5.5. Use of Impulsive Noise Thresholds at Large Ranges

  1. For any noise of a given amplitude and frequency content, impulsive noise has a greater potential to cause auditory injury than a similar magnitude non-impulsive noise (Southall et al., 2007; 2019; 2021; NMFS, 2018; von Benda-Beckmann et al., 2022). For highly impulsive noises such as those generated by impact piling, UXO detonations and seismic source arrays, the interaction with the seafloor and the water column is complex. In these cases, due to a combination of dispersion (i.e. where the waveform elongates), multiple reflections from the sea surface and seafloor and molecular absorption of high frequency energy, the noise is unlikely to still be impulsive in character once it has propagated some distance (Hastie et al., 2019; Martin et al., 2020; Southall et al., 2019; Southall, 2021). This transition in the acoustic characteristics therefore has implications with respect to which threshold values should be used (impulsive vs. non impulsive criteria) and, consequently, the distances at which potential injury effects may occur.
  2. This acoustic wave elongation effect is particularly pronounced at larger ranges of several kilometres and, in particular, it is considered highly unlikely that predicted PTS or TTS ranges for impulsive noise which are found to be in the tens of kilometres are realistic (Southall, 2021). However, the precise range at which the transition from impulsive to non-impulsive noise occurs is difficult to define precisely, not least because the transition also depends on the response of the marine mammals’ ear. Consequently, there is currently no consensus as to the range at which this transition occurs or indeed the measure of impulsivity which can be used to determine which threshold should be applied (Southall, 2021). However, evidence for impact pile driving and seismic source arrays does indicate that some measures of impulsivity change markedly within 10 km of the source (Hastie et al., 2019). Additionally, the draft NMFS (2018) guidance suggested 3 km as a transition range, but this was removed from the final document.
  3. The cross-over between impulsive and non-impulsive noise is an area of ongoing research and there are a number of potential methods for determining the cross-over point being investigated, such as the kurtosis metric, and the loss of high frequency energy from the spectrum (above 10 kHz, e.g. Southall, 2021). In the meantime, it is considered that any predicted injury ranges in the tens of kilometres are almost certainly an overly precautionary interpretation of existing criteria (Southall, 2021).
  4. As disturbance ranges are likely to extend beyond the range at which injury (PTS or TTS) could occur, this transition from impulsive to continuous noise is likely to have even more impact on the disturbance range (e.g. Southall et al., 2021). For example, where dose-response relationships have been derived based on exposure to impulsive noises, particularly where these have been derived based on experiments relatively close to the impulsive source, then extrapolation of the dose-response relationship to larger ranges could be misleading. This is particularly true where the dose-response relationship has been derived using parameters such as unweighted single pulse SEL or rms(T90) SPL, which does not take into account the characteristics (e.g. frequency content of impulsivity) of the noise. Consequently, great caution should be used when interpreting potential disturbance ranges in the order of tens of kilometres. Where appropriate, these should be considered alongside an understanding of potential background noise levels in order to understand the distances at which noises related to an impulsive source may be detected.

6. Source Noise Levels

6.1. General

  1. Underwater noise source level is usually quantified using a dB scale with values generally referenced to 1 μPa pressure amplitude as if measured at a distance of 1 m from a hypothetical, infinitesimally small point source (sometimes referred to as the SL). This quantity is often referred to as an equivalent monopole source level. In practice, it is not usually possible to measure noise at 1 m from a large structure, which is more akin to a distributed noise source, but the source level metric allows comparisons and reporting of different source noise emissions on a like-for-like basis, as well as a standard input parameter for noise propagation models. In reality, for a large noise source such as a monopile, seismic source array or vessel, the source level value at this conceptual point at 1 m from the (theoretical, infinitesimally small) acoustic centre does not exist. Furthermore, the energy is distributed across the source and does not all emanate from this imagined acoustic centre point. Therefore, the stated sound pressure level at 1 m does not occur at any point in space for these large sources. In the acoustic near field (i.e. close to the source), the sound pressure level will be significantly lower than the value predicted by the SL.
  2. A wealth of experimental data and literature-based information is available for quantifying the noise emission from different construction operations. This information, which allows us to predict with a good degree of accuracy the noise generated by a source at discrete frequencies in one-third octave bands, will be employed to characterise their acoustic emission in the underwater environment. Sections 6.2 to 6.7 detail the types of noise sources present during different construction activities, their potential signatures in different frequency bands, and acoustic levels.

