Temperature-dependent hysteresis effects on EM induction instruments: An example of single-frequency multi-coil array instruments

Abstract

Non-invasive electromagnetic (EM) induction has been used in agriculture, earth science and archaeology. This is because the apparent electrical conductivity (ECa or quadrature response – mSm−1) and the apparent magnetic susceptibility (in-phase response – ppt) which they measure are related to soil properties such as clay, soil mineralogy, salinity and soil moisture as well as buried metal objects. Although the accuracy issues of the single-coil array EM38 meter have been widely discussed, the accuracy issues of the next generation, multi-coil (perpendicular-PRP and horizontal-HCP) array DUALEM meter, particularly the instrument drift, have little been reported. In this study, the diurnal drifts of a DUALEM-421S and a DUALEM-21S were studied at single locations (for a 24h period) and along a 480-m transect (at five typical operation times). Based on the experiment results of the two DUALEM instruments, it was found that the ECa readings of the PRP arrays were more stable than those of the HCP arrays. The reverse was true for in-phase measurements. Specifically, during the diurnal cooling and heating phases, ECa measurements of the HCP arrays and in-phase PRP arrays showed different correlations with ambient temperature, which can be defined as instrument-specific and temperature-dependent hysteresis effects. In addition, the stability of ECa and in-phase measurements increased with array length and was much less compared to the theoretical values. It was suggested a similar experiment should be conducted for the DUALEM instruments before the DUALEM surveys and repeated DUALEM surveys for mapping the spatio-temporal variations in soil properties should be carried out at the similar temperature (i.e., similar ambient temperature and within the same warming or cooling phase). In addition, shading the instruments with non-conductive thermal insulation should be adopted and drift correction procedures should be applied to improve the quality of the measurements of the EM instruments.