MAPPING OF MAGNETIC ANOMALIES IN SOME PARTS OF NIGERIA CHAD BASIN, USING AEROMAGNETIC DATA

Thesis Abstract

Aeromagnetic data over some part of the Chad Basin, Nigeria was analyzed. The aeromagnetic anomaly map, its qualitative and quantitative interpretation helped in identifying the nature and depth of the magnetic sources in the study region. The regional-residual separation was performed with a first order polynomial using oasis montaj software. The residual data obtained was contoured into 3-D maps and this revealed two prominent magnetic anomaly source intensities; the low frequency anomaly and the high frequency anomaly sources. The low frequency anomalies emanates from deep seated bodies in areas with thicker sediments and may be viewed as the magnetic basement depth. The high frequency anomaly emanates from shallow seated geologic bodies in areas of shallower sediments and may be attributed to the volcanic rocks that intruded into the sedimentary formation and this could possibly be responsible for the mineralization found in the area. The maximum magnetic field amplitude is greater than 223 nT mostly above the earth surface while the lowest is less than 96 nT. Varying magnetic highs and lows have been found in the study area which might likely indicate undulating basement surface and likely traps. Depth to the basement of the basin structure ranges from about 0.5 km in the southern part of the study area and gets deeper toward the northern part up to 3.0 km. Lineament and structural complexity map and magnetic depth map values have been utilized to construct the interpretation of the main subsurface structures affecting the studied area, which correlate with the previous studies of the Chad Basin. The lineaments map shows the long regional trends which control the subsurface structure beneath the studied area in an ENE–WSW, WNW–ESE and E–W dominant structural trends directions. Results of the Spectral analysis method enabled delineation of the lithology and mapping the depths of subsurface geologic structures. The study indicated that there is a major source of disturbance at the southern end of the map which trends in an approximately NE-SW direction and is a possible target for feature analysis as it has very high magnetic field anomaly.            

Also, the Standard Euler 3D plots in clear terms showed that the shallow depth of about 214 m for the possible causative sources. The depths in the eastern part of the area are not uniform. In the study area, maximum depth is greater than 770m and is found mostly at the southwestern part of the area where the highest magnetic field intensity amplitude from TMI is located. Therefore, this area has the thickest sedimentary rocks and could be the most probable areas for future analysis. The Energy Power Spectrum calculated average depth to the top of regional sources is about 7 km whereas the shallow sources have depth of about 1.5 km as an average depth to their sources. The depth to the top of the intrusive causative targets was calculated from the aeromagnetic map using spectral analysis method. It was found that the depth result generally ranges from 285.6 m (outcropping and shallow magnetic bodies) to 5400.3 m (deep lying magnetic bodies).