Correction of phase errors due to leakage currents in wideband EIT field measurements on soil and sediments

Abstract

Electrical impedance tomography (EIT) is a promising method to characterize important hydrological properties of soil, sediments, and rocks. The characterization is based on the analysis of the phase response of the complex electrical conductivity in a broad frequency range (i.e. mHz to kHz). However, it is challenging to measure the small phase response of low-polarizable soils and rocks in the higher frequency range up to 10 kHz. In order to achieve the required phase accuracy in the kHz frequency range, an optimized measurement system and advanced model-based processing methods have been developed. Recently, EIT measurements at sites with low electrical conductivity have shown a new dominating phase error related to capacitive leakage currents between cable shields and soil. In order to correct this phase error, we developed an advanced finite element model that considers both leakage currents and capacitive coupling between the soil and the cable shields in the reconstruction of the complex electrical conductivity distribution. This advanced model also takes into account potential measurement errors due to high electrode impedances. The use of this advanced model reduced the new dominating error for media with low electrical conductivity. It was also found that the amount of leakage current is an additional indicator for data quality that can be used for data filtering. After application of a novel data filter based on the leakage current and the use of the advanced modelling approach, the phase error of the measured transfer impedances above 100 Hz was significantly reduced by a factor of 6 or more at 10 kHz. In addition, physically implausible positive phase values were effectively eliminated. The new correction method now enables the reconstruction of the complex electrical conductivity for frequencies up to 10 kHz at field sites with a low electrical conductivity.

Field-scale comparison of frequency- and time-domain spectral induced polarization

“…Recently, methods have been presented for modelling-based removal of both inductive and capacitive coupling, for borehole applications (Zhao et al 2013), cross-hole applications (Zhao et al 2014) and surface measurements (Zimmermann et al 2016;Kang & Oldenburg 2017). All these methods require an accurate knowledge of the cable relative positions for the removal of the inductive coupling, and fan-like cable layouts have been proposed for surface applications (Zimmermann et al 2016). This limits the actual usability of the correction in the field considerably, especially when electrode distances bigger than 1 m are used.…”

“…Hence, low-noise sequences with nested dipoles, like the gradient array, are still very difficult to use in big-scale field FD applications. Furthermore, Zimmermann et al (2016) points out that you actually need a model-dependent correction, which takes the leakage current and the total parasitic capacity between system ground and the soil into account.…”

Induced polarization applied to biogeophysics: recent advances and future prospects

“…Dahlin and Zhou 2006;Flores-Orozco et al 2013;Kelter et al 2018). Correction methods for capacitive and inductive coupling effects have also been developed for borehole (Zhao et al 2013(Zhao et al , 2015, and surface FDIP measurements (Zimmermann et al, 2019), but have not yet been transferred to commercially available measurement equipment.…”

Spectral Induced Polarization imaging to investigate an ice-rich mountain permafrost site in Switzerland

“…Laboratory studies have demonstrated the sensitivity of SIP measurements to changes in temperature above the freezing point (e.g., Binley et al, 2010;Zisser et al, 2010) and in frozen samples (e.g., Kemna et al, 2014a;.…”

“…The main challenge regarding the conduction of field SIP measurements corresponds to the contamination of the data at frequencies above 1 Hz by electromagnetic (EM) coupling (e.g., Flores Orozco et al, 2021;Pelton et al, 1978;Zimmermann et al, 2019). EM coupling may result in self-induction of the near subsurface materials, cross-talking between cables, and the https://doi.org/10.5194/tc-2021-234 Preprint.…”