Evaluation of a Lock-in-Amplifier-Based Spectral Induced Polarization Method for Soil Characterization

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A method of measuring spectral-induced polarization (SIP) by employing an SR810 DSP lock-in amplifier has been proposed. The method, to be used for characterizing soils, was calibrated with known resistors, and tested for a resistive-capacitive (RC) circuit, which closely simulated a soil sample in terms of its electrical properties. A suitable sample-electrodes configuration was selected by evaluating the impedance spectroscopy of three pairs of current electrode and seven pairs of potential electrode. The current electrodes conduct a launched sinusoidal current wave and the potential electrodes provide the voltage at their locations. The method imparted negligible influence of capacitive coupling below 100 Hz frequency of the applied electric field, and measured voltage phase (φ(ω)) within 1 mrad accuracy; the noise level in the measurement was the least at <10 Hz frequency. An input current density of 0.35 A/m2 induced negligible noise in φ(ω) and was found optimum for the method. The resistance-dependent response of the circuit elements affected the phase and impedance spectra of the RC circuit, and hence a correction for the system's transfer function was employed. A geometric factor arising from the sample-electrodes configuration was found independent of frequency of the electric field and conductivity of the sample. The current electrodes of porous bronze plate generated negligible φ(ω) below 100 Hz frequency and were superior to chrome nickel and platinum electrodes. The potential electrodes of platinum, titanium and stainless steel rods resulted in the negligible surface polarization below 100 Hz frequency, with the superior result by the platinum electrodes. The lock-in amplifier can thus reliably measure SIP from suitable sample-electrodes configurations.

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