1. Introduction

  1. This technical report presents the results of a desktop study undertaken by Seiche Ltd. considering the potential effects of underwater noise on the marine environment from the development of the Ossian Array (hereafter simply referred to as the ‘Array’).
  2. The Array, which is to be located within the site boundary, is located in the North Sea, approximately 80 km east of the Aberdeenshire coast ( Figure 1.1   Open ▸ ). The planned activities within the site boundary fall into four phases: pre-construction, construction, operation and maintenance, and decommissioning. Within each of these four phases, different underwater noise sources are identified. These noise sources are both continuous and intermittent in characteristics.
  3. Noise is readily transmitted into the underwater environment and there is potential for the noise emissions from construction, operation and maintenance and decommissioning of the Array to adversely affect marine mammals and fish. At a close range from a noise source with high noise levels, permanent or temporary hearing damage may occur to marine species, while at a very close range gross physical trauma is possible. At wider ranges, the introduction of any additional noise could potentially cause short term behavioural changes, for example the ability of a species to communicate and to determine the presence of predators, food, underwater features and obstructions. It should be noted, however, despite the topic being an area of active research, current scientific literature is unclear on whether/how close range or short-term impacts may translate to long term population level impacts.
  4. The primary purpose of this technical report is to present the likely distances at which the onset of potential auditory injury (i.e. Permanent Threshold Shifts (PTS) in hearing) and behavioural effects on different marine species may occur when exposed to the different anthropogenic noises that occur during different developmental phases of the Array. The results from this technical report have been used to inform the following chapters of the Array Environmental Impact Assessment (EIA) Report in order to determine the potential impact of underwater noise on marine species:
  • volume 2, chapter 9: fish and shellfish ecology;
  • volume 2, chapter 10: marine mammals; and
  • volume 2, chapter 12: commercial fisheries.
  1. Consequently, the sensitivity of species, magnitude of potential impact and significance of effect from underwater noise associated with the Array are addressed within the relevant chapters (volume 2, chapters 9, 10 and 12).
  2. This technical report uses noise propagation models to calculate the impact ranges to marine mammals and fish for each phase of the Array. Key modelled sources include:
  • clearance of Unexploded Ordnance (UXO);
  • geophysical and geotechnical surveys;
  • impact piling; and
  • vessels and other non-impulsive sources.

Figure 1.1:
Location of the Array

Figure 1.1: Location of the Array

 

2. Study Area

  1. No specific study area has been outlined for underwater noise as this is defined by the receptors and discussed within the relevant topics listed in paragraph 4.
  2. The modelled area is approximately 146,000 km2 and covers the Array and an area extending up to 200 km from the site boundary up to land.
  3. Bathymetry data used within the modelling was obtained from the General Bathymetric Chart of the Oceans (GEBCO). The GEBCO 2021 Grid is a global terrain model for ocean and land, providing elevation data, in metres, on a 15 arc-second interval grid.
  4. To produce a representative sound speed profile, conductivity, temperature, and depth (CTD) data were obtained from the National Oceanic Atmospheric Administration (NOAA) service WODselect for the closest sample point to the development (NOAA, 2023).

3. Acoustic Concepts and Terminology

  1. Noise travels through water as vibrations of the fluid particles in a series of pressure waves. These waves comprise a series of alternating compressions (positive pressure) and rarefactions (negative pressure). As noise consists of variations in pressure, the unit for measuring noise is usually referenced to a unit of pressure, the Pascal (Pa). The decibel (dB) is a logarithmic ratio scale used to communicate the large range of acoustic pressures that can be perceived or detected, with a known pressure amplitude chosen as a reference value (i.e. 0 dB). In the case of underwater noise, the reference value (Pref) is taken as 1 μPa, whereas the airborne noise is usually referenced to a pressure of 20 μPa. To convert from a sound pressure level referenced to 20 μPa to a sound pressure referenced to 1 μPa, a factor of 20 log (20/1) (i.e. 26 dB has to be added to the former quantity). Thus 60 dB re 20 μPa is the same as 86 dB re 1 μPa, although differences in sound speeds and different densities mean that the decibel level difference in sound intensity is much more than 26 dB when converting pressure from air to water. All underwater sound pressure levels in this report are quantified in dB re 1 μPa.
  2. There are several descriptors used to characterise a sound wave. The difference between the lowest pressure variation (rarefaction) and the highest-pressure variation (compression) is called the peak-to-peak (or pk-pk) sound pressure level. The difference between the highest variation (either positive or negative) and the mean pressure is called the peak pressure level. Lastly, the Root Mean Square (rms) sound pressure level is used as a description of the average amplitude of the variations in pressure over a specific time window. Decibel values reported should always be quoted along with the Pref value employed during calculations. For example, the measured sound pressure level (SPLrms) value of a pulse may be reported as 100 dB re 1 µPa. These descriptions are shown graphically in Figure 3.1   Open ▸ .

