the sea, with constant speed of sound, follows a spherical spreading law at short distance and cylindrical spreading at longer distance. A combination of the two spreading laws gives for distances R greater than the ocean depth Z in metres, the asymptotic loss behaviour (see Section 3.5.3):


20log(Z) + 10log(R/Z) However, it is well know that the


Figure 3.95: Auditory and startle thresholds for codfish, which are hearing generalists with medium hearing ability. The audiogram (black curve) gives the faintest sounds that can be heard at each frequency. The startle response level (red curve) is assumed to be around 80 dB above the known hearing threshold. The red curve is displayed as a smoothed version of the black curve, added around 80 dB. Fish species react very differently to sound. Therefore, any generalisation about the effects of sound on fish should be made with care. The reactions of fish to anthropogenic sound are expected to depend on the sound spectrum and level, as well as the context (e.g. location, temperature, physiological state, age, body size, etc.)


180


bathymetry and the composition of the ocean bottom, whether soft or hard, is important for long-range propagation. In addition, sound propagation changes with the oceanographic conditions and thereby the season. Terefore, if one wants to scientifically consider sound propagation under specified ocean conditions, one has two options: to measure or model seismic sound propagation. Obviously, it would be costly to measure sound propagation from seismic activity in the water column everywhere where seismic is acquired. Te realistic alternative is to develop mathematical-acoustic simulation models which describe how sound propagates in the sea at long distances from the seismic source. Inputs to such acoustic models are source information and available geological and oceanographic information.


3.12.7 Simulation Models


Combined with knowledge of fish hearing and their startle thresholds, simulation models can be used to estimate the distances at which various fish species are affected by seismic activity. Te ultimate goal is to develop an acoustic- biological model to use in the design and planning of seismic surveys, such that possible disturbance to fishing interest is minimised. Maybe, in the future, just as acousticians compute noise maps around airports due to airplane takeoff,


160 140 120 100 80 60 10 100 Frequency (Hz) 1000


underwater acousticians will be able to compute sound propagation maps in the sea due to seismic shooting? In summary, seismic surveys may introduce a behavioural


change in fish in the vicinity of the seismic source. Te radius of the affected zone will depend on many variables, like the local physical conditions of the sea, the food supply for the fish and the behavioural patterns of the fish. Fish with natural habituation will be more steadfast than shoals of fish migrating through an area. Terefore it may be difficult to accurately determine the exact impact of seismic on the behaviour of fish. However, as long as their prey does not vanish, the steadfast fish will return.


148


Threshold level (dB re µPa)


Lasse Amundsen, modified from Karlsen (2010)


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