Delineating hazardous material without touching — AEM mapping of Norwegian alum shale

Abstract

Alum shale is a particular type of metamorphic clay that occurs in Cambro-Ordovician metasediment units throughout southern Scandinavia. Owing to its geochemical composition, it is considered a large environmental challenge in Norway (Endre and Sormo, 2015) and poses a significant site hazard for infrastructure projects (Endre, 2014). Swedish alum shale has such a high Uranium content, that is was mined as a Uranium ore during the mid-1900s (Dyni, 2006). Norwegian alum shale typically contains more than 15 g/kg sulphides and 60-200 mg/kg Uranium. It is consequently a source for radon gas posing a risk to human health. The sulphides oxidize to produce sulphuric acid with pH down to 2-1 once the shale is exposed to air and water, posing a hazard to concrete and metal constructions and the environment. Finally, the most critical geotechnical risk connected to alum shale is its intense swelling owing to oxidation that has damaged properties in the vicinity of building sites that exposed alum shale units and consequently initiated swelling. Prior knowledge of the existence and extent of this hazardous shale is consequently a major risk management factor for underground building activities in areas prone to alum shale. The state of practice for identifying alum shale and other acid producing black shales classified by the Norwegian Environment Agency (Endre and Sormo, 2015), is primarily based on geochemistry and an understanding of the local geology. Geochemical identification is based on the balance between acidification- and neutralisation potential (Pabst et al., 2016) the geological risk for projects in alum shale prone areas is consequently high. In the worst case, a project may have to be abandoned, for example if the excavated shale volume exceeds the capacity of special waste landfills in the area. Geophysical delineation can decrease the geological risk and increases the efficiency of detailed volume assessments. Since 2014, we have been studying how sulphide and uranium content of various black shale types relate to electrical properties. Sulphide-rich alum shale has very low resistivity (~0.1 Ωm) (Lysdahl et al., 2015) and a strong induced polarization signature (Lysdahl et al., 2016). In the following we show the relationship between geochemically classified shale types and resistivity measurements directly on drill cores as well as surface electrical resistivity tomography (ERT) data. Two case histories show black shale units even at depths exceeding 80 metres based on airborne electromagnetic (AEM) surveying. Here we only focus on resistivity, for further studies on the polarization behaviour of alum shale please see Bazin et al. (2015) and (2017).