ABSTRACT
This project work was carried out at
Ireukpen-Ozalla Road Axis Ekpoma, Esan-West Local Government Area Edo State.
Using seismic refraction prospecting method to examine the cause(s) of the
persistent failure of the road. An ABEM TERRALOC MARK-6 Seismometer was used as
the recording instrument and twelve geophones as wave detectors in series with
one another. The geophones were spread at predetermined distances and the
impact of a sledge hammer on a flat plate served as the source of generating
seismic waves. The “SERCOM1” software was employed in the interpretation of the
field result for the forward and reversed shooting respectively. From which the
subsurface reveals two layers of velocities, which are 980ms-1 and
1283ms-1 for the forward shooting, 851ms-1 and 1276ms-1
for the reversed shooting respectively. The investigation also shows that at
twenty three metres (23 m) from the surface of the forward shooting, clay
deposit could be discovered. And even at thirteen metres (13 m) from the
surface of the reversed shooting clay deposit could also be encountered. The
form of road failure identified in this study is due to subsidence associated
with clayey material and the delineated clayey water absorbing sections which
are major geologic factors responsible for road failure in the area.
TABLE OF CONTENTS
Title page…………………………….ii
Certification…………………………..iii
Dedication……………………………..iv
Acknowledgement……………………v
Table of content……………………vii
Abstract……………………………….xiii
CHAPTER ONE: Meaning and Basic
Application of Geophysics
1.1
Introduction……………………..1
1.2
Location
of Study Area……………….2
1.3
Statement
of the Problem………… 7
1.4
Aim
and Objectives of the Study……..7
1.5 Significance of the
Study………….……...8
1.6 Meaning of
Geophysics……….………….8
1.6.1 Division of
Geophysics……….10
1.6.2 Uses of Geophysics……………10
1.7 Geophysical Techniques………………11
1.7.1 Data Acquisition………………11
1.7.2 Interpretation………………..12
1.8 General Geology of Study Area
…13
1.8.1 Regional Setting……………13
1.9 Stratigraphic
Nomenclature….…..14
1.10 Clay and its Mineral Deposit……16
1.10.1 Properties of Clay………..17
1.10.2 Formation of Clay………18
CHAPTER TWO: Geophysical
Surveying Methods
2.1 Introduction………………….……..21
2.2 Seismic Waves…………………….……..23
2.2.1 Types of Seismic Waves…………23
2.3 Classification of Geophysical
Survey Method…25
2.3.1 Natural Field Method……………25
2.3.2 Artificial Source Method………25
2.4 Gravity Methods…………………..……26
2.4.1 Principles of Gravity Method…27
2.4.2 Gravimeters………………………..28
2.4.3 Gravity Data Analysis………….29
2.4.4 Interpretation of Gravity Data……30
2.4.5 Application of Gravity Survey……32
2.5 Electrical
Methods………………………….32
2.5.1 Theoretical Background …………33
2.5.2 Types of Resistivity Survey
Techiques…35
2.5.3 Array System in Resistivity
Survey..36
2.5.4 Interpretation of Resistivity………38
2.5.5 Application……………………………………….…….39
2.5.6 Limitations………………………………………..……40
2.6 Magnetic
Method………………………….………………….41
2.6.1 Theoretical Background……………………………41
2.6.2 Equipment/Instruments……………………..……43
2.6.3 Data Acquisition and
Processing…………………44
2.6.4 Application……………………………………………..45
2.7 Radiometric
Method……………………………………..…36
2.7.1 Instrumentation and
Measurements…….……49
2.7.2 Interpretation………………………………………….51
2.7.3 Application………………………………………………51
2.8 Well Logging
Method………………………………………..51
2.8.1 Electrical
Logging…………………………………..52
CHAPTER THREE: Materials and
Method
3.1
Introduction………………………………………………….57
3.1.1 System Connection…………………………..……60
3.1.2 Preliminary Operation
Check……………………61
3.2 Theoretical Analysis of
Method…………………...……63
3.2.1 Two Layer Case with Horizontal
Interface……65
3.2.2 Three Layer Case with Horizontal
Interface…69
3.2.3 Multilayer Model with Horizontal
Interface…71
3.2.4 Dipping-Layer Case with Planner
Interface….72
3.2.5 The Hidden/Blind Layer
Problem………….……76
3.3 Field
Layout…………………………………………………...80
3.4 Corrections Method in seismic
Refraction……………81
3.4.1 Weathering
Correction…………………………...…82
3.4.2 Elevation
Correction……………………………..….83
3.5 Interpretation of Refraction
Records…………..………84
3.5.1 Travel-time equations
Methods………………....85
3.5.2 Delay-Time Method………………………………….85
3.5.3 Wavefront Method…………………………………..87
3.6 Application of Seismic
Refraction Surveying….…….88
3.6.1 Hydrological Survey……………………………….88
3.6.2 Engineering and Environmental
Surveys…..89
3.6.3 Crystal Seismology 90
CHAPTER FOUR: Results and
Discussion
4.1 Mode of Refraction Operation 91
4.2 Interpretation 103
CHAPTER FIVE: Conclusion and
Recommendation
5.1 Conclusion 105
5.2 Recommendation 106
References: 107
CHAPTER ONE
MEANING AND BASIC APPLICATION OF GEOPHYSICS
1.1 INTRODUCTION
It is to be noted that the
geophysical method of prospecting and delineation of anomalous zone in the
subsurface extend its wide application to buried material (both of economic and
non economic values) (Ozegin, K.O. et al., 2007). Also geological factors are
not often considered as precipitators of road failure even though the highway
pavement is founded on the geology (Momoh et al., 2008).
