In August 2006, Magnetic Data Acquisition System (MAGDAS) was installed in
the University of Ilorin (UNILORIN) by experts from Space Environmental Research
Centre (SERC) of Kyushu University, Japan under the auspices of the International
Helio-physical Year (IHY) and Government of Japan (Maeda,
2008; Rabiu et al., 2009). Such MAGDAS system
has been previously installed in Cote dvoire and Ethiopia. Subsequently,
in 2008, some other units were installed across Africa (Maeda,
2008; Moodley et al., 2008). However, in March
2010, more magnetometers were installed in the UNILORIN observatory. During
the installation, the team of Japanese Scientists from SERC remarked verbally
that the MAGDAS facility in UNILORIN is believed to be to be one of the best
functioning observatories in Africa and may probably become a centre of excellence
in magnetic studies in Africa.
In view of the fact that there is a possibility of the observatory gaining
international recognition, there is the need to adequately secure the area from
acute water shortages for observers and researchers. Hence, a vision for an
independent source of water for the observatory would be imperative. In line
with this goal; a study of the groundwater condition at the observatory has
been made. Groundwater is the water that lies beneath the ground surface, filling
the pore space between grains in bodies of sediment and clastic sedimentary
rock and filling cracks and crevices in all types of rock (Plummer
et al., 1999). The source of groundwater is rain and snow that falls
to the ground. A portion of this precipitation percolates down into the ground
to become groundwater. How much precipitation soaks into the ground is influenced
by climate, land slope, soil and rock type and vegetation. In general, approximately
15% of total precipitation ends up as groundwater, but that varies locally and
regionally from 1 to 20%. Despite the fact that global water distribution shows
that groundwater is about 0.61%, it is surprisingly, about 60 times as plentiful
as fresh water in lakes and rivers on the surface (Plummer
et al., 1999).
Studies have showed that groundwater could be explored using electrical resistivity
methods (Olorunfemi and Fasoyi, 1993; Olasehinde,
1999; Alile et al., 2008). Previous work
has been done within the University of Ilorin while utilizing electrical resistivity
technique. Fracture pattern was elucidated using radial electrical resistivity
sounding and profiling by Olasehinde et al. (1986)
and at the same time they evaluated a prospective site for a weir on River Oyun
using same electrical methods. Nwankwo et al. (2004)
determined the structural disposition of the area using electrical resistivity
pseudosection method while a combination of ground magnetic and electrical resistivity
studies were used by Olasehinde (1999) to evaluate the
groundwater potential of the Southern portion of the university with great success.
Where it is difficult to locate aquifers such as water-saturated zones in hard
rock, it is also difficult to select suitable sites for water drilling. The
2D resistivity technique has improved the chance of drilling successes by identifying
the fractured and weathered zones in these areas (Singh
et al., 2006). Therefore, the use of such technique for groundwater
exploration has earned an important place in recent years despite some interpretive
limitations (Dogara et al., 1998; Singh
et al., 2006). It is therefore expected that the results obtained
from this work would produce detailed groundwater condition and recommend areas
within the observatory were deep tube wells could be located. This would definitely
meet the objective of securing the observatory from acute water shortages for
both observers and researchers.
MATERIALS AND METHODS
Basic theoretical considerations: The fundamental equation for resistivity
survey is derived from Ohms law (Grant and West, 1965;
Dobrin and Savit, 1988):
where, ρ is resistivity, R is resistance, L is length of homogenous conducting cylinder and A is cross sectional area.
For the solid earth, whose material is predominantly made up of silicates and basically non-conductors, the presence of water in the pore spaces of the soil and in the rock fractures enhances the conductivity of the earth when an electrical current I is passed through it, thus making the rock a semi-conductor. Since the earth is not like a straight wire and it is anisotropic, then Eq. 1 is thus customized to:
where, ΔV is change in voltage and r is the radius of current electrodes small hemisphere.
Since, the earth is not homogeneous, Eq. 2 is used to define
an apparent resistivity ρa which is the resistivity the earth
would have if it were homogeneous (Grant and West, 1965):
where, 2πr is then defined as a geometrical factor (G) fixed for a given electrode configuration.
The Schlumberger configuration was used in this study. The geometric factor G is thus given as:
where, AB is current electrode spacing and MN is spacing between potential electrodes.
Location and geological setting: The area of study lies entirely within
the basement rocks in the Western part of Central Nigeria bounded by longitudes
4° 39-4° 42E and latitudes 8° 28-8° 30N
(Fig. 1). This area falls within the Eastern part of Ilorin,
a semi-arid region of Nigeria with vegetation mainly of the guinea savannah
type with shrubs and undergrowth. Rugged troughs and crests due to erosions
characterize the topography of the area.
map of Nigeria showing the surveyed area
layout at 20 m VES station interval
The area is drained by rivers and streams such as Oyun river and river Ile-apa
as a tributary of River Niger (Nwankwo et al., 2004).
The rocks are mainly banded gneiss, sheared gneiss and augen gneiss intruded
by granodiorites and granites at the Southeast. The structural fabric is mainly
a North-South trending fracture system dominated by a Southerly plunging (6°-10°)
anticlinorium with a gentle Westerly dipping limb is depicted (Olasehinde,
1999; Olasehinde et al., 1986). The rocks
within the basement complex of South-West Nigeria has been classified into five
major groups: Migmatite-Gneiss complex which comprises gneisses, quartzite,
calc silicates rocks, biotite hornblende schist and amphibolites; slightly migmatised
to unmigmatised para-schists and meta-igneous rocks; Charnockitic rocks; Older
granites; and Unmetamorphosed dolerite dykes, which comprises pegmatite, quartz
veins and doleritic dykes (Rahaman, 1973).
