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7/25/2019 Jurnal Resis 1
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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).
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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)
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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
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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
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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).
Fig. 7: Inverse model resistivity section for profile 2
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).
Fig. 9: Inverse model resistivity section for profile 4
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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).
Fig. 10: Inverse model resistivity section for profile 5
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.
Fig. 11: Inverse model resistivity section for profile 6
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).
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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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