Interpretation of Aeromagnetic Data from Upper Benue Basin, Nigeria Using Automated Techniques

Abstract

The Benue basin is a major geological formation underlying a large part of Nigeria. It is a part of the broader Central African rift system. The Upper Benue basin being part of Benue basin is believed to be rift valley and is expected to be a major depositional basin, because rifting structures are often good sites for mineralization. The strategic economic importance and the availability of data from the study area arose the interest of many researchers including this present work to focus their attention on the area in search of geological features that are favourable to mineral deposition in the basin. The area investigated covers from 07 0 00'-08 0 30'N and from 11 0 00'-12 0 00'E. In this work, the interpretation of the data extracted from the aeromagnetic maps of the basin was carried out using automated techniques involving the analytic signal, horizontal gradient magnitude and the log power spectrum techniques to delineate linear geologic structures such as faults, contacts, joints and fractures within the study area in a bid to unravel the gross sub surface geology of the area which would in no doubt help in better understanding and characterization of the area investigated. The residual magnetic field was subjected to three filtering techniques, the analytic signal, horizontal gradient method and the log power spectrum. A grid of analytic signal technique was employed to study source parameters which include location, depth and susceptibility contrast of identified magnetic anomalies in the basement rocks. The horizontal gradient method was used to study the fault and contact pattern in the study area while Power spectrum technique was also used to further study the depth estimates to the magnetic basement. Results from analytic signal technique showed that the basement in the study area is segmented by faults whose depth ranges between 0.5km and 10.5km with an overall average depth ranging between 1.13km to 5.88km. The estimated depths were contoured to portray the basement isobaths for the study area. The horizontal gradient method revealed a minimum depth or sedimentary cover ranging between 0.01km to 2km and faults pattern trending northeast-southwest (NE-SE), Eastnortheast-westsouthwest (ENE-WSW) and westnorthwest-eastsoutheast (WNW-ESE). Depth estimates from the log power spectrum revealed two major grabens or sub-basins with depth ranging between 1.22km and 3.45km for depth to magnetic basement while those ranging between 0.01km and 1.49km are identified with shallow sources which are due to near surface intrusives. Based on the result obtained it was revealed that the study area is divided into three basinal structures; the deep sources ranging between 6.5km and 10.5km are associated with some layers of intra-crustal discontinuities; an indicator to feature volcanic eruption in the area. The intermediate depths between 3.5km and 5.5km correspond generally to the top of intrusive masses occurring within the basement, a depth deep enough for possible hydrocarbon deposit. Shallow depths between 0.01km and 2.5km are attributed to shallow intrusive bodies or near surface basement rocks probably isolated bodies of ironstones formation concealed within the sedimentary pile.