Spectral ReEvaluation of the Magnetic Basement Depth over Yola arm of Upper Benue Trough Nigeria Using Aeromagnetic data


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1 Stadard Scietific Research ad Essays Vol2 (9): , Septeber 2014 (ISSN: ) Research Article Spectral ReEvaluatio of the Magetic Baseet Depth over Yola ar of Upper Beue Trough Nigeria Usig Aeroagetic data *1 Eberga TT ad 2 Chigbu Tiothy 1 Departet of Physics/Electroics, Federal Polytechic, Nekede Owerri Io State, Nigeria 2 Departet of Physics/Electroics, Abia state Polytechic, Aba Abia state *Correspodig Author Eail: Accepted 03 Septeber Abstract The aeroagetic data have bee used to reevaluate parts of the Upper Beue Trough Nigeria usig spectral aalysis techique i order to appraise the ieral accuulatio potetial of the area. The regioal field was separated with a first order polyoial usig prolific progra. The residual data was subdivided ito 24 spectral blocks usig OASIS MONTAJ software progra. Two proiet agetic depth source layers were idetified. The deeper source depth values obtaied rages fro 1.56k to 2.92k with a average depth of 2.37k as the agetic baseet depth while for the shallower sources, the depth values rages fro 1.17k to 0.98k with a average depth of 0.55k. The shallow depth source is attributed to the volcaic rocks that itruded the sedietary foratio ad this could possibly be resposible for the ieralizatio foud i parts of the study area. Keywords: Spectral aalysis, Upper Beue Trough, OASIS, MONTAJ, Magetic baseet depth, shallow depth source INTRODUCTION This data was used to reevaluate the baseet depth of soe parts the U p p e r Beue trough usig the spectral aalysis techique. Aog the ajor previous studies usig old aeroagetic data i Nigeria iclude Ofoegbu (1984, 1985, 1986), Ahed (1991), Ofoegbu ad Ouoha (1991), Owueesi (1996) ad so o. Nwachukwu(1985) usig geocheical techique cosidered the Middle Beue Trough to have high prospects for hydrocarbo withi the trough. Mappig of agetic baseet depth beeath sedietary cover is oe of the key fuctios of aeroagetic survey ad its iterpretatios. This study therefore ais to appraise the ieral potetial of the study area by accoplishig the followig objectives: deteriatio of sedietary thickess withi the study area, ad agetic baseet topography through surface plots. The Beue trough is part of the log stretch ar of the Cetral Africa rift syste origiatig fro the early Cretaceous riftig of the cetral West Africa baseet uplift (Sauel et al.,2011). The trough has bee categorized ito three differet zoes ad they are: the Lower Beue Trough at the souther part, the Middle Beue Trough at the ceter while the Upper Beue Trough at the Norther part. The geology of the Middle Beue Trough is characterized by the presece of thick sedietary cover of varied copositio whose age rages fro Albia to Maastrichtia (Obaje, 2004). The AsuRiver Group of arie origi is the oldest deposited sediet i this area followed by Ezeaku Foratio, keaa/awe Foratio, Awgu Foratio ad Lafia Sadstoe which is the yougest sediet (Obaje, 2004). More o
2 Stad. Sci. Res. Essays Eberga ad Tiothy 365 this geology could be foud i the work of Cratchley ad Joes (1965), Burke et al. (1970); Offodile (1976), Osazuwa et al. (1981) ad Offoegbu (1985). LOCATION AND GEOLOGY OF THE STUDY AREA The study area is located withi the Upper Beue Trough. Beue Trough is defied as a itercotietal Cretaceous basi about 1000k i legth stretchig i a NESW directio ad restig ucoforably upo the Pre Cabria Baseet. I the Yola, the oldest sediets belog to the Bia sadstoe ad Yolde Foratio which outcrop i the ajor part of the study area Eze Aku Shale Group ad Yola Foratio are also preset i the study area. The Bia Sadstoe ad Yolde Foratio are variable sequece of sadstoes ad shale which ark the trasitio fro Cotietal to Marie sedietatio. The Upper part of the foratio (Bia sadstoe ad Yolde) cotai blueblack shales (Carter et al., 1963). Fro the ap of the study area (Figure 1), it is observed that o the souther part of the area, the baseet coplex outcrops o the surface. The baseet rocks are aily quartz, feldspathic, biotite, horblede, geisses, quartzites, arbles ad calcsilicate rocks Figure 1. Geology ap of Upper Beue Tough
