A case study of aeromagnetic data interpretation of Nsukka area, Enugu State, Nigeria, for hydrocarbon exploration

The aeromagnetic data of Nsukka area was interpreted qualitatively and quantitatively. Standard Euler deconvolution, Source Parameter Imaging (SPI), Forward and Inverse modeling techniques were employed in quantitative interpretation with the aim of determining depth/thickness of the sedimentary Basin, magnetic susceptibilities and type of mineralization prevalent in the area. Oasis montaj 6.4.2 software and potent Q 4.10.07 software were employed in the data analysis. Forward and inverse modeling estimated depths for profiles 1, 2, 3, 4 and 5 were 1644, 2285, 1972, 2193 and 1200 m respectively, with respective susceptibility values of 0.0031, 0.0073, 1.4493, 0.0069 and 0.0016 which indicate dominance of iron rich minerals like limonite, hematite,pyrrhotite and pyrite, and forms lateritic caps on sandstones. Results from SPI estimated depth ranges from 151.6 m (outcropping and shallow magnetic bodies) to 3082.7 m (deep lying magnetic bodies). Depths of shallow magnetic sources resulting from lateritic bodies in the outcrops within the study area as estimated by Euler depths for the four different structural index (SI = 0.5, 1, 2, 3) ranges from 7.99 to 128.93 m. 35 to 150 m depth are good potential water reservoirs for Nsukka and environs. Depths of 1644 to 3082.7 m show sufficiently thick sediments suitable for hydrocarbon accumulation.


