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GEOPHYSICAL INVESTIGATION OF RESISTIVITY LAYERS AND THEIR CLASSIFICATION USING ELECTRICAL RESISTIVITY METHOD

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Schlumberger Array: Electrical Resistivity Methods, Part 2

ABSTRACT

The aim of this study is to assess the resistivity layer in some selected area of Birnin-Kebbi, Nigeria. This was carried out by evaluating the geoelectrical and hydrogeological characteristics of the aquifer in the area. The Vertical Electrical Sounding (VES) technique using the Schlumberger array configuration was applied to investigate the geoelectrical characteristics. The data obtained were interpreted first by partial curve matching, computer iteration techniques. The result of interpretation indicates the presence of (3-5) three to five geo electric layers which are composed of fine sand formation clayey sand and loose sand mud stones peaty soil medium grain. The result also revealed the presence of confined aquifer located in layer (4) four and unconfined in layer (3) three. The various geoelectrical interpretations and hydrogeological results depict that the best sites for locating wells or boreholes in Birnin-Kebbi within study areas are VES stations 2, 3, 4 and 5. While probability of getting good ground water in VES station 1 is very infinitesimal, this might result from the presence of high accumulated pollutant in the station.

TABLE OF CONTENT
CERTIFICATION…………………………………………………………………………………………………….. ii
DEDICATION…………………………………………………………………………………………………………. iii
ACKNOWLEDGEMENTS……………………………………………………………………………………….. iv
TABLE OF CONTENT……………………………………………………………………………………………… v
LIST OF FIGURES…………………………………………………………………………………………………. vii
LIST OF TABLES…………………………………………………………………………………………………… vii
ABSTRACT…………………………………………………………………………………………………………… viii
CHAPTER ONE……………………………………………………………………………………………………….. 1
1.0     INTRODUCTION…………………………………………………………………………………………. 1
1.1     GEOPHYSICS……………………………………………………………………………………………… 2
1.2     HYDROGEOLOGICAL PARAMETER…………………………………………………………. 2
1.2.1       POROSITY…………………………………………………………………………………………… 2
1.2.2       PERMEABILITY………………………………………………………………………………….. 3
1.3    SIGNIFICANCE OF THE STUDY…………………………………………………………………. 3
1.4    STATEMENT OF THE PROBLEM…………………………………………………………………. 3
1.5    DESCRIPTION OF STUDY AREA………………………………………………………………… 4
1.6    AIM AND OBJECTIVES OF THE STUDY…………………………………………………….. 5
1.7   SCOPE AND LIMITATION……………………………………………………………………………. 6
CHAPTER TWO……………………………………………………………………………………………………….. 7
LITERATURE REVIEW…………………………………………………………………………………………… 7
2.0   INTRODUCTION…………………………………………………………………………………………… 7
2.1   PHYSIOGRAPHY AND CLIMATE………………………………………………………………… 9
2.2   GEOLOGY AND HYDROGEOLOGY……………………………………………………………. 9
2.3   GROUNDWATER FUNDAMENTALS…………………………………………………………. 11
2.4   DEFINITIONS OF TERMS…………………………………………………………………………… 11
2.5   TYPES OF AQUIFERS…………………………………………………………………………………. 12
2.5.1       Unconfined Aquifers…………………………………………………………………………….. 12
2.5.2       Confined Aquifers………………………………………………………………………………… 12
CHAPTER THREE………………………………………………………………………………………………….. 14
3.0   METHODOLOGY………………………………………………………………………………………… 14
3.1   ELECTRICAL RESISTIVTY SURVEY…………………………………………………………. 15
3.2   THEORY OF ELECTRICAL RESISTIVITY SURVEY…………………………………… 16
3.3   THE SCHLUMBERGER ARRAY…………………………………………………………………. 16
3.4   THE WENNER ARRANGEMENT………………………………………………………………… 17
3.5   PRINCIPLE OF ELECTRICAL RESISTIVITY SURVEY………………………………. 18
CHAPTER FOUR……………………………………………………………………………………………………. 20
4.0   DATA PROCESSING AND INTERPRETATION OF RESULT………………………. 20
4.1   DATA PRESENTATION-……………………………………………………………………………… 20
4.2   INTERPRETATION OF VES RESULT………………………………………………………….. 20
CHAPTER FIVE……………………………………………………………………………………………………… 29
CONCLUSION AND RECOMMENDATION………………………………………………………….. 29
5.0   CONCLUSION…………………………………………………………………………………………….. 29
5.1   RECOMMENDATIONS……………………………………………………………………………….. 29
REFERENCE………………………………………………………………………………………………………. 30
APPENDIX 1: RAW DATA FOR VES 1………………………………………………………………….. 32
APPENIX II: RAW DATA FOR VES 2……………………………………………………………………. 33
APPENDIX III: RAW DATA FOR VES 3………………………………………………………………… 34
APPENDIX IV: RAW DATA FOR VES 4………………………………………………………………… 35
APPENDIX V: RAW DATA FOR VES 5…………………………………………………………………. 36
APPENDIX VI: During Data Acquisition…………………………………………………………………… 37

             CHAPTER ONE

1.0     INTRODUCTION

In the shallow subsurface, the presence of water controls much of the conductivity variation. Measurement of resistivity (inverse of conductivity) is, in general, a measure of water saturation and connectivity of pore space. This is because water has a low resistivity and electric current will follow the path of least resistance. Increasing saturation, increasing salinity of the underground water, increasing porosity of rock (water-filled voids) and increasing number of fractures (water-filled) all tend to decrease measured resistivity. Increasing compaction of soils or rock units will expel water and effectively increase resistivity. Air, with naturally high resistivity, results in the opposite response compared to water when filling voids. Whereas the presence of water will reduce resistivity, the presence of air in voids should increase subsurface resistivity (Amadi & Nurudeen, 1990).

