SUBSURFACE INVESTIGATION OF PARTS OF BENUE TROUGH AND BORNU BASIN, NORTHEAST NIGERIA, USING AEROMAGNETIC DATA

Abstract

ABSTRACT The geophysical investigation of the subsurface structures of parts of Benue Trough and Bornu Basin, northeast Nigeria, using aeromagnetic data was carried out in this study. The area under investigation is bounded by latitude 9.50 N to 12.00N and longitude 9.50 E to 12.00 E. It is covered by 25 aeromagnetic maps. The aeromagnetic maps were digitized on a 3 km by 3 km grid and later compiled to produce a combined aeromagnetic data file of the study area. The 3 km spacing interval imposed a Nyquist frequency of 0.167 km-1 while the data file generated comprised of 7921 data points. The data obtained were subjected to filtering process so as to obtain residual data necessary for interpretation. The residual data were subjected to structural analysis using Centre for Exploration Targeting (CET) grid plug-ins, 2D subsurface modelling and depth analysis using spectral analysis and source parameter imaging. The filtering process separated the deep seated (long wavelength) anomaly from the shallow (short wavelength) anomaly. The low magnetic values tend towards northern portion of the study area. The magnetic (structural) trends in the study area were east-west, northeast southwest, and northwest-southeast trends with the dominating structural trend in the area as northwest-southeast. There were various magnetic lineaments in the study area. These lineaments were suggested to be extension of landward Oceanic fracture zones. The results of CET structural analysis showed that the two basins (Bornu and Benue Trough) had similar structural relationship with more structural activities in Bornu Basin, the basement complex region and the volcanic areas at the eastern part. It also showed that structural similarities exist between the surface geology and the surface lineament map of the area. The maximum sedimentary thickness obtained with spectral analysis was 3.72 km. This occurred in the central part around Gombe and south of Damaturu and Bulkachuwa. The result of the Source Parameter Imaging (SPI) has its maximum sedimentary thickness of about 5.0 km around Gombe, Ako Gombe, Bulkachuwa and Damaturu areas. The result of 2D modelling showed that the sedimentary thicknesses ranged from 0.0 km to a maximum of 4.60 km. The maximum sedimentary thicknesses were found around Gombe, Ako Gombe, Bulkachuwa and Damaturu areas, with a value of about 3.00 km to 4.60 km. The maximum sedimentary thicknesses obtained, which range between 3.72 km to about 4.60 km are adequate for the hosting of hydrocarbons. The minimum sedimentary thickness delineated by these methods could be found around Bauchi axis in the basement complex region, Kaltungo and the volcanic areas in the eastern part of the survey area. The results of these analytical methods were all in agreement. The boundary between the Bornu Basin and the Upper Benue Trough was successfully delineated through trend analysis of the total magnetic intensity, upward continuation filter and 2D modelling of the subsurface structures. The end results of these methods showed that Upper Benue Trough was separated from the Bornu Basin at about latitude 11.00 N. This area corresponds to ―Dumbulwa-Bage High‖. However, the subsurface lithology obtained from 2D modelling of the residual field showed the presence of two lithological units. The sedimentary rock unit underlined by the basement rock consists of shales, sandstones, limestones, siltstones, clay and non-marine facies, while the Basement rock units were composed of pegmatite, granite gneiss and migmatites