DEPARTMENT OF GEOSCIENCES Analysis of the Aeromagnetic Anomalies of the Chad Basin in Maiduguri North-eastern, Nigeria   Ishola, K.S., Okoye, B., Adeoti, L., Oyedele, K.F. Department of Geosciences, University of Lagos, Akoka, Lagos.   DEPARTMENT OF GEOSCIENCES UNIVERSITY OF LAGOS OCTOBER, 2017. LASUFoSC 2017

Geology of the Study Area Methodology Results & Discussion OUTLINE Introduction Aim of the Study Literature Review Geology of the Study Area Methodology Results & Discussion Conclusion and Recommendation

INTRODUCTION Magnetic surveys are carried out to investigate subsurface geology on the basis of magnetic anomalies in the Earth’s magnetic field resulting from the magnetic properties of the underlying rocks (Ikumbur et al., 2013). In addition, aeromagnetic surveys have been applied at the early stage of petroleum exploration to determine depth and major structure of crystalline basement rocks underlying sedimentary basins (Abdulsalam et al, 2011). Consequently, spectral analysis has proved to be a powerful and convenient tool in the processing and interpretation of potential field geophysical data (Nwankwo et al., 2008). Hence, this paper focuses on the application of aeromagnetic anomalies in delineating magnetic structures underlying the study area with the hope that hydrocarbon potential of the area could be inferred.

Determine the depth to magnetic basement. AIM AND OBJECTIVES OF THE STUDY The main goals of this study are to extract magnetic anomalies of geologic interest and to determine the depth to magnetic sources giving rise to magnetic anomalies. The main objectives are to: Carry out a qualitative interpretation of aeromagnetic anomalies in order to reveal structural features such as faults, and fractures. Determine the depth to magnetic basement. Generate geologic models of the depth to magnetic basement. Obtain a 3-D view of the Basin.

LITERATURE REVIEW Presently, the magnetic method is by far the most widely used of all geophysical survey methods (Chinwuko et al., 2014). Onuba et al., (2008) used the Least Square technique to analyse aeromagnetic data over Upper Benue Trough of Northeastern Nigeria in order to determine the variations in sedimentary thickness over the magnetic basement. Anakwuba et al. (2011) used aeromagnetic data of the Maiduguri – Dikwa depression in Northeastern Nigeria to established the main shallow crustal structural features of the area. Anudu et al., (2012) analysed an aeromagnetic data over Wamba and its adjoining areas in north-central Nigeria with a view to mapping metalliferous mineral deposits in basement rocks, delineate structural lineaments. In another study, Kasidi and Nur (2012) carried out a study on aeromagnetic Data over Mutum-Biyu and Environs, North–Eastern, Nigeria to provide an average sediment thickness and structural patterns through the use of data enhancement techniques. Chinwuko et al.(2013) conducted a study on four aeromagnetic maps representing parts of southern Chad Basin and Upper Benue Trough using spectral analysis and 2.0 D modeling.

LOCATION OF THE STUDY AREA The area of study is in North eastern, Bornu Basin, Nigeria. It lies between latitudes 11o 30I N- 12o 30I N and longitudes 11o 00I E- 12o 00I E (Fig. 1). The study area has an area of about 12,100 square kilometers. It is bounded on the north by Niger (Country), on the east by Borno State, on the south by Gombe State, and on the west by Jigawa State. The study areas includes: Damaturu, Potiskum, Dapchi and Biriri.

Fig.1: Map of Nigeria showing the study area

Geology of the area The study area North-eastern Nigeria is situated within Chad Basin (CB) an intra-continental basin that owes its immediate origin to the existence of a number of peripheral uplifts (Anakwuba et al. 2011). The Chad Formation thicken towards Maiduguri axis and thins out towards Potiskum where the Keri Keri Formation is exposed. The Kerri-Kerri Formation in the north-east is part of the Tertiary deposits. The Chad Formation overlies Kerri-Kerri Formation and is composed of basal sands and gravels with greenish clays above, the latter containing some minor bands of sands. Fig. 2: Geological map of the Nigerian sector of the Chad Basin (After Genik, 1992).

