Baseline General

Sound in water travels as compressional waves in which water particles are alternately compressed and decompressed. The sound speed in water is nearly five times faster than in atmospheric air, which is due to density and compressibility differences between the two media.

Marine life is sensitive to sound (acoustic) pressure and particle motion, or both, depending on the type of sensory systems they possess (Verfuß et al., 2015).

Sound is always present in the underwater environment, irrespective of the status of the sea. A commonly accepted division of sound is natural versus anthropogenic generated sound, where natural generated sound encompasses all kinds of events that are produced by either animals or geophysical processes, while anthropogenic generated sound is produced by humans. The

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primary sources of natural sound in sea are gas bubbles mainly produced by breaking waves.

Examples of geophysical processes which are sporadically occurring are rain, waves, ice, thunder, seismic activity, and thermal noise. Natural sounds also include biological sounds (animal

vocalization) produced by, for example, cetaceans, seals, fish, and crustaceans. Anthropogenic sources include, for example, ships, piling, sonars, seismic airguns, underwater explosions, and operational infrastructure noise (Verfuß et al., 2015).

The definitions of the various parameters used to characterize noise levels are given in Section 5.1.5 Underwater noise.

The typical sound pressure levels and their respective frequency ranges caused by natural and anthropogenic sources in the sea are shown in Figure 9-26. The loudest acoustical sources with the main energy in the low frequency region are earthquakes and underwater explosions, followed by biological (animal) sound, spanning a wide frequency range up to and including the ultrasound region. Figure 9-26 also shows the relationship between ambient noise levels and sea state levels (Verfuß et al., 2015).

The source level of underwater sounds varies. Generally, lightning strikes, seismic eruptions and underwater explosions are some of the loudest sound sources, and have source levels of 260-280 dB re 1 μPa at 1 m. Loud ships can also generate high noise levels, with source levels of up to 190 dB re 1 μPa at 1 m. Sound sources can also be biological; dolphins have been known to have source levels of approximately 230 dB re 1 μPa at 1 m, whilst cod, when they grunt, can produce sounds with source levels of approximately 150 dB re 1 μPa at 1 m (Verfuß et al., 2015). Quieter sound sources such as wind and rain, with sound levels of 40-90 dB re 1 μPa. Monitoring for the Nord Stream 2 project showed that the average noise levels within the main shipping lanes ranged from 100-130 dB re 1 µPa in the 50-200 Hz frequency range (Rambøll / Nord Stream 2 AG, 2017a).

Underwater noise in the Baltic Sea

As part of a project to study the influence of anthropogenic noise on the Baltic Sea (the Baltic Sea Information on the Acoustic Soundscape (BIAS) project), a series of measurements were undertaken over one year (2014) at 38 locations covering the whole Baltic Sea. These

measurements have been used as a basis for numerical modelling of the underwater noise in the entire Baltic Sea. Input data comprised measurements of ship noise close to the main shipping routes, and the model results have been calibrated against the measurements carried out far from the main shipping routes (Tougaard et al., 2017). A portion of the modelling results is shown in Figure 9-25.

Document ID: PL1-RAM-12-Z02-RA-00003-EN 176/433 Figure 9-25 Underwater spectrum level map of noise in the Baltic Sea Picture generated by numerical modelling based on shipping traffic data and measurements conducted in June 2014 by the BIAS project, represented as median values at the 125 Hz one-third octave band. The map includes both natural and human-induced noise. The largest shipping lanes are clearly shown (from SYKE, 2017).

Figure 9-25 shows that there is a close correlation between underwater noise levels and the density of ship traffic; the highest noise levels are associated with the major shipping lanes.

Document ID: PL1-RAM-12-Z02-RA-00003-EN 177/433 Figure 9-26 Underwater noise spectrum levels in the deep ocean and Baltic Sea, including both natural and anthropogenic sources (after Verfuß et al., 2015).

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9.5.2 Impact assessment

The results of underwater noise propagation modelling for the noise-emitting construction activities are reviewed in Section 5.1.5. The applied model for calculating underwater sound propagation is “Parabolic”. The underwater sound propagation has been modelled and calculated in the commercial software program dBSea, version 2.2.

Offshore construction activities such as rock installations, trenching, pipe-lay, anchor handling and ship traffic are characterised as continuous noise sources. As described in Section 5.1.5, the underwater noise generated from the construction activities is not distinguishable from ambient noise levels, as the background levels in the Baltic Sea (with large volumes of ship traffic) are relatively high. Hence only noise from munitions clearance is included in the underwater noise propagation modelling.

Due to the route design strategy, munitions

The impact on underwater noise (or underwater sound) as a receptor is irrelevant, as it is the marine life perceiving the noise that can be impacted. The impacts on the biological receptors, such as invertebrates, fish and marine mammals are assessed in Sections 9.11 (benthic fauna), 9.12 (fish) and 9.13 (marine mammals). The subject will therefore not be assessed further in this section.

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