The subsoil investigations serve to describe the properties of the subsurface and to assess the seabed soil as an environmental asset. Knowledge of the properties of the seabed and the geological structure of the subsurface permits preliminary planning with regard to the foundations to be used and the wind turbine locations. They enable interested companies to make an initial estimate of the costs incurred for the construction of the wind farm.
On the open sea, wind turbines are constantly exposed to the forces of wind, waves and ocean currents. This poses a particular challenge. The designated areas for offshore wind energy lie in water depths of 20-50 m. Depending on the water depth, different types of foundations are available on which the turbines can be erected. The choice of a suitable foundation type is a decisive cost factor and depends on the local properties of the subsoil. Frequently, both the wind turbines themselves and the transformer platform are founded in the seabed with the aid of pile foundations.
Hydrographic mapping of the seabed
Seabed surveying using a multibeam echosounder
The aim of the hydrographic mapping is a detailed description of the seafloor. For the investigations, a survey vessel is equipped with various hydroacoustic measuring instruments. Similar to an echosounder, these are based on the use of artificial sound sources that produce an acoustic image of the seabed:
Multibeam echosounder: An essential basis for investigating the seabed condition is the spatial bathymetric survey by means of a multibeam echosounder. From this, a terrain model of the seabed is created, from which the water depths can be derived.
Side scan sonar: The side scan sonar provides a spatial acoustic image of the seabed surface. In contrast to the multibeam echosounder, the backscatter amplitudes are of particular interest for the evaluation, which can be interpreted in connection with soil samples with regard to the sediment distribution on the seabed. Video recordings also provide information about the sediments and the colonisation of the seabed by benthic organisms.
Subbottom profiler: In order to gain insights into the uppermost meters of the subsurface, subbottom profilers are used. The acoustic image of these provides information about the vertical sequence of fine sediment layers.
Combining these methods and merging their results allows an interpretation of the seabed surface with regard to the sediment types. From this interpretation, potential seafloor habitats (biotopes), such as geogenic reefs, can be determined.
Geophysical investigation of the subsurface
In addition to hydrographic mapping, the investigation of the deeper subsurface takes place. In particular, reflection seismic methods are used for this purpose, which make it possible to map the structural composition of the subsurface. The area is investigated using inline and crossline profiles so that a three-dimensional representation of the subsurface is obtained (fence diagrams). For the investigations, a survey vessel is equipped with a variety of geophysical measuring equipment:
Subbottom profiler: Subbottom profilers are attached directly to the ship or towed in the direct vicinity of the ship. The data obtained with the aid of a subbottom profiler are used to identify fine sediment layers in the upper metres of the subsoil.
Single-/multichannel seismics: Single-channel seismics and multichannel seismics require the towing of a seismic source that generates a signal at regular intervals. A tube equipped with measuring sensors (streamer) is towed to record the signals reflected in the subsurface. These signals allow to draw conclusions about the geological structure of the subsurface down to an investigation depth of 80 m below the seabed.
The preliminary result is a geophysical 3D subsurface model, which is used for further planning of the geotechnical investigations.
Based on the geophysical exploration, the geotechnical sampling of the subsoil is planned and carried out with the aid of drilling vessels. The geotechnical investigations are carried out according to DIN EN 1993 and its normative references.
Drilling and probing are determined on the basis of the subsurface conditions. Based on the drill cores, the sampling and the required sample quantity are then determined with regard to the laboratory tests and the respectively required technology. From the results of the laboratory tests, properties of the sediment such as particle size distribution, water content and water permeability are derived. Furthermore, the knowledge gained is incorporated into the interpretation of the geophysical 3D subsurface model.
All geotechnical work is accompanied in cooperation with the Federal Institute for Hydraulic Engineering (BAW).
Preparation of the geological report and the underground model
Subssoil model of a wind farm
A comprehensive geological report summarises the results of the disciplines hydrographic mapping, geophysical investigation and geotechnical investigation. The report provides a description of the subsoil conditions through a geological interpretation of these individual disciplines.
From the results of the above-mentioned disciplines, additionally a 3D subsurface model is created. This product is made available to interested companies via open source software. The 3D subsurface model provides a three-dimensional overview of the condition of the subsoil.
The geological report together with the 3D subsurface model represent the planning basis and support, for example, determining the plant locations as well as selecting suitable foundation types.
The results of hydrographic mapping are primarily used to describe the soil as an environmental asset. On the basis of the side scan sonar and video investigations and the sediment sampling, information on the sediment composition and surface structure of the seabed can be derived. Subsequently, on the basis of a model wind farm, a description and evaluation of various influencing factors is carried out with regard to their extent, duration and intensity. Then a prognosis of the structural and functional influence can be made.
This consideration is incorporated into the strategic environmental assessment (SUP).