INTERNATIONAL JOURNAL OF SCIENTIFIC & TECHNOLOGY … · 2013-08-05 · INTERNATIONAL JOURNAL OF...

INTERNATIONAL JOURNAL OF SCIENTIFIC & TECHNOLOGY RESEARCH VOLUME 2, ISSUE 8, AUGUST 2013 ISSN 2277-8616

153 IJSTR©2013 www.ijstr.org

Interpretation Of Aeromagnetic Anomalies Over Some Parts Of Lower Benue Trough Using

Spectral Analysis Technique

David. I. Igwesi, Marius N Umego

ABSTRACT: Four aeromagnetic map sheets of ½0 x ½

0 on a scale of 1: 100,000 covering some parts of Lower Benue Trough were analysed using

spectral techniques to estimate the average depth of magnetic sources. The result indicates a two-layered source model. The deeper magnetic sources are located atdepths which vary between 1.16 km and 6.13 km, with an average depth of 3.03 km, representing magnetic basement surface. The depths to the shallower magnetic sources vary from 0.06 km and 0.37 km, with an average depth of 0.22 km showing the presence of magnetic intrusive bodies within the sediments. Profiles taken from the area indicate that the topography of the basement is undulatingwith an anticlinal structure over Abakaliki area. The average depth to basement of 3.03 km to the magnetic source suggests enough sedimentary thickness for hydrocarbon accumulation. The undulatingof the basement surface possibly provides traps for hydrocarbon.Thus, the possibility of hydrocarbon accumulation cannot be ruled out. Key words: Aeromagnetic Sheets, Spectral Analysis, Least Square, Lower Benue Trough

————————————————————

INTRODUCTION The geographical coordinates of the study area lie between 8.0E and 9.0E of longitude and between 6.0N and 7.0N of latitude, covering an area of approximately 121,000 km

2.

The major towns are Abakaliki, Ogoja, Bansara and Ejekwe (Fig. 1). The study area is characterized by a variety of lithological units, which include many types of igneous, metamorphic and sedimentary rocks. The main factors responsible for the sedimentation within the study area during the Cretaceous are the progressive sea level rise from Albian-Maastrichtian leading to worldwide transgression, regression and local tectonics [15]. Spectral analysis has proved to be a powerful and convenient tool in the processing and interpretation of potential field geophysical data. It seeks to describe the frequency content of a signal based on a finite set of data. Its advantage is that the spectral domain expressions of the anomalies are generally vastly simple as compared to the expressions of the anomalies in the space domain. Furthermore, the noise associated with potential field data generally has high frequency and by restricting the interpretation to low frequencies, considerable improvement in the interpretation is possible [5], [12].

An important property of spectral analysis is that features with given direction in spectral domain are transformed into a feature with only one direction in the spectral domain. Variation in magnetic susceptibility combined with other geophysical data and known geology provide important information about the regional geology especially where rock outcrops are scarce or absent and also helps to develop priorities for follow-up in the most prospective areas. The present study therefore is a spectral analysis of aeromagnetic anomalies of some parts of Lower Benue Trough with the aim of ascertaining the variability of basement depths under the sedimentary cover and to appraise the hydrocarbon accumulation potential.

GEOLOGY OF THE STUDY AREA The Benue Trough is a sedimentary basin located in Nigeria, extending from the Gulf of Guinea in the South to the Chad Basin in the North. It is believed to have originated from a 'pull-apart' basin associated with the opening of the Atlantic Ocean which ended in Early Tertiary with the development of the Tertiary Niger Delta [16], [3]. The Benue Trough is characterized by extensive magmatic activities as evidenced by the widespread occurrence of intrusive and extrusive rocks [10]. These rocks are the result of the tectonic activities within the trough. The Benue Trough originated from Early Cretaceous rifting of the central West African basement uplift. It forms a regional structure which is exposed from the northern frame of the Niger Delta and runs north-eastwards for about l000km to underneath Lake Chad, where it terminates. Regionally, the Benue Trough is part of an Early Cretaceous rift complex known as the West and Central African Rift System. The Trough is subdivided into Lower, Middle and Upper Benue Troughs. The Lower Benue Trough is underlain by a thick sedimentary sequence deposited during the Cretaceous and made up of Albianshales, subordinate siltstones of the Asu River Group and the presence of volcanic

_______________________________

David Igwesi is a Lecturer at the Department of Physics and Industrial Physics, Nnamdi Azikiwe University. He holds a Master of Science (M.Sc.) in Applied Geophysics from the Same University. He is a member of Nigerian Institute of Physics.

