CHAPTER ONE

INTRODUCTION

1.1 GENERAL STATEMENT:

Groundwater is one essential but necessary substitute to surface water in

every society. It’s no doubt a hidden; replenish able resource whose occurrence

and distribution greatly varies according to the local as well as regional geology,

Hydrogeological settings and to an extent the nature of human activities on the

hand.

Groundwater occurrence in a Precambrian Basement terrain is hosted within

zones of weathering and fracturing which often are not continuous in vertical and

lateral extent (Jeff, 2006). There is a steady rise in the demand for ground water in

most hard rock areas most of which cannot boast of any constant surface source of

water supply (Adam, 1994). The failure rate in most ground water project recorded

in Basement Complex aquifers has informed the general acceptance of a

geophysical survey as a compulsory prerequisite for any successful water well

drilling project (Dan Hassan, 1999). The electrical resistivity method involving the

vertical electrical sounding (VES) technique is extensively gaining application in

environmental, groundwater and engineering geophysical investigations.

1

Integrating electrical resistivity method of geophysical study with Basement

subsurface structure trends provides a very useful tool in predicting ground water

distributing pattern in a Basement Complex rocks terrains. Vertical electrical

sounding (VES) as a method of geophysical exploration measures the change of

formation resistivity with depth. Based on this analogy, a study was carried out in

Ayaran village in Akoko Edo Local Government Area of Edo State which is within

the Basement Complex terrain of South Western Nigeria and four of such sounding

were conducted to investigate the subsurface for borehole sinking.

A first qualitative interpretation of the geo-electric sounding curves gives a general

outline of the geologic settings of the area from which assertion can be made on

the geo-electric layers and the nature of these layers.

Top soil, sedimentary layers, weathering front and basement (massive or fractured)

are typical of Basement Complex terrain and as such water is most likely to be

found in joints and fractures. The electrical methods have proved versatile in

determining such aquifers pertinent in these areas.

2

TYPICAL RESISTIVITY OF SOME EARTH MATERIALS

Dry (Ωm) Wet (Ωm)

Top soil 200-2400 45-250

Dari crust 400-1600 270-380

Clay 1-100

Alluvium & Sand 800-2500 100-800

Highly weathered/

fractured rock

300-106

Massive bedrock 1000-106

Granite >102-106

Shale 10-104

Gabbro 103-106

Schist 10-104

Sandstone 1-108

Fig. 1: shoeing typical resistivity of some earth materials.

3

Fig.2: showing the geology map of Nigeria

1.2 AIMS AND OBJECTIVES

To establish that ground water development in Basement Complex is facilitated

by proper geophysical investigation prior to drilling.

To provide avenue to get use to VES data acquisition and interpretation.

To show the role of vertical electrical sounding in groundwater exploration in

hard rock areas.

4

To provide information on the existing subsurface layering in the study area for

the purpose of planning and executing successful bore hole drilling

programmes.

To define the nature and distribution of ground water in a typical Basement

Complex aquifers.

1.3 SCOPE OF STUDY

It has been observed that improper subsurface investigation can result in

failure of borehole schemes. With a proper knowledge of the subsurface

configuration, this project has as its aim, the utilization of the results for the

sinking of bore holes.

The scope of work intends carrying out extensive geophysical (electrical

resistivity) survey within the study area, rudimentary geological mapping,

literature review and computer interpretation which will inform the

recommendation to be adopted with respect to bore hole citing.

1.4 GEOLOGY OF THE STUDY AREA

Ayanra village is located within latitudes N 07° 30’ and N 07⁰ 26’ and

longitudes E 06⁰ 53’ and E 06° 00’. The town is situated along the Auchi road at

the Southern part and Ikhakumo towards the North. The major problem of the

5

study area is its lack of sufficient and safe water supply for domestic uses. The

major source of domestic water for the inhabitant in the study area is from both

hand-dug well and a stream at Oshunba. However, this stream is polluted by

activities of the local farmers especially during the dry season for fermentation of

cassava, washing of melon, clothes, passing of feaces resulting in unhygienic

nature making it unsafe for drinking and domestic uses. The area is located in the

Northern part of Edo

State in Nigeria and the rocks here belong to the crystalline basement of

South Western Nigeria. The area is composed generally of low lying Basement

rocks. The area is underlain by ferruginised sandstone, quartz, rich sandstone, rich

sandstone (non- ferruginised) and clay stone. This clastic sediment underlies

migmatitegneiss Basement Complex. The south western basement complex is one

of the three Basement Complexes in the country, the other two are the north central

and the South Eastern Basement complexes. The south western basement complex

of Nigeria lies to the rest West African in late pre-cambrian region to early

Paleozoic orogenesis. It extends westward and continues till Ghana. The basement

complex like the other two basement complexes has two major group of rocks.

