Soil mapping using electro-magnetic methods: development of a unified constitutive model.
In the context of sustainable development, there is a growing interest in mapping soil characteristics and land use, in particular to evaluate threats to soil quality. Fast non-invasive geophysical surveys have been used to estimate the areal distribution of soil properties, such as moisture content, texture and salinity. All geophysical methodologies rely on constitutive relationships to convert the measured variables in the corresponding properties of interest. Among the most used methodologies are those that measure soil electrical properties, in particular DC electrical conductivity and permittivity in the static limit. Traditionally, these two variables have been treated separately, that is, different constitutive models have been used to parameterize and link the measured response of the soil to its hydrological state and its texture. A number of works however have shown that these two quantities depend to a large extent on the same soil characteristics, and can therefore be jointly studied. The aim of this study was to develop unified conceptual framework to link electro-magnetic variables to soil properties. A new petrophysical equation – suitable to model DC conductivity and permittivity – was derived combining the Hashin and Shtrickman upper and lower bounds with Archie’s law. The model was further extended to simulate low-frequency (induced) polarization (IP). The contribution of both EDL and membrane polarization was incorporated. Comparison with experimental data demonstrated that the constitutive equation is able to reproduce satisfactorily measurements of DC conductivity and permittivity, even when all parameters were estimated independently. Concerning IP measurements, the model showed a behavior overall consistent with the observations, even for variably saturated conditions. Discrepancies between measurements and observation were however found. These raise a number of theoretical questions that will be presented and discussed.