Laboratory investigation of high-frequency electromagnetic induction measurements for macro-scale relaxation signatures

Abstract

SUMMARY Direct contact, frequency-dependent, electrical relaxation effects in soils are well documented in both conductivity and dielectric permittivity-dominated frequency regimes. These relaxation signatures result from charge displacement in physical processes at varying scales ranging from conductivity-dominated ionic transport at soil–grain pore-fluid interfaces to permittivity-dominated water molecule rotation. Given the physical mechanisms associated with these relaxation signatures, it is possible to gain vital soil characteristics needed for a variety of civil and environmental applications. Yet, at the field scale, these direct contact geophysical methods are generally time-consuming and cumbersome hindering the extent at which data can be reasonably acquired. Here, we evaluate a standoff high-frequency electromagnetic induction (HFEMI) instrument, designed for remote, non-contact detection of non-metallic unexploded ordinance and repurposed as a means to measure soil relaxation effects remotely. The instrument offers a proven interrogation range of 100 kHz–10 MHz but has the ability to record data as low as 100 Hz. In this laboratory study, we demonstrate the effects of sample volume using NaCl solutions, where greater volume samples are necessary to overcome low signal-to-noise signatures. Further, pyrite, a mineral known to exhibit relaxation signatures at low frequencies was used in sand mixtures of varying concentrations and varying pore-fluid conductivity to explore the lower frequency range of the HFEMI instrument. The resulting measured HFEMI responses demonstrated sensitivity to the changes in conductivity due to increased pyrite and pore-fluid NaCl concentrations at high frequencies, but the lower frequency range of 100 Hz to 100 kHz was dominated by system noise, disallowing interrogation of low-frequency relaxation effects. This initial investigation compiles the comparative knowledge for relaxation responses in both direct electrical and standoff electromagnetic measurements while demonstrating high-frequency signatures of low-conductivity soils and sample volume effects. As a result of this research, a new EMI system is in development addressing a lower range of frequencies for continued relaxation process investigations.

Application of Disturbance Observer-Based Fast Terminal Sliding Mode Control for Asynchronous Motors in Remote Electrical Conductivity Control of Fertigation Systems

“…However, these traditional approaches fall short in addressing the challenges of remote electrical conductivity control's nonlinear and dynamic nature, especially in dealing with irregular fluctuations [21]. The complexity arises from continuous variations in moisture levels, soil characteristics, and environmental factors [22]. Consequently, there remains a gap in developing a control system that effectively integrates robustness, precision, and adaptability, particularly under variable and unpredictable agricultural conditions [23].…”

Experimental and numerical analysis of dielectric polarization effects in near‐surface earth materials in the 100 Hz–10 MHz frequency range: First interpretation paths

“…Due to technical limitations, most of the corresponding devices use low frequencies in the 10-100 kHz range, but several attempts reached the 300 kHz-3 MHz medium frequency band, with both frequency (Glaser et al, 2022;Kessouri et al, 2016) and time-domain (Auken et al, 2019;Wright et al, 2000) instruments. In these frequency ranges, between frequencies dominated by conductivity and those by dielectric permittivity, electrical polarization cannot be neglected in comparison with conductivity (Glaser et al, 2023;Loewer et al, 2017;, and it is necessary to consider the dielectric permittivity together with the electrical conductivity and the magnetic susceptibility. Some interpretations of field results acquired with EMI (e.g., Simon et al, 2019) and electrostatic instruments (e.g., Schamper et al, 2021;Souffaché et al, 2016) led to apparent relative permittivity in accordance with the published laboratory measurements.…”