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1 Serial No University of Nigeria Virtual Library Author 1 ONUOHA, K. M. Author 2 OFOEGBU, Charics O. Author 3 Title M. Ahmed Nur. Spectral Analysis of Aeromagnetic Data Over the Middle Benue Trough, Nigeria Keywords Description Category Publisher Spectral Analysis of Aeromagnetic Data Over the Middle Benue Trough, Nigeria Physical Science Nigerian Mining and Geosciences Society (NMGS) Publication Date 1994 Signature

2 Journal of Mining and Geology Vol No.2, pp. Nigerian Mining and Geosciences Society (NMGS) - Printed in Nigeria SPECTRAL ANALYSIS OF AEROMAGNETIC DATA OVER THE MIDDLE BENUE TROUGH, NIGERIA *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 &.:rechnology P0l1-Ha rcoul1 ABSTRACT Two-dimensional spectra! analyses of aeromagnetic data has been cultied out to detennine the average depths of magnetic sources in the Middle Benue valley of Nigeria. The thickness of the sedimentary sequences overlying the crystalline basement in th.is part of the trough has thus been detennined in this way. From the point of view of petrolem exploration, the results are important since they indicate that a reasonable thickness 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 1600 and 5000 m, The shallower depth source model probably indicates the presence of intrusions of igneous rocks, and their average depths lie between 60 and 1200 m. the results obtained compare favourably with those from gravity studies, and confu1l1 the. existence of a deep sub-basin in the Lafia - Keana - Awe area. INTRODUCTION TIle Benue Trough of Nigeria is a major tectonic feature in West Africa, This Cretaceous structure, elongated roughly NE-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 is concerned with the Middle Benue Trough, an area well noted for the OCCUlTence of several minet als of economic significance, notably coal, batytes, lead and zinc, and numerous 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. The detennination 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 by the 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 been described in some details by Offodile (1976). The oldest' sediments belong to the Asu River Group (see Fig. 1) and consist of shales and siltstones of marine origin, represenring the first Middle Albian transgression into the Benue v..uey. Rock units belonging to this fot111ationoutcrop along tbe axis of the Ke311aAnticline to the east of the town of Keana (Offodile, 176). The Asu River FOl111ationis overlain by transitional beds of the Awe FOl1118tion,which oonsi of flaggy, whitish, medium to coarse-grained sandstones interbedded with carbonaceous shales or clays from which brine springs issue continuously. The Awe Formation marks the beginning of the regressive phase of the Albian sea 81dis overlain by continental fluviatile sands d the Keana FOl111ation(Late Cenomanian-Early Turonian). ;;;" 211

3 212. SOON 8 30'E god LEGEND R EC E NT r rr Alluvium TE R TlARY vvvvolcanics l Basalts. Trachytes and Rhyolites) MACHST RICHTIAN ;. 0 Lafia Fo,mationlShal es. Sandstones ) SENONI AN Awgu FormationlShales Sandstones S Limesto n es) LATE ALBIAN.::.'1Awe SKeana Formation lsandstones) AL B I AN Asu River Formation - Complex lshales) PA LEOZOIC lj Undifferentiated Ba sement Formation Boundary Anticline Syncline 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 AEROMAGNETIC and represent the Turonian to Early Maastrichtian sediments DATA in the Middle Benue Trough. The Ezeaku Formation consists The application of spectral analysis to the interpretation of essentially of calcareous shales, micaceous fine to medium - aeromagnetic anomalies allows an estimates of the depth of grained 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 used shales, sandstones and local seanls of coal. TIle Lafia involved Fourier transfonuation of the digitized Formation 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 mutones and clays (Offodile, 1976). developed (Bhattacharyya, 1966; Spector and Grant, 1970; The age of the formation is Maastrichtian. 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 Ofoegbu outcrops on the surface and cover the northern fringes of the and Onuoha (1991) to analyze the data over the Middle study area. The rocks here are mainly granulitic gneisses, Benue. Given a residual magnetic anomaly map of migmatites, Older Granites, Younger Granites, porphyries dimensions L x L, digitized at equal intervals, the residual and rhyolites. total intensity anomaly values can be expressed in tenns caa double Fourier series expansion:

