A Study of the Middle Benue Trough (Nigeria) Based on Geological Application and Analyses of Spectra of Aeromagnetic Data

Abstract

With an upbeat in petroleum exploration efforts in the Middle and Upper Benue Trough, emphasis is increasingly laid on the structural setting of the basin. This work reports the results of the analysis of aeromagnetic data over the Middle Benue Trough, Nigeria. It is well known that the expression of the spectrum (radial and angular) for the total-field magnetic anomaly field has been extensively used in a variety of geologic applications ranging from depth estimates to magnetic body tops and bottoms and estimates of polarization directions and other linear features. In the same vein, matched filters are capable of separating magnetic anomalies of individual magnetic layers from the total magnetic field anomaly. Consequently, the spectra of the different layers can be separated through the matching of the observed spectra to the spectra of equivalent sources at different depths. This technique can also yield dipole equivalent layers, which might be magnetic ensemble discontinuities. The angular spectrum of the magnetic anomaly, on the other hand, is capable of yielding information on the directions of polarization vectors, as this spectrum is usually maximum in these directions. These processing tools have been briefly examined and applied to the aeromagnetic anomaly data of the Middle Benue Trough, Nigeria. The performances of the radial spectrum and matched filtered output of the field are compared. Results from the plot of log radial spectrum against the frequency numbers indicate five discernable linear segments giving depths corresponding to magnetic layers ranging from 20.62 km (highest) to 0.26 km (lowest). Results from the spectral matching yielded three dipole equivalent source layers at 0.89, 4.33, and 18.22 km, giving two corridor bands. With the sensed magnetic discontinuities at depths of 18.10 to 20.50 km (from ground level), the temperature gradient probably over the area lies between 28.80 and 25.40°C/km for the Curie temperature of 520°C. Finally, we have computed the angular spectrum of the field over this area. It is found that the angular spectrum in the mid and high frequency bands shows two peaks at 62° and 145°. Also there are peaks corresponding to the direction of polarization and the rectangular tapered window used in the analysis.

Keywords:

• angular spectrum
• aeromagnetic anomalies
• Benue trough
• Curie point depth
• Fourier analysis
• magnetic anomalies
• petroleum exploration
• radial spectrum