Estimation of Magnetic Basement Depths Beneath the Abeokuta Area, South West Nigeria from Aeromagnetic Data Using Power Spectrum
This study presents the result of spectral analysis of aeromagnetic data of Abeokuta area, a basement complex of South-western Nigeria. Aeromagnetic survey is a powerful tool in delineating the regional geology of buried basement terrain. The purpose of magnetic surveying is to investigate the subsurface based on the variation in the observed magnetic field result from the differences in the magnetic properties of the underlying rocks, or in some cases cultural sources. In order to achieve the aim of this study, matched pass filtering operation was performed on digitised aeromagnetic data of the area under consideration to separate magnetic anomalies produce by shallow features and cultural features separated from anomalies produced by deeper geologic unit. Two-dimensional spectral analysis revealed that the magnetic sources are mainly distributed at two levels. The shallow source is depth 0.467 km below ground level is inferred to be due to the intrusions within the region. The deeper sources depth, which is 2.797 km below ground, is attributed to the underlying basement. The mean shallow depth and deeper depth are 0.467 km and 2.797 km, respectively. The shallow depth of 0.467 km might probably due to intrusive within the sediment while deeper depth of 2.797 km is attributed to underlying basement. Variation in magnetic depth values reveals the general trends in the magnetic basement surface.
Received: November 22, 2012;
Accepted: February 01, 2013;
Published: March 19, 2013
The early use of potential field methods in petroleum was to map sedimentary
basin thickness Airborne geophysical surveys are an extremely important aspect
of modern geophysics. Compared with ground surveys airborne surveys allow faster
and usually cheaper coverage, of large areas. At the largest reconnaissance
scale, the most common airborne surveys are aeromagnetic surveys, as used in
government reconnaissance surveys. Until the last couple of decades the other
primary application of airborne surveys was in mineral exploration (Nabighian
et al., 2005). Over the last decade there has been increase in the use
of airborne magnetic and more recently, gravity in the petroleum exploration
industry but high-resolution surveys are used to investigate basement trends
and intra-formational structures. High resolution methods are now being applied
in the groundwater, environmental and engineering studies (Nabighian
et al., 2005; Grauch et al., 2006).
Magnetic method is one of the most economical geophysical techniques to delineate
the subsurface structures. Generally, aeromagnetic anomaly maps reflect the
lateral variations in the earth's magnetic field. These variations are related
to changes of structures, magnetic susceptibility or remanent magnetization.
It was observed that sedimentary rocks have low magnetic properties compared
to metamorphic and igneous rocks, which have greater magnetic properties (intensity
and susceptibility). Therefore airborne magnetic surveys are useful to map geologic
structure on or inside the basement rocks or to detect magnetic minerals directly.
Previous study has shown that Abeokuta is underlain by Precambrian rocks typical
of the basement Complex of Nigeria (Rahaman, 1976). Some
of the main rock types found in this area are granite-gneiss. This study deals
with an estimation of shallow (residual) and deeper (regional) depth from the
observed digitized aeromagnetic data of Abeokuta area using power spectral techniques.
Thus, spectrum is a transformation of data from time or space domain to frequency
or wave number domain respectively (Igboama and Ugwu, 2004).
Location of the study area: The study was carried out in Abeokuta area of Ogun State, South western Nigeria. The area is located within longitude 3°00 E to 3° 30 E and latitude 7°00 N to 7°30 N covering an area 55x55 km, which is 3,025 km2. Ogun state is bounded in the west by Benin Republic, in the south by Lagos, in the north by Oyo/Osun and in the east by Ondo State.
Abeokuta is one of the most prominent urban settlements in the South-western
Nigeria. The gneiss-migmatite complex is the most widespread rock formation
within the study area. It comprises gneisses, quartzite, calcsilicate, biotite-hornblende
schist and amphibolites (Rahaman, 1976). The older granites
and around the Abeokuta, are of late Precambrian to early Palaeozoic in age
and are magmatic in the origin (Jones and Hockey, 1964).
