geoelectric assessment as an aid to geotechnical investigation at a proposed residential development...
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ORIGINAL PAPER
Geoelectric assessment as an aid to geotechnical investigationat a proposed residential development site in Ilubirin, Lagos,
Southwestern Nigeria
L. Adeoti1 & A. O. Ojo1 & R. B. Adegbola2 & O. O. Fasakin1
Received: 18 November 2014 /Accepted: 20 January 2016# Saudi Society for Geosciences 2016
Abstract The study focuses on the application of electrical
resistivity methods as a guide for a detailed geotechnical in-vestigation due to the inhomogeneity of the soil materials cum
inability of the drilling beyond 38 m from the three initially
drilled geotechnical boreholes which did not allow for proper
foundation decisions and thus necessitated the research. Two-
dimensional (2D) electrical resistivity data were acquired
using the Wenner array along ten (10) traverses of about
500 m long, and forty-four (44) vertical electrical sounding
(VES) data were acquired along the various traverses using
the Schlumberger array with a maximum spread of 620 m.
The VES data were interpreted using partial curve matching
technique and one-dimensional (1D) computer iteration using
WinRESIST software. The 2D dataset were processed using
DIPROWin software. The inversion of the 2D resistivity data
was constraint by the VES results and available borehole data.
The results of the interpretation of the electrical resistivity data
reveal that the lithological units underlying the study area
compose of clay/peat, clayey sand, sandy clay, and sand.
The results of the interpretation of the electrical resistivity data
guided by the range of resistivity (ρ) values and knowledge of
the geology of the area reveal that the lithological units under-
lying the study area are composed of peat (ρ < 10 Ωm), clay
(10 > ρ < 100 Ωm), clayey sand (100 > ρ < 200 Ωm), sandy
clay (200 > ρ < 300 Ωm), and sand (ρ > 300 Ωm). The various
layers are intercalated with each other, and thickness values
vary from one location to another up to a maximum depth of
70 m. The VES results show that the clayey material underliesthe sand-filled topsoil and there are indications of competent
sand layers at depth beneath these clayey layers in the study
area. The 2D pseudosections reflect that the different litholog-
ical units are intercalated with varying thicknesses across the
study area. Thus, the study reveals that the subsoil within the
study area is quite inhomogeneous and great care and exper-
tise is required for developing the site. Anonymously low-
resistivity areas delineated along each of the traverses are lo-
cations recommended for targeted geotechnical investigation
prior to construction.
Keywords Geotechnical . Geoelectric . Electrical resistivity .
Wenner . Schlumberger . Lithological . Pseudosections
Introduction
In the last decade, the use of geophysics in civil and environ-
mental engineering has become a promising approach. Before
then, much money has been wasted by covering sites with
regular grids of boreholes and expensive programs of routine
tests rather than targeting the investigation towards anomalous
areas where information is required (McDowell et al. 2002).
Because every geotechnical in situ survey gives discontinuous
one-dimensional (1D) information of the subsurface condi-
tion, the variability of the near-surface ground may complicate
such site investigations and budgetary constraints may also
limit the number of boreholes that will be drilled. Hence, at
an early stage of site investigation, it may be beneficial to
undertake a reconnaissance geophysical survey which can
provide either a 2D or 3D image of the subsurface that can
be used to identify areas of the site which should be investi-
gated by drilling (i.e., those where anomalous results are
* L. Adeoti
1 Department of Geosciences, University of Lagos, Akoka,
Lagos, Nigeria
2 Department of Physics, Lagos State University, Lagos, Nigeria
Arab J Geosci (2016) 9:338
DOI 10.1007/s12517-016-2334-9
http://crossmark.crossref.org/dialog/?doi=10.1007/s12517-016-2334-9&domain=pdf
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obtained) so as to further investigate areas with potentially
dangerous subsurface conditions and make correlation be-
tween boreholes.
