amount of input data to describe the bathymetry, sound speed profiles, and sediment properties in the local area. Such information may not always be available, and any model is only as accurate as its input data. In addition, to describe the propagation of short broadband pulses, typically this type of model would be run at a number of discrete frequencies in order to predict the transmission loss at all the frequencies present in the pulse, and this requires greater computational power (and time).
185.
It should also be noted that the accuracy of any model depends on accurate representation of the source. The source in the case of marine piling is very complex, with noise being radiated from the surface of the pile itself, and with noise also being launched directly into the sea-bed by the impact of the pile through the sediment. Currently, a perfect model does not exist for such a complex distributed source, and representations of the source in terms of simplified idealised sources such as point sources and line sources will inevitably limit the accuracy of predictions. This is particularly true for the acoustic field close to the pile (in the near-field), and possibly for greater ranges where sound propagating through the sea-bed re-enters the water column.
9.9.2.5 Choice of model 186. A propagation model must be adopted in order to make any attempt to estimate the acoustic field at ranges other than those where measurements have been made. For example, to estimate the acoustic field within a few hundred metres of the source from measurements made at greater ranges. Similarly, if the source is to be described in terms of simplified concepts such as source level (useful, for example, if there is a desire to make comparisons with other sources), a propagation loss model is needed in order to estimate the transmission loss required to derive the source level. For the work described here, the model adopted is the Energy flux model described by Weston (Weston 1976). This propagates the sound energy in the water column, and takes full account of geometric spreading, interaction with boundaries, modal propagation in shallow-water, frequency-dependent absorption in the water and seabed, and scattering from the sea-surface (caused by wave agitation). The implementation of this model has been has benchmarked by NPL against several other standard models such as methods based on normal modes such as Kraken (c 1991) and CSNAP (Ferla et al. 1996), as well as the RAM parabolic equation solution (Collins 1993), and the OASES wave-number integration code (Goh and Schmidt 1996). The Weston model decomposes the acoustic field into one-third octave band levels and propagates each frequency band independently, recombining the frequency bands at a new range to calculate the broadband levels.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Appendix 9.1 Underwater Noise Modelling 90
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