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

Abstract

Unforeseen, challenging ground 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, an integrated process we call airborne geoscanning. 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 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: identify major fractured zones, weaker rock units and rock cover. These insights can be highly valuable for tunnel design and construction projects worldwide.