The design of the wind-turbines foundations in near-offshore contexts requires the characterization of the physical parameters in sub-marine geological media, including the determination of their spatial and temporal variations. The propagation velocity of shear seismic waves is a crucial mechanical parameter for geotechnical issues. DC-Electrical resistivity is a complementary parameter that enables to discriminate between lithological layers. However, within the first 20 miles from the French Atlantic coast, the geological structures are too complex for the hypothesis of stratified medium to be considered valid. Consequently, seismic methods based on the analysis of the surface waves dispersion and the usual geo-electrical 2D-imaginig tools can not be used. To overcome this challenge the PROSE project aims to define the feasibility of seismic and DC-electrical resistivity methods able to take high spatial variations into account for imaging undersea geological media in the near-offshore area. The PROSE project is also dedicated to the development of monitoring techniques for the subsoil media in the vicinity of the wind-turbines foundations, both during their installation and their service life since the temporal evolution of the surrounding sediments is currently essentially unknown and difficult to assess.
Scientific advances and innovation
Towards a better knowledge of near-offshore geological media and their spatial variability: analysis of the potential to acquire and process geophysical data : 1) concerning seismic data acquisition carried out at sea-bottom for quantifying S velocity down to 50 m deep in the aim to overcome the inversion of surface waves phase velocity dispersion ; 2) concerning geo-electrical data, by taking the undersea environment into account to overcome the loss in sensitivity to resistivity contrasts between saturated sediments.
Towards the monitoring of undersea geological media around foundations in near-offshore context : analysis of the ability of seismic and electrical approaches to assess changes in terms of spatial and temporal resolution, and sensitivity during the installation as well as during the service life of a wind-turbine.
Expected technical and economic impact
- Feasibility of geophysical methodologies in near-offshore for characterizing the mechanical and geo-electrical parameters of the subsoil including their spatial and temporal variations.
- Decrease of the cost through a non-destructive imaging methodology by reducing the number of core drilling
Key project milestones
- September 2016 - Kick-off
- April 2018 - Go no Go before in-situ tests based on results in laboratory for the adaptation of seismic source
- January 2019 - Comparison of numerical tests and experimental results acquired at reduced scale
- September 2019 - Definition of the methodological feasibility
Numerical tests with the codes developed in LPG and IFSTTAR labs for the seismic and electrical (COMSOL based) approaches respectively
- experimental tests in laboratory with measurement benches adapted to reduced scales developed in the lab : GERS/GeoEND/IFSTTAR,
- Measurement tests at the SEM-REV site driven by Centrale de Nantes