Lithological mapping of Ogun State, Southwestern Nigeria, using aeroradiospectrometry

Abstract

Airborne gamma-ray spectrometric has gained more recognition over decades, since it serves majorly as a geological mapping tool in most areas. This method utilizes the variation in the three primordial radionuclide concentrations, U, K and Th, to study the lithology of different bedrocks. It measures the earth surface natural radionuclides, which enhances the distribution of soil- and rock-type formations. Ogun State, the study area, is bounded by geographical coordinates of easting 3.0°–5.0° and northing 6.2°–7.8° of tropical rainforest region, Southwestern Nigeria. The present study aims at mapping the distribution of radioelements and delineates the anomalies associated with various interpreted radiometric lithological (IRL) units using the three radioelements U (ppm), Th (ppm), K (%) and the total count (TC) in Ur and their ratios, U/K, U/Th and Th/K. The dose rates were also considered in the study. The lithology was described in terms of three formations; basement, basement–sediment and sediment. With the aid of Geosoft software (Oasis montaj) and Golden Surfer 10 software, the contour map was obtained which showed the trend of distribution of radioelements alongside their ratios. The uranium migration index was also carried out to check the regions associated with uranium leaching. The result revealed that uranium is more concentrated in NW and partly NE directions of the study area. This may be due to the presence of shale and possibly phosphate-bearing rock and metamorphozised granitic rocks while thorium has the largest deposit due to quarry activity which is associated with blasting of rocks and deposition from dumpsites. The study also revealed that only 2 out of all the 47 IRL units, A1 and C4, which belong to basement and basement–sediment formations have their mean dose value to be 1.51 mSv/yr and 1.43 mSv/yr which are above the 1 mS/yr recommended limit.