Seismic method has successfully help
in the search and exploitation of the subsurface. In particular, seismic
refraction method is commonly used to get detailed information of the
subsurface lithology, geologic setting (mapping), locating refracting
interfaces separating layers of different seismic velocity, subsurface mapping,
lithological boundary differentiation, engineering geophysics and static
correction.
It is sad to note, that, the
perennial or incessant failure and poor rehabilitation work on these roads has
become a very common phenomena and a source of concern. Generally, in seismic
refraction surveying technique, the method uses seismic energy that returns to
the surface of the Earth after travelling through the ground along refracted
ray paths. The vast majority of refraction surveying is carried out along
profile lines which are arranged to be sufficiently long to ensure that
refracted arrivals from target layers are recorded as first arrivals for at
least half the length of the line. It involves putting the first geophone
relatively far away from the shot point, and the shot and detector are on the
same line. Consequently, the ABEM TERRALOC MARK-6 were used. And a hand held
hammer was used to generate the source energy.
Many factors causes road failure,
these include;
Geogical,
geomophological/geotechnical, road usage, poor or bad construction practices
and maintenances. The influence of geology and geomorphology in the design and
construction phases may not have been adequately considered. The problem could
also be as a result of inadequate knowledge of the characteristics and behavior
of residual soils and not putting the bearing capacity of rocks in relation to
vehicular traffic into consideration. Furthermore, the geological factors in
road failure covers the nature of soils (i.e. laterite) the near surface
geological sequence, existence of geological structure like cavities, ancient
stress, channels and shear zones, near surface geological sequence. Sometimes
there is the presence of some concealed subsurface geological structure as well
as rock weakness or deficiency. One or more of the aforementioned factors has
been noticed to have contributed in some of our highway and rail track failure. For the purpose of
information, geomorphological factors are concerned or related to topography
and surface/subsurface drainage systems. Also, subsurface geologic sequence and
concealed geological structure can be mapped by geophysical method, hence its
relevance (Ozegin et al., 2007).
1.2
LOCATION OF STUDY AREA
The study area lies along the
Iruekpen-Ozalla Road, in Esan-West Local Government Area, Ekpoma, Edo State,
Nigeria. It has its headquarters in the town of Ekpoma, with an area of 502km2
and a population of 125,842 people, according to the 2006 national census. The
study area is located on 614411211N, 60813611E
as obtained from a reliable geographical positioning system (GPS) meter. The
Local Government Area is bounded on the South by Orhionmwon Local Government
Area, on the East, by Esan Central Local Government Area (L.G.A), on the West
by Uhumwonde and on the North by Owan West L.G.A. The people of the local
government are basically subsistent farmers and petty traders.
It is thickly forested with a moderate
temperature between 200C to 300C and a climate which is
predominately rainforest characteristics by two seasons, that is, dry and wet
season. Its topography is generally undulating (Ewanlen, T.A., 2010).
1.3
STATEMENT OF THE PROBLEM
The perennial road failure experience
at Iruekpen-Ozalla Road Axis Ekpoma, Easn West L.G.A has been characterised by
different problems, these include; loss of precious lives, properties,
transportation challenges and environmental degradation. Similarly, there has
be several cases of gully erosion in the neighboring communities and its
environs. Therefore finding the cause(s) why this road is constantly failing is
the focus of this study.
1.4
AIM AND OBJECTIVES OF THE STUDY
1.4.1 Aim
The aim of this study is to
investigate the causes (s) of the failure of this Iruekpen-Ozalla road axis in
Easn-West L.G.A Ekpoma.
1.4.2 Objectives
i.
