Topographic values in the study area range from 80m to 170m. The rock type comprises Basement Complex at the west bounded by River Niger, and sedimentary rock type in the east. Density measurements of various rocks were taken with the highest bulk density from the metamorphic schist (2.77gm/cm3), followed by igneous rock olivine gabbro (2.73gm/cm3), and sandstone (2.35gm/cm3). Results of gravity survey revealed a mean Bouguer anomaly of +12.15 mgals and a mean free air anomaly of +22.0 mgals. Interpretation of gravity measurements revealed the existence of a fracture at Gboloko NE-SW axis, a synclinal fold axis at about 5.5km west of Gboloko (between the Staurolite Schist and Cordierite-Tourmaline schist). The Basement-Sedimentary boundary is characterized by a drop in residual Bouguer anomaly from positive to negative at about 6km east of Gboloko. The thickness of the sediments is about 0.90km at the northern part of the Basement-Sedimentary boundary, and about 2.0km in the south, thus suggesting a progressive increase in sedimentary thickness at the western edge of the Benue trough. The Free-air anomaly ‘highs’ correspond to Bouguer anomaly ‘highs’ and tied to areas of high topography and bands of weathered, lateritized sediments. The Bouguer anomaly profiles exhibited reliable signature changes at the rock boundaries, thus a supportive tool for delineation of those border areas.

1. 1
IJGM
Bouguer anomaly and free-air anomaly correlation
signatures in parts of Benue and Kogi States of Nigeria
Madu Anthony Joseph Chinenyeze1*
and Onuoha Mosto K2
1*
Department of Geology, Michael Okpara University of Agriculture Umudike, Abia State, Nigeria.
2
PTDF Professorial Chair, Department of Geology, University of Nigeria Nsukka (UNN), Enugu State, Nigeria.
Topographic values in the study area range from 80m to 170m. The rock type comprises
Basement Complex at the west bounded by River Niger, and sedimentary rock type in the east.
Density measurements of various rocks were taken with the highest bulk density from the
metamorphic schist (2.77gm/cm3), followed by igneous rock olivine gabbro (2.73gm/cm3), and
sandstone (2.35gm/cm3). Results of gravity survey revealed a mean Bouguer anomaly of +12.15
mgals and a mean free air anomaly of +22.0 mgals. Interpretation of gravity measurements
revealed the existence of a fracture at Gboloko NE-SW axis, a synclinal fold axis at about 5.5km
west of Gboloko (between the Staurolite Schist and Cordierite-Tourmaline schist). The
Basement-Sedimentary boundary is characterized by a drop in residual Bouguer anomaly from
positive to negative at about 6km east of Gboloko. The thickness of the sediments is about
0.90km at the northern part of the Basement-Sedimentary boundary, and about 2.0km in the
south, thus suggesting a progressive increase in sedimentary thickness at the western edge of
the Benue trough. The Free-air anomaly ‘highs’ correspond to Bouguer anomaly ‘highs’ and tied
to areas of high topography and bands of weathered, lateritized sediments. The Bouguer
anomaly profiles exhibited reliable signature changes at the rock boundaries, thus a supportive
tool for delineation of those border areas.
Keywords: Free-air anomalies, Bouguer anomaly, Gravity profiles, Basement complex rocks, bulk density.
INTRODUCTION
The gravity study area lies between latitudes 7
o
33’N and
7
o
48’N, and longitudes 6
0
45’E and 7
0
00’E. The area is
bordered in the north by the River Benue, in the west by
the River Niger and falls under the southeast quadrant of
Topographical Map-Sheet 247 of the Nigerian Ministry of
Lands and Surveys. The area covers Kogi State, and
Benue State, Nigeria.
The area has an average range of elevation of 80m to
170m (250ft to 550ft). But there are also topographic lows
and highs, which are respectively characterized by
quaternary Alluvium that adjoins the River Benue in the
south, and a ridge of sediments which flanks the
Basement Complex in the East. The ridge of sediments
on the eastern portion is about 304m (1000ft) above
mean sea level, while the Basement Complex area has
an intermediate topography.
*Corresponding author: Department of Geology, Michael
Okpara University of Agriculture Umudike, Abia State, Nigeria.
Tel.:+234 8064169362, Email: ajchinenyezem07@yahoo.com.
Co-author: Tel.: +234 8037793601, Email:
mosto.onuoha@gmail.com
International Journal Geology and Mining
Vol. 2(1), pp. 030-037, August, 2016. © www.premierpublishers.org. ISSN: 0907-3409x
Research Article

