particular area.
Although there is an obvious reduction in resistivity value to 13325 Ω-m in this layer
which indicate the presence of water-bearing horizon, a borehole penetrating to a
depth of 215m may strike water but may easily become unproductive.
The second profile(VES II) follows almost a similar trend as profile I and
gave an eight-layer section (Tab. 4.6).The perched aquifer of thickness of about 102m
occurs in the sixth layer covering depth ranges from about 33m to about 135m. The
water table occurs in the eight -layer at a depth of about 220m. This assertion is made
from the obvious drastic decrease in resistivity in the last layer indicating the presence
of water body. A depth 220m and more will give a good yield of water since it is
within a permanent water saturation zone.
VES III (Tab.4.6) is a seven-layer section with the water table appearing
within the seventh layer at a depth of about 222m. This is indicated by the drastic drag
93
down on the resistivity value from 20158 Ω-m in the sixth layer to 8520 Ω-m in the
seventh layer. A hanging water table could be obtained in the fifth layer at a depth of
about 10m. The resisitivity value of this layer is 1258Ω-m.
VES IV (Tab.4.6) gave a seven-layer section with the water table occurring at
the seventh layer at a depth of about 220m. The thickness of the aquiferous layer
could not be established by the survey hence a borehole drilled to a depth of about
220m and beyond within this area is possibly in the region of permanent water
saturation. A perched aquifer whose thickness is about 21m is suggested to occur in
the depth ranges of 14m to 35m.
VESV (Tab.4.6) generated a seven-layer section with the water table occurring
in the last layer at a depth of about 215m. The resistivity value in the seventh layer is
3895 Ω-m compared with the resistivity value of 13520 Ω-m obtained in the sixth
layer. The decrease in the resistivity value probably results from the presence of a
water bearing layer: the aquifer at this depth. A borehole penetrating to a depth of
about 215m is probably within the zone of permanent water saturation and will yield
water readily.
VESVI (Tab.4.6) is also interpreted as a seven-layer section and follows a
similar trend as the fifth profile. The water table exists at a depth of about 218m in the
seventh layer. A borehole is expected to penetrate to a depth of about 218m within
this region for a good yield of water.
The seventh profile (VESVII) is also a seven-layer geoelecric section. The
layer resistivity values increase from the first layer to the sixth layer where it has a
value of 11005Ω-m (Tab.4.6). The seventh layer has a low resistivity value of 2875
Ω-m. This lowering of resistivity in the seventh layer is probably due to the fact the
seventh layer is within a water saturated zone; hence the water table is suggested to
exist at a vertical depth of about 212m in this region.
94
VES VIII is seven-layer region whose resistivity values increased gently from
the first layer to the third layer. Reduction in resistivity is observed in the fourth layer.
There is another increase in the fifth layer through the sixth after which another
gradual decrease in resistivity is observed in the seventh layer (Tab.4.6). The gradual
reduction in the resistivity values from 7852 Ω-m to 5879 Ω-m at the infinite depth
layer suggests the presence of water-bearing rock. a borehole is expected to penetrate
beyond 218m depth in this station for efficient yield of groundwater
In the ninth profile (VES IX) a seven- layer earth model is suggested. The
resistivity is seen to increase from the second layer up to the sixth layer (Tab.4.6).The
value in the sixth layer is 9758Ω-m whereas in the seventh layer, it is 1025 Ω-m. This
decrease is drastic and probably results from the presence of water bearing rock layer.
This suggests the existence of the water table at a depth of about 155m. Therefore, in
this region, a borehole within a depth of about 200m will give a good yield of water as
it probably exists within the zone of permanent water saturation.
By the end of the above analyses and applying similar judgments to all the
VES data, table 4.7 was generated. The table shows in summary the minimum depth
at each VES location to which boreholes should be drilled for efficient yield of
groundwater.
4.5 Discussion of survey result in terms of the subsurface condition
The variation of resistivity of a particular rock or sediment is enormous and is
greatly controlled by the percentage of water content within the pore spaces
and layers of rock. Table 2.2(a) shows the bulk resistivity variation of some rock
types.
95
Table 4.7: Estimated depth of water bearing aquifer at the VES points.