To obtain data from survey using seismic refraction 2-D array in all
parts of the study area.
ii.
To quantitatively analyze the data obtained
iii.
To determine the presence of clay deposits within the study area.
iv.
To determine the thickness of clay deposit using seismic refraction
method.
v.
To identify zones of weakness.
1.5
SIGNIFICANCE OF THE STUDY
It is hoped that a research work like
this type can provide useful information for best policies and solutions, aim
at minimizing the loss of lives and properties, help fast track economic
development in Iruekpen and environs. Which can also be replicated in other
parts of the country that possibly share similar problem?
1.6
MEANING OF GEOPHYSICS
Geophysics is the science which deals
with investigating the Earth, using the method and techniques of physic. The
physical properties of the Earth materials (rock, air, and water masses) such
as density, elasticity, magnetization, and electrical conductivity all allow
inference about those materials to be made from measurement of corresponding
physical field-gravity, seismic waves, magnetic fields, and various kinds of
electrical fields (Encarta 2008).
In a more robust definition,
geophysics is a non-destructive and non-invasive Earth science (that is, the
study of the Earth and one or more of its part) that uses the very latest
science and technology in instrument, data acquisition and advanced computer
modeling and interpretation in subsurface exploration.
We use seismic, magnetic,
electromagnetic, radiometric and gravitational technologies and techniques to
determine the structure and composition of natural (and sometimes artificial)
material below the Earth’s surface without the need for drill or excavation.
1.6.1 Division of Geophysics
The two great division of geophysics
conventionally are labelled as; Global Geophysics and Exploration Geophysics.
1.
Global Geophysics
In global geophysics, we find studies
of; Earthquakes, physical oceanography, the main magnetic field, studies of the
Earth’s thermal state, meteorology amongst others.
2. Exploration Geophysics
In exploration geophysics, we find
the same physical studies applied, usually to the search for resources such as
oil, gas, minerals, water and building stone. Seismic prospecting method is the
most common geophysical survey method used.
1.6.2 Uses of Geophysics
Geophysics
can be used in many diverse situations such as: Hydrocarbon, mineral and
underground water exploration, Archaeology, Oceanography, Atmospherics,
Planetary science, Astronomy and Astrophysics, Geohazards, such as volcanoes
and earthquakes as well as urban utility mapping etc.
1.7 GEOPHYSICAL TECHNIQUES
Geophysicist
uses a variety of scientific techniques to determine subsurface structure of
the Earth and other bodies.
The main geophysical techniques used
are:
i.
Seismic (reflection and refraction)
ii.
Radiometric and ground penetrating radar (GPR)
iii.
Electrical
iv.
Magnetism and electromagnetism
v.
Gravity
1.7.1 Data Acquisition
Methods
such as seismic, sonar and GPR can be classified as active where a signal is
generated into the medium being analyzed. Because the different layers within
the medium have different density, part of the signal is reflected back to its
surface as the signal passes through the layers. Other equipment and
instrumentation (geophones or hydrophones) is then used to detect the signal
and record its new properties.
Other
methods such as magnetism, electromagnetism (induce polarization) and gravity
are passive in that instrumentation is used to detect changes in the medium
properties due to variation in its density and content. For example, a body of
iron-ore will have much higher magnetic and gravitational properties than the
Earth surrounding it.
1.7.2 Interpretation
Once the data has been collected in the field it can then be analyzed using powerful
computer and sophisticated software applications. After analysis, 2D and 3D
maps of the subsurface, magnetic or gravitational structure of the test area
are generated. In the case of resources exploration, based on the results of
the data analysis, the exploration team (made up of geophysicist, geologist,
petroleum, drilling, production and reservoir engineers) will determines the
most promising sites to continues with further exploration.
1.8 GENERAL GEOLOGY OF STUDY
AREA
1.8.1 Regional Setting
The
Niger Delta is situated on the gulf of Guinea on the West of Central Africa. It
built out into the Altantic Ocean of the mouth of the Niger-Benue River system.
The Delta’s is one of the world largest with the sub-aerial portion covering
about 75,000km and extending more than 300km from the apex to mouth (short,
K.G. and stable, A.J.I., 1967). Accumulating of marine sediments in the basin
probably commenced in Albian time, after the opening of the South Atlantic
Ocean between Africa and South America continents through delta. Its structure
and stratigraphy have been controlled by the interplay between rates of
sediments supply and subsidence. It is a typical wave and tidal dominated
Delta. One of the most striking features of the Delta is the sandy nature of
the sediments.
1.9 STRATIGRAPHIC NOMENCLATURE
As in many deltaic areas, it is
extremely difficult to define a satisfactory stratigraphic nomenclature.
