GEOPHYSICAL MAPPING OF NEAR- SURFACE FEATURES ALONG IRUEKPEN - IFON HIGHWAY SOUTH-SOUTHERN, NIGERIA

Abstract

The Iruekpen-Ifon road is of economic importance, linking several towns in Edo and Ondo States. However, the recurring failures of sections of the road had made it deplorable. There is dearth of information on the subsurface features that may be responsible for these failures. Hence, this study was designed to identify and characterise the subsurface features that may be responsible for the failures. Three main sections of the 60 km Iruekpen-Ifon highway that have experienced recurring failures were identified. Very Low Frequency - Electromagnetic (VLF-EM) field data were taken in three traverses (T1, T2 and T3) measuring 210 m, 600 m and 240 m at the sections. The VLF-EM data were processed and interpreted. Two electrical resistivity techniques; Vertical Electrical Sounding (VES) and 2-D Electrical Resistivity Tomography (ERT) were employed. In all, forty two (42) VES were carried out using Schlumberger configuration array with current electrode spacing varying from 1.00 to 100.00 m at locations 1, 2 and 3 having 7, 10 and 25 VES, respectively. The data were plotted on bi-logarithmic graph and were interpreted using partial curve matching with the aid of master curves. The outcome of these served as input for numerical iteration using appropriate software. The derived geoelectric parameters were used to generate the Dar Zarrouk second order parameters for the co-efficient of anisotropy (). A 2-D ERT using dipole-dipole electrode configuration with inter-station separation of 10 m and an expansion factor that varied from 1 to 5 was also carried out at the identified sections. The ERT data were processed and inverted to generate subsurface model beneath the sections. The VLF-EM data presented positive anomalies which indicated presence of conductive subsurface features at 32.0 m, 160.0 m and 200. 0 m for T1; 80.0 m, 175.0 m, 212.0 m, 245.0 m, 380.0 m, 420.0 m, 500.0 m and 550.0 m for T2; 42.0 m, 102.0 m, 143.0 m, 169.0 m and 200.0 m for T3 from the reference stations. The VES present resistivity distribution for locations 1, 2 and 3, which ranged from Ωm with probed depth of 24.97 m; Ωm with probed depth of 49.10 m; and Ωm with probed depth of 24.97 m, respectively. The values of  ranged from , with the relatively higher values ( ) suggesting that the subsurface rocks in these areas are intensely fractured. The geoelectric section delineated: the top section (lateritic/clayey sand) with resistivity values varying from Ωm and m in thickness; clay with characteristic low resistivity values ( Ωm) and thickness of m. The cause of the recurring failures on Iruekpen – Ifon highway is as a result of clay formation, underlain by highly fractured subsurface rocks environment.