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International Journal of Science and Technology Volume 4 No. 4, April, 2015

IJST © 2015 – IJST Publications UK. All rights reserved. 156

2-D Electrical Resistivity Tomography for Groundwater Exploration in Hard

Rock Terrain

Ayodeji Jayeoba and Michael Adeyinka OladunjoyeDepartment of Geology, University of Ibadan, Ibadan Nigeria

ABSTRACT

In an attempt to examine the hydrogeological potential of the recently acquired land for University of Ibadan Cooperative Housing

Estate located at Alabata near Ibadan, south-western Nigeria, 2D electrical resistivity tomography (ERT) was utilized. The 2-D

resistivity imaging technique involves the Wenner electrode array configuration. Field data were obtained at six electrical resistivity

tomography lines with lengths varying from 140 m to 220 m. The field data were subjected to inversion in order to remove

geometrical effects from the pseudo-section and produce an image of true depth and true formation resistivity using Res2DINv

software. Three layers were revealed, which are top layer, weathered layer, and fractured/fresh basement rock. The overburdenthickness is relatively shallow ranging from 3 m to 13.4 m. The resistivity of the fractured/fresh basement rock varies from 146-

Ohm-m to 850-Ohm-m. In general, the results show that groundwater exploration and development is feasible in the study area.

Keywords: Hydrogeological Potential, Alabata, 2-D Resistivity Imaging, Inversion, Partially Weathered/Fractured Layer, Groundwater

1. INTRODUCTION

Water is one of the mankind’s most vital resources. An

adequate supply of water is one of the pre-requisites for

development and industrial growth. In areas where surface

water is not available, groundwater constitutes significant part

of active freshwater resources of the world and is obviouslydependable source for all the needs. Exploration of

groundwater in hard rock terrain is a very challenging and

difficult task when the promising groundwater zones are

associated with fractured and fissured media. In thisenvironment, the groundwater potential depends mainly on

the thickness of the weathered/fractured layer overlying the

basement (Al-Garni, 2009). The weathered material, which

constitutes the overburden, has high porosity and contains a

significant amount of water, and, at the same time, it presents

low permeability due to its relatively high clay content

(Barker, 2001). According to Sharma and Baranwal (2005),

fractures are the primary source to store and allow movement

of groundwater in hard rock areas. The size and location ofthe fractures, interconnection of the fractures, and amount of

the material that may be clogging the fractures and recharging

sources determine how much water one can get out of the

hard rock. Boreholes, which intersect fractures, but which are

not overlain by thick saturated weathered material, cannot be

expected to provide high yields in the long term. Boreholes

which penetrate saturated weathered material but which find

no fractures in the bedrock are likely to provide sufficient

yield for a hand pump only (Louis et al 2002). Fractures in ageologic medium can greatly influence its hydrogeological

characteristics. They can increase the hydraulic conductivity

of an otherwise impermeable rock or soil by orders of

magnitude in the dominant fracture directions. Therefore,knowledge of the presence, extent, intensity, and direction offractures is desirable for any hydraulic engineering project

(Louis et al., 2002). Hence, the location of potential fracture

zones in hard rock area is extremely important to yield large

amounts of groundwater.

The University of Ibadan cooperative housing estate at

Alabata area of Ibadan is located in the basement complex ofsouthwestern Nigeria. The study area is underlain by banded

gneiss, which is concealed. The development of the estate

requires a detailed groundwater evaluation of the area due to

the erratic nature of groundwater availability in the basement

complex.

Electrical resistivity methods have been employed in

groundwater exploration (Sharma and baranwal, 2005). The

traditional resistivity method of sounding and profiling gives1-D model of the subsurface, which is not adequate in

mapping areas of complex subsurface geology. In addition,

the basic sounding interpretation assumption of horizontally

stratified earth model, which do not match the localgeological model, and failure of the profiling method to map

changes in resistivity with depth are the major limitations ofthese methods (Griffiths and Barke, 1993). Electrical

resistivity tomography (ERT) provides a more realistic 2-D

resistivity model of the subsurface, where resistivity changes

in the vertical as well as in the horizontal direction along the

survey line are mapped continuously even in the presence of

geological and topographical complexities (Loke, 2000). The2D ERT method has been a powerful technique to investigate

shallow subsurface electrical structures in various

environments (Yang et al., 2002; Hauck et al., 2003; Cheng et

al., 2008; Crook et al., 2008). Studies have showed that 2D

electrical resistivity tomography has been employed in bedrock detection, geological mapping and groundwater

investigation (Zhou et al., 2004: Hsu et al., 2010 and Rao et

al., 2013).