However, three formation names are in wide spread use. They are;
Akata formation, Agbada formation and
Benin formation (Marron, P. 1967).
Akata Formation (Marine Shale’s)
This lithofacies is composed of shales, clays
and silts at the box of the known Delta sequence. They contain a few streaks of
sand, possibly of turbiditic origin and were deposited holomarine (Delta front
to deeper marine) environment. The thickness of this sequence is not known for
certain but many reads 7000m in the central part of the Delta. They crop at
offshore along the continental slope and onshore in the north Easthern parts of
the Delta. The marine shale forms the base of the sequence in each deposit belt
and range from Paleocene to Holocene age.
Agbada Formation (Paralic Clastics)
It
is represented by an alteration of sands, silts and clay in various proportions
and thickness, representing cyclic sequence of off lap units.
The
paralic clastics are the truly deltaic portion of the sequence and were
deposited in a number of Delta front, Delta topset and fluvio-deltaic
environments. The paralic sequence is present in all deposit belt and ranges in
age from Eocene to Pleistocene. A maximum thickness of more than 300m, under
this formation Edo State lies.
Benin Formation (Continental Sand)
The
shallowest part of the sequence is composed almost entirely of non-marine sand.
It was deposited in alluvial or upper coastal environments following a
Southward sloft of deltaic deposition into a new deposit belt. The oldest
continental sands are probably Oligocene, although they lack fauna and are
impossible to date directly.
1.10 CLAY AND ITS MINERAL DEPOSIT
Clay
is a naturally occurring material composed primary of fine-grained minerals and
part of the sedimentary rock formation. It show plasticity through a variable
range of water content and can be hardened when dried or fired. The mineral,
phyllosilicate, is found in clay deposit. It can be differentiated from other
fine-grained soils by differences in size or mineralogy for example silts soil,
which has larger particle size than clays.
1.10.1 Properties of Clay
The
advancement in technology like the x-ray diffraction technology has helped in
analyzing the molecular particles of clay. Hence, the term clay is applied both
to materials having a particles size of less than two micrometers and the
family of minerals that has similar chemical compositions and common crystal
structural characteristic. However, clay sized crystals of other minerals such
as quartz; carbonated metal oxides are also constituents of clay. Its colour
may range from dull grey to deep orange-red depending on the soil content.
Clay
properties include: plasticity, shrinkage under firing and air drying, fineness
of grain, colour after firing, hardness, cohesion, and capacity of the surface
to take decoration.
Clay
minerals have strong affinity for water. That is water molecules are attracted
to clay mineral surface, because when a little clay is added to water, a slurry
forms. Because the clay distributes itself evenly throughout the water. Hence,
it is properly utilized by the paint industry to dispense pigment evenly
throughout paint.
Another
important property of clay minerals is their ability to exchange ions
relatively to the charged surface of clay mineral. Hence, clay can be an
important vehicle for transporting and widely dispersing contaminant from one
area to another.
1.10.2 Formation of Clay
The
geological condition which clay and its minerals occur include: continental and
marine sediments, soil horizons, volcanic deposits, geothermal fields,
weathering rock formations. Most clay minerals form where rock are in contact
with water, air or steam. Also extensive alteration of rocks of clay minerals
can produce relatively pure clay deposits that are of economic interest for
example drilling mud and ceramic (Hillier, S. 1995).
Furthermore,
it is also to be noted that some of the factors that causes clay formation are;
erosion, Diagenesis and weathering.
Erosion:
The
transport and deposition of clay and its minerals produce by eroding older
continental and marine rocks and soils are important parts of the cycle that
form sedimentary rock. The ancient sedimentary rock record is composed of about
70 percent mudstone (containing 50% clay-sized and shale which are coarser than
mudstone and may contain clay sized particles).
Today,
sedimentary environment that contain muds cover 60% of marine continental
shelves and 40% of deep ocean basins. Therefore, clay is a critical component
of both ancient and modern sedimentary environment.
Diagenesis:
It
is the in- place alteration of mineral to more stable forms, excluding surfical
alteration (which is weathering). It occurs, for example, when minerals stable
in one depositional environment are exposed to another by burial and
compaction. For example, silicate minerals like, quartz, feldspars are
transformed during diagensis to more stable clay mineral by dissolution and re-crystallization.
Weathering:
The
primary way that clay and its minerals form at the Earth surface today, is the
weathering of rock and soil. Hence, weathering is the process that involves
physical disaggregation and chemical decomposition that change original mineral
to clay minerals. Weathering is uneven and many stage of breakdown may be found
in the same clay sample.
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