Characterization of seismic signals induced by the operation of wind turbines in North Rhine-Westphalia (NRW), Germany

In recent years, the impact of wind turbines (WTs) on seismological stations has been noticed, since WT-induced ground motions perturbed the seismic background noise level at seismological monitoring sites. The resulting deterioration of the recording quality at seismic stations leads to a conflict between seismological network and WT operators. As a first step towards the solution of the conflict - or at least a peaceful coexistence - it is of paramount importance to understand the characteristics of seismic signals generated by WTs. For this study, a 6.5-week measurement campaign was conducted at a wind park (WP) consisting of five 2 MW turbines in North Rhine-Westphalia (Germany) with eleven mobile seismic stations installed at distances of 1–10 km from the WP. At each measurement point, power spectral density (PSD) spectra are calculated and correlated with different operating states of the WTs. Changes in the operating states of the WT are reflected in the noise level at the seismic stations with different distances to the WP. An analysis of the radiated frequencies from the WT foundation into the subsurface is carried out by performing shutdown and switch-on tests at the WP. In this way, seismic signals generated by WTs are identified and used to illustrate how frequency-dependent peaks in WT-induced seismic noise are attenuated with distance. The attenuation of the peaks can be described by a power-law decay proportional to r^−b, with r as the distance between station and WT and b as the decay parameter with values between 2.4 and 5.5. The relationship between the noise level at a seismic station and the number of WTs in operation, that behave as interacting sources for the background noise level, could be determined as $\sim \sqrt{N}$, with N being the number of switched-on WTs.