Geophysics is non-destructive, and it often serves as a re-
liable and cost-effective means of imaging the subsurface be-
tween and below boreholes and for determining the in situ
bulk properties of soil and rock (Neil et al. 2008). The use of
geophysical methods confers advantages as they generally
speed up the process of investigation. They provide continu-
ous streams of information not otherwise available in discrete
sampling or invasive procedures and give advance informa-
tion on what to expect for a given locality. This information
can be used to guide a more detailed soil exploration, thereby
offering considerable savings in both time and money. The
application of geophysical investigations can enhance the re-
liability and speed of geotechnical investigations and reduce
Fig. 1 Geological map of Lagos state showing the study area (modified after Jones and Hockey 1964)
Fig. 2 Geophysical data acquisition map
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the cost of the investigation as the number of boreholes re-
quired for adequate definition of subsurface conditions can be
greatly reduced if geophysics is used as a basis for selecting
borehole locations in order to optimize the layout of excava-
tions. This will make the borehole data being technically more
representative of ground conditions than they might have been
otherwise.
Several geophysical methods can be applied in engineering
investigation, but the use of the electrical resistivity method
has been favored and successfully applied to solve a wide
variety of engineering problems (Olorunfemi and Meshida
1987; Barker 1997; Adepelumi and Olorunfemi 2000;
Olorunfemi et al. 2004; Soupios et al. 2007; Omoloyole et
al. 2008; Adepelumi et al. 2009; Adeoti et al. 2009; Fatoba
et al. 2010; Ayolabi et al. 2010; Fatoba et al. 2013) particularly
in aiding geotechnical investigation (Akintorinwa and Adelusi
2009; Akintorinwa and Adesoji 2009; Adeyemo and
Omosuyi 2012; Salami et al. 2012). This is because of the
wide range of resistivity values found in nature and the fact
that it is one of the simplest and less costly geophysical
methods available. The method is best suited to the determi-
nation of depths to the bedrock and detecting the presence of
structural features in the bedrock or potentially dangerous
subsurface conditions before the erection of any engineering
structure (Soupios et al. 2007).
Due to urbanization and geometrically increasing popula-
tion in Lagos, Southwestern part of Nigeria, a section of
Ilubirin in Lagos Island Local Government Area was
Fig. 3 Correlation between VES
1, 41, and 42 and boreholes 1, 2,
and 3, respectively
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reclaimed by sand filling in order to provide further land for
housing projects/development purposes. In this study area,
three randomly geotechnical boreholes were initially drilled
to reveal the nature of the subsurface information, but the in-
homogeneity of the soil materials cum inability to drill beyond
38 m did not allow for proper foundation decisions. Thus,
these limitations informed the application of vertical electrical
sounding (VES) and 2D resistivity survey with a view to properly revealing laterally and vertically the subsurface ge-
ology as a guide for a detailed geotechnical investigation prior
to construction.
Geological setting of the study area
The study area (Ilubirin) located between latitudes 60 27′ 40″
N and 60 27′ 58″ N and longitudes 30 23′ 45″ E and 30 24′ 10″
E lies within Lagos State in the Southwestern part of Nigeria.
The state overlies the Dahomey Basin, which extends almost
from Accra in Ghana through the Republic of Togo and Beninto Nigeria where it is separated from the Niger Delta basin by
the Okitipupa Rigde at the Benin hinge flank. According to
Jones and Hockey (1964), the geology of Southwestern
Nigeria reveals a sedimentary basin which is classified under
five major formations according to their geological formation
age, namely the Littoral and the Lagoon deposits, Coastal
Plain sands, the Ilaro Formation, the Ewekoro Formation,
and the Abeokuta formation overlying the crystalline base-
ment complex with their ages ranging from Recent to
Cretaceous (Fig. 1).
As part of the Nigerian sector of the Benin Basin, the
Quaternary geology of the study area comprises the Benin
Formation (Miocene to Recent), recent littoral alluvium, and
lagoon/coastal plain sand deposits (Pugh 1954; Durotoye
1975; Longe et al. 1987; Jones and Hockey 1964). The allu-
vial deposits consist mainly of sands (Jones 1960; Halstead
1971), littoral and, lagoon sediments formed between two
barrier beaches (Adeyemi 1972) and coastal plain sands.
