A spectral induced polarization instrument using square-wave current injection to track critical zone processes: application to long-term monitoring of a wetland

SUMMARY In the last two decades, the improvement of both instruments and theory, as well as the broadened scope of applications, led to a spectacular development of the use of induced polarization. In particular, the richness of complex conductivity spectra is driving the scientific community towards vast deployment of this measurement method often referred to as spectral induced polarization (SIP). In this contribution, we describe an innovative multichannel instrument that we develop for fast monitoring of critical zone processes. The spectral content of a signal with line spectrum resulting from square-wave current is exploited by injecting successively three square-wave currents with periods of 1, 10 and 100 s, covering the frequency range of 10−2 to 102 Hz in less than 4 min. One data set consists of eight successive current injections at different depths. For each current injection, the electrical potential is simultaneously measured at seven dipoles. The time-series are recorded with a 2 kHz sampling rate, allowing to calculate by Fourier transform the amplitude and phase spectra up to 1 kHz for each quadrupole. The complex conductivity data was validated by a comparison with the commercial SIP-Fuchs instrument, despite a significant discrepancy below 0.1 Hz which may be due to a worse signal-to-noise ratio at low frequencies. The prototype version of the instrument has been installed in 2018 at a wetland at Ploemeur-Guidel hydrogeological observatory to monitor reactive processes with high spatial resolution across the top metre of soil. The instrumental device, controlled by a Gantner data acquisition system connected to a solar panel, is fully autonomous and consumes little energy. Acquisitions are made several times a day and recorded on a SD card. Seven-year continuous monitoring highlights significant temporal variations of both subsurface resistivity and phase angle. The absence of correlation between resistivity and phase variations in the continuously saturated soil thickness highlights the potential of the system to monitor and separate different types of dynamics processes, such as groundwater/surface water mixing and mineral precipitation/dissolution.