These are;

1. Migmatite-gneiss complex which comprises biotite and biotite horn blend,

gneisses with intercalated amphibolites and

6

2. Slightly migmatised, layered, medium grained granite and gneiss.

1.4.1 VEGETATION:

The area is part of the tropical rain forest belt of the South Western Nigeria.

There are two seasons, the rainy season which begins in May and ends in October

and the dry season which runs from November to April. The forest has been

drastically reduced due to persistent farming and bush burning. The area is of rural

setting and the inhabitants practice peasant farming. They grow cash crop mostly

cocoa and palm produce. Some practice mixed cropping such as yam, cassava,

melon, maize, pepper, beans, onions, and vegetables. Some of them engage in

fishing as part time job from the river available in the village.

7

Fig .3: geologic map/ location of VES point of the study area

1.4.2 Groundwater occurrence in the study area:

The hydrogeologic settings of the area is a typical of any Basement Complex

terrain and groundwater in such terrain is usually found in two situations

(Bannerman and Ayibotele 1984): E.Y. Mbiinibe et al, Continental J. Earth

Sciences 5 (1): 56-63, 2010. Fractured poorly decomposed or fresh rock overlain

by a relatively deep zone of well decomposed rock and the fractured rock

8

Groundwater is known to be more promising within granular alterite and the

transition zone immediately overlying the fresh bed rock (Chilton and Smith

Carrington 1984). In the study area, groundwater was identified to occur within the

weathered mantle developed on the crystalline rocks mainly migmatite, The zone

of weathering is relatively regular within VES 1 and VES 4 and slightly irregular

as confirmed by the variations in depth to bedrock which varies from rocks having

experienced prolonged weathering and tectonism which has given rise to thick

weathered mantle of 11-13m.

Fig.4: Map showing the ground water province of Nigeria

9

1.4.3 Hydrogeology of study area:

The hydro geologic settings of the study area are typical of any Basement

Complex terrain. Usually, in hard rocks, storage of water depends mostly on the

total thickness of the weathered and fractured zones and the yield here compared

with that of alluvial and sedimentary area is very small (R.K. Verma 1950).

Aquifers are formed in these hard rock areas from weathered and fractured zones

with the extent of weathering being depended on the presence of fractures at depth

and surficial morphological features. Thus, the geology of the area suggests good

hydraulic characteristics in terms of groundwater storage in the weathered zone.

Fissures on fresh rock joints tend to close at a depth of about 70m below which

there will practically be a limited circulation of ground water (M’ Kireld ,1950).

Isolated water may form below reservoirs mainly within fractured rocks and

pockets of weathered rocks with varying porosity and permeability of these

isolated reservoirs resulting in widely variable yields. The main source of recharge

in the area is through precipitation during the wet season. The main

hydrogeological unit in the area is the weathered zone.

10

Typical thickness of various layers in hard rock is given below:

Layer No Rock Type Thickness (M)

1. Top soil 1 to 2

2. Weathered layer 10 to 20

3. Semi weathered layer/fractured 10 to 20

4. Hard rock Up to infinity

Fig.5: showing typical thickness of various layers in hard rock

Interracial and fracture porosities are common in weathered rocks in which

the clay are present as a result of the feldspar content in these rocks, thereby

reducing the permeability to some extent. Fracture porosity is common in jointed

and fracture rocks and these rocks are able to yield sufficient quantities of water to

meet the needs of a small community.

1.5 GEOPHYSICAL METHODS IN HYDROGEOLOGICAL STUDY

In the area of ground water study, the utilized methods include:

1. Gravity method

2. Electrical resistivity method

3. Seismic refraction method

4. Electrical self- potential method

11

Of these five (5) methods listed above, the most commonly used, especially

for detailed exploration are the seismic refraction and electrical resistivity methods

(Koefed 1979, Telford et al 1976,) Lennose 1962,Vanderbeghe). However, the

most electrical resistivity method employing the Schlumberger array are used for

the study due to its low cost of field operation, its ability to detect local

inhomogeneities and its ability to investigate the change in formation resistivity

with depth.

1.6 PREVIOUS WORK

The literature work dealing with evaluation of ground water potential in the

weathered zone of the crystalline basement is diversified, some works in

weathering profiles are available (e.g. Oviei 1969, Renva 1964). Hydrogeologists

have been able to understand the occurrence of ground water in regoliths

(Omosinbola 1950). It was noted that most of the aquifers in the regolith of the

crystalline basement rocks are mostly of the perched type caused by irregular

weathering pattern of the rock.