Topographically, the land on the northern fringe of the state
has soils that do not rise very much above sea level.
Material and methods
The electrical resistivity method adopted for the geophysical
survey uses artificial source of current ( I ) introduced into the
ground through a set of metallic rods referred to as the current
electrodes while a measurement of the potentials generated
was made using another set of metallic rods referred to as
the potential electrodes. The PASI-16GL resistivity meter
was used to measure the potential difference (ΔV) from which
the resistance ( R) is computed ( R = ΔV/ I ) and converted into
apparent resistivity (ρa ) (Geometric factor * Resistance). The
true resistivity of the subsurface was subsequently estimated
by inversion using a computer program. For the purpose of
this research, the Schlumberger array was used for the acqui-
sition of vertical electrical sounding (VES) because of its com-
parable depth of inve stigation, speed, and convenie nce.
Likewise, the plethora of interpretation material for the
Schlumberger array also makes it attractive for depth sound-
ing. The Wenner array was used for the acquisition of the 2D
resistivity dataset. Wenner array is relatively sensitive to ver-
tical changes in the subsurface resistivity (Loke 2004); there-
fore, it is good in resolving vertical changes (i.e., horizontal
structures) which can effectively help to delineate the lateral
extent of different lithologies in the study area. Compared to
other arrays, the Wenner array has a moderate depth of inves-
tigation and the strongest signal strength which is an important
factor because the survey was carried out in Ilubirin area
which has high background noise. A constant spacing be-
tween adjacent electrodes was used along each profile and
increased to deeper depth.
The data acquisition layout for the survey is shown in Fig.
2, and the orientations of our traverses were roughly perpen-
dicular to NE-SW which is the general strike of geological
features in the study area. This was done to achieve optimum
resolution of the subsurface structures. Ten (10) traverses of
about 500 m along the NW-SE direction were established, and
the 2D resistivity datasets were acquired along these traverses
(a)
(b)
Fig. 4 Typical VES curves in the study area: a KQH-type curve, b QH-
type curve
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in two segments of 0 – 250 m and 250 – 500 m. In addition, a
total of forty-four (44) VES points were occupied along the
ten traverses. Three of the VES data were acquired beside
three boreholes located within the study area as shown in
Fig. 2. The spread length of the Schlumberger current elec-
trode varied from 2 to 620 m while for Wenner profiling, the
Table 1 VES interpretation results and lithological description
VES no. Type curve Resistivity (ρ1….ρn)Ωm//thickness (h1…..hn)m Lithological description
1 KQ 34.7208.6,27.5,8.8//0.5,2.4,3.3 Clayey topsoil, clayey sand, clay, clay/peat
2 QQH 2603.5150.8,87,6.1207.3//1,9.6,25.5,18.6 Sandy topsoil, sandy clay, clay, clay/peat, sandy clay
3 KH 84.3270.2,16.6152.3//0.7,0.6,21 Clayey topsoil, clayey sand, clay/peat, sandy clay
4 KQH 306.1409.8,89,8.1//0.6,1.7,2.7,9.2 Sandy topsoil, sand, clay, clay/peat, clay
5 QQH 732.6585.1,83.53.8,26.3//0.7,2.8,13.4,14.2 Sandy topsoil, sand, clay, clay/peat, clay
6 QH 1668.1152.5,16.7,98//0.9,2.4,20.8 Sandy topsoil, clayey sand, clay, clay
7 KQH 59.4124.7,12.2,1.4,25//0.6,1.7,6.5,8.2 Clayey topsoil, sandy clay, clay/peat, clay/peat, clay
