Urban seismic site investigations for a new metro in central Copenhagen: Near surface imaging using reflection, refraction and VSP methods

Abstract

Planning and execution of large infrastructure projects benefit from detailed mapping of the subsurface through geophysical methods. However applying geophysical methods in an urban environment requires careful preparation to ensure success in areas where source of noise and present infrastructure may compromise the results. We present and discuss the results of a seismic investigation conducted in Copenhagen, Denmark. The surveys included seismic refraction, reflection and “walk-away” Vertical Seismic Profilings (VSP). The overall aim was to produce complementary information needed for geological, geotechnical and groundwater modeling assessments related to the planned Cityringen underground metro project. The specific objectives were to map structural features and delineate the top surface of a limestone sequence. The surface seismic surveys were comprised of six separate survey lines with a combined receiver length of 12.8 km. The vertical seismic profilings were made at the available boreholes adjacent to the surface seismic surveys. Reflection analysis was performed for all surface seismic surveys and refraction tomography was applied to both surface surveys and VSP. The tomography results indicate the velocities at the interface between sediments and limestone to be between 1700 ms −1 and 2000 ms −1. Additionally, localities with buried valleys along the planned metro line were identified. The reflection analyses show the presence of a fault system across the study area. Data quality was affected mainly by infrastructure and external sources of noise on receivers. The use of two modalities of surveying with a large scale surface data acquisition combined with borehole “walk-away” as control data allowed to integrate the results with known geology. In general, the results demonstrated that it is possible to successfully use the seismic method in an urban environment provided a detailed planning and execution is implemented.