6.2. Types of Noise Sources

  1. The noise sources and activities which were investigated during the development of this technical report are summarised in Table 6.1   Open ▸ .

 

Table 6.1:
Summary of Noise Sources and Activities Included in the Underwater Noise Technical Report

Table 6.1: Summary of Noise Sources and Activities Included in the Underwater Noise Technical Report

 

  1. Noise sources included in Table 6.1   Open ▸ are considered in more detail in the following sections.

6.3. Pre-construction Phase

6.3.1. Geophysical Surveys

  1. Several sonar-like survey types will potentially be used for the pre-construction site investigation geophysical surveys. During the survey, a transmitter emits an acoustic signal directly toward the seabed (or alongside, at an angle to the seabed, in the case of side scan techniques). The equipment likely to be used can typically work at a range of signal frequencies, depending on the distance to the bottom and the required resolution. The signal is highly directional and acts as a beam, with the energy narrowly concentrated within a few degrees of the direction in which it is aimed. The signal is emitted in pulses, the length of which can be varied as per the survey requirements. The assumed pulse rate, pulse width and beam width used in the assessment are based on a review of typical units used in other similar surveys. It should be noted that sonar like survey sources are classed as non-impulsive noise because they generally comprise a single (or multiple discrete) frequency (e.g. a sine wave or swept sine wave) as opposed to a broadband signal with high kurtosis, high peak pressures and rapid rise times.
  2. The characteristics assumed for each device modelled in this technical report are summarised in Table 6.2   Open ▸ . these sources are considered to be continuous (non-impulsive).


Table 6.2:
Typical Sonar Based Survey Equipment Parameters Used in Assessment

Table 6.2: Typical Sonar Based Survey Equipment Parameters Used in Assessment

 

  1. The assumed pulse rate has been used to calculate the SEL, which is normalised to 1 s, from the rms sound pressure level. Directivity corrections were calculated based on the transducer dimensions and ping frequency and taken from manufacturer’s datasheets. It is important to note that directivity will vary significantly with frequency, but that these directivity values have been used in line with the modelling assumptions stated in Table 6.2   Open ▸ .
  2. Directivity corrections have been applied to the source noise level data based on directivity characteristics for the proposed sources. Directivity factors were derived based on source take-off angle for an animal on the seabed. This results in a larger correction (reduction in level) due to directivity at distances further from the source than for receivers close to the source.
  3. At distances closer to the source (i.e. less than the water depth), no directivity correction is made because the animal could be directly underneath the source. As the source to receiver range increases, the take-off angle between the source and animal becomes larger. Hence, when the range to source is large in comparison to the water depth, the effects of the source's directivity will have a much greater bearing on the received noise level. Once the range to source becomes larger than the water column depth then the source directivity effects will become increasingly more important.
  4. Unlike the sonar like survey sources, the UHRS source is likely to utilise a sparker, which produces an impulsive, broadband source signal. The parameters used in the underwater noise modelling are summarised in Table 6.3   Open ▸ .

 

Table 6.3:
Typical UHRS Survey Equipment Parameters Used in Assessment

Table 6.3: Typical UHRS Survey Equipment Parameters Used in Assessment

 

6.3.2. Geotechnical Surveys

  1. Source noise data for the proposed CPTs was reported by Erbe and McPherson (2017). In this report, the SEL measurements at two different sites in Western Australia at a measured distance of 10 m were presented. The signature is generally broadband in nature with levels measured generally 20 dB above the baseline noise levels. The report also refers to other paths for acoustic energy including direct air to water transmission and other multipath directions, which implied that measured noise level is strongly dependant on depth and range from the source. The third octave band SEL levels from the CPT extracted are presented in Table 6.4   Open ▸ .