International Journal of Science and Technology (IJST) – Volume 4 No. 3, March, 2015


In order to properly plan for the management of groundwater

resources in this estate, a hydrogeological characterization of

the area was carefully carried out using 2D electrical

resistivity tomography method.

Site Description and Geological Setting

The study area is located in Alabata, Ibadan, southwestern

Nigeria (Fig 1). It is confined within latitudes 70 34.970 and

7035.138 and longitudes 3052.180 and 3052.0. The study areais characterized by relatively gentle undulating terrain with

elevations of between 265 and 278 m above mean sea level

(msl). The study area has a tropical wet and dry climate with

a lengthy wet season and relatively constant temperatures

throughout the course of the year. The wet season runs from

March to October and November to February form the dry

season. The mean total rainfall is 1420.06 mm with two peaks

of rainfall in June and September. The mean maximum

temperature is 26.46 C, minimum 21.42 C and the relative

humidity is 74.55% (NIMET, 2011). The survey area is

underlain by the Precambrian basement complex rock of

southwestern Nigeria. The basement rock, which underlain

the study area is metamorphic rock, mostly undifferentiated

migmatite-gneiss, quartzite-schist, banded gneiss and granite

gneiss, underlie the area (Afenkhare, 2012). Figure 2highlights the local geology of study area, which falls in the

area underlain by banded gneiss. The coarse-grained banded

gneiss was low-lying. It strikes approximately north-south

with minor folds. There are quartz and pegmatite intrusions

occurring concordantly with the rock’s strike direction.

Fig. 1: Location of the study area

Fig. 2: Geological map of the study area (Afenkhare, 2012)

http://en.wikipedia.org/wiki/Tropical_wet_and_dry_climate

http://en.wikipedia.org/wiki/Wet_season

http://en.wikipedia.org/wiki/Wet_season

http://en.wikipedia.org/wiki/Tropical_wet_and_dry_climate





2. METHODOLOGY

2-D electrical resistivity tomography (ERT) is now mainly

carried out with a multi-electrode resistivity meter system.

Such surveys use a number (usually 25 to 100) of electrodes

laid out in a straight line with a constant spacing. A

computer-controlled system is then used to automaticallyselect the active electrodes for each measure (Griffith and

Barker, 1993) (Fig. 3).

In this study, field resistivity data were obtained along six

traverses. Traverses 1, 2 and 6 were oriented along N-S

azimuth while traverses 3, 4, and 5 were along E-W direction

with the traverse ranging between 140 m to 200 m long (Fig.

4). Wenner array with 5 m electrode spacing and a maximum

of five levels were attained for each of the traverse.

According to Loke (2000), the characteristics of an array that

should be considered in choosing array for field survey are (i)

the sensitivity of the array to vertical and horizontal changes

in the subsurface resistivity, (ii) the depth of investigation,

(iii) the horizontal data coverage and (iv) the signal strength.

Wenner array was considered for this field survey because it

is sensitive to vertical changes in the subsurface resistivity

below the centre of the array i.e. is good in resolving vertical

changes (horizontal structures), it has moderate depth ofinvestigation and the strongest signal strength compare with

other resistivity arrays.

The raw data were processed and interpreted using

RES2DINV software. The data were filtered to remove bad

datum points and inverted to estimate the true resistivity of

the subsurface. Model refinement option of the “Inversion”

menu was used to take care of the large resistivity variations

near the ground surface.

