HYDROGEOPHYSICAL CHARACTERIZATION OF AQUIFER UNITS
IN PART OF JOS YOUNGER GRANITE PROVINCE, NORTH
CENTRAL NIGERIA.
1
Yinusa Ayodele Asiwaju-Bello, 2Sunday Bayode, 3Joy Omowumi Ololade and 4Olusegun Adekoyejo
Dada.
1,3
Department of Applied Geology, Federal University of Technology, Akure, Nigeria.
Department of Applied Geophysics, Federal University of Technology, Akure, Nigeria.
4
Department of Marine Science & Technology, Federal University of Technology, Akure, Nigeria.
2
E:mail:
1
ayoasiwajubello@futa.edu.ng; 2sbayode@futa.edu.ng; 3omowumijoy25@yahoo.com;
4
oadada@futa.edu.ng,
ABSTRACT
This work was carried out in the non-orogenic Younger Granite terrain of North Central Nigeria.
Geoelectric method involving horizontal profiling using Wenner array and Vertical Electrical Sounding
using Schlumberger array, was employed in field data collection. Twenty-nine traverses, at spacing of 25m,
were established for horizontal profiling study with station separation of 25m along each traverse. A total
of 371 stations were occupied. Apparent resistivity data from horizontal profiling were used to generate
iso-resistivity maps for different horizons and these were used to characterize the study area into Low
(ρ<700Ω-m), Medium (ρ=700-1400Ω-m), and High (ρ>1400Ω-m) apparent resistivity zones. The low
(ρ<700Ω-m) apparent resistivity values of a = 40m found in the central, north east and north western parts
of the investigated site are indicative of fractured/highly weathered rocks in the area. Based on profiling
results, twenty-one VES stations were established.
The geoelectric sections from VES result were used to delineate six subsurface geoelectric layers.
These are topsoil, weathered rock material, hard fresh rock (boulders), partly weathered rocks,
fractured/highly weathered rock material and fresh bedrock. The delineated weathered rock material,
partly weathered rocks and the fractured/highly weathered rock material horizons were identified with
resistivity values ranging from 300 – 600, 600 – 1,000 and 100 – 600 ohm-metres, respectively and
thickness values varying from 2 – 20, 5 – 22 and 6.5 – 56 metres, respectively. The investigation revealed
existence of limited discrete zones of weathered and fractured rocks at deep depths which can support
medium to high groundwater abstraction in the investigated area.
Keywords:
Geoelectric, Fracture, Weathered, Aquifer, Groundwater, Younger Granite
1.0
INTRODUCTION
Hydrogeophysical survey was carried out
at the permanent site of National Institute of
Mining & Geosciences campus, Jos, Nigeria, an
area that is underlain by Younger Granite rocks.
The survey was meant to delineate all areas within
the site having substantial accumulation of
groundwater in the subsurface.
1.1
Geology and Geomorphology of the
Locality
Jos, the study area lies within the Younger
Granite Province of Nigeria (Fig. 1). The Younger
Granite rocks are non-orogenic high-level plutonic
igneous rocks that were emplaced in ring forms in
parts of central Nigeria around the Jurassic period,
about 150 - 162 million years
ago (Buchanan et al, 1971) The dominant
lithologic unit in the locality is porphyritic
hornblende biotite granite which is part of the JosBukuru ring complex rocks (Buchanan et al, 1971).
The top soil at the study site was the reddish to
brownish lateritic material which usually harden
during the dry season.
1.2
Hydrogeology of the Crystalline Rock
Environment
Crystalline rocks are generally non-porous
and depend on acquisition of secondary porosity to
be able to store water. The presence, extent and
1
characteristics of groundwater storage units in this
area are dependent on the depth and degree of
weathering as well as intensity of fracturing of the
underlying rocks. Thickness of the weathered
overburden, i.e. depth to the fresh bedrock, is
normally directly proportional to groundwater
storage capacity of any available aquifer unit in the
subsurface (Olorunfemi, 1990). Availability of
fractures at depth is expected to complement the
overburden effect and determine borehole yield in
the terrain (Olayinka and Olorunfemi, 1992).
Younger Granite rocks are known for
their hardness and induration. They have not been
subjected to significant weathering to deep depths
except in few areas where fracturing must have
occurred. An investigation for groundwater in such
rocks is more of a survey of the available fracture
zones that are expectedly limited in the rocks
(Anudu et al, 2008).
