Caratteristiche del rumore sismico nei pressi del Rilevatore di Onde Gravitazionali VIRGO(Cascina, Pisa)

We present analyses of the noise wavefield in the vicinity of VIRGO, the Italy-France gravitational wave observatory located close to Pisa, Italy, with special reference to the vibrations induced by a nearby wind park. The spectral contribution of the wind turbines is investigated using (i) on-site measurements, (ii) correlation of spectral amplitudes with wind speed, (iii) directional properties determined via multichannel measurements, and (iv) attenuation of signal amplitude with distance. Among the different spectral peaks thus discriminated, the one at frequency 1.7 Hz has been associated the greatest power, and under particular conditions it can be observed at distances from the wind park as large as 11 km. The spatial decay of amplitudes exhibits a complicate pattern, that we interpret in terms of the combination of direct surface waves and body waves refracted at a deep (~ 800 m) interface between the plio-pleistocenic marine, fluvial and lacustrine sediments and the Miocene carbonate basement. We then investigated the local soil responses using H/V spectral ratios. We found significant amplification of the horizontal components over two frequency bands: the first, centered at about 0.35 Hz; the second, between 0.7 Hz and 2 Hz. These frequency peaks, likely due to resonance effects in shallow, soft layers, are observed uniformly throughout the investigated area, thus confirming the homogeneity of the shallow geological setting.