Spectral impedance measurements are receiving increased attention with regard to the characterization of soils, sediments and rocks, particularly in terms of the internal rock structure, the mineralogical composition and the chemistry of fluids contained in porous rocks. In fluid-saturated, porous sedimentary rocks, which are of particular relevance for many hydrological and environmental problems, the polarization processes that give rise to an observed phase shift between input current and output voltage signals are caused by the interaction of the electrolyte in the pores of the rock with electrically charged mineral surfaces. However, this phase response is relatively weak, typically smaller than 10 mrad and sometimes even of the order of only 1 mrad. In order to reliably measure such phase responses in the relevant frequency range, a high-accuracy impedance spectrometer is required. This system must allow phase measurements between 1 mHz and 1 kHz with a phase accuracy better than 0.1 mrad. In this paper, we present a new impedance spectrometer which meets these requirements. It is based on the four-point measurement method and offers a measurement range from 1 mHz to 45 kHz. Furthermore, we present design information for the sample holder and the electrodes, and methods for performing numerical corrections to reduce measurement errors. The overall accuracy of the setup was validated using water and sand with well-defined polarizable objects.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.