Acoustic propagation properties in the seasonal change environment of shallow sea area

The seasonal change of hydrological environment in shallow water has significant influence on acoustic propagation. The objective of this study is to determine the effects of seasonal change environment on the acoustic field in a typical shallow water area. According to analysis of climatological hydrological data, the sound speed profile (SSP) shows an intense seasonal change under the influence of monsoon. In winter, a complete mixed layer is formed under the effect of air-sea interaction, which leads to surface duct between surface and bottom boundary. In summer, a strong thermocline (temperature gradient > 0.5°/m) is formed because of a warm surface and a cold water mass near the bottom, which leads to a poor duct and fast propagation attenuation. A Gaussian beam model was used to calculate the sound pressure fields in spring, summer, autumn and winter for a 20 kHz sound source in the depth of surface and bottom separately, and the transmission losses(TL) in the different environments were compared. It is found that TL variation could be over 30 dB with seasonal environmental change. The sound level reaches a minimum when thermocline is strengthened in summer, and it reaches its maximum when a surface duct is formed in winter. Under the effect of seasonal environment change, together with the change of source depth, the fluctuation of sound effective arrival range could be as far as 5-8km. When the source is near the bottom, a special duct below the thermocline is identified in the summer. The TL in the upper layer is about 10 dB greater than that in other seasons at the range of 10 km; conversely, TL in the lower layer is even slightly less than that in other seasons. The results indicate that sound propagation is sensitive to seasonal environmental change, so the environmental adaptability should be a key factor to be considered in the design of underwater long baseline systems and acoustic detection systems.