Seismic and Electrical Resistivity Investigations for the Planning of a Tunnel in Oslo Outskirts

Oslo municipality is presently planning bus and car tunnels to facilitate its accessibility and increase traffic efficiency. Urban environment is usually a challenge for geophysical pre-investigations because of the various sources of noise, vibrations and restriction both in time and space. These technical challenges were overcome with the use of a newly developed seismic streamer specifically designed for noisy urban areas, from an industry-academia partnership. A total of 3.5 km long seismic data along 14 profiles were acquired for the tunnels pre-investigation with the main goals of (1) obtaining information about depth to bedrock, (2) detecting potential weakness zones, and (3) optimizing the number of drillings and their locations for a follow-up study. In addition, six electrical resistivity tomography profiles were acquired near the planned tunnel alignments. Inversion of first breaks and electrical resistivity data provides a seamless depth to bedrock interface that is in most places in good agreement with the nearby geotechnical soundings. In addition, the geophysical sections reveal the bedrock undulation character and provide some indication of weakness zones. This case study also illustrates that if the pre-investigation had been based only on boreholes, it would have overseen a potential difficulty during excavation.