Modeling substrate-borne vibrations generated by offshore monopile installation

Impact and vibratory pile driving for increasingly large offshore monopiles generate low frequency substrate-borne vibrations that radiate from the pile. These waves contain compression and shear wave components that propagate in sediments with poroelastic properties, potentially impacting benthic fauna around the monopile installation site. Additionally, these waves are coupled to the water column and can propagate along the interface between the water and seafloor, exciting additional acoustic waves and contributing to increased sound levels in the water column. Currently, no methods exist to mitigate the effects of these interface waves on marine ecosystems, so understanding how they are generated and the extent to which they propagate is imperative for assessing noise impacts on all marine species. The generation of these vibroacoustic waves is investigated for both vibratory and impact pile driving in a simulated 3D marine environment using numerical modelling. Near field models developed using the SPECFEM3D-Cartesian package are coupled to a Parabolic Equation model adapted to include poroelastic wave motion for far field propagation. Sediment properties are derived from data collected during recent benthic surveys. Predictions of particle motion for simulated ground waves are compared to measurements of ground vibration taken at recent or ongoing offshore windfarm installation sites.