MATERIALS & METHODS Digitization of aeromagnetic map The data (aeromagnetic sheets) were obtained from National Geological Survey Agency, Abuja. Regional airborne magnetic surveys over the entire Bornu Basin and adjoining areas were carried out using 3 Scrintrex magnetometers. Sheet 63 (Dapchi), Sheet 64 (Biriri), Sheet 86 (Potiskum) and Sheet 87 (Damaturu). The procedures involved in this study include: Digitization of aeromagnetic map Separation of magnetic data Generation of magnetic anomaly maps Analysis of magnetic anomaly data.

DIGITIZATION … The aeromagnetic maps were digitized along flight lines with a spacing of 2km. The flight line and tie-line trends were 135o and 45o respectively The intersection points were picked Contoured map using geological softwares.

The digitized magnetic data contain both the regional and residual SEPARATION OF AEROMAGNETIC DATA The digitized magnetic data contain both the regional and residual anomalies. To Interpret the residual data, the regional data was removed from the data. 8107.16 – 3.71811x – 2.7127y In this case, a linear trend surface was fitted on to the digitized aeromagnetic data by a multiple regression technique for the purpose of removing the regional magnetic gradient. The surface linear equation on the data can be given by: P(x, y) = a + bx – cy - - - (1) where, a , b & c are the constants; x & y are distances in x & y- directions ; P(x , y) is the magnetic value at x and y coordinates. The trend surface equation (regional gradient) becomes: P(x, y) = 8107.16 – 3.7811x – 2.7127y - - - (2)

MATHEMATICAL BACKGROUND Discrete Fourier Transform (DFT) is the mathematical tool used in this study. DFT is applied to regularly spaced data such as the aeromagnetic data. The Fourier Transform is represented mathematically as shown below: -------- (3) where, Y(x) is the magnetic reading, an & bn= Fourier amplitudes, L=length of the cross section of the anomaly, n= harmonic number of the partial wave

MATHEMATICAL BACKGROUND CONT’D After calculating the values of the parameters above, a graph of the logarithm of the amplitude against frequency is plotted. COMPUTATION OF DEPTH TO BASEMENT The gradient of the linear segment was evaluated and the depth to the basement was calculated according to Spector and Grant, (1970), Z = - ML /2π - - - (4) where, Z = depth to basement M = gradient of the linear segment L = length of the cross section of the anomaly

QUALITATIVE INTERPRETATION RESULTS AND DISCUSSION The magnetic anomaly maps were both qualitatively and quantitatively interpreted. QUALITATIVE INTERPRETATION The qualitative interpretation is done by visual inspection of the total magnetic intensity (TMI), residual anomaly and analytic signal maps. Fig. 4 is a TMI map of the study Area, while Fig. 5 is the Residual Magnetic Anomaly map produced after subtracting the regional magnetic field. The TMI field ranges from 7800 to 8290 nT in the study area. The higher magnetic anomalies values are found in the eastern and southern parts, and lower values in the northern and central regions (Figs.3 and 4). Figures 4 and 5 also show that the area is composed of three main magnetic regions; northeastern, southwestern and centeral region. The north-eastern and south-western regions are characterized by high magnetic anomalies trending in the NW-SE and W-E directions.

QUALITATIVE INTERPRETATION The total magnetic intensity map of the study Area (Fig. 4) shows there is a large magnetic anomaly of 8550nT at Birin area of northeastern part of the study area. Towards the southern parts (Damaturu and Potiskum areas), there is another anomaly of small size and its maximum amplitude is 7705nT and 7650., while towards the Dapchi area of northwestern part of the map, there is a long anomaly size with maximum intensity of 7925nT. Fig.4: Total Magnetic Field Intensity Map (Contour Interval ~ 15nT)

