EIT Measurement System with High Phase Accuracy for the Imaging of Spectral Induced Polarization Properties of Soils and Sediments

E. Zimmermann1, A. Kemna2, J. Berwix1 and W. Glaas1

1. Central Institute for Electronics (ZEL), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany, Email: e.zimmermann@fz-juelich.de

2. Institute of Chemistry and Dynamics of the Geosphere, Agrosphere (ICG-4), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany

ABSTRACT

A powerful tool for the structural characterization of material inside objects is electrical impedance tomography (EIT), combined with the capabilities of impedance spectroscopy. This tool determines the complex resistivity magnitude and phase images at a set of different measurement frequencies. We are particularly interested in the application of such an advanced approach for the improved characterization of soils and sediments, which show only a weak polarizability. Here, typical phase values lie between 1 and 10 mrad, requiring instrumentation with relatively high phase resolution and accuracy. Here, we present a new spectral EIT data acquisition system for laboratory applications, operating in the frequency range from 1 mHz to 45 kHz, which was developed to meet these requirements. In this context, we also present a new measurement method based on current injection swapping, which results in significantly improved phase images, in particular for higher measurement frequencies. The system is equipped with 96 electrodes with integrated amplifiers for electric potential measurements and 32 electrodes with integrated switches for current injection. The capacitive load of the potential electrodes and the inactive current electrodes is less than 5 pF and 1 pF, respectively. The data acquisition is based on 104 high-resolution 24-bit ADCs that permit simultaneous sampling. The system and the new measurement method were tested on a water-filled tank and column which contained different 2D and 3D targets (metallic and biological objects). The tests proved a phase accuracy of 1 mrad for frequencies up to 1 kHz and higher, resulting in a clear discrimination of the objects on the basis of the reconstructed phase images.

Keywords EIT, imaging, impedance spectroscopy, spectral induced polarization, electrical soil and sediment properties