1983). A moderate level source transmitting over an efficient propagation path may produce the same received sound pressure level as a higher level source transmitting through a lossy propagation path (i.e. relatively higher TL). In deep water, variations in water properties strongly affect the sound propagation. In shallow water, effects due to the surface and bottom become more influential. Variations in bathymetry (depth) can have a significant effect on the transmission of the sound, and for piling noise significant proportions of the sound may be transmitted through the seabed itself.
173. The sound speed profile may be divided into several layers. Just below the surface is what is sometimes called the surface layer where the speed is susceptible to daily changes due to heating, cooling and wind action. This is followed by a seasonal thermocline, a region characterised by a negative sound speed gradient due to the decrease in temperature with depth. Below the main thermocline and extending into the deep ocean is the deep isothermal layer, which is nearly constant in temperature at about 4 ºC. In this layer, the sound speed increases with depth due to the increasing hydrostatic pressure. Between the thermocline and the isothermal layer is a sound speed minimum, toward which sound tends to be bent by the action of refraction. Some of the sound from a source placed in this channel can be trapped within the channel and travel great distances without appreciable losses due to surface or bottom reflections. Whilst spreading losses will still occur, they are reduced from spherical spreading and in certain cases may approximate to cylindrical spreading. The variation with salinity is less of an influence in deep water, but can have a strong influence where water layers of different salinity are mixing, for example at the estuaries of fresh-water rivers.
174.
In shallow water around the UK coast, the sound speed is less likely to vary strongly with depth due to the shallow conditions, and the often rapid tidal flow which leads to a mixed isothermal water column.
175. The sound speed is such an important oceanographic parameter that it is routinely measured as a function of depth. This may be done using an instrument such as a velocimeter, which measures the time for a high frequency pulse to travel over a known path. Alternatively, a measurement is made of the conductivity (to derive salinity), temperature and depth using a CTD meter with the sound speed calculated form empirically-derived relationships.
9.9.2.2 Shallow water specific environmental dependence 176. One effect not always appreciated is that shallow water channels do not allow the propagation of low frequency signals due to the wave-guide effect of the channel (Urick 1983; Jensen et al. 2000). This effect means that there will be a lower cut-off
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Appendix 9.1 Underwater Noise Modelling 86
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