12

CHAPTER TWO

METHODOLOGY

2.1 RESISTIVITY METHOD:

The electrical resistivity method employed in this study is

the Schlumberger array configuration . Electrical prospecting

makes use of a variety of principles, each based on some

electrical properties or characteristics of the materials in the

earth (Egbai and Asokhia, 1998). In this method, measurements

were made with increasing separation between the electrodes

about the midpoint. The instrument used for data acquisition was

the ABEM 3000 SAS Terrameter having an inbuilt booster. This

equipment has the ability of computing and displaying the

apparent resistivity on the subsurface with the input data of the

current electrode AB and potential electrode MN separation. There

are a lot of geophysical methods which use measurement of voltages or magnetic

fields associated with electric currents flowing in the ground. The current may be

natural but are more often artificially generated by direct contact or

electromagnetic induction. Two types of arrays are in common use, the Wenner

array, and the Schlumberger array. Arrays can be use for either profiling and/or

depth sounding, often refer to as electric trenching or drilling respectively.

13

Electrical resistivity surveys are used routinely in mining, coal, geothermal,

engineering application, hydrogeological investigation (Zoldy, 1964, Al’pon et al

1966, Kienetz 1966). They are also use in oil and gas exploration (Keller, 1968,

Eadie, 1981, Spies, 1983). Their relatively recent use for sensing buried wastes and

waste mining is documented in Stoilar and Roux (1965). For the purpose of this

investigation, however, Vertical Electrical Sounding method of electrical

resistivity survey was adopted to investigate the electrical properties of the ground

in vertical discontinuities.

2.2 PRINCIPLES OF RESISTIVITY SURVEY:

The resistivity sounding method was first adopted by Conrad Schlumberger

(1912). This method involve the introduction of artificially generated current to the

ground. The generated potential differences are measured at the surface and

subsequently compared to the pattern of potential differences expected from

homogenous ground. The interpretation of the measurement is based on the

assumption that the subsurface consists of a sequence of distinct layers of finite

thicknesses, each of these layers is assumed to be electrically homogenous and

isotropic and the boundary between subsequent layers are assumed to be

horizontal.

14

These assumptions present only a very ideal description of the real

conditions that exist in the subsurface. The nature of the deviations of the real

subsurface conditions provides information on the form and electrical properties of

subsurface inhomogeneities.

The resistivity of a material is defined as the resistance (R) in Ω between the

opposite face of the unit cube of the material (Keary and Brooks). For a cylinder,

the resistivity is usually represented by (𝜌). The resistance (R) across a unit length

(L) of cross sectional area (A) is expressed as 𝜌¿R.A/L (Ohms)

……………………………………...1

Resistivity (𝜌) becomes apparent resistivity (𝜌a) when there is a deviation

in the assumption of homogenous materials to inhomogenous materials. All

resistivity measurements in general use require the measurement of resistance (R)

and the geometric factors used to calculate the apparent resistivity (𝜌a) can be

calculate from the first principles. Consider a current passing through homogenous

materials such as a cylinder, it will cause a potential drop (-𝛿v) between the end of

the element. The current in a conductor is generally equal to the voltage across it

divided by a constant; the resistance. This principle is known as Ohms law. The

resistance (R) is measured in Ohm when current (I) is in amps and voltage (V) is in

volts. Ohms law is related mathematically

15

V = IR……………………………………………………..2

Substituting equation 2 into 1

𝜌.L = V/I. A 𝜌I/A = V/L. 𝜌I……………………………………………………………..3

Where V/L = potential gradient through the element in volt m-1 and I = current

density in amps m-2.

But consider a single current electrode on the surface of a medium of

uniform resistivity (𝜌). At the far end is the current sink away from the electrode.

The current, flow in form of a hemisphere shell away from the electrode centre at

the source of a distance, r. Therefore the surface area is 2πr2, so the current density

(I) is given by

i = I/2πr2………………………………………………..4

From equation (3), the potential gradient (v) associated with the current density (i)

is given by

V/r = -𝜌i = 𝜌I/2πr2

16

Current flow line

v

equipotential surface

Fig. 6: showing current flow from a single surface electrode

Note that the minus sign only indicate that the current is acting / flowing in an

opposite direction.

By integration with respect to V and r

𝛿v = 𝜌I/2πr2 𝛿r

∫ =𝜌.I/2π∫1/r2

17

𝛿v

v

V = 𝜌I/2π [r-2+1 / -2 +1]+C V = 𝜌.I/2πr…………………………………………………………………5

Equation (5) allows us to calculate the potential at any point on or below the

surface of a homogenous half space. The potential gradient across electrode (C1

and C2) will be

Vp1 = 𝜌I/2π [1/r1 – 1/r2]……………………………………...6

Where r1 = distance from current electrode C1 to potential electrode P1.

R2 = distance from potential electrode P1 to current electrode C2,

Similarly,

Vp2 = 𝜌I/2π [1/r3 – 1/r4]…………………………………....................7Where r3 = distance from current electrode C1 to potential electrode P2. R4= distance from potential electrode P1 to current electrode C2.Therefore the potential difference across the circuit is 𝚫V = Vp1 – Vp2 𝚫V = 𝜌I/2π [1/r1 – 1/r2] – [1/r3 – 1/r4] is a function of the electrode separation and it is a measure of

18

the amount of earth that contribute to the resistivity while the 2π represent the half space covered by the circuit. Thus, 𝜌a = 𝚫V/I. 2π/ [1/r1 – 1/r2] – [1/r3 – 1/r4]. Hence, 𝜌a is the apparent resistivity, it enables us to determine the change that occur in the character of the surface where inhomogenities exist, as the electrodes which are arranged on a line and then separation is increased in a systematic manner with increasing depth of penetration.