Fig. 3: The arrangement of electrode for a 2-D electrical survey and the sequence of measurements used to build up a pseudosection (Loke,

2000)

Fig. 4: Location map showing the 2-D electrical resistivity tomography profiles





3. RESULTS

The results of the inverse models are displayed as cross

sections of the true resistivity distribution of the subsurface

with depth along each profile (Figs. 6 and 7). The RMS errors

obtained in the inverted models were between a minimum of

3.6 to a maximum of 6.3 %. The models show a range of both low resistivity and relatively high resistivity zones.

The sensitivity function refers to the degree to which a

change in the resistivity of a section of the subsurface will

influence the potential measured by the array. The higher the

value of the sensitivity function, the greater is the influence of

the subsurface region on the measurement (Loke, 2004). The

way to reduce such ambiguity is to use additional

data/information.

The soil profile distinguished three major subsoil

stratifications for the site as indicated by the hand dug well of

6.2m in depth and Vertical Electrical Sounding survey carriedout close to the well which is 7.2m to basement (Fig. 5).

Fig. 5: Stratigraphic column from the hand dug well and VES point within the site

Profil e 1

Profile 1 (Fig. 6) located at the western part of the study area

(Fig. 4) trends N-S direction to with a length of 220 m. Theupper part of the layer revealed resistive materials as the top

layer, which has resistivity value of between 117-Ohm-m and

1000-Ohm-m and is about 4 m thick. The top layer was

interpreted as lateritic clay/sandy clay. This layer is underlain

by a conductive layer, which is about 5 m thick with

resistivity value between 10-Ohm-m to 117-Ohm – m, which isinterpreted as clay/clayey sand/sandy clay. This layer was

absent between lateral distance of 150 m to 170 m where the

resistivity of the material is 117-Ohm-m. It coincides with a basement rise. Below this layer is the basement rock with the

upper part interpreted as fractured/saturated basement, which

has resistivity value of between 372 to 850-Ohm-m. The high

resistivity values (1942-Ohm-m to 4439-Ohm-m) observed at

the depth range of 10 to 13.4 m at the southern part of the

section showed that the subsurface materials are resistive andwas interpreted as basement rock.

Fig. 6: Inverse model resistivity section for profile 1

Profil e 2

Figure 7 showed the inverted section of 2-Dimensional

imaging of profile 2 located at the eastern section of the studyarea trending N-S direction (Fig. 4). The maximum length of

the profile is 210 m. The profile showed top layer of about

4m thick with resistivity values ranging from 60-Ohm-m to

1000-ohm-m at the southern part of the profile due to the

presence of low lying coarse-grained banded gneiss outcrop.

The variation in the resistivity reveals the in-homogeneity

along the top layer. This layer is underlain by moreconductive layer, which has resistivity value of between 30-

Ohm-m to 110-Ohm-m and thinned out towards the northern

and southern ends of the profile with considerable thickness





between 40m and 80m. The layer is interpreted as clay/clayey

sand. Below this layer is fractured/saturated basement with

resistivity values ranges from 180-Ohm-m to 700-Ohm-m,

which formed the upper section of the basement rock. It has

uniform thickness of 6m across the profile. The base of this

profile was occupied by the basement rock, which has

resistivity value of 1241-Ohm-m and above. The depth to the

basement ranges between 3 m to 13.4 m (fig. 7). The uneven

thickness across the profile line indicates that there was no

uniform weathering across the profile line (fig. 7).


Profil e 3

Profile 3, located at the southern part of the study area (Fig.4)

trends W-E direction to a length of 145 m. The inverted

section of 2-Dimensional imaging shown in Figure 8 revealedtwo layers (top/weathered layer and fractured/fresh

basement). The top of the profile is occupied by clay/clayey

sand with resistivity value of between 40 to 120-Ohm-m,

which has a thickness of 5 m at the western end and 10 m at

the eastern end of the profile. This was followed by the

fractured/fresh basement, with the upper part of the basement

has resistivity value of between 200 to 800-Ohm-m, and has

uniform thickness across the profile line. This was interpreted

as fractured basement while the deeper part with hasresistivity value of 1000-Ohm-m and above marked the fresh

basement. At the west end of the profile, the basement rock is

closer to the surface than the east end while the weathered

layer has uniform thickness across the profile with different

depth across the profile. The weathering is more pronounce at

eastern end of the profile than the western end, which resulted

in thick clay/clayey sand at the eastern end (fig. 8).

