The sound propagation of seismic airguns differs under a range of environmental conditions. Until now, few studies have examined the characteristics and propagation of seismic airgun sounds in shallow waters. To investigate this, researchers from Aarhus University in Denmark analysed airgun pulses in shallow water to determine the potential effects on small marine mammals.
Seismic airguns are designed to produce low frequency pulses at around 50Hz. However, studies have shown that although most airgun energy is in the low frequency range, they have very broadband characteristics with noise also produced at mid to high frequencies. These higher frequencies are likely to overlap with the hearing sensitivities of smaller marine mammals such as harbour porpoise, seals, sea lions and many species of dolphin. This is of particular concern in shallower waters where low frequency sounds propagate poorly with comparatively more energy in the medium to high frequencies.
This study was conducted in shallow waters with a depth of ~15m and a sandy substrate. Scientists recorded data from an airgun fired at different pressures and made sound recordings at various distances from the airgun, ranging between 6 m and 1.3 km. Although the majority of energy from the airgun was at low frequencies with peak frequencies between 50 – 90 Hz, much higher frequency components were recorded at over 10 kHz at 1.3 km. At further distances from the source, higher frequencies were detected before the low frequencies and peak frequency increased with increasing distance.
These higher frequency components overlap with the hearing range of smaller marine mammals such as harbour seals and harbour porpoise. The authors state:
Our findings here suggest that harbour porpoises and harbour seals are unlikely to experience TTS at distances beyond 500m from a single airgun, and thus a 500m shutdown zone should be sufficient to avoid auditory injury in these two species for small airguns.
Although the authors concluded the there was a low probability of auditory trauma as a result of a single airgun being fired in shallow waters, they noted the potential for significant behavioural responses at distances up to several kilometers from the airgun. Future studies might benefit from examining the individual and cumulative impacts of airgun sounds on animals located outside the boundaries of typical mitigation zones. It is worth noting that while this paper provides good data for the propagation of sound from a specific airgun at one site; sound propagation depends on a variety of factors other than just depth, such as: sea state, salinity, temperature, bathymetry etc. It would be very interesting to replicate this type of study in a range of different conditions to observe how the sound propagation changes.
SMRU Consulting has been involved in several projects assessing the potential impacts of underwater noise on marine mammals. To see what we’ve been up to, see below:
- Risk of noise disturbance to marine mammals from sub bottom profile survey activities
- Underwater noise monitoring at the site of the Queensferry Crossing Bridge
- Underwater noise and marine mammal monitoring and mitigation at the Queens Dock expansion
Looking for more information? Check out the full paper here.