Hydrogeophysical approach for identification of layered structures of the vadose zone from electrical resistivity data

The electric resistivity survey and borehole collection of resistivity data are one of the oldest geophysical tools for characterization of the vadose zone. A current trend is to conduct such surveys in a tomographic manner, which requires significant computational resources. We present a simple, semianalytical approach to delineate multiple layers in partially saturated soils from resistivity and saturation measurements taken at several depths along a borehole. The number of layers and their hydraulic properties are assumed to be known. The proposed inversion algorithm is computationally efficient and can serve either as a stand-alone tool for layer delineation or as an autonomous module in a more comprehensive geophysical survey. It is most robust when each layer is sampled at least once. When one or more layers have not been sampled, the algorithm's robustness (convergence) depends on the accuracy of an initial guess (e.g., expert knowledge and other hard or soft data). We provide a detailed analysis of the algorithm's convergence and identify potential pitfalls.