The use of High Resolution Multichannel Seismic Data and Geotechnical Records to Investigate Near Surface Seabed Structures in the North Sea

Due to highly variable seabed characteristics within very small areas in the North Sea, the near surface investigation becomes a dominating aspect in order to ensure an economic design of offshore structures, e.g. for the foundation design of windmills. The aim of a research project at the Research Center Ocean Margins (RCOM) at the University of Bremen, Germany, is the geophysical seabed characterization and its correlation with geotechnical data. In the course of the project a new shallow-water multi-channel seismic system was developed to collect high-resolution multichannel seismic data and a new dynamic triaxial testing unit was designed to simulate the effects of cyclic loads on the sedimentary strata. During two cruises in 2005 and 2007, which were carried out in a cooperation between the RCOM and the BGR (Hannover, Germany) as well as the Senkenberg Institute (Wilhelmshaven, Germany), respectively, two designated wind farm areas in the shallow North Sea were visited to collect high resolution multichannel seismic data, very high resolution acoustic data as well as swath sounder bathymetry data. Additionally, cores for geotechnical measurements were drilled with a vibrocore system. As seismic source, a Mini GI Gun of 0.2 L primary chamber volume was used, the seismic signals were recorded with a 50 meter long streamer of 48- channels, which are realised by single hydrophones evenly distributed at a spacing of 1 m. In both study areas the seismic data reveal a complex and heterogonous subsurface. The main structural features are filled channels of different dimensions, depths, and filling characteristics. These channels are often cut into each other. The reliefs of the channels vary between just a few meters and up to 50 m, their widths reach values between 200 m and 2000 m. In one of the study areas at least two stages of channel development could be identified. Ongoing work on the seismic data and cores should establish in the future a correlation between the seismic stratigraphy and the geotechnical parameters.