NON-INVASIVE GEOPHYSICAL APPROACH FOR CRUDE-OIL SPILL MONITORING IN SHALLOW COASTAL AREA OF NIGER DELTA

Dense Non-aqueous Phase Liquids characterization in coastal environments remains a difficult task due to soil and fluid heterogeneity. Understanding crude oil contamination in the Niger Delta has thus far been done primarily through point sampling, with little emphasis paid to the spatiotemporal variation in crude oil plume generation and migration. To assess the spatiotemporal variation associated with crude oil migration in the shallow subsurface, time-lapse electrical resistivity monitoring of a crude oil spillage simulation under different subsurface conditions was carried out. A 3D experimental sandbox with dimensions of 200cm/60cm/40cm was constructed using 10 mm plexiglass. The sandbox was filled to a height of 40 cm using 2 mm sand, and a 20 cm headspace was maintained to create an aerobic condition. At the initial stage of each experiment, 40 kg of sand was mixed with 1 liter of water to create moist sand, after which the mixture was flushed over 12 hours to remove suspended fine particles and the water table was set at 10 cm. In simulating crude oil spillage, two liters of crude oil were poured over the surface and monitored for 36 hours. A surface array of 98 electrodes with a 2 cm electrode spacing was installed along two profiles with a 12 cm inter-profile separation. Continuous time-lapse measurements were acquired using a dipole-dipole array before, during, and 36 hours after the injection of crude oil. Gravity-induced vertical migration was seen to dominate at the early stage of the experiment under saturated and unsaturated conditions, but distinct lateral migration was evident over time. Over time, the concentration of the crude plume created shows 150% to 400% increase in resistivity values. An increase in solute dispersivity was observed for unsaturated soil compared to saturated soil, thus establishing a positive relationship between dispersion and water saturation. The occurrence of pockets of high resistivity structures linked with localized permeable zones demonstrates the sensitivity of electrical resistivity measurement to subtle but measurable anisotropy in soil pores' distribution. The electrical resistivity technique has been shown to be successful in imaging the non-ideal behavior of crude oil pollutants and associated spatial changes in the pore-size distribution.