Low-frequency Janus-Helmholtz transducers for great-depth acoustical oceanography
|Author(s)||Le Gall Yves1|
|Affiliation(s)||1 : IFREMER, Unité Navires et Systèmes embarqués, Service Acoustique Sous-Marine, BP 70, 29280 Plouzané, France|
|Meeting||Sonar Transducers '99, Birmingham, 1999|
|Source||Proceedings of the Institute of Acoustics, Sonar Transducers '99, Birmingham, 1999, vol. 21|
|Abstract||Initially designed for low frequency active sonar, the double-ended Janus-Helmholtz Acoustic Source (JHAS) has attractive applications for acoustical oceanography, namely Ocean Acoustic Tomography (OAT) and Very High Resolution (VHR) reflection seismic.
In 1994, IFREMER launched the development of a new autonomous instrumentation adapted to OAT basin-scale studies (@ 1000 km). Due to its low-frequency and high-power properties, the JHAS technology was well-suited to these long range applications. Thanks to a technological evolution of the JHAS concept, the problem of high hydrostatic pressures has been solved. The first very low frequency (VLF) JHAS prototype was optimized to work in two frequency bands located around 250 and 400 Hz. The large electrical Q-factor values, the small TVR values and the limited available power (500 VA) prevented a use between both resonances (250 Hz) with enough bandwidth and sound level. A use around the second resonance (400 Hz) was then decided and 600 km ranges were obtained during Cambios experiment.
Following the description with working principles and the evolution of the JHAS technology, the performances of this first VLF prototype are presented with experimental results. A second JHAS prototype with a better coupling between both resonances, allowing a use around 250 Hz with a 70 Hz bandwidth and a constant sound level at any depth, will be achieved in 1999.
Furthermore, the large-bandwidth depth-unlimited JHAS concept appeared very promising in soil survey, compared to conventional VHR seismic sources.