Pagecontent:
Multifrequency Seismoacoustic Survey of a planned Wind Farm Area
Introduction
In November 2005 a seismoacoustic survey was performed in the southern German Bight with the research vessel FK Senckenberg. The main purpose of that cruise was, to obtain high resolution multichannel seismic data in the shallow waters off the Eastern Frisian Islands.
Three days of the cruise were used to survey a designated wind farm area off Borkum with a multitude of acoustic system. Although FK Senckenberg is a comparably small vessel, it was possible to operate in parallel the Bremen shallow water multichannel seismic system with two different seismic sources, an Innomar SES2000 parametric subbottom profiler, a RESON Seabat multibeam sonar and a QTC seafloor classification sounder.
Survey Design
The study area has a rectangular shape with an extension of approximately 7 x 1.5 km. This area was surveyed on 28 parallel lines in total at an average spacing of 60 m. Because of the demand for high lateral resolution at a given shot rate of 4 seconds for the multichannel seimics the ship speed was limited to 3.5 knots. Hence the line spacing of 60 m was a good compromise to get total coverage of swath bathymetry and a good threedimensional impression of subbottom features in the seismic on one hand and the available ship time on the other hand. Unfortunately weather conditions in the North Sea in November 2005 were very rough, which made the navigation quite difficult for the ship and the survey had to be interrupted after the 8th profile.
First results
Multichannel Seismics
The seismic data reveals that the wind farm area is located above the eastern flank of an updoming structure, probably a salt dome. The flank is located in depth of 170 to 280 meters. The near-surface structure is quite heterogeneous and characterized by numerous filled channels, often cut into each other. The relief of the channels varies between just a few meters up to 40 m, their widths are between 500 m and 2 km. Some channels show prograding filling sequences indicating a riverine origin. The on-going work will include enhanced processing of the data as well as comparison and linking with geological and geotechnical results derived from drilled cores.