Fig. 8: Inverse model resistivity section for profiles 3

Profil e 4

Profile 4, located at the northern section of the study area,

trends E-W direction to a length of 170m (fig. 4). The upper

part of the layer revealed materials with resistivity values

ranging between 120-Ohm-m to 400-Ohm-m, which wasinterpreted as sandy clay/lateritic clay. The thickness of the

layer varies from < 1.25m towards the eastern end of the

profile to 6.5m towards the western end of the profile. The

intermediate layer has resistivity values ranging between 40-

Ohm-m to 100-Ohm-m. The materials were interpreted as

clay/clayey sand with thickness varies between 3.5m to 12m.

The degree of weathering in this profile is high, resulting in

high percentage of the clay/clayey sand in the profile.Beneath this layer is the fractured/fresh basement with

resistivity ranges from 200-ohm-m to 1072-Ohm-m. The

upper section of this layer was interpreted as moderately

weathered/fractured basement with thickness of about 5 m.

The lower part of the layer represents the basement rock, with

resistivity above 1000-Ohm-m and dips westward (Fig. 9).






Profil e 5

Profile 5, located across the centre of the study area, trends

W-E direction to a length of 170 m (Fig. 4). The inverted

section of 2-Dimensional imaging of the profile (Fig. 7b)

showed that a thin moderately resistive layer occupies the top

of the profile with thickness varying from < 1.25 to 4 m. It

has resistivity values of between 80-Ohm-m to 130-Ohm-m,which was interpreted as the clayey sand/sandy clay. The

intermediate layer has low resistivity values ranging from 10-

Ohm-m to 80-Ohm-m, which was interpreted as the wet

clay/clay/clay sand. It has thickness varying from 3m to 4m

across the profile. The high resistivity zone of the basement

rock was identified at a depth of 10m to 12.5m. The

resistivity values vary between 200-Ohm-m to 1227-Ohm-m.

The resistivity between 200-Ohm-m to 700-Ohm-m at the

upper part of the layer was the fractured basement while therest was the fresh basement (Fig.7b).


Profil e 6

Figure 11 showed the inverted section of 2-Dimensional

imaging of profile 6 located at the centre of the study area and

trending N-S direction to a length of 210 m (Fig. 4). The top

layer of this profile is occupied by a layer interpreted asclay/clayey sand/lateritic clay, which has resistivity value

ranging from 60-Ohm-m to 450 -Ohm-m with uneven

thickness across the profile (<1.25 to 4 m). Underlain the

layer was more conductive layer with varying thickness

across the profile. The layer exposed as part of top layer insome points across the profile. The resistivity of the layer

ranges between 10-Ohm-m and 110-Ohm-m. The materials

occupying this layer were interpreted as wet-clay/clay/clayey-

sand. After this layer is the fractured basement with resistivity

value ranging from 169-Ohm-m to 800-Ohm-m, whichformed the upper part of the basement rock. The thickness of

the section ranges between 4m to 6m across the profile. The

fresh basement rock was identified with high resistivity

ranging from 1270 to 4865-Ohm-m (Fig. 6b). This layer was

found at the northern end of the profile protruding to form asyncline while pocket of it was found at the southern end ofthe profile. A vertical fracture was evident at 183 m lateral

distance.


DISCUSSION

Bedrock depth and basement topography are important

factors in terms of groundwater prospecting (Kumar, 2012).

In basement complex terrain, the geological structure

normally encountered is characterized by the existence of a

hard rock basement overlain by a weathered overburden ofvariable thickness (Louis et al., 2002). According to Olayinka

et al., (1997), the typical geological sequence in a basementcomplex terrain consists of top layer, highly weathered layer,

which is mostly clay/clayey sand (Wright, 1992),

fractured/fresh basement rock. The interpretation of data

resulting from the 2D ERT provide information on the lateral

variations of formations, depth to basement rocks, resistivity

of the weathered/fractured basement, presence or absence offracture zones, and aquifer potential.

