INTEGRATED GEOPHYSICAL INVESTIGATION OF YEMOO GROVE ARCHAEOLOGICAL SITE IN ILE-IFE, OSUN STATE, SOUTHWEST NIGERIA

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Ife Journal of Science vol. 17, no. 3 (2015)
INTEGRATED GEOPHYSICAL INVESTIGATION OF YEMOO GROVE
ARCHAEOLOGICAL SITE IN ILE-IFE, OSUN STATE, SOUTHWEST NIGERIA.
1Dept. of Geology, Obafemi Awolowo University, Ile-Ife, Nigeria
2Natural History Museum, ObafemiAwolowo University, Ile-Ife, Nigeria
3Department of History, The College of Williams and Mary, Williamsburg, Virginia, USA.
4Institute of Ecology and Environmental Studies, Obafemi Awolowo University, Ile-Ife, Nigeria
(Received: 3rd August, 2015; Accepted: 15th August, 2015)
Integrated geophysical investigation involving the electrical resistivity and magnetic methods was carried out at
Yemoo Grove archaeological site at Ile-Ife in Osun State, Southwest Nigeria. The resistivity survey involved 1-D
Vertical Electrical Sounding (VES) with the Schlumberger array and 2-D Dipole-Dipole resistivity imaging
(profiling) technique. The VES data were interpreted quantitatively by partial curve matching and computer
assisted 1-D forward modelling with the W-GeoSoft/WinSev 5.1 software. The 2-D Dipole-Dipole data were
inverted to 2-D subsurface structure with the DIPRO for Windows software. The VES delineated a
stratigraphic sequence composed of the topsoil, lateritic clay and weathered basement within the upper 14.0 m.
The topsoil, which is the most archaeologically relevant horizon, had thicknesses of up to 2.0 m. The 2-D
structures identified four priority zones with relatively thick topsoil (up to 2.0 m) along the existing trench and at
the shoulders of the flanking ridges. Three of these zones were excavated plus one random location.
Archaeological artefacts ranging from ceramics, potsherd pavement, charcoal and stone concretion were
identified within the excavated priority zones while nothing was found at the randomly located site. The 2-D
structure along one of the traverses identified a terraced surface on the underlying lateritic layer. This was
corroborated by excavation. No metallic artefact was identified. The geophysical results provided a reliable guide
to archaeological prospection at the Yemoo Grove Site.
Keywords: Geophysics, Archaeology, Artefacts, Subsurface Imaging
1 2 3 1 1
Olorunfemi, M.O. ; Ogunfolakan, B.A. ; Chouin, G.L. ; Oni, A.G. ; Okunubi, M. O. and
4
Akinwumiju, A. S.
ABSTRACT
553
INTRODUCTION
Feder (1999) defined archaeology as the study of
the lives of past people through the physical
remains they left behind. Archaeological
investigation usually involves visual inspection and
classification of groups of objects (artefacts);
laboratory studies involving analysis and dating of
recovered objects and samples from buried
archaeological sites and geophysical investigations
including remote sensing. The former two
methods require destructive excavations at
archaeological sites. Pre-excavation geophysical
investigations, which are non-invasive and
environmental friendly, are virtually becoming a
routine because surfaces of archaeological sites
have often times been altered by anthropogenic
activities such as farming and physical (urban)
development. Also locations, exact nature and
extent of archaeological objects are often not
known. Geophysical investigations are also
engaged to reduce cost of excavation by
constraining (guiding) locations to be excavated.
Geop h ysica l m e t hods are rele v ant in
archaeological prospection because of the
existence of detectable contrasts in physical
properties, such as resistivity or conductivity,
magnetic susceptibility, density and elasticity,
between archaeological artefacts and the
surrounding earth materials. This makes the
electrical resistivity, electromagnetic, ground
penetrating radar, magnetic, microgravity and
even seismic methods of geophysical prospecting
relevant (Clark, 1996; Gaffney and Gater, 2003;
Witten, 2006; Oswin, 2009; Dolphin, 2011 and
Dalan, 2012). Geophysical investigations have
been used with considerable success in
archaeological prospection (Eluyemi et al., 2012a;
Kuhne et al., 2013; Fassbinder et al., 2014 and
Fassbinder, 2015). Geochemical analysis, direct
metal detection and aerial photographs/remote
sensing have also found useful applications in
archaeological prospection (Kennedy and Jones,
2009; Eluyemi et al., 2012b).
