Comparison of Surface Duct Energy Leakage with Bottom-Bounce Energy of Close Range Propagation

For close range (almost 8–15 km) propagation, bottom-bounce energy (BBE) usually suffers from a great transmission loss due to a large grazing angle interacting with the ocean bottom, and the surface duct energy leakage (SDEL) might be able to make a significant contribution to shadow zone insonification. This study aims at making a comparison between SDEL and BBE in a shadow zone with a source depth of 50 m for close range propagation. Analysis of experimental data shows that for lower frequencies the SDEL can be comparable with the BBE up to a range of 12 km. Numerical simulations suggest that the transmission loss (TL) of 90 dB is a proper value to quantify the role of the SDEL in shadow zone insonification. When TL of the SDEL is about 90 dB, it is believed to be comparable with the BBE. Studies on the effect of mix layer depth (MLD) on the SDEL indicate that larger MLD and lower frequencies can help to create favorable conditions for SDEL and that there exists an optimal MLD for SDEL at certain frequency. Statistics of MLD distribution in the South China Sea (SCS) show that winter is favorable for SDEL and the situation reverses in summer. Eddies are known to occur frequently in the SCS and the study on the effect of eddies on the SDEL suggests that the presence of an eddy will influence the SDEL significantly by modifying the MLD.