32

Estimation of Magnetic Basement Depth of Aboh in

Midwestern Nigeria Using Aeromagnetic Data

Ijeh, B. I & Anyadiegwu, F.C.

Department of Physics,

Michael Okpara University of Agriculture, Umudike, Abia State, Nigeria

Fzinoz@gmail.com

ABSTRACT An airborne magnetic data was analyzed and interpreted employing qualitative and quantitative tools,

such as regional map, residual map, vertical derivative maps, reduction to pole and spectral depth

estimate, with the aim of investigating the basement topography, identifying and delineating the structures

associated with the basin; identifying the trends and patterns of such structures and making inferences

about their relationship with basin formation and dynamics. The spectral analysis result of the

aeromagnetic data of the study area obtained using Oasis Montaj software revealed a two depth source

models. An average depth of 11.944km was obtained for the basement rock.

Keywords: Spectral Analysis, Magnetic Anomalies, Basement Depth, Aboh.

1.0 INTRODUCTION

The interpretation of aeromagnetic maps has moved from the interpretation of basement structure to a

detailed examination of structures and lithologic variations in the sedimentary section. In some of the

sedimentary basins, magnetic anomalies arise from secondary mineralization along fault planes, which are

often revealed on aeromagnetic maps as surface linear features. If the magnetic units in basement occur at

the basement surface, then the depth determinations for these will map the basin floor morphology and its

structure (Onyedim et al., 2006). Some lineaments patterns have been defined to be the most favorable

structural condition in control of various mineral deposits. They include the traces of major regional

lineaments, the intersection of major lineaments or both major (regional) and local lineaments, lineaments

of the tensional nature, local highest concentration (or density) of lineament and lineament associated

with circular features. Linear features are clearly discernible (Onyewuchi et al, 2012) on aeromagnetic

maps and often indicate the form and position of individual folds, faults reins lithological contacts and

other geologic features that may lead to the location of individual mineral deposits. The often indicate the

general geometry of subsurface structures and geomorphic features expressed as lineament and classify

the according to their spatial and directional attributes, it would be necessary to process the aeromagnetic

data in a manner that would enhance trends and facilitate the computation of location and depth to

magnetic source.

The purpose of this study therefore is to investigate the basement topography, identify and delineate the

structures associated with the basin, identify the trends and patterns of such structures and to make

inferences about their relationship with basin formation and dynamics.

2.0 Geology of Study Area The area of study is in Delta State in the Niger Delta region and lies between latitudes 5˚30`- 6˚00` North

and longitudes 6˚30`- 7˚00` East and is in the coastal sedimentary basin of Nigeria has been the scene of

three depositional cycles. The Niger Delta is situated at the apex of the Gulf of Guinea on the west coast

of Africa. The onshore portion of the Niger Delta province is delineated by the geology of the southern

Nigeria and southwestern Cameroon. It is one of the world’s most prolific deltaic hydrocarbon provinces

and is the youngest sub-basin of the Benue Trough. Both the marine and mixed continental deposition

environments characterize the Niger Delta. The Tertiary section of the Niger delta is divided into three

International Journal of Innovative Environmental Studies Research 6(2):32-44, April-June, 2018

© SEAHI PUBLICATIONS, 2018 www.seahipaj.org ISSN: 2354-2918

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formations representing prograding depositional facies that are distinguished mostly on the basis of sand-

shale ratios (Short and Stauble, 1967). Sedimentary deposits in the basin have been divided into three

large-scale lithostratigraphic units: The Akata Formation at the base of the Delta, of marine origin and is

composed of thick shale sequences (potential source rock), turbidite sand (potential reservoirs in deep

water) and minor amounts of clay and silt. Beginning from the Paleocene and through the recent, the

Akata Formation was formed when the terrestrial organic matter and clays were transported to the deep

water areas characterized by low energy conditions and oxygen deficiency (Stacher 1995). The Formation

underlies the entire delta and is typically over-pressured. Turbidity current likely deposited deep water

sands within the upper Akata Formation during the development of the delta (Burke, 1972). The

deposition of the overlying Agbada Formation, the major petroleum –bearing unit, began in the Eocene

and continues in the Recent. The Formation is of 3700m thick and represents the actual deltaic portion of

the sequence. In the lower Agbada Formation, shale and stone beds are deposited in equal proportion.

