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Abstract In the frame of a Research and Development (R&D) project dedicated to quantitative ground modelling, Underwater Multichannel Analysis Surface Waves (UMASW) data were acquired within an offshore windfarm area currently in development in North Sea. The ultimate goal of the project described in this paper was to test the added value of the shear wave velocity (Vs) interpreted from the UMASW, particularly as a propagation tool of the geotechnical information. This requires a suitable Vs dataset within the foundation depth and the quantification of the associated uncertainties. The Vs of the ground is generally accessed in one dimension (1D) through geotechnical borehole (P-S-Primary & Shear seismic wave- logging or laboratory sample testing) or through S-CPT (Seismic Cone Penetration Test) in-situ testing within the offshore wind industry. The present acquisition was then an opportunity to compare both UMASW and borehole results, and to assess the spatial variability of the Vs around the boreholes. The UMASW acquisition was designed to allow a pseudo three-dimensional (3D) gridding of the data, with a 10m bin size, and to reach a minimal signal penetration depth of 60m below seabed (bsb), compatible with the geotechnical investigation. Two block areas of about 400m × 300m, each centered around a borehole location, have been investigated together with a corridor area of 2300m × 20m tying the blocks. The spread used for the acquisition includes a sledge equipped with a 60cu.in airgun and a 166m long streamer with 48 channels. The sledge, dragged on the seabed, was tracked along intended 10m spaced lines with a shot point interval of 10m. The data processing was done for each shot and includes the manual picking of the dispersive curves and a 1D inversion modelling. The pseudo 3D gridding was then carried out by interpolation with the closest neighbors. The results are found to be of good quality, with clear dispersive curves and a signal penetration reaching about 80m bsb. The standard deviation of the processed Vs remains generally below 3%. A first comparison of the Vs with the existing 2D Ultra High Resolution Seismic (UHRS) reflection data shows a good stratigraphic match with the key seismic horizons, while detailing some significant variability across the corresponding units. While UMASW, together with seismic refraction, is regularly used to characterize the ground for burial or trenching purpose, it appears to also bring valuable design parameters for the offshore wind turbine foundations.
Unterseh et al. (Mon,) studied this question.