High-efficient tunable sound sources for ocean and bottom tomography, 15 years of operating history

Abstract

In November, 2001, the R/V “Point Sur” departed from the Moss Landing Marine Laboratories pier with the first prototype of a high-Q, tunable organ pipe projector. The first test of the tunable Teledyne Webb Research (TWR) organ-pipe was successfully conducted on 11.09.2001. The TWR sound source demonstrated exceptional performance. It was coherent, efficient, powerful, and had unlimited operational depth, as well as a minimum level of high frequency harmonic content. The projector uses a narrow-band, highly efficient sound resonator, which is tuned to match the frequency and phase of a reference frequency-modulated signal. A finite element simulation shows the structural acoustics of the tunable resonator. The results of such simulation for the organ pipe with the octave frequency band are presented and discussed. This sound source was built for the Naval Postgraduate School (Monterey, CA) for studying temperature variability in the California Current. Since 2001, many deep-water ocean acoustic experiments have used this type of TWR sound source. The operating history of the sound sources is reviewed here. The first variant of the TWR sound source tuned the frequency of the resonator tube over the bandwidth 200–300 Hz. Modifications with 140–205 Hz, 500–1000 Hz and 800–1200 Hz frequency sweeps have been built. The transmission duration can vary from one second to a few minutes. In October 2002 a TWR sound source was deployed on the top of Hoke Seamount. It transmitted 135-s linear frequency sweeps for ocean acoustic tomography and RAFOS 80-s narrow-band chirps for two years until October 2004. The signal was received with a good signal-to-noise ratio at 700 km range. During 2004 and 2005 the TWR sound source system was used as a part of the instrumentation for the Meridional Overturning Variability Experiment (MOVE) of the International program CLIVAR. The sound source was deployed near the French island of Guadeloupe and transmitted tomographic signals to measure the temperature of the North Atlantic cold deep waters during 2004– 2005. After solving the engineering problems found in the first sea trials, a modified deep-water version of the sound source was built for long-term deployment.