However, the upper portion is mostly sand with only minor shale inter-beds. The third Formation

overlying the Agbada Formation, the Benin Formation, a continental late Eocene to Recent deposit of

Alluvium and coastal plain sands.

3.0 METHODOLOGY

3.1 Data Acquisition

The aeromagnetic data used for this were obtained from the Geological Survey Agency of

Nigeria (GSAN). The data were acquired and complied by Fairet Surveys Ltd, during an airborne

geophysical survey between May to December, 1975 as part of the nation-wide aeromagnetic

survey which was completed in 1976. flight line direction was NNW -SSE at station spacing of

2km with flight line spacing of 20km at an altitude of about 150 m. Tie lines were flown in an

ENE -WSN direction. Regional correction of the magnetic data was based on the

International Geomagnetic Reference field (IG RF), (epoch date 1 of January, 1974).

3.2 Methods

For qualitative analysis, Reduction to Pole (RTP), Regional- Residual Separation, and Vertical

derivative were employed. While spectral analysis was employed for magnetic depth estimation.

4.0 MAP PRESENTATION AND ANALYSIS

4.1 Total Magnetic Intensity Map

The resultant total magnetic field map gotten after digitization along flight lines at a spacing distance of

5km is presented in figure below. The general magnetic susceptibility of basement rocks and the inherent

variation is shown in figs 1and 2. Areas of strong positive anomalies likely indicate a higher

concentration of magnetically susceptible minerals (principally magnetite).Similarly; areas with broad

magnetic lows are likely areas of low magnetic concentration and therefore lower susceptibility.

Interpretation of this map is to infer how the variations in this susceptibility affect the overlying

sedimentary section.

Ijeh & Anyadiegwu….. Int. J. Innovative Environ. Studies Res. 6(2):32-44, 2018

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Figure 1: Total magnetic intensity contour map of the study area

Figure 2: Total magnetic intensity contour map of Aboh presented as a pixel image (Raster)

Ijeh & Anyadiegwu….. Int. J. Innovative Environ. Studies Res. 6(2):32-44, 2018

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Figure 3: 3-D Wireframe Plot of the Magnetic Field Intensity of the Study Area

4.2 3-D Wire Frame Plot of the Magnetic Intensity of the Study Area

The 3-D plot of the study area shows the morphology of the magnetic basement surface. This plot

describes the surface of the basement of study area as smooth and subsidence. However, the basement

surface of the study area creates less tectonic picture and smooth surface. This feature the basement

uplifts from the subsidence basement areas. The basement topographies exhibit a smooth surface area.

4.3 Regional-Residual Separation In practice, the polynomial is rarely extended beyond the fourth order. The regional trend is represented

by a straight line or generally, by a smooth polynomial curve while the residual is represented by

contours. The plots below represent First to fourth degree trend (polynomial) surfaces of the regional and

residual fields of the aeromagnetic data with the first degree regional separation often representing the

trend of the study area. The first degree residual map of the study area having negative values indicate the

presence of nonmagnetic material like coal, silt, limestone, sand and sandstone.

Ijeh & Anyadiegwu….. Int. J. Innovative Environ. Studies Res. 6(2):32-44, 2018

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5.5

5.6

5.7

5.8

5.9

6.0

5.5

5.6

5.7

5.8

5.9

6.0

6. 5 6. 6 6. 7 6. 8 6. 9 7. 0

6. 5 6. 6 6. 7 6. 8 6. 9 7. 0

0. 025 0 0. 025 0. 05

( m et ers)

Non Linear Filter map of the aeromagnetic data

7584.47589.57592.57594.77596.17596.87597.37597.67598.07598.47598.77599.17599.47599.77600.07600.37600.67600.97601.47602.07602.77603.37604.27604.97605.87606.87607.97609.17610.37611.67613.07614.87616.77618.77621.07623.57626.57631.1

Figure 4: Non Linear Filter of the Aeromagnetic Data of the Study Area.

Figure 5: First Degree Regional Fields of Aeromagnetic Field of the Study Area.

Ijeh & Anyadiegwu….. Int. J. Innovative Environ. Studies Res. 6(2):32-44, 2018

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Figure 6: Second Degree Region Field of the Aeromagnetic Field of Study Area.

Figure 7: Third Degree Regional Fields of Aeromagnetic Field of the Study Area.

