Combination of geological mapping and geophysical surveys for surface-subsurface structures imaging in Mini-Campus and Methodist Ago-Iwoye NE Areas, Southwestern Nigeria

Abstract

Electrical resistivity imaging and geological mapping were used to study the geology and delineate geologic structures at the Northeastern part of Ago Iwoye, Southwestern Nigeria. Two areas which include Mini-Campus Olabisi Onabanjo University (MCOOU) and Methodist Comprehensive High School (MCHS), Ago-Iwoye were studied. Electrical resistivity tomography (ERT) survey with Wenner configuration was done to reveal the horizontal and vertical variations in the subsurface geology; Azimuthal resistivity survey (ARS) was used to determine the orientation of the subsurface fractures. Both Wenner and Schlumberger array methods were employed. Measurement for each array were made about a fixed central point at an increment of 45° with respect to the reference axis using maximum electrode expansion of AB/2 at 130 m. Petrographic description and structural mapping were also carried out on three (3) outcrops nearest to the survey area in order to correlate the subsurface and surface geology and geologic structures. Petrographic study of rocks showed that the rock exposures are foliated biotite gneisses with mineralogical composition of quartz, plagioclase feldspar, biotite, and microcline. The orientation of joints (fractures) is dominantly ENE-WSW, E-W, and WNW-ESE. The electrical profiling revealed four geo-electric layers, clayey topsoil, clayey sand, sandy layer and weathered bedrock, the fresh bedrock/basement occurred at depth of 15.9 and 19.8 m subsurface at MCOOU and MCHS, respectively. The ARS showed that there is significant anisotropy between 30 and 65 m with fracture occurrence at 39 and 30 m at Methodist and Mini-campus; these fractures are oriented NW-SE, N-S and NE-SW, respectively. The coefficient of electrical anisotropy indicates that the intensity of fracturing at Methodist opens with depth at one part and later became constant with depth at other parts while that of Mini-campus is constant with depth. Fractures orientation suggests that surface structures are not deep seated and were produced by dissimilar tectonic event relative to subsurface fractures. Key words: Geology, fractures, orientation, anisotropy, electrical resistivity tomography (ERT), Azimuthal resistivity survey (ARS).