3 Eberga ad Tiothy 366 MATERIALS AND METHOD The data used for the study were copiled usig the geological ap of the study area coprisig Nua (sheet 196), Dog (sheet 195), Jada (sheet 217), Kiri (sheet 237),Moki (sheet 216) ad Tougo (sheet 236),each at a scale of 1:100,000 ad satellite iage  Ladsat 5 TM, Bads 432 with wavelegths μ (ear IR), 0.69 μ (Red) ad μ (gree). It has high spatial resolutio of 30x30 groud area.. Filterig (High pass filterig): A local cotrast ehaceet ethod for the purpose of highlightig liear features or edges faults, fractures ad joits, draiage patters, ad geological boudaries. Filterig is a way of separatig sigals of differet wavelegth to isolate ad hece ehaces ay aoalous feature with a certai wavelegth. A rule of thub is that the wavelegth of a aoaly divided by three or four is approxiately equal to the depth at which the body producig the aoaly is buried. Thus filterig ca be used to ehace aoalies produced by features i a give depth rage. Traditioal filterig ca be either low pass (Regioal) or high pass (Residual). Thus the techique is soeties referred to as RegioalResidual Separatio. Badpass filterig isolates wavelegths betwee userdefied upper ad lower cutoff liits. Figure 2. Total agetic aoaly ap of Upper Beue Trough THE THEORETICAL BACKGROUND OF SPECTRAL ANALYSIS The power spectru coputes the thickess of the sedietary basi ad that of the crustal Moho depth (Spector ad Grat, 1970). The spectral depth ethod is based o the priciple that a agetic field easured at the surface ca therefore be cosidered the itegral of agetic sigatures fro all depths. The power spectru of the surface field ca
4 Eberga ad Tiothy 367 be used to idetify average depths of source esebles (Spector ad Grat, 1970). This sae techique ca be used to attept idetificatio of the characteristic depth of the agetic baseet, o a ovig data widow basis, erely by selectig the steepest ad therefore deepest straightlie seget of the power spectru, assuig that this part of the spectru is sourced cosistetly by baseet surface agetic cotrasts. A depth solutio is calculated for the power spectru derived fro each grid subset, ad is located at the cetre of the widow. Overlappig the widows creates a regular, coprehesive set of depth estiates. This approach ca be autoated, with the liitatio however that the least squares bestfit straight lie seget is always calculated over the sae poits of the power spectru, which if perfored aually would ot ecessarily be the case. Depth results are geerated for the etire dataset usig differet wave uber rages ad widow sizes. A potetial field grid ay be cosidered to represet a series of copoets of differet wavelegth ad directio. The logarith of the power of the sigal at each wavelegth ca be plotted agaist wavelegth, regardless of directio, to produce a power spectru. The power spectru is ofte observed to be broke up ito a series of straight lie segets. Each lie seget represets the cuulative respose of a discrete eseble of sources at a give depth. The depth is directly proportioal to the slope of the lie seget. Filterig such that the power spectru is a sigle straight lie ca thus ehace the effects fro sources at ay chose depth at the expese of effects fro deeper or shallower sources. It is a dataadaptive process ivolvig spectral shapig. As such, it perfors sigificatly better tha arbitrary traditioal filterig techiques described above. The applicatio of spectral aalysis to