8 KQH 437.2,1628.2123.8,15.9128.3//0.5,2.1,1.7,54 Sandy topsoil, sand, sandy clay, clay, sandy clay
9 QH 2567.6208.5,37.9258.4//0.8,2.2,21.8 Sandy topsoil, clayey sand, clay, clayey sand
10 QH 2467.8139.5,14.4129.1//0.5,1.5,27.8 Sandy topsoil, sandy clay, clay/peat, sandy clay
11 KHA 375.2740.3,27.4,72.9626.7//0.5,1.9,1.8,44 Sandy topsoil, sand, clay, clay, sand
12 QH 745.4,66.6,12,160.1//0.8,4.3,48 Sandy topsoil, clay, clay/peat, sandy clay
13 QH 3909.2473.1,28.4115.7//0.7,1.8,16.1 Sandy topsoil, sand, clay sandy clay
14 KH 26.7156.9,7.8,1900.6//0.6,1.5,16.4 Clayey topsoil, sandy clay, clay/peat, sand
15 KQH 27.6153,11.9,7224.9//0.4,1.6,2.6,29.2 Clayey topsoil, sandy clay, clay/peat, clay/peat, clayey sand
16 QHK 1217.8152.9,7.5,57.6,7.8//0.6,2.9,10.3,55.3 Sandy topsoil, sandy clay, clay/peat, clay, clay/peat
17 KQH 464,2588.2183,20,441.5//0.3,1.5,6.6,25.7 Sandy topsoil, sand, sandy clay, clay, sand
18 KH 87.7201,14.8168.9//0.4,3,15.1 Clayey topsoil, sandy clay, clay, sandy clay
19 KH 121.4193.8,19.9222.2//0.5,1.6,22.8 Sandy topsoil, clayey sand, clay, clayey sand
20 KQH 38.7143.1,32.1,3.2,47.6//0.5,3.3,14.8,27.6 Clayey topsoil, sandy clay, clay, clay/peat, clay
21 QQ 2987.8428.7,34.3,13.6//0.9,1.5,4.2 Sandy topsoil, sand, clay, clay
22 KQH 2341.6,2544.1,92.9,32.8252.9//0.6,0.8,7.4,42 Sandy topsoil, sand, clay, clay, clayey sand
23 QQH 2065.1463,64.1,46.9611.3//0.8,1.2,3.5,68.8 Sandy topsoil, sand, clay, clay, sand
24 QQH 1137.3157.4,36.6,14.2207.7//1,2.9,3.7,57 Sandy topsoil, sandy clay, clay, clay, clayey sand
25 KQ 276.4372.8155.6,13.6//0.5,3.4,36.8 Sandy topsoil, sand, sandy clay, clay
26 KHK 257.3319.4,14.2418.8,45.6//0.6,2.4,2.9,37.5 Sandy topsoil, sand, clay, sand, clay
27 KQQ 319.6896.6,36,35.1878.6//0.5,1.3,0.4,71.7 Sandy topsoil, sand, clay, clay, sand
28 QH 1760.8792.9187.5696.5//0.8,2.2,18 Sandy topsoil, sand, sandy clay, sand29 QHK 1226.1563.3,37.9423.8,23.4//0.8,2.1,8.7,22.6 Sandy topsoil, sand, clay, sand, clay
30 QHK 8439.9390.6,41.3300.8,66.3//0.6,2.7,2.4,75.4 Sandy topsoil, sand, clay, sand, clay
31 QQH 3445.5305.2,40.2,5,54.2//1.2,2.2,7.2,10.9 Sandy topsoil, sand, clay, clay/peat, clay
32 KQH 635.7,1750.2,44.1,10.7136.2//0.5,1.8,11,57.4 Sandy topsoil, sand, clay, clay/peat, sandy clay
33 QH 3247.9201.7,29.5270//0.8,3.1,42.5 Sandy topsoil, clayey sand, clay, clayey sand
34 QH 3592.7675.3,34.7397.5//0.6,1.8,36.1 Sandy topsoil, sand, clay, sand
35 QH 559.6170.7,41.2,4080//0.9,4,65.9 Sandy topsoil, sandy clay, clay, sand
36 QQH 1210.8758.7,53,4.8,61.1//0.6,3.3,13.2,16.5 Sandy topsoil, sand, clay, clay/peat, clay
37 QHA 5350.5,1496.4,99.6110.1841.8//0.8,2,1,67.7 Sandy topsoil, sand, clay, sandy clay, sand
38 QHA 8405.3499.7,37.2273,80.4//0.7,2.2,3104.4 Sandy topsoil, sand, clay, clayey sand, clay
39 KQH 711,2295.8163.2,12.3177.8//0.4,1.8,5.5,11.8 Sandy topsoil, sand, sandy clay, clay/peat, sandy clay
40 KQQ 776.1,1710.1414.8,38.7,12.4//0.5,1.5,0.5,5.8 Sandy topsoil, sand, sand, clay, clay/peat
41 QHA 374.3129.5,30.6113.4,1550//0.7,2,7.2,49.9 Sandy topsoil, sandy clay, clay, sandy clay, sand
42 K 351.3,1188.1,20.4//0.7,1.8 Sandy topsoil, sand, clay
43 KH 34.6106.9,16.8,1098.5//0.6,3.3,10.2 Clayey topsoil, clay/sandy clay, clay/peat, sand
44 KHA 128.8735.3,14.8,69.1175.6//0.5,0.9,5.9,15 Clayey topsoil, sand, clay/peat, clay, sandy clay
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electrode spacing varied from 10 to 80 m. The Garmin Etrexmodel Global Positioning System (GPS) handset was used to
record geographical coordinates (in degrees) of the VES sta-
tions and the traverse lines.