Dr Marius Umego is a Professor in the Department

Physics and Industrial Physics, Nnamdi Azikiwe University. He holds Ph.D. degree in Applied Geophysics from Ahmadu Bello University, Zaria. He is a Fellow of Nigerian Institute of Physics. His research interests are in the areas of Magnetic and Gravity methods of Geophysical Exploration.



Fig. 1 Map of Nigeria showing the study area.



and pyroclastic materials forming elongated conical hills in the cores of the anticlinal structures. The AlbianAbakalikishales (Formation) are an inlier in the medial portion of the Abakalikisub-basin. It is continuous for about 180km stretching from Oju, north of the Workum hills in the northern part through Abakaliki to Lokpaukwu in the southern limit [19]. It was the Santonian deformation that fragmented the Lower Benue Trough into the Abakaliki anticlinorium and the flanking Anambra and Afikpo synclines [3]. The northern limit of the Lower Benue Trough

corresponds to the Gboko transform fault that was recognized by Whiteman [20] while the eastern limit covers

the Lokpanta areas.

DATA SOURCE AND ANALYSIS This work which covers four aeromagnetic map sheet numbers 289, 290,303 and 304 namedEjekwe, Ogoja, Abakaliki and Bansara respectively are located in the Lower Benue

Fig. 2 Total Magnetic Field Intensity of the Study area contoured at interval of 20nT after IGRF has been subtracted.

Abakaliki .

Ogoja .

Bansara

Ejekwe

.

.



Trough. These maps are the product of Nigerian Geological Survey Agency, which undertook aeromagnetic survey and digitizing of aeromagnetic data in some parts of Nigeria between 2005 and 2009. The data were collected at a nominal flight altitude of 80 meters along N-S flight lines spaced approximately 1000 meters apart. The maps are on a scale of 1: 100,000 and half – degree sheets each contoured mostly at 20nT intervals covering a total of 121000km

2 (Fig. 2). Diurnal variation effects on the

magnetic field, which arise due to solar activities, wererecorded using additional unit of base station

magnetometer (the ScintrexCS3CesiumVapour). Also, International Geomagnetic Reference Field (IGRF) was subtracted from the total magnetic measurements to get rid of the regional gradient of the earth’s magnetic field due to the continual changes in the magnitude and direction of the earth’s magnetic field from one place to another [2]. The study area was subdivided into sixteen cells for the purpose of spectral depth determination. The sixteen cells represent a square grid of 19 by 19 residual field points, which corresponds to ¼

0 by ¼

0. Each of these sections covers a

total area of 22.5km by 22.5km.

Fig.3 Residual Map of the study area contoured at 15nT intervals

Abakaliki .

Ogoja .

Ejekwe

Bansara

.

.



The analysis of the data was carried out at the Nigerian Geological Survey Agency (NGSA), Abuja. The digitized data was delivered in Oasis MontajGeosoft raster file format; therefore, the Oasis Montaj programme obtained from the NGSA was utilized to perform the analysis. The regional field was computed as a two dimensional first-degree polynomial surface using a computer programme known as POLYFIT. The residual values were obtained by subtracting values of the regional field from the total magnetic (Fig. 2) value at the grid points and the values were contoured as the residual map (Fig. 3). Spector and Grant [17] stated that the depth factor invariably dominates the shape of the radially averaged power spectrum of magnetic data. Therefore, if the energy spectrum of the Residual total magnetic field intensity anomaly over a rectangular/square block is considered, the expression for the energy spectrum transcribed in polar coordinates, according to Spector and Grant, [17] is given by:

)()(),()1(4),(2222222 kT

trhr RRrSeeKrE

….. (1)

For the power spectrum

),()1(4),( 22222 rSeeKrE trhr

…………………. (2)

And taking the average of equation 2 with respect to θ gives

)()1(4)( 22222 rSeeKrE trhr

...................................... (3)

o

drSrS ),(1

)(2 .. (4)

The ensemble average depth h is observed only in the factor e

-2hr

Thus,

hr

hree

hrhr

4

)2sinh(22

……….... (5)

