Coordinateurs du projet
Context
The development of fixed wind turbines in shallow waters (near offshore areas) raises the question of how to characterize the mechanical parameters of the seabed in marine environments, their spatial variability, and their evolution over time. The velocity of S-waves is one of the parameters of interest for the geotechnical design of foundations. Electrical resistivity is a discriminating property for distinguishing lithological levels, which can be complementary to geological and geotechnical interpretation. However, in the 20 nautical mile zones off the Atlantic coast in the Pays de la Loire region, the surface environments are structurally complex and the assumption of a flat stratified environment can no longer be made. Seismic methods based on the analysis of surface wave dispersion and standard electrical imaging practices in a slightly variable marine environment are reaching their limits of validity. Faced with these obstacles, the PROSE project aims to determine the feasibility of imaging and monitoring techniques for the geology of shallow underwater sediments using geophysical, seismic, and electrical methods adapted to the reconnaissance of environments with high spatial variability. Furthermore, the installation of single-pile wind turbine foundations can cause local changes to the subsoil. These potential local changes to the subsoil are not well understood and are difficult to assess. They therefore require the development of permanent monitoring methodologies.
Scientific breakthroughs and innovation
“Towards a better understanding of the near-offshore subsoil and its spatial variability”: analysis of the potential for acquiring and exploiting 1) seabed seismic data for S-wave quantification over the first 50 meters with a view to surpassing surface wave phase velocity inversion methodologies, and 2) electrical data, taking into account seawater, which allows effective coupling of the electrodes with the medium but contributes to dispersing a significant portion of the injected electrical current and reducing the electrical resistivity contrasts between saturated sediments.
“Towards near-offshore subsoil monitoring in the immediate vicinity of large piles”: analysis of the feasibility of near-offshore subsoil monitoring using permanent seismic and electrical sensor devices in the presence of a fixed wind turbine or during its installation.
Expected technical and economic impact
- Feasibility of near-offshore geophysical methodologies for characterizing the mechanical and geoelectric parameters of the subsoil and their spatial and temporal variations
- Cost reduction through a non-destructive imaging methodology by limiting the number of core samples
Demonstrator
Numerical tests using codes developed at LPG for seismic data and codes developed at Gustave Eiffel University for electrical data, based on the COMSOL tool.
- Experimental tests in the laboratory using small-scale measurement benches (MUSC bench and acoustic tank) developed at the GERS/GeoEND/Gustave Eiffel University laboratory.
- Measurement tests at the SEM-REV site piloted by the École Centrale de Nantes.
Results
Within the various components of the PROSE project, the actions carried out to date are summarized below:
Active seismic imaging:
This issue has been addressed both digitally and experimentally on a small scale. The digital methods developed are based on an innovative adaptation of reconstruction methods using random exploration of environmental parameters (particle swarm). The resulting imaging tool demonstrates the ability to use surface waves to recover spatial variations in S-wave velocities for average offshore geological contexts. In this way, these techniques make it possible to go beyond the assumptions of the methods traditionally used in the North Sea, which assume a flat stratified medium (1D). The proposed method is based on measurements recorded at the surface of the seabed. In order to test this new approach experimentally, an experimental tank was developed and tested using submersible sensors. This small-scale study demonstrated the ability to obtain experimental surface measurement data (source and seabed sensors) at a scale of 1/100 by reproducing the generation of surface waves expected numerically.
In order to offer a measurement device that can be used on actual sites, the PROSE project aims to develop a seismic source that can be positioned on the seabed and is light enough to be easily deployed. Initial tests using sparker-type sources show the ability to generate surface waves in the desired frequency range. Additional tests in deeper water are needed to confirm full feasibility.
Geoelectric imaging
Subsurface resistivity tomography provides an image showing areas that conduct electrical current more or less effectively. This image can then be used to identify and map the contours of geological formations in the subsurface. This method can also be used in underwater environments. To take the measurements, a boat tows a line of electrodes along the seabed. However, the fact that seawater is highly conductive greatly limits the method. The aim here is to explore the capabilities of this tomography approach to help identify the top 10 meters of the subsurface under 30 to 40 meters of seawater. This geological context was simplified and then simulated, not only by computer calculation, but also in a 1/100 scale model, with sand and water in a plastic tank and a miniaturized line of electrodes at the bottom of the water. These simulations made it possible to specify the conditions under which resistivity tomography remains relevant and the information it provides.
Seismic monitoring:
Experiments were conducted to establish the feasibility of monitoring the mechanical properties of the subsoil around a wind turbine by applying seismic background noise listening techniques; on the one hand, during the foundation installation phase by pile driving (analogous terrestrial experimental site, Gouvieux, EDF-EN), and during operation (Bouin onshore wind farm, EDF-EN). The measurements collected show that, in both cases, the seismic noise generated by the pile (pile driving or wind turbine pile) has a frequency content that allows the first 10-20 meters of the subsoil to be investigated and monitored without the application of an external seismic source.