1 Introduction

Underwater sound has the potential to affect marine life in different ways depending on its sound level and characteristics. Richardson et al. (1995) defined four zones of sound influence which vary with distance from the source and level. These are:

• The zone of audibility: this is the area within which the animal can detect the sound. Audibility itself does not implicitly mean that the sound will affect the marine life.
• The zone of masking: this is defined as the area within which sound can interfere with the detection of other sounds such as communication or echolocation clicks. This zone is very hard to estimate due to a paucity of data relating to how marine mammals detect sound in relation to masking levels (for example, humans can hear tones well below the numeric value of the overall sound level).
• The zone of responsiveness: this is defined as the area within which the animal responds either behaviourally or physiologically. The zone of responsiveness is usually smaller than the zone of audibility because, as stated previously, audibility does not necessarily evoke a reaction.
• The zone of injury/hearing loss: this is the area where the sound level is high enough to cause tissue damage in the ear. This can be classified as either Temporary Threshold Shift (TTS) or Permanent Threshold Shift (PTS). At even closer ranges, and for very high intensity sound sources (e.g. underwater explosions), physical trauma or even death are possible.

For this study, it is the zones of injury and disturbance (i.e. responsiveness) that are under consideration (there is insufficient scientific evidence to properly evaluate masking). To determine the potential spatial range of injury and disturbance effects from subsea noise, a review has been undertaken of available evidence, including international guidance and scientific literature. The following sections summarise the relevant thresholds for onset of effects and describe the evidence base used to derive them.

This technical note provides details of the proposed noise modelling methodology for piling as part of the Ossian offshore wind farm as follows:

• Proposed injury and disturbance thresholds.
• Pile source level determination.
• Sound propagation modelling methodology.
• Sound exposure calculations.

We anticipate a response to this methodology statement within 28 days of the receipt of this document. Agreement with the approach is to be ideally reached before the modelling commences. Should a response not be received within 28 days, it will be assumed that MD-LOT are in agreement with the method proposed.

The Scoping Opinion has been received and considered in the development of the underwater noise modelling methodology.

2 Proposed injury and disturbance thresholds

2.1 Marine mammals

Sound propagation models can be constructed to allow the received sound level at different distances from the source to be calculated. To determine the consequence of these received levels on any marine mammals which might experience such sound emissions, it is necessary to relate the levels to known or estimated potential impact thresholds. The auditory injury (PTS/TTS) threshold criteria proposed by Southall et al. (2019) are based on a combination of un-weighted peak pressure levels and marine mammal hearing weighted Sound Exposure Level (SEL). The hearing weighting function is designed to represent the frequency characteristics (bandwidth and sound level) for each group within which acoustic signals can be perceived and therefore assumed to have auditory effects. The proposed PTS and TTS thresholds to be employed for Ossian are summarised in Table 2.1.

Table 2.1:  Summary of PTS and TTS onset acoustic thresholds for impulsive sounds (Southall et al., 2019; Table 6).

Disturbance of marine mammals due to impulsive sound from piling activity will be assessed quantitatively by considering the proportional response of individuals exposed to decreasing sound levels with increasing distance from the sound source. Empirical evidence from piling studies at the Beatrice Offshore Wind Farm (Moray Firth, Scotland) (Graham et al., 2019) and Horns Rev Offshore Wind Farm (Brandt et al., 2011) demonstrated that the probability of occurrence of harbour porpoise Phocaena phocoena (measured as porpoise positive minutes) increased exponentially moving further away from the source. Graham et al. (2019) showed a 100% probability of disturbance at an (un-weighted) SEL of 180  (dB) re 1μPa2s, 50% at 155dB re 1μPa2s and dropping to approximately 0% at an SEL of 120dB re 1μPa2s and the data were subsequently used to develop a dose-response curve.

Similarly, a telemetry study undertaken by Russell et al. (2016) investigating the behaviour of tagged harbour seals Phoca vitulina during pile driving at the Lincs Offshore Wind Farm in the Wash found that there was a proportional response at different received sound levels. Dividing the study area into a 5 km x 5 km grid, the authors modelled SELss levels and matched these to corresponding densities of harbour seals in the same grids during periods of non-piling versus piling to show change in usage. The study found that there was a significant decrease during piling activities at predicted received SEL levels of between 142 and 151 dB re 1 µPa2s.

The approach to be employed for Ossian is therefore to plot un-weighted single pulse SEL contours in 5 dB increments and apply the appropriate dose-response curve to estimate the number of animals that would be disturbed by sound from the piling within each stepped contour. For cetaceans, the dose-response curve will be applied from the Beatrice Offshore Wind Farm data (Graham et al., 2019) whilst for pinnipeds the dose-response curve will be applied using Whyte et al. (2020).