Dimensional modeling of the major structures underlying

Journal of Natural Sciences Research www.iiste.org
ISSN 2224-3186 (Paper) ISSN 2225-0921 (Online)
Vol.2, No.5, 2012

2-1/2 Dimensional Modeling of the Major Structures Underlying
Dong and Shelleng of the Upper Benue Valley, Using GM-SYS
Computer Modeling
M. Omeje*, N. B. Ibrahim*, O. Meludu** and P.E. Ugwuoke***
Maxico3333@yahoo.co.uk, noorddin@utm.my,P.E ugwuoke@yahoo.com
*Department of Physics, Faculty of Science, Universiti Teknologi Malaysia,
**Department of Physics, Federal University of Technology, Yola, Nigeria,
***National Centre for Energy Research and Development, University of Nigeria, Nsukka
Abstract: Airborne magnetic data over part of Dong and Shelleng area of the upper Benue Valley, N-E
basement complex of Nigeria's was carried out to investigate the deep and thick structures of the underlying
geology and their relationship with granitic intrusion. These features are interpreted in terms of deep geologic
structures, and/or susceptibility variations within basement rocks. The residual separation exercise revealed the
area is underlain by a NE-SW regional trends, which shows it is in agreement with Benue trough, presumed they
have the same structure control. Furthermore, the 3-D residual magnetic anomaly map suggests Dong is likely to
be the sedimentary basin. It shows that the magnetic field there is smoother, which is masked off by the effects
of the stronger magnetic basement. High amplitude of the anomalies which outcrop at eastern part of Dong
where the profile cut across 6 could be the presence of Ngurore basalts. The thickness revealed by the model (D-
D1), Northwestern part Numan has a maximum thickness of 1.4km and minimum of 0.2km. In D-D1 modeling, it
revealed a porphyritic granite with non- uniform thickness of about 1.1Km and minimum of 0.17Km with a
width approximately 2.9Km. The first sedimentary layer reveals has a thickness of about 1.0Km and the second
varies from 1.0 to 1.6km thick. The width is about 4km.The Alluvium thickness increases progressively between
0.2Km to 0.7Km and the width of about 18km, reveals by the model. The thickness of the basin varies between
0.9km to 2.9Km. The model reveals the width of the basin to about 136Km.
Key Word: Geologic Map, Total Field Map, Regional Field, Residual field and 3-D Residual Field, Model D-D1

1.0 Introduction
The purpose of the magnetic survey is to detect rocks or minerals possessing unusual magnetic properties; which
reveal themselves by causing disturbances or anomalies in the intensity of the earth’s magnetic field. However,
there are differences which increase the complexity of magnetic interpretation (Keary and Brooks, 1984).
Sometimes individual magnetic anomalies are found; which can clearly be distinguished from neighboring
effects. This is simple appearance and can be to a single magnetized body. In such cases, quantitative methods
can be used most effectively to deduce the shape and size of the magnetic formation.
Potential fields are inherently ambiguous. Theoretically, an infinite number of solutions may be obtained for the
physical property distribution, giving rise to a specific anomaly. However, only a limited number can be due to
geological factor.
In this regard, the choice of modeling is required to detect such geological features. There are several methods of
modeling bodies. These methods are divided into three. Two-Dimensional (2D) techniques are usually applied in
modeling elongated bodies with length to width ratio greater than 10, as in the case of dykes. Thus, the length is
assumed to be infinite. In 3-D techniques, many simple geometric forms have been used to model magnetic
anomaly fields. Some of these forms, such as the vertical and inclined prisms, have been particularly successful,
especially in near surface exploration work. A simple geometric form which can be used in a closed-shaped
arrangement for modeling anomalies is the finite depth, vertical-sided rectangular prism developed by
Bhattacharyya (1964). The study proposed a closed-form equation for the total field magnetic anomalies of
prismatic models. Complicated 3-D shapes can also be modeled by other methods such as the Talwani method
(1965). In this technique, the outline of the body is defined by horizontal polygonal shaped depth contours. A
large number of sides in the polygons and highly dense contours can accurately represent any complex structure
or sloping boundaries. In calculation, these contours are replaced effectively by horizontal laminae. Therefore,
the magnetic effect of the body can be calculated by numerical integration with respect to depth.
The 2-1/2D modeling technique is an improvement of the 2D and approximation to the 3-D model. In this
method, the length is made finite though considerably longer than the width of the body. The technique applied
in any survey depends on the structures intend to model and the purpose of the survey. Cosequently, the two
dimensional (2D) technique can readily be used to quantitatively model dykes (such as schist) since their length