Abeokuta falls within the basement complex of the geological setting of south-western
Nigeria (Fig. 1). The basement complex rocks of Pre-Cambrian
age are made up of older and younger granites, with the younger and older sedimentary
rocks of the both tertiary and secondary ages. The area is underlain by basement
rocks, which cover about 40% of landmass in Nigeria (Fig. 2)
Aeromagnetic data: Abeokuta area is covered by an aeromagnetic survey conducted by Nigeria Geological Survey Agency of Nigeria in 2006. The aeromagnetic data were obtained using a proton precession magnetometer with a resolution of 0.01 nT. Fugro Airborne Surveys carried out the airborne geophysical work. Aeromagnetic surveys were flown at 500 m-line spacing and 80 m-terrain clearance. The flight line direction was in the direction 135 azimuths while the tie line direction was in 45 azimuths. The average magnetic inclination and declination across the survey was 9.750 and 1.300, respectively. The geomagnetic gradient was removed from the data using International Geomagnetic Reference Field (IGRF).
Power spectrum: Spectral analysis of potential field data has been used
extensively over the years to derive depth to certain geological features (Spector
and Grant, 1970; Hahn et al., 1976; Connard
et al., 1983; Gracia-Abdeslem and Ness, 1994)
or the curie-temperature isotherm, (Okubo and Matsunaga,
1994; Shuey et al., 1977; Blakely,
1988). Spectral analysis is the process of calculating and interpreting
the spectrum of the potential field data. The spectral depth method is based
on the principle that a magnetic field measured at the surface can be considered
as an integral of magnetic signature from all depths (Rabeh,
2009). The power spectrum of a surface field can be used to identify average
but maximum depth of source ensemble (Spector and Grant,
|| Geological map of Ogun State showing the study area
|| Basement geological map of Nigeria, (Obaje,
2009) geological setting
Hence, spectral analysis method is suitable in providing the average depth value to the top of statistical ensemble of blocks of anomalous bodies. These anomalous sources can be interpreted in terms of subsurface structures.
Indirect interpretation, the information such as the maximum depth at which
the body could lie and depth estimates of the centre of the body is obtained
directly from the magnetic anomaly map. Inherent ambiguity may lead to infinite
number of different configurations that may result in identical magnetic anomalies
at the surface. Many researchers have used the calculation of the power spectrum
from the Fourier coefficients to obtain the average depth to the disturbing
surface or equivalently the average depth to the top of the disturbing body
(Spector and Grant, 1970). It is necessary to define
the power spectrum of a magnetic anomaly in relation to the average depth of
the disturbing interface. It is also important to point out that the final equations
are dependent on the definition of the wave number in the Fourier transform.
For an anomaly with n data points the solution of Laplace equation in 2-D is:
where, wave number k is define as k = 1/λ and Ak is therefore the amplitude coefficient of the spectrum:
for z = 0 Eq. 2, can be written as:
Then Eq. 2 can be rewritten as:
Then the power spectrum Pk is defined as:
The plot of log P against frequency reflects the average depth to the disturbing interface. The interpretation requires the best-fit line through the lowest frequency of the spectrum.
Therefore, the average depth can be estimated from the plot of Eq.
where, h = Depth to the magnetic source (Albora and Ucan,
RESULTS AND DISCUSSION
The total magnetic intensity (Fig. 3) over Abeokuta Area
showed magnetic signature ranging from-10 nT to 102 nT. The magnetic high of
magnitude 102 nT observed in some part of Northwestern and Northeastern part
of the study area which could be as a result of presence of gneiss-migmatite
complex is the most widespread rock formation in the area under consideration.
These compared favorably well with geologic map on Fig. 1.
The variation in the magnitude of the earths magnetic field (Fig.