The inverted sections of all the profiles have been classified

into three layers, based on the lithologies provided by the

hand dug well present on the site and the Vertical ElectricalSounding surveyed close to the well (fig. 5).





Generally, the distribution of the subsurface soil resistivity in

the inversion models (figs. 6 - 11) shows a wide variation in

the soil resistivity and at different depth along the profiles.

The resistivity of the three-layer case reveals the top layer is

characterized by its relatively high resistivity across the study

site with the resistivity value ranges between 80-Ohm-m and

1000-Ohm-m with thickness varies from <1.25m to 10m.

This layer is factually absent across profile 3, which is

located at the southern end of the site. The variation in the

resistivity values depict the in homogeneity nature of thelayer. The subsurface heterogeneity comes from the presence

of clayey sand/sandy clay with lateritic clay and sometimes

outcrop. The intermediate layer is characterized with

relatively low resistivity values ranging from 10-Ohm-m to

120-Ohm-m and thickness vary from 3m to 12m. The layer is

characterised by the dominance of clay/clayey sand (Olayinka

et al., 2004). The layer is hydro-geologically good because

the weathered material, which constitutes it, has high porosity

and contains a significant amount of water, but presents low

permeability due to its relatively high clay content (Barker,

2001). The layer serves as the main groundwater aquifer in

this study as revealed in figures 6, 7,9,10 and 11.

The fractured/fresh basement underlies the main aquiferous

zone (unconfined aquifer) with resistivity ranging between

146-Ohm-m to 88674-Ohm-m. This layer is divided to two

sections with the upper segment represents fractured

basement with resistivity values vary from 169-Ohm-m in profile 6 to 850-Ohm-m in profile 1. The thickness of the

fractured basement ranges between 4 to 8 m and it serves as

confined aquiferous zones. The overburden thickness varies

across the survey area with some portions having thickness

above 13.4 m. Geophysical studies in south-western basementcomplex of Nigeria have identified thick overburden as zones

of high groundwater potentials (Olorunfemi and Okhue,1992; Oladapo et al., 2004, Oyedele and Olayinka, 2012). In

addition, Olayinka et al., (2004) classified moderately

weathered/fractured basement in the basement complex of

Nigeria with resistivity ranging from 100 to 800-Ohm-m as

good groundwater aquifer. The cusps/drop pattern observed

between the layer and basement rock (Figs. 6, 7, 9 and 11)

show differential weathering and fracturing in association

with gneissic host rock especially near the contact and such

areas are hydro-geologically potential site for groundwater

reserves and target for groundwater development. According

to Louis et al., (2002), groundwater favourable areas are

locations where the intense fracturing of the basement rock

has produced extensive or local thickening of overburdenmaterial.

The 2D sections clearly show the basement rock delineated

with resistivity of the order of 1000-Ohm-m to 58674-Ohm-

m. The bedrock topography is variable with the thickness of

overburden between 3 m and above 13.4 m (fig. 9). The

trough areas (thick overburden) will act as promising target

for groundwater development.

4. CONCLUSION

2D electrical resistivity tomography has provided a clear view

of the lithological units, weathering profiles and geologicalstructures favourable for groundwater exploration and

development in the study area. The analyses of the inverted

sections along the profiles clearly show three divisions viz.,

top layer, which is clayey sand/sandy clay/lateritic clay;

weathered layer which contains clayey materials and the

fractured/fresh basement with the fractured part acting as the

aquifer in the area. The overburden thickness varies from 3.0

m to above 13.4 m. The water-bearing unit in the area of

study is the regolith (weathered basement) derived mostly

from in-situ weathered crystalline rocks. The bedrock

depressions and the fractured zones being groundwater

collecting centres are priority areas for groundwater

development. Based on the results, groundwater explorationand development in the area should be targeted towards the

fractured basement in areas with relatively thick overburden.

Acknowledgement

The authors acknowledged Mr. Isaac O. Babatunde with

profound appreciation for the assistance during the data

acquisition. Sincere thanks are given to the anonymous

reviewers.

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