2. 2
Madu and Onuoha 031
The geologic setting comprises an assemblage of
heterogeneity of the rock types within the study area with
division of the terrain sedimentary terrain, the Basement
Complex rocks and the igneous intrusive (Adeleye,
1976). It has a sedimentary terrain which comprises
outcrops of light colour and sandstone. Some parts of the
sediments have been weathered and intensely lateritized.
In consequence, bands and pockets of laterites and
ferruginized sandstone exist. The banks of some big
streams are bordered with boulders of sandstone while
most hand specimens were collected from fossil bearing
boulders. Colour changes in the various samples of the
medium-grained sandstone are from light to grey, brown,
reddish brown and dark brown. These colour variations
reflect different degrees of ferruginization and/or
laterization.
These sediments, deeply weathered, occur east of
Nyankpo area also cover the entire Emi-Eronu, in the
Northeastern portion of the study area. The prominent
escarpment in the topographic map showing a range of
hills of height about 250m (1140ft) constitutes the
easterly boundary of the Basement Complex. The degree
of weathering and ferruginization of the sediments
indicates a considerable similarity to the characteristic of
the Upper Coal Measures. This gives credence to the
result of previous geological mapping of the area by
(Ekwueme, 1981) in which the sedimentary terrain of this
area has been broadly identified as the Coal Measures,
of Campanian-Maaestrichtian age. The Northern fringe of
the study area is delimited by Quaternary Alluvium which
adjoins the southern bank of the River Benue. This
portion bears little or no good outcrops, but constitutes
favourable soil for agriculture.
The Basement complex portion consists of a variety of
schists ranging from quartz-bearing mica-schists to
quartz-free schist (Rahaman, 1976). Samples of the
quartz-mica schist are characterized by white and dark
bands which respectively show the preferred alignment of
quartz and mica (biotite-muscovite) minerals. Other types
of schists in the area are staurolite schist and Cordierite-
Tourmaline schist which have been affected by the
intrusion of olivine gabbro and granite bodies respectively
(Cratchley and Jones, 1965). A discrete exposure of
gabbro outcrop has an approximate extent of 15m x 20m;
and is contiguously located with the Straurolite Schist,
with a minimum separation of about 40m. The actual
extent and thickness of the olivine gabbro would be
estimated from the quantitative interpretation of gravity
and magnetic data. Adjoining the Cordierite-Tourmaline
schist westwards, are the Granite gneiss and migmatite
which have been subjected to intense granitic and
pegmatitic intrusions. The Bouguer anomaly and Free-air
anomaly profiles simultaneously indicate density and
topogragphic changes (Dobrin, 1983), and offer
themselves to delineation markers at borders of changing
rock types (Ojo and Ajakaiye, 1976). Major lithologic
boundary outlines synergize or fall into phase with axes
of inflexion of the Bouguer anomaly, as well as the Free-
air anomaly especially with consistent topography
(Telford, et al, 1976).
OBJECTIVE OF SURVEY
The survey was aimed at using geophysical methods,
specifically gravity method to delineate major rock
boundaries. The presence of different major rock types:
the Basement Complex metamorphic rocks, igneous, and
sedimentary is associated with change in densities along
gravity profiles, giving rise to significant density contrasts
at lithologic contacts or boundaries.
METHODOLOGY
A reconnaissance survey preceded the detailed gravity
survey. The Canadian Scintrex Gravimeter Model CG-2
was used in carrying out the survey. The elevation of
each gravity station was obtained by an American Paulin
System Altimeter, measuring to an accuracy level of
0.0304m (1ft). It involved gravity measurement at
stations, whose elevation values were determined at the
same time. Selected gravity measurements were
conducted covering several stations along the three
traverses. The three traverses were of spread 8km, 15km
and 16km (fig.1). The observation intervals varied from
500m to 2km.
The Gravimeter Model CG-2 instrument measures
gravity values to the accuracy of 0.01 mgal.
There was paucity of previous geophysical investigation
in the area, but detailed geological mapping has been
done. The quantitative modelling computation for the
gravity anomalies was conducted using the Iterative
system procedure.
RESULTS AND ANALYSIS
Shintaku-Gboloko Profiles
The gravity profile (A-B) cuts across various lithologic
units comprising granite gneiss, a strip of pegmatite
intrusive into the host rock metamorphic granite gneiss
extensive outcrop. The latter was adjoined by extent of
Cordierite-toumaline schist, followed by the 2km
extensive spread of Staurollite Schist. The profile shows
repeatability of geologic features displayed on the

