Influence of the measurement configuration for the assessment of underwater noise radiated from ships in shallow water

In the context of the increase of human activities in coastal areas, awareness of the impact on marine mammals has risen. Following the Maritime Strategy Framework Directive of the European Union in 2008, actions have been undertaken to achieve good environmental status of the European seas. For instance, one of the topics is the reduction of the underwater noise, which is mainly due to commercial traffic. In order to set guidelines and propose solutions to reduce the radiated noise from commercial ships, there is a need to define a procedure to measure the underwater sound from ships. In 2016, the ISO committee on underwater acoustics has published a standard describing the experimental procedure to measure the underwater sound from ships, under the reference ISO17208-1. This procedure is intended for deep water environments, i.e. for environments with a minimum depth of 150 m or 1.5 times the overall ship length. A second part, not published yet, has been written to correct the measured data from the reflection of the acoustic waves on the sea surface, effect known as the Lloyd's mirror effect. In some maritime areas, it can be difficult to find trial zones with sufficient water depths and the measurements can only be done in shallow waters. It is well known that in a shallow water environment, it is difficult to assess the level of a sound source, because of the multiple reflections of the acoustic waves on the bottom and on the sea surface. The aim of this study is to understand which parameters influence the sound measured by a hydrophone array in such a configuration, assuming that the source level is known. Knowing the environment, the sound level measured from each hydrophone on the array is calculated using the open source underwater propagation toolbox AcTUP. The levels are then corrected by the distance and quadratically summed over all the hydrophones, according to the procedure described in ISO17208-1 for deep water measurements. At low frequencies, the radiation of the source is similar to a dipole because of the Lloyd's mirror effect. At high frequencies, the third-octave bands level tends to a constant number with respect to frequency. It can be shown that the constant value depends on the ratio of water depth to distance to source, and to the sea floor properties. The influence of different parameters is successively investigated: number of hydrophones, sea bottom properties, speed of sound profile. Based on the simulations, empirical formulas are put forward in order to correct the effect of the shallow water environment.