The residual total magnetic field intensity values were used to obtain the two dimensional Fourier Transform from which the spectrum were extracted. The average spectrum of the partial waves falling within the frequency range is calculated and the resulting values together constitute the radial spectrum of the anomalous field [5], [8], [18]. Finally the logarithm of the energy spectrum versus frequency on a linear scale was plotted and the linear segments identified on each plots. Each linear segment has a group of points due to anomalies caused by bodies occurring within a particular depth. If Z is the mean depth of the layer, the depth factor for this ensemble of anomalies is exp (-2Z). Thus the logarithmic plot of the radial spectrum would give a straight line whose slope is 2Z. The mean depth of burial of the ensemble is thus obtained from:

2

MZ …………….. (6)

where M is the slope of the graph. The results of the plot of logarithm of the energy spectrum versus frequency are shown on Fig. 4. Two lines drawn on the graph represent the two linear segments. By evaluating the slopes of the graphs, the average depths to magnetic sources for each of the sixteen cells were calculated using equation (6) (Table 1). The values of Z1 (Table 1) represent the deep magnetic sources while the values Z2 represent magnetic sources within the sediments. The value Z1 is assumed to represent the basement in their various locations. Fig. 5 is the contour map of the basement surface, and fig 6 is the 3-dimensional surface plot for the basement in the study area. Similarly Fig. 7 and Fig. 8 represent the contour map of the shallow source depths and their 3-dimensional surface plot respectively.

0

5

10

15

20

25

0 1 2 3

Ln E

Frequency (Rad/km)

Graph of Ln E vs Freq. for Cell 2

0

5

10

15

20

25

0 1 2 3

Ln E

Frequency (Rad/km)

Graph of LnE vs Freq. for Cell 3



0

5

10

15

20

0 1 2 3

Ln E

Frequency (Rad/km)


0

5

10

15

20

0 1 2 3

Ln E

Frequency (Rad/km)


0

5

10

15

20

0 1 2 3

Ln E

Frequency (Rad/km)


0

5

10

15

20

25

0 1 2 3

Ln E

Frequency (Rad/km)


0

5

10

15

20

0 1 2 3

Ln E

Frequecy (Rad/km)


0

5

10

15

20

25

0 1 2 3Ln

EFrequency (Rad/km)


0

5

10

15

20

25

0 1 2 3

Ln E

Frequency (Rad/km)


0

5

10

15

20

0 1 2 3

Ln E

Frequency (Rad/km)

Graph of Ln E vs freq for Cell 11



Fig. 4 Plots of the logarithm of energy (E) versus Frequency for each of the 16 cells.

16

17

18

19

20

0 1 2 3

Ln E

Frequency (Rad/km)

Graph of Ln E vs freq for Cell 12

0

5

10

15

20

25

0 1 2 3

Ln E

Frequency (Rad/km)


0

5

10

15

20

25

0 1 2 3

Ln E

Frequency (Rad/km)


0

5

10

15

20

25

30

0 1 2 3

Ln E

Frequency (Rad/km)


0

5

10

15

20

25

0 1 2 3

Ln E

Frequency (Rad/km)




Table 1 Spectral Depths for the Sixteen (16) blocks of 22½ x 22½ grid size.

Spectral Cell

Centre Value Deep source depth Z1 = -M1/2 (km)

Shallow sources depth Z2 = -M2/2 (km)

X

Y

1 8.13 6.13 2.989262 0.158071

2 8.13 6.38 5.731544 0.223083

3 8.13 6.63 6.125839 0.098003

4 8.13 6.88 1.225503 0.200391

5 8.38 6.13 4.711409 0.275307

6 8.38 6.38 2.241611 0.203992

7 8.38 6.63 3.870470 0.258233

8 8.38 6.88 1.157718 0.188887

9 8.63 6.13 2.597987 0.226805

10 8.63 6.38 3.801678 0.055683

11 8.63 6.63 1.211218 0.342561

12 8.63 6.88 1.974832 0.078375

13 8.88 6.13 2.017450 0.294343

14 8.88 6.38 1.855705 0.366064

15 8.88 6.63 2.381544 0.201123

16 8.88 6.88 4.516779 0.293975

Two profiles AA’ and BB’ of lengths 110km and 106km long respectively were taken as indicated on Fig. 5. The plots (Fig. 9) indicate that the basement surface in the study is undulating. These undulations may affect the structure of the overlying sedimentary sequences and thus act as possible traps for hydrocarbon accumulation in the study area.

