Comparative Investigation of Prospecting Accuracies of Resistivity Instruments in Subsurface Groundwater Delineation in Basement Complex of South Western Nigeria

Abstract

Geophysical investigation of groundwater potential was carried out within Osun State University, Main Campus, Osogbo, South-western Nigeria using ABEM, PASI and IGIS resistivity instruments with the aim of investigating their prospecting accuracies for
groundwater. Vertical electric sounding (VES) and 2D Wenner array techniques were employed in the investigation with the central reference located close to an already drilled borehole. The data obtained were first subjected to partial curve matching and later iterated and modeled using the WinResisit software. Using ABEM SAS, the topsoil was predicted to be composed of sandy layer of resistivity value of 207.5 Ωm and of about 1.0 m, PASI predicted a clayey topsoil of 30.8 Ωm and about 1.0m thick while the IGIS instrument revealed a concretional lateritic topsoil of about 817.7 Ωm and 2.4 meters thick. The aquiferous layer was predicted by ABEM SAS to be clayey sand and conformably overlie the basement rock with resistivity of 98.3 Ωm and about 18 m thick. PASI revealed a waterbearing zone to be a clay interval with resistivity of 25 Ωm and of about 1.7 m thick. The output of the three instruments do not correlate. The geoelectric interpretation of ABEM data closely correlates with that of the existing borehole but PASI and IGIS data do not. This is probably because the ABEM has signal averaging sensor which records four
consecutive cycles of resistivity values before display their mean value but PASI and IGIS have one-shot voltage-current signal stacking. The ABEM instrument gave resistivity and geoelectric values closest to that of the existing borehole. Hence, it is a more reliable instrument than PASI or IGIS that operate on single-shot voltage-current signal stacking