Determination of physico-mechanical parameters of soil extract by the method of impedance eddy current probing

Abstract

Purpose. To develop and test a non-contact triparametric impedance eddy current method for simultaneous monitoring of physico-chemical parameters of soil extracts (soil-to-water ratio 1:5), including the effective geometric parameter of the probing zone, specific electrical conductivity, and temperature of the liquid phase, using a transformer-type impedance eddy current transducer (IEC) with a longitudinal electromagnetic field at multiple frequencies. To evaluate the method's sensitivity to changes in the electrophysical properties of soils in areas of technogenic load, particularly due to military actions. Methods. The study was conducted using a transformer-type impedance eddy current transducer with two identical channels (RP – for probing and heating the sample, OP – for reference measurements). An AC generator, frequency meter, voltmeters, phase meter, and nickel resistance thermometers were employed. Probing was performed at two multiple frequencies (f = 40 MHz) with a current of 20 mA and magnetic field strength H₀ ≈ 280 A/m in the temperature range of 10–30 °C. Measured were the total EMFs EΣ₁ₜ, EΣ₂ₜ, reference EMFs E₀₁, E₀₂, and phase shift angles φ₀₁ₜ, φ₀₂ₜ. Parameters were calculated using a system of analytical relations linking the amplitude and phase characteristics of the signals to specific electrical conductivity σₜ, temperature t, geometric parameter of the probing zone aеф, equivalent concentration of heavy metals Cme, and pH (accounting for the temperature coefficient α = 0.0132 1 / °C and calibration coefficients of the pH model). Results. Theoretical analysis showed that the phase component of the signal is more sensitive to changes in the electrochemical properties of the medium, while the amplitude component reflects the combined influence of conductivity and temperature. The use of multiple frequencies enabled separation of the contributions of the controlled parameters and reduction of their mutual correlation. Experimental results confirmed the high sensitivity of the method to technogenic changes: in background soils, values of σₜ, pH, and Cme corresponded to natural indicators of a weakly acidic state; in degraded soils (post-explosions), a sharp increase in conductivity (6–10 times), a decrease in pH to a strongly acidic level (1.5–3.7), and an increase in Cme by 1.5 times were observed due to mobilization of heavy metals (Fe, Cu, Zn, Pb) and destruction of the buffer system. The obtained values correlate well with classical electrochemical methods, while the non-contact nature eliminates polarization errors and ensures stability in aggressive environments. Conclusions. The proposed triparametric impedance eddy current method is an effective tool for non-contact comprehensive monitoring of the physico-chemical state of soil extracts, surpassing traditional conductometric and potentiometric methods in speed, stability, and capability to work with aggressive samples. The method is promising for ecological monitoring of soils in zones of technogenic and military load, assessment of degradation degree, control of reclamation measures efficiency, and further development of multiparametric impedance systems in agroecology