YEMOO GROVE (known to the local
community as Ita Yemoo), in Ile-Ife, southwest
Nigeria, is a renowned archaeological site and one

554
of such sites being overseen by the National
Commission for Museum and Monuments under
the Federal Ministry of Culture and Tourism.
Archaeological works started at this site in 1957 by
Frank Willet. However, prior to Frank Willet's
visit, seven bronze sculptures were discovered by
builder's labourers during construction works at
the site. Apart from bronze sculptures, other
artefacts that have been recovered at this site
include terra-cottas, stone sculptures, ritual
vessels, glass beads, fused but unused glass,
fragments of crucibles for making glass and
potsherd pavements (Willet, 1959).
The French Government, through the third co-
author, has recently provided funding for the
excavation of part of the archaeological site. A
preliminary integrated geophysical investigation
of the site was embarked upon as a fast
recognisance technique capable of providing
information on artefact-laden vs barren ground
that can be used to guide excavation works
thereby reducing cost. The geophysical
investigation involved the electrical resistivity and
magnetic methods.
Description of the Project Environment
The Yemoo Grove archaeological site is situated
in the ancient city of Ile-Ife in Ife East Local
Government Area (LGA) of Osun State, Nigeria
OSUN
STATE
Figure 1: Map Showing the Study Site in Ile-Ife, Ife Central LGA of Osun State, Nigeria
Olorunfemi et al.: Integrated Geophysical Investigation of Yemoo Grove

555
o o
geographic co-ordinates of Latitudes 7 29' 40'' - 7
o o
29' 42'' and Longitudes 4 34' 04''- 4 34' 08'' (or
Northings 828642 to 828703 mN and Eastings
672998 to 673121 mE)(Fig. 2). The study area is
underlain by the Precambrian Basement Complex
rocks (Fig. 3). The archaeological site is located
within pegmatised schist characterized by
clay/lateritic clay topsoil overlying a clayey
weathered basement. Rock outcrops are rare
except along river and stream channels.
Figure 2: Map of the Premises of Yemoo Groove Showing Geophysical Traverses
andVertical Electrical Sounding Stations
Figure 3: Geological Map of the Area around Ile-Ife
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556
METHODOLOGY
The geophysical investigation involved the
electrical resistivity and magnetic methods. Five
traverses (TR 1-4, each 20 m long and TR 5, 33 m
long) were established across an existing trench, as
shown in Figure 2. The traverses were marked out
at 1 m interval and all the marked stations were
geo-referenced. 2-D resistivity imaging with
Dipole-Dipole array was carried out along
traverses TR 1&2 while magnetic profiling was
carried out along all the five traverses. The Dipole-
Dipole profiling utilized 1 m dipole length and
expansion factor, n, that varied from 1-5. Total
field magnetic measurements were made at 1 m
inter val w i t h the Proton P r e c ession
Magnetometer. Six (6) Schlumberger Vertical
Electrical Sounding (VES) stations were carried
out along traverses TR 1&2 with the locations
constrained by the 2-D images. The electrode
spacing (AB/2) was varied from 1 to 32 m. The
resistivity measurements were made with PASI
16gl Digital Resistivity Meter.
The VES data were presented as sounding curves
(Fig. 4) and interpreted quantitatively by partial
curve matching and computer assisted 1-D
forward modelling with the W-GeoSoft/WinSev
5.1 software. The VES interpretation results were
used to generate geoelectric sections along the
two traverses occupied. The 2-D Dipole-Dipole
data were inverted into 2-D subsurface images
(resistivity structures) using the DIPRO for
Windows V. 4.0 software.