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to width is usually greater than 10. Alternatively, the three dimensional (3D) modeling technique can be
conveniently used in modeling batholiths while moderately elongated slabs can be modeled using the 2-1/2D
method (Talwani, 1965; Gemerle, et al.,1991).
The purpose of this research is to model the shape and depth of the major structures underlying part of Dong and
Shelleng which is model D-D1 , cut across the area at 45 degrees.

2.0 Location of the Study Area
The upper Benue Valley is a northeasterly structured, about 1000 km long; and between75 to 150 km wide. The
Yola arm is an easterly branch of the trough. The study area covered lies between latitude 90 30/ N and 100 00/ N
and longitude 110 30/ E and 120 30/ E, which are Numan, Guyuk ,Shellem and Dong. This work will only focus
on the profile model of D-D1 which is part of Dong and Shelleng of the study at 450.
2.1 Geology
As part of this study, a geological survey of part of the area was undertaken with emphasis on structure. The
following gives a brief description of the lithological units found and structural features. The geological map is
presented in figure1.0.

2.1.1 Lithological Unit:
Basement Complex
The basement rock units in the north and south parts of the study area are made up of older granites, gneisses and
migmatite. However, basic extrusive (basalt) and intrusive also occur. Whilst sedimentary rock units comprising
mainly sandstones, limestone, clays and shales outcrop within the central portion of the study area (Cater et al.,
1963).
Volcanic Rocks and Igneous Intrusion
The extensive igneous activity which took place on the lower Cretaceous of Nigeria was most intense during the
late Albian time (Ofoegbu 1985a). The intrusions are widely distributed throughout the area; and the younger
intrusive are represented by volcanic plugs, dyke, silts in the study area. Smaller dykes of basic masses;
occasionally produce contact metamorphism in the sediments, characteristic of a short time interval between
segmentation, folding and intrusion (Cratchely and Jones 1965).
Alluvium: Quaternary -Recent
The Quaternary alluvium is a superficial deposit; comprising mainly argillaceous stuff called Benue valley
alluvium. The soil is black to dark-gray clayey sediment formed by the weathering of volcanic (basaltic) and
shaley rocks. It is marshy and covered by shrubs forming stripes of 500m to 1km radius range, beyond which
they are intercalated with poorly sorted sands and gritty clays (typical of the Benue valley Alluvium). In the
study area, the Quaternary Alluvium uncomformably overlies the Bima formation obscuring it in most sections.
Bima Sandstone
The Bima sandstone is classified into three layers: the lower, middle and upper Bima. The upper Bima consists
of medium to coarse grained feldspathatic sandstone. In the Lamurde anticline, the maximal thickness reaches up
to 1, 700m but may vary considerably elsewhere. The sequence was deposited under fluvaital to deltaic
environment (Cater et al, 1963) with late Albian/Cenomanian age assigned to it. The Middle Bima is composed
of very coarse grained, feldspathic sand stone with bands of clay, shale and occasional calcareous sandstones. In
the Lamurde anticline, the thickness is 800m but may vary elsewhere. This sequence was laid down in fluvaital
and the deltaic environment (Carter et la 1963) with a tentative middle Albian age assigned to it. Lower Bima is
the oldest sediment known in the area which appears in the core of the Lamurde anticline comprising coarse
grained feldspathic sandstone. It also alternate with red, purple shale and occasional band of calcareous
sandstones and silts stone. The visible thickness in the Lamurde anticline is 390m (Cater et al, 1963). This basal
sequence was deposited in lacustrine environment with a brief marine incursion (Cater et al, 1963). An upper
Aptian/ Albian age has been recently assigned to this part of Bima sandstone assuming palynological date.
Figure 1.0 shows the geologic map of the study area.