3. 3
Int. J. Geol. Mining 032
Figure 1. Showing topographic conditions and traverses of Gravity Profiles.
composited profiles, establishing a correlation of some
features. The residual gravity low, adjacent to Gboloko
(Gravity base station), has been interpreted as a
signature of an existing fracture. The stacked profiles
reveal the existence of a fold axis.
The correlation of structures in the various profiles along
with the illustration of the deductions can be observed
about the area.
From the plot of the gravity measurements in figure 3 (a),
there is a revelation of occurrence of a fracture at
Gboloko NE-SW axis, a synclinal fold axis at about 5.5km
west of Gboloko, between the Staurolite Schist and
Cordierite-Tourmaline schist. See figure 3b.
A graphical subtraction of the regional Bouguer anomaly
from the observed anomaly resulted in the residual
anomaly for the profile. The regional anomaly curve
modified the Bouguer gravity features into residual
positive and negative anomalies. The graphical method
of estimation of regional anomaly trend used by
(Artsybasheu and Kogbe, 1976) was adopted in the
modeling by iterative method that yielded best fit to the
observed anomaly.

4. 4
Madu and Onuoha 033
Figure 2. Gravity measurement showing Bouguer anomaly Profiles superimposed on Geologic map with rock boundaries.
The Profile runs East-West, from Gboloko in the east
(Base Station), to Shintaku in the west. It has a mean
Bouguer anomaly of about +11.5 mgals, with remarkable
“lows” and “highs” defining a maximum Bouguer anomaly
of +23 mgals and a minimum of +7 mgals, at about Eluti
and Kpata respectively.

5. 5
Int. J. Geol. Mining 034
Figure 3. Bouguer anomaly and free-air anomaly signatures in relation with rock changes.
Figure 4. Geologic Cross section along the Gravity Profile B-C
Figure 4. Geologic Cross section along the Gravity Profile C-D
Figures 3, 4, and 5 show a comparison of Bouguer
anomaly and Free-air anomaly along the profile of
varying rock formation and corresponding density
contrasts. The density of the rock type from the River
Niger at the Western border comprising meta-sediments
and gneisses are both of relative lower than the densities

6. 6
Madu and Onuoha 035
Table 1. Densities of the Metamorphic Rocks
Rock type No of samples Dry density range
(gm/cm
3
)
Mean dry
density g/cm
3
Saturated density
range g/cm
3
Mean wet density
g/cm
3
Schist 14 2.33 – 2.77 2.55 2.43 – 2.89 2.66
Gneiss 2 2.20 – 2.48 2.34 2.40 – 2.56 2.48
Table 2. Densities of the Igneous Rocks
Rock type No of samples Dry/saturated
density range
(gm/cm)
Mean dry/ weight
density
Gabbro 4 2.54 – 2.73 2.64
Granite 3 2.39 – 2.58 2.49
Pegmatite 1 2.34
Table 3. Densities of the Sedimentary Rocks
Rock type No of
samples
Dry density
range (gm/cm
3
)
Mean dry
density g/cm
3
Saturated density
range g/cm
3
Mean wet density
g/cm
3
Sandstone 7 1.85 – 2.55 2.10 1.91 - 2.45 2.18
of adjoining migmatites and Schists members of the
metamorphic Basement Complex rocks. Tables 1 to 3
depict the trend of densities ranging from low values at
the sedimentary sandstone increasing with the igneous
intrusive rocks and the Basement rocks.
Results of the gravity survey shows that the study area
has a mean Bouguer anomaly of +12.15 mgals and a
mean free air anomaly of +22.0 mgals. Basement
complex / sedimentary boundary is characterized by a fall
in residual Bouguer anomaly from positive to negative at
about 6km East of Gboloko,.
The Gravity profile B-C which traversed from the area of
metamorphic and structural activity at Gboloko as
depicted in figure 4 to Odugbo area, a sedimentary
terrain of almost homogeneous lithology. The
geophysical signature indicated a transition from the
metamorphic portion of staurollite schist (of wet density
2.89gm/cm3) and quartz-mica-schists (2.55gm/cm3) to
the sedimentary rock (Coal Measures of wet density 1.85
to 2.45gm/cm3) showing significant marked density
contrast. See Tables 1 and 3.
A 2-dimensional modeling was generated which
produced a fairly good fit to the observed anomaly of
Profile B-C (figure 6). The result showed that the cause of
the anomaly is a sphere. The fitting model of a spherical
body has a radius of about 1.20km, and the depth to the
top of the body approximately 100m. The model reveals
that the source of the anomaly is likely an igneous body
of density 2.51 gm/cm
3
. This is in contrast to the density
of the host Basement Complex rock 2.72 gm/cm3,
yielding the density contrast of 0,21gm/cm
3
. Appendix 1
is an example of computation of best fits involved.
CONCLUSION
There is a synergy or relational trend of the Bouguer
anomaly and the Free-air anomaly in the area. They
occur as indicators of lithologic changes as well as
topographic variation. Portions of appreciable density
contrast are generally identifiable as negative Bouguer or
Free-air anomalies.
REFERENCES
Adeleye, D. R. (1976). The Geology of the Middle Niger
Basin. In Geology of Nigeria. (Ed) Kogbe C.
A.,Elizabethan Publ. Co, Lagos Pp. 283 – 287.
Artsybashev, V.A., and Kogbe, C.A..1974. Crustal
structure of the Benue Valley Area Nigeria.
Geologische Rundschau, Vol. 64, No.2.
Cratchley, C. R., and Jones G. P., (1965). An
interpretation of the Geology and Gravity anomalies of
the Benue Valley, Nigeria. Overseas Geological survey,
Geophysical Paper No. 1, 26p.

