High-resolution reflection seismic imaging for the planning of a double-train-track tunnel in the city of Varberg, southwest Sweden
A newly developed broadband digital-based seismic landstreamer system was employed for the planning of a double-train-track tunnel in the city of Varberg, southwest Sweden. Twenty-five seismic profiles, totalling more than 7.5 km of data, were acquired using a 2- to 4-m receiver and source spacing. At places where it was not possible to move the streamer such as road crossings, wireless recorders connected to 28-Hz geophones were used. In addition to the earlier refraction data analysis and first-break traveltime tomographic modelling, reflection processing of the data was considered in this study, given the realisation of reflections in raw shot gathers and their good quality. Bedrock is strongly reflective in most cases but is not evident in the sections when it gets near the surface. Bedrock undulation is noticeable in most reflection sections, and at one occasion, strong diffraction is observed in the bedrock or near to it. The diffraction is originated, not known during the survey, from a 400-m3 cylindrical-shaped (of about 3-m-height and 13-m-diameter) concrete-made fire-protection water tank sited in the bedrock and used in emergency situations. Reflection seismic data greatly complement the tomographic models and support deep bedrock where the excavation of the tunnel is planned in downtown Varberg. This interpretation implies different reinforcements and tunnel construction methods (e.g., roofed concrete) at this section of the tunnel. In addition, weakness zones associated with fracture systems are inferred from the reflection characteristics and in conjunction with the velocity models requiring verification by additional boreholes.