Inversion géoacoustique passive en milieux petits fonds à partir de signaux représentatifs des émissions de cétacés

This paper presents a geoacoustic inversion scheme relying on the inversion of signal close to marine mammals vocalizations on a single hydrophone in shallow waters. Our method makes best use of the multipath propagation of vocal calls and of their time-frequency signature to extract an observable used by the inversion algorithm to estimate sea bottom features. This method requires vocalizations with at least three resolved paths (direct, first bottom reflected and first surface reflected paths), which implies a 300 meter range around the hydrophone for 100 meter water depth. Using time-frequency tools, each emitted call is localized and processed to provide its corresponding couple (grazing angle, reflexion coefficient). Through the processing of several vocalizations, we get a curve of the reflexion coefficient according to grazing angles. This curve feeds a geoacoustic inversion algorithm run with a genetic algorithm. In this paper, we describe both the theory and performances of our method. We demonstrate its validity on generic real signals emitted through a controlled acoustic source in the Gulf of Lion (10 ms duration chirps with [1 kHz-2 kHz] bandwidth). An application on delphinids higher frequency calls is also mentioned but the inversion step for those signals will need further functions to assess and correct the directivity losses. MOTS-CLÉS : acoustique passive, inversion, géoacoustique, mammifères marins.