INTRODUCTION
The search for mineral deposits and hydrocarbon has been a major business challenge in Nigeria since the precolonial era and the 1960's respectively.The bedrock of Nigeria's economy before the discovery of oil had been the solid minerals and agricultural sectors, but currently, it is the oil and gas sector.Over 80% of the country's revenue comes from export and domestic sales of oil and gas.As the hydrocarbon potentials of the prolific Niger Delta becomes depleted or in the near future may be exhausted due to continuous exploitation, attention needs to be shifted to other sedimentary Basins.The lower Benue Trough, Nsukka in particular, is one of those  (Nwachukwu, 1972).
Basins being suspected to have high hydrocarbon potential, besides other economic mineral deposits concentration.The Benue Trough of Nigeria is a major tectonic feature in West Africa.It is an elongated rifted depression that trends NE-SW from the south, where it merges with the Niger Delta, to the north, where its sediments are part of the Chad Basin successions.The Benue Trough attains an approximate length of 800 km.It has a width of 130 to 150 km and is filled with Cretaceous sediments whose ages range from Middle Albian to Maestrichtian.Their thickness varies from about 6 km in the Lower Benue to about 5 km in the Upper Benue.The origin and evolution of the Benue Trough of Nigeria is fairly documented (Wright, 1976;Petters, 1978;Ofoegbu, 1985).Generally, the Benue Trough is believed to have been formed when the South America separated from Africa (Petters, 1978).The major component units of the Lower Benue Trough include the Anambra Basin, the Abakaliki Anticlinorium and the Afikpo Syncline.
The Nsukka area is a sedimentary Basin within the Anambra Basin which is part of the lower Benue Trough.There was a speculation in 2007 that a part of Nsukka area (Towns within Uzo-Uwani, Opi Agu and Eha-Ndiagu, Figure 1) has a good quantity of oil.An Oil company visited the area and carried out a preliminary survey work.Though their report is not yet made known to the public, but some geoscientists believe that the area contains more gas than oil.
There are some published works in the lower Benue Trough based on aeromagnetic data interpretations which employed different methods (Ofoegbu, 1984;Okeke, 1992;Onwuemesi, 1997;Ugbor and Okeke, 2010;Onu et al., 2011;Onuba et al., 2011;Ugwu and Ezema, 2012;Adetona and Abu, 2013;Igwesi and Umego, 2013;Ugwu et al., 2013;Ezema et al., 2014).The publications are based on the depths to basements/magnetic source bodies over the Lower Benue Trough and Anambra Basin in which Nsukka and environs falls, though no specific work had been carried out on Nsukka area.Ofoegbu (1984) interpreted the aeromagnetic anomalies over the Lower and Middle Benue Trough in terms of basic intrusive bodies which may occur either within the Cretaceous rocks or within the basement or both.He obtained a depth ranging from 0.5 to 7 km in the lower and middle Benue Basin and concluded that, although some of the observed anomalies could be attributed to the basement underneath the Benue Trough, the magnetic anomalies could not be satisfactorily interpreted fully in terms of a basement of variable topography.Such an interpretation leads to a basement of too high a magnetization, basement outcrops and too thick a sedimentary cover in places not in agreement with the known geology of the area.The results of his study suggested that the magnetic anomalies over the Lower and Middle Trough are in the main caused by sizeable intrusive bodies of basic composition which can either lie within the sedimentary rocks or within the basement or both.The magnetic anomalies over the Lower and Middle Benue Trough are therefore due to the combined effect of a basement of variable thickness and topography and basic intrusive bodies.Onwuemesi (1997) evaluated the depth to the basement (sedimentary thickness) in the Anambra Basin which is part of the lower Benue Trough to vary from 0.9 to 5.6 km.In the work of Igwesi and Umego (2013), they interpreted the aeromagnetic anomalies over some part of lower Benue Trough, using Spectral analysis which indicated a two layer source model with depth for deeper magnetic source ranging from 1.16 to 6.13 km with average of 3.03 km and depth to the shallower magnetic source ranging from 0.016 to 0.37 km with average of 0.22 km.Onu et al. (2011) estimated that the depths to the magnetic source bodies in the lower Benue Trough and some adjoining areas vary from 0.518 to 8.65 km with a mean depth of 3.513 km (for deeper magnetic source bodies) and 0.235 to 3.91 km with a mean depth of 1.389 km (for shallower magnetic source bodies).Onuba et al. (2011) interpreted the aeromagnetic anomalies over Okigwe area (Okigwe is within the Lower Benue Trough) and estimated the depth to basement using slope methods.They established two depth models varying from 2.18 to 4.91 km for deeper sources while the shallower sources vary from 0.55 to 1.82 km.However, they did not recommend hydrocarbon exploration in the area since the area has low thickness of sediments on the average.Adetona and Abu (2013) estimated the thickness of sedimentation within the Lower Benue Basin and Upper Anambra Basin.They employed spectral depth analysis which used the radial average energy spectrum to obtain a depth of 7.3 km and source parameter imaging to obtain a depth of 9.847 km.Also, Ezema et al. (2014) used forward and inverse modeling (potent software) to interpret the aeromagnetic data of Abakaliki which showed maximum depths of 4.96 to 9.8 km with minimum depths ranging from 0.12 to 0.17 km.These studies were carried out within the Lower Benue Trough in which Nsukka area falls, but none of the studies was carried out specifically in Nsukka area.This prompted our carrying out this work in Nsukka area, hence the first of its kind in that area.
The purpose of this study is to interpret qualitatively and quantitatively the aeromagnetic data of Nsukka area in southeastern Nigeria, using the standard Euler deconvolution, source parameter imaging (SPI) and forward and inverse modeling methods.These will include the determination of: (i) the susceptibilities of rock types in the area, (ii) the depth of the anomalous bodies, (iii) the dip, plunge and type of body causing the magnetic anomaly, and (iv) possible mineralization in the area.The results of this study will be compared with the results of previous works carried out in the lower Benue Trough and it will throw more light to the knowledge of subsurface structure in Nsukka area.