4 JOURNAL OF MINING AND GEOLOGY VOL NO 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 sum P: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 wavethe frequency of this wave is given by: f = (n )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 work were acquired along a series of E -W profiles with a spacing of 2 km, an average flight elevation of 152 m above the ground surface, and a nominal tie-line spacing of 20 km. The geomagneti<.:gradient was removed from the data using the Intemational Geomagnetic Reference Field (I G R F) fommla. TIle following sheets on a scale of 1:100,00 were used: 210, 211, 311, 312, 212, 313, 251,252, and 253. These cover the area within latitudes 7 52'N -900'N and longitudes 8 30'E 'E. Digitization was at 2 km intervals and the resultant magnetic fields was then contoured at intervals of 20 nt (seefig. 2). prior to the contouring, a plane surface was fitted to the data by multi-regression least squares analysis. lllis was done in order to remove the regional field. The expression obtained for the regional fields-t(r) was: T(R) = x y wherex andy arewlits of spacing / (4)

5 c:;j I.;.., Figure J: Residual Field over Middle Benue Trough as contoured mp (contour Interval =20 nt) The regional field values were subtracted from the observed data to obtain the residual anomaly values. The residual field over the study area is shown on Figure 3. In order to carry out the spectral analysis, the study area was divided into blocks containing 16 x 16 data points. In doing this. we have ensured that essential parts of each anomaly were not cut by the blocks. The analysis ws implemented using a FORTRAN program DEPT3 which is a modified version of the program SPECTRDEP used earlier by Ofoegbu and Onuoha (1991). As is very well known, several practical problems arise when wc apply the Discrete Fourier transformation (DET) to real data during spectral analysis. Some of such problems include aliasing, Gibb's phenomenon and those associated with the odd and even symmetries of the real and inlaginary pa/1s of the Fourier transformation. In the present paper we took care of using the approach of Ofoegbu and Onuoha (1991). Further discussion of these problems (1991). and their amelioration can be found in Nur TIle spectra obtained from the analysis of the sixteen blocks which make up the study area are shown in Fig. 4 and 5 while Table 1 snmlarizes the depth estimates obtained for the blocks. DISCUSSION TIle results suggest the existence of two main source depths for blocks 5 to 16 which covers over 75% of the area studies (see table 1). A single depth source model accqwlts for the magnetic anomalies in the northermost portion of the area. TIlis is not surprising since the basement complex outcrops near the surface in parts of this area, and the regolith in most places is quite thin here.

6 JOURNAL OF MINING ANO GEOLOGY VOL NO _5' h , : 5. " " 548 ". Iffq..3 5' '128;K689 I!L.OCK 2 -; 4, " c '14 :4'" -..." 4-0 " Froq.-'4'a ' 3,96 I "BLOCK 3 BLOCK 4 t,.,.., r "" '''''. ' " - il 5.Q.. ] e 4. "!' 4, BLOCK 5 Frtquuey ; ::: Go ;; BLOCK 7 F,equlncy -:5'2B.. : i 4-9 Fiequuey -' 4'80 I BLOCK '82 Go 4'6 " "'4' " F,equlney BLOCK 8 Table / Figllre 4: Spectra obtained for blocks 1-8 1: Summary of the depths for different 16 x 16 blocks obtained from Figures 4 and 5. All depths D 1 and D2 are in!em. 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 = = = = = = = =0.737 Block 9 Block 10 Blockll Bklock 12 Dl =3250 Dl =2.132 Dl =1.593 Dl = = = = =0.737 Block 13 Block 14 Block 15 Block 16 Dl =2.696 Dl =3.252 Dl = = = = = = O.814 For the greater part of the area studied. the deeper source (of the two depth-source model) lies at a depth that varies between 2500 and 3250 m. Sediment thicknesses in excess of 4900 m were obtained in a particular block to the soud1of the area. The thickest sedimentary sequences were found to overly 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 between 3000 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 sedimentary cover averages about 3200 m. It thus appears, that for the area under 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 Trough as 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 Middle Benue Trough from a spectral analysis of the aeromagnetic data over the area. Sediment thicknesses vary across the area, with the thickness cover found in the Keana-Awe region where the basement lies at depth of over 4900 m. The-results have confirmed the irregular nature of the floor of Benue valley, with sub-basinal structures being separated by harst. like features or basement ridges. From the point of view of sediment thicknesses alone, the sub-basins south and SE of Lafia should provide good prospects for hydrocarbon exploratjoo.

