Fluid migration in the vadose zone from 3-D inversion of resistivity monitoring data

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Abstract
The movement of a small plume of fresh water through the vadose zone was monitored using surface resistivity tomography and pole‐pole potential measurements. Sets of potential measurements on a square grid at several times throughout the experiment show gradual, progressive, and systematic development of low‐resistivity zones that are inferred to be loci of fluid concentration. A procedure for inverting percentage changes in potentials is developed here and used to map maximum potential changes of 13% into resistivity decreases of up to 40% through 3-D resistivity inversion. The resulting patterns are complex, with both resistivity increases and decreases needed to match the observed data. The resistivity reductions show a spatial connection to the plume’s source and are suggestive of the fluid migration. Resistivity increases generally appear to form rims surrounding the decreases and may be artifacts of the inversion process. However, some isolated increases in the surface layer may be caused by evaporation of fluid from previous precipitation events. Interpretation of the complex patterns may limit the usefulness of this method for monitoring fluid migration. Nonetheless, the resulting pattern of resistivity reductions may show details of fluid migration that are unavailable with more conventional monitoring techniques. In this experiment, comparison of the volume with reduced resistivity with the volume of injected water predicts only a 0.5% increase in saturated porosity, demonstrating that fluid flow in the vadose zone was most likely controlled by the distribution of fine‐grained clays and silts and occurred by capillary action.