RESULTS The result of the spectral analysis of aeromagnetic data over the basin indicates the existence of two main magnetic source depths as summarized in Table1. The deeper magnetic sources represented by the low frequency components of the spectrum are considered to reflect magnetic bodies on the basement surface. These deeper magnetic sources lie at depths that vary between 1.16km and 6.13km with an average of 3.03km. On the other hand, the shallow magnetic horizon which lie at depths that vary between 0.06 km and 0.37 km with an average depth of 0.22 km represented by the high frequency components can be attributed to the effect of magnetic bodies intruded into the sediments. A generalized depth to magnetic basement map (Fig. 5) produced from the results of the spectral analysis over the study area reveals a basement depth with average depth of 3.03 km.



Fig. 5 Contour map of the magnetic basement depth (contour Interval = 0.2).

Fig.6 3-dimensional surface plot of the magnetic basement

B

B’

A A’



Fig.7 Contour map of the Shallow magnetic source depths (contour interval =0.2).

Fig. 8 3-dimensional surface plot of the shallow magnetic sources depths.



Fig. 9 Profile AA’ taken from the deep magnetic source depth.

The surface plot (Fig. 7) shows that the topography of the basement surface indicates a general trend from shallow in the northeast and southwest sections to deep in the northwest section of the area. The magnetic basement map portrays a series of ridges and troughs over the area with some depressions here and there (Fig. 9 and Fig. 10). Three zones of very thick sedimentary cover in the study area as indicated in Table 1 include the following:

i. Average depths of 5.73km and 6.13km which are the very thick sedimentary cover in the area were obtained at an area northwest of Abakaliki (cells 2 & 3). This area is thought to represents a sub-basin known as Abakaliki sub-basin within the Lower Benue Trough, which extended outside of the study area and towards the Anambra Basin.

ii. The second very thick sedimentary cover is in an area south of Abakaliki (cell 5) which has a thickness of 4.71km. It is assumed to be a south-easterly extension of the Abakaliki sub-basin.

iii. An average depth of 4.52km was obtained in an area northeast of Ogoja (cell 16). This may likely to be northeast extension of the Abakaliki sub-basin.

-6

-5

-4

-3

-2

-1

0

0 20 40 60 80 100 120D

ep

th (

km)

Distance (km)

Profile AA'



Fig. 10 Profile BB’ taken from the deep magnetic sourcedepth.

DISCUSSION Previous magnetic interpretation carried out in the study area by Ofoegbu and Onuoha [11] showed that the basement depth vary between 1.2 km and 2.5 km. Also, Onwuemesi[13] obtained the sedimentary thickness variations of the Ananmbra basin to be between 0.9km and 5.6km. Similarly, Madufor[7] indicated that the thickness of the Abakaliki shale ranges from 1.95km to 5.09km. According to Kogbe [6], the average thickness of sediments of Albian to Coniacian in the Lower Benue Trough is about 3.3km. On the other hand, Etuk, et al [4] pointed out that the shales and other grained sediments of the Lokpaukwu-Uturu-Ishiagu areas of Lower Benue Trough had attained more than 6km in thickness. These results are in close agreement with the present study with an average depth of 3.03km for the magnetic basement in the area. The profile AA’ and BB’reveal that the basement surface presents a successive pattern of crests and troughs. The crests or antiforms strike generally in the NE-SWdirections. This strong E-W trend of all closures in the map (Fig. 3) which indicate fracture systems as suggested by Wright [21] and Ball [1] represent important lines of weakness in the continental crust and could be loci of economic mineralization. This goes to show that the depositions in the Lower Benue Trough were controlled by movement of tectonic phases which resulted from the separation of the African Continent from South America [9].

CONCLUSION From the spectral analysis result obtained, the study area holds enough thickness for the accumulation of hydrocarbon. The magnetic basementsurface plot of the study area reveals that the basement surface presents a

successive pattern of crests and troughs. It also indicates that the shape of the basement is irregular and associated with deep faults resulting from fractures. These faults in the Lower Benue Trough share the same history with the formation of the Benue Trough, and are probably related to the initial formation and evolution of the Benue Trough during continental drift.

ACKNOWLEDGEMENT The authors express gratitude to the Nigerian Geological Survey Agency (NGSA) for making available the digital aeromagnetic data for this work.

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-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0 20 40 60 80 100 120

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pth

(km

)Distance (km)

Profile BB'



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