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  Figure 2. Left: simplified schematic showing the types of sound generated as a result of a hammer striking a pile. Sound pressure is radiated into the water at an angle relative to the pile axis, compressional and shear waves are generated in the sediment, and interface waves propagate along the seafloor boundary. Right: finite-element output for the pile driving of a vertical steel pile in 12 meters of water. The seafloor is at 12 meters depth (black horizontal line). The acoustic pressure in the water (<12 meters) and the particle velocity in the sediment (>12 meters) generated from a hammer strike are shown. Various wave phenomena can be seen, including the sound pressure wave radiated at an angle from the pile into the water and the resulting body and interface waves in the sediment. Reprinted/adapted from Popper and Hawkins, 2016, with permission from Springer.
over long distances and generate large-amplitude oscilla- tions along the water-sediment boundary that have the potential to affect marine life living close to or within the seafloor sediment that is sensitive to this type of distur- bance (Popper and Hawkins, 2018). The amplitude of the interface wave decays exponentially away from the inter- face, and, therefore, any disturbance will be noticeable only within a distance of a few wavelengths from the seafloor (Tsouvalas and Metrikine, 2016).
Measuring the Radiated Sound
The total number of hammer strikes required to drive a pile to its final penetration depth could range between 500 to more than 5,000, with the hammer striking the pile between 15 and 60 times per minute (Matuschek and Betke, 2009). On average, a jacket foundation requires three times more hammer strikes to install than a monopile and will result in a longer total piling time because the jacket design requires multiple piles to secure the structure to the seabed as opposed to a single pile for the monopile design (Norro et al., 2013). To characterize the impulsive sound generated during each hammer strike as part of impact pile driving, the sound exposure level (SEL) and peak sound pressure
level metrics can be used. The SEL is a measure of the energy within a signal and allows for the total energy of sounds with different durations to be compared. It is defined as the time integral of the squared sound pres- sure reported in units of decibels re 1 μPa2s. This metric can be used to describe the sound levels from a single strike (SELss) and cumulated across multiple hammer strikes or over the duration of the piling activity (SELcum). When assessing the potential effect of impul- sive sounds on the physiology of marine mammals and fishes, the peak sound pressure level and SEL are used (Popper et al., 2014; Southall et al., 2019).
A standard measurement method is important to ensure that independent measurements made at different wind farms can be compared. An approach for measuring and characterizing the underwater sound generated during impact pile driving is defined through the International Organization of Standardization (ISO) 18406 document (2017), which is the standard for measurements of radi- ated underwater sound from impact pile driving. In this standard, a combination of range-varying hydrophone deployments and fixed-range measurements are recom- mended to capture variation in the resulting sound field
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