Geology and stratigraphy of the study area
The Nsukka sheet (covering the aeromagnetic data of Nsukka area) lies between latitudes 6°30΄ and 7°00ʹ North and longitudes 7°00ʹ and 7°30ʹ East.It covers a total surface area of approximately 3,961 km 2 .Nsukka is a sedimentary Basin within the Anambra Basin underlain by rocks which range in age from Coniacian to Paleocene.Anambra Basin consist of six major rock formations namely Enugu shale, Agwu shale, Mamu formation, Ajali formation, Nsukka formation and Imo shale formation.These rocks are grouped into four formations namely: Mamu formation, Ajali Sandstone, Nsukka Formation, and Imo Shale Formation (Nwajide and Reijers, 1995;Onwuemesi, 1995).Sediments deposited within this time interval occur in four distinct physiographic provinces, namely the Cross River plains, the Escarpment, the Plateau and the Anambra Plains.It was discovered that over 3,965 m of sediments comprising shales, sandstones, limestone and coal, were deposited in the area.Their environment of deposition varied from marine, through brackish water to entirely continental.Some of these sediments are of considerable economic importance and contain reserves of coal, natural gas, glass sands and considerable prospects for liquid hydrocarbon.The soil is rich and sustains a virile rural agriculture.The highest point in the area is about 590 m above datum plane but the lowlands generally have heights below 250 m above sea level.There are several elongated and conical hills separated by dry and wet valleys.

Source of data
Data covering the Nsukka sheet was acquired from the Nigerian Geological Survey Agency.This survey was conducted in three phases between 2005 and 2010 as part of a major project known as the Sustainable Management for Mineral Resources by FURGO Airborne surveys.The survey has a Tie-line spacing of 500 m, the flight line along East-West direction with 100 m spacing, altitude of 80 m and terrain clearance of 100 m.Nsukka data was recorded in digitized form (X, Y, Z text file) after removing the geomagnetic gradient from the raw data using International Geomagnetic Reference Field (IGRF), 2010, with intensity of 33095 nT (nanoTesla), angles of inclination and declination of -13.988° and -1.902°, respectively.The X and Y represent the longitude and latitude of Nsukka in meters respectively, while the Z represents the magnetic field intensity measured in nanoTesla.

METHODS AND DATA ANALYSIS
Qualitative as well as quantitative interpretations were employed in this work.Qualitative interpretation of the field data was first carried out by inspecting the total magnetic intensity (TMI) grid of the study area.The total magnetic intensity map of the area was produced into maps which are in colour aggregate.Source Parameter Imaging (SPI), standard Euler deconvolution and Forward and Inverse modeling methods were employed in quantitative interpretation.
The initial stages of quantitative magnetic data interpretation involved the application of mathematical filters (reduction to pole, upward-downward continuation, first vertical derivative and horizontal derivative) to observed data.The specific goals of these filters vary, depending on the situation.The general purpose is to enhance anomalies of interest and to gain some basic information on source location or magnetization.The upward projection (upward continuation) operation smoothen the anomalies obtained at the ground surface by projecting the surface mathematically upward above the original datum (Revees, 2005).By implementation of reduction to pole on both the amplitude and phase spectra of the original TMI grid, the shapes of the magnetic anomalies were simplified so that they appeared like the positive anomalies located directly above the source expected for induce magnetized bodies at the magnetic pole where the angle of inclination is 90° and zero declination.
A derivative helped to sharpen the edges of anomaly and enhanced shallow features (Revees, 2005).This includes first and second vertical derivatives, and horizontal derivative.Computation of the first vertical derivative in an aeromagnetic survey is equivalent to observing the vertical gradient with a magnetic gradiometer with advantages of sharpening the edges of magnetic anomalies, enhancing shallow magnetic sources, suppressing deeper magnetic sources and giving a better resolution of closelyspaced sources.Horizontal derivative was also calculated in the x and y directions.
Source parameter imaging, standard Euler deconvolution, forward and inverse modeling were used to evaluate the depths of magnetic source bodies.Source parameter imaging (SPI) method calculated source parameters for gridded magnetic data.The method assumes either a 2D slopping contact or a 2D dipping thinsheet model and is based on the complex analytic signal.The SPI depth of magnetic data was determined using Oasis Montaj software and employing the first vertical derivatives and horizontal gradient.SPI method made the task of interpreting magnetic data significantly easier.This model was displayed on an image and the correct depth estimate for each anomaly determined.The Euler deconvolution utilizes Euler's homogeneity relation proposed by Thompson (1982) and (Reid et al., 1990).It is a valuable tool for locating the position and depth of anomalous sources.OASIS MONTAJ software was employed in computing the Euler-3D image and depth.
Potent Q 4.10.07software was used for the modeling and inversion of the anomalies after getting preliminary information about anomaly causative sources.Potent is a program for modeling the magnetic and gravitational effects of subsurface.It provides a highly interactive 3-D environment that, among other applications, is well suited for detailed ore body modeling for mineral exploration.The main concepts in potent Q 4.10.07include: Observation, inversion, model, visualization and calculation.Interpretation of magnetic field data using potent software started with observation of the image of the observed data.The first step in interpreting the observed data was to take profiles on the field image.In interpreting the observed data, five profiles were taken along different parts of the field image.