2.0
METHOD OF STUDY
A total of twenty nine (29) traverses of
about 100 – 500m long with inter-traverse
separation of 25m were established in an
approximately east – west directions across the
study area. The geoelectric study involved the
horizontal profiling (HP) and a follow-up
investigation with the Vertical Electrical Sounding
(VES) technique (Fig. 2). The Wenner Array was
used for the horizontal profiling while the
Schlumberger Array was adopted for the VES
technique. Horizontal profiling measurements, with
electrode spacing of a = 20m and a = 40m, were
carried out along the 29 traverses.
Figure 1: Simplified map of Younger Granite Province of Nigeria showing Jos locality (after MacLeod et al,
1971). Inset is map of Nigeria showing Jos.
Generally, this profiling technique was employed
as a means of delineating possible areas with
characteristic low apparent resistivity values that
are diagnostic of deep-seated weathered layer and
fractured zones which can be probed further with
the VES technique (Keller and Frischnecht, 1966;
Asiwaju-Bello, 1999). The horizontal profiling data
was used to generate the iso-resistivity maps using
SURFER, a graphic computer software.
Based on areas where low resistivity
values were observed for electrode spacing of a =
20 and a = 40m, a total of 21 controlled VES was
2
conducted within the study area (Fig. 2). ABEM
Terrameter model SAS-1000 instrument was used
for the data acquisition. Schlumberger electrode
array configuration was employed for the VES
using a maximum current electrode spacing (AB/2)
of 100m. The VES data were interpreted using
manual partial curve matching technique to obtain
the initial geoelectric parameters which were later
subjected to 1-D computer iteration using
WinResist, a data modelling computer software.
The results obtained were used to construct geoelectric sections.
3.0
RESULTS AND DISCUSSION
The results obtained were presented as
summary table, iso-resistivity maps (Figs 3 & 4),,
depth-sounding curves (Fig.5) and geoelectric
sections (Figs. 7 and 8). Figure 3 shows the general
apparent resistivity distribution for the electrode
spacing a = 20m.
The obtained values were grouped into
Low (ρ<700Ω-m), Medium (ρ=700-1400Ω-m), and
High (ρ>1400Ω-m) apparent resistivity zones. The
low (ρ<700Ω-m) apparent resistivity values found
in the central, north east and north western part of
the investigated site are indicative of materials in
the upper most 7m of the earth in the area. The
map is therefore indicative of features at shallow
depths.
Figure 2: Location and Data Acquisition Map of the Study Area.
3
The a = 40m apparent resistivity map (Fig. 4) also
display low apparent resistivity values (ρ<700Ω-m)
which correlated well with the areas delineated as
low apparent resistivity values for a = 20m. The
low apparent resistivity (ρ<700Ω-m) zones is
straddled with medium to high apparent resistivity
(ρ>700Ω-m) zones alligned in the southwest –
northeast directions and occupy a small area in the
north western part of the study area. The medium
to high apparent resistivity zones suggest nearsurface bedrock while the low apparent resistivity
zones are indicative of relatively deep bedrock,
weathered layer and deep-seated fractures.
Typical depth sounding curves obtained for the
area are H, KH, QH, HKH, KQH, KHKH and
QHKH. A summary of the VES interpretation
results are in Table 1. Three geoelectric sections
were constructed across the study area (Figs. 6, 7
and 8). These are along the line of section A – A”,
which runs east – west, and sections B – B” and C
– C” running in the north – south directions (Fig.
4). The three geoelectric sections delineated six
subsurface geoelectric units. These are topsoil,
weathered rock material, hard fresh rock
(boulders),
partly-weathered
rocks,
fractured/highly weathered rock material and fresh
bedrock.
The first layer is the Topsoil. Its resistivity
values generally range from 320 – 5500 ohm-m
and its thickness varies from 0.8 – 2.2m. It is made
up of sandy and clayey materials which may be
indurated in some cases.The second layer is the
weathered rock material. The resistivity value
range from 300 – 600 ohm-m and the thickness
varies from 2 – 20m. The layer is composed of
sandy clay and clayey sand.
The third layer is the hard fresh rock (boulders).
The resistivity values range from > 1500 ohm-m
and its thickness varies from 2 – 42m. It is
composed of fresh rock material in boulder forms.
