GEOPHYSICAL STUDY OF AKURE-OBA ILE ROAD, SOUTHWESTERN NIGERIA

Abstract

Geophysical study has been undertaken on the 4.72 km Federal roadway linking Akure with Obaile town in southwestern Nigeria with the aim of identifying failure vulnerable segments of the newly constructed road. The road is underlain by Precambrian Basement Complex rocks consisting of Migmatite-gneiss and older granites lithologic units. The geophysical methods adopted for the study are Magnetic, Electromagnetic (Very Low Frequency) and Electrical resistivity (Dipoledipole) methods. The geophysical traverse was established along the roadway. Station interval of 10 m was adopted for the profiling (Very Low Frequency and Magnetic methods), and 25 m was adopted for the Dipole-dipole with the expansion factor (n) varied from 1 to 4. The Magnetic values obtained range between 30679 and 37165 nT. Magnetic profile and geomagnetic section generated indicate variable overburden thickness ranging between 9 m and 171 m. Field data obtained from the VLF-EM range between -42.5 and 35.8% for the raw real and -46.1 - 48.3% for the filtered real. The combined plots of raw real and filtered real VLF-EM presents relatively high conductive values at sections 75 – 150 m, 250 – 300 m, 750 – 900 m, 1450 – 1500 m, 1600 – 1650 m, 2010 – 2120 m, 2530 – 2570 m, 3200 – 3350 m, which are suggestive of fractured/faulted zones or conductive clay bodies. The 2-D resistivity structure generated from Dipole – Dipole technique reveal the presence of uneven bederock and discontinuities which are indicative of near surface linear features such as fracture, faults, and joints along the roadway. The correlation between the anomalous zones delineated from the results shows some sections of the roadway to be founded on conductive and low resistivity sub-base to depths in excess of 10 m. Sections 80 – 500 m (Owena Mass Transport/Leprosarium), 1100 - 1420 m (Energy petrol/ODIEC office area), 2000 – 2200 m, 2425 - 2650 m (Children Home - Wexford College), 3000 - 3250 m (Oluwaseun Plaza - Olusile Oil) are vulnerable and thus require some mitigation measures to ensure long time performance of the rehabilitated roadway