Lithologic Delineation using Radiometric Mapping: A Case Study of South-Western Nigeria

Abstract

The survey area covered a total traverse distance of about 91km taken primarily across the general trend of the strike and or bedding plane of rock formations in the area. The traverses which included: Ibadan-Lagos expressway (29km); Shagamu-Ijebu-Ode road (28km); Oru-Ijebu-Ode road (13km) and Ore-Benin road (21km) were made mainly across the sedimentary formations of Southwestern Nigeria. Two gamma radiation detecting instruments: scintillometer and spectrometer were used. At intervals of 1km, gamma radiation values as reflected by each instrument were noted. Results were tabulated and plots of gamma radiations in counts per second (cps) against distance (km) were plotted for each traverse and for the two instruments for the same coordinates. Although each instrument has its peculiar gamma radiation values, they show the same graph trend which reflects the differences in gamma radiation. It is a reflection of the concentration of Uranium (U), Thorium (Th) and Potassium (K) in the rock formations traversed. From the radioactivity graph, various rock formations traversed were delineated. Comparison was made with the geological maps of the area. These were found to march approximately and in some cases almost exactly with the corresponding formations delineated on the radiometric graphs. It was also found that the rise and fall on the graphs reflected the radioactive content of the formations. The survey also established variability in radioactive elements contents in Ewekoro and Abeokuta formations and this led to the production of radioactivity maps of the area.