structural trends and spectral depth analysis

7/27/2019 Structural Trends and Spectral Depth Analysis

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Indian Journal of Science and Technology Vol. 4 No. 11 (Nov 2011) ISSN: 0974- 6846

Research article “Magnetic field of Naraguta” Akanbi & Mangset

Indian Society for Education and Environment (iSee) http://www.indjst.org Indian J.Sci.Techno

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Fig.1.TopographicalMapofstudyArea(modifiedfromGSNmapseries)

StructuraltrendsandspectraldepthanalysisoftheresidualmagneticfieldofNaragutaarea,Northcentral,Nigeria.

E.S.AkanbiandW.E.Mangset

DepartmentofPhysics,UniversityofJos,Nigeria

[email protected] Abstract

Theresultsofamagneticstudyof theNaragutaarea,NorthCentralNigeriaarepresentedhere.Regional-residuaseparationwascarriedoutusingLeastSquaremethod.The resultantregionalmaprevealeda regionaltrendinginaNW-SE direction.The magnetic residual values range from -487.828nT to +405.447nT. Short to moderately longdislocationswerefoundin theresidualmagneticmap.SeveralmagneticclosuresofvarioussizeswerealsonoticedWhilethe discontinuities indicate thepresenceofminor tomoderately longgeologic fault in thearea, theclosuresdepictthetypeandsizeoftheanomaliesthatliebeneaththearea.MagneticlineamentstrendinginaNE-SWdirectionhasbeenidentifiedtopassthroughthestudyarea.Thisconcentrationofmagneticlineamentsmaybeconnectedwiththeoccurrenceofyoungergranitesinthestudyareasincealmostallknownyoungergranitecomplexesliewithintheregiondominatedbythistrends. AlsoaprominentENE-WSWanomaly lowhasbeen identified intheJos-Bukurucomplex.ThetrendoftheanomalyisbelievedtoberoughlyparalleltothatofthemainstructuralfeatureoftheBenuetroughindicatingtheprobabilitythattheBenuetroughandtheyoungergraniteintrusivearerelatedconsideringthe

stress,whichinitiatedtheformationofthesefeatures.2-Dspectralanalysisofthemagneticanomaliesovertheareahasbeencarriedoutinanefforttoestimatethedepthtomagneticsources.Theresultshowsthatthedeepersourceshaveanaveragedepthof2.03Kmwhiletheshallowersourceshaveanaveragedepthof265m.Keywords:Lineaments,Trend,MagneticAnomalies,YoungerGranites.

IntroductionThestudyareaissituatedinNorthCentral

Nigeria and covers an area of about2970.25km

2anditisboundedbylatitudes9

o30

’

and 10 o

00’and longitude 8

o30

’ and 9

o00

’. It

includesJosareaandsurrounding towns suchasBukuru,Bassa,Hoss,Vom,Barakinladietc.This area covers Naraguta topographic map

(sheet 168) published by the Federal SurveyDepartment on scale 1:100 000 (Fig.1). Thestudyareahashighrelieffeaturesandelevationranges from 1800m-5300m above sea level.Thehighlandareaisaffectedbyweatheringanderosion with laterite covering most parts. Thesceneryoftheareavariesfromlevelplainandplateausurfacesalmostdevoidofexposedrocktorugged,deeplydissectedmassifsdevelopedon the more resistant rock types. Throughoutthearea, there isaclose relationshipbetweenrocktypeandscenery.TheJos–Plateauowesits preservation largely to the closeconcentration of resistant younger and older

granites,andindeedalmostalltheuplandareascoincidewithoutcropsofoneofthesetworocks(Macleodetal.,1971).Theyoungergranitesinparticularwiththeirsharpcontrasttobasementrocks,aregenerallymarkedbyanabruptbreakofslopeattheirmargins.Theoldergraniteswiththeirwidelyspacedjointingarecharacterizedbysmooth rounded inselbergs. The youngergranites generally give rise to a more ruggedtopography with steep rocky hills and joint






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controlled valleys. Apart from the granites, basementrocks generally form low-lying, poorly exposed areasexceptwheredissectedaroundtheplateaumargins.Thebasaltsproducesomeof themost prominent landscapefeatures of the Plateau, especially the laterite- cappedmesasofdecomposedolderbasaltandthenewerbasalt

volcanoes with their steep sides scored by shallowgullies.AgeneralsuccessionaccordingtoMacleod etal.(1971)isshowninTable1;alsovidegeologicalmap(Fig2).

Themostcommonmineralintheareaiscassiterite.