Resistivity measurements are associated with varying depths depending on the separation of the current and potential electrodes in the survey, and can be interpreted in terms of a lithologic and/or geo-hydrologic model of the subsurface. Data are termed apparent resistivity because the resistivity values measured are actually averages over the total current path length but are plotted at one depth point for each potential electrode pair. Two dimensional images of the subsurface apparent resistivity variation are called pseudo-­sections. Data plotted in cross-section is a simplistic representation of actual, complex current flow paths. Computer modeling can help interpret geoelectric data in terms of more accurate earth models (Burke, 1996).

The failure rate in must ground water project recorded in basement complex aquifars as informed the general acceptance of a geophysical survey as compulsory prerequisite to any successful water well drilling (Olurunfemi et al., 1999). The electrical resistivity method involving the vertical electrical sounded (VES) techniques is intensively gaining application in environmental, ground water and engineering geophysical investigations (Abubakar, 1965).

1.1     GEOPHYSICS

Geophysics is the science, which deals with investigation of the Earth using the principles of Physics. The physical properties of the Earth (rocks, air and water masses) such as density, elasticity, magnetic susceptibility and electrical conductivity all allow inference about those materials to be measured corresponding to physical fields such as gravity, seismic waves, magnetic fields and electrical fields etc. Geophysics is an interdisciplinary physical science that incorporates Physics, Mathematics, Geology and to some extent Chemistry. Geophysics can be divided into two main branches:

  1. Global Geophysics which studies large-scale problems relating to the earth’s structure as a whole and dynamic behaviour (Coruh, 1988).
  2. Exploration geophysics that is concern with the harnessing the potentials of the earth crust and perhaps the upper mantle.

1.2     HYDROGEOLOGICAL PARAMETER

Ground water and resistivity layers are characterized by a certain number of parameters which geophysical method are trying to determine from surface measurement, mostly indirectly, but sometimes directly most usually parameter are porosity and permeability.

1.2.1    POROSITY

The porosity is the ratio between the volumes of the pores and that of the rock. When dealing with saturated layers (under the water level, that is to say under the vadose zone where the pores are filled with air and with water), the water content is equal to the porosity.

Being a ratio, the porosity is expressed in percentage. The total porosity also includes the water located in clay, even if clay is impermeable. For the exploitation of water, it is important to determine the porosity of free water (water which can move), and hydro geologists speak of the effective porosity which is the ratio of the volume of the pores which are interconnected to the volume of the rock. As an order of magnitude, the effective porosity can be for instance 80% of the free water porosity. The porosity of a fissured rock can be a few percent, which of a gravel or a sand of the order of 30%.Porosity represents a measure of how much water a body of rock can hold, expressed as a percentage of the rock’s volume (Amadi, 2011).

1.2.2    PERMEABILITY

The permeability (which, is actually the hydraulic conductivity) is the ability of a material to let a water current flow through it when a hydraulic pressure is applied, can be defined on a sample of rock by the Darcy law:

If the porosity is almost zero the permeability is necessarily also very weak. Thus, permeability is dependent on the porosity character of the rock, as only porous rocks can be permeable to water (Amadi, 2011).

1.3    SIGNIFICANCE OF THE STUDY

The study of geophysical investigation of resistivity layer and classification using vertical electrical sounding (VES) method could provide base-line information which could be useful to both governmental and non-governmental agencies interested in the country (Nigeria). The study would in addition aid in demonstrating an alternative technology for exploiting different layers and classification in the study area that may be relevant to students of geophysics, hydrogeology, soil science and -researchers in similar disciplines.

1.4    STATEMENT OF THE PROBLEM

The different climatic conditions among regions of the world and/or marked spatial variations in weathering depth are reflected in the characteristics of regolith aquifers. The effects of these factors could be seen in the relationship that exists between saturated zone thickness and weathering depth. It applies the use of the Vertical Electrical Sounding (VES) technique. The Vertical Electrical Sounding (VES) is a geo electrical method for measuring vertical changes of electrical resistivity. The method has been recognized to be more appropriate for geophysical investigation of resistivity layer and hydro geological study of sedimentary basin (Kelly & Stanislaw, 1993).

The reason for its wide use is due to the fact that the instrument is simple to handle and relatively cheap compared to other techniques available for geophysical investigation of resistivity layer and classification. Also, the field logistics are easy and straightforward while the analysis of data is less tedious and economical. Therefore, many researchers have used this method for the determination of aquifer boundaries.

1.5    DESCRIPTION OF STUDY AREA

Kebbi state is a state in northern Nigeria. Birnin-Kebbi is a town, located in the southeast of Kebbi State lies within latitude12o 16’42N 4o27’6”E and longitude 12.27833oN 4.45167oE. The geology of Birnin-Kebbi local government area is sedimentary type, belonging to the Sokoto Rima Group and Gwandu Formation (Kogbe, 1976). The Gwandu group consists of massive clay grits, interbedded with sandstone and the Rima Group consist of clays, mudstones and siltstones it has an average elevation of 300 m above sea level, with Tropical Continental climate characterized by wet and dry seasons. The area has a long dry season of 8 to 9 months and a short rainy season of 4 to 5 months. The vegetation of the area is Sudan type. Birnin-Kebbi local government has a human population of 28,149. Boreholes have been drilled successfully in some communities of the local government. The major activity of people residing in this area is agriculture, and the main crops grown are rice, millet, sorghum, tobacco and onions. The study area is shown in the figure below.

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