3) is to detect local changes in the properties of the underlying geology
and also show that there is sizeable quantity of different magnetic deposit
structures on the location.
||Total intensity magnetic map of the study area (Long-wavelength
magnetic anomalies are produced by regional geologic sources, cultural features
and shallow geologic sources produce short-wavelength anomalies)
Figure 3 shows combination signals of regional, residual
and noise but with the help of matched filtering, separation of signals was
made possible as can be seen from Fig. 4 and 5,
In Fig. 4 there is a noise layer containing low-amplitude, very short-wavelength magnetic noise largely unrelated to geologic sources, a layer corresponding to near-surface magnetic sources, and Fig. 6 is an excellent representation of the field without the noise and contains the magnetic anomalies from the deepest and broadest features of the geology.
Graphs of the logarithms of the spectral energies against frequencies were
plotted as in Fig. 6, from which depths were computed. This
shows that the magnetic anomalies originate at two distinct mean depth levels.
Two linear segments can be drawn from each graph: regional and residual components.
The gradient of each linear segments were evaluated using equation
8 (Albora and Ucan, 2008) and was used to calculate
the depth to the causative bodies (deep and shallow sources); where h and m
are the depth and gradient, respectively.
||Residual field of the total magnetic intensity map of Abeokuta
(contains magnetic anomalies produced by shallow geologic sources and cultural
||Regional field of the total magnetic intensity map of Abeokuta
(contains the magnetic anomalies from the deepest and broadest features
of the geology)
||Radially averaged power spectrum for the magnetic data (curves
representing the power spectra of simple equivalent magnetic layers)
Depth estimates from spectral analysis of magnetic data along the profile indicate
a two-depth source model. This is in agreement with earlier study by Bansal
et al. (2010), where the application of scaling spectral method
on Bouguer anomaly of Kucth indicates variation in the depth of anomaly sources.
The mean depth of the deeper sources (2.797 km) could be identified with the
basement while the mean depth of the shallower sources (0.426 km) could be identified
as near surface intrusive and local changes on the earth surface. The magnetic
basement depth range from 0.467-2.797 km compare well with what obtained within
Ibadan area by Olowofela et al. (2011) because
airborne magnetic data of Abeokuta dovetail into part of Ibadan. The result
obtained is in support with the earlier study by Kasidi
and Ndatuwong, 2008 on aeromagnetic data over Longuda plateau and environs
where the mean depth of shallow sources was 0.591 and 2.26 km for the depth
to deeper magnetic sources. Hassanein (2001) used Filon
Fourier spectral analysis and obtained 0.7 and 6.0 km for residual and regional
However, the study carried out by Nwankwo et al.
(2008) on Sedimentary Formation of Northern Nupe basin found depth to magnetic
basement to vary from 0.52-4.38 km while depth range of 0.24-1.74 km was attributed
to shallow sources. Also, Onuba et al. (2011)
evaluate aeromagnetic anomalies over okigwe Area, South-eastern Nigeria using
Half-slope method to obtained on the average depth of the deeper magnetic sources
ranging from 2.0-4.99 km while the shallow magnetic sources ranges from 0.4-1.99
The results of this spectral analysis show clearly the variation along profiles in the surface of magnetic basement across the study area. The depth of the deeper sources 2.797 km and is believed to correspond to the surface of the magnetic basement in the study area. The shallower depth, 0.426 km, may refer to some major magnetic units, uplifted basement surface as well as to some local magnetic features.
These results therefore demonstrate the applicability of the spectral method of magnetic interpretation in estimating depths to the surface of magnetic basement in a basement complex.
The authors appreciate the Nigerian Geological Survey Agency, Abuja Nigeria for providing the Aeromagnetic data for this research work.
1: Albora, A.M. and O.N. Ucan, 2008. Gravity anomaly separation using 2-D wavelet approach and average depth calculation. Geophysics, 58: 395-404.
Direct Link |
2: Bansal, A.R., V.K. Rao, V.P. Dimri and K.K. Babu, 2010. Heterogeneity in kutch (India) an intraplate seismic region from gravity data. Proceedings of the EGM International Workshop, April 11-14, 2010, Capri, Italy -
3: Blakely, R.J., 1988. Curie temperature isotherm analysis and tectonic implications of aeromagnetic data from Nevada. J. Geophys. Res., 93: 11817-11832.
