Performance of electrical spectroscopy using a RESPER probe to measure salinity and water content of concrete and terrestrial soil

Abstract

This report discusses the performance of electrical spectroscopy using a resistivity/ permittivity (RESPER) probe to measure salinity s and volumetric content θW of water in concrete and terrestrial soil. A RESPER probe is an induction device for spectroscopy which performs simultaneous noninvasive measurements of electrical resistivity 1/σ and relative dielectric permittivity εr of a subjacent medium. Numerical simulations show that a RESPER probe can measure σ and ε with inaccuracies below a predefined limit (10%) up to the high frequency band. Conductivity is related to salinity, and dielectric permittivity to volumetric water content using suitably refined theoretical models that are consistent with predictions of the Archie and Topp empirical laws. The better the agreement, the lower the hygroscopic water content and the higher the s; so closer agreement is reached with concrete containing almost no bonded water molecules, provided these are characterized by a high σ. The novelty here is application of a mathematical–physical model to the propagation of measurement errors, based on a sensitivity functions tool. The inaccuracy of salinity (water content) is the ratio (product) between the conductivity (permittivity) inaccuracy, as specified by the probe, and the sensitivity function of the salinity (water content) relative to the conductivity (permittivity), derived from the constitutive equations of the medium. The main result is the model prediction that the lower the inaccuracy of the measurements of s and θW (decreasing by as much as an order of magnitude, from 10% to 1%), the higher the σ; so the inaccuracy for soil is lower. The proposed physical explanation is that water molecules are mostly dispersed as H+ and OH- ions throughout the volume of concrete, but are almost all concentrated as bonded H2O molecules only at the surface of soil.