the iterpretatio of aeroagetic aoalies is ow sufficietly well established (Bhattacharyya, 1966, 1968; Spector ad Grat, 1970; Mishra ad Naidu, 1974; Nah et al., 1976). The ethod allows a estiate of depth of agetized blocks of varyig depth, width, thickess ad agetizatio. Most approaches used ivolve Fourier trasforatio of the digitized aeroagetic data to copute the eergy (or aplitude) spectru. This is plotted o the Logarithic scale agaist frequecy. The plot shows the straight lie segets which decrease i slope with icreasig frequecy. The slopes of the segets yield estiates of depths to agetic sources. Give a residual agetic aoaly ap of diesio L L digitized at equal itervals, the residual itervals, the residual total itesity aoaly values ca be expressed i ters of double Fourier series expasio: T( x, y) N M 1 1 P 2 cos( )( x y) Q L 2 si[( )( x y)]...(1) L Where L diesio of the block, P ad Q are Fourier aplitude ad N, M are the uber of grid poits alog the x ad y directios respectively. Equatio (1) ca be cobied ito a sigle partial wave thus: p 2 cos[( )( x y)] Q L 2 si[( )( x y)] C L 2 cos[( )( x y) (2) L Where ( P 2 ) ( Q ) 2 ( C ) 2...(3) ad is the appropriate phase agle. Each ( C ) is the aplitude of the partial wave. The frequecy of this wave is give by: 2 2 F is called the frequecy of the wave. Siilarly, usig the coplex for, the two diesioal Fourier trasfor pair ay be writte (Bhattacharryya, 1966; Bath, 1974; Oppehei ad Schafer, 1975) G( U, V ) g( x, y) e j( ux vy) dxdy...(4) 1 g( x, y) 4 j( ux vy) G( U, V ) e dxdy 2...(5)
5 Eberga ad Tiothy 368 Where u ad v are the agular frequecies i the x ad y directios respectively. G (u,v) whe broke up ito its real ad iagiary parts is give by G (u,v) = P (u,v + jq(u,v)...(6) The eergy desity spectru or siply the eergy spectru is E (u, v) + G (u,v) 2 = P 2 +Q 2.(7) The Aalysis (Quatitative iterpretatio) The followig operatios were perfored o the acquired digitized aeroagetic data leadig to the quatitative deteriatio of depth to agetic sources RegioalResidual separatio The regioal field was reoved fro the total agetic itesity data (observed data) to obtai the residual data with a first order polyoial usig prolific progra. The residual data was subdivided ito 16 spectral blocks allowig spectral probe of 12.5k by 12.5k area for 15iute by 15iute widowig. Divisios ito Spectral cells ad Widowig For the purpose of easier hadlig of the large data ivolved, the four residual blocks of the study area was subdivided ito 24 spectral cells (labeled i table 1) of 12.5k by 12.5k i order to accoodate loger wavelegth so that depth up to about 12k could be ivestigated. Each sigal was the widowed 15 iutes by 15 iutes. Table 1. Twety four (24) spectral cells TOUNGO A NUMAN A MONKIN A KIRI A JADA A DONG A TOUNGO B NUMAN B MONKIN B KIRI B JADA B DONG B TOUNGO C NUMAN C MONKIN C KIRI C JADA C DONG C TOUNGO D NUMAN D MONKIN D KIRI D JADA D DONG D Geeratio of radial eergy spectru: Digital sigal processig software (OASIS MONTAJ) progra eployig the fast Fourier trasfor techique was used to trasfor the residual agetic data ito the radial eergy spectru for each block. The average radial power spectru was calculated ad displayed i a seilog figure of aplitude versus frequecy. Plots of Log of Eergy ad the frequecy: Spector ad Grat (1970) have show that the Logpower spectru of the source have a liear gradiet whose agitude is depedet upo the depth of the source. Graphs of logarith of the spectral eergy agaist frequecies for the 24 spectral cells was plotted ad show o figure 3
6 Eberga ad Tiothy 369 Power spectru plots of Dog sheet Power spectru plots of Nua sheet
7 Eberga ad Tiothy 370 Power spectru plots of Jada sheet Power spectru plots of Tougo sheet
