A 20 Hz giant magnetostrictive source for monitoring of global ocean variability
Toshiaki Nakamura, Tomoyuki Kanaizumi, and Iwao Nakano (JAMSTEC)
This paper describes a concept of very low frequency (20 Hz) source for monitoring of global ocean variability of 10 Mm scale in long term observation of over ten years. A low frequency (200 Hz) source has been developed for ocean acoustic tomography of 1000 km scale by JAMSTEC. Our 200 Hz source has an octagonal barrel shape, and consists of eight driving units by the use of giant magnetostrivtive rods. A giant magnetostrictive source has the advantage of small size, high transmitting level and long life span. We intend to apply the technology of this 200 Hz source to very low frequency sources. We have already conducted a long term operation test of the driving units of the equivalent time to ten years, and proved its long life. However, Iower frequency transmission requires larger transmitting level of the source. It means larger displacement of a giant magnetositrictive material is required than that of the present 200 Hz source. We are now planning two types of 20 Hz sources. The one is an ordinary barrel type by the use of the several giant magnetostrictive rods which has larger diameter and larger prestress than the present source.The other one is a bender bar type which made of giant magnetostrictive plates. The latter is expected to have a smaller size than the former because the displacement of bending plates can be larger than rods. Some theoretical results are shown for an extremely small 20 Hz source within 1 m diameter and 1 m height.