The magnetic data were corrected for diurnal
variation and offset and the residual magnetic
field presented as magnetic profiles.
Fig. 4 Typical VES Type Curves and the Interpretation Models
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557
RESULTS AND DISCUSSION
Characteristic VES Curves and the Subsurface
Sequence
With the exception of VES 3, the K type curve
characterized the study area. The geoelectric
sections (Fig. 5) delineated three subsurface
geologic layers in the upper 14.0 m along traverses
TR 1&2. These include the topsoil, laterite
(lateritic clay) and the weathered basement. The
topsoil resistivity varied from 43-1515 ohm-m and
is composed of clay/sandy clay. The thicknesses
varied from 0.7-2.0 m. This layer is the most
relevant in archaeological prospection, at this site.
The lateritic clay second layer displayed
resistivities and thicknesses that varied from 171-
1952 ohm-m and 3.4-6.2 m respectively. The
upper segment of the horizon could be relevant in
archaeological prospection. The weathered
basement layer resistivity values varied from 120-
247 ohm-m. This horizon is composed of sandy
clay.
Fig. 5 Geoelectric Section Beneath (a) Traverse TR 1 (b) Traverse TR2
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558
2-D Subsurface Image
Figures 6(c) and 7(c) present the 2-D resistivity
structures (2-D images) along traverses TR 1&2
respectively. The inversion of the Dipole-Dipole
data compensated for topographic variation along
both traverses. The 2-D structures imaged the
upper 3.0 m of the subsurface sequence – the
topsoil and the upper segment of the lateritic
layer, in the geoelectric sections (see Fig. 5).
Fig. 6(a) Observed Pseudosection (b) Theoretical Pseudosection and (c) 2-D Resistivity Structure
along Traverse TR 1
Along traverse TR 1, Figure 6 imaged the topsoil in
blue/green colour with resistivity values varying
from 70-248 ohm-m (68-116 ohm-m from VES)
and thicknesses of 0.5-1.5 m (0.7-1.1 from VES).
The underlying layer, in brownish/red colour, is
the lateritic clay layer with resistivity values varying
from 253-1157 ohm-m (171-1052 from VES).
The lateritic layer outcrops between stations 11-12
and 16-20. This is corroborated by the geoelectric
section (Fig. 5a). The subsurface image showed a
terraced surface of the laterite on the southwest
flank. It is suspected that this ancient structure
must have been created when the trench was
being dug, for easy access. The topsoil is the
stratigraphic unit of importance in archaeological
prospection at this site and is relatively thick (up to
2.0 m) beneath the trench (between stations 6-9
with centre at station 8) and on the northeast flank
beneath station 3 (up to 2.0 m thick).
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559
Along traverse TR 2, Figure 7 imaged the topsoil
also in blue/green colour with resistivity values
ranging from 45-223 ohm-m (43-151 ohm-m from
VES) and thicknesses of between 0.5 and 2.0 m
(0.8-2.0 m from VES). The lateritic layer, in
brownish/red colour, had resistivity values varying
from 227-824 ohm-m (329-772 ohm-m from
VES). The zones of archaeological interest with
significantly thick (up to 2.0 m) topsoil are the
trench (between stations 6 and 10 with centre at
station 9) and the crest of the ridge on the
southwest flank between stations 11 and 14 with
centre at station 13. The very low resistivity zone
(in deep blue colour) between stations 14 and 18
represents an effluent chamber (soak away).
Figures 8a-e show the residual magnetic profiles
along the five traverses. The magnetic
measurements at the northeast edge of traverses
TR 1 & 4 (Figs. 8a&d) were influenced by an
installed wire gauze at the top of an adjoining
brick wall fence while the magnetic anomaly
around station 5 along traverse TR 2 (Fig. 8b) was
due to a steel reinforced standing pillar. The
anomalous zone at the southwest edge of same
profile was due to a metal rod reinforced top of an
effluent chamber (soak away). The magnetic
profiles along traverses TR 1,2 and 4 (Figs. 8a,b
&d) which cut across the trench (Fig. 2) show
relatively flat magnetic anomaly indicating non-
existence of a metallic artefact along the tunnel.
