Abstract
A 200-Hz low-frequency sound source for ocean acoustic tomography was developed by using a supermagnetostrictive material made of the rare-earth metal (Tb, Dy)Fe2 aimed at increasing the transmission level and the reduction of the sound source size. A sound-pressure level above 190 dB (re: 1 μPa at 1 m) with a frequency bandwidth of over 50 Hz were obtained in the depth range between 800 and 1200 m. Field tests for the long-range propagation of several hundreds of kilometers were carried out near the Japan Trench in November 1993. The sound source was moored at a depth of 1242 m and the hydrophone array was moored at a depth of 1385 m. The distance between the sound source and the hydrophone array was 621 km. Received data were recorded with a sufficient signal-to-noise ratio after achieving beamforming to discriminate the sound waves incident upon the hydrophone array from the upper and lower direction. The signals were also received by the acoustic monitoring system suspended from the ship at the distance of 772 km from the sound source. From these results, it is confirmed that this sound source has the capability of 1000-km propagation necessary for the tomography transceiver systems.
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