G.P.S
Readings
VES Location G.L ρ
(Ωm)
Thickness(m) Depth
(m)
Remark
N 06055
i
E 07023
i
Elevation
344.0m
I Erike 6-
8
2255
13325
55.00
Infinity
85.0
>215.0
Perched aquifer ;about 85m deep
Main aquifer.
N 06053
i
E 07022
i
Elevation
369.0m
II Alor-Uno 6
8
1587
2250
102.0
Infinity
135
>220.0
Perched aquifer: about 135m deep
Main aquifer from the depth of about
220.0m
N 06052
i
E 07023
i
Elevation
400.0m
III Amaogbo 5
7
1258
8520
20.0
Infinity
30.0
>222.0
Perched aquifer
Main aquifer.
N 06052
i
E 07023
i
Elevation
369.0m
IV Isiuja 5
7
680
3542
21.0
Infinity
35
>220
Shallow perched aquifer about 35m
deep.
Main aquifer from the depth of about
220.0m
N 06054
i
E 07024
i
Elevation
366.0m
V Eluagu 7 3895 Infinity >215.0 Main aquifer from the depth of about
215.0m.
N 06054
i
E 07025
i
Elevation
359.0m
VI Ochikum 7 5574 Infinity >218.0 Main aquifer originating from the depth
of about 218.0m
N 06053
i
E 07024
i
Elevation
413.0m
VII Amaugwu 7 2875 Infinity >212.0 Main aquifer from a depth of about
212.0m
N 06057
i
E 07025
i
Elevation
342.0m
VIII Ibagwa 4
7
1145
5879
11.50
Infinity
14.0
>218.0
Perched water table about 14.0m deep
Main aquifer.
N 06056
i
E 07024
i
Elevation
343.0m
IX Itchi
Rd.Ibagwa
7 1025 Infinity >155.5 Main aquifer.
96
whereas table 2.2 (b) shows the variation of resistivity of rock types and sediments as
a function of percentage water content. Clearly pictured from the tables is that the
resistivity of rocks is largely controlled by rock texture and water content. For the
above reason, a sample of granite with 0% water content (dry) has a resistiviy value of
about 1010
Ώm. The same sample with 0.19% water content has resistivity of about
1.8x106 Ώm and with 0.31% water content; its resistivity is about 4.4x10
3 Ώm. Thus,
it is obvious that the presence of water in a rock changes the resistivity of the sample
drastically hence the applicability of resistivity variation in establishing regions of the
subsurface saturated with water from other non-saturated substrata.
More so, the capacity of a rock sample to contain and transmit water is in turn
controlled by porosity and permeability. Porosity is a measure of the percentage of
void spaces within the rock and controls water storage capacity of the rock or soil
sample while permeability measures the degree of interconnection of the pore spaces
and determines the water transmission capacity of the sample. The two quantities
(porosity and permeability) are important determinant factors of aquifer types.
Porosity in rocks can be broadly categorized as intergranular, jointed or vugular
porosity. Intergranular porosity can be found in consolidated rocks of sedimentary
origin while jointed porosity mainly occurs in basement rocks. Solution channels in
limestones or gas bubbles from volcanic rocks create the third category of porosity
called vugular porosity. Consequently, for a rock to conduct, pore spaces must be
interconnected and filled with fluid. In all three types of porosity, the pore volume
may consist of two parts; the larger voids (storage pores) and the finer interconnecting
pores. The implication of the above is that a rock with a high ratio of interconnected
pores to storage pores has a higher permeability than a rock in which the opposite is
true. Therefore clay (shale) has lower resistivity than sandstone.
97
Having seen that electrical character of rock is chiefly determined by rock
texture and water content, it then suffices to apply the resistivity contrast in
establishing formational boundaries. This follows from the fact that factors such as
fossils that are used by geologists in establishing boundaries between layers have no
effect on the electrical properties of rock. Consequently however, the resistivity layers
(geoelectric sections) therefore, do no necessarily correspond separately with each of
the geologic or lithostratigraphic sections of the subsurface layers but rather combined
sections of approximately common resistivity under a single bed. Therefore, with
reference to the information available from tables 2.2 and 4.3 and figure 3.3, the
lithostratigraphic profile (Fig 4.5) was constructed. The profile indicates that within
the deepest probe of each VES profile, the last two layers are lithologically composed
of sandstone unit of different texture. A very thick layer of dry coarse grained
sandstone underneath which is found the watertable which marks the top of the stable
aquifer of medium- coarse grained sandstone unit of indeterminate thickness exists..