7. 7
Int. J. Geol. Mining 036
Dobrin, M. B. (1983). Introduction to Geophysical
Prospecting. Mc Graw-Hill Book Co., London. 3
rd
Ed.,
576p.
Ekwueme, B.B., (1981). Mineralogy and Petrology of the
Metamorphosed rocks in SE Lokoja, Nigeria.
Unpublished M.Sc. Thesis, Department of Geology,
University of Nigeria Nsukka (UNN).
Ojo, S. B., and Ajakaiye, D. E., (1976). Preliminary
Interpretation of Gravity measurements in the Middle
Niger Basin Area, Nigeria. In Geology of Nigeria. (Ed)
Kogbe C. A., Elizabeth an Publ. Co. Lagos, Pp. 295 –
307.
Rahaman, M. A., (1976). Review of the Basement
Geology of South Western Nigeria. In Geology of
Nigeria (Ed) Kogbe C. A., Elizabethan Publ. Co. Lagos.
Pp. 41 – 58.
Telford, W. N., Geldert, L. P., Sheriff, R. E., and Keys, D.
A., (1976). Applied Geophysics. Oxford Univ. Press
New York. 860p
Accepted 08 July, 2016.
Citation: Madu AJC, Onuoha MK (2016). Bouguer
anomaly and free-air anomaly correlation signatures in
parts of Benue and Kogi States of Nigeria. International
Journal Geology and Mining 2(1): 030-037.
Copyright: © 2016 Madu and Onuoha. This is an open-
access article distributed under the terms of the Creative
Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium,
provided the original author and source are cited.

8. 8
Madu and Onuoha 037
APPENDIX 1
Computation of regression line
Equation of a straight line: Y = a0 + a1 x.
Where
n = Number of Gravity base station measurements
x = Time of measurement (in hours)
Y = observed gravity (in mgals) at the base station, at time x.
a1 = regression coefficient
a0 = intercept at Y-trend.
# n = 5 X XY Y X
2
1 1 3547.8015 3547.0815 1.0
2 3 10667.2416 3555.7472 9.0
3 5.125 18200.3582 3551.2894 26.2655
4 7.12 25344.8440 3559.6691 50.6944
5 9.27 32958.1427 3555.3552 85.9329
n=5 ∑X =25.5250 ∑XY = 90718.3880 ∑Y = 17769.8624 ∑X
2
= 172.8928
= 0.9084
Where â1 = 0.9084
= 3553.9725
= 5.1030
ao = 3553.9725 – 0.9084 (5.103)
= 3549.3369
The equation of the line then becomes;
Regression line for 28/9/83 – Gravity survey:
Y 3549.3369 3550.2453 3557.5125
X 0 1 9