1. INTRODUCTION
We are planning to develop a very low frequency source such as 20Hz and deploy the sea floor of the 1000m depth near Japan. The sound is transmitted to the whole Pacific Ocean and received at the around the Pacific ocean. The objective of this system is a long term observation of the cahnge of water temperature of global scale like the present ATOC Project. However, recently, it was reported the lower frequency of the source is better than 70Hz of the ATOC, because the more stable sound propagation is obtained due to the less effect ofinternal waves. So, we need to develop a very low frequecy source aorund 20-30Hz. and now under basic research.
The ATOC project which observes the water temperature variation of the gyre scale has been conducted by the Scripps Institution of Oceanography, Univ. of California, the Applied Physics Laboratories, Univ. of Washington and other several institutions. The performance of 70Hz frequency and 195dB (1μPa at 1m) transmitting level is required for the sound source to realize 10Mm propagation distance. It is also one of the necessary conditions to guarantee the continuous operation for ten years, since the observation for this period has been scheduled in the plan. We carried out the acceleration test in order to examine the durability of grant magnetrostrictive driving unit developed for the 200Hz source in the condition of corresponding to the case in which the ATOC source would be used for 10 years.