RESULTS
The total magnetic intensity map of Nsukka area was produced into map (Figure 3) from the qualitative interpretation, which is in different colour aggregate.The magnetic intensity of the area ranges from -74.72 nT to 147.24 nT.The area is marked by the high (pink colour) and low (blue colour) magnetic signatures.The variation in magnetic intensity could be as a result of degree of strike, variation in depth, difference in magnetic susceptibility, difference in lithology, dip and plunge.
The total magnetic intensity data was projected 500 m above the original datum plane.The upward continuation of the total magnetic field intensity (TMI) map is shown in Figure 4.The magnetic intensity of the area ranges from a minimum value of -65.82 nT to a maximum value of 137.95 nT as shown in the upward continuation map (Figure 4).
Figure 5 shows the reduction to pole of the upward continuation of the TMI grid.The magnetic intensity of the area ranging from -136.13 to 227.73 nT was obtained   after reducing the upward continuation (UC) TMI map to magnetic pole.The filtered TMI grid was used for quantitative interpretation of depths to magnetic anomalies.
In computing the SPI depth of magnetic data, Oasis Montaj software was employed.Using the first vertical derivatives and horizontal gradient, the SPI depth was computed.Different magnetic depths and susceptibilities contrast within the study area are indicated by the gridded SPI depth map and legend.Figure 6 (derivative grids) shows that the magnetic bodies are not restricted to a particular location.In Figure 7, the negative depth values shown in the SPI legend depicts the depths of buried magnetic bodies, which may be deep seated basement rocks or near surface intrusive while the positive values depicts outcropping magnetic bodies.The pink colour generally indicates areas occupied by shallow magnetic bodies, while the blue colour depicts areas of deep lying magnetic bodies ranging from -379.43 to -3082.7 m.SPI depth result generally ranges from 151.6 m (outcropping and shallow magnetic bodies) to -3082.7 m (deep lying magnetic bodies).In computing the Euler depth and to produce the Euler depth map and legend, Standard Euler deconvolution (Euler 3-D) interpretation was carried out in three dimensions by employing Oasis montaj software.For four different stuctural index (SI = 0.5, 1, 2, 3), four Euler 3D maps were generated as shown in Figure 8(a, b, c, d).There is no Euler solution (depth) for the particular structural index used as indicated by the areas in the maps without magnetic signatures or colour (depth).The pink colour indicates shallow magnetic bodies, while the blue colour indicates deep lying magnetic bodies (Figure 8).Depths to lateritic bodies and outcrops in the study area that are magnetic because they contain ironstone are signified by positive depth values, while the negative depth values are depths of shallow magnetic bodies below the datum plane The Euler 3D depth grid for structural index 0.5, 1, 2, and 3 is shown in Figure 8. Depth ranges for SI = 0.5 is from 9.47 to 124.02 m; for SI = 1, depth ranges from 22.51 to 125.29 m; for SI = 2, depth ranges from 39.05 to 120.30 m; and for SI = 3, depth ranges from 7.99 to 128.93 m.The Euler depths were estimated using vertical derivatives in three dimension (x, y, z).Vertical derivatives enhances shallow magnetic bodies.Hence, depths of shallow magnetic bodies or anomalies for different structural index are displayed by Euler method.
Using a single component data (TMI) for multiple bodies, the inversion procedure was performed.The geographic coordinates (X, Y and Z) at which the software (Potent Q) calculated the field due to the model were provided by the data points in the sample of observed data.They further provided the observed field values against which the calculated field values were compared.The root mean square (RMS) difference between the observed and calculated field values was attempted to be minimized by the inversion algorithm.At the end of the inversion, the RMS value was displayed.The RMS value decreased as the fit between the observed and calculated field continues to improve, until a reasonable inversion result was achieved.Less than 5% of root mean square value was set as an acceptable error margin.Five profiles were taken in this modeling and each profile was expected to have a degree of strike, dip and plunge where the observed values matched well with the calculated values.The blue curves in Figure 10 (a, b, c,  d, e) represent the observed field values while the red curves represent the calculated field values.The forward modeling being a trial and error method, the shape, position and physical properties of the model were adjusted in order to obtain a good correlation between the calculated field and the observed field data.Using potent software, the field of the model was then calculated.Five bodies were modeled and using the  upward continuation TMI dataset, inversion method was performed.The potent software used geographic coordinates x, y and z provided by the TMI grid with the field values of the observed field to compare the calculated field values.The difference between the calculated and observed field values is represented by a root mean square value (RMS) which is displayed at the end of each inversion.The RMS value is less than 5% in the inversion model which helped in getting an improved result and match between the observed and calculated field.The five model profiles taken from the upward continuation TMI grid (Figure 9) are shown in Figure 11  (a, b, c, d, e).The shape of the body causing the anomaly is a sphere and its measurement was given in   terms of radius (3610 m) as shown in the model profile in Figure 11d.Therefore, its ambiguous values of length and height were not considered.The results of the forward and inverse modeling are summarized in Table 1.The susceptibility values obtained from the model profiles 1, 2, 3, 4 and 5 are 0.0031, 0.0073, 1.4493, 0.0069 and 0.0016 respectively, with respective depths of 1644,2285,1972,2193 and 1200 m.Dominance of iron rich minerals like limonite, haematite, pyrrhotite and pyrite are indicated by the susceptibility values and are typically sandstones and ironstones or oxides of iron which are magnetic in nature.Table 2 shows the average magnetic susceptibilities of some mineral (Telford et al., 1990).

