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University of Nigeria Virtual Library

Serial No

Author 1

ONUOHA, K. M.

Author 2 OFOEGBU, Charics O.

Author 3

M. Ahmed Nur.

Title Spectral Analysis of Aeromagnetic

Data Over the Middle Benue Trough, Nigeria

Keywords

Description Spectral Analysis of Aeromagnetic

Data Over the Middle Benue Trough, Nigeria

Category

Physical Science

Publisher Nigerian Mining and Geosciences Society (NMGS)

Publication Date

1994

Signature

Journal of Mining and Geology Vol. 30 1994 No.2, pp. 211-217@ Nigerian Mining and Geosciences Society (NMGS) - Printed in Nigeria

SPECTRAL ANALYSIS OF AEROMAGNETIC DATA OVER THE MIDDLE BENUE TROUGH, NIGERIA

1116--2775

*M. Ahmed Nur, *K. Mosto Onuoha and **Charics O. Ofoegbu*Dep311ment of Geology, University of Nigeria, Nsukka

**Depat1ment of Physics, Rivers State University of Science & .:rechnologyP0l1-Ha rcoUl1

ABSTRACT

Two-dimensional spectra! analyses of aeromagnetic data has been cUlTied out to detennine the average depths of magnetic sources in theMiddle Benue valley of Nigeria. The thickness of the sedimentary sequences overlying the crystalline basement in th.is part of the trough has thusbeen detennined in this way. From the point of view of petrolem exploration, the results are important since they indicate that a reasonablethickness of Cretaceous sediments exist in this are.

TIle analysis indicates a two-depth sourcemode1with the depth to the deeper sources (identified with the basement) varying between 1600and 5000 m, The shallower depth source model probably indicates the presence of intrusions of igneous rocks, and their average depths lie between60 and 1200 m. the results obtained compare favourably with those from gravity studies, and confu1l1 the. existence of a deep sub-basin in theLafia - Keana - Awe area.

INTRODUCTION

TIle Benue Trough of Nigeria is a major tectonic feature

in West Africa, This Cretaceous structure, elongated roughlyNE-SW, extends from the Niger Delta to the Lake Chad

Basin. The origin and evolution of the Benue Trouh is now

fairly well documented (Wright, 1968; Burke et aI., 1971;Nwachukwu, 1972; Olade, 1975; Bcnkhelil, 1982;' Ofoegbu,

1985; Benkhelil, 1988; Ojoh, 1992). TIle present paper isconcerned with the Middle Benue Trough, an area well notedfor the OCCUlTence of several minet als of economic

significance, notably coal, batytes, lead and zinc, andnumerous saline to name, but a few. More recently the Federal

Govemment of Nigeria gr311ted prospecting licenses to

several oil companies to search for oi~and gas in concessions

located onshore, notably in the Lower and Middle Benue

Trough,

TIle petroleum potentials of the Benue Trough remain a

topical issue, but serious exploration activities (e.g. seismic

data acquisition) have confined to date mainly to the Lower

Benue Trough. There are indications, however, that detailed

exploration work in the Middle Benue could start soon. Thedetennination of sediment thicknesses above basement, and

the delineation of major faults and stru9tures, represent an

indispensable initial step in the assessment of the petroleum

potentials an area. The present paper presents the results of

;;;"

the analysis of aeromagnetic data over area about 16,400 sq.km in the Middle Benue Trough. The variation of the

bascment depths has ben detcl1nined from a two- dimensional

spectral analysis of aeromagnetic data previously acquires bythe Geological Survcys of Nigeria. An analysis similar to the

one given here has earlier been presented fOf the AbakaIiki

Anticlinorium of thc lower Benue Trough by Ofocgbu an~Onuoha (1991).

GEOLOGY OF THE AREA

TIle geology of the Middle Benue Trough has beendescribed in some details by Offodile (1976). The oldest'sediments belong to the Asu River Group (see Fig. 1) andconsist of shales and siltstones of marine origin, represenringthe first Middle Albian transgression into the Benue v..uey.Rock units belonging to this fOt111ationoutcrop along tbe axisof the Ke311aAnticline to the east of the town of Keana

(Offodile, 1~76). The Asu River FOl111ationis overlain bytransitional beds of the Awe FOl1118tion,which oonsi~ offlaggy, whitish, medium to coarse-grained sandstonesinterbedded with carbonaceous shales or clays from whichbrine springs issue continuously. The Awe Formation marksthe beginning of the regressive phase of the Albian sea 81disoverlain by continental fluviatile sands d the KeanaFOl111ation(Late Cenomanian-Early Turonian).

