A high-accuracy impedance spectrometer for measuring sediments with low polarizability

Abstract

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.