This paper proposes a computer model that simulates the emission of acoustic signals produced by a wave energy device. It analyses these signals with the aid of the audiogram of marine mammals, in this case the Harbour seal. This enables us to estimate the levels of acoustic noise experienced by marine mammals due to the presence of ocean deployed devices.
The author acknowledges the Irish Research Council (IRC) and the Marine and Environmental Sensing Technology Hub (MESTECH) who co-funded the work.
To present a model that estimates the impact of acoustic noise by a marine energy device on marine animals, along with a reverberation of sound signals due to bottom surface influence in the acoustic medium.
It can be deduced that sound pressure level values attenuate faster with distance as frequencies increase. Due to the reverberation and interaction of sound signals from the bottom surface, it is difficult to relate the decrease of SPL with distance as a result of frequency in the model with bottom surface influence. The audiogram of the harbour seal is above the SPL values for all frequencies at 51 meters from the source for the model without bottom surface influence. Habour seals which are approximately 51 m and beyond the sound source do not receive these sound signals. This is because the signals’ sound pressure levels are below their hearing threshold for these frequency values. It could be deduced that certain frequency components could be heard by harbour seals well beyond the 51 m mark. However certain frequency components cannot be heard by the harbour seal even if they go closer than 51m to the sound source.
Sound signals bounce off the bottom surface of the acoustic medium and the extent of the reverberations depends on several factors like the individual frequency component. As shown, the higher the frequency component the higher the scattering effect of sound pressure level values as a result of the surface bottom interface. The bottom surface of this model comprises of a soft bottom bed sandy material which gives some allowance for attenuation of sound signals. In the case of a ‘rock bottom’ surface, the SPL values will be more pronounced. It is important to note that marine energy devices could be set as an array of multiple devices. Hence the combined SPL values will exceed that of a single device. This results in a larger affecting area radius for marine species.
Ikpekha, O.; Soberon, F.; Daniels, S. (2014). Modelling the Propagation of Underwater Acoustic Signals of a Marine Energy Device Using Finite Element Method. Paper Presented at the International Conference on Renewable Energies and Power Quality (ICREPQ’14), Cordoba, Spain. https://tethys.pnnl.gov/publications/modelling-propagation-underwater-acoustic-signals-marine-energy-device-using-finite