AT-SEA EXPERIMENTAL EVALUATION OF THE INFLUENCE OF ENVIRONMENTAL FLUCTUATIONS ON THE ACOUSTIC COHERENCE RADIUS

Sound propagation in a fluctuating medium can be impacted by local space and time variation of the celerity. In the ocean, this environmental variability is mainly due to internal waves, tides or gyres and is well studied by oceanographers. More specifically, major research tackles the issue of characterizing the impact of these internal waves on a measured signal propagated throughout the perturbed medium. One important quantity defined in this literature is the ''coherence radius'': a metric that evaluates the distance between two sensors on which the received signals can be considered as correlated. It can be related to the level of spatial fluctuations under a high frequency approximation. Hence, integrated as prior knowledge on the propagating medium, the coherence radius can be very useful to determine the degree of corruption of a measured signal for typical inverse problems in underwater acoustics, such as tomography and source localization. In this work, we present different results from the analysis of the ALMA 2017 measurement campaign. ALMA (for Acoustic Laboratory for Marine Applications) is an autonomous acoustic observatory designed by DGA Naval Systems in 2012 and deployed in 2017 in the Saint-Florent Gulf, in the Western Mediterranean Sea. We specifically aim at assessing the intensity of environmental and acoustic fluctuations on sound waves propagated from a known source to a set of vertical line arrays and at comparing it to the theoretical predictions. Results suggest a strong relationship between the coherence radius and temperature fluctuations as well as other phenomena such as tides or water-mixing areas. Given hypotheses made about the influence of fluctuations on the radius of coherence, we further discuss future experimental settings to validate or discard these hypotheses.