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Dec 2013. Vol. 4, No. 6 ISSN2305-8269

International Journal of Engineering and Applied Sciences © 2012 - 2013 EAAS & ARF. All rights reserved www.eaas-journal.org

43

GEOELECTRICAL INVESTIGATION OF AN EXISTING DAM

WITHIN A BASEMENT COMPLEX TERRAIN, SOUTHWESTERN

NIGERIA

1OLADUNJOYE H.T.,

2OLASUNKANMI N.K. and

3OLATUNJI S.

1OLABISI ONABANJO UNIVERSITY, AGO-IWOYE, NIGERIA, [email protected]

2LADOKE AKINTOLA UNIVERSITY OF TECHNOLOGY, OGBOMOSHO, NIGERIA,

[email protected] 3UNIVERSITY OF ILORIN, ILORIN, NIGERIA, [email protected]

ABSTRACT

An existing dam site within the main campus of University of Ilorin, located around the southern flank of the

Nigerian basement complex in West central part of Nigeria was geoelectrically investigated using Vertical

Electrical Sounding (VES) technique of Schlumberger and Horizontal Resistivity Profiling (HRP) of Wenner

Array electrode configuration methods. The objectives are to investigate the vertical contact, lateral changes in

the geologic setting, the fracture pattern to evaluate possible dam seepage along the dam axis or the river

banks. Twenty seven (27) VES stations on eight profiles with maximum electrode spacing (AB) of 100 m and five

(5) HRP of 120 m were established along the flank of the dam. The lithology units delineated are the low

resistivity layer (top soil) , gravelly zone (mostly saturated) and weathered/fresh basement with resistivity

range of about 29-120 Ωm and thickness range of 0.8-3.1 m, 90- 200 Ωm with thickness range of 1.8-11.3 m

and 600-2577 Ωm at depth of 10 m downward respectively. Vertical Electrical Sounding (VES) revealed three

major lithological units delineated as the topsoil, weak/gravelly zone and the fracture/fresh basement. The

geoelectric sections showed low resistivity regions which might be due to percolation of water beneath 4

stations of which two (i.e. VES 1 and 10) are along the dam reservoir and the two starts at the depth of 10 m,

extending downward. The geoelectric maps show high resistivity range of about 29-2613 Ωm and thickness

range of 0.8-3.1 m. This is underlain by relatively low resistivity layer whose values loiters around 100 Ωm with

thickness range of 1.8-11.3 m, (remain values of fresh basement with its depth). This indicates near surface

bedrock straddled with basement depression or fracture. Though the weathered basement shows no surface

manifestation, but the significant water reduction can be experienced which is considered as responsible for low

recharge as the season changes. Different types of vertical contacts were able to delineate with the aid of HRP.

The pseudosections also revealed the depth to unexposed granitic intrusion that appeared in form of dyke in

some profiles and various vertical contacts were identified in the areas.

KEY WORDS: Dyke, HRP, VES, Seepage, Geometric factor, Bedrock

Introduction

Dams are vital elements which stores water for

purposes such as human consumption, food

production through irrigation, electricity

production, industrial use, recreation, flood

protection etc. It is a barrier of concrete or earth

that is built across a river or stream to obstruct or

control the flow of water, especially in order to

create a reservoir. Dam failure is usually fatal

which involves destruction of lives, properties and

the ecosystem 1. Hence, it is important to note that

once a dam is completed, it represents a potential

risk in the future, because of several factors acting

on the structures2. History has shown that flood

disaster is not a recent phenomenon, and that its

destructive tendencies are sometimes enormous.

For instance, in Nigeria, report has it that serious

flood disasters have occurred in Ibadan (1985,

1987 and 1990), Oshogbo (1992, 1996, and 2002),

Yobe (2000), Akure (1996, 2000, 2002, 2004 and

2006) and the coastal cities of Lagos, Ogun, Port

Harcourt, Calabar, Uyo, and Warri3, 4

.

University of Ilorin Dam is the studied dam which

is located within basement complex of Nigeria.

(Figure-1)5. It lies entirely within the basement

rocks in the Western part of Central Nigeria

bounded by longitudes 4° 39'52" to 4° 40'0"E and

latitudes 8° 27'5" to 8° 28'5"N. The study site is

easily accessible through a moderately wide

motorable road that eased the fieldwork (Figure-2).

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44

The topography of the area is relatively plain with

minor undulations (Figure). The city is drained

mainly by Asa River and its tributary river such as

Aluko, Alalubosa, Okun, Osere, Agba and Oyun

River which forms the drainage basin. The main

river within the campus is river Oyun which is

dendritic in nature flows from South-East to North-

West.

