EIT measurement system with high phase accuracy for the imaging of spectral induced polarization properties of soils and sediments

Abstract

A powerful method for the non-invasive structural characterization of material is electrical impedance tomography (EIT) combined with the capabilities of impedance spectroscopy. This method 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 only show a weak polarizability. Here, typical phase values lie between 1 and 20 mrad only, requiring instrumentation with relatively high phase resolution and accuracy. In this paper, we present a new spectral EIT data acquisition system for laboratory applications, which operates in the frequency range from 1 mHz to 45 kHz and which was developed to meet these requirements. In this context, we also present a new measurement method based on current injection swapping, which leads to significantly improved phase images, particularly for higher measurement frequencies. The system and the new measurement method are tested on a water-filled tank and column containing different 2D and 3D targets (metallic and biological objects). The tests prove a phase accuracy of 1 mrad for frequencies of up to 1 kHz and higher, resulting in a clear discrimination of the objects on the basis of the reconstructed phase images.

Phase correction of electromagnetic coupling effects in cross-borehole EIT measurements

“…impedance spectrometers have been developed to measure the complex electrical resistivity with sufficient accuracy in the mHz to kHz frequency range (Zimmermann et al 2008). Spectral Electrical Impedance Tomography (EIT) is a new method which combines the diagnostic potential of SIP with the imaging capability of tomography (see e.g.…”

“…To image the spectral phase response of low-polarizable soil and sediment samples, a spectral EIT data acquisition system was developed by Zimmermann et al (2008), which reaches a phase accuracy of better than 1 mrad at 1 kHz in the laboratory under optimal conditions. Recently, electrode chains and logging tools have been developed to allow borehole EIT measurements in the mHz to kHz range using the EIT data acquisition system described in Zimmermann et al (2008).…”

“…The EIT system used in this study is an extension of the EIT laboratory system (Zimmermann et al 2008(Zimmermann et al , 2010) that was designed to make accurate impedance measurement using two electrodes for current injection and two electrodes for potential measurement. The system has 40 channels that can be used for current injection and potential measurement at each electrode.…”

“…The complex resistivity distribution was reconstructed in 1D (z-direction) using a 3D forward model (Zimmermann et al 2008) of the subsurface, as already done in Zhao et al (2013). The regularization (smoothing) in the inversion was applied to the z-direction to obtain a smooth profile and to stabilize the reconstruction.…”

“…Vanhala 1997, Slater and Lesmes 2002, Binley et al 2005. Several studies have reported that the phase angle of the complex electrical resistivity varies with the frequency (Binley et al 2005, Leroy et al 2008, Zimmermann et al 2008, Breede et al 2012 and advanced…”