The dry sandstone unit has a thickness range of 85.0m (thinnest) to 192.0m (thickest)
at different locations. The watertable assumes a gently undulating trend following the
general relief pattern of the surveyed area. They occurred within the depth ranges of
212.0m-222.0m in the majority of the portions of the surveyed area and appear
drastically reduced towards the north. The northern region which is at lower elevation
appears to border with the recharge zone hence the 155.0m water table depth obtained
within the northern portion.
4.6 Correlation of the survey result with geology/borehole data
The area under study is composed mainly of sandstone. Recognized from
some of the outcrops exposed within the area of study and also mentioned in earlier
references in this work that Nsukka formation lies conformably on the Ajali
98
sandstone. The lateritic cap of Nsukka formation contains clay. The ratio of storage
pore to the connecting pore is high in clay implying high porosity and low
permeability and therefore low value of resistivity in sandstone with clay intercalation
since there would be water in the pores. The underlying Ajali sandstone has high
permeability which implies overlaying Nsukka formation would have lower resistivity
than Ajali sandstone. Owing to the high permeability to the Ajali sandstone, it will
yield significant quantities of water; hence for the survey area, Ajali sandstone
constitutes the major aquifer.
Correlation of the results of survey was achieved using information obtained
from logged boreholes of known litostratigraphic description within and around the
survey area. Borehole BH.A (Fig.3.3a) is central to most of the VES centres and was
used for the correlation. BH.A is about 1-1.5km from the centre of VESI, VESII,
VESIII, VESVI, and VESVII and is closest, about 500m away to VESV thus, it
furnishes a good control for the correlation and interpretation of the resistivity
measurement. The maximum depth penetrated by BH.A is 190m. This depth suggests
that the water table occurs at a depth greater than 190m in the location. Resistivity
survey measurement within this location (from VESV) suggests a depth of 215m for
the top of low resistivity water-bearing layer. This depth of 215m from the survey
result is the depth to the water table. Profile V is used for this initial correlation due to
its close proximity to BH.A. Thus to a fair approximation, both VESV profile and log
of BH.A diagnose the same subsurface condition. The low resistivity 3895Ωm in the
seventh layer of VES V (Fig.4.4) is a zone of permanent water saturation and
corresponds to a layer of stable aquifer of medium-coarse grain sandstone. The above
correlation analysis would also hold for other profiles within the same geologic
formation: Hence figure 4.5 summarizes the correlation of the entire geoelectric
sections with the boreholes (B.H.A) logs.
99
Borehole BH.I log (Fig. 3.3b) was similarly utilized in the correlation of the result of
the resistivity measurement. Being about 1km away from the centre of VESIII and
about 2km away from the centre of VESIV, BH.I was juxtaposed with the geoelectric
sections of VESIII and VESIV for initial correlation as shown in fig.4.6. BH.I
penetrated to a maximum depth of about 236m approximately and is efficiently
productive. The VES measurement of profiles III and IV suggest 222m and 220m as
the depth to the water table within these locations respectively. In VESIII section, a
drastic drop in the resistivity value to 8520Ωm is obtained in the seventh layer which
begins at 222m depth below the surface. The drop in resistivity is ascribed to the
presence of a water saturated layer. The same trend is obtained in VESIV. The
resistivity value dropped from 12500Ωm in the sixth layer to 3542Ωm in the seventh
layer at a depth of 220m.Thus the VES measurement of profiles III and IV establishes
that the water table occurs at the depths of 222m and 220m in the location. T he
correlation to a fair approximation is in agreement with the information of borehole
(BH.I). Figure 4.7 presents the correlation of the entire geoelectric section with
borehole (B.H.I) log.
