Using electrical anisotropy for structural characterization of sediments: an experimental validation study

Improved characterization of subsurface heterogeneity is important to better understand a range of key processes in the hydrologic cycle, such as overland flow, infiltration into the soil, evaporation to the atmosphere, and transpiration by plants. Recently, synthetic modelling studies have shown that information on subsurface heterogeneity can be obtained from the anisotropy in electrical resistivity. The objective of this paper is to experimentally validate the findings of this synthetic modelling study. In order to do so, we developed a new measurement procedure to determine the effective complex electrical resistivity from a set of current injections and voltage measurements on a heterogeneous sample. A synthetic modelling study showed that this new measurement procedure was able to reproduce the results of the previous study that showed how the electrical properties and the correlation length ratio of bimodal distributions of two materials can be obtained from the effective complex electrical resistivity measured in two perpendicular directions. After validation of the new measurement approach, we constructed two bimodal distributions in a 2D measurement cell and applied the newly developed measurement strategy. We were able to estimate the electrical properties, the volume fraction, and the correlation length ratio with good accuracy from the complex resistivity measurements in two directions. The remaining differences were attributed to variations in sediment thickness that occurred during sample preparation. We conclude that anisotropy should not be ignored but embraced when dealing with subsurface heterogeneity and that proper interpretation of anisotropy may actually be used to characterize subsurface heterogeneity.