3.0 Materials and Method
3.1 Data Acquisition
The data sets used in this study were compiled from four aeromagnetic maps with index numbers of 174,196,175
and 195, for Numan, Dong, Guyuk and Shelleng respectively. These maps were obtained from the Geological

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Vol.2, No.5, 2012

survey's Agency, Kaduna, Nigeria. The maps were produced between the year 1974 and 1980 and published on
1/20 by 1/20 sheets contoured manly at 10 gammas interval. The maps are published on scale 1:100,000. The
present work covers 196 and 175. The contour lines in all the maps were dense; therefore, it was easy to adopt
the flight line digitization used with very minimal errors of human judgment. The magnetic values were plotted
at the 10nT (gamma) intervals. The maps were numbered and named according to the places covered for easy
reference. A total of 340 maps covered the entire country. The actual magnetic values were reduced by 25,000
gammas before plotting the contour maps (Huntings, 1976). Therefore, the value of 25,000 gammas should be
added to the contour values to obtain the actual magnetic field at a given point.
The maps were digitized on a grid of 1km imposing a Niquisit Frequency of ½ km-1 so that the narrowest
magnetic feature that can be defined by the digitized data has a width of “2” km. The anomalies are wider than
2km and therefore, lie in a frequency range from aliasing, which do not occur with a 1km digitizing grid.
Consequently, the aeromagnetic map was digitized such that the data was stored in 55x 55 coding sheets. Each
record contains the boundary longitudes and latitudes, the map number, and name of the town overflow. The
data were entered into a computer file, thereafter became the input file for a computer program. It picked all the
data points row by row, calculate their longitude and latitude and the magnetic value for coordinates
respectively. Each output file for this operation was given a meaningful name for easy identification. The three
dimensional coordinates from x, y, z, is made to be acceptable to a contouring package “SURFER 7.0." This is a
menu-driven interactive computer programme, which places each magnetic data point according to their
longitude and latitude bearing and thereafter produces contour maps to ensure that they correspond with their
respective original maps. The maps are figures.2, 3, 4 and 5.
The compilation of the aeromagnetic maps of Numan, Dong, Guyuk and Shelleng form the composite map
figure 6. The most flexible and applicable analytical known as Polynomial fitting method was used for
determining regional magnetic field (Skeels, 1967; Johnson, 1969 and Dobrin 1976). Usually by least square,
which is the mathematical describable surface giving the closest fit to the magnetic field produces regional field,
and the residual field is the difference between the magnetic-field values as actually mapped and the regional
field thus determined. The resultant map is shown in figure 7. The subtracting values of the regional filed from
the total magnetic field value at grid points produce the residual field (shown in figure 8a ) . The magnetic ( high
and low) was also determined as presented in figure 8b. Further analyses of the residual field map generated the
3-D residual anomaly field is presented figure 9.

3.2 Gravity/Magnetic Software System (GM-SYS)
GM-SYS, written by Gemperle et al, (1991) is a programme used for the easy interactive modeling of 2D and
optionally 2½D geological cross section, with the ability to quickly calculate and display the gravity or magnetic
response from the cross section. The 2½D option was used to calculate the magnetic response within the study
area. The method is used for calculating the magnetic response based on the methods of Talwani et al;
(1959);Talwani and Heirtzler; (1964) using the algorithms described in Wen and Bevis (1987). However, the
2½D calculations are based on Rasmussen and Pedersen (1979).
The computer program can give a composite model of the thickness of the basin, the surface topography of the
basement below the basin, the structures and depth of any intrusion into the sedimentary terrain.

3.3 The Modeling of major anomalies
The magnetic residual map (figure8a) of the study area is complicated as expected in magnetic studies.
Nevertheless, some major anomalies stand out which profiles were drawn across on the magnetic residual
anomaly map as shown in figure.(10). For modeling the shape, depth of the causative bodies and the thickness of
the sedimentary basin within the profile D-D1 that cut across Dong and Shelleng at 45 degrees to the area. The
anomalies were selected for modeling; numbered 6 for easy identification (figure10). Profiles D-D1 runs across
anomalies 6. Additionally, the DD1 profile was drawn in order to estimate the width of the causative basin; it
was necessary to draw profiles perpendicular to the strike length. Less disturbed area was chosen so that the
effect of topography rather than lithology will lead to better accuracy in modeling the profile. The basin is
assumed to be underlain by granite gneiss, which is the dominant rock type of the basement complex of
Nigeria.Furthermore, migmatite often associated with basaltic rock and intrusive as well as sedimentary rock
units comprising mainly sandstones, limestones, clays outer or within the central portion are also present (Carter
et al, 1963). However, a study by Ajakaiye et al, (1991) reported ranges of 0.0059 – 0.0097 SI units and 0.15 –
1.030 SI units respectively for granite – gneiss as the magnetic susceptibility values in the neighboring Benue
Trough. Another study by Ojo (1990) used a value of 0.12 SI units for the granite gneiss of the Nupe Basin. The
value used in this study was 0.016 SI, since the modeling package uses the Gaussian’s unit, this value translates