211

212.

SOON

R EC E NT

TE R TlARYMACHST RICHTIANSENONI ANLATE ALBIANAL B I ANPA LEOZOIC

8°30'ELEGEND

r rr AlluviumvVvVOlcanics l Basalts. Trachytes and Rhyolites);. 0 Lafia Fo,mationlShal es. Sandstones )

Awgu FormationlShales Sandstones S Limesto n es).::.'1Awe SKeana Formation lSandstones)

Asu River Formation -Complex lShales)

lJ Undifferentiated Ba sementFormation BoundaryAnticlineSyncline

god go30E

Figure 1: Geological map of the Middle Benue Trough (After Geological Survey Department of Nigeria, 1984)

The Ezeaku, Agwu and Lafia Fonuations are also present SPECTRAL ANALYSIS OF THE AEROMAGNETICand represent the Turonian to Early Maastrichtian sediments DATAin the Middle Benue Trough. The Ezeaku Formation consists The application of spectral analysis to the interpretation ofessentially of calcareous shales, micaceous fine to medium - aeromagnetic anomalies allows an estimates of the depth ofgrained friable sandstones, andoccasional beds oflimestones. an ensemble of magnetic blocks of varying depth, width,The Coniacian Agwu Fonuation consists mainly of black thickness and magnetization. Most of the approaches usedshales, sandstones and local seanlS of coal. TIle Lafia involved Fourier transfonuation of the digitizedFormation is the youngest fomlation reported in the Middle aeromagnetic data to compute the energy (or amplitude)Benue. It consists of coarse-grained ferruginous sandstones, spectrum. The method is now sufficiently very well

red loose sand, flaggy mu~tones and clays (Offodile, 1976). developed (Bhattacharyya, 1966; Spector and Grant, 1970;The age of the formation isMaastrichtian. Mishra and Naidu, 1974, Hahn et al,., 1976).

Figure 1 also indicates, that the basement complex In the present paper, we adopted the approach of Ofoegbuoutcrops on the surface and cover the northern fringes of the and Onuoha (1991) to analyze the data over the Middlestudy area. The rocks here are mainly granulitic gneisses, Benue. Given a residual magnetic anomaly map ofmigmatites, Older Granites, Younger Granites, porphyries dimensions L x L, digitized at equal intervals, the residualand rhyolites. total intensity anomaly values can be expressed in tenns cAa

double Fourier series expansion:

JOURNAL OF MINING AND GEOLOGY VOL. 3 199.4 NO.2 213

T(x,y) = :E:EP:1cas [(2Jt/L)(nx + my)) + Q ~ sin [(2Jt/L)(nx + my)] (1)

where L =dimension of the block, P:1 and Q:1 are Fourier amplitudes, and N,M are the number of grid points along the x

andy directions,respectively.The sumP:1 cas [(2Jt/L}(nx+ my)]+ Q~ sin [(2Jt/LHnx+ my)]

representsa singlepartialwavefor which:(P rJ2 + (Q :1)2 = (C:h)2 (2)

C :1 is the anlplitude of the partial wave~the frequency of this wave is given by:

f ~ = (n2 + 012)112

~~:~J

II"N

.a8\r--

..... ".:so'

Figure 2: Aeromagnetic Field over Middle Scnue Trough as contoured map (contour Interval =20 nT)

TIle original aeromagnetic data provided for this workwere acquired along a series of E -W profiles with a spacingof 2 km, an average flight elevation of 152 m above theground surface, and a nominal tie-line spacing of 20 km. Thegeomagneti<.:gradient was removed from the data using theIntemational Geomagnetic Reference Field (I G R F)fommla. TIle following sheets on a scale of 1:100,00 wereused: 210, 211, 311, 312, 212, 313, 251,252, and 253. Thesecover thearea within latitudes 7°52'N -900'N and longitudes

8°30'E - 9°38'E. Digitization was at 2 km intervals and the

resultant magnetic fields was then contoured at intervals of20 nT (seeFig. 2). prior to the contouring, a plane surface wasfitted to the data by multi-regression least squares analysis.lllis was done in order to remove the regional field. Theexpression obtained for the regional fields-T(R) was:

T(R) =7898.675 + 0.008x - 0.0427y

wherex andy arewlitsof spacing.

