Geophysical Investigation of Impact of Geologic Structures on Preferred Percolating Paths of an Oil-based Leachate within a Migmatite-Gneiss Environment: A Case study of Federal University of Technology, Akure Campus (Nigeria)

Abstract

The study was aimed at determining the lateral and vertical flow of an oil-based leachate within the Migmatite-Gneiss environment with time and to delineate the presence of possible bedrock structures that can enhance the leachate movement. The method of study adopted combines the use of VLF-EM profiling, Electrical Resistivity Tomography survey (Dipole-dipole) and Vertical Electrical Sounding (VES). The surveys were carried out at four times with regular interval of four-week. A particular point along a feeder road within the Federal University of Technology, Akure campus was polluted with used black engine oil. The location was left to settle for two weeks before the first round of survey involving the above mentioned methods was carried out. The results show that the oil-based leachate percolated towards the south which is an uphill direction. Each successive results showed a gradual progression of the leachate in different directions; laterally and vertically. At the fourteenth week, which is the fourth round of survey the leachate was observed to have percolated down to the depth of about 10-15 m, which is rather too fast for a leachate moving according to normal gravity flow. This indicated that probably geologic structures; perhaps faults, fracture and void could be present at the subsurface beneath the polluted point. The VES result presented as geoelectric section confirms this speculation as the bedrock layer beneath VES 1, 2 and 3 have resistivity values that range between 230 and 347 ohm-m.

The integrated geophysical method adopted for this study successfully helps in delineating the oil-based leachate flow direction, extent and preferred paths.

Keywords: Electrical resistivity tomography, vertical electrical sounding, oil-based leachate, bedrock structures and bedrock fracture.