A new modular instrumentation for ocean acoustic tomography, present status and future trends

In 1994, IFREMER launched the development of a new modular instrumentation for meso-scale OCEAN Acoustic Tomography (OAT). Previous instruments made by Woods Hole Oceanographic Institution (WHOI), Webb Research Corporation (WRC) and IFREMER are actually not adapted to basin scale studies (@1000 km). The new autonomous instrument is made up of a wide-band unlimited depth acoustic source, a high efficiency class D power amplifier, a programmable multi-function receiver, a long baseline positioning system and a low power high stability clock. Because of great depth and high efficiency requirements, the Janus-Helmholtz transducer technology was well suited to the long range OAT applications. For the 1997 CAMBIOS experiment, the instrument has been deployed in Atlantic at a 600 m depth. The working central frequency is 400 Hz and 600 km ranges are expected. In parallel to the source development, a 1 Gbyte storage capacity autonomous receiver able to work in various frequency bands has been realised. Fully compatible with the acoustic source, it can be used for standard OAT experiments. Following a description of the present instrumentation and the first result of CAMBIOS experiment, the evolution of the autonomous instrument for the near future is presented: a new optimised 250 Hz Janus-Helmholtz transducer for basin scale studies, a high stability rubidium clock, a four hydrophones receiving array and a multi-channel receiver, and the possibility to make a reliable acoustic control of all instruments from surface.