7 f. f!l. j \, "', BLOCK8 h 4 ;4' I ,...""" 8LOCK 10 4'. _& -4 & i I4'..,... n '-.c,,,80. I 4.8 B"'- SLaCK II 8LOCK 12 il... J:'. j588 :'234'878. "-..,, 8LOCK'3 -t"'.. I... J... 'n '-... BLOC.. -"35 j..,... :- ll 05-8!"8 t5'" c. -;" I. 4 " c, BLOC.'4!... 'U "" c, BLOC. 16 Figllre5: Spectra obtained for blocks 9-16 REFERENCES ADIGHUE, C. (1981): A gravity interpretation of the Benue Trough Nigeria. Tectonophysics, vol. 79, AJAYI, C.O. and AJAKAIYE, D.E. (1986): Structure deduced from gravity data in the Middle Benue, Nigeria.J. Afric. Earth Scielice, vol. 5, BENKHELIL, J. (1982): Benue trough aild Benue Chain. Geol. Mag. vol. 119, BENKHELIL, J. (1988): Structure et evolution geodynan1ique de Basin intracontinentale de la Benoue (Nigeria). Bull. Centres Explor. Prod. Elf-Aquitaine, vol. 1207, I BHAIT ACHARYYA, B.K. (1966): Continous spectrum of the total magnetic field anomaly due to a rectangular prismatic body. Geophysics, vol. 31, BURKE, K; DESSAUVAGIE, T.F. J., and WHITEMAN, A. J. (1971): Opening of the Gulf of Guinea aild geological history of the Benue depressionaild Niger Delta. Nature Phys. Sci., vol. 233,-sr::5S. CRATCHLEY, C.R. and JONES, G.P. (1966): An interpretation of geology and gravity anomalies of the Benue Valley, Nigeria. overseas Geol. Surv.. London, Geophys. Paper, No.1. I HAHN, A., KIND. E.G., aild MISHRA, D.C. (1976): Depth estimation of magnetic sources by meai1sof Fourier amplitude spectra. Geophys. Prospect., vol. 24, ISHRA, D.C., and NAIDU, P.S. (1974): Two- dimensional power spectrum and anlysis of aeromagnetic fields. Geophysis - Prospect., vol. 22, NUR, M.A. (1991): Spectral analysis of aeromagnetic data over the Middle Benue trough, Nigeria. Univ. of Nigeria, 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, OFFODILE, M.E. (1976) TIle Geology of the Middle Benue, Nigeria. Pub!. paleonto!. Inst. University of Uppsala, Sp. vol. No pp. OFOEGBU, C.O. (1984): Interpretation of aeromagnetic anomalies over the Lower and Middle Benue Trough of Nigeria. Geophys. J. R. Astr. Soc., Vol. 79, OFOEGBU, C.O. (1985): A review of the geology of the Benue Trough of Nigeria. J. Afr. EaI1hSci., vol. 3, OFOEOBU, C.O. aildonuoha, K.M. (1991): Analysis of magnetic data over the Abakaliki Anticlinorium of the Lower Benue Trough, Nigeria. Marine & Petrol. Geol., vol OJOH, K.A. (1992): The southem pm of the Benue Trough (Nigeria) Cretaceous stratigraphy, basin analysis, paleoceailography aildgeodynamic evolution in the equatoria domain of the South Atlantic. Nig. Assoc.

8 JOURNAL OF MINING AND GEOLOGY VaL NO.2 Petrol. Expl. Bull. vol. 7, 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 , WRIGHT, J.B. (1968): South Atlantic continental drift and the Benue Trough. Tectonophysics. vol /