Geophysical Characterization of Basement Rocks and Groundwater Potentials Using Electrical Sounding Data from Odeda Quarry Site, South-western, Nigeria
This study was carried to characterize the basement rocks and groundwater potentials
by delineating the resistivity and overburden thickness of subsurface lithology
of Odeda quarry, south-western, Nigeria. Vertical electrical soundings were
carried out at fifteen locations using Schlumberger electrode arrangement. The
maximum current electrode spread and potential electrode separation were 200
and 5 m, respectively. Data acquired were interpreted using the manual partial
curve matching method and a fast computer iteration technique to generate the
geoelectric layer of various resistivity values and thicknesses. The geoelectric
sections revealed the lithological sequence as: topsoil, weathered layer and
fresh bedrock. The overburden coefficient of anisotropy also revealed that,
the underlying basement rock is suspected be granite gneiss/granite-schist.
The overburden coefficient of anisotropy was calculated for each sounding location
and this ranges from 1.00 to 2.30. This was used in delineating the study area
into region underlain by granite gneiss (λ between 1.00 and 1.03) and those
regions with granite-schist (λ between 1.30 and 2.30) as the underlying
basement rocks. At locations where current terminated at the fresh bedrock region,
the thicknesses were undetermined. Groundwater potential is presumed to be very
low within the study area as outcrops of gneissic rocks dominate the area. Locations
where the regolith is of appreciable thickness, the resistivity of the layer
suggests a material medium likely composed of clay/sandy clay or clayey sand
which are not good aquiferous media from which groundwater could be extracted.
Received: October 12, 2012;
Accepted: December 24, 2012;
Published: March 19, 2013
Electrical resistivity technique has been widely employed in groundwater exploration,
depth to bedrock determination and basement rock characterization (Zohdy
et al., 1974; Beck, 1981; Olorunfemi
and Okhue, 1992). Geoelectric investigation of Odeda quarry site was carried
out with a view of understanding the subsurface lithology and hydrogeological
setting to characterize the groundwater potentials. Geophysical survey of basement
rock is useful because such surveys are more time and cost efficient than collecting
data through borehole drilling and coring technique. Geophysical surveys provide
non-invasive spatial images of the subsurface, while borehole data only provides
information of the subsurface close to a well. Beacon and
Jones (1988) and Caruther and Smith (1992) have
all shown the significance of the use of electrical resistivity techniques for
sitting wells and boreholes in crystalline basement aquifers in sub Sahara Africa.
However, interpretation of the geophysical data can be ambiguous if clear correlations
are not established between the geophysical attributes and the geologic properties
coupled with the stratigraphic properties of the subsurface (Koster
and Harry, 2005). Given the usual sparsity of core and geophysical logs,
establishing such a correlation is often problematic. As good alternative, geophysical
studies of outcrops strata in basement complex terrain are often used (Wolfe
and Richard, 1996; Hubbard and Rubin, 2000). Water
content affects a variety of geophysical attributes, such as electrical resistivity,
seismic, radar velocity and reflectivity (Heigold et
al., 1979; Kelly and Frohlich, 1985; Mazac
et al., 1988; Metwaly et al., 2010;
Smith and Jol, 1995; Doser et
al., 2004). Outcrop studies provide a robust qualitative tool for identifying
major stratigraphic features on the basis of their resistivity character (Koster
and Harry, 2005).
Weathering is not a uniform phenomenon in any environment and results in heterogeneous
and hydrological characteristics of the rock formations. The conceptual structure
of hard rocks is that of a fresh basement overlain by materials which have undergone
different stages of weathering. Groundwater availability is therefore attributed
to weathering in the overburden and basement surface. Basement weathering presents
themselves as zones of disintegration (KOrowe et
al., 2008). These zones appear as low electrical resistivity anomalies
compared to the massive basement rocks that surround them. Consequently, basement
troughs with deep weathering are points of disintegration which are hydro-geologically
viable as far as groundwater aquifers are concerned (Ahmed
et al., 1988; KOrowe et al., 2008).
MATERIALS AND METHODS
Study area and the geology: This study area revealed outcrops in a heterogeneous, unconfined and unconsolidated rock exposed at the Odeda quarry site, south-western Nigeria. Odeda is in the north central region of Ogun State and has accessible and well-connected roads and foot paths. The quarry has two phases; the abandoned site and the new site. This study covered part of the new site to the extreme of the abandoned site. The quarry site lies within the southwest basement complex of Nigeria. In general, the surface is covered with granitic boulder of outcrops of different sizes with thick sage brush between the boulders. Topographically, the site consists of steep natural slopes with several intervening drainage courses. Structural blocks and intervening valleys characterize this physiographic region. The area is underlain directly by crystalline formations (Precambrian to upper Cambrian) of the basement complex of south-western Nigeria. The prolonged weathering of the crystalline rocks has led to the development of regoliths of varying thicknesses which in effect reduced the conductivity of the parent rocks. The degree of weathering also depends on the depth of the rock beneath the earth surface. When the rock is very close to the surface, weathering is faster because it can be easily affected by rainwater and other weathering agents. But when the rock is buried far deep within the earth, weathering rate is reduced. The physical changes in rock material as a result of weathering are referred to as litho-facie changes.