However, the magnetic profile (Fig. 8e) observed
along the trench (TR 5) identified an anomalous
zone beneath a refuse dump. The magnetic
profile along traverse TR 3 taken outside the
trench displayed two low amplitude peak negative
Fig. 7(a) Observed Pseudosection (b) Theoretical Pseudosection and (c) 2-D Resistivity Structure
along Traverse TR 2
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magnetic anomalies due to buried metal pipes. The
magnetic survey, therefore, did not identify any
metallic artefact within the survey area.
Fig. 8 Magnetic Profiles Along (a) Traverse TR 1 (b) Traverse TR 2 (c) Traverse TR 3
(b) Traverse TR 4 and (e) Traverse TR 5
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Followed-up Archaeological Excavation
Along traverse TR 1, priority (1) (station 8) was
excavated to a depth of 1.9 m and was rested on
the lateritic layer. Priority (2) (station 3) was
excavated to 0.67 m depth and abandoned before
the lateritic layer was reached, while station 16 was
also excavated but abandoned on lateritic layer at a
depth of about 0.4 m. Along traverse TR 2,
priority (2) was excavated to a depth of 1.7 m.
Archaeological artefacts such as ceramics and
charcoal were found at priority(1) along traverse
TR1 while charcoal, potsherd pavement (Fig. 9)
and stone concretion were found at priority(2)
along traverse TR 2. Excavation works at priority
(2) along traverse TR 1 was prematurely
terminated at shallow depth with some artefacts
recovered while priority (1) along traverse TR 2
was not excavated. Excavation at priority (1) along
traverse TR 1 from station 8 (trench) and up to the
ridge crest on the southwest flank identified a
terraced surface of the underlying laterite (Fig. 10).
No artefact was found at the randomly selected
location (station 16) along traverse TR 1.
CONCLUSION
The Yemoo Grove archaeological site in Ile-Ife,
southwest Nigeria, has been investigated using
the electrical resistivity and magnetic methods of
geophysical prospecting. The electrical resistivity
method involving 1-D (VES) and 2-D (Dipole-
Dipole) subsurface images delineated the
stratigraphy in the upper 14.0 m of the subsurface
sequence. The VES identified an archaeologically
relevant topsoil with thicknesses of up to 2.0 m
while the 2-D structures identified priority zones
with relatively thick topsoil along the existing
trench and at the shoulders of the flanking ridges.
Archaeological artefacts including ceramics,
potsherd pavement, charcoal, stone concretion
were identified within the priority zones (fig. The
2-D structure along traverse TR 1 identified a
terraced surface of the underlying lateritic layer
which was corroborated by excavation. The
magnetic survey did not identify any metallic
artefact in the study area.
Deductions from the Geophysical Investigation
The table below presents suspected artefact-laden zones identified from the results of the geophysical
investigation.
Traverse TR 1
Zone of Archaeological
interest
Depth Range (m) Classification
Stations 2-4 with centre at
station 3
0 – 2 Priority(2)
Stations 6-10 with centre at
station 8 (trench)
0 – 2 Priority(1)
Traverse TR 2
Station 6-10 with centre at
station 9
(trench)
0 – 2 Priority(1)
Stations 11-14 with centre at
station 13
0 – 2 Priority(2)
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Fig. 9 Excavation Works at Priority Site 2 along Traverse (TR) 2
Showing Identified Stone Concretion and Potsherd Pavement.
Stones
Potsherd Pavement
Terraced Laterite
Surface
Fig. 10 Correlation of Geophysical Result and Cross Section of Excavation Portion along Traverse TR 1
Olorunfemi et al.: Integrated Geophysical Investigation of Yemoo Grove

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