Seismic Noise by Wind Farms: A Case Study from the VIRGO Gravitational Wave Observatory, Italy

1 We present analyses of the noise wavefield in the vicinity of VIRGO, 2 the Italy-France gravitational wave observatory located close to Pisa, 3 Italy, with special reference to the vibrations induced by a nearby 4 wind park. The spectral contribution of the wind turbines is investi5 gated using (i) on-site measurements, (ii) correlation of spectral am6 plitudes with wind speed, (iii) directional properties determined via 7 multichannel measurements, and (iv) attenuation of signal amplitude 8 with distance. Among the different spectral peaks thus discriminated, 9 the one at frequency 1.7 Hz has associated the greatest power, and 10 under particular conditions it can be observed at distances as large as 11 11 km from the wind park. The spatial decay of amplitudes exhibits 12 a complicate pattern, that we interpret in terms of the combination 13 of direct surface waves and body waves refracted at a deep (≈ 800 m) 14 interface between the plio-pleistocenic marine, fluvial and lacustrine 15 sediments and the Miocene carbonate basement. We develop a model 16 for wave attenuation which allows determining the amplitude of the 17 radiation from individual turbines, which is estimated on the order of 18 300-400 μms/ √ Hz for wind speeds over the 8-14 m/s range. On 19 the base of this model, we then develop a predictive relationship for 20 assessing the possible impact of future, project wind farms. 21