2.3 ELECTRODE CONFIGURATION:

This has to do with the manner in which the electrodes are arranged in

conducting an electrical resistivity survey.

These include the Wenner array which is widely used, with a vast amount of

interpretational literature; Two electrode array; gradient array used principally in

reconnaissance work, Dipole-Dipole (Eltran) array; popular in induced polarization

work, Pole-Dipole array; Square array; Multi-electrode array, and the

Schlumberger array; the only array to rival the Wenner in availability of

interpretational material. The Schlumberger array, in which only two electrodes are

19

moved, which is often preferred for speed and convenience was adopted in this

study. Consider a situation where the current sink (P1 and P2), which is the

electrode at a finite distance from the source (C1 and C2), which is the positive

electrode.

20

C1 P1 P2 C2

L L

Fig.7: showing Schlumberger Electrode Configuration

2.4 SITE SELECTION:

Site selection is extremely significant in all sounding works especially in the

Schlumberger array, which is very sensitive to condition around the closely spaced

inner electrodes. A location where the upper layer is very inhomogenous is

unsuitable for an array centre. Directions of expansion are often constrained by

topography. There may be only one direction in which electrodes can be taken in

sufficient distance in a straight line. Also, paved environment is not suitable

because it affects the conductivity of the electrodes.

In choosing the site of resistivity sounding measurement and in particular

positioning of the potential electrode in this study, adequate attention was given to

the erroneous effect of near surface inhomogeneities upon the measurement, such

21

I

V

as roads, ditches, wire fences, and buried metallic object like pipelines. Therefore,

if an inhomogeneity occurs close to the potential electrode, its effect is to alter the

potential difference measured.

Based on the above precautions, a total of four (4) evenly spaced VES site

were occupied in this study. In choosing the VES site in this study, it was ensured

that there was at least 150m of cleared straight line on both sides.

2.5 FIELDWORK AND EQUIPMENT:

Resistivity measurement were made using the Schlumberger array which

consists of two sets of electrodes, potential electrodes and the current electrodes

arranged in a straight line with a fixed point of array. Each of the electrodes

consists of metal stakes driven into the ground by hammer. Each of the electrodes

is then connected to ABEM AC Terrameter (ABEM SAS 3000), with cables made

of flexible multistrand insulated wires of several hundreds of metres in length.

The purpose of resistivity sounding is to investigate the change of the

formation resistivity with depth and this can be achieved by changing the distance

between the current electrodes, so that the depth range to which the current

penetrates is changed. Measurements were carried out such that there are six

equally spaced points on a decade of a log scale. The end result of the field

22

measurement is the computation of resistivity values and the thicknesses of the

layers. These are then plotted on a log-log paper.

Resistivity survey requires instruments and some means of making contact

with the ground, such as cables and electrodes. For the purpose of this survey,

metal bar electrodes were used. The cables used in resistivity measurement are

normally single core, multistrand copper wires insulated with PVC. The thickness

is usually dictated by the need for mechanical strength rather than low resistance.

The contact resistance which is the major limitation on current flow depends on

moisture and contact area.

The source of AC current for this survey was the ABEM SAS 3000,

TERRAMETER. This is a resistivity meter with reasonably high sensitivity. The

equipment is strong, potable and easy to use. This instrument has high penetration

capability (0-600m), which makes it suitable for subsurface investigation. It is also

very accurate to the tone + 3-10% for readings as low as 0.01 – 0.001 ohms.

23

Fig.8: showing the equipment used in acquiring VES in a given location

2.6 PRECAUTIONS:

Precautions are of great significance in any geophysical work. These were

strictly adhered to during the cause of the data acquisition and they include the

following:

1. The effect of lateral inhomogeneities close to the potential electrodes has to be

considered; the effect is to alter the potential difference measured.

24

CABLE

ELECTRODE

TAPE GPS ABEM TERRAMETER

2. Current cables must never be connected to or disconnected from the electrodes

while the current source is switch-on.

3. Grasses are cleared around the electrodes to prevent current leakage.

4. Stringent safety precaution was also observed in the whole length of the

current cables for passers-by and livestock.

5. Care was also taken not to allow the cable to become tangled, which can cause

permanent kinks.