4: Connard, G., R. Couch and M. Gemperle, 1983. Analysis of aeromagnetic measurements from the cascade range in central oregon. Geophysics, 48: 376-390.
CrossRef | Direct Link |
5: Grauch, V.J.S., M.R. Hundson, A.M. Scot and J.S. Caine, 2006. Source of along-strike variation in magnetic anomalies related to ultra sedimentary faults: A case study from Rio Granda Rifts, USA. Explor. Geophys., 37: 372-378.
Direct Link |
6: Hassanein, H., 2001. Using filon fourier spectral analysis technique for filtering of aeromagnetic data and outlining subsurface structures of helma-madrakah area kingdom of Saudi Arabia. JKAU: Earth Sci., 12: 105-125.
Direct Link |
7: Igboama, W.N. and N.U. Ugwu, 2004. Basement depths in the anambra basin determined by one-dimensional spectral analysis of aeromagnetic data. J. Applied Sci., 7: 4411-4418.
8: Jones, H.A. and R.D. Hockey, 1964. The geology of parts of south western Nigeria. Bull. Geol. Surv. Nigeria, 31: 101-101.
9: Kasidi, S. and L.G. Ndatuwong, 2008. Spectral analysis of aeromagnetic data over longuda plateau and environs North-Eastern Nigeria. Continental J. Earth Sci., 3: 28-32.
10: Nabighian, M., V. Grauch, R. Hansen, T. Lafehr and Y. Li et al., 2005. The historical development of the magnetic method in exploration. Geophysics, 70: 33ND-61ND.
CrossRef | Direct Link |
11: Obaje, N.G., 2009. Geology and Mineral Resources of Nigeria. Springer, Berlin, Germany, ISBN-13: 9783540926849, Pages: 221
12: Okubo, Y. and T. Matsunaga, 1994. Curie point depth in Northeast Japan and its correlation with regional thermal structure and seismicity. J. Geophys. Res., 99: 22363-22371.
13: Olowofela, J.A., B.S. Badmus, S.A. Ganiyu, O.T. Olurin and P. Babatunde, 2011. Source location and depth estimation from digitized aeromagnetic data acquired from a basement complex formation. Earth Sci. India, 4: 136-142.
Direct Link |
14: Onuba, L.N., G.K. Anudu, O.I. Chiaghanam and E.K. Anakwuba, 2011. Evaluation of aeromagnetic anomalies over Okigwe area South-Eastern Nigeria. Res. J. Environ. Earth Sci., 3: 498-507.
Direct Link |
15: Rahaman, M.A., 1976. A Review of the Basement Geology of South Western Nigeria. In: Geology of Nigeria, Kogbe, C.A. (Ed.). Elizabethan Publishing, Surulere, Lagos State, Nigeria, pp: 41-58
16: Shuey, R.T., D.K. Schellinger, A.C. Tripp and L.B. Alley, 1977. Curie depth determination from aeromagnetic spectra. Geophys. J. Int., 50: 75-101.
CrossRef | Direct Link |
17: Spector, A. and F. Grant, 1970. Statistical models for interpreting aeromagnetic data. Geophysics, 35: 293-302.
CrossRef | Direct Link |
18: Rabeh, T., 2009. Prospecting for the ferromagnetic mineral accumulations using the magnetic method at the Eastern Desert, Egypt. Geophys. Eng., 6: 401-411.
19: Gracia-Abdeslem, J. and G.E. Ness, 1994. Inversion of power spectrum from anomaly. Geophysics, 59: 381-401.
20: Hahn, A., E. Kind and D.C. Mishra, 1976. Depth Estimation of magnetic source by means of fourier amplitude spectral. Geophys. Prospect., 24: 287-305.
21: Nwankwo, D.I., T.I. Owoseni, D.A. Usilo, I. Obinyan, A.C. Uche and I.C. Onyema, 2008. Hydrochemistry and plankton dynamics of Kuramo lagoon. Life Sci. J., 5: 50-55.