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Vol.2, No.5, 2012

to 0.0013 Gaussian units using a conversion factor of 4π from SI unit to Gaussian unit. The average ambient
magnetic field, magnetic inclination and declination values used for this study are 33000gammar, 40 and 50
respectively.

4.0 Result:
The result indicates that Southeastern part of the Dong area possesses the most dense closures. Some major
closures were observed in the upper Eastern part of Dong particularly in the middle area located towards NE-
SW. The total field map indicated lineament closures focusing NE-SW direction.
The regional map of the study area computed with polynomial coefficients revealed of 5nT in the lineament and
trending NE-SW as presented in Figure: 7. The subtracted values of the regional filed from the total magnetic
field at grid points generated a residual and the high and low magnetic field observe in figure 8a & 8b. Further
analyses of the residual field map in Figure 8a generated 3-D residual anomaly field. Some parts in the study
were observed to be smoother, particularly the northeastern part of Dong and Southeastern part of Guyuk as
observed in Figure.9.
Using available geological information and magnetic susceptibility as stated in (Figure11), bodies were fitted to
the anomalies and subsequently adjusted until a good fit was obtained between the calculated anomalies and
observed residual anomalies. This infinite number of models is possible in this way; but geological and other
constraints often limit these to be only acceptable.
Results of (figure11) show the model of profile D-D1 which cut across Dong and Shelleng at 45degrees to the
study area.

4.1 Discussion
From the results, the residual map (Figure 8a), there are noticeable closures, NE-SW & E-W
trending features, which the situation looks different because the contour lines of the total residual magnetic field
are concentrated more in the Northern & Southern portion of the study area. This could be that the basement
complex outcropping there is not homogeneous. Mineralogically, in addition to the presence of basement faults
in the area granitic and pegmatitic dykes were also observed. This inferred zone may be interpreted as major
fracture zones of the weakness within the trough. The NE-SW lineaments (found predominantly in pan Africa
granite) were observed towards the northeast and souteastern part of the study area. However, with some
extension to the southwestern part.The NE-SW structures are found to be faults and foliation. This structural
direction is related to the West African rift system made up of the north easterly Benue Trough and Cameroon
volcanic line (Flitton, 1983) (Bassey, 2006). These features are foliation and shear zones trending towards NW-
SE features. The lineaments are among the longest and invariably are of deep crustal origin (Odeyemi et al,
1999). East-West lineaments are the youngest as they cross cut structures of all other directions. They are
mainly basement normal faults, while others are granites. .
This profile passes through west of Dong and cut across the field at 45 degrees to the east south of
Shelleng (figure11). It revealed the porphyritic granite which has maximum thickness about 1.1km and
minimum of about 0.17km. The width is about 2.9km. The first sedimentary layer has thickness of about 1.0km
and the second sediment varies from 1.0 to 1.6km thick. They have width of about 4km, the alluvium was
reveled from the profile which has maximum thickness of about 0.7km and minimum of 0.2km thick. The width
is about 18km. The Ngurore basalt which the profiles reveal along the NW part of Numan has the maximum
thickness of 1.4km and minimum 0.2km. The width is about 31km. The thickness of the basin and the width are
3km and 131km respectively. The basement migmatite gneiss outcrop Northeast of Shelleng with thickness of
about 0.2km.The model inferred the basin width to about 136Km.This is in agreement with spectral depth
estimation of Nur.A (2001)
.
4.2 Conclusion
The regional field has shed light on the geology and structure of this part of Nigeria's basement complex that has
been relatively understudied. The study revealed NE-SW trending, which correlate with the geology of the area
showing that the Benue trough trends NE-SW, indicating that the underlying geological structures have the same
structural control with the trough. The study which also revealed NE-SW structures as the youngest in the
Nigerian Basement (Olumide 1988; Ene and Mbono, 1988; Ekwueme, 1994 ).The major lineaments observed to
have been accounted for in terms of faults and foliation. The lineaments have the regional extension into Niger,
Chad and Cameroon.