From the foregoing analysis presented from the correlations above, BH.A
penetrated to depth of 190m. This depth is neither suggested nor supported by this
present research work. Fathomable however is that BH.A has not penetrated the stable
aquiferous horizon hence is not efficient. From VES measurement of profile V,a
depth of about 210m -220m should be penetrated by a borehole in order to be
efficiently productive all year round. In the areas within the vicinity of VESIII and
VESIV, boreholes are expected to penetrate beyond 220m as suggested by resistivity
measurements and supported by the borehole information. More so, the result of a
geophysical survey (Fig.4.8) carried out within the region in May 2005 by Felgra
Links Nigeria Limited recommended that borehole should penetrate well over the
depth of 199m within the area.
100
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
240
250
Top lateritic sand
Extremely coarse sand soil
Medium-coarse grained sand stone with clay
intercalation
Extremely coarse sandstone
Clay intercalations
Extremely coarse sandstone with clay
Medium grained sandstone
Medium-coarse grained sandstone
Well sorted/scattered medium grain
sandstone.
Coarse grained sandstone
End of bore hole.
650Ωm
6
85
2Ω
m
1352
0Ω
m
3
895
Ωm
Water table
215m
Water saturated medium-coarse grain
sand stone
Dry medium-fine
grained sandstone
Dry sand
Wet sand
Top lateritic sand
15m
45m
Bore hole data VES V resistivity layers Scale (m)
2.5m
Fig. 4.4 Correlation of Obukpa bore hole (no. 32) data with geoelectrical
resistivity layer data of VES V.
101
1234
5Ω
m
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
240
250
7542
Ωm
15
87
Ωm
20
158
Ωm
1258
Ωm
1
25
00
Ωm
680
Ωm
1
35
20
Ωm
6852
Ωm
14
528
Ωm
1052
Ω
m
11
005
Ω
750Ω
m
8572
Ωm
7
852
Ωm
4578
Ωm
975
8Ω
m
2580
Ωm
2250
Ωm
8
52
0Ω
m
3
542
Ωm
3
895
Ωm
55
74
Ωm
2
875
Ωm
587
9Ω
m
1
025
Ωm
VE
S I
VE
S V
I
VE
S V
VE
S V
II
1332
5Ω
m
1781
5Ω
m
2255
Ωm
13520Ω
m
Fig. 4.5: Correlation of bore hole data with geoelectric sections
178
45
Ωm
123
45
Ωm
2
015
8Ω
m
12
500
Ωm
1
35
20
Ωm
1
452
8Ω
m
11
005
Ωm
7
852
Ωm
9758
Ωm
VE
S V
III
VE
S I
X
VE
S I
II
VE
S I
I
VE
S I
V
Depth (m)
12250Ωm 9550Ωm 4525Ωm 4135Ωm 650Ωm 2785Ωm 750Ωm
1145Ωm 1899Ωm Top lateritic sand
Extremely coarse sand soil
Medium-coarse grained sand stone with clay intercalation
Extremely coarse sandstone
Clay intercalations
Extremely coarse sandstone with clay
Medium grained sandstone
Medium-coarse grained sandstone
Well sorted/scattered medium grain sandstone.
Coarse grained sandstone
End of bore hole.
Bore hole data
Water table
102
Fig. 4.6: Correlation of VES III and IV geoelectric section with bore hole
(BHI) data
2
015
8Ω
m
12
500
Ωm
VE
S I
II
VE
S I
V
Depth (m)
4525Ωm
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
240
250
20
158
Ωm
1258
Ωm
1
25
00
Ωm
680
Ωm
4135Ωm
Reddish lateritic soil
Dark brownish coarse sand
Brownish gravelly sand
Light brownish coarse sand
Light brownish-white medium grain sand
Light brownish medium-coarse grain sand
Brownish clay
BH
.I
0
85
20
Ωm
35
42
Ωm
2.5
103
Light brownish-white
medium-coarse grain
sand
Light brownish-white
medium grain sand
Light brown coarse
sand sandl
Brown gravelly sand
Dark brown coarse
sand
Reddish lateritic
soil
Fig. 4.7: Correlation of borehole data with geoelectric sections
178
45
Ωm
123
45
Ωm
2
015
8Ω
m
12
500
Ωm
1
35
20
Ωm
1
452
8Ω
m
11
005
Ωm
7
852
Ωm
9758
Ωm
VE
S V
III
VE
S I
X
VE
S I
II
VE
S I
I
VE
S I
V
Depth (m)
12250Ωm 9550Ωm 4525Ωm 4135Ωm 650Ωm 2785Ωm 750Ωm
1145Ωm 1899Ωm
Water table
1234
5Ω
m
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
240
250
7
542
Ωm
15
87
Ωm
20
158
Ωm
1258
Ωm
1
25
00
Ωm
680
Ωm
1
35
20
Ωm
6852
Ωm
14
528
Ωm
1052
Ω
m
11
005
Ω
750Ω
m
8572
Ωm
7
852
Ωm
4578
Ωm
975
8Ω
m
2580
Ωm
2250
Ωm
8
52
0Ω
m
3
542
Ωm
3
895
Ωm
55
74
Ωm
2
875
Ωm
587
9Ω
m
1
025
Ωm
VE
S I
VE
S V
I
VE
S V
VE
S V
II
1332
5Ω
m
1781
5Ω
m
2255
Ωm
13520Ω
m
Clay
Borehole data
104
60
199.0
115.0
9.0
6.0
2.0
0.6
00m Sand
Sand
Sand
Sand
Dry sand
Dry sand
Wet sand
20000
30000
800
100
0
300
10000
