Design and operation of a long-term monitoring system for spectral electrical impedance tomography (sEIT)

Abstract

Abstract. Spectral electrical impedance tomography (sEIT) is increasingly used to characterise the structure of subsurface systems using measurements in the megahertz to kilohertz range. Additionally, hydrogeophysical and biogeophysical processes are characterised and monitored using sEIT. The method combines multiple, spatially distributed, spectroscopic measurements with tomographic inversion algorithms to obtain images of the complex electrical resistivity distribution in the subsurface at various frequencies. Spectral polarisation measurements provide additional information about the systems under investigation and can be used to reduce ambiguities that occur if only the in-phase resistivity values are analysed. However, spectral impedance measurements are very sensitive to details of the measurement setup as well as to external noise and error components. Despite promising technical progress in improving measurement quality as well as progress in the characterisation and understanding of static polarisation signatures of the subsurface, long-term (i.e. multi-month to multi-year) monitoring attempts with fixed setups are still rare. Yet, measurement targets often show inherent non-stationarity that would require monitoring for a proper system characterisation. With the aim of improving operating foundations for similar endeavours, we here report on the design and field deployment of a permanently installed monitoring system for sEIT data. The specific aim of this monitoring installation is the characterisation of crop root evolution over a full growing season, requiring multiple measurements per day over multiple months to capture relevant system dynamics. In this contribution, we discuss the general layout and design of the monitoring setup, including the data acquisition system, additional on-site equipment, required corrections to improve data quality for high frequencies, data management and remote-processing facilities used to analyse the measured data. The choice and installation of electrodes, cables and measurement configurations are discussed and quality parameters are used for the continuous assessment of system functioning and data quality. Exemplary analysis results of the first season of operation highlight the importance of continuous quality control. It is also found that proper cable elevation decreased capacitive leakage currents and in combination with the correction of inductive effects led to consistent tomographic results up to 1 kHz measurement frequency. Overall, the successful operation of an sEIT monitoring system over multiple months with multiple daily tomographic measurements was achieved.

Linking horizontal crosshole GPR variability with root image information for maize crops

“…(2020, 2021), and Weigand et al. (2022). Other soil and root measurements were analyzed by Cai et al.…”

“…The deviations in horizontal distance between R UT rhizotubes were less than 0.02 m. Additionally, in order to make the measurements within the rhizotubes, various sensors were installed within the soil to measure the soil water potential (tensiometers and MPS‐2 sensors manufactured by UMA GmbH and Decagon Devices, Inc., respectively), soil water content (time domain reflectometer in‐house production), and soil temperature (MPS‐2 sensors). Above ground at R LT , there was a monitoring system for sEIT installed 3.5 m away from the trench (Weigand et al., 2022).…”

“…The belowground data acquired within the MR facilities between 2016 and 2021 can be found in Lärm et al (2023). The belowground geophysical measurements were analyzed by Cai et al (2016), Klotzsche, Lärm, et al (2019), Yu et al (2020), and Weigand et al (2022. Other soil and root measurements were analyzed by Cai et al (2017Cai et al ( , 2018 and Morandage et al (2021).…”

“…Only data between 2.5 and 6 m were considered because GPR data from close to the access trench where the soil sensors were located were not reliable and were therefore excluded. The 1.2‐m depth in Plot 3 of R LT was not measured due to a broken rhizotube, and a data gap is present between 2.8 and 3.3 m along the rhizotubes at 0.2‐m depth due to the installed sEIT line (Weigand et al., 2022), which affected the GPR measurements.…”

Multi-year belowground data of minirhizotron facilities in Selhausen

“…In addition to the measurements (GPR and root images) that can be performed within the rhizotubes, various soil sensors are embedded within the soil (see Soil Sensor Data section). Above ground at R lt , there is a monitoring system for spectral electrical impedance tomography (sEIT) 45 .…”