The nature of the subsurface of the German North Sea and Baltic Sea, as in many other relevant regions off the coasts of Denmark, England, or Poland, is shaped by past ice ages, which are responsible for complex geological depositional geology, including some spectacular underground canyons filled with marine sediments. In addition to this geological complexity, ice age environments are characterized by widespread deposits of rock fragments transported by glaciers, which can pose a lasting threat to successful offshore foundation construction. Furthermore, organic material and gas deposits are found in many places in the near-surface layers of the sub-surface which can lead to destabilization effects. A subsoil map that is as detailed as possible is therefore crucial in order to determine the basic suitability of potential development sites for the construction of offshore wind farms. Figure 1 illustrates how high-resolution seismic data similar to non-invasive imaging in medicine can be used to derive an initial sub-surface model for a preliminary investigation carried out by Fraunhofer Institute for Wind Energy Systems IWES in the North Sea. Since the direct, invasive collection of geotechnically relevant data, for example by means of Cone Penetration Tests (CPTs), is cost-intensive and therefore only possible at specific locations, the integration of imaging techniques such as high-resolution marine seismics can provide effective added value in minimizing the risks associated with foundation construction in the context of offshore wind farm development.
© Fraunhofer IWES
Geophysical preliminary exploration of offshore wind farm sites
As part of the tendering process for potential sites in the German North Sea and Baltic Sea, Fraunhofer IWES has been working for many years with the federal agency responsible for site allocation – the German Federal Maritime and Hydrographic Agency (BSH) – and, together with partners, is conducting ship-based geophysical preliminary surveys, particularly in the German North Sea. As part of the 2025 Site Development Plan, further extensive measurement campaigns are planned in the North Sea. The ship-based geophysical surveys use seismic methods similar to those used in the marine oil and gas industry, but specifically adapted to the challenges of shallow waters and capable of achieving significantly higher resolutions. Similar to the ultrasound method used in medicine, this allows detailed images of the top 100 meters below the seabed, which are relevant for foundations, to be generated. These images are invaluable for geological interpretation and subsequent ground model building.
Risk minimization through object detection
While a comprehensive and large-scale characterization of the subsoil is sufficient for the planning approval of offshore wind farms, every detail is crucial when constructing individual wind turbine foundations, as small-scale obstructions such as solid ice age boulders or inaccurately measured subsea cable routes and contaminated sites from World War II (unexploded ordnance – UxO) can pose significant installation risks. Due to the small size of these disruptive elements, a patented multi-channel seismic measurement system developed specifically for this purpose at Fraunhofer IWES has been used successfully for several years in industrial projects for targeted risk assessment in German, Polish, and English waters. In 2023, measurements were taken with this specialized system at all planned wind turbine sites and cable routes for Poland’s first offshore wind farm, Baltic Power, and made an important contribution to ensuring that the first foundation installations carried out in early 2025 were uneventful and successful. Figure 2 shows how the measurement system is towed behind the ship together with two controlled seismic sources for object detection for Baltic Power.
Further developments through applied research
At Fraunhofer IWES, we are actively conducting research in close cooperation with industry partners to improve measurement technology and optimize evaluation methods for sub-surface assessment and hazard analysis. The aim is to minimize risks and speed up offshore wind farm development, thereby making the often very time-consuming planning phase safer and shorter. To increase resolution, for example, the digital sampling rate of seismic recordings will be doubled this year to enable even more reliable detection and characterization of small-scale geological structures and disturbances. While the measurement system, which specializes in object detection, is being made more weather-resistant with specifically optimized sensor configuration, a miniaturized version of this towed frame is currently under development, which can be guided close to the seabed at greater depths using an ROTVs (remotely operated towed vehicles) and thus enables more reliable location of submarine cables and promises direct characterization and classification of UxO objects. The two publicly funded research projects SASACD and IRAV are specifically investigating these issues.
Related article:
Putting the seabed under the microscope: ultra-high-resolution multichannel seismics