Figure 3.1:
Graphical Representation of Acoustic Wave Descriptors

Figure 3.1: Graphical Representation of Acoustic Wave Descriptors

 

  1. The SPLrms is defined as:

  1. The magnitude of the rms sound pressure level for an impulsive noise (such as airguns from a seismic survey source) will depend upon the integration time, T, used for the calculation (Madsen, 2005). It has become customary to utilise the T90 time period for calculating and reporting rms sound pressure levels[1]. This T90 time period is the interval over which the cumulative energy curve rises from 5% to 95% of the total energy and therefore contains 90% of the sound energy.
  2. Another useful measure of noise used in underwater acoustics is the Sound Exposure Level (SEL). This descriptor is used as a measure of the total sound energy of an event or a number of events (e.g. over the course of a day) and is normalised to one second. This allows the total acoustic energy contained in events lasting a different amount of time to be compared on a like for like basis[2]. The SEL is defined as:

where is the integration time of the noise “event”, is the squared sound pressure at a time and is the reference time-integrated squared sound pressure of 1 µPa2s.

  1. The frequency of the noise is the rate at which the acoustic oscillations occur in the medium (air/water) and is measured in cycles per second, or Hertz (Hz). When noise is measured in a way which approximates to how a human would perceive it using an A-weighting filter on a noise level meter, the resulting level is described in values of dBA. However, the hearing capability of marine species is not the same as humans, with marine mammals hearing over a wider range of frequencies and with a different sensitivity. It is therefore important to understand how an animal’s hearing varies over its entire frequency range to assess the effects of anthropogenic noise on marine mammals. Consequently, use can be made of frequency weighting scales (M-weighting) to determine the level of the noise in comparison with the auditory response of the animal concerned. A comparison between the typical hearing response curves for fish, humans and marine mammals is shown in Figure 3.2   Open ▸ [3].

Figure 3.2:
Comparison Between Hearing Thresholds of Different Animals

Figure 3.2: Comparison Between Hearing Thresholds of Different Animals

 

  1. The broadband acoustic power (i.e. containing all the possible frequencies) emitted by a noise source, measured/modelled at a location within the Array is generally split into and reported in a series of frequency bands. In marine acoustics, the spectrum is generally reported in standard one-third octave band frequencies, where an octave represents a doubling in noise frequency[4].
  2. The source level is the sound pressure level of an equivalent and infinitesimally small version of the source (known as point source) at a hypothetical distance of 1 m from it. The source level is commonly used in combination with the Transmission Loss (TL) associated with the environment to obtain the Received Level (RL) at distances from (in the far field of) the source. The far field distance is chosen so that the behaviour of a distributed source[5] can be approximated to that of a point source. Source levels do not indicate the real sound pressure level at 1 m. TL at a frequency of interest is defined as the loss of acoustic energy as the signal propagates from a hypothetical (point) source location to the chosen receiver location. The TL is dependent on water depth, source depth, receiver depth, frequency, geology, and environmental conditions. The TL values are generally evaluated using an acoustic propagation model (various numerical methods exist) accounting for these dependencies.

 

The RL is the noise level of the acoustic signal recorded (or modelled) at a given location, that corresponds to the acoustic pressure/energy generated by a known active noise source. This considers the acoustic output of a source and is modified by propagation effects. This RL value is strongly dependant on the source, environmental properties, geological properties, and measurement location/depth. The RL is reported in dB either in rms or peak-to-peak sound pressure level (SPL), and SEL metrics, within the relevant one-third octave band frequencies. The RL is related to the SL as (where TL is the transmission loss of the acoustic energy within the survey region):

  1. The directional dependence of the source signature and the variation of TL with azimuthal direction (which is strongly dependent on bathymetry) are generally combined and interpolated to report a two-Dimensional (2-D) plot of the RL around the chosen source point up to a chosen distance.