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.