Despite series of geotechnical studies preceding the

construction of dams, there are still number of

problems that dams are prone to. Such problems

can be caused by existence of concealed

fracture/faults, fissures, joints or shears which can

greatly reduce the reservoir capacity of the dam.

Thus, dams require ongoing maintenance,

monitoring, safety inspections, and sometimes even

rehabilitation to continue safe service. The

objectives of this work are to investigate the

vertical contact, lateral changes in the geologic

setting, the fracture pattern and possible dam

seepage along the dam axis and its banks for safety

of the dam.

different dam breach parameter estimators that are

normally used to describe the physical

characteristics of a dam can be compared in the

study of behavior of dam. These parameters

comparison must go along with their computation

if they are within the specific model. the empirical

and embankments erosion process models had been

used as case study for this comparison6. integrated

geophysical methods to test a concrete dam, in

which Seismic geoelectrical, and GPR methods

were used. The geoelectrical method was used to

detect a possible water saturation of the area which

might have effect of degradation of the mechanical

parameters of the dam body. Its geoelectrical study

makes use of dipole-dipole array at 2 m electrode

spacing7.

Vertical Electrical Sounding (VES) was used to

reveal a network of lineaments pressured to be

fractured in the geologic section of a dam situated

in a basement complex8. Momoh et al used

geoelctrical investigation of a dam site in Maro

area of central basement terrain of Nigeria using

about 64 VES of Schlumberger array. The

interpretation revealed 4 geoelectric layers viz: top

soil, laterite, weathered layer, resistive bedrock.

They were able to suggest the dam should be along

investigated transverse without reservoir. This

study makes use of the combination of Profiling

and Sounding method in studying the the static and

dynamic properties of this dam. Likewise the

lithological arrangement in conjunction with

seepage characteristics was observed from

modeling and its interpretation9.

This study was embarked upon to integrate

sounding and profiling along the dam axis and the

river bank. Schlumberger array using vertical

electrical sounding was employed to compliment

and probe the profiling information acquired

through Wenner array of the profiling. The

methods was able to give high resolutions in the

profiling and the sounding technique. This is

because a lot of vertical contacts were observed

from the profiling technique due to the

configuration employed.

Figure-1: Geologic Map of Nigeria (After Oyawoye 1964)

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Figure-2: Road Map of University of Ilorin Showing the Study Area (Olasunkanmi et al)

MATERIALS AND METHODS

Electrical resistivity method of geophysical survey

using Vertical Electrical Sounding (VES) technique

of Schlumberger and Horizontal Resistivity

Profiling (HRP) of Wenner array electrode

configurations was used to achieve the stated

objectives of this work. Twenty seven VES

stations on eight profiles with maximum electrode

spacing (AB) of 100 m and five HRP of 120 m

were established along the West-East flank of the

dam. HRP involves the process where current and

potential electrodes are maintained at a fixed

separation and progressively moved along the

traverse after each measurement.

VES involves the process where the current and

potential electrodes are maintained at same relative

spacing and the whole spread is progressively

expanded along the profile.

Theory: The fundamental equations are derivable

from Ohm’s laws. The electric potential Vr at any

point P distance r from a point electrode emitting

an electric current I in an infinite homogenous and

isotropic medium of Resistivity ρ is given by10, 11

For a semi-finite medium, this is the simplest Earth

model, and with both current and potential point-

electrodes placed at the Earth’s surface.

( )

Irrespective of surface location and electrode

spread, the resistivity is constant in a homogenous

and isotropic ground. However, it does vary with

the relative positions of electrodes when there is

presence of subsurface inhomogeneities and any

computed value is known as apparent resistivity ρa.

( )

For a semi-finite medium, which is the simplest

earth model, and with both current and potential

point-electrodes placed at the earth’s surface,

8° 29'

8° 28'

4° 39' 4° 40'

4° 39' 4° 40'

8° 29'

8° 28'

STAFF QUARTERS

UNJLORIN PRY SCHOOL

UNILORIN SECONDARY SCHOOL

UNILORIN DAM

RECEPTION GROUND

WORKS (ADMIN)

PG HOTEL

SCHOOL CLINIC

SUGBUILDING

GT BANKSKY BANKUNILORIN

CENTRALMOSQUE

SENATEBUILDING

SUGAR RESERCH INSTITUTE

FACULTY OF ENGINEERING

MAIN CAMPUS GATE

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Figure-3: The Generalized Form of the Electrode Configuration Used in Resistivity Measurements.

