Unforeseen rock conditions are a risk factor that often leads to delays and extra costs during construction works. There are two geological conditions that are considered important to identify, manage or possibly even avoid, for safe and efficient underground constructions. The first is poor quality rock and the second is a large leakage of groundwater into the tunnel. Water passages are often the parts of a rock tunnel where there is the greatest risk of problems with poor rock quality and water leakage.

The traditional method for finding the fresh rock surface is soil/rock probing, which, however, provides only punctual information. Moreover, extensive and expensive logistics are required to probe on the water.

Integrated geophysical surveys, using both seismic refraction (SR) and geoelectric tomography (ERT), can provide valuable information about the geological structures, which together with traditional geotechnical survey results can create synergy effects and improve the end result.

The project is a feasibility test of DAS (Distributed Acoustic Sensing) for mid-scale underwater seismic surveys. DAS uses fiber optics as seismic sensors and there are several advantages instead of traditional sensors:

  • Lighter and thinner cables enable to use smaller and cheaper boats that are easy to manoeuvre and can be used in shallow water.

  • Co-located ERT and SR cables eliminate uncertainty of sensor positions.

  • Simultaneous acquisitions of ERT and SR via DAS is possible, because the is no signal interference between the methods. This means a considerable reduction of the measurement time in the field.

  • Increased resolution compared to traditional hydrophone cables that usually have a 5 m hydrophone distance.

This project was carried out during the years 2018-2020 at the Department of Technical Geology at Lund University, Lund University. Funding came from BeFo (Stiftelsen Bergteknisk Forskning, id: 214) and Vinnova (id: 2018-00643), and support from Hydroresearch AB, especially Sam Johansson, and Silixa Ltd. Matteo Rossi and Roger Wisén are the main authors of the final report and have performed all seismic measurements and analysis of data.

The project’s reference group supported the project and consisted of Björn Lund (Uppsala University), Claes Vahlberg and Fredrik Kullander (Swedish Defense Research Institute), Tara Wood (Ramboll), Lars Nilsson (NCC), Martin Holmén (Swedish Geotechnical Institute) and Per Tengborg (BeFo). 

Stockholm 2021
Per Tengborg