Ryukyu Islands Waters Near Sound Field Analysis

The sounding velocity field in sea area by the Ryukyu is stuided by accessing Generalized Digital Environmental Model data base. The distribution rules about the depth corresponding to the minimal sound velocity value are analyzed particularly. Profile along the Guntu Archipelago and profile across Amami Archipelago are studied to find out the above character variations in them. There are series of conclusions. First, there is a sea area about the north of Hangzhou bay coastwise area where the minimal sounding velocity value always appear on sea surface in winter, spring and autumn and stabile surface sound channel exists. Along the coast of Zhejiang and Fujian , in the range of 30m topography isoline, the minimal sounding velocity value appears on the surface in summer. Second, sea water mixed very strongly and ascendant flow stand out in the eastern 130 km domain of the Okinawawa islands. It lead to that the minimal sounding speed depth value in winter and summer is greater 400m than that in spring and autumn. Sound signal whose frequency is higher than lOHz attenuated very fast in top level (10m below surface). Transmission loss vary wi th source and receiver location in water.