(

) (

)

(

) (

)

But,

For the HRP;

since, ( )

Thus,

Where R is the measured resistance and is the Geometric Constant which is a function of the electrode

configurations employed during the survey.

RESULT AND DISCUSSION

Field Curves: The result of the Horizontal

Resistivity Profiling (HRP) and the Vertical

Electrical Sounding (VES) conducted around the

Unilorin dam is presented both in Linear Plots and

2-D Resistivity Structure along the traverses

surveyed. The geologic equivalence models of

lateral and depth resistivity variations are

juxtaposed with the linear plots along all the

Horizontal Resistivity Profiles as shown in Figure-

4 below. The linear plots were obtained by plotting

the profiling midpoints against their instantaneous

apparent resistivity value. These plots were able to

depict the weak zones as profiling progresses on

the transverse. Field inspection gives/ helps in

detailed understanding of these plots. From the

plots, profiles beside the dam bank depicted low

resistivity values compared to profiles along the

river bank. This might be due to saturation of these

profiles as a result of the pressure of the volume of

the water in the dam bank.

Graph of Apparent Resistivity(ohm-m) against Midpoints(m)

App

aren

t Res

istiv

ity(m

)

Midpoints(m)

53.950

120.170

186.390

252.610

318.830

385.050

2.000 26.200 50.400 74.600 98.800 123.000

Wet SandLaterite Wet Sand

Late

rite

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Figure-4: HRP Curves with Inferred Geologic Equivalence.

Figure-5: Typified Resistivity Field Curves of Sampled VES stations

The Vertical Electrical Multilayer Sounding field

curves obtained after curve matching and computer

iteration showed various types of curves which

were determined by the relationship existing

between the layer resistivity values ρ1, ρ2, ρ3 ... ρn. It

was discovered that the curves are mostly H-type

(23 VES stations), while 2 VES stations showed A-

type, 1 VES station showed HA-type and 1 VES

station showed HKH-type. Summary of the

formation layer thickness, classification of the

resistivity sounding curves and samples of the

curves are presented on table-1 below

TABLE-1: Depth Sounding Interpretation Result

Location

Layer 1 Layer 2 Layer 3 Layer 4 Layer 5

h1 (m) ρ1 (Ωm) h2 (m) ρ2 (Ωm) h3 (m) ρ3 (Ωm) ρ4 (Ωm) h4 (m) ρ5 (Ωm) h5 (m)

VES 1 1.1 783 10.2 258 -- 893.1 -- -- -- --

VES 2 1.4 2343.1 8.8 142.6 -- 4366.7 -- -- -- --

VES 3 1.0 2612.8 11.3 118.5 -- 1317.3 -- -- -- --

VES 4 1.2 1489.1 5.4 119.6 -- 1326 -- -- -- --

VES 5 2.5 514.2 9.1 143.4 -- 3483.9 -- -- -- --

VES 6 1.7 1464.4 9.3 148.6 -- 3566.4 -- -- -- --

VES 7 1.2 2039.8 8.3 187.5 -- 4376.9 -- -- -- --

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VES 8 1.8 724.5 3.8 117.6 -- 2056.3 -- -- -- --

VES 9 1.8 1111.2 8.8 164.2 -- 5690.7 -- -- -- --

VES 10 1.3 1785.7 9.4 170.5 13.8 549.1 -- 3234.6 -- --

VES 11 1.6 840.3 5.5 119.3 -- 2089.8 -- -- -- --

VES 12 1.9 628 5.9 157.6 -- 2506.4 -- -- -- --

VES 13 2.7 28.8 -- 2297.6 -- -- -- -- --

VES 14 1 611 7.7 189.3 -- 1667.4 -- -- -- --

VES 15 1.5 975.6 4.5 123.2 -- 1627.3 -- -- -- --

VES 16 1 122.3 4.9 67.6 -- 2121.6 -- -- -- --

VES 17 1 608.6 3.4 120.2 -- 2014.1 -- -- -- --

VES 18 1 1276.3 8.9 142.7 -- 2639.9 -- -- -- --

VES 19 3 470.6 5.9 146.9 -- 622 -- -- -- --

VES 20 1.1 397.6 1.8 198.5 3.2 1232.5 6.6 164.6 -- 3820.3

VES 21 1.2 626.9 12.1 316.8 -- 2091.4 -- -- -- --

VES 22 0.8 487.1 4.7 87.1 -- 3258 -- -- -- --

VES 23 3.1 75.7 4.9 270.7 -- 2165.2 -- -- -- --

VES 24 1.2 217.8 4.8 107.1 -- 2780.2 -- -- -- --

VES 25 0.8 290 5.1 91.2 -- 3235.4 -- -- -- --

VES26 1.2 221.7 3.7 63.2 -- 3083.6 -- -- -- --

VES27 0.9 759 5.2 110.8 -- 2834.1 -- -- -- --

TABLE-2: Classification of the Resistivity Sounding Curves.

