Transient Resistivity Variation due to Air Entrapment under Ponding Condition

Abstract

Physical properties of the unsaturated zone during percolation are subjected to temporal variations. An attempt is made to understand the changes through electrical resistivity modulation. A pilot scale experiment was conducted over medium textured sandy-gravel soil intercalated with regolith cover of considerable thickness. Wenner (alpha) electrical profiling with electrode spacing‟s of 2 & 5 m was adopted over a grid area of 4 m x 4m, with inter-station interval of 0.5m. The apparent resistivity was measured at each stations of alternate profile lines during different time periods of infiltration test conducted at the centre of the grid area. Prominent changes in apparent resistivity are observed due to influence of infiltrating moisture flux. The analysis on changes in apparent resistivity at depth (D~1.0m) between pre and post infiltration test helped in identifying the area of influence due to saturation. A significant rise in apparent resistivity at the bottom of wetting front at depth (D~2.5m) was noticed, attributable to the migration and displacement of air from the moisture occupied pore spaces. The inferred depth of moisture migration from the resistivity profile results concurred with the depth evolved through tritium injected tracer studies. The resistivity investigations also indicated preferred flow direction of the percolating moisture at deeper levels confirmed by the tracer movement. The results demonstrate the utility of electrical profiling in mapping the subsurface for better understanding of the moisture migration pathways and its associated processes.