The basement complex area of Nigeria (Fig. 1) is composed
predominantly of migmatitic and granitic gneisses, quartzite, slightly migmatised
to unmigmatised meta-sedimentary schist and meta-igneous rocks, charnockitic,
gabbroic and diorite rocks and the members of the older granite suite mainly
granites, granodiorites and syenites.
|| General geological map of Nigeria
The migmatite (gneissic quartzite) is mostly widespread in the basement complex
of south-western Nigeria (Fig. 2) which comprises of gneisses,
quartzite, cal-silicate rocks, biotic (hornblende schist) and amphibolites.
The slightly migmatised to unmigmatised paraschist and meta-igneous rock are
described as younger or newer metasediments. Charnockites occur west of Ibadan
as dyke-like bodies scattered over a wide area. Jones and
Hockey (1964) recognised three main groups of granites: an early phase comprising
granodiorites and quartz diorites; a main phase, comprising coarse porphyrite,
hornblende granite, syenite and coarse porphytic biotite granite; and a late
phase comprising homogeneous granites, dykes, pegmatites and aplite.
Data acquisition and interpretation: In this study, a total of fifteen VES (Vertical Electrical Sounding) stations were carried out (Fig. 3). ABEM Terrameter SAS 300B and its accessories were used. Consecutive readings of resistance were taken automatically by the Terrameter and the result stacked and ranged continuously thereby increasing the signal-to-noise ratio of measured values. The apparent resistivity values were obtained using the product of apparent resistance and geometric factor.
For the manual data interpretation, apparent resistivity values were plotted
against half current electrode separation (AB/2) on a log-log graph. Partial
curve-matching was performed using the Schlumberger array master curve and auxiliary
curves to determine the layer resistivity values and thicknesses. The results
obtained from the manual curve-matching procedure served as initial model for
the fast computer iteration technique using software RESIST (Velpen,
1988) for further refinement of the results.
|| Ogun State geology map showing the study location
|| Data acquisition map of the study area
Samples of the curves obtained after a number of iterations until the model
generated for all VES curves are totally resolved with minimum RMS error, are
presented in Fig. 4a-c.
||VES (a) 1, (b) 4 and (c) 6 computer iterated results
RESULTS AND DISCUSSION
The results of the electrical soundings carried out in this study area are
presented in tables (Table 1) and geoelectric sections (Fig.
5a-c). The fifteen locations sounded at the quarry site
were used to characterize the subsurface lithology to a depth of about 200 m.
Based on the interpretation of the VES resistivity data, the inferred geoelectric
sections (Fig. 5a-c) revealed the thickness
of the regolith in the range 2.2 to 16.3 m.
||Geoelectrical section beneath profile (a) A-A' (VES 1-6),
(b) B-B' (VES 7-11) and (c) C-C' (VES 12-15)
|| Apparent resistivity using the fabricated resistivity equipment
The resistivity of the weathered layer varies from a minimum of 63.9 Ωm
to a maximum of 456.5 Ωm. The basement rock is suspected to be granite
of different litho-facie changes. The lateral resistivity changes in Odeda granite
can be attributed to the nature of the rock, degrees of compaction, depth of
burial and other geological features.
The qualitative interpretation of the data revealed a minimum of three geoelectric
layers and a maximum of five geoelectric layers. The curve types obtained in
soundings over a horizontally stratified earth is indicative of the number of
layers as well as the electrode configuration adopted (Zohdy
et al., 1974). The curve type A occurred mostly in this study and
it reveals a steady increase in resistivity with depth as the current electrode
spread is increased. The steeply rising segment of the curves at a large electrode
distance indicates the characteristic high resistivity of basement rocks (Olayinka,
1990; Olayinka and Sogbetun, 2002). The weathered
layer (materials above the basement) is very thin in most locations, a manifestation
that the site is clustered with outcrops.
Granite gneiss/schist which underlain the quarry site can be classified into
three litho-facie changes:
||Basement rocks with resistivity values greater than 25,000
Ωm. These are highly compacted, dense and fine grain
||Basement rocks with resistivity values in the range 10,000 to 25,000 Ωm.
These are compacted, mixed with sandstone and less dense
||Basement rocks with resistivity below 10,000 Ωm; very porous as well
as having cracks
The resistivity values of granite-gneiss/schist range between 2136.5 and 36462.7
Ωm and thicknesses between 1.8 and 11.0 m. The overburden coefficient of
anisotropy (Christensen, 2000; Olatinsu,
2003) was calculated for each sounding location and this ranges from 1.00
to 2.30. This was used in delineating the study area into region underlain by
granitic gneiss (λ between 1.00 and 1.03) and those regions with granite-schist
(λ between 1.30 and 2.30) as the underlying basement rocks.
Possible locations where granite rock are close to the surface for exploration
and possible excavation can be found mainly along traverse A-A with VES
1-6. The depth to bedrock is significantly high at VES 8, 10 and 11 and as a
result, could be less cost effective because of the extra cost in reaching these
depths. However at VES 13, 14 and 15 where there is a possible existence of
fracture units of moderate resistivity (above 250 Ωm), groundwater exploitation
may be difficult, as the results of this study revealed most areas of appreciable
overburden thickness to compose of sandy clay, clay or clayey sand. These media
are not suitable aquiferous media for groundwater extraction.
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