Other minerals in the area include columbite, wolfram,pyrochlore, fergosonite, thorite, zircon, monazite,xenotime, beryllium minerals, molybdenite, cryolite andother minor minerals such as topaz, gelana, pyrite,arsenopyrite,bismuthinite,andchalcopyrite.Inthispaperthe trends of the Residual magnetic field map areanalyzed qualitatively and depths to magnetic sourcesarecalculatedbystatisticalspectralanalysismethods.

TheoryLeastsquaremethodThe least square method (LSM) is usually used inestimating the residual component of Bougue(Abdelrahman et al., 1989), magnetic (Abdelrahman eal., 1996; Akanbi & Udensi, 2006) and self-potentia

(Abdelrahmanetal., 1997)anomalies. In this studytheLSM approach by Nettleton (1976) was applied. Themethod consists of matching the regional field by apolynomialsurfaceofloworder.Thetreatmentisbased

onstatisticaltheory.AssumingthattheequationforthesurfacethatbestfitsthedateisT(x,y)=A0=A1x+A2y(1)Where T would be computed value of theregionalforthecoordinatesx,y.A0,A1andA2areconstantswhicharetobedetermined.Theresidual(R)wouldbeR=B–T(2)With B being the observed total magnetic field

valueandTtheregionalsurfacevalue.StatisticalspectraldepthanalysisSpector (1968) and Spector & Grant (1970)

developed a 2-D spectral depth determination methodTheirmodelassumesthatanuncorrelateddistributionofmagneticsourcesexistsatanumberofdepthintervalsinthe geologic column. The evolution of spectral analysishas some important precursor, by which one tried topresent data only in a simple 2-D format. The mos

important of these precursors isthe harmonic analysis orFourieseries expansion of a given timeseries of data. According toFourier’s theorem,any function

(t) satisfying certain restrictionscan be expressed as a sum oinfinite number of sinusoidaterms.

In the general case, f(t)canstand for any function such asdisplacement, particle velocity,acceleration, temperature,rainfall, wind velocity,geomagnetic field intensity etc.The phenomenon e.g.geomagneticfieldcanalsobeafunction of f(x). To study the

characteristics of the residualfield,thedataisfirsttransformedfrom space to the frequencydomainandthentheirfrequencycharacteristicsareanalyzed.Forthe purpose of analyzingaeromagnetic maps, thesubsurfaceisassumedtoconsistof a number of independentensembles of rectangular,

Table1.Geologicsequenceofstudyarea

Geologictime Rocktype Occurrence

Tertiary-Quaternary

Newerbasalts Lava flows and volcanic cone,lava flows now largelydecomposed,overlyingalluvium

Jurassic Youngergranite Granites,porphyriesandrhyolites

Precambrian tolowerPaleozoic

Crystallinebasement

Migmatites, Gneisses & oldergranite

Fig.2.Digitizedgeologicalmapofthestudy(ModifiedfromG.S.N)






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vertical sided parallelepiped. If there are two sets ofsourcesthentheycanberecognizedbymarkedchangein spectra decay rate. The energy spectrum of thedoubleensemblewillthenconsistoftwoparts.Thefirst,whichrelatestothedeepersources,isrelativelystrongat low frequencies, and decays away rapidly. The

second, which arises from the shallower ensemble ofsources, dominates the high frequency end of thespectrum(Spector&Grant,1970).Ingeneralcase,theradial spectrum may be conveniently approximated bystraight line segments, the slopes of which relate todepthsofthepossiblemagneticlayers(Spector&Grant,1970; Hahnetal., 1976).Thepowerspectrumderivedfrom a two-dimensional dataset such as a grid ofresidual magnetic data, also has inherently a two-dimensionalform.Ifthefrequencyunitisinradiansperkilometer the mean depthof burialof the ensemble isgivenby

(3)Wheremistheslopeofthebestfittingstraightline.If,however, the frequency unit is in cyclesper kilometer,thecorrespondingrelationcanbeexpressedas

(4)The use of the Discrete Fourier Transform

introduces the problem of aliasing and the truncationeffects(Gibb’sphenomenon).In thisstudy,aliasingwasreduced by the digitizing interval used. The map wasdigitized on a 1km x 1km grid system. The spacing

imposes a Nyquist frequency of km-1

.

Thus, the narrowest magnetic anomaly thatcan be defined by the digitized data has awidth of 2km. Previous studies with crustalmagneticanomalies(Hall,1968&1974)showthat this spacing is suitable for the portrayalandinterpretationofmagneticanomalies.Thetruncationeffectariseswhenlimitedportionofanaeromagneticmap issubjected toFouriersynthesis;itisdifficulttoreconstructthesharpedgesoftheanomalywithalimitednumberoffrequencies. This truncation leads to theintroductionofspuriousoscillationaroundtheregion of discontinuity.Thismeans that falsefrequencies will be introduced into the

spectrum. The truncation effect was reducedby applying a cosine tapper to the observeddatabeforeFouriertransformation (Kangkolo,1996; Akanbi & Udensi, 2006). It has beenfound (Pal et al., 1978) that in the use ofspectral approach to magnetic source depthdeterminations, the error in depth predictionincreaseswiththedepthofsourceandisalsorelatedtothemapsize.Themapsizerequiredfor adequate results should be much larger

(about 10 times) the required target depth. The lowfrequency components in the energy spectrum aregeneratedfromthedeepestlayerswhoselocationsaremost likely in error thus, it isadvisable in the generalmethodheretoignorethefirstfewpointsintheenergyspectrum.

MaterialsandmethodThe data sets used for this research are

1. Aeromagnetic map covering Naraguta sheet 168 (1100000),2.GeologicmapcoveringNaragutasheet168(1:100000)and3.GeologicreportcoveringtheareabyMacleodetal.(1971).

The software used include: ILWIS 3.2(academic)SURFER8,REGRES.FORandSPEC.FOR.Thefollowingprocedurewasadoptedforthestudy:(i) The Naraguta (sheet 168) aeromagnetic map was

digitizedandreproducedusingILWIS3.2(Academic)(ii) Theregionalmagneticfieldwasdeterminedbyfitting

a2dimensionalfirstdegreepolynomialsurfacetothe

total data using the leastsquare method. This wasachievedusingREGRES.FORwhichisbasedontheleastsquarestechnique.

(iii) The residual magnetic field was obtained bysubtracting the regional field values from the totamagneticfieldvaluesatgridcrosspoints.

(iv) Depths to the magnetic layers of the residuamagnetic field were determined usingSPEC.FORaFORTRAN program which is based on statisticaspectralanalysis.

(v) Theresultantmapswereinterpretedqualitativelyandquantitatively.

Fig.3.DigitizedtotalaeromagneticfieldmapofNaraguta

Sheet,168.(modifiedfromG.S.N)






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Resultpresentationanddiscussion Structuraltrendanalysis

ThepicturethatemergesfromamagneticcontourmapsuchasFig.3isonethatshowsthesuperpositionof disturbances of notably different order of sizes.Larger features produce magnetic anomalies that are

smoothoverconsiderabledistancesandarecausedbythe deeper heterogeneity of the earth’s crust. Thesesmooth trends are referred to as regional trends,regionalfieldsorsimplyregionals.Smaller,morelocalsourcesaccount for sharperanomaly shapes ofmorerestrictedarealextent.Thesearesuperimposedontheregional fields but frequently camouflaged by them.Though theyaresmaller localdisturbanceswhich aresecondary in size, they are of primary importance.These are the residual anomalies, residual field orsimply residuals which may provide evidence of theexistence of mineral ore bodies or reservoir-typestructures.Forpotentialfielddata(suchasmagneticor

gravity) to be interpreted and or used for furtheranalysis, the residual anomalies must be separatedfrom the regional background field. Least squaresmethod wasusedforregional-residual separationandtheequationoftheregionalobtainedinthisworkisT(x,y)=32851.98–0.00148x-0.00013y(5)Fromthisrelationtheregionalgradientsalonganylinewere calculated.The resultantmapis shown inFig.4.The regional trends in an approximate NW-SEdirection. A program was used to derive the residual

magneticvaluesbysubtractingthevaluesoftheregionalfieldfromthetotalmagneticfieldvaluesatgridcrosspoints.Thecontourmapofthe residuavalues(Fig.5)showsthatthemagneticresiduavaluesrangefrom-487.828nTto+405.447nT

Negative residuals dominate the study areabecause the study area is close to themagneticequator.Thefeaturesonthemaparenot linear i.e. we have closures on the mapwhich indicates the anomalous conditions inthesubsurface.CircularpatternonFig.5couldbeassociatedwiththepresenceoforebodiesgranitic aswell asbasicorebodieswhile thelongnarrowpatternsseenmoreintheNorthtothe North-Central portion of the map couldprobablybeduetodikes,tectonicshearzonesisoclinically folded strata with magneticimpregnationorlongorebodies.Thenatureoftheclosuredepictsthedepthofburialandsize

of intrusions that are within the basemenunderlying the area.Short tomoderately longdislocationswerealsonoticedinthemagneticmapwhichisindicativeofshorttomoderatelylong geologic faults. Long narrow anomalyfeatures known as magnetic lineamentstrending in NE-SW direction (A-B) has beenidentifiedtopassthroughthestudyarea.TheworkofBuser(1966)inthestudywhichcoversNigeriaanditssurroundingcountries,

450000 455000 460000 465000 470000 475000 480000 485000 490000 495000 500000

LONGITUDE

1050000

1055000

1060000

1065000

1070000

1075000

1080000

1085000

1090000

1095000

1100000

1105000

L A T I T U D E

0 10000 20000 30000 40000

Contour interval=5nT

10Km = 10000 map units

map units

Fig.4.Regionalmagneticmapofstudyarea.

450000 455000 460000 465000 470000 475000 480000 485000 490000 495000 500000

LONGITUDE

1050000

1055000

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1065000

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1100000

1105000

L A T I T U D E

0 10000 20000 30000 40000

10Km= 10000 map units

map units

Contour interval=20nT

Axes in UTM

B

A

Fig.5.Airbornemagneticresidualanomalymapofstudyarea

Bukuru






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establishedtheexistenceofpaleostructures,whichhavedirected events like tectonic movements, intrusions,metamorphism,sedimentation,mineralization,volcanismand drainage. He identified them asstriking ina NNE-SSW direction. These structures are in the form ofculminations(crests)anddepressions.Theyinfluencethe

major tectonic features in these regions to anapproximateNE-SW direction. Ajakaiye et al. (1991) intheir interpretation of aeromagnetic data across theNigeriancontinentalmassidentifiedtheNE-SWtrendinganomaliesasdominantmagneticfeaturesofmostof thisshieldarea.TheydeducedthattheselineamentscoincidewithmajorstructuraltrendssuchastheBenuetroughinNigeria,fracturesintheoceaniccrustoftheWestAfricancoast,EburneansynclineintheCote-D’ivoireandcanbetraced to the lineaments inGuyana and EasternBrazil.TheyshowedthatontheshorelineamentsinWestAfricaare the extensions of the St Paul’s, Romanche, Chainand Charcot fracture zones. These fracture zones are

believedtobepartofthemajorzonesofweaknessinthecrustthatpredatetheopeningoftheAtlanticOceanandwere reactivated during the early stage of continentalrifting. These authors further pointed out thisconcentration of magnetic lineaments appeared to beconnectedwiththeoccurrenceofYoungerGranitessincealmost all known Younger granite complexes lie withintheregiondominatedbythistrends.Alsofromfig.5mostof the anomalies in the area trend largely in NE-SWdirectionwhileafewtrendE-W,N-Sdirection.AnENE-WSWanomalylowhasbeenidentifiedintheJos-Bukurucomplex. This agrees with the study carried out by Ajakaiye (1982). In their study, a prominent ENE-WSWanomalylowiscentredattheJos–Bukurucomplexwitha

trend which is roughly parallel to that of the mainstructuralfeatureoftheBenuetrough.TheysuggestthattheoriginsoftwomajorstructuralfeaturesinNigeriathatistheBenuetroughandtheYoungerGraniteintrusivearerelated considering the stress, which initiated theformation of these features. Also from Fig.5 it can beseenthatthelowmagneticanomalyvaluesdominatethestudy area probably because the biotitegranites which are the major rock typeformingtheplutonsintheringcomplexesare characterized by very low magneticanomaliessincetheyarepooriniron.

StatisticalspectralanalysisThe residual total magnetic field ofFig.5wasusedtodeterminethedepthtomagnetic sources within the Naragutaarea using statistical spectral analysis.Graph of the logarithm of the spectralenergiesagainstfrequenciesobtainedforthe area is shown in Fig.6. Two linearsegments were drawn from the graph.Thegradientsof the linearsegmentwas

evaluated and used to calculate the depth to thecausativebodies.Theresultshowsthatthefirstsegmenthasanaveragedepth(Z1)of2.03Kmwhilethesecondsegmenthasanaveragedepth(Z2)of265m.

We may attribute the first segment to the deepesourcestoandthesecondsegmenttothesurfacerocks.

The deeper sourcesare probably thought tobecausedby the crystalline metamorphic basement rocks of thearea which consists of migmatites, gneisses and OldeGranites.While theshallowersourcesareprobablyduetotherhyoliticrocksthatdirectlyoverliethemetamorphicbasement, Pleistocene cassiterite bearing alluviumand/orQuaternarytoRecentbasaltlava flowshavefilledthebroadPleistocenevalleys.Conclusion

The Residual aeromagnetic map over Naraguta incentralNigeriahasbeeninterpretedinthisstudy.Circulapatterns on the map could be associated with the

presence of ore bodies, granitic as well as basic orebodieswhilethe longnarrowpatternsseenmorein theNorth to the North-Central portion of the map couldprobably be due to dikes, tectonic shear zonesisoclinically folded strata with magnetic impregnationolong ore bodies. Short to moderately long dislocationswerealsonoticedinthemagneticmapwhichisindicativeof short to moderately long geologic faults. MagneticlineamentshavebeenidentifiedtopassthroughtheareaLow magneticanomaly valuesdominate the studyareaprobablybecausethestudyareaisclosetothemagneticequatorandthatthebiotitegraniteswhicharethemajorocktypefoundintheareaarecharacterizedbyverylowmagneticanomaliessincetheyarepooriniron.Alsoan

ENE-WSWanomalylowhas been identified in the JosBukurucomplexwithatrendwhichisroughlyparalleltothat of the main structural feature of the Benue troughTheresultshowsthatthefirstlayerhasanaveragedepthof265mwhilethesecondlayerhasanaveragedeptho2.03Km.

Fig6.Energyspectrumofstudyarea

Deep-seated

contribution (z1)Spectrum dominated by near- surface

contribution (z2)






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AcknowledgementThe authors would like to thank Dr. Kola Lawal of

AhmaduBelloUniversity,Zaria,KadunaState,Nigeriaformaking the REGRES.FOR and SPEC.FOR programsavailablefortheprocessingoftheaeromagneticdata.

References1. Abdelrahman EM, Bayoumi AI, Abdelhady YEM,

Ghobashi MM and El- Araby HM (1989) Gravityinterpretation using correlation factors betweensuccessive least squares residual anomalies.Geophys.54,614-621.

2. AbdelrahmanEM,El-ArabyAA,HMAmmarAAandHassanein(1997)A leastsquareapproachto shapedetermination from residual self potential anomalies.Pure&Appl.Geophys.150,121-128.

3. Abdelrahman EM and Sharafeldin SM (1996) Aniterative least squares approach to depthdetermination from residual magnetic anomaliesdue

tothindykes.J.Appl.Geophys.34,213-220.4. Ajakaiye DE (1982) Aeromagnetic anomalies acrossthe Nigerian younger granite province. 20

TH

anniversary Proce. Nigerian Mining & Geosci. soc.,edited byOluyide PO, MbonuWCand Onuugu SA.19(1),266.

5. Ajakaiye DE, Hall DH, Ashiekaa JA and Udensi EE(1991)MagneticanomaliesintheNigeriancontinentalmassbased onaeromagneticsurveys.Tectonophys.192,211-230.

6. Akanbi ES and Udensi EE (2006) Structural trendsand spectral depth analysis of the residual field ofPategi area, Nigeria, using Aeromagnetic Data.NigerianJ.Phys.18(2),271-276.

7. Buser H (1966) Paleostructures of Nigeria andadjacent Countries. Geotectonic Res. Vol. 24, E.Schweizerbart Sci. Publi., Johannesstr. 3A D-70176Stuttgart,Germany.

8. Hahn A, Kind EG and Mashira DC (1976) Depthestimationofmagneticsourcesbymeans ofFourieramplitudespectra.Geophys.Prospec.24,287-308.

9. Hall DH (1968) Regional magnetic anomalies,magnetic units and crustal structure in the Kenoradistrict of ontario. Canadian J. Earth Sci. pp: 1227-1296.

10. Hall DH (1974) Long wavelength aeromagneticanomalies and deep crustal magnetization in

manitoba and Northern ontario, Canada. Can. J.Geophys.40,403-430.11. Kangkolo R (1996) A detailed interpretation of

aeromagnetic field overmamfebasinofNigeria andCameroun. PhD Thesis, Ahmadu Bello Univ. Zaria,Nigeria.

12. MacleodWN, TurnerDC and WrightEP(1971)TheGeology of the jos plateau. Geological Survey ofNigeria.Bulletin32.1,11-15.

13. Nettleton LL (1976) Gravity and magnetic in oiprospecting.NY.McGraw-HillBookCo.Inc.pp:464

14. Pal PC,Khuran KKandUnnikrishnnaP (1978) Twoexamples of spectral approach to source depthestimationingravityandmagnetics.PAGEOPH.117772-783.

15. Spector A (1968) Spectral analysis of aeromagneticmaps.PhDthesis,Dept.Phys.Univ.ofToronto.

16. SpectorAandGrantFS(1970)StatisticalmodelsfointerpretingAeromagneticdata.Geophys.35(2),293302.



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