Ijeh & Anyadiegwu….. Int. J. Innovative Environ. Studies Res. 6(2):32-44, 2018

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5.5

5.6

5.7

5.8

5.9

6.0

5.5

5.6

5.7

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6. 5 6. 6 6. 7 6. 8 6. 9 7. 0

6. 5 6. 6 6. 7 6. 8 6. 9 7. 0

-40

-30

-2 0

-10

-10

-10

01

01

0 1

10

02 20

02

02

20

02

03

30

0

0

0

0

0

0

0

0. 025 0 0. 025 0. 05

( m et ers)

First degree residual map of the study area

-19.8-16.3-13.9-11.5-9.2-7.3-5.5-3.9-2.6-1.2-0.10.91.52.12.73.23.74.24.65.05.45.65.75.96.36.97.68.49.6

11.212.814.917.019.321.724.528.633.4

Figure 8: First Degree Residual Map of the Study Area

5.5

5.6

5.7

5.8

5.9

6.0

5.5

5.6

5.7

5.8

5.9

6.0

6. 5 6. 6 6. 7 6. 8 6. 9 7. 0

6. 5 6. 6 6. 7 6. 8 6. 9 7. 0

0527

003 7

7300

7450

745

0

00

57

7500

75

00

7550

05 57

75

50

05

57

7650

76

50

7700

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77

0047

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67

76

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006 7

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67

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67

7600

0067

00

67

760

0

7600

7600

00

67

0. 025 0 0. 025 0. 05

( m et ers)

Reduction to pole map of the aeromagnetic data

7440.57517.27547.07567.27579.87586.07590.67593.77596.97598.87600.37601.87603.47604.87606.17607.27608.17609.17610.07610.97611.67612.27612.87613.27613.57613.97614.67615.47616.47617.47618.57620.17622.37625.47628.17631.87640.07662.1

Figure 9: Reduction to Pole Map of the Aeromagnetic Data

Ijeh & Anyadiegwu….. Int. J. Innovative Environ. Studies Res. 6(2):32-44, 2018

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5.5

5.6

5.7

5.8

5.9

6.0

5.5

5.6

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6. 5 6. 6 6. 7 6. 8 6. 9 7. 0

6. 5 6. 6 6. 7 6. 8 6. 9 7. 0

0 005 -

0053-

0 003-

00

51-

- 10 00

0001-

-10

00

-100

0

-100

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-500

005-

-500

-50

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0. 025 0 0. 025 0. 05

( m et ers)

First vertical derivative map of the study area

-2165.3-1049.5

-643.3-423.3-299.8-224.2-153.7-108.1

-73.9-48.6-25.1

-2.215.829.741.853.865.873.078.483.689.596.3

102.9109.0115.6122.9129.8134.9144.4159.9176.5195.1217.8255.5319.3412.6541.3

1151.7

Figure 10: First Vertical Derivative Map of the Study Area

Figure 11: Contour map of the shallow magnetic depth source layer

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Figure 12: Spectral depth determination (block A) of the basement shown as a power log

radiance plot

Figure 13: Spectral depth determination (block B) of the basement shown as a power log radiance

plot

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Figure 14: Spectral depth determination (block C) of the basement shown as a power log

radiance plot

Figure 15: Spectral depth determination (block D) of the basement shown as a power log radiance

plot

Ijeh & Anyadiegwu….. Int. J. Innovative Environ. Studies Res. 6(2):32-44, 2018

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TOWN

LATITUDE

LONGITUDE

DEPTHS (KM)

ABOH

X1 X2 Y1 Y2 Z1 Z2

6.50 6.75 5.75 6.00 0.895 12.908

6.75 7.00 5.75 6.00 1.738 11.732

6.50 6.75 5.50 5.75 0.988 10.238

6.75 7.00 5.00 5.50 0.675 13.650

Figure 16: Contour map of the deeper magnetic depth source layer (basement depth)

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Figure 17: Contour map of the deeper magnetic depth source layer (basement depth)

The first layer depth (D1), is the depth to the shallower source represented by the second segment of the

spectrum. This layer (D1) varies from 0.975km to 1.738km, with an average of 1.207km. The second

layer depth (D2) varies from 10.238km to 13.650km, with an average of 11.944km (figs 12 to 15). This

layer may be attributed to magnetic rocks intruded onto the basement surface. Another probable origin of

the magnetic anomalies contributing to this layer is the lateral variations in basement susceptibilities. It

can be deduced that the D2 values obtained from the spectral plots represent the average depths to the

basement complex in the blocks considered (table 2). Depth to basement map estimated from spectral

inversion of the area was generated (figs.16 and 17). The maps reveal the sedimentary thickness, as

thinning towards the NE direction. This direction coincides with areas were the basement outcrops. The

sedimentary thickness of this area ranges from 10.2km to 13.6km. The colour codes show the depth in

km.