DISCUSSION
The depths estimated for shallow magnetic bodies and deep lying magnetic bodies using Standard Euler deconvolution, source parameter imaging (SPI) and forward and inverse modeling methods are within the same range.The depths deduced from the forward and inverse modeling are 1644,2285,1972,2193 and 1200 m for profiles 1, 2, 3, 4 and 5 respectively (Figure 11a (Telford et al., 1990).Wright et al. (1985) that the minimum thickness of the sediment required for the commencement of oil formation from marine organic remains would be 2300 m (2.3 km).

Rocks
Euler depths for the four different structural index (SI = 0.5, 1, 2, 3) ranges from 9.47 to 128.93 m which are depths of shallow magnetic sources resulting from lateritic bodies in the outcrops within Nsukka area and is within the range of depth for shallow magnetic source bodies as estimated using SPI method.Depths of 35 to 150 m as obtained in this work are good potential water reservoirs in Nsukka area.This is consistence with the assertion of Anomohanran (2013), and Ezeh and Ugwu (2010) that the depth of fresh and good quality aquifer reservoirs in Nsukka area is from 33 to 150 m.

Comparison with some aeromagnetic studies carried out in the Lower Benue Trough
Comparing the results obtained in this work with the results obtained by other researchers in the Lower Benue Trough, Onwuemesi (1997) evaluated the depth to the basement (sedimentary thickness) in the Anambra Basin to vary from 0.9 to 5.6 km.His depth is slightly higher than the depths obtained from this study.Igwesi and Umego (2013) obtained a two layer source model with depth for deeper magnetic source ranging from 1.16 to 6.13 km with average of 3.03 km and depth to the shallower magnetic source ranging from 0.016 to 0.37 km with average of 0.22 km.Their estimated depth agrees with the depth estimated from this study especially within the shallow source.Onu et al. (2011) estimated that the depths to the magnetic source bodies in the lower Benue Trough and some adjoining areas vary from 0.518 to 8.65 km with a mean depth of 3.513 km (for deeper magnetic source bodies) and 0.235 to 3.91 km with a mean depth of 1.389 km (for shallower magnetic source bodies), which are greater than the depth estimated from this work.Onuba et al. (2011) estimated the depth to basement over Okigwe areas and established two depth models varying from 2.18 to 4.91 km for deeper sources while the shallower sources vary from 0.55 to 1.82 km.However, they did not recommend hydrocarbon exploration in the area since the area has low thickness of sediments on the average.Adetona and Abu (2013) estimated the thickness of sedimentation within the Lower Benue Basin and Upper Anambra Basin by employing spectral depth analysis and obtained a depth of 7.3 km and source parameter imaging to obtain a depth of 9.847 km.This result is also greater than our result.Ezema et al. (2014) estimated maximum depths of 4.96 to 9.8 km and minimum depths of 0.12 to 0.17 km over Abakaliki area.Their estimated depth agrees with our depth for the shallow source.The results of some of these works agree with the results of the present study especially within the shallow source.It should be noted that none of these works were carried out in Nsukka and its environs, but in other parts of Lower Benue Trough.

