An electromagnetic (EM) tool working in the medium-frequency range allows the determination of the electric conductivity and dielectric permittivity of soils with a single measurement. It brings information about different state parameters of soils, especially their water and clay contents for a significant volume of investigation. To investigate these properties, a medium-frequency-range EM prototype, the CE120, was built using a perpendicular coil Slingram configuration with a working frequency of 1.56 MHz and a fixed coil spacing of 1.2 m. This configuration was chosen using modeling with the purpose of measuring electric resistivities up to a few thousands of ohm-meters and relative dielectric permittivities as low as two. These thresholds match the expected parameters values in the medium frequency range. Moreover, the CE120 characteristics allowed for an investigation depth between 2 and 2.5 m, depending on the nature of the soil. The prototype was tested on two different soils with water variations: sandy alluvia and clay-loam soil. The electric resistivities of the sandy alluvia can reach [Formula: see text], which is higher than the detection threshold of the CE120. Only the measured dielectric permittivity can be interpreted in terms of water contrasts. Four different zones corresponding to four different water contents were detected. For the clay-loam soil, the electric conductivity and dielectric permittivity measure the water content variations created by the wheat roots. At 1.56 MHz, the high-frequency equations linking the dielectric permittivity to the volumetric water content have a limited validity. Laboratory measurements for each different soil type are necessary to deduce empirical relations. In both cases, the results are coherent with the measured mass water content variations.
Metrological support of acoustic measuring devices in the medium frequency range