(4)

/

~-----

214

c:;j~

...

I.;

..,

Figure J: Residual Field over Middle Benue Trough as contoured m~p (contour Interval =20 nT)

The regional field values were subtracted from theobserved data to obtain the residual anomaly values. Theresidual field over the study area is shown on Figure 3.

In order to carry out the spectral analysis, the study areawas divided into blocks containing 16 x 16 data points. Indoing this. we have ensured that essential parts of eachanomaly were not cut by the blocks. The analysis wsimplemented using a FORTRAN program DEPT3 which is amodified version of the program SPECTRDEP used earlierby Ofoegbu and Onuoha (1991). As is very well known,several practical problems arise when wc apply the DiscreteFourier transformation (DET) to real data during spectralanalysis. Some of such problems include aliasing, Gibb'sphenomenon and those associated with the odd and evensymmetries of the real and inlaginary pa/1s of the Fouriertransformation. In the present paper we took care of using the

approach of Ofoegbu and Onuoha (1991). Further discussion

of these problems and their amelioration can be found in Nur(1991).

TIle spectra obtained from the analysis of the sixteenblocks which make up the study area are shown in Fig. 4 and5 while Table 1 snmlarizes the depth estimates obtained forthe blocks.

DISCUSSION

TIle results suggest the existence of two main sourcedepths for blocks 5 to 16 which covers over 75% of the areastudies (see table 1). A single depth source model aCCQWltsfor the magnetic anomalies in the northermost portion of thearea. TIlis is not surprising since the basement complexoutcrops near the surface in parts of this area, and the regolithin most places is quite thin here.

JOURNAL OF MINING ANO GEOLOGY VOL. 3 1994 NO.2 215

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Figllre 4: Spectra obtained for blocks 1 -8/

Table 1: Summary of the depths for different 16 x 16 blocksobtained from Figures 4 and 5. All depths D 1 andD2 are in !em.

For the greater part of the area studied. the deeper source(of the two depth-source model) lies at a depth that variesbetween 2500 and 3250 m. Sediment thicknessesin excess of

4900 m were obtained in a particular block to the soud1of thearea. The thickest sedimentary sequences were found tooverly the following areas: Keana-Awe (block 14 and 15),Lafia-Obi area (block 9) and NE of Lafia-Obi area (block 6).Inthe Keana-Awe area, the depth to basement varies between3000 and 4900 m. This confirms the results of gravity studies(e.g. Ajayi and Akakaiye, 1986) which earlier indicated the

existence of a deeper sub-basin in this area.

In the Lafia-Obi area the thickness of the sedimentarycover averages about 3200m. It thus appears, that for the areaunder consideration, there exist two sub-basinal structures,

that are separated by a basement ridge. This confirms the. 'horst and graben" structure of the floor of the Benue Troughas obtained from previous gravity and aeromagnetic studies(e.g. Adighije. 1981; Cratchley and Jones, 1965; Ofoegbu,1984). .

CONCLUSIONS

Basement depths have been determined for the MiddleBenue Trough from a spectral analysis of the aeromagneticdata over the area. Sediment thicknesses vary across the area,with the thickness cover found in the Keana-Awe regionwhere the basement lies at depth of over 4900 m. The-resultshave confirmed the irregular nature of the floor of Benuevalley, with sub-basinal structures being separated by harst.like features or basement ridges. From the point of view ofsediment thicknesses alone, the sub-basins south and SE ofLafia should provide good prospects for hydrocarbonexploratjoo.

