The META-WT project is built on a 3-weeks seismic experiment in Germany with a dense seismic array of 400 three-component geophones that will cover (1) a 2.5 km x 2.5 km wind farm with almost 200 wind turbines (WTs) and (2) a 20-km long radial line from the center of the wind farm to the north with one geophone every 0. 75 km. New sensing technologies (fiber optic cable (DAS), rotation sensors, and Lidar vibrometer) will be used as complementary components for obtaining (3) highly resolved spatial information of the seismic wave-field and (4) investigation of modal analysis for each WT as well as (5) local 6 component wavefield information (rotation + displacement) at the bottom of some WTs.

WTs can act as both active and passive structures for seismic waves. We thus aim to study: (1) the seismic wavefield scattered through the multiple WTs when excited by continuous ambient noise ( 1 Hz); (2) the superimposed self-generated vibrations of the set of WTs transmitted to the ground as seismic noise (0.5-10 Hz) when exited by wind. When several mechanical resonators couple to each other at a sub-wavelength scale, their combined vibration leads to an overall large-scale effect- the so-called metamaterial physics.

Unlike classical seismic metamaterials approaches that aim at designing a set of coupled spring-mass resonators to cancel seismic surface waves, we investigate here the role of the WTs themselves as the set of coupled resonators that create a meta-surface with physical properties for seismic wave propagation. One of the consequences of the field of WTs described as a seismic metamaterial are the measurement of anomalous dispersion curves and frequency gaps for surface waves, i.e., frequencies with no wave propagation possible.

The measurement of the seismic wave-field along a 20-km long radial line will add constraints on the incoming low-frequency wave field as well as on the radiation pattern of the set of WTs and the correlation of this spatial pattern with wind at intermediate scale. The measurements will be compared to 3D numerical simulations. Superimposing the seismic wave-field emitted from each WT, we will obtain constructive or destructive interference patterns, especially around the resonances of these structures. Each WT will be scanned by a Lidar vibrometer to analyse the WT vibration modes whereas the DAS system and several rotational sensors provide observations of the near-field motion complexity aiming to quantify the radiation of the WT vibrations into the ground.

The proposed experiment will be the first field test of metamaterial physics applied to a larger spatial scale in seismology after the successful recording of meter scale metamaterial properties of the subsurface within the METAFORET project. It is complementary to recently completed research projects which aimed at investigating vibration and infrasound emissions in the near-field of single wind turbines and mitigating disturbing effect on seismic stations.

Updated on 11 September 2024