Characterizing the seismic signal of individual wind turbines in the Borgsweer area

Abstract

Wind turbines (WTs) emit seismic signals due to the rotation of the blades and the movement of the tower. These signals can be characterized based on frequency, attenuation, the influence of wind speed and type of seismic waves. The objective of this research is to characterize the seismic signals of individual WTs in the Borgsweer area, Groningen, the Netherlands. This is of great important, since the seismic WT signal can interfere with the monitoring of induced earthquakes in the area. It is important to characterize the seismic signal of WTs since it can be used in the future to study the inversion of subterranean parameters and to update velocity and attenuation models. WT signals are a good source for seismic interferometry due to the continuous signal and stable distribution. A spectral analysis is conducted to untangle the individual signal from the signal of multiple WTs in two steps: I) finding the local combined WT signal by producing power-spectrum densities (PSDs), and II) using a cross-correlation beamforming algorithm to localize the individual WTs. The input to this research is 33 days of recorded noise by the NAM Borgsweer flexible three-component array. The small spacing of the stations provides the opportunity to better characterize individual signals of WTs in contrast to other studies that only were able to detect WT parks due to the large spacing between their stations. A dispersion curve was constructed to estimate the travel times of the seismic waves needed for beamforming. Unfortunately the shallow subsurface was too heterogeneous for the dispersion curve to continue at frequencies higher than 1 Hz. The heterogeneous subsurface leads to an incomplete beamforming localization of the WTs in the area, since the algorithm assumes a homogeneous subsurface. The radiation pattern and the type of seismic wave are characterized as a function of wind direction. Using the PSDs, the influence of wind speed on the signal is determined. Eigenmode frequencies due to the movement of the tower are found for the Delfzijl North (0.63 Hz) and South (0.31 Hz) wind park. Multiples of these frequencies follow the physics of standing waves on a clamped beam. A prominent peak near 2.1 Hz is likely related to the 3P blade rotation.