Figure 1 is the total field of the magnetic data presented as a contour map with the magnetic

intensity values ranging from 7590 -7635γ while the total field is presented in figure 3 as a 3-D

wireframe plot. The first to third degree regional and residuals are shown in figures 5, 6, 7 and 8

respectively with trend direction in the NW-SE, E-W and N-S direction, with the NW-SE trend being

dominant. A n interesting feature of most of these local anomalies is the fact that they appear

to have a NW-SE elongation trend. The short wavelength anomalies on the aero magnetic

profiles are caused by the variation in the magnetization due to existence of the very thin

intrusions occurring at shallow depths. The medium and long wavelength anomalies on the

aeromagnetic map are due to the magnetization from deeply seated intrusive bodies of

asthenospheric origin. The information derived from the analysis of the prominent magnetic

anomalies of the aeromagnetic map of the study area revealed that most of the anomalies

have a mean anomaly width of 5km.

Depth to source interpretation of aero magnetic field data pro vides important information

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on basin architecture for petroleum exploration and for mapping areas where basement is

shallow enough for mineral exploration. Magnetic basement is an assemblage of rocks that

underlies sedimentary basins and may also outcrop in places. If the magnetic units in the

basement occur at the basement surface, then depth d e terminations for this will map the

basin floor morphology, relief and structure (Onyedim, 20 06). The result of spectral analysis of

the study area indicated a two-depth source mo del with the depth to the deeper sources

identified with the crystalline basement(figure 16).

5.0 CONCLUSION The spectral analysis result of the aeromagnetic data of the study area shows a two depth source model.

The first layer brought out by the above analysis can be attributed mostly to magnetic rocks intruded into

the sedimentary formations, together with few that are extruded onto the surface. The second layer may

be attributed to magnetic rocks intruded onto the basement surface. Another probable origin of magnetic

anomalies contributing to this layer is lateral variations in basement susceptibilities. It can be deduced that

D2 values obtained from the spectral plots represent the average depths to basement complex in the blocks

considered. However the average depth of the basement rock is 11.944km which is high. Geothermal

gradient increase with respect to increasing depth in the earth’s interior. Away from tectonic plate

boundaries, the temperature of about 25˚C to 30˚C per km depth (1˚C per 70feet depth) is observed in

most regions. So due to this increase in temperature thermal maturation (source rocks) are found to occur

and this favor hydrocarbon generation and also the exploration of mineral.

REFERENCES

Burke,K.C, Dessauvagie, T.F and Whiteman,A.J(1970): Geological Histroy of the Benue Valley and

Adjacent Area. In: T.F.J Dessauvagie and A.J. Whitemaned) African Journal, University Press,

Ibadan,pp. 187-205.

Onyedim, G.C and Awoyemi, E.A (2006): aeromagnetic Imaging of the Basement Morphology in the

part of the middle Benue Trough. Journal of mining and Geology. Vol. 42(2), pp. 157-163.

Onyewuchi, R. A, Opara, A.I, Ahiarakwem, C.A, and Oko, F.U (2012): Geological Interpretations

Inferred From Airborne Magnetic and Landsat Data: Case Study of Nkalagu Area, Southeastern

Nigeria: International Journal of Science and Technology. Volume 2 No.4. Spector, A and Grant F.S (1970): Statistical Models for interpreting Aeromagnetic map. Geophysics,

35.293-302

Short, K.C., and Stauble, A.J., (1967). Outline geology of the Niger Delta. Am. Assoc. Petrol. Geol. Bull.

51, 761-779.

Stacher, P., 1995, Present understanding of the Niger Delta hydrocarbon habitat, in, Oti, M.N., and

Postma, G., eds., Geology of Deltas: Rotterdam, A.A. Balkema, p. 257-267.