2.7 DATA ACQUISITION:

A total of four (4) VES readings were taken using a terrameter and two sets

of electrodes; potential and current electrode arrange in a straight line with a fixed

point of array. The first point of consideration when using the schlumberger array

is that of station. The sounding stationing has to be sited on a long and straight

stretch of hand on a flat terrain, so as to minimize error of measurement and

interpretation. Limited separation of 0.2m was used for the potential electrodes and

this was increased on when it became too small for reliable reading. On the whole,

a total of four VES were made with half current electrode separation (AB/2) from 1.0

to about 147m. At least two readings were taken with the same values of AB as the

MN values were gradually increased. The sequence was 1.0, 1.47, 2.15, 3.16, 4.48,

10.0, 14.7, 21.5, 31.6, 46.8, 68.1, 100, 147 metres. This increase gives good

25

sampling intervals on a logarithmetric plot. Apparent resistivity values were

calculated by means of the usual constants based on the schlumberger electrode

array and plotted in double logarithm paper against half electrode separation AB/2.

The readings from the field data were illustrated as electrical resistivity sounding

curves. These curves represent the changes of apparent resistivity (𝜌a) as a

function of half current electrode separation.

2.8 REDUCTION OF FIELD DATA:

The current electrode and potential electrode are spaced away from each

other, with the potential electrode at a fixed point about a center position in exact

log spacing sequence. As the current electrode distance increases, the potential

difference reduced, until point is reached where the voltage-drop becomes too

small to be exactly measured and thus, the potential electrode has to be moved

further apart to a distance such that a fixed ratio is maintained.

The resulting sounding curve derived by plotting 𝜌a (Ω-m) against AB/2 (m)

at location on a logarithmic sheet will thus, consists of a number of separate

segments (fig.9). The reasons for these are not far-fetched. In the first place,

measurements are made with a symmetrical electrode configuration in which the

ratio of the potential electrode to current electrode has a finite value, changing the

distance between the measuring potential electrode tend to alter this ratio and thus,

26

alter the apparent resistivity which is dependent on it. The other reason is

attributable to the existence of near surface inhomogeneities which affect current

distribution pattern and the current density, thus reflecting in the resistivity

measurement (Koefoed, 1979).

For an easy interpretation to be made, the segments of the curves has to be

join by moving all the segment parallel to the resistivity axis so that a continuous

curve is formed. To do this, overlap reading must be made. Ideally, there should be

at least three such at each change-over but two are more usual and one is

unfortunately the norm (J. Milson, 1989).

By multiplying all the apparent resistivity (𝜌a) values at the beginning or

end of each segment by a constant factor depending on whether the segment is to

be raised if the sequence is increasing downward or dropped if the sequence is

decreasing downward. If the segment is to be raised, the constant factor is obtained

by dividing the higher apparent resistivity values with the lower values. While to

drop the higher apparent resistivity values of the segment, the constant factor is

obtained by dividing the lower value by the higher value at the point of change-

over, then, multiply all values below it by the constant factor.

27

100

𝜌a (Ωm)

10

10 100

AB/2 (m)

Fig.9: Diagram showing unadjusted curve

With the procedure above, it is assumed that the inhomogeneities is small

compared to the distance between the current electrode. Hence, the current-field

that would exist if the inhomogeneities were absent is very nearly homogenous in

horizontal direction. The linking removes the shift or jump in the curve, thus a

smooth curve is formed which can be interpreted by matching it with master curves

along side the auxiliary curves as recommended by Orellana and Moorney (1966)

or by a suitable computer program.

In this study, all the field curves were subjected to the smoothing procedure

described above and were later followed by computer assisted iterative

interpretation procedure as described by Zohdy, 1989.

28

2.9 INTERPRETATION PROCEDURE:

There are basically two methods of interpreting geo-electric data sounding

data.

1. The time consuming traditional method of interpretation; such as auxiliary

point technique (Zohdy, 1965.) or curve matching procedure using albums of

theoretical curves (Orellana and Mooney, 1966)

2. The other is the direct iterative computer assisted interpretation method.

The auxiliary point method, first published by Ebert (1943) involves

matching a small segment of the plotted curve with families of master curves (two

layer curves) and auxiliary curves (three layer curves) having equal modules as the

plotted curves. There are four types of curve which are employed in this

interpretation. These include the Ascending or A-type curve, the Descending or Q-

type curve, the Bowl shaped or K-type curve and the Bell shaped or H-type curve

which is the predominant in the case of this project. The A and Q-type layer curves

are two layer curves while the H and K-types are three layer curves.

The acquired data is plotted on a transparent log-log paper with 𝜌a on the Y-

axis and AB/2 on the X-axis. The transparent log-log paper is then superimposed on

the families of the master curves such that the co-ordinates of the master curve and

the plotted curve are parallel. One sheet is moved relative to the other, keeping the

29

vertical axis parallel until a segment of the field curve fits one of the families of the

master curve.