The partly weathered rock is the fourth layer with
the resistivity value ranging from 600 to 1000
ohm-m and its thickness varying from 5 – 22m.
The layer is made up of partly-weathered granitic
rocks.
Figure 3: Apparent Resistivity Distribution for HP of ɑ=20m.
4
Figure 4: Apparent Resistivity Distribution for HP of ɑ=40m.
The fifth layer is the fractured/highly weathered
rock material. The resistivity values range from
100 – 600 ohm-m and the thickness varies from 6.5
– 56m. The layer is the most reliable aquiferous
horizon within the subsurface in the area.
The last layer is the fresh granite bedrock. The
resistivity values are generally above 1500 ohm-m.
Depth to bedrock ranges from 16.5 to over 72
metres.
Both the weathered rock material and the partlyweathered rocks occur at shallow depths and their
resistivity values will allow area only limited
hydraulic conductivity characteristics. The small
amount of storage in them will be prone to easy
pollution from the ground surface. However, where
found, they may constitute low to medium
hydrogeological significance and will readily serve
the needs of shallow hand-dug wells.
The fractured/highly weathered rock material has
thickness varying from 6.5 to over 56 metres, with
layer resistivity from 100 to 600Ω-m. This unit is
encountered everywhere along all the traverses but
at varying depths which is not less than 15m below
ground surface. It is a unit that is of highest
hydrogeological significance in the area and it is
expected to be a large reservoir of groundwater at
deeper depth. Areas with large thickness are
expected to support boreholes with high yields
(Olorunfemi, 1990). In most places, the hard rock
unit of low permeability overlies this aquiferous
zone making it more difficult for pollution by
infiltrating water.
5
6
Figure 5: Typical VES Curves Obtained in the Study Area.
Table 1: Summary of VES Interpretation Results for the Study Area.
S/N
STATION
No.
1
TR 3,4
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Layer 1
= 1400
T= 1
TR 3,7
= 560
T= 1
TR 3,15
= 1550
T= 1
TR 4,6
= 320
T= 1
TR 7,1
= 1040
T= 0.5
TR 7,8
= 2900
T= 1
TR 8,13 =1800
T= 1
TR 9,1
= 2300
T=1
TR 13,16 = 750
T= 1
TR 15,3
=1400
T= 1
TR 15,13 = 3500
T= 1
TR 15,15 = 1050
T= 1
TR 16,12 = 1750
T= 1
TR 17,4
= 480
T= 1
TR 17,14 = 1900
T= 1
TR 18,1
= 1500
T= 1
TR 19,9
= 2050
T= 1.5
TR 19,18 = 4800
T= 1
TR 21,18 = 5500
T= 1
TR 29,5
= 2600
T= 1
TR 29,7
= 5000
T= 1.5
GEOELECTRIC LAYER PROPERTIES
(resistivity, , in ohm-metres; Thickness, T, in metres)
Layer 2
Layer 3
Layer 4
Layer 5
Layer 6
= 700
= 1420
T= 7
T= 
= 168
= 3075
= 624
= 
T= 5
T= 6.5
T= ??
T= 
= 7750
= 1560
= 504
= 
T= 1
T= 8
T= ??
T= 
= 256
= 540
= 204
= 
T= 3
T= 45
T= ??
T= 
= 2080
= 630
= 1850
T= 13
T= 58
T= 
= 3625
= 1020
= 2750
T= 1
T= 20
T= 
= 1440
= 450
= 1150
T= 1
T= 34
T= 
= 8050
= 1440
= 300
= 24000
T= 1
T= 20
T= 21
T= 
= 1125
= 135
= 650
T= 1
T= 1.5
T= 
= 280
= 1500
= 82.5
= 1050
T= 11.5
T= 19.5
T= 2
T= 
= 700
= 37500
= 650
= 
T= 11.5
T= 16.5
T= ??
T= 
= 525
= 1240
= 285
= 6750
T= 2
T= 6
T= 6.5
T= 
= 1138
= 1500
= 420
= 
T= 15
T= 14
T= ??
T= 
= 960
= 487.5
= 2100
= 550
= 
T= 2
T= 5
T= 16
T= ??
T= 
= 950
= 3500
= 420
= 31500
T= 14
T= 12
T= 30
T= α
= 1875
= 850
= 174
= 900
T= 1
T= 17
T=9
T= α
= 820
= 2975
= 280
= 7500
T= 12.5
T= 7.5
T= 8
T= α
= 1440
= 750
= 3045
= 31.25
= α
T= 7
T= 21
T= 21
T= ??