Apparent resistivity (Ωm) Lithology
Fig. 4.8: Result of resistivity
survey at Ibagwa road showing
geoelectric layers and lithology
(Felgra Links )
105
4.7 Conclusion
The results of apparent resistivity measurements and interpretation of the field
data generated during this survey presented a two eight-layered and seven seven-
layered resistivity structure. The VES result of the entire profile indicated that the
water table occurs within an approximate depth of 210m within the time of the
survey. This depth of 210m is not absolute as it is seasonally varying being depends
on the quantity of rainfall, rate of recharge of the aquifer and discharge. The
interpretation of the nine resistivity curves over Obukpa and the surroundings within
geologic terrain often referred to as Ajali formation which bears false-bedded
sandstone with associated clay and shale intervals in the bottom section indicates that
the area has abundant groundwater potential.
However existing data show that very few of the already existing boreholes
within these regions were drilled down to this depth. The implication is that just the
surface of the aquifer horizon within this region has been penetrated by these
boreholes hence the possible reason for the failure of the borehole to yield significant
quantities of water all year round more especially during the dry seasons after heavy
drawdown. Existence of a productive borehole (BH.I) around the study is a field
confirmation of the groundwater potentialities of the geologic formation of the study
area
The above finding is fundamental to finding a lasting solution to water scarcity
problem as it would stand as a first guide to anybody sinking a borehole.
Furthermore, the resistivity values of the different layers reached and the
borehole log establish the lithology of the various geoelectric layers to be composed
mainly of Ajali sandstone which rank second as the most prolific aquifer in Nigeria.
The area possesses high groundwater potential.
106
4.8 Recommendation
For there to be an exhaustive geophysical survey within the area of study,
there should be a corresponding extensive geological mapping/survey. The area of
study has very scanty qualitative/quantitative geological information hence
geological correlation of geophysical survey result is inadequate or incomplete. A
preliminary and more advanced geophysical survey using at least two methods and
more sophisticated instrument is therefore recommended. For example, resistivity
method with spontaneous polarization methods should be carried out within these
areas for a better and wide-reaching result.
The department of Physics and Astronomy should partner with some co-
operate organizations and multinational firms involved in geophysical survey work to
avail students the opportunity to have an in-depth practical knowledge of their
academic effort. The department should also procure some of the equipment
(Gravimeter, terrameter, etc) for geophysical fieldwork and software packages to
enhance research work.
Since the formation in the area is mostly sandy and the water table deep
seated, the drilling method recommended is the direct rotary method using a highly
efficient rig. Enough quantity of drilling fluid/ chemical should be used to forestall the
incidence of bit stuck as well as collapse
Geophysical logging of borehole should be undertaken and supervised by an
experienced geologist/geophysist/water engineer before well completion.
Boreholes should be drilled to at least a depth of about 820ft (about 250m) to
ensure constant water supply in these areas. Having established that the region
possesses a very high ground water yielding potential, more individuals are
encouraged to involve in borehole for water supply business.
107
This will not only ensure the availability of the commodity all year round but
is also a source of huge revenue to the owner. After all, ground water is the only
source of potable water apart from rainfall within this region.
108
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