The computer assisted interpretation technique in resistivity sounding is

based on a 9-point digital lineal filter method of Ghosh (1970) to compute the

theoretical resistivity curve for a given set of layer parameters, or a 20-point digital

filter of O’Neil’s for models with layer parameters having resistivity contrast of <

1/25 for any two cons layers (Koefoed, 1979). Two stages are involved in

computational interpretation. The first stage involves the computation of the

“resistivity transform” of the sample values from the layer parameters. This is done

by the use of the Perekis Recurrence Relation given as;

Ti = [Ti + 1 + 𝜌 tan h (𝜆ti)]/[1 + Ti + 1 tan h (𝜆ti)/𝜌I]

Where I= 1, 2, 3,…….ni; denotes the number of subsurface layers.

Ti= resistivity transform corresponding to the ith layer.

Ti and 𝜌I= thickness and resistivity of the ith layer respectively.

The second stage involves using suitable computer program to evaluate the

equation given above; computed 𝜌a values were obtained for each measurement.

And by the process of “trial and error”, the model parameters were adjusted to

attain a good match between the field curves and the computed theoretical curves.

30

AB/2

AB2

𝜌a

Fig. 7 Showing .10: types of

curves

AB/2

Pa

CHAPTER THREE

31

a

𝜌a

AB/2

H –CURVE OR BOWL SHAPE

K – CURVE OR BELL SHAPE

A – CURVE OR ASCENDING

Q – CURVE OR DESCENDING

3.1 RESULTS OF DATA ACQUISITION INTERPRETATION

Results and interpretation of the soundings generated from the study area are

presented below as field and computed data, layer earth model and plot of apparent

resistivity (ρa) vs current electrode spacing (AB/2) and from which geo-electrical

sections were drawn. All the curves fall within the H or bowl type of Kalenov

classification (1957).

The observed field data were used to produce depth sounding curves. The

qualitative interpretation of field sounding curves were subjected to partial curve

matching techniques using two layer apparent resistivity curves. The sounding

curves were obtained as a result of plotting the apparent resistivity values from the

field work against electrode spacing. The results of the curved matched values

were iterated using the resist software (Vander Velpen, 1988). The computer

modeling utilized the quantitative Interpretation (curve matching) result to obtain

the layer resistivities and Thicknesses of the subsurface under investigation. This is

shown in the table below:

Results of Data Acquisition

32

Schlumberger Array

Nothing: 26928.5 Easting: 21467.6

Elevation: 332m

VES1

Point AB/2 (m) MN/2(m) Apparent Resistivity

(Ω m)

1 1.00 0.40 150

2 1.47 0.40 123

3 2.15 0.40 89

4 3.16 0.40 64

5 4.64 2.00 51.71

6 6.61 2.00 57.82

7 10.00 2.00 75

8 14.70 2.00 95

9 21.50 10.00 122.06

10 31.60 10.00 160

11 46.40 10.00 249

12 68.10 10.00 340

13 100.00 10.00 550

Fig.11.1a

Layered Model

33

S/N RESISTIVITY

(ohm-m)

THICKNES

S (meters)

DEPTH

(meters)

ELEVATION

(meters)

1 169.59 0.87885 0.87885 331.2

2 36.543 2.3620 3.2409 328.76

3 86.707 4.0909 7.3317 324.67

4 162.29 11.352 18.683 313.32

5 4756.4

Fig 11.1b:

34

3.2 RESULT OF VES INTERPRETATION

VES1

It shows an H or (Bowl) shape ascending type curve. From the model, there are

five interpreted geo-electrical sections. The first geo-electrical layer (GL1)

corresponds to the top soil which has a resistivity value of 169.59Ωm with a

thickness of 0.88m. The second and third layers (GL- 2 and GL-3) with resistivity

values of 36.543Ωm and 86.707Ωm and thicknesses of 2.3620m and 4.0909m

represent the clay, sandy clay layer. The fourth layer (GL- 4) which has a

resistivity value of 162.29Ωm and a thickness of 11.352m is interpreted as the

(sand layer) weathered zone. The fifth layer (GL-5) is interpreted as the fresh

Basement with a resistivity value of 4756.4Ωm and an infinite thickness.

35

Results of Data Acquisition

Schlumberger Array

Nothing: 26940.6 Easting: 21451.8

Elevation: 315m

VES 2

Point AB/2 (m) MN/2(m) Apparent Resistivity

(Ω m)

1 1.00 0.40 205.46

2 1.47 0.40 89.29

3 2.15 0.40 39.56

4 3.16 0.40 29.13

5 4.64 2.00 36.16

6 6.81 2.00 55.00

7 10.00 2.00 80.00

8 14.70 2.00 115.00

9 21.50 10.00 151.01

10 31.60 10.00 218.50

11 46.40 10.00 320.00

12 68.10 10.00 482.52

Fig. 11.2a:

36

Layered Model

S/N

RESISTIVITY

(ohm-m)

THICKNES

S (meters)

DEPTH

(meters)

ELEVATION

(meters)

1 451.91 0.48617 0.49617 314.51

2 17.642 0.86214 1.3483 313.65

3 15.298 0.97528 2.3236 312.68

4 344.10 3.3615 5.6851 309.31

5 680.29 6.5102 12.195 302.80

6 4172.2

Fig.11.2b

37

RESULT OF VES INTERPRETATION

VES2

This shows an H type curve. There are six geo-electrical layers from the model.