TYPE

CURVES

RESISTIVITY

MODEL

MODEL

FREQUENCY

LOCATION

(VES)

A ρ1 < ρ2 2 13,23

H ρ1 > ρ2 < ρ3 23 1,2,3,4,5,6,7,8,9,11,12,14,15,16,17,18,19,21,22,24,25,26,27

HA ρ1 > ρ2 < ρ3 < ρ4 1 10

HKH ρ1>ρ2<ρ3>ρ4<ρ5 1 20

TOTAL 27

The Topography of the area: The Global

Positioning System (GPS) reading which gives the

coordinates and elevation of an area was used to

obtain the surface topographic map of the area. The

map shows the undulation, rugged troughs and crests

due to erosions which characterize the topography of

the area. The flow net of the dammed river, the dam

axis, the dam reserve and the overflow part of the

dam is shown on the map (Figure-6).

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Figure-6: Topographic Map of the Study Area

Pseudosections: This is a phase diagram

showing the fields of different equilibrium mineral

assemblages for a single bulk-rock composition.

Apparent resistivity pseudosections were obtained

by contouring the apparent resistivity values, in

ohm-meters, in the vertical and horizontal

directions.The lithological arrangement predicted

by the linear plots shows that area around the dam

comprises of wet sand underlain by lateritic sand,

saturated gravel, weathered basement and basement

complex. In the same vein, the lithological

arrangement of profiles beside the stream flank

depicts combination of literate soil, gravelly layer,

and basement complex.

In the case of VES, the horizontal location

of the point is placed at the mid-point of the set of

electrodes used to make that measurement. The

vertical location of the plotting point is placed at a

distance which is proportional to the separation

between the electrodes.

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Figure-7: HRP Pseudosections

Fig. 5: Vertical Psuodosections

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Figure-8 VES Pseudosections

Figure-9: Resistivity of Layered Model at 10m Depth

The sections show apparent resistivity value

variations and the characteristics at different

depths. The resistivity variation from the horizontal

profiling in the surface layer have the range of 28.8

Ωm to 2612 Ωm, which shows that the texture of

the rock constituent is hard and dry due to high

exposure to intense radiation from the sun. At a

depth of about 4 m to 35 m, a relatively weak zone

with apparent resistivity value ranging from 100

Ωm to 342 Ωm is shown in most profiles and a

Dyke-like structure was observed at depth of about

10.0 m with resistivity ranges between 1515 Ωm -

12735 Ωm as shown on the 2D structure for profile

1. It forms a laterally elongated structure in the

form of void. The weak zone could be as a result of

water percolation or as the tip of a weathered rock

of the basement rock underlying the area.

Conclusion

The results revealed the presence of near

surface basement which are expected to serve as

the sealing barrier for water retention in the

reservoir. These basements show no fracture

signature which might be a threat of the failure of

the dam. Some vertical contacts which look like

fractures were observed along the river bank, these

features aligned and agreed with past work on

River Oyun which suggested to have controlled

fracture13

. The anomalous percolation zone/weak

zone obtained at near-surface in the HRP resistivity

structure showed the adverse effect of careless

domestic activities around the study area. These

activities if not properly checked or controlled

might tamper with the sealing barrier which will be

dangerous to the dam. It can be concluded that

Wenner configuration with multiple spacings (a)

ranging from 5m, 10m, 15m, 20m, 30m is good

enough in identifying and precisely locating

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geologic vertical contacts, weak zones, faults, and

fractures. The fracture pattern of the study area is

unevenly distributed. Also the depth to the

basement in the study area is shallow. The low

resistivity areas, weak zones, delineated by the

geoelectric map present little or no risk of reservoir

water seepage but may be the water table or the

area is extensively marshy, capable of retaining

water to the ground surface level. The suspected

fractured basement observed beneath some

sounding points along the dam bank, started at the

depth of 10 m and proceeds downward as

explained in figure-9. These can be considered

inimical to the continued water retention or zone of

anomalous seepage but no surface manifestation

and significant water reduction experienced. The

presence of fracture is generally accompanied by

high fluid streaming potential and substantial water

can be lost through the fractures thus initiating the

weakening of the dam foundation

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