Subsurface electrical resistivity imaging and electromagnetic conductivity profiling at a proposed construction site at Adekunle Ajasin University campus, Akungba-Akoko, South-Western Nigeria

Abstract

Electrical resistivity imaging and co-planar loop electromagnetic conductivity measurements have been employed to delineate the bedrock topography and subsurface distribution of structural features at a construction site on the campus of Adekunle Ajasin University. The goal was to map the subsurface geological disposition and to identify features which could adversely impact on the ongoing construction works. Data were collected along three geophysical traverses using the ABEM1000 Terrameter unit and the EM34 Co-planar loop electromagnetic system. Traverse 1 trend roughly east-west for a total length of 155m while traverses 2 and 3 are trend north-south for 80m each. The collection of electrical resistivity data utilized the dipole-dipole array with an electrode separation of 5 m and maximum dipole spread of 35m. Observed field data were processed and inverted using a finite-element modelling inversion algorithm. Conductivity were collected at 10m coil separation and plotted against station positions. A station separation of 5m was maintained on all traverses for both data sets. Three geo-electric layers were delineated; a thin highly discontinuous low resistivity layer representing the humus-rich top soil, a continuous consistently high resistivity layer with a highly rugose upper layer and several near vertical scarps representing the fresh bedrock and intermediate resistivity layer representing the weathered bedrock separating the upper low resistivity and basal high resistivity signatures. Overburden thickness rarely exceeds 4 m with the exception of the deep weathering section in the southwest and at fracture points with the bedrock. Perennial groundwater flow at the site is restricted to the southeastern sections of the site.