The first geo-electric layer (GL-1) with resistivity value 451.91Ωm with a

thickness of 0.48617m which is the top soil. The second and third geo-electric

layers with resistivity values of 17.642Ωm and 15.298Ωm has thicknesses of

0.86214m and 1.75m is interpreted as the clay layer. The fourth geo-electrical layer

with resistivity value of 344.10Ωm and thickness of 3.3615m represent the

silty/sandy layer or the slightly weathered zone. The fifth geo-electrical layer with

resistivity value of 680.29Ωm and thickness of 6.51m is interpreted as the

weathered or fractured zone. The sixth geo-electrical layer is the fresh Basement

and also the last layer with resistivity value of 4172.2Ωm and an infinite thickness.

38

RESULTS OF DATA ACQUISITION

Schlumberger Array

Nothing: 26933.9 Easting: 21467.9

Elevation: 329m

VES3

Point AB/2 (m) MN/2 (m) Apparent Resistivity

(Ω m)

1 1.00 0.40 109.62

2 1.47 0.40 92.79

3 2.15 0.40 76.68

4 3.16 0.40 65.76

5 4.64 0.40 58.50

6 6.81 4.00 70.98

7 10.00 4.00 104.80

8 14.70 4.00 149.80

9 21.50 4.00 217.80

10 31.60 4.00 320.00

11 46.40 20.00 460.00

12 68.10 20.00 640.00

Fig. 11.3a:

39

Layered Model

S/N

RESISTIVIT

Y (ohm-m)

THICKNESS

(meters)

DEPTH

(meters)

ELEVATION

(meters)

1 120.47 0.75473 0. 75473 328.25

2 64.508 1.1007 1.8555 327.14

3 21.096 1.4618 3.3173 325.68

4 863.97 2.9612 6.2785 322.72

5 7203.7

Fig. 11.3b:

RESULTS OF VES INTERPRETATION

40

VES3

It is an H type of curve with five geo-electrical layer based on the modeled layer.

The first geo-electrical layer represents the top soil with resistivity of 120.47Ωm

and thickness of about 0.755m. The second and third geo-electrical layers are

interpreted to be the clay/silty layer with resistivity values of 64.508Ωm and

21.096Ωm with thicknesses of 1.1007m and 1.4618m. The fourth geo-electrical

layer with resistivity value of 863.97Ωm with thickness of 2.9612m is interpreted

as the weathered zone or fractured layer. The fifth and the last geo-electrical layer

has resistivity value 7203.7Ωm with an infinite thickness and it represent the fresh

Basement.

RESULTS OF DATA ACQUISITION

Schlumberger Array

41

Nothing: 26938.2 Easting: 21471.6

Elevation: 342m

VES 4

Point AB/2 (m) MN/2 (m) Apparent resistivity

(Ω m)

1 1.00 0.40 250.00

2 1.47 0.40 190.00

3 2.15 0.40 150.89

4 3.16 0.40 130.00

5 4.64 0.40 120.00

6 6.81 0.40 123.00

7 10.00 4.00 132.00

8 14.70 4.00 180.00

9 21.50 4.00 234.00

10 31.60 4.00 324.00

11 46.40 20.00 434.70

12 68.10 20.00 579.15

Fig.11.4a:

Modeled Layer

RESISTIVIT

Y (ohm-m)

THICKNESS

(meters)

DEPTH

(meters)

ELEVATION

(meters)42

S/N

1 366.10 0.48777 0. 48777 341.51

2 126.02 2.0573 2.5451 339.45

3 88.476 4.7952 7.3402 334.66

4 789.51 12.961 20.302 321.70

5 1737.7

Fig.11.4b:

RESULTS OF VES INTERPRETATION

VES4

43

It displays an H type of curve. There are five geo-electrical layers based on the

modeling. The first geo-electrical layer is the topsoil with resistivity of 366.10Ωm

and thickness of 0.58m. The second geo-electrical layer has resistivity of

126.02Ωm and a thickness of 2.17m which is interpreted as the clay sand layer.

There is a drop of resistivity value which is 88.476Ωm with a thickness of 4.85m

indicating a clay silty layer. The fourth geo-electrical layer represents the

weathered layer with resistivity value of 789.51Ωm a thickness of 12.96m. The last

layer is the fifth with resistivity value of 1737.7Ωm with an infinite thickness

represents the fresh Basement.