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More so, residual field legend shows the magnitude of an anomaly from -200 to 120 nT. From the 3-D residual
field, the area is characterized by residual positive anomalies ranging from 60 to 90 nT. In this study, it is
assumed that the anomalous magnetic field of the crustal rocks is due to induced magnetization in the area. It
could be the fact that some locations have a high component of remnant magnetism in the anomalies' field.
High-sensitivity aeromagnetic data over the area contain both low-amplitude, linear anomalies produced by
structurally deformed magnetized layers near the top of the sedimentary section and high amplitude, broad
anomalies produced within the basement.
Matched-filtering has been successful in separating the anomalies produced by these two source regions. In this
work, it is assumed that the anomalous magnetic field of crusted rocks is due to induced magnetization from 3-D
residual field. This could be due to the fact that some locations may have a high component of remnant
magnetization in the anomalous field. In conclusion this research calls for more geophysical studies within the
area to correlate the result of this work Therefore, a thorough and widespread determination of the direction and
magnitudes is recommended before further studies are carried out in the area. More so, studies on the
susceptibilities of the rocks in the area should be carried out as these will enhance the results of the further
studies.

Acknowledgment
I give thanks to Prof. Dr Noorddin Ibrahim for his impact in the supervision of this paper to this point and the
entire physics Department of Universiti Tecknologi Malaysia. My regards also goes to the Dr. Osita Meludu for
his guidance in during this research and the entire Department of Physics, Federal University of Technology,
Yola, Nigeria.
The GM-SYS 2½D modeling computer packages made available to this researcher by, I. B Osazuwa (Ahmadu
Bello University Zaria, Nigeria), satisfies the need enumerated above for this studies, regards for your
contribution.

Reference

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continental and mass based on aeromagnetic surveys Tectonophysics, 192: 211-230.
Bassey N.E (2006): Structuer of Madagali hill's NE Nigeria from
Airborne magnetic and satellite data over Hawal basement complex. Global journal of
geological science volume 4 No 1. Pg
Cratchely C.B and Jones. G.P. (1965). An interpretation of the Geologic and
gravity anomalies of the Benue Valley, Nigeria Geophysics paper.
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Dobrin, M (1976): Introduction to Geophysical Prospecting. McGraw-Hill book
Co(Third Edition) New York.
Ekwueme, B.N. 1994. Structureal features of southern Obudu Plateau, Bamenda Massif, SE Nigeria:
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Basement Complex, facts, fallacies and new frontiers. In: P.O. Oluiyide (coordinator),
Precambrian geology of Nigeria. Geol. Sur.Nigeria Pub... pp53-67
Fitton J.G 1983 Active versus passive continental rifting: evidence from
The West African Rift system. Tectnophysics: 94:473-481
Huntings Geology and Geophysical Ltd (1976):Aieborne Magnetometer survey
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Johnson, W.W.(1969).The least-squres method of interpreting magnetic
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exploration: Geophysics, 70, no. 6, 33ND–61ND
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Trough, Nigeria. Dept. of Geology, Federal University
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Over southwest of Chad Basin, Nigeria.J.M.Geol.37 (2) pp.9-98
Odeyemy, I.B, ANIFOWOSE, Y.B. AND ASIWUJA-BELLO Y.A.1999:
Multtechnique graphical analysis of fractures from remote sensing
images of Basement regions of Nigeria.J.MIN.GEO.35 (1) PP.9-21
Ofuegbu .C.O. (1985). Interpretation of a linear Magnetic features near Mutum
Biyu, Nigeria.
Ojo, S. B. (1990). Origin of a major Magnetic anomaly in the Middle Niger Basin,
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Olumide, P.O.1988.Structural trends in the Nigerian Basement Complex. In:
P.O Oluiyide. (Coordinator), precambrian Geology of Nigeria,
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Osazuwa, I. B., (1981): analysis of the structure of part of upper Benue Rift valley
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Rasmussen, R. and Pedersen, L.B. (1979): End corrections in potential field
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Syberg, F.J.R., 1972, A Fourier method for the regional-residual problem of
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Talwani, M. (1965): Computation with the help of Magnetic Anomalies caused by arbitrary. Geophysics
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subroutines: Geophysics, 52, 232 -238

