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

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.