In order to obtain a good final model, the data must be
of very good quality. As a rule of thumb, prior to inver-
sion, we plotted the resistivity data from the 2-D survey
using the pseudosection contouring method as well as a
profile plot. From the pseudosection plot, we manually
picked out bad apparent resistivity measurements from
the dataset. Such bad measurements stood out as points
with unusually high or low values which might be caused
by some sort of failure during the survey. Since we ac-quired our data manually, we had the privilege of cross-
checking and verifying our readings; therefore, the per-
centage of bad data points is negligible in our case. In
addition, our resistivity meter was usually checked for
possible wire leakages, poor electrode contact, and current
injection issues before taking several measurements which
are averaged to suppress random noise.
The VES curves were interpreted using the conventional
partial curv e matching tech nique (Orella na and Mooney
1966; Bhattacharya and Patra 1968) to obtain an initial
model as input into an inversion procedure using a modified
Marquardt-Levenberg inversion algorithm involving the
WinResist Software (Vander Velpen and Sporry 1993).
The acquired 2D resistivity dataset were inverted to obtain
a 2D image of the subsurface using DIPROfWin software.
The 2.5-D inversion code solves the forward problem of
electrical resistivity using finite difference modeling
(FDM) or finite element modeling (FEM) approximations.
For our study, we used the FDM component of the software
to generate theoretical dataset for several models. In the
inversion step, the theoretical dataset was compared to the
observed resistivity data and an attempt to fit both data wasachieved using a smoothness-constrained least-squares in-
version algorithm and an active constraint balancing (ACB)
to achieve stable results. The ACB method was used to
determine the spatially varying Lagrangian multiplier at
each of the parameterized blocks of the model during the
inversion process to enhance both resolution and stability
(Yi et al. 2003). As the inversion proceeded, the method
aimed to iteratively minimize the difference between the
synthetic and observed resistivity fields by updating the
model until a reasonable fit was achieved. The quality of
the data fit was expressed in terms of the RMS error, and
once the misfit is less than 5 %, the iteration was stoppedand the observed field pseudosection, the computed theo-
retical data pseudosection, and the inverted subsurface 2D
resistivity structure were outputted. As a constraint to the
inversion process, we supplied a priori information in terms
of the number of layers from the VES interpretation results
and available borehole information.
Results and discussion
Correlation between borehole and VES model
For optimum interpretation of geophysical survey data, it is
important that adequate direct control is available; such con-
trol can be provided by boreholes or trial pits. Hence, para-
metric soundings at VES 1, 41, and 44 were carried out beside
three boreholes, which had been dug prior to the geophysical
investigation. This has assisted in correlating the results of
both methods and also to probe deeper where necessary.
This is also to help constrain and aid the interpretation of the
geophysical results. The lithological information of boreholes
Fig. 5 Frequency chart for
different VES-type curves
obtained in the study area.