Conclusion
Interpretation of aeromagnetic data of Nsukka area has been done qualitatively and quantitatively.Source parameter imaging (SPI), standard Euler deconvolution and forward and inverse modeling methods were employed in quantitative interpretation.The estimated depths from the forward and inverse modeling method are 1644,2285,1972,2193 and 1200 m for profiles 1, 2, 3, 4, 5 (Figure 11a, b, c, d and e) respectively.The susceptibility values obtained from the model profiles 1, 2, 3, 4 and 5 are 0.0031, 0.0073, 1.4493, 0.0069 and 0.0016 respectively which indicate dominance of iron rich minerals like limonite, hematite, pyrrhotite, and pyrite and forms lateritic caps on sandstones.Depth result from SPI ranges from 151.6 m (outcropping and shallow magnetic bodies) to -3082.7 m (deep lying magnetic bodies).Sediment thickness of 1644 m to 3082.7 m is sufficient for hydrocarbon accumulation.This agrees with the work of Wright et al. (1985) which asserts that the minimum thickness of the sediment required for the commencement of oil formation from marine organic remains would be 2300 m (2.3 km).From Euler depth results, the depth of contact body ranges from 9.47 m to 124.02 m for SI = 0.5; depth of dyke, silt and thin sheet body ranges from 22.51 m to 125.29 m for SI = 1; depth of line source body ranges from 39.05 m to 120.30 m for SI = 2; and the depth of spherical and compact body like ellipsoid ranges from 7.99 m to 128.93 m for SI = 3. Depths of shallow magnetic sources resulting from lateritic bodies in the outcrops within the study area ranges from 9.47 to 128.93 m (Euler depth for the four different structural index) and is within the range of depth for shallow magnetic source bodies as estimated using SPI method.35 to 150 m depths are good potential water reservoirs (Anomohanran, 2013;Ezeh and Ugwu, 2010) within Nsukka area.The depths obtained in this work fairly agrees with the depths estimated by many previous researchers (Ofoegbu, 1984;Onwumesi, 1997;Onu et al., 2011;Onuba et al., 2011;Igwesi and Umego, 2013;Adetona and Abu, 2013;Ezema et al., 2014) in lower Benue Trough and Anambra Basin in which Nsukka area falls.Results from SPI and modeling techniques show that Nsukka area has sufficiently thick sediments suitable for hydrocarbon accumulation.This study is a case study which provides a good example of the use of aeromagnetic data for basinal studies in a structurally complex mineral rich Basin.

Figure 2 .
Figure 2. Geological map of the study area.

Figure 3 .
Figure 3.Total magnetic intensity (TMI) map of the study area.

Figure 4 .
Figure 4. Upward continuation (UC) map and legend of the study area.

Figure 5 .
Figure 5. Reduction to pole (RTP) map and legend of the study area.

Figure 7 .
Figure 7. SPI depth map and legend of the study area.

Table 1 .
Summary of Forward and Inverse modeling results. .

Table 2 .
Magnetic susceptibilities of some mineral