Continuous seismic signals of wind turbines (WTs) have been analyzed at 13 sites of the Gräfenberg (GRF) array in Germany. The stations of the GRF array have operated continuously for 40 years and comprise the longest available digital broadband array data set. By comparing time spans before and after installation of WTs in the vicinity of the stations, their influence on background noise can be quantified. Here, a strong dependence is shown between local wind speed and the observed effects on noise spectra. Station sites with WTs within distances up to 5 km are exposed to significant disturbance in the background noise; even at distances of 15 km such signals are still visible. The geological setting at GRF with sedimentary layer below all stations seems to favor propagation of these signals. Moreover, we observe different decay patterns for signals below and above 2 Hz, which could be related to the geometry of this layer. Overall, our observations clearly document deteriorating effects of WTs to highly sensitive seismological stations.