UNDERWATER SOUND PROPAGATION MODELLING

Sound propagation modelling is used to predict the variation in noise levels at different distances from a sound source (e.g. seismic airgun, hammering of a monopile). Data are compared with proposed injury and behavioural response criteria for different species, for example, marine mammals (Southall et al., 2007) and fish (Popper et al., 2014), to determine potential impacts on marine life. ACCOBAMS guidelines require results to be precautionary enough to handle large levels of uncertainty, especially when extrapolating from other species (http://accobams.org).

To estimate the received level at a receptor location for a source of given source level and frequency, it is necessary to model the transmission loss through the path from source to receiver. To model this transmission loss/sound propagation appropriately, local data are require (OSC, 2009; Urick, 1983). Models, for example, require airgun configuration and characteristics, sound speed profiles, bottom topography, water depth, sound channels, etc.

A very simple approach to model underwater propagation is to consider the simple geometrical spreading laws given by using the formula

TL = N log(r) +αr

The disadvantage of using this method is that it fails entirely to take into account the influence of the environment on the propagation of sound and hence propagation loss may be under or over-estimated by a considerable amount. The solution to this is to make use of more sophisticated modelling techniques. For example, a Range-dependent Acoustic Model (RAM) Parabolic Equation (PE) code can be used. This model utilises a solution of the acoustic-wave equation using the parabolic approximation.

Results are able to inform regulators and operators on appropriate mitigation/exclusion zones to be used during offshore work (e.g. a seismic survey or piling operations). Some guidelines suggest that these should be verified in the field. Ocean Science Consulting Ltd (OSC, www.osc.co.uk) is able to provide both underwater sound propagation modelling and in-field measurements (Jiang et al., 2015; OSC, 2012; OSC, 2015; Todd and White, 2010).

Potential sound sources during dredging operations. Source: Todd et al. (2015).

Potential sound sources during dredging operations. Source: Todd et al. (2015).

REFERENCES

ACCOBAMS Guidelines to address the impact of anthropogenic noise on cetaceans in the ACCOBAMS area.
Agreement on the Conservation of Cetaceans in the Black Sea Mediterranean Sea and Contigous Atlantic Area. 9 pp.
Jiang, J, Todd, VLG, Gardiner, JC, and Todd, IB (2015): Measurements of underwater conductor hammering noise:
compliance with the German UBA limit and relevance to the harbour porpoise (Phocoena phocoena): Euronoise 2015. Acoustical Society of the Netherlands (NAG) and Belgian Acoustical Society (ABAV), Maastricht, The Netherlands pp. 6.
OSC (2009): Prediction of the radiated noise levels around a BHP Billington FPSO Platform, Western Autralia.
Report prepared for Gardline Environmenal Services Ltd. Ocean Science Consulting Ltd. , Belhaven, Dunbar. 61 pp.
OSC (2012): Marine Mammal Observer (MMO) and underwater noise monitoring report for Noble George Sauvageau jack-up
rig move (Q8-A to A6-A) and conductor hammering. Technical report no 8 to Wintershall Holding GmbH. Ocean Science Consulting Limited, Belhaven Dunbar, Scotland. 41 pp.
OSC (2015): Underwater noise monitoring report for piling operations at the Kentish Flats Extension (KFE) Offshore
Wind Farm 25/04–13/05/15. Technical Report No. 2 for Vattenfall Wind Power Ltd. Ocean Science Consulting Limited, Belhaven Dunbar, Scotland. 37 pp.
Popper, AN, Hawkins, AD, Fay, RR, Mann, DA, Bartol, S, Carlson, TJ, Coombs, S, Ellison, WT, Gentry, RL, and Halvorsen,
MB (2014): ASA S3/SC1.4 TR-2014 Sound Exposure Guidelines for Fishes and Sea Turtles: A Technical Report Prepared by ANSI-Accredited Standards Committee S3/SC1 and Registered with ANSI. Springer.
Southall, BL, Bowles, AE, Ellison, WT, Finneran, JJ, Gentry, RL, Greene, CR, Kastak, D, Ketten, DR, Miller, JH, Nachtigall,
PE, Richardson, WJ, Thomas, JA, and Tyack, PL (2007): Marine mammal noise exposure criteria, initial scientific recommendations: Aquatic Mammals, Vol. 33. pp. 411-414.
Todd, VLG, Todd, IB, Gardiner, JC, Morrin, ECN, MacPherson, NA, DiMrzio, NA, and Thomsen, F (2015): A review of
direct and indirect impacts of marine dredging activities on marine mammals. ICES Journal of Marine Science 72, 328-340.
Todd, VLG, and White, PR (2010): Proximate measurements of acoustic emissions associated with the installation and operation
of an exploration jack-up drilling-rig in the North Sea. Springer, Cork. Urick, RJ (1983): Principles of underwater sound (3rd Edition). McGraw-Hill, New York, USA.

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