CHAPTER FOUR

DISCUSSION

44

4.1 DISCUSSION OF THE RESULTS:

To study the possible variation of the subsurface in Ayanra for the exploration of

water, a total of four VES were measured and interpreted. To this end, contour

maps were generated. They are as follows;

1. Overburden thickness

2. Thickness of the fractured zone

3. Basement Elevation

4. Surface Elevation

45

Fig.11: showing the overburden thickness

OVERBURDEN THICKNESS

From the overburden contour map, the green colour indicates the regions

with the thickest overburden while the red colour indicate the region with thin

overburden, also known as the danger zone. The central region of the map is

dominated by the red colour thus, it represent the thinnest or shallowest region and

46

it is the least productive in terms of water prospecting. While the green colour

dominates the edge of the map thus, representing the thickest region of the map.

The importance of this map is to delineate the cut-off limit where water can be

drilled. From the map, it is evident that VES four with the thickest overburden

(7.34m) with respect to its fractured zone thickness, is the most productive.

Followed by VES one (7.33m) and then VES two.VES three with a very thin

overburden (3.32m) is a dangerous zone and should be ignore in other to cut down

cost during exploration.

47

Fig.12: showing the fractured zone

THICKNESS OF THE FRACTURED ZONE

This is the most significant when exploring for groundwater in a Basement

Complex as it indicates how productive a particular area is going to be with respect

to the other area by looking critically at the thickness values alongside its

overburden thickness thereby reducing the cost of drilling unnecessary amount of

boreholes. And it also help to decipher the type of drilling that should be

undertaken and the most suitable location for such exercise. From the fractured

zone map shown above, it is obvious that the most productive weathered zone is

located at VES four, followed by VES one and VES number two and this are

represented as the blue region on the map while the region coloured red indicates

the least productive zone due to how thin the layer is when compared to its high

resistivity value of 863.97Ωm. It is advisable that a total of two boreholes can be

drill through a depth of 11m – 13m at location four and one respectively.

48

Fig.12: Map showing surface Elevation

SURFACE ELEVATION

Water will normally flow from a region of high topography to a region of

low topography and this is evident from the above map as water will be expected

to flow from VES location four with the highest elevation to VES one location

probably due to the high rate of fractural connectivity as a result of secondary

porosity and unable to flow to location three due to massive blockage of

unweathered granitic rock with a very poor connectivity. This result also supports

49

the high prospect of location four and one. The presence of a stream along VES

two and four also show how water can take advantage of topography i.e. the river

will not be able to flow to a higher elevation (location two) thus flowing to location

four and through it also flow to location one whose fractured zones have a very

good connectivity with that of VES four. This also applies to the rainy seasons.

Meaning that location two may not be able to hold water for a very long time due

to its position.

50

Fig.13: Map showing the Basement Elevation

BASEMENT ELEVATION

Due to the several tectonic events that may have taken place in a specific

Basement Complex, the basement elevation might have been altered for a good

number of times which could have resulted to what we have in the above map.

From the map above, we can see here again that VES four with the highest

basement elevation is also shown to be the most productive. This may not always

accurately correspond to the above maps due to the effects of tectonism which

51

could have been a fault displacing a formal high location to a lower position, but to

a large extent it does support the results of the other maps and still show VES four

as the most productive.

A summary of the VES data interpreted according to the modeling is shown below.

VES

No

NORTHINGS

Deg min Sec

EASTINGS

Deg min Sec

Type

Description

Modeled

Layer

No

Overburden

Thickness

Fractured

Zone

Thickness

Basement

Elevation

Surface

Elevation

1 7 28 48.5 5 57 47.

6

H 5 7.33 11.35 324.67 332

2 7 29 01.0 5 57 31.

8

H 6 5.75 6.51 309.31 315

3 7 28 53.9 5 57 47.

9

H 5 3.32 2.96 325.68 329

4 7 28 58.2 5 57 51.

6

H 5 7.34 12.96 334.66 342

Figure.14: Showing the Interpreted Geo-electric Parameters

52

CHAPTER FIVE

SUMMARY AND CONCLUSION

Geophysical survey methods are now widely used for the investigation of

the subsurface geology. The electrical resistivity techniques carried out in Ayanra

is to investigate the nature and distribution of groundwater in weathered zones.

From the interpretation, different sections of the subsurface geology in the

Basement terrain (area studied) were revealed and made known including the

target zone (fractured zone) where groundwater occurs. This zone of interest is

known to exist at a depth range of 7.3 to 7.4m with a thickness of about 11to13m.

Therefore, it is advisable to drill the first borehole at VES 4 location because it has

the thickest overburden and productive window. At least two boreholes should be

drilled at a depth of 7-8 m. Electrical resistivity survey is very fast and the

equipment used in carrying out the operation is relatively cheap and easy to

operate when compare to other geophysical field method. Results of this study

have gone to some extent to prove that electrical survey is a practical tool for

obtaining significant geological subsurface information.

53

5.2. RECOMMENDATION

I recommend electrical resistivity method as an effective geophysical

approach to investigating groundwater distribution in Basement rocks before

proper borehole drilling is done, since it’s cheaper and safes time among other

known geophysical methods.

54

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56

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57