Pulsed Electromagnetic Sounding of a Geological Environment with Time-Varying Magnetic and Electrical Properties

Transient responses recorded in Transient/Time-Domain Electromagnetics (TEM/TDEM) are generated and formed in the studied geological environment by several physical processes. The induction process (IN) is generated by eddy currents, which depend on the distribution of electrical conductivity in the probed volume. The superparamagnetic (SPM) effect occurs upon remagnetization of single-domain (SD) magnetite grains in the rock. The nature of polarization processes (PP) is associated with flow of electric current through a rock with frequency dependent electrical conductivity. Inductively induced polarization (IIP) is generated by eddy currents in a dispersive electrically conductive medium. The induced polarization process (IP) can be produced by displacement currents, in antenna loops with distributed capacitance and resistance. IN and IIP show at early stages of transient responses, while IN, IP and SPM appear at late stages. The relaxation rates of the processes differ: IN and IIP processes are fast while IP and SPM are slow. IN and SPM decay in phase, while IP and IIP decay in antiphase with IN. In TEM, information about the geoelectric structure of the medium is extracted from IN, while IIP, IP, and SPM usually considered noise. However, the presence or absence of polarization and magnetic effects in the recorded responses can only be identified when they dominate, dramatically distorting the shape of the IN responses. The presence of weak IIP, IP, and SPM may become apparent as late as in the process of inversion and interpretation of TEM data. Monitoring of transient responses has shown that IIP, IP, and SPM processes form in the soil layer and change over time. Variations in IIP and IP are modulated by a diurnal cycle, with early afternoon maxima and nocturnal minima. It is established that time-multiplied SPM responses in TEM magnetic antenna can be represented by a convolution of the particle volume distribution function with a narrow-band filter. SPM effects are sensitive to soil moisture and orientation of magnetizing field relative to the Earth’s magnetic field. Due to internal biological processes in the soil colloidal complex, the SPM particles contained within it clump together, forming clusters. This effect shifts the particle size distribution and, thus, the decay pattern of SPM responses. The processes of cluster formation and breakdown continuously alternate, generate anomalies in the recorded TEM signals. IP and SPM processes are typically concomitant, but the balance between of antiphase SPM–IP effects is extremely unstable over time, with polarity-flipping variations in TEM signals times greater than the inductive responses of induced polarization of the medium. This paper presents the results of three-year monitoring of the IIP, IP, and SPM effects and explores previously unknown SPM properties of soils.