Introduction
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 Sørmo, 2015) and poses a significant site hazard for infrastructure projects (Endre, 2014). 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 Sørmo, 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.
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 meters 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).
Conclusion
Norwegian alum shale is a serious, geological risk for geotechnical projects because of its geochemical composition and consequent environmental hazards. On the other hand, its richness of sulphides and uranium makes it a very practical target for geophysical investigations. AEM provides both the resolution and band width to map and delineate alum shale in a very efficient manner, given a sufficient project size. For smaller projects, ground-based resistivity measurements can fill the gap and can further provide higher resolution than the AEM data.
Acknowledgement
The majority of the presented work has been performed for the Norwegian Public Roads Administration and we are grateful for funding and permission to publish. Some of the detailed analysis has been funded by the NGI sabbatical fund. GHS carried out parts of her studies within a joint NGI Perth and CSIRO Perth research co-operation funded by CSIRO. Our former colleague Erik Endre has been instrumental in the initial phases of this work especially with respect to the interpretation of resistivity of Norwegian grey and black shales.
Reference
The full paper can be requested through the download link above or found directly at www.researchgate.net:
Pfaffhuber, A. A., Lysdahl, A. O. K., Sørmo, E., Bazin, S., Skurdal, G. H., Thomassen, T., Anschütz, H., and Scheibz, J. 2017. Delineating hazardous material without touching - AEM mapping of Norwegian alum shale, First Break, 35 35-39.
