From microstructure to subsurface characterization. Spectral information from field scale time domain induced polarization

Abstract: A large variety of subsurface infrastructure projects of different types can gain valuable information from tomographic measurements of subsurface physical properties. With resistivity and time domain spectral induced polarization (IP), the distributions of the conductive and capacitive properties of the subsurface are obtained. Technical developments of time domain IP equipment and new inversion algorithms have led to the possibility of collecting large amounts of data and invert for spectral IP parameters. In this way, much more information about the subsurface can be extracted from a single measurement compared to what was possible previously. It is well-known that spectral IP effects arise through redistribution of ions at the pore scale, and that spectral IP parameters can be linked to microscale surface chemical and structural properties. However, much remains to be known about how to analyse and interpret spectral IP parameters from field scale time domain IP measurements. Therefore, the main objective of this licentiate thesis is to focus on these issues. In soils contaminated with free phase non-aqueous phase liquids (NAPLs), the contaminant phase affects the microgeometry inside the pore spaces of the soil. The results from paper I show that different configurations of the NAPL phase likely affect the spectral IP response of the soil in different ways. This is exemplified by measurements at a field site, where IP responses are absent in the NAPL source zone. In contrast, elevated IP responses are observed in the degradation zone, where the NAPL probably is configured as isolated droplets in the pore space. In order to investigate probable sources to observed varying IP responses in a Cretaceous limestone bedrock, the measured spectral IP parameters were compared with the investigations of the microstructure and composition of thin sections in paper II. Several characteristics that can affect the measurements were found, such as presence of certain minerals and varying texture of the limestone. However, more research is needed in order to understand the polarization mechanisms in limestones. Although most previous research has been carried out in sand- and claystones, there is a great potential to reveal textural and structural properties in any kind of bedrock by measurements of spectral IP parameters.