T= α
= 825
= 1350
= 32.5
= α
T= 4.5
T= 28.5
T= ??
T= α
= 1300
= 700
= 1245
T= 2
T= 7
T= α
= 1000
= 2725
= 138
= 5625
T= 17.5
T= 11
T= 14
T= α
CURVE
TYPE
H
HKH
KQH
HKH
KH
KH
QH
KQH
KH
HKH
HKH
HKH
HKH
KHKH
HKH
KQH
HKH
QHKH
HKH
QH
HKH
7
Figure 6: Geoelectric Section Along Line A-A”.
Figure 7: Geoelectric Section along Line B-B”.
8
Figuere 8: Geoelectric Section along Line C-C”.
4.0
CONCLUSION
A
hydrogeophysical
investigation,
the units of hydrogeological significance with
employing horizontal profiling and Vertical
moderate to high layer resistivity values of 300 –
Electrical Sounding techniques, was carried out at
600, 600 – 1000 and 100 – 600 ohm-metres,
the permanent site of National Institute of Mining
respectively. Their thickness values range from 2 –
& Geosciences, Jos, Nigeria. The apparent
20, 5 – 22 and 6.5 – 56 metres, respectively.
resistivity values obtained for the electrode spacing
The investigation also revealed that the low
a = 20m and a = 40m were used to characterize the
apparent resistivity value (<700Ω-m) zones
area into Low (ρ<700Ω-m), Medium (ρ=700obtained using horizontal profiling correspond with
1400Ω-m), and High (ρ>1400Ω-m) apparent
areas of low layer resistivities (<600Ω-m) in the
resistivity zones. The low apparent resistivity
geoelectric sections obtained from VES which are
values (ρ<700Ω-m) observed for a = 40m suggest
indicative of fractured/highly weathered zones at
the presence of weathered layer/fractured bedrock
deeper depth which can support medium to high
at deeper depths. KH type curve and its variants
yielding boreholes in the study area.
constitute the dominant geoelectric curves that
characterise the prevailing weathering pattern of
REFERENCES
the Younger Granite rocks in the study area.Anudu,
The G. K., A. G. Onwuemesi, N. E. Ajaegwu, L. N.
geoelectric sections constructed were used to
Onuba and A. O. Omali. 2008. Electrical
delineate six subsurface geoelectric layers. These
resistivity investigation for groundwater
are topsoil, weathered rock material, hard fresh
in the Basement Complex terrain: a case
rock (boulders), partly weathered rocks,
study of Idi-Ayunre and its environs, Oyo
fractured/highly weathered rock material and the
State, Southwestern Nigeria. Natural and
fresh bedrock.
Applied Sciences Journal, 9(2), 1-12.
The delineated horizons of weathered rock
Asiwaju-Bello, Y.A. 1999. Electrical Resistivity
material,
partly
weathered
rocks
and
Mapping of Structures Controlling a
fractured/highly weathered rock material constitute
Spring System: A Case Study of the
Federal University of Technology.
Buchanan, M.S; MacLeod, W.N. and Turner, D.C.
FUTAJEET. 1(1):7-16.
1971. The Geology of the Jos Plateau.
9
Explanation of 1:100,000 sheets Nos. 147,
148, 168, 169, 189 and 190. Volume 2.
Younger Granite Complexes. Geological
Survey of Nigeria Bulletin No. 32.
Keller, G.V. and Frischnecht, F.C. 1966. Electrical
Models in Geophysical Prospecting.
Pergamon Press: Oxford, UK.
MacLeod, W.N; Turner, D.C. and Wright, E.P.
1971. The Geology of the Jos Plateau.
Explanation of 1:100,000 sheets Nos. 147,
148, 168, 169, 189 and 190. Volume 1.
General Geology. Geological Survey of
Nigeria Bulletin No. 32.
Olayinka, A.I. and Olorunfemi, M.O. 1992.
Determination
of
Geoelectrical
Characteristics in Okene area and
implications for Borehole Siting. Journal
of Mining and Geology. 28(2); 403-412.
Olorunfemi, M. O. 1990. “The Hydrogeological
implication of topographic variation with
overburden thickness in Basement
Complex area of south Western Nigeria.”
Journal of Mining and Geology, 26(1),
145-229.
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