Environmental statement - Flyndre and Cawdor - Maersk Oil

Flyndre and Cawdor Environmental Statement
Section 5 Assessment of Potential Impacts and Control Measures
signal 40‐1000 Hz are expected to be above ambient noise levels out to a distance of approximately
25 km, beyond this they will fall below ambient levels and are expected to be indistinguishable from
background noise levels.
Figure 5‐3 Frequency output profiles for piling sound at source (1 m) and 20 km, shown is the
audiogram for the harbour porpoise (adapted from Kastelein, Hagedoorn, & Au, (2002))
Overlain on Figure 5‐3 is the audiogram of the harbour porpoise, which illustrates the species’
frequency specific hearing ability. The modelling predicts that as some components of the piling
signal are above both the ambient noise level and the hearing threshold it is possible that the piling
noise could be audible to high frequency hearing cetaceans such as the harbour porpoise out to a
distance of 20 km, although the zone of audibility of the sound will be dependent upon the prevailing
weather conditions at the time (e.g. waves and sea state influence ambient noise levels). No
appropriate audiograms are available to use for any low (e.g. minke whales) or mid‐frequency
cetaceans (e.g. bottlenose dolphins).
For marine mammals, hearing impairment can occur when sound levels are high and, in the case of
transient noise sources, such as pile driving, when marine mammals are exposed to repeated sounds.
The hearing loss can occur in two forms:
Temporary Threshold Shift (TTS): On exposure to noise the ear’s sensitivity level will
decrease as a measure to protect against damage. This process is referred to as a temporary
shift in the threshold of hearing, and generally returns to normal in 24 hours.
Permanent Threshold Shift (PTS): A permanent change in the threshold of hearing caused by
a sound level, or cumulative exposure of a sound level that is capable of causing irreversible
damage to the ear.
On the basis of observed cetacean physiological and behavioural responses to anthropogenic sound
Southall, et al. (2007) proposed precautionary noise exposure criteria for injury and behavioural
responses. These criteria are currently considered the best available and are based on quantitative
sound level and exposure thresholds over which PTS‐onset could occur for different groups of species.
By comparing the modelled sound pressure outputs against the Southall thresholds, the sound levels
will not be considered capable of causing Permanent Threshold Shift (PTS) to cetaceans, the range at
which Temporary Threshold Shift extends to is a maximum of 1 m from the pile driver (Table 5‐7).
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