7.7. UXO Noise Modelling

7.7.1. High Order Detonation

  1. Acoustic modelling for UXO clearance has been undertaken using the methodology described in Soloway and Dahl (2014) and Arons (1954). The equation provides a simple relationship between distance from an explosion and the weight of the charge (or equivalent trinitrotoluene (TNT) weight) but does not take into account bottom topography or sediment characteristics:

where W is the equivalent TNT charge weight and R is the distance from source to receiver.

  1. Since the charge is assumed to be freely standing in mid-water, unlike a UXO which would be resting on the seabed and could potentially be buried, degraded or subject to other significant attenuation, this estimation of the source level can be considered conservative.
  2. According to Soloway and Dahl (2014), the SEL can be estimated using the following equation:

Figure 7.5:
Assumed Explosive Spectrum Shape Used to Estimate Hearing Weighting Corrections to SEL reproduced from Weston (1960)

Figure 7.5: Assumed Explosive Spectrum Shape Used to Estimate Hearing Weighting Corrections to SEL reproduced from Weston (1960)

 

  1. In order to compare against the marine mammal hearing weighted thresholds, it is necessary to apply the frequency dependent weighting functions at each distance from the source. This was accomplished by determining a transfer function between unweighted and weighted SEL values at various distances based on an assumed spectrum shape (refer to Figure 7.5   Open ▸ ) and taking into account molecular absorption at various ranges. Furthermore, because there is potential for more than one UXO clearance event per day (a maximum of two per day is assumed) then it is also necessary to take this into account in the exposure calculation.

7.7.2. Low Order Techniques

  1. According to Robinson et al. (2020), deflagration (a specific method of low order UXO clearance) results in a much lower amplitude of peak sound pressure than high order detonations. The study concluded that peak sound pressure during deflagration is due only to the size of the shaped charge used to initiate deflagration and, consequently, that the acoustic output can be predicted for deflagration if the size of the shaped charge is known.
  2. Acoustic modelling for low order techniques (such as deflagration) has therefore been based on the methodology described in section 7.7.1 for high order detonations, using a smaller donor charge size ( Table 6.6   Open ▸ ).

8. Noise Modelling Results

8.1. Pre-construction Phase

  1. The estimated ranges for auditory injury to marine mammals due to various proposed activities undertaken during the pre-construction site investigation surveying phase of the operations are presented in this section. These include geophysical and geotechnical survey activities, UXO clearance and supported vessel activities.
  2. The potential ranges presented for injury and behavioural response are not a clearly delineated ‘line’ where an impact will occur on one side and not on the other. Potential impact is more probabilistic; in reality, dose dependency in PTS onset, individual variations, and uncertainties regarding behavioural response and swim speed/direction combine to create a probability field around the source location. Defining a single distance around this area of probability allows visualisation of the spatial extent of different source types and levels and allows comparison of the impacts on a like-for-like basis.

8.1.1. Geophysical and Geotechnical Surveys

  1. Geophysical surveying includes many sonar like noise sources and the resulting injury and disturbance ranges for marine mammals are presented in Table 8.1   Open ▸ , based on a comparison to the non-impulsive thresholds set out in Southall et al. (2019). Table 8.2   Open ▸ presents the results for geotechnical investigations. CPT distances are based on a comparison to the Southall et al. (2019) thresholds for impulsive noise (with the distances presented in brackets for peak SPL thresholds) whereas borehole drilling and vibro-core results are compared against the non-impulsive thresholds. Borehole drilling source levels were reported as 142 dB to 145 dB re 1 µPa rms at 1 m, indicating little to no disturbance.
  2. The potential impact distances from these operations vary based on their frequencies of operation and source levels and are rounded to the nearest 5 m. It should be noted that, for the sonar like survey sources, many of the injury ranges are limited to circa 75 m as this is the approximate water depth in the area. Sonar like systems have very strong directivity which effectively means that there is only potential for injury when a marine mammal is directly underneath the noise source. Once the animal moves outside of the main beam, there is significantly reduced potential for injury. The same is true in many cases for TTS where an animal is only exposed to enough energy to cause TTS when inside the direct beam of the sonar like source. For this reason, many of the TTS and PTS ranges are similar (i.e. limited by the depth of the water). Disturbance thresholds are as shown in Table 5.3   Open ▸ for impulsive and non-impulsive sources respectively, noting that impulsive sources have both a strong and a mild disturbance threshold.

Table 8.1:
Potential Impact Ranges (m) for Marine Mammals During the Various Geophysical Investigation Activities Based on Comparison to Southall et al. (2019) SEL Thresholds

Table 8.1: Potential Impact Ranges (m) for Marine Mammals During the Various Geophysical Investigation Activities Based on Comparison to Southall et al. (2019) SEL Thresholds

 

Table 8.2:
Potential Impact Ranges for Geotechnical Site Investigation Activities Based on Comparison to Southall et al. (2019) SEL Thresholds (Comparison to Ranges for Peak SPL Where Threshold was Exceeded Shown in Brackets)

Table 8.2: Potential Impact Ranges for Geotechnical Site Investigation Activities Based on Comparison to Southall et al. (2019) SEL Thresholds (Comparison to Ranges for Peak SPL Where Threshold was Exceeded Shown in Brackets)

 

8.1.2. Vessels

  1. The potential impact ranges for vessels are included in section 8.4, which summarises the vessel modelling results for all phases of the Array.

8.1.3. UXO Clearance

  1. The predicted injury ranges for low order disposal are presented in Table 8.3   Open ▸ and for high order detonation of UXOs in Table 8.4   Open ▸ . All UXO injury and disturbance ranges are based on a comparison to the relevant impulsive noise thresholds as set out in section 5.3.2.
  2. It should be noted that, due to a combination of dispersion (i.e. where the waveform elongates), multiple reflections from the sea surface and seabed and molecular absorption of high frequency energy, the noise is unlikely to still be impulsive in character once it has propagated more than a few kilometres. Consequently, great caution should be used when interpreting any results with predicted injury ranges in the order of tens of kilometres. Furthermore, the modelling assumes that the UXO acts like a charge suspended in open water whereas in reality it is likely to be partially buried in the sediment. In addition, it is possible that the explosive material will have deteriorated over time meaning that the predicted noise levels are likely to be over-estimated. In combination, these factors mean that the results should be treated as precautionary potential impact ranges which are likely to be significantly lower than predicted.

 

Table 8.3:
Potential Impact Ranges for Low Order and Low Yield UXO Clearance Activities

Table 8.3: Potential Impact Ranges for Low Order and Low Yield UXO Clearance Activities

 

Table 8.4:
Potential Impact Ranges for High Order Clearance of UXOs

Table 8.4: Potential Impact Ranges for High Order Clearance of UXOs