 

Table 6.4:
CPT Source Levels in Different Third Octave Band Frequencies (SEL Metric) Used for the Assessment (Erbe and McPherson, 2017)

Table 6.4: CPT Source Levels in Different Third Octave Band Frequencies (SEL Metric) Used for the Assessment (Erbe and McPherson, 2017)

 

  1. Seismic CPT noise is classified as impulsive at source since it has a rapid rise time and a high peak sound pressure level of 220 dB re 1 µPa (pk), compared to a SEL of 189 dB re 1 µPa2s.
  2. The seismic CPT test is typically conducted at various depths for each location every three to five minutes with between 10 and 20 strikes per depth.
  3. It should be noted that if non-seismic CPT were to be used, the noise would be considered non-impulsive if it produced any noise at all, and therefore the assessment of seismic CPT is considered precautionary. As piston core and box core methods are lower in sound energy, CPT has been used as a maximum design scenario across all core measurement techniques.
  4. Measurements of a vibro-core test show underwater source sound pressure levels of approximately 187 dB re 1 µPa re 1 m (rms) (Reiser et al., 2011). The SEL has been calculated based on a one hour sample time which, it is understood, is the typical maximum time required for each sample. The vessel would then move on to the next location and take the next sample with approximately one hour break between each operation. The vibro-core noise is considered to be continuous (non-impulsive).

 

Table 6.5:
Vibro-Core Source Levels Used in the Assessment

Table 6.5: Vibro-Core Source Levels Used in the Assessment

 

  1. The frequency spectral shape for vibro-coring is presented in Figure 6.1   Open ▸ .

Figure 6.1:
Frequency Spectral Shape Used for Vibro-Coring

Figure 6.1: Frequency Spectral Shape Used for Vibro-Coring

 

  1. Source levels for borehole drilling was reported in Erbe and McPherson (2017), with source levels of 142 dB to 145 dB re 1 µPa re 1 m (rms). A set of one third octave band levels, calculated from the spectrum presented in the paper are shown in Figure 6.2   Open ▸ .

Figure 6.2:
Borehole Drilling Source Level Spectrum Shape Used in the Assessment

Figure 6.2: Borehole Drilling Source Level Spectrum Shape Used in the Assessment

 

  1. As for other non-impulsive sources, the impact assessment criteria is the SEL metric for a receptor moving away from the source.

6.3.3. UXO Clearance

  1. The precise details and locations of potential UXOs is unknown at this time. For the purposes of this assessment, it has been assumed that the Maximum Design Scenario (MDS) will be clearance of UXO with a Net Explosive Quantity (NEQ) of 698 kg cleared by either low-order or high order techniques. Low-order techniques are not always possible and are dependent upon the individual situations surrounding each UXO. The value of 698 kg is derived from a site-specific desktop review undertaken by Ordtek (2022) to understand the potential maximum charge weight and likelihood based on past military activity.
  2. There are a number of low-order and low-yield techniques available for the clearance of UXO, with the development of new techniques being a subject of ongoing research. For example, one such technique (deflagration) uses a single charge of 30 g to 80 g NEQ which is placed in close proximity to the UXO to target a specific entry point. When detonated, a shaped charge penetrates the casing of the UXO to introduce a small, clinical plasma jet into the main explosive filling. The intention is to excite the explosive molecules within the main filling to generate enough pressure to burst the UXO casing, producing a deflagration of the main filling and neutralising the UXO.
  3. Recent controlled experiments showed low-order deflagration to result in a substantial reduction in acoustic output over traditional high order methods, with SPLpk and SEL being typically significantly lower for the deflagration of the same size munition, and with the acoustic output being proportional to the size of the shaped charge, rather than the size of the UXO itself (Robinson et al., 2020). Using this low-order deflagration method, the probability of a low order outcome is high; however, there is a small inherent risk with these clearance methods that the UXO will detonate or deflagrate violently resulting in higher noise level emissions.
  4. It is possible that there will be residual explosive material remaining on the seabed following the use of low-order techniques for unexploded ordnance disposal. In this case, and only for debris of sufficient size to be a risk to fishing activities, recovery will be performed which includes the potential use of a small (500 g) ‘clearing shot’.
  5. As a last resort, if it is not possible to carry out low-order or low-yield clearance techniques, it may be necessary to carry out a high order detonation of the UXO. The underwater noise modelling has been undertaken for a range of charge configurations as set out in Table 6.6   Open ▸ .

 

Table 6.6:
Details of UXO and their Relevant Charge Sizes Employed for Modelling

Table 6.6: Details of UXO and their Relevant Charge Sizes Employed for Modelling

 

  1. The source levels for UXO are included within the terms for propagation modelling and are described in section 7.7.

6.3.4. Vessels

  1. Use of vessels is addressed in section 6.7 for all phases of the Array.