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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.
RÉSUMÉ
Cet article présente un procédé d’inversion géoacoustique passif à partir de signaux représentatifs de vocalises de mammifères marins et d’un unique hydrophone en milieux petits fonds. Notre méthode utilise au mieux la propagation en multitrajets des vocalises et leur signature temps-fréquence pour extraire un observable permettant d’estimer par inversion les propriétés du fond marin. Il s’applique aux vocalises présentant au moins trois trajets résolus (direct, fond, surface) ce qui implique une portée d’environ 300 m autour de l’hydrophone pour des hauteurs d’eau de 100 m. A partir d’outils temps-fréquence, chaque émission est localisée et permet d’estimer le couple (rasance, coefficient de réflexion) lui correspondant. Le cumul d’un ensemble d’émissions permet de mesurer une courbe du coefficient de réflexion en fonction des angles de rasance. Cette donnée alimente un algorithme d’inversion géoacoustique mis en œuvre à travers un algorithme génétique. Dans cet article, nous détaillons la théorie de la méthode et nous étudions ses performances. Nous démontrons la validité de la méthode sur des signaux génériques réels émis par une source contrôlée dans le Golfe du Lion (rampes de fré-quences de 1 kHz à 2 kHz de durée 10 ms). Ces données permettent de valider la méthode sur des signaux aux caractéristiques très proches de celles de vocalises de mysticètes (baleines à bosse par exemple). Le cas des sifflements de delphinidés (ex : dauphins communs), plus hauts en fréquence, est également abordé.
passive acoustics, inversion, geoacoustics, marine mammals
MOTS-CLÉS
acoustique passive, inversion, géoacoustique, mammifères marins
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