Identification of seismic waves radiated from offshore wind turbines highlighted by an ocean-bottom DAS array

Performances of sub-seafloor imaging using natural ambient seismic noise induced by sea waves are constrained by offshore wind turbines that generate mechanical vibrations transmitted into the ground. To better understand the interferences between such natural and anthropogenic seismic waves, we analyse an open access ocean-bottom DAS dataset acquired on a 40-km submarine power cable located near a park of heterogeneous types of wind farms. The duration of the available recordings is limited to 1 hour with a sampling rate of 10 Hz. We determine the power spectral density along the cable and identify characteristic frequency contents of different seismic waves: the natural ambient seismic noise dominates in the 0.3-3 Hz frequency band whereas anthropogenic seismic waves generated by monopile and jacket wind turbines dominate at frequencies larger than 0.75 and 2 Hz, respectively. The spatial correlation of DAS signals is used to define common-source gathers that highlight significant interferences between linear time-lag patterns associated with natural ambient noise and hyperbolic ones linked to anthropogenic seismic waves. The results also highlight two different seismic sources that contribute to the natural ambient seismic noise associated to different propagation patterns. We also demonstrate that the cumulative contribution of multiple consecutive active wind turbines amplifies the axial strain measured along the ocean-bottom DAS array. As a consequence, the extraction of dispersion curves from the ambient seismic noise remains challenging when seismic waves are recorded close to wind farms. This apparent drawback however provides great potential in using wind turbines as active sources of seismic waves to monitor the surrounding sub-seafloor.