Borehole data (Fig. 3) in thestudy
area form a basis for interpreting
the 2D resistivity images. The
inverted 2D resistivity structure
beneath traverses one (1) to five
(5) andtraverses six(6 ) toten(10)
are shown (Fig. 6 (1a – 5b) andFig. 7 (6a – 10b)). Segment a of
each traverse covers a lateral
distance of 0 – 250 m while
segment b covers a lateral
distance of 250 – 500 m making
each traverse 500 m long
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1 – 3 in correlation with the results of the parametric VES
sounding is presented (Fig. 3). The results obtained from the
correlation informed the use of different ranges of resistivity
values to classify the different lithology in the study area.
RMS error = 0.060 RMS error = 0.059
RMS error = 0.053 RMS error=0.056
RMS error = 0.062RMS error = 0.057
RMS error = 0.059 RMS error = 0.056
RMS error = 0.052RMS error = 0.054
Fig. 6 2D Resistivity structure beneath traverses 1 – 5. Segment (a) is from 0 to 250 m while segment (b) is from 250 to 500 m
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Fig. 7 2D resistivity structure beneath traverses 6 – 10. Segment (a) is from 0 to 250 m while segment (b) is from 250 to 500 m
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Vertical electrical sounding results
Samples of the resistivity curves are presented (Fig. 4). The
summary of interpreted VES results and lithological descrip-
tion is also presented (Table 1). The resistivity curves obtained
from the survey are the KQ-, QQH-, KH-, KQH-, QH-, KHA-,
QHK-, QQ-, KHK-, KQQ-, QHA-, and K-type curves with
the KQH- and QH types being dominant (Fig. 5). The litho-logical interpretation of the VES subsurface models reveals
that the study area is underlain by topsoil (mostly sandy),
sandy clay/clayey sand, clay/peat, and sand layers.
The dominant-type curve (Fig. 5) shows that resistivity
generally decreases with an increase in depth and later in-
creases. This indicates that beneath the sand-filled topsoil,
there is a strong indication of clayey material and subsequent-
ly a competent sandy material at depth.
For construction purposes in the study area, appropriate
foundation such as piling is recommended since the clayey
material lying beneath the sand-filled layer is inimical to
building construction due to differential settlement resultingfrom poor load-bearing capacity.
2D electrical resistivity imaging results
The inversion of the 2D resistivity data was constraint using the
VES results as shown (Table 1) and discussed in a previous
section. The number of layers, which is an input into the inver-
sion process, was determined from the VES results. Likewise, the
understanding of the relationship between resistivity values and
lithology obtained from the borehole data (Fig. 3) in the study
area formed a basis for interpreting the 2D resistivity images.
The 2D subsurface images obtained from the inversion of the
Wenner resistivity data show both the lateral and vertical varia-
tions in subsurface resistivity across the different traverses. The
2D images reveal four major subsurface lithology which are peat
(generally in blue color band with resistivity values less than
10 Ωm), clay (generally in lime/ light orange/ yellow color band
with resistivity values less than 100Ωm), sandy clay/clayey sand
(generally in deep orange/ red color band with resistivity values
ranging from 101 to 199 Ωm for sandy clay and 200 – 299 Ωm
for clayey sand), and sand (generally in purple color band with
resistivity values greater than 300 Ωm). A description of the
lithology is denoted on the subsurface image obtained across
each traverses as presented (Fig. 6 (1a – 5b) and Fig. 7 (6a – 10b)).
For further geotechnical investigation, boreholes should be
targeted at anomalously low resistivity areas that are generally
in blue/lime color band across the different traverses.
Conclusions
From the results of the geophysical investigation, it can be
concluded that the subsurface lithology underlying the sand-
filled area is generally clayey. However, there are patches of
sand unit along the different traverses at different depths.
Since clayey layers are inimical to the development of engi-
neering structures due to differential settlement resulting from
poor load-bearing capacity, it is recommended that a suitable
foundation such as pilling is constructed.
As a guide to further geotechnical investigation, boreholes
are recommended to be drilled at anomalous areas (areas withvery low resistivity
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