Field survey: A four day ground resistivity survey was carried out at MAGDAS Observatory, University of Ilorin in February 2009 using PELI 1300, a low frequency potable terrameter. A total of 18 Vertical Electrical Soundings (VES) with 20 m station interval was completed in 3 even parallel profiles for two dimensional (2D) groundwater investigations (Fig. 2).
The depth to the basement rock in the study area was not expected to be too
deep (Nwankwo et al., 2004); however, for proper
depth probing a total current electrode spread of 200 m was used with the Schlumberger
RESULTS AND DISCUSSION
The apparent resistivities of the VES stations plotted on a log-log graph against corresponding half Schlumberger were interpreted by computer iteration technique. The curves, which are predominantly A-type (1<ρ2<ρ3) and H-type (ρ1>ρ2<ρ3), are shown in Fig. 3. The resistivity values of some common water bearing rocks are 1-100 (Ωm), 10-800 (Ωm) and 10-100 (Ωm) for clay, alluvium and fresh groundwater respectively. These resistivity values and that of some other common rocks are presented in Table 1.
In order to consider the hydro-electrical characteristics of the study area, the results were then used to develop series of 2D resistivity pseudo-sections (Fig. 4) at various depths and 2D thickness section (Fig. 5) of the aquiferous zone as determined from the interpretation of results. These maps are profiles of resistivity and thickness contrasts of rock layers within a depth extent in the survey area.
plots of some VES stations. (a) VES11, (b) VES22, (c) VES24, (d) VES25,
(e) VES32 and (f) VES36
||2D resistivity contour map of the first/second layers. (a)
Resistivity contour map of the surface Layer. Contour interval = 50 m, (b)
2D resistivity contour map of the study area at AB/2=15 m. Contour interval
= 50 m, (c) 2D resistivity contour map of the study area at AB/2=25 m. Contour
interval = 100 m, (d) 2D resistivity contour map of the study area at AB/2=30m.
Contour interval = 100 m, (e) 2D resistivity contour map of the study area
at AB/2=35m. Contour interval = 100 m
Contour map of the second layer thickness
The purpose of selecting multiple layers was to distinguish the different lithological
units and to convert the resistivity values into a geological reasonable picture.
The analysis of the field data shows that study area is embedded with three
geoelectrical layers. The resistivity of the first layer varies from 200 to
500 Ωm, interpreted as top lateritic soils. The resistivity of the second
layer ranges from 10 to 500 Ωm, which was interpreted as weathered rock
containing water. This layer also have water-saturated weathered zone. Lower
resistivity values in the area are either due to silty or clayey soils. The
third layer resistivity has values above 900, interpreted as fresh basement
rocks. Although, there is limited published geophysical work for the study area,
these results are consistent with available findings for the university area
(Olasehinde et al., 1986; Olasehinde,
1999; Nwankwo et al., 2004). They had earlier
revealed that the structural characteristics of the area are made up of three
Based on the model classification in this study, the distribution of resistivities in the surface layer is not uniform. Figure 4a shows that the resistivity values of the surface layer ranges from 200 to 500 Ωm. The figure indicates that the apparent resistivity values are increasingly spreading from the North-West area to other parts in the study area. The texture of the rock in this layer was found to be generally hard and dry due to high exposure to intense heat from the sun.
Figure 4b-e were prepared to see how the
resistivity values of the weathered basement and aquiferous zone varies from
place to place at different depths with the aim of delineating the most favourable
area and depth for groundwater exploitation as well as delineating possible
structures or structural trends, if any, within the layer of the weathered basement.
The resistivity variations at 15 m depth (Fig. 4b) shows that
very low resistivity values (<100 Ωm) are found around the South central
portion of the study area and resistivity values not more than 150 Ωm are
also dominated in NW-SW region. At 25 m depth, Fig. 4c shows
that the variations of the resistivity within these two regions (that is, South
Central and NW-SW) are constant. However, at 35 m depth, Fig.
4d reveals that there is a diminutive increase in the resistivity values
of both portions but these fall below 300 Ωm. Figure 4b-e
has reveal that there exist some consistencies of low resistivities at these
two areas right from 15-35 m depth. These areas suggests plausible targets for
groundwater exploitation, however to make reasonable judgement the thickness
variations of the aquifer in these areas are to be considered (Dogara
et al., 1998).
Therefore, Fig. 5 depicts the second layer thickness variations
as well as the aquiferous zone of the study area. As can be deduced from the
map, the thickness of the weathered basement has a maximum of 30 m. Considering
the two inferred plausible areas for groundwater target, it has been discovered
that the South central area has thickness less than 10 m while a thickness of
30 m exist within the NW-SW axes. Studies have shown that aquifer thickness
is of utmost important (Dogara et al., 1998),
the larger the thickness, the larger amount of water the place can hold and
vice-versa. Consequently, the most favourable target for siting groundwater
tube in the study area would be NW-SW, particularly, the central part of the
Author thanks Bayo Geophysical Services, Sango Ilorin for releasing the PELI 1300 Terrameter used in this study.