Analysis and interpretation of Ilorin aeromagnetic data, North-Central, Nigeria, using geostatistical techniques

The use of geostatistical approaches in the structural dependency analysis and interpretation of magnetic response in conjunction with bedrock composition cannot be de-emphasised. This study utilised the two geostatistical tools namely; variogram and kriging in investigating the structural and spatial dependence index of the magnetic variation in different lithological bedrock constituents. Aeromagnetic data of Ilorin sheet 223 obtained from Nigeria Geological Survey Agency was analysed using a geostatistical approach. Nine lithological units (L1 - L9) were assessed such that their magnetic structural dependency was delineated by fitting the variogram models while the spatial magnetic variation was estimated using ordinary kriging method. Three variogram models; Spherical, Exponential and Gaussian models were adopted. The Nugget Sill Ratio (NSR) and Coefficient of Variability (CV) were as well deduced. Variogram cloud together with box and whisker plot were used to delineate the anomalous magnetic responses across the units. From the fitted models, the results accounted for NSR in the range of 4.1 - 46.2%, 4.0 - 42.4% and 4.1 - 43.7% for Spherical, Exponential and Gaussian models, respectively. Two structural dependencies levels; strong and moderate, were estimated across the nine lithological units. Six units revealed strong autocorrelation levels while three units give rise to a moderate level. Based on kriging estimation, three distinct levels; low, moderate and high magnetic response were identified across the nine lithological units. The least (- 482.6 nT) and highest (401.5 nT) magnetic responses were found associated with L6 lithological unit. Low variability (homogeneity) was found associated with four units (CV < 60%), while five units accounted for heterogeneous response (CV > 60%) in the source of magnetic intensity. In this study, geostatistical methods have proven to be good in estimating the magnetic structural and spatial dependency associated with Ilorin lithological bedrock units.