Characterization of the seismic wave field radiated by a wind turbine

Abstract

Due to the increased expansion of wind turbines (WTs) in the last two decades, seismologists have recently observed an impact of disturbing signals at seismic stations. Ground vibrations generated by WTs perturb the seismic background noise level at seismological monitoring sites causing the performance and functional capability of a seismological network to be limited which leads a conflict between operators of WTs and seismological institutions. With regard to a conflict settlement, the research project MISS (Minderung der Störwirkung von Windenergieanlagen auf seismologische Stationen) was initiated to investigate in detail the seismic signals emitted by a WT in order to identify and discuss methods to reduce the disturbance impact on seismic stations. A part of this research project is presented in this study and deals with movement patterns of a WT tower, foundation and the immediate subsurface as well as the amplitude decay behavior of the seismic noise with increasing distances to the WT. For this purpose, short- and long-term measurements were conducted at a single WT of the “Bürgerwindpark A31 Hohe Mark” located in Heiden (NRW, Germany). First, the first four eigenfrequencies of the WT tower are identified at 0.3 Hz, 1.1 Hz, 3.25 Hz and 6.0 Hz using 1.5-h recordings of sensors that were installed in different heights on the inner wall of the tower. From the measurements, linear, elliptical and circular motion patterns could be observed at the tower. The results of the long-term measurements using 15 mobile seismic stations installed over 1 month in two circular arrays with distances of 100 and 200 m around the WT and two additional stations, one deployed on the foundation and one directly besides in the field, also show linear, elliptical and circular particle movements of the foundation and shallow underground. By correlating power spectral density (PSD) spectra and vertical displacements with the prevailing wind direction, it can be shown that at the eigenfrequencies 0.3, 3.25 and 6.0 Hz Rayleigh waves are radiated in crosswind direction from the WT. In downwind direction, the wave field is dominated by the Love wave type. In contrast, at 1.1 Hz, the dominating wave type emitted by the WT in crosswind direction is Love wave and in downwind direction mainly of the Rayleigh wave type. In order to estimate an amplitude decay relationship of the WT-induced seismic waves, a 1.5-h line-array measurement with 36 sensors installed 100–1000 m from the WT and a sensor spacing of 25 m was used to determine attenuation curves of ground motion velocities proportional to r^−b, with r as the distance between sensor and WT and b as the decay parameter, for the four eigenfrequencies of the tower. The calculated b-values are increasing with frequency from 0.30 to 0.86.