High-resolution offshore reflection seismic investigation of the Stockholm Bypass tunnel

Boat-towed high-frequency, short source and receiver spacing, reflection seismic data, 16 profiles and in total 3884 m long, were acquired in 2008 for the planning of the Stockholm Bypass multilane (3 lanes in each direction in two different tubes) underground motorway tunnel designed to ease the ever increasing car traffic on the city and neighbouring regions. The planned bypass will be about 21 km long of which 18 km will be in the form of bedrock tunnel. The planned tunnel will intersect three water passages at where the tunnel will be at around 40-50 m depth. In this study, the seismic data along two of three water passages have been processed and interpreted. Due to the relatively shallow water depth (< 10 m), the main challenge for the reflection data processing was the interferences of strong multiples from lake sediments and bedrock. After a number of tests, it was found that conventional processing methods could not attenuate multiples effectively. Therefore, an optimized workflow based on predictive deconvolution de-multiple method was developed. The new workflow proved to be effective at suppressing multiple reflections, while primary reflections as well as diffraction signals could be well preserved. After carefully attenuating the multiples in the pre-stack and post-stack domains, processing continued with time-to-depth conversion for data interpretations. To reduce uncertainty with time-to-depth conversion errors, bathymetry data available from the study area were used to match the water-sediment interface that also generated a clear reflection in the data. Bedrock surface shows strong undulations, which is typical for the Scandinavian geology from steep valleys to sometimes sub-horizontal at some parts of the water passages. Nevertheless, a dominant bedrock valley-type direction can be recognized striking in the same direction as the water passages. The planned tunnel at the nearest point is estimated to locate approximately 19 m below bedrock surface, which is important factor for the excavation planning of the tunnel and its reinforcements. The steep valley-shaped bedrock may also imply a zone of weakness, fault and/or fracture zone, where the water passages were formed and the reflection seismic data clearly depict this shape under the overlying saucer-shaped unconsolidated sediments