Airborne geoscanning as a site investigation tool in large-scale tunnelling projects: a synthesis of case studies from Norway and India

We present a slightly abridged and adapted translation of the paper “Airborne geoscannin g as a site investigation tool in large-scale tunnelling projects: a synthesis of case studies from Norway and India” by Norwegian and Indian researchers (Rasmussen et al., 2021). It was published in 2021 in the journal “Earth and Environmental Science” by the publishing company of the British scientific society “Institute of Physics” (IOP) that is now virtually international. It is an open access article under the CC BY 3.0 license that allows it to be distributed, translated, adapted, and supplemented, provided that the types of changes are noted and the original source is referred to. In our case, the full reference to the original paper (Rasmussen et al., 2021) used for the presented translation is given in the end. Unforeseen, challenging grounding conditions are a major obstacle for infrastructure development, including tunnel construction. Addressing this risk with traditional, intrusive ground investigations can be costly, sometime prohibitively so. In this paper, we present airborne geoscanning, a more efficient site investigation method that integrates airborne geophysics with other datasets to produce ground models. We primarily employ helicopter- based time-domain electromagnetics (AEM), a method that images differences in electrical resistivity in the subsurface. When available, we can combine geophysical data with ancillary datasets for more sophisticated interpretation. Such an integrated process we call airborne geoscanning. The integration techniques range from simple clustering analysis that support planning of follow-up ground investigations to customised artificial neural networks that automatically detect interfaces like the top of rock. Using examples from projects in Norway and India, we illustrate the strengths and weaknesses of using airborne geophysics for tunnelling projects. Using these case studies, we demonstrate three key insights that airborne geoscanning can provide to tunnelling engineers, i. e. identify major fractured zones, weaker rock units and rock cover thickness. These insights can be highly valuable for tunnel design and construction projects worldwide.