Block 1 Block 2 Block 3 Block 4

Dl =0.242 Dl =0.731 J)l =0.447 Dl =0.066

Block 5 Block 6 Block 7 Block 8

Dl =2.631 01 =3.283 01 =2.771 01 =2.514

02 =0.800 02 =1.066 02 =1.212 02 =0.737

Block 9 Block 10 Blockll Bklock 12

Dl =3250 Dl =2.132 Dl =1.593 Dl =2.086

02 =0.800 02 =0.873 02 =0.658 02 =0.737

Block 13 Block 14 Block 15 Block 16

Dl =2.696 Dl =3.252 Dl =4.938 01 =2.645

02 =0.554 02 =0.546 02 =0.642 02= O.814

216

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Spectra obtained for blocks 9-16

REFERENCES

~ISHRA, D.C., and NAIDU, P.S. (1974): Two- dimensional

power spectrum and anlysis of aeromagnetic fields.Geophysis - Prospect., vol. 22, 345 - 353.

NUR, M.A. (1991): Spectral analysis of aeromagnetic dataover the Middle Benue trough, Nigeria. Univ. ofNigeria, Nsukka, unpublished M.Sc. Thesis.

NWACHUKWU, S.O. (1972): The tectonic evolution of the

southem p0l1ion of the Benue Trough, Nigeria.Geol. Mag., vol. 109,411 - 419.

OFFODILE, M.E. (1976) TIleGeology of the Middle Benue,Nigeria. Pub!. paleonto!. Inst. University ofUppsala, Sp. vol. No.4. 166pp.

OFOEGBU, C.O. (1984): Interpretation of aeromagneticanomalies over the Lower and Middle BenueTrough of Nigeria. Geophys. J. R. Astr. Soc., Vol.

79,813 - 823. .OFOEGBU, C.O. (1985): A review of the geology of the

Benue Trough of Nigeria. J. Afr. EaI1hSci., vol. 3,283 - 291.

OFOEOBU, C.O. aIldONUOHA, K.M. (1991): Analysis ofmagnetic data over the Abakaliki Anticlinorium ofthe Lower Benue Trough, Nigeria. Marine & Petrol.Geol., vol. 8. 174- 183.

OJOH, K.A. (1992): The southem pm of the Benue Trough(Nigeria) Cretaceous stratigraphy, basin analysis,paleoceaIlography aIldgeodynamic evolution in theequatoria domain of the South Atlantic. Nig. Assoc.

ADIGHUE, C. (1981): A gravity interpretation of the BenueTrough Nigeria. Tectonophysics, vol. 79,126 -136.

AJAYI, C.O. and AJAKAIYE, D.E. (1986): Structurededuced from gravity data in the Middle Benue,Nigeria.J. Afric. Earth Scielice, vol. 5, 359 - 369.

BENKHELIL, J. (1982): Benue trough aIld Benue Chain.Geol. Mag. vol. 119, 155 - 168.

BENKHELIL, J. (1988): Structure et evolutiongeodynan1ique de Basin intracontinentale de laBenoue (Nigeria). Bull. Centres Explor. Prod.Elf-Aquitaine, vol. 1207,29 - 128.

.I BHAIT ACHARYYA, B.K. (1966): Continous spectrum ofthe total magnetic field anomaly due to a rectangularprismatic body. Geophysics, vol. 31,97 - 121.

BURKE, K; DESSAUVAGIE, T.F. J., and WHITEMAN, A.

J. (1971): Opening of the Gulf of Guinea aIldgeological hiStory of the Benue depressionaIldNiger Delta. Nature Phys. Sci., vol. 233,-sr::5S.

CRATCHLEY, C.R. and JONES, G.P. (1966): An

interpretation of geology and gravity anomalies ofthe Benue Valley, Nigeria. overseas Geol. Surv.

. London, Geophys. Paper, No.1.I HAHN, A., KIND. E.G., aIldMISHRA, D.C. (1976): Depth

estimation of magnetic sources by meaI1Sof Fourieramplitude spectra. Geophys. Prospect., vol. 24, 287- 308.

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Petrol. Expl. Bull. vol. 7, 131 - 152.OLADE, M.A. (1975): Evolution of Nigeria's Benue Trough

(Aulacogen) a tectonic model. Geol. Mag., vol. 112,/ 575: 583.

""SPECTOR, A., and GRANT, F.S. (1970): Statistical models

for interpreting aeromagnetic data. Geophysics, vol.

217

35, 293 - 302.WRIGHT, J.B. (1968): South Atlantic continental drift and

the Benue Trough. Tectonophysics. vol. 6. 301 -310.

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