Analysis of Remanent Magnetisation Effects on Aeromagnetic Interpretation in the Nigerian Basement Complex

This study analyses the effects of remanent magnetisation on the interpretation of aeromagnetic anomalies in the Nigerian basement complex, developing a methodology for identifying and compensating for remanence effects in mineral exploration interpretations. Magnetic interpretation of basement complex aeromagnetic data typically assumes that rock magnetisation is induced by the present geomagnetic field. However, many basement rock types including granites, amphibolites, and iron formations carry significant remanent magnetisation from ancient magnetic field orientations that creates anomaly shapes and orientations inconsistent with induced magnetisation models, leading to interpretation errors. This study analyses aeromagnetic data from selected NGSA map sheets in Kwara, Ogun, and Ondo States where rock samples are available for palaeomagnetic analysis. Rock sampling collects oriented hand specimens from 45 outcrops representing key basement lithologies for alternating field demagnetisation in the palaeomagnetics laboratory. Koenigsberger ratios are computed from natural remanent magnetisation and susceptibility measurements. The aeromagnetic data are reprocessed using total field, RTP assuming induced magnetisation, and enhanced analytic signal that is insensitive to magnetisation direction. Systematic comparison between RTP-derived and analytic signal-based structural interpretations identifies discrepancies attributable to remanence effects. Findings reveal Koenigsberger ratios exceeding 1.0 in 28 percent of sampled rocks, primarily from Precambrian granites and banded iron formations, indicating significant remanence effects. Structural interpretations differ by more than 20 degrees in azimuth in areas where Koenigsberger ratio exceeds 2.0. The study develops a remanence screening workflow for Nigerian basement aeromagnetic interpretation.

Keywords: remanent magnetisation, aeromagnetic interpretation, Nigerian basement complex, Koenigsberger ratio, palaeomagnetics.