The Relationship Between Morpho-structural Features and Borehole Yield in Ilesha Schist Belt, Southwestern Nigeria: Results from Geophysical Studies

Abstract

Aeromagnetic data and Landsat image, were assessed to establish a relationship between morphostructural feature and borehole yield of a sub-basin of Osun basin in the Northern part of Ilesha, southwestern Nigeria. Four morphostructural factors (Geomorphological unit, Slope, regional structure, and lineament) were considered. Geomorphological units were classified based on digital elevation model (DEM), Lineaments were identified and extracted using edge detection technique, slope were generated from the digital data derived from Landsat imagery, and the regional structure were extracted using magnetic data filtering and enhancement techniques processes. The results of morphostructural features shows five (5) various geomorphic units; (Denudation hill, linear ridge, pediment inselberg complex, pediment and moderately weathered Pediplain) and four (4) main structural trends (NE-SW, NW-SE, ENE-WSW and E-W directions). The relationship between the morphostructural factors and borehole yield were assessed using spatial global autocorrelation and spearman rank correlation. Moran's I global tests for dependence shows that geomorphological units, and regional structures were clustered given the zscores of 2.46 and 1.99, p-values of 0.01 and 0.04, Moran's I index value of 0.08 and 0.06 respectively. Likewise, borehole yield, lineament and slope were dispersed given the z-scores of 0.89, -0.43 and -1.15, p-values of 0.38, 0.66 and 0.25, Moran's I index value of -0.06, -0.04 and -0.07 respectively. The Spearman's rank correlation for the four independent variables (Geomorphological unit, Slope, Fracture, and Lineament) are statistically significant to the observed borehole yield, with correlation coefficients of -0.332, 0.137, -0.031, 0.200 respectively. The study concluded that slope and lineament are capable of favoring high borehole yield indicative of active surface recharge mechanism that is prominent in Schist belts and regional structures have little or no contribution to borehole yield in schist belts due to the influence of developed clayey filling resulting from the deep weathering processes.