8 Eberga ad Tiothy 371 Power spectru plots of Moki sheet Power spectru plots of Kiri sheet Figure 3. Spectral plots of log (E) agaist Frequecy (rad/sec) For each cells two liear segets could be idetified which iplies that there are two agetic source layers i the study area. Each liear seget group poits are due to aoalies caused by bodies occurrig withi a particular depth rage. The lie seget i the higher frequecy rage is fro the shallow sources ad the lower haroics are idicative of sources fro deep  seated bodies
9 Eberga ad Tiothy 372 Table 2. Depth estiates of the first ad secod agetic layers for the 24 spectral blocks ad their coordiates SPECTRAL BLOCK LONGITUDE LATITUDE DEPTH KM X 1 X 1 Y 1 Y 1 D 1 D 2 Kiri A Kiri B Kiri C Kiri D Moki A Moki B Moki C Moki D Jada A Jada B Jada C Jada D Dog A Dog B Dog C Dog D Tougo A Tougo B Tougo C Tougo D Nua A Nua B Nua C Nua D AVERAGE DEPTH The deep aoaly source depth averagig 2.373k deep could possibly represet the agetic baseet surface of the study area while the other depth averagig 0.554k could possibly represet the shallow source DISCUSSIONS Spectral aalysis of the aeroagetic data of the U p p e r Beue trough has revealed two ai agetic aoaly sources depth as show o table 2. These are the deep agetic aoaly sources ad the shallow agetic aoaly sources. The deep source aoalies vary betwee 2.373k ad 0.554k ad this represets the agetic baseet depth. The shallow aoaly sources with a average of 0.554k ad this ay be regarded as the agetic itrusios ito the sediet, probably through the agatic activities ad could be resposible for the leadzic ieralizatio foud i the area. Active agatiss have bee reported i the Beue Trough with Ofoegbu ad Odigi (1989) cofirig the close associatios betwee agatiss, ieralizatios ad fractures i the area. CONCLUSION These results obtaied fro the use of the ew high resolutio data have show soe siilarities with those fro the previous old data. However, they could be ore depedable ad exacts owig to the high resolutio ature of the 2009 data ore tha the 1970s data i ters of terrai clearace, lie spacig ad iproveet i techology sice the. Also the stressful work of digitizig a ap ad the likely error that could be itroduced durig the process have all bee eliiated because the 2009 data is i a digitized forat. The depth values estiated fro spectral aalysis over part of the U p p e r Beue is a idicatio that ieral prospectig should be itesified.
10 Eberga ad Tiothy 373 Refereces Bhattacharrya BK(1966). Cotiuous Spectru of t he total agetic field aoaly due to rectagular prisatic body, Geophysics. 31: , Cratchley CR, Joes GP(1965). A iterpretatio of Geology ad Gravity aoalies of the Beue Valley Nigeria Geophy. Pap. Overseas Geol. Surv.Lodo. No. 1. Mishra DC, Naidu PS(1974). Twodietioal Power spectral aalysis of aeroagetic fields, Geophysics Prop. 22: Ofoegbu CO(1984a). Aeroagetic aoalies over the lower ad iddle Beue Trough Nigeria; Nigeria J. Mi. Geol. 21: Ofoegbu CO(1984b). A odel for the tectoic evolutio of the Beue Trough of Nigeria. Geologische Rudschau. 73: Ofoegbu CO(1985a). A review of the geology of the Beue Trough of the Nigeria. J. Afr. Sci. 3: Ofoegbu CO(1985c). Log Wavelegth aoalies ad the crustal structure udereath the Beue Trough a surroudig regios. Nig. J. Mi. Geol.22: Ofoegbu CO(1986). A preliiary result of a pseudogravity study of the Beue Trough. Nig. J. Afr. Earth Sci. 5: Ofoegbu CO, Odigi MI(1989). Baseet structures ad ore ieralizatio i the Beue Trough, I the Beue Trough: Structure ad Evolutio (ed. Ofoegbu, C.0,) Earth Evol. Sci. Pp Ofoegbu CO, Ouoha KM(1991). Aalysis of agetic data over the Abakaliki Aticlioriu of the Lower Beue Trough, Nigeria. Marie ad Petr.Geol. 8: Owueesi AG(1996). OeDiesioal Spectral Aalysis of Aeroagetic Aoalies ad Curie Depth Isother I The Aabra Basi Of Nigeria; J. Geodyaics.23(2): Spector A, Grat FS(1970). Statistical odels for iterpretig aeroagetic Data; Geophysics. 25: