Interpretation of high resolution aeromagnetic data oversouthern Benue Trough, southeastern Nigeria

I A Oha1,∗, K M Onuoha1, A N Nwegbu2 and A U Abba2

1Department of Geology, University of Nigeria, Nsukka, Nigeria.2Nigerian Geological Survey Agency, Abuja, Nigeria.

∗Corresponding author. e-mail: ifeanyi.oha@unn.edu.ng

High resolution airborne magnetic data of parts of the southern Benue Trough were digitally processed andanalyzed in order to estimate the depth of magnetic sources and to map the distribution and orientationof subsurface structural features. Enhancement techniques applied include, reduction to pole/equator(RTP/RTE), first and second vertical derivatives, horizontal gradients and analytic signal. Results fromthese procedures show that at least 40% of the sedimentary basin contain shallow (<200 m) magmaticbodies, which in most cases are intermediate to mafic intrusive and hyperbysal rocks, and may occuras sills, dikes or batholiths. Magnetic lineaments with a predominant NE–SW trend appear to be moredensely distributed around the basement rocks of the Oban Hills and metamorphosed rocks around theWorkum Hills. 3D standard Euler deconvolution and Source Parameter Imaging (SPITM) techniqueswere employed for depth estimation. Results from the two methods show similar depth estimates. Themaximum depth to basement values for 3D Euler and SPI are 4.40 and 4.85 km with mean depths of0.42 and 0.37 km, respectively. Results of 2D modelling of magnetic profiles drawn perpendicular tomajor anomalies in the study area reveal the existence of deep seated faults which may have controlledthe emplacement of intrusive bodies in the basin. The abundance of intrusive bodies in the study arearenders this part of the southern Nigerian sedimentary basins unattractive for petroleum exploration.However, the area possesses high potential for large accumulation of base metal mineralization.

1. Introduction

Following the release of the first aeromagnetic datacollected over most parts of Nigeria by the Geo-logical Survey of Nigeria (GSN) in 1974, attemptswere made by early researchers to interpret thedata both qualitatively and quantitatively. Studiesinvolving the interpretation of aeromagnetic dataover the Benue Trough have revealed the existenceof block faulting and numerous intrusive bodies(Osazuwa et al. 1981; Ajakaiye 1981; Ofoegbu 1984,1985; Ofoegbu and Mohan 1990; Ofoegbu andOnuoha 1991). Various depth estimate techniqueswere applied on the data and results obtained

showed that estimates of the thickness of sedimen-tary rocks obtained by different authors agree fairlywell with each other. For example, Osazuwa et al.(1981) obtained a depth range of 0.9–4.9 km and0.9–2.2 km in the northern Benue Trough frommagnetic and gravity data respectively, while Nur(2000) combining two-dimensional spectral analysisand Hilbert transform of magnetic data reportedthe existence of two main source depths in parts ofthe northern Benue Trough, with the deepest sourcelying between 1.5 and 2.25 km. In the southern BenueTrough, Ofoegbu (1984) found that the thicknessof sediments vary between 0.5 and 7 km. Ofoegbuand Onuoha (1991) from the results derived from

Keywords. Airborne magnetic data; magnetic sources; magmatic bodies; depth estimation; 2D modelling; Benue Trough.

J. Earth Syst. Sci. 125, No. 2, March 2016, pp. 369–385c© Indian Academy of Sciences 369

370 I A Oha et al.

2D spectral analysis identified the existence of twomain source depths in parts of the southern BenueTrough, the deeper source lying at a depth of 1.3–2.5 km, while the shallower depths were generally<250 m. Recently, Obi et al. (2010) carried outdepth estimates based on ‘SAKI’ modelling, powerspectrum and horizontal gradient magnitude andobserved that sediment thickness range between1.0 and 4.0 km.

Between 2005 and 2009, the Federal Ministry ofMines and Steel Development engaged the servicesof Fugro Airborne Surveys Limited, to acquire highresolution data of 500 m line spacing and 80 mterrain clearance for most parts of Nigeria. Therelease of these data by the Nigerian GeologicalSurvey Agency (NGSA) in 2010 coincided with theavailability of high speed and robust computer pro-grams, making it increasingly possible to generate

Table 1. Characteristics of existing aeromagnetic data for the study area.

Line TerrainDate spacing clearance Flight

Data available (m) (m) line direction Data format

Old data 1974 2000 200 E–W Hard copy contour maps ona scale of 1:100,000

New data 2010 500 80 NW–SE Digital formats in .gdb,.xls (excel), PDF, JPEG, etc.

Figure 1. The study area as part of the West African and Central African rift system.

Interpretation of aeromagnetic data over the southern Benue Trough 371

subtler results and more detailed information. Thisstudy aims at utilizing newly acquired high res-olution airborne magnetic data to generate moredetailed information on the structural frameworkand distribution of igneous bodies in the basin.Table 1 accounts for the improvement in the newdigital high resolution data over the existing datain which most of the works earlier cited are basedupon.The southern Benue Trough includes the

southernmost part of the Benue Trough, which is amajor sedimentary basin in Africa, stretching over1000 km in length with width ranging between 150and 250 km (figure 1). It is part of the CretaceousWest African Rift System (WARS) which can betraced along a distance of about 4000 km fromNigeria, running northwards into the neighboringRepublic of Niger and terminates in Libya (Binksand Fairhead 1992).The various mechanisms that are being proposed

for the formation of the Benue Trough, have gen-erated a lot of controversy. However, the mostpopular theories include:

i) Tensional movement resulting in a rift (King1950; Cratchley and Jones 1965).

ii) Horst and graben genesis related to the Creta-ceous opening of the Gulf of Guinea (Stoneley1966).

iii) Asthenospheric uplift or mantle plume, blockfaulting, crustal stretching and thinning andemplacement of igneous bodies in the litho-sphere (Olade 1975; Bott 1976; Adighije 1979;Fairhead and Okereke 1986).

iv) Large scale wrenching leading to the formationof numerous pull-apart basins (Benkhelil andRobineau 1983; Benkhelil 1986, 1989).

The inherent genetic ambiguity, structural com-plexity, coupled with the existence of igneous bod-ies and base metal mineralization in the basin hasgenerated a lot of interest amongst geoscientists.The studies have been multi-dimensional involvinggeophysical, geochemical, structural and petrolog-ical studies. As newer data and tools are generatedand developed, it becomes increasingly possible tomodel and characterize the basin.

2. Regional geology

The sedimentary fill of the southern Benue Troughwas controlled by cycles of transgressions andregressions accompanied by interferences of localtectonics. Figure 2 is a simplified geological map ofthe southern Benue Trough. Three cycles of basinfill have been recorded in the basin, they include;(i) the Neocomian–Cenomanian Asu River Group,(ii) the Early to Late Turonian Eze–Aku Group

and (iii) the Coniacian–Santonian Awgu Group(Ojoh 1992; Umeji 2007). The Asu River Grouprepresents the earliest clastic fill of the LowerBenue Trough and occupies the core of the basin.It consists of about 3000 m thick basal arkosicsandstones and middle and upper marine shales.Overlying the Asu River Group is the TuronianEze–Aku Group which consists of fossiliferous cal-careous sandstones, shales and limestones. Thesesediments are noted as the most extensive marinesedimentation deposited when the Mediterraneanor Tethys Sea linked up with the Atlantic acrossthe Sahara (Ojoh 1992).The overlying Awgu Group consists of the grey-

blue Awgu–Ndeaboh shale and the medium-to-coarse grained bioturbated Agbani sandstones.Large scale cross-stratification and herringbonecross-bedding are common primary and sedimen-tary structures in the Agbani sandstone. Reyment(1965) considered the Agbani sandstone as a timeequivalent of the Awgu shale, conversely, Cratchleyand Jones (1965) thought that it is a late depositof thick sandstone beds which accompanied theshallowing of the Coniacian Sea. The Awgu shaleis estimated to be about 900 m thick (Benkhelil1986). The Santonian represented a period of tec-tonic and igneous activity, when the sediments ofthe Abakaliki–Benue Trough were folded, upliftedand intruded by igneous rocks, leading to low-grade metamorphism in some cases (Ojoh 1992).The sediments of the Benue Trough were erodedand variously overstepped by the Campanian andMaastrichtian beds of the Anambra Basin. Thehorst and graben structure in the Calabar Flankof the Benue Trough were initiated at this time(Whiteman 1982; Reijers 1996).The study area also includes the Oban and

Bamenda Massifs, which are composed of crys-talline metamorphic rocks of Neo-Proterozoic age.These rocks are mainly migmatitic gneisses, bandedgneisses, coarse porphyritic granites, pegmatiteswith thin dykes of intermediate–basic intrusive andhyperbysal rocks. They generally underlie the sed-imentary fills of the Lower Benue Trough. Youngersediments of Campanian–Paleocene age are shownin the southwestern tip of the geological map(figure 2). It is known that they belong to two youn-ger adjacentbasins, namely, the Afikpo and Anambrabasins. Sedimentation ended in the Lower BenueTrough after the Santonian and this was as a resultof widespread uplift consequent of a pervasive com-pressive event. This led to the development of twobasins, the Afikpo Syncline to the south and theAnambra Basin to the west. The Campanian rep-resents the oldest sediments in both the Afikpo andAnambra basins and sedimentation may have con-tinued until the end of the Cretaceous. The struc-tural history and stratigraphic successions of

372 I A Oha et al.

Figure 2. Generalized geologic map of the study area.

these basins have been well documented elsewhere(Nwajide and Reijers 1996; Akaegbobi and Boboye1999; Obi and Okogbue 2003; Odigi and Amajor2009).

3. Data and methods

3.1 Data

The data used for this research form part ofthe new high resolution digital airborne data formost parts of Nigeria acquired between 2005 and2009. They were collected in two phases, withPhase I data acquired between May and September2007, while Phase II data was acquired during theperiod September 2007–August 2009. The entiredata included 1,930,000 line km of magnetic andradiometric surveys flown at 500 m line spacingand 80 m terrain clearance. Data acquisition wascarried out by Fugro Airborne Surveys.A subset of the nationwide grid covering the

study area (lat. 5◦30′–7◦00′N and long. 7◦30′–9◦00′E) was made available by the Nigerian Geo-logical Survey Agency (NGSA). The survey for

most of the Benue Trough was flown along theNW–SE direction (i.e., perpendicular to the axis ofthe basin). The geomagnetic gradient was removedfrom the data using the International GeomagneticReference Field (IGRF) formula for 2005. This newdata offers a lot of advantages in resolution andformat over the old data (acquired in 1974) inwhich most of the earlier interpretations were made(see table 1).

3.2 Methods

3.2.1 Data processing

Data processing including editing and initial fil-tering was performed by the preliminary process-ing contractors, Paterson Grant and Watson Ltd(PGW). This includes cultural editing to correctfor rough effect due to interference from substan-tial infrastructure in the area. Diurnal variationsin the airborne magnetometer data were correctedby subtraction of the filtered and IGRF correctedground station data. Thereafter, the magnetic datawas IGRF corrected using the 2005 model. After

Interpretation of aeromagnetic data over the southern Benue Trough 373

the diurnal and IGRF corrections, a levelling pro-cedure was applied to account for a number ofeffects including data differences at intersections oftie and traverse line recordings. The products wereinterpolated into a regular grid with cell size of125 m (one quarter of the flight line spacing) usinga minimum curvature algorithm with a constantelevation of 80 m. This final product is the totalmagnetic intensity (TMI) map.

3.2.2 Data analysis and interpretation techniques

Data analysis and interpretation can be broadlydivided into two main procedures which includedata enhancement and forward modelling. Enhan-cement is necessary since the TMI data displaysgross interpretation limitation. In order to enhancesubtle anomalies which, in many cases, are anoma-lies of interest, a number of filters are applied to theraw TMI data. This was done in the spatial fre-quency domain by the introduction of Fast FourierTransform (FFT). The enhancement routines per-formed in this work include, reduction to pole (RTP),reduction to equator (RTE), first and second ver-tical derivatives (1VD, 2VD), horizontal gradient(HG), analytic signal (AS), standard Euler decon-vulation and source parameter imaging (SPI).The last two are essentially depth-estimation fil-ters, while the others enhance or suppress certainanomalies invariably aiding the interpretation pro-cess. More elaborate description of the principle,theory, application and formulation of these rou-tines are given elsewhere (Nabighian 1972, 1984;Thompson 1982; Roest et al. 1992; Thurston et al.1999, 2002; Reeves 2005; Nabighian et al. 2005;Whitehead and Musselman 2008). Only a briefpresentation of these routines is outlined here.The reduction to pole (RTP) filter simplifies

interpretation of anomalies by reconstructing themagnetic field as if it were at the pole. (i.e., ver-tical magnetic field and declination of zero). Ver-tical bodies hence will produce induced magneticanomalies that are centred on the body and aresymmetrical.First and second vertical derivatives emphasize

shallower anomalies and can be calculated eitherin the space or frequency domains. Before thedigital age, use of second vertical derivative fordelineating and estimating depths to the basementformed the basis of aeromagnetic interpretation(Nabighian et al. 2005).Many modern methods for edge detection and

depth to source estimation rely on horizontal andvertical derivatives. For this study, the first andsecond vertical derivatives were generated in OasisMontaj using the MAGMAP GX.The amplitude of the analytic signal (total gra-

dient), possesses considerable advantage over the

maximum horizontal gradient, due to its lack ofdependence on dip and magnetization direction, atleast in 2D (Nabighian et al. 2005). Thus, the ana-lytic signal performs well at all magnetic latitudes.This notion is extended to 3D by Roest et al. (1992)and represented using the expression.

|A(x, y)| =

√

√

√

√

[

(

∂T

∂x

)2

+

(

∂T

∂y

)2

+

(

∂T

∂z

)2]

(1)

where |A(x, y)| is the amplitude of the analytic sig-nal at (x, y). T is the observed magnetic field at(x, y).Thompson (1982) expressed the Euler’s homo-

geneity as:

(x−x0)∂T

∂x+(y−y0)

∂T

∂y+(z−z0)

∂T

∂z= N(B−T )

(2)

where x0, y0, z0 is the position of the magneticbody. T is total field measured at (x, y, z). N is thedegree of homogeneity which can be interpreted asthe structural index (SI). B is background valueof the TMI. The Standard 3D Euler method isbased on Euler’s homogeneity equation, whichrelates the potential field and its gradient compo-nents to the location of the sources, by the degreeof homogeneity N . The method makes use of astructural index in addition to producing depthestimates. The structural index combined withdepth estimates have the potential to identify andcompute depth estimates for a variety of geologicstructures such as faults, magnetic contacts, dykes,and sills.The source parameter imaging (SPI) technique

is based on the principle of complex analytic sig-nal and computes source parameters from grid-ded magnetic data. It requires first and secondorder derivatives and is thus susceptible to noise inthe data and interference effects (Nabighian et al.2005). The wavenumber theory is that for verticalcontacts, the poles of the local wave number definethe inverse of depth. In other words, depth is givenby the expression

Depth =1

Kmax

(3)

where Kmax is the peak value of the local wavenumber K over the step source. It can be shownthat

Kmax =

√

√

√

√

[

(

∂Tilt

∂x

)2

+

(

∂Tilt

∂y

)2]

(4)

374 I A Oha et al.

and Tilt = arctan (VDR/HGRAD)

= arctan

⎡

⎢

⎢

⎣

(∂T/∂z)√

[

(∂T/∂x)2+ (∂T/∂y)

2]

⎤

⎥

⎥

⎦

. (5)

Solution grids using the SPI technique show theedge locations, depths, dips and susceptibility con-trasts. Hence, the SPI map more closely resem-bles geology than either the magnetic map or itsderivatives. The technique works best for isolated2D sources such as contacts, thin sheet edges, orhorizontal cylinders (Nabighian et al. 2005). Theratio of the vertical gradient to the horizontalderivative shown in equation (4) has been definedas the tilt angle (Miller and Singh 1994). The tiltangle acts as an automatic gain filter and is seenhere as an excellent edge detector.In this study, forward modelling of two profiles

across the study area were performed using GM-SYS software. The GM-SYS program is based on

the method of Talwani et al. (1959) and Talwaniand Heirtzler (1964).

4. Results

4.1 Edge detection

The RTP, vertical and horizontal derivative gridsdisplay useful edge information. The RTP trans-formation has correctly placed the peaks over thesource, magnetic highs are observed over the base-ment Oban Hills, the Abakaliki magmatic areawhich contains pyroclastics, the metamorphosedand magma invaded Workum Hills (outcroppingbetween Wanakom and Wanakonde) and the Ishi-agu area (figure 3). The cause of the magneticanomaly around the Enyigba area cannot be clearlyascertained from this image, the absence of intru-sives and hyperbysal rocks (based on field andborehole data) further complicates it. Prominentmagnetic lows are observed between Afikpo andAka–Eze to the south, around Eha–Amufu to the

Figure 3. Reduction to pole (RTP) grid with amplitude correction=40.

Interpretation of aeromagnetic data over the southern Benue Trough 375

northwest of the study area and Bansara–Ogojaaxis to the northeast.First and second vertical derivatives (1VD and

2VD) of the TMI were performed in order toenhance shallow sources. The 2VD grid (figure 4)tends to reveal more features, but is noisy. The2VD grid is upward continued to a height of 300 min order to smoothen the image. The 1VD grid canbe displayed in grey scale so as to enhance linearfeatures, which may be dikes, sills, geologic con-tacts, faults, fractures etc. When compared withthe tilt angle map, it was observed that the latterpossesses greater potential for edge detection. Thefeatures are digitized onscreen and superimposedon the tilt angle map (figure 5). The horizontalgradient display is characteristically employed inthe enhancement of linear features from aeromag-netic data. The magmatic induced patterns in thestudy area are observed and are accompanied by

numerous linear features. The analytic signal gridsimplifies the interpretation by placing the ano-maly peaks directly above the source. The intrusivebodies around Ishiagu, Workum Hills, Igumale,Obubra, Ikom and the basement rocks around theOban Hills are clearly defined in the analytic signalgrid of the study area (figure 6).

4.2 Source depth estimates

Depth estimates from the SPI image shows the dis-tribution of deep basins. It is observed in figure 7that 70–80% of the study area contain sources thatare relatively shallow (<1 km). The deep parts ofthe basin are generally small, isolated pockets andmay attain depths of up to 4.6 km. Most of theintrusives, hyperbysal and volcanics are generallyless than 300 m deep. The basement areas around

k

Figure 4. Second vertical derivative (2VD) image.

376 I A Oha et al.

Figure 5. Tilt angle map (in radians) with interpreted magnetic lineaments overlaid.

the Oban Hills are generally exposed and whereburied they rarely exceed a depth of 300 m.Four 3D Euler maps are displayed in figure 8

with structural index (S.I) values of 0, 1.0, 2.0 and3.0. The Euler images are similar to the SPI image(figure 7) except that in some locations the Eulerimages lack solutions. Table 2 compares SPI depthestimates with Euler (SI = 3.0) for certain deepbasin locations in the study area.

4.3 Modelling

2D modelling of profiles AA′ and BB′ was car-ried out in order to verify the distribution of litho-logic units and the structural framework of thesubsurface. Three main rock types were built intothe model, based on their contrasting susceptibilityvalues. The overlying sedimentary rocks were assig-ned susceptibility values of zero; the metamorphic

Interpretation of aeromagnetic data over the southern Benue Trough 377

k

Figure 6. Analytical signal image, revealing the extent and shapes of shallow and deep seated intrusive bodies. (AA′ andBB′ are profile lines for the models in figure 9).

basement was assigned values ranging from 0.001to 0.003 cgs, while the intermediate to basic intru-sives were assigned values ranging from 0.003 to0.006 cgs. The raw TMI image of the study areawas used in building the model.Profile AA′ is a NNW–SSE trending section with

an approximate length of 63 km. It starts froma point approximately 18 km north of Onitcha–Uburu, through Afikpo and terminates aroundUgep (see figure 9a). The 2D model reveals afaulted basement with depth range between 1.8and 2.5 km and numerous intermediate to maficsills and dikes, most of them occurring at depthsbetween 0 and 500 m. The outcropping sill north ofAfikpo is observed around the 48,000 m point along

the profile and appears to be closely associated tonormal faulting at depth (see figure 9a).Profile BB′ also trends in the NNW–SSE direc-

tion with an approximate length of 65 km. It startsfrom a point approximately 12 km northwest ofthe Workum Hills and passes through Wanikandeand Wanakom, terminating close to Bansara (seefigure 6). The structure of the area is typicallythat of an anticline with the oldest sediments(Albian Asu River Group) at the core and youngersediments at the flanks. The underlying basementrocks display a characteristic horst and grabenstructure with an obvious interplay between tec-tonism and magmatism. Numerous bodies of intru-sive igneous rocks with diameter ranging from 1

378 I A Oha et al.

Figure 7. Source parameter imaging (SPI) grid, showing approximate depths to intrusive bodies and the underlyingbasement complex rocks. 1–7 represent positions where the basins are thick and are explained in table 2.

to 8 km occupy the core of the anticlinal struc-ture around the Workum Hills and can be tracedfor close to 30 km along the profile. Emplacementof the intrusive bodies are observed to have beenclosely related to large dip-slip faulting at depthsin excess of 3 km. A thick intermediate to maficintrusive body (14,000 m along BB′) may havecompensated for the large depth to source (about2.2 km) observed on the SPI image at thatpoint. The area around Bansara also has depth tobasement in excess of 2 km. For other parts of thesection, depth to basement is generally below 2 kmand most of the intrusive rocks at the core of thelocal anticlinal feature are outcropping.

5. Discussion and conclusion

The 529 magnetic lineaments extracted from theenhanced aeromagnetic image represents deepseated fractures, dykes, sills and vents. It isobserved that the distribution (figure 10a) is closelycontrolled by the host lithology, as basement rocksand intrusives are seen to display higher density ofthese features. Figure 10 highlights the similaritiesand differences in trend and length between lin-eaments digitized from enhanced Landsat7 ETM+data (Oha 2014) with the magnetic lineamentgenerated from this study. The dominant trendobserved from the rose diagram (figure 10b, c)

Interp

retatio

nofaero

magn

eticdata

overthesouthern

BenueTrough

379Figure 8. 3D standard Euler deconvolution with structural index (SI) = 0, 1, 2 and 3. (1–7 on SI = 3 grid represent positions where the basins are thick and are explainedin table 2).

380 I A Oha et al.

Table 2. Comparative depth to basement estimates for selected (deep basin) locations in thestudy area.

Sl. no. Location Latitude Longitude Euler (SI = 3) SPI (km)

1 Southeast of Bansara 6.41◦N 8.57◦E No solution 3.80

2 Afikpo–Akaeze 5.82◦N 7.76◦E 3.37 km 3.85

3 Northwest of Oshirigwe 6.80◦N 8.31◦E 2.88 km 2.29

4 South of Enugu 6.43◦N 7.67◦E No solution 4.61

5 West of Ogoja 6.60◦N 8.55◦E No solution 3.64

6 Southwest of Eha–Amufu 6.65◦N 7.68◦E No solution 2.14

7 Southwest of Ugep 5.59◦N 7.96◦E 3.84 km 3.73

is consistent with the major (NE–SW) structuraltrend of the Benue Trough. However, there is amarked deviation in subordinate trends from whatis observed on Landsat and on the field. The N–Sand NW–SE subordinate trends reported in thearea (Ezepue 1984; Oden 2012; Oha 2014), are notprominent in the filtered aeromagnetic maps. Thisimplies that these trends do not persist at depth.Results from aeromagnetic data interpretation

suggests the demarcation of six magmatic centresin the study area (figure 11). They include, Ishi-agu, Ugep, Obubra, Ikom, Wanikande and Igumalemagmatic centres. At these areas, igneous bodieseither outcrop or they are covered by thin overbur-den of not more than 100 m. Depth to basementestimation using Euler deconvolution and sourceparameter imaging (SPI) gave depths between 2.2and 4.8 km for areas where there are no intrusivebodies (see table 2).Spatial distribution of these bodies shown on the

300 m upward continued second vertical deriva-tive map (figure 11) reveal that about 60% of thestudy area is covered by either basement rocks orintrusives. This includes some parts of Oban andBamenda Massifs, which also outcrop in the studyarea. These rocks are observed from the models(figure 9) to include large volumes of intrusive,hyperbysal and volcanic rocks of highly varied com-position. These large volumes of igneous bodiescombined with their widespread occurrence maybe responsible for the observed close spatial asso-ciation with the Pb–Zn–Ba mineralizations in thestudy area (Olade and Morton 1985; Akande andMucke 1993; Oha 2014). It is inferred that theobserved close spatial association between mineral-ized veins and intermediate to mafic intrusive rocksin the study area does not have genetic implication.Furthermore, it is obvious from field observa-tions that the intrusives predate the mineraliza-tion. In some places, barite-dominated veinlets areseen cross-cutting late Cenomanian to Santonianintrusives (Oha 2014).Depth estimates from this study agrees fairly

with results from previous works. The works ofOsazuwa et al. (1981); Ajakaiye (1981); Ofoegbu

(1984, 1985); Ofoegbu and Mohan (1990); Ofoegbuand Onuoha (1991); Nur (2000) and Obi et al.(2010) gave depth to deeper source range in theBenue Trough as 2.5–7 km. Whereas, the deepestparts of the basin from this study is about 6 km,the high frequency of shallower bodies (<200 m)as displayed by the powerful visualizing capabil-ity of Oasis Montaj software offers a remarkableaddition to the existing knowledge on distributionof intrusives in the Lower Benue Trough. Some ofthe magmatic centres reported in this work werepreviously reported (Obiora and Charan 2010),but their extent and spatial distribution were notclearly demarcated.The proliferation of near surface intrusives in

the study area as shown in the various filters and2D model is an indication of widespread pervasivemagmatism in the Lower Benue Trough. Thisconsequently may have adversely increased tem-perature ranges in the basin leading to possibleovermaturation of potential source rocks. Hence,the abundance of igneous bodies in the LowerBenue Trough may have adversely affected thepetroleum potential of the Lower Benue Trough.Notwithstanding, reports of extensive mineraliza-tion of Pb, Zn, Cu and Ba mineralization arewell documented (Farrington 1952; Orajaka 1965;Olade 1976; Ezepue 1984; Olade and Morton 1985;Akande et al. 1988; Akande and Mucke 1993; Oha2014). Although, recent work in the basin sug-gests the deposits may not be genetically relatedto the igneous bodies (Oha 2014), understandingtheir presence and disposition presents a usefulexploration guide.Based on the results and inferences generated

from this work, we conclude that the Lower BenueTrough contains not only pockets of magmatic bod-ies as earlier thought, but include enormous andwidely distributed magmatic rocks. These rocksgenerally cover approximately 60% of the studyarea and are either outcropping or concealed by rel-atively thin overburden. Whereas, in most areas,depth to basement range between 2 and 2.5 km,a few isolated portions of the basin show depthestimates in excess of 4 km. The existence of deep

Interpretation of aeromagnetic data over the southern Benue Trough 381

Figure 9. 2D model for profiles AA′ and BB′.

seated (200–500 m) magmatic lineament is con-firmed and tends to deviate in trend from surfacelinear trends (lineaments) obtained from fieldwork

and interpreted Landsat7 ETM+ data. The pro-liferation of the basin by large scale magma-tism renders the basin unattractive for petroleum

382 I A Oha et al.

Figure 10. (a) Combined lineament map (from aeromagnetic and Landsat 7 ETM+ data) for the study area. (b) Rose plotfor lineaments from Landsat data. (c) Rose plot for lineaments from aeromagnetic data.

Interpretation of aeromagnetic data over the southern Benue Trough 383

k

Figure 11. 2VD image upward continued to 300 m. A are basement rocks of the Oban Hills. B represents basaltic rocksaround Ikom. C are basement rocks. D are basaltic/doleritic rocks outcropping around Obubra. E represents the meta-morphose Workum Hills and the magmatic centres around Wanikande and Wanakom. F represents near surface intrusivesassociated with barite mineralization around Gabu-Oshina. G represents the Igumale area characterized by numerous nearsurface intrusives. H represents the Abakaliki area with its pyroclastics and associated rocks. I represents the Ishiagumagmatic area.

exploration, but a deeper understanding of thedistribution and disposition of these igneous bodiespresents a valuable base metal exploration tool.

Acknowledgements

The authors are grateful to the former DG ofNGSA, Prof. S Malamo who granted the firstauthor a 6-month intenship at NGSA Abuja, in2011. Dr O Okunola was on ground to offer use-ful suggestions and advice during the internship.Ria Tinion of Geosoft South Africa, provided the

software (Oasis Montaj) used for data process-ing and interpretation. The quality of this paperhas improved enormously as a result of vital com-ments and contributions made by two anonymousreviewers.

References

Adighije C 1979 Gravity field of Benue Trough, Nigeria;Nature, London 282 199–201.

Ajakaiye D E 1981 Geophysical investigation in the Benue-Trough. A review; Earth Evolution. Arbitrary shape; In:

384 I A Oha et al.

Computers in the Minerals industries. Part 1 (ed.) ParksG A, Stanford University Publ. Geol. Sci. 9 464–480.

Akaegbobi I M and Boboye O A 1999 Textural, struc-tural features and microfossil assemblage relation-ship as a delineating criteria for the stratigraphicboundary between Mamu Formation and Nkporo Shalewithin the Anambra Basin, Nigeria; NAPE Bull. 14(2)193–207.

Akande S O and Mucke A 1993 Co-existing copper sulphidesand sulphosalts in the Abakaliki Pb–Zn deposit, LowerBenue Trough and their genetic implications; J. Mineral.Petrol. 47 183–192.

Akande S O, Horn E E and Reutel C 1988 Mineralogy, fluidinclusion and genesis of the Arufu and Akwana Pb–Zn–Fmineralization, middle Benue Trough, Nigeria; J. AfricanEarth Sci. 7 167–180.

Benkhelil J 1986 Structure et evolution geodynamique dubasin intracontinental del la Benoue (Nigeria); These,Univ. Nice, 226p.

Benkhelil J 1989 The origin and evolution of the CretaceousBenue Trough (Nigeria); J. African Earth Sci. 8 251–282.

Benkhelil J and Robineau B 1983 Le fosse de la Benoueest-ilun rift? Bull. Centres Rech. Explor. Prod. Elf-Aquitine7 315–321.

Binks R M and Fairhead J D 1992 A plate tectonic settingfor Mesozoic rifts of west and central Africa; Tectonophys.213 141–151.

Bott M H P 1976 Formation of sedimentary basins of grabentype by extension of the continental crust; Tectonophys.36 77–86.

Cratchley C R and Jones G P 1965 An interpretation ofthe geology and gravity anomalies of the Benue valley,Nigeria; Overseas Geol. Surv. Geophysical Paper No. 1.

Ezepue M C 1984 The geologic setting of lead-zinc depositsat Ishiagu, southeastern Nigeria; J. African Earth Sci. 297–101.

Fairhead J D and Okereke C S 1986 A regional gravity studyof the west African rift system in Nigeria and Cameroonand its tectonic interpretation; Tectonophys. 143 141–159.

Farrington J L 1952 A preliminary description of the Nige-rian lead-zinc field; Econ. Geol. 47 583–608.

King L C 1950 Outline and disruption of Gondwanaland;Geol. Mag. 87 353–359.

Miller H G and Singh V 1994 Potential field tilt – a newconcept for location of potential field sources; J. Appl.Geophys. 32 213–217.

Nabighian M N 1972 The analytic signal of two-dimensionalmagnetic bodies with polygonal cross-section: Its prop-erties and use for automated anomaly interpretation;Geophysics 37 507–517.

Nabighian M N 1984 Toward a three-dimensional auto-matic interpretation of potential field data via generalizedHilbert transforms: Fundamental relations; Geophysics49 780–786.

Nabighian M N, Grauch V J S, Hansen R O, LaFehr TR, Li Y, Peirce J W, Philips J D and Ruder M E 2005The historical development of the magnetic method inexploration; Geophysics 70 33–61.

Nur A 2000 Analysis of aeromagnetic data over the Yolaarm of the Upper Benue Trough, Nigeria; J. Mining andGeology 36 77–84.

Nwajide C S and Reijers T J A 1996 Geology of the southernAnambra Basin; In: Selected chapters on Geology (ed.)Reijers T J A, SPDC, Warri, pp. 133–148.

Obi G C and Okogbue C O 2003 Sedimentary response totectonism in the Campanian–Maastrichtian succession,Anambra Basin, southeastern Nigeria; J. African EarthSci. 38 99–108.

Obi D A, Okereke C S, Obei B C and George A M 2010 Aero-magnetic modelling of subsurface intrusives and its impli-cation on hydrocarbon evaluation of the Lower BenueTrough, Nigeria; European J. Sci. Res. 47 347–361.

Obiora S C and Charan S N 2010 Geochemical constraintson the origin of some intrusive igneous rocks from theLower Benue rift, southeastern Nigeria; J. African EarthSci. 58 197–210.

Oden M I 2012 Barite veins in the Benue Trough: Field char-acteristics, the quality issue and some tectonic implica-tions; Environ. Nat. Resour. Res. 2(2) 21–32.

Odigi M I and Amajor L C 2009 Brittle deformation ofthe Afikpo Basin, SE Nigeria: Evidence for a terminalCretaceous extensional regime in the Lower BenueTrough; Chin. J. Geochem. 28(4) 369–376.

Ofoegbu C O 1984 Interpretation of aeromagnetic anoma-lies over the Lower and Middle Benue Trough of Nigeria;Geophys. J. Roy. Astron. Soc. 79 813–823.

Ofoegbu C O 1985 A review of the geology of the BenueTrough of Nigeria; J. African Earth Sci. 3 283–291.

Ofoegbu C O and Mohan N L 1990 Interpretationof aeromagnetic anomalies over part of southeasternNigeria using three-dimensional Hilbert transformation;PAGEOPH 134 13–29.

Ofoegbu C O and Onuoha K M 1991 Analysis of magneticdata over the Abakaliki Anticlinolium of the Lower BenueTrough, Nigeria; Marine Petrol. Geol. 8 174–183.

Oha I A 2014 Integration of geologic, remote sensing andairborne geophysical data for regional exploration of lead-zinc-barium mineralization in parts of the southern BenueTrough, south-east Nigeria; Unpublished PhD Thesis,University of Nigeria, Nsukka, 201p.

Ojoh K A 1992 The southern part of the Benue Trough(Nigeria) Cretaceous stratigraphy, basin analysis, paleo-oceanography and geodynamic evolution in the equato-rial domain of the south Atlantic; Nigerian Association ofPetroleum Explorationists (NAPE). Bull. 7(2) 131–152.

Olade M A 1975 Evolution of Nigeria’s Benue Trough(Aulacogen): A tectonic model; Geol. Mag. 112 575–581.

Olade M A 1976 On the gensis of the lead-zinc depositsin Nigerians Benue rift (aulacogen) a re-interpretation;Nigerian J. Mining Geol. 13 20–27.

Olade M A and Morton R D 1985 Origin of lead-zinc min-eralization in the southern Benue Trough, Nigeria, fluidinclusion and trace element studies; Mineralium Deposita20 76–80.

Orajaka S 1965 Geology of Enyigba, Ameri and Ameka lead-zinc mines; J. Geol. 3 49–51.

Osazuwa I B, Ajakaiye D E and Verheijen P J T 1981 Analy-sis of the structure of part of the Upper Benue Rift Valleyon the basis of new geophysical data; Earth Evol. Sci. 2126–134.

Reeves C V 2005 Aeromagnetic Surveys, Principles, Practiceand Interpretation; Geosoft, 155p.

Reijers T J A 1996 Selected chapters on geology ; Shell Petrol.Dev. Co. (Nigeria) Publ., 197p.

Reyment R A 1965 Aspects of the Geology of Nigeria; IbadanUniversity Press, 145p.

Roest W R, Verhoef J and Pilkington 1992 Magnetic inter-pretation using the 3D analytic signal; Geophysics 57116–125.

Stoneley R 1966 The Niger Delta region in the light of thetheory of continental drift; Geol. Mag. 103 385–397.

Talwani M and Heirtzler J R 1964 Computation of mag-netic anomalies caused by two-dimensional bodies of arbi-trary shape; In: Computers in the mineral industries, Part1 (ed.) Parks G A, Stanford Univ. Publ., Geol. Sci. 9464–480.

Talwani M, Worzel J L and Landisman M 1959Rapid gravity computations for two-dimensional bodies

Interpretation of aeromagnetic data over the southern Benue Trough 385

with application to the Mendocino submarine fracturezone; J. Geophys. Res. 64 49–59.

Thompson D T 1982 EULDPH – A new technique formaking computer-assisted depth estimates from magneticdata; Geophysics 47 31–37.

Thurston J, Guillion J C and Smith R S 1999 Model-

independent depth estimation with the SPITM method;69th Annual International Meeting, SEG, ExpandedAbstracts, pp. 403–406.

Thurston J, Smith R S and Guillion J C 2002 A multi-model method for depth estimation from magnetic data;Geophysics 67 555–561.

Umeji O P 2007 Late Albian to Campanian palynostratigra-phy of southeastern Nigerian sedimentary Basins; Unpub-lished Ph.D Thesis, Department of Geology, University ofNigeria, Nsukka, 280p.

Whiteman A J 1982 Nigeria: Its petroleum geology resour-ces and potentials; Graham and Trotman, London,394p.

Whitehead N and Musselman C 2008 Montaj Grav/MagInterpretation: Processing, Analysis and VisualizationSystem for 3D Inversion of Potential Field Data for Oasismontaj v6.3, Geosoft Incorporated, 85 Richmond St. W.,Toronto, Ontario, M5H 2C9, Canada.

MS received 30 January 2015; revised 29 October 2015; accepted 8 November 2015