Abstract
As countries explore renewable energy opportunities, offshore wind farms are being developed along coastlines. To construct these wind farms, piles are driven into the seafloor as foundational structures for the turbines. Pile driving involves risk of noise pollution that can affect marine fauna. To mitigate these risks, risk assessments and permits are required and are based on the best oceanographic understanding and acoustic modeling available at the time of the permitting process. This project aims to facilitate improved risk prediction by recommending the most efficient acoustic models and ranking the most influential environmental parameters at two representative sites. Many underwater sound sources, like sonar transmitters, appear point-like from a distance when they have a maximum dimension that is small enough. That is, they radiate sound in all directions into their far field, where sound propagates spherically away from the source, as if it originated from a point. Even large sources (e.g., ships, airgun arrays) have this far-field property because the ocean is deeper than the wavelengths these sources emit. If the water is deep enough, it is possible to get far enough away from these sources for them to behave as if they were point sources—the basic requirement for a far field. A sound source with this salient property of having a far field can be characterized in terms of its source level (SL). The SL typically is obtained from the far-field sound pressure level (SPL, i.e., ????) and is scaled depending on the propagation pathway and distance the sound travels from the source to the receiving location. When the SL value is derived based on spherical spreading, only the distance in which the sound travels to the far field is needed (not the direction). Pile driving involves a source (the monopile) that extends from the surface to the seafloor and into the sediment. Being a large, cylindrical source in shallow water, the requirement for attaining a far field will never be met. There is no position anywhere in the water column to which the sound arrives after some propagation loss (PL) as if from a point source. This means that a pile driver does not have a SL, which also means that traditional methods of calculating the SPL using the sonar equation (SPL = SL - PL) cannot be applied.