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facilitate comparison with the large number of existing measurements at this range from other wind farm sites (Robinson and Theobald, 2017).
Frequency Content of Hammer Strikes
Impact pile driving radiates considerable levels of low- frequency impulsive noise into the environment. The majority of the energy in the resulting broadband sound field is found below 2 kHz, with spectral peaks between 100 and 400 Hz (Figure 3, top; Matuschek and Betke, 2009) , where the dispersion of shallow-water acoustic modes is present (Frisk, 1994). Measurements taken during wind farm construction in the North Sea showed similar spectra resulting from the piling of a monopile and jacket foundation (Norro et al., 2013).
Azimuthal Dependence of Radiated
Sound Fields
The installation of jacket foundations sometimes requires piles to be driven on an angle inside the legs of the foun- dation. For example, the legs of the jacket foundations at the BIWF were hollow, steel members that were inclined inward at an angle of roughly 13° and piles were impact driven into the legs to secure the foundation to the seabed (Figure 4). The nonaxisymmetric orientation of the pile relative to the seabed causes an azimuthal dependence to the radiated sound field, which can result in a significant
 Figure 3. Top: time-frequency representation of hammer strikes during impact pile driving at the Block Island Wind Farm (BIWF) recorded at a range of 7.5 kilometers and roughly at midwater depth. Bottom: time-frequency representation of the acoustic signals around 71 Hz hypothesized to be due to the operation of 1 turbine at the BIWF measured near the seafloor at a range of 50 meters while fin whales were vocalizing at 20 Hz. The received wind turbine sounds were measured at a level of 100 dB re 1 μPa root-means-square (rms) while the fin whale vocalizations were measured at a level of 125 dB re 1 μPa peak.
 Figure 4. Jacket foundation in the water to the right of the pile-driving barge at the BIWF, with a steel pile section inserted into each leg at an angle of roughly 13° prior to piling. The hammer is shown positioned on one of the piles in preparation to drive the pile into the seafloor.
with both distance and changing source characteristics. The source characteristics and resulting sound level radiated into the environment will vary during a piling sequence due to changes in the hammer strike energy, penetration depth of the pile, and depth-dependent seabed properties. Usually, the piling event will begin with hammer strikes at a lower energy before increasing to a higher strike energy to drive the pile deeper into the seafloor. As the length of the pile driven into the seafloor increases, it has the poten- tial to encounter sediment layers with different properties that would influence the resulting radiated sound levels. This variation could be adequately captured on stationary measurement systems, ideally deployed at multiple ranges but with at least one deployed at a range of 750 meters to
16 Acoustics Today • Summer 2020

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