Method for characterization of soil electrical resistivity based on Wenner measurements by means of Nelder–Mead algorithm and FEM calculations

Soil resistivity is the key parameter influencing resistance and potential distribution on the soil surface for grounding systems and electrodes for HVDC power transmission. The paper presents a novel approach that does not rely on analytical expressions or simplifications inherent in the Wenner and classical characterization methods for obtaining a layered soil model. The new method and corresponding computational tool are implemented as an iterative process using a finite element method coupled with a numerical parameter estimation solver. The novel method is applied on soil with a very thin and highly resistive surface layer compared to a resistivity of a lower layer, where accurate implementation of the classical method would require Wenner measurements for short electrode spacings a. Since the classical characterization methods rely on the assumption that the electrode driving depth b is much smaller than a, it is practically (physically) impossible to drive the electrodes to sufficiently small depth b. The application of the novel method and a comparison with the classical approach are presented on examples including normal soils, one hypothetical soil with a thin highly resistive surface layer, and one experimental desert location. The application of the proposed method leads to an extreme increase in the characterization accuracy for the case of ideal two-layer soil with thin high resistivity topsoil justifying considerable computational time.