Recent instrumental developments at IFREMER.
1 Introduction.
OAT, based on precise measurement of the ocean impulse response requires low frequency acoustic source with a large bandwidth (for the resolution) and a high sound level (for the precision). OAT infers from the accurate measurement of some sound propagation parameters (mainly travel times but also arrival angles or sound waves amplitude) the characteristics of the insonified ocean volume (temperature or density fields, currents...). The measured parameters are not the one directly of interest and those ones are obtained through the resolution of an inverse problem. Independently of the propagation range, the time resolution must be at least 10ms (i. e. at least 100Hz bandwidth) to separate the ray arrivals in time [4]. Ambient noise in the ocean is a natural limit to the achievable precision but other effects like internal waves scatterring and ray interferences decrease the measurement precision as well. Moreover, limitations are imposed by the instrument itself and the experiment geometry : timebase precision, instrument self noise, instrument positionning... However, it is possible to measure travel time variations of 100ms with an incertainty of a few milliseconds [5], [6]. The signal to noise ratio (SNR) at the receiver or equivalently the source level is of primary interest which, for a given precision, fix the maximum range. A propagation range of 1000km can be reached either with a source at 250Hz and a sound level of 190dB ref 1μPa at 1m or a source at 400Hz and a sound level of 197dB ref 1μPa at 1m.