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110 30′E 120 30′E

100N 100N

90N 90N

110 30′E 120 30′E

Fig. 1: Geologic Map of the study area after Nigeria Geolog. Surv. Agen:2005

LEGEND
Pcg = Granite

Qr = Basult
Qa = River Alluvium

Kby = Feldspatic sandstone, clay, shales, calcareous sandstone and shelly

Kp = shale and limestone

Pcb = Gneiss-migmatite (complex)

Ky = Medium and coarse grain feldspatic sandstone
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Vol.2, No.5, 2012

50

45

40

35

30

25

20

15

10

5

0
0 5 10 15 20 25 30 35 40 45 50 55

FIG 2: Aeromagnetic Map of Numan Contoured at interval of 10nT

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Vol.2, No.5, 2012

55

50

45

40

35

30

25

20

15

10

5

0
0 5 10 15 20 25 30 35 40 45 50 55

FIG.3: Aeromagnetic map of Dong contoured at interval of 10nT

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Vol.2, No.5, 2012

55

50

45

40

35

30

25

20

15

10

5

0
0 5 10 15 20 25 30 35 40 45 50

Figure.4: Aeromagnetic map of Guyuk contoured at interval of 10nT

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Vol.2, No.5, 2012

55

50

45

40

35

30

25

20

15

10

5

0
0 5 10 15 20 25 30 35 40 45 50 55

Figure.5: Aeromagnetic map of Shelleng contoured at interval of 10nT

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Vol.2, No.5, 2012

10

9.9

9.8

9.7

9.6

9.5

9.4

9.3

9.2

9.1

9
11.5 11.6 11.7 11.8 11.9 12 12.1 12.2 12.3 12.4 12.5

Fig6: Total Aeromagnetic field of the study area contoured at interval of 10nT

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10

9.9

9.8

9.7

9.6

9.5

9.4

9.3

9.2

9.1

9
11.5 11.6 11.7 11.8 11.9 12 12.1 12.2 12.3 12.4 12.5

Fig 7: Regional map of the study area contoured at interval of 5nT

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10

9.9

9.8

9.7

9.6

9.5

9.4

9.3

9.2

9.1

9
11.5 11.6 11.7 11.8 11.9 12 12.1 12.2 12.3 12.4 12.5

Figure 8a: Residual map of the study area contoured at interval of 20nT

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10

9.9
120
100
9.8
80
9.7 60
40
9.6 20
0
9.5
-20
-40
9.4
-60

9.3 -80
-100
9.2 -120
-140
9.1 -160
-180
9
11.5 11.6 11.7 11.8 11.9 12 12.1 12.2 12.3 12.4 12.5 -200

Fig8b: Residual magnetic map of the study area showing magnetic high and low contoured at
interval of 20nT

.

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.NORTH . SHELLENG
.GUYUK
nT
.NUMAN

.DONG

Figure 9: 3-D Residual magnetic anomaly map of the study area

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A1 B1 E1

10 D1
1
9.9

9.8 SHELLENG
.GUYUK
9.7
8
9.6
2
9.5
6

9.4 .NUMAN
9.3 5
DONG
9.2

9.1

3 4 7
9 D
11.5 11.6 11.7 11.8 A 11.9 12
B 12.1 12.2 12.3 12.4 12.5
E
Fig.10: Residual Magnetic Map of the study area With Profiles contoured at 20nT
Intervals

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Figure 11: Model profile D-D1

LEGEND

Pcg = Porphyritic Granite

Kby = Feldspatic sandstone, clay, shales, calcareous sandstone and shelly limestone

Qa = River Alluvium

Qr = Ugurore Basalt

Pcb = Gneiss-Migmatite (complex)

67

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Dimensional modeling of the major structures underlying

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Dimensional modeling of the major structures underlying