Interpretation of Aeromagnetic Anomalies over Abeokuta, Southwest Nigeria, Using Spectral Depth Technique

One of the geophysical techniques used in imaging the subsurface is the magnetic method via spectral analysis. To estimate the depth to basement and highlight spatial variations of various basement and sedimentary structures, an aeromagnetic data which lies within longitude 3⁰ 16’E to 3⁰ 25’E and latitude 7⁰ 06’N to 7⁰ 13’N was processed, analyzed and interpreted using spectral depth analytical technique with the aid of Arc GIS,Ms Excel and Origin Pro 8 Software’s. In this study, Regional-residual separation was applied on the composite data and thereafter band pass filtering applied on the data using Ms excel software of the Orgin Pro 8. This filtering sectioned the residual into 9 spectral cells upon which Fast Fourier Transform (FFT) was performed. The FFT decomposed the residual into its frequency components and energy spectral segments. Log of the energy spectral was thus plotted against the radial frequency component. The plotted graphs revealed and identified two source depth models, D_A and D_B. This estimated depths as evidenced with the depth contour map aided in delineating prospective areas. D_A represents depth due to the long wavelength parts while D_B is indicative of depth due to the short wavelength components. D_Aand D_Bvary from 1.9 km to 4.3 km but with an average depth of about 3.6 km and 0.09 km to 1.2 km but with an average of 0.9 km respectively. The average depth of 3.6 km suggests slight sedimentary thickness within the study. Hence, the possibility of the study area to slightly add to the economic reserve of the country cannot be ruled out when seismic sections are run over the area but where the data is a High resolution aeromagnetic data (HRAM), the result will probably be better.

Keywords
Aeromagnetic Data; Geochemistry; Abeokuta; spectral cells; intrusive; Basement