Fig. 5: Residual anomaly map (RAM) of the study area The RAM of the study area (Fig. 5) shows positive and negative magnetic anomalies, which are distributed throughout the area. Maximum magnetic value (325 nT) was recorded at the easterrn part and the minimum value (- 425 nT) was recorded at the western part. The closely spaced linear sub-parallel orientation of contours in the eastern and southern parts of the study area suggests that faults or local fractured zones may possibly pass through these areas. Most of the anomalous features trend in the Northeast-Southwest and minor ones trend East- West directions. Fig. 5: Residual anomaly map (RAM) of the study area (Contour Interval ~ 25nT)

Fig. 6: Analytical Signal Map showing the The main trend of the lineaments is NE-SW, while few trend E-W (Fig. 6). Fig. 6: Analytical Signal Map showing the Magnetic Lineament within the study area

QUANTITATIVE INTERPRETATION The quantitative interpretation of the geophysical anomalies is often based on the analysis of data observed along selected profiles (Fig.7) . Fig. 7: RAM of the study area with cross sections (Contour Interval ~ 25nT)

Some sample profiles (Fig Some sample profiles (Fig. 8) have been selected for detailed interpretation. These serve as representatives of the others as they behave almost in the same way. Fig.8: Selected cross sections along NW-SE direction

Thus, graphs of the logarithms of the amplitude against frequencies obtained for the various profiles were obtained (Fig. 9). Linear segment from the low frequency portion of the spectral, representing contributions from the deep-seated causative bodies could be drawn from each graph. Fig.9a: Spectral analysis graph for various anomaly 1-6

Table 1: Basement depth determined from spectral analysis The computed average depths to the basement ranges from 0.58 to 5.37km (Table 1). Pls confirm these values I think you should say the estimated average depths to the basement ranges from 0.68km (shallow) to 2.51 km (deep) Table 1: Basement depth determined from spectral analysis Profile name Profile direction Anomaly Shallower Depth (km) Deeper Depth (km A - Al NW-SE 1 0.58 3.21 Along Potiskum 2 0.74 2.91 3 0.72 2.74 B - Bl (Along Dapchi & 4 0.91 1.34 Potiskum) 5 0.45 2.93 6 0.63 3.08 C - Cl (Along Dapchi & 7 0.89 2.01 Damaturu) 8 0.76 2.84 9 0.82 3.42 D - Dl (Along Birin & 10 0.33 1.38 11 0.67 2.63 12 3.33 Q - Ql (Along Birin) 13 0.42 1.58 14 0.62 1.79 Average   0.68 2.51

GEOLOGIC MODELLING OF MAGNETIC ANOMALIES Fig.10a: Geologic modelling of anomalies along profile A-B

BASEMENT TOPOGRAPHY MODEL The shallow magnetic sources vary from 0.33 to 0.91km with an average of 0.68km Fig.11: Shallower magnetic sources map of the study area. (Contour Interval ~ 0.025 km)

Fig.11: Deeper magnetic sources map of the study area (Contour Interval ~ 0.10 km)

Fig. 12: 3D Surface Plot for the basement topography of the study area

CONCLUSIONS Some of the conclusions from this study are as follows: The area is intensely fractured with major regional faults trending in NE-SW direction. This conforms with the trend obtained in some previous studies in the Benue Trough. Two depth sources were obtained in the study area; the deeper magnetic sources vary from 1.34 to 3.42km, whereas the shallow magnetic sources vary from 0.33 to 0.91km. The study area is of rift origin due to Benue Trough origin. The basement map of the study area shows clearly that the sedimentary cover is generally high towards the southern and central parts and therefore it is likely to favour hydrocarbon formation. Based on the estimated sedimentary thickness (2.01-3.42km) , the possibility of hydrocarbon generation in the southern and central parts of the study area is realistic.

RECOMMENDATIONS In order to ascertain the hydrocarbon potential of the study area, other geophysical survey such as seismic acquisition should be carried out. Sample cuttings in future wells to constrain the ages of the section using biostratigraphy. Knowledge of sedimentology, stratigraphy and structural geological should be integrated with aeromagnetic data.

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