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Dielectric characterisation of soil

  • Hilhorst, M.A.
Publication Date
Jan 01, 1998
Wageningen University and Researchcenter Publications
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The potential of dielectric measuring techniques for soil characterisation has not been fully explored. This is attributed to the complex and incomplete theory on dielectrics, as well as to the lack of sensors suited for practical applications.The theory on dielectric properties of soils is described, evaluated, and expanded. Colloidal polarisation of soil particles appears to be negligible. The polarisability of air bubbles in the soil matrix is made plausible. The Maxwell-Wagner effect is expressed in the form of a Debye function. A soil texture parameter is introduced that can be derived from dielectric measurements at three frequencies. Newly derived are a relationship between the soil water matric pressure and the dielectric relaxation frequency, a dielectric mixture equation with depolarisation factors that account for electromagnetic field refractions at the boundary between two soil materials, and a model to predict permittivity versus frequency from soil porosity, water content, and matric pressure.A model of sensors for Frequency Domain (FD) measurements as well as for Time Domain Reflectometry (TDR) is described. An integrated circuit (ASIC) has been developed that is based on synchronous detection and is intended for practical low-cost dielectric sensors. Algorithms correct for phase errors, parasitic impedances of the ASIC and electrical length of electrodes and wiring. These elements are incorporated in a new FD sensor, operated at 20 MHz.The new theory is tested in different ways using the new FD sensor and TDR. Calibration curves of water content versus electrical permittivity of different soil types compare reasonably well with predicted curves. The Maxwell-Wagner effect increases with increasing water content and specific surface area. The electrical conductivity of the extracted soil solution can be determined by simultaneous measurements of the electrical permittivity and bulk conductivity. This method proved accurate for glass beads and for most tested soils. Soil layers polluted with chlorinated solvents or oil are detected by measuring the same parameters as function of depth. The frequency dependence of the bulk electrical conductivity, attributed to the Maxwell-Wagner effect, is analysed by measurements at three frequencies.Hydrating concrete is shown to simulates the dielectric behaviour of soils of different textures. Its dielectric spectrum from 10 MHz to I GHz illustrates the effect of water binding (> 100 MHz) and the Maxwell-Wagner effect (< 100 MHz). Around 100 MHz concrete exhibits only small changes of the dielectric properties; this is known to occur also for soils of different textures. The compressive strength of concrete appears to be predictable from the electrical permittivity at 20 MHz, due to the Maxwell-Wagner effect.Due to the simplification to apply a single sine wave rather than a pulse or step function, existing theory is inadequate to correct TDR measurements of water content for the effect of electrical conductivity. TDR electrical conductivity measurements are found to be low-frequency (< 3 MHz) measurements.

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