A new scheme to estimate the nature of superficial sediment with dolphin whistles
Résumé
Three sessions of acoustics records were performed by our own in 2009 and 2010 in Bay of Biscay and Ushant area. Autonomous recorders AURAL from Multi Electronique Inc. (http://www.multi-electronique.com/) were moored in shallow water (depth ~ 130 m) at a 85 m depth to acquire sounds at a 32672 Hz sampling rate and with a 16 bit resolution. An exhaustive exploration of these data with a home-made whistles detector indicates that the 2009‟s records present many 2-hours patches filled with whistles from common or bottlenose dolphins whereas the 2010‟s records present only four 2-hour patches filled with whistles from Risso dolphins or pilot whales. A rapid inspection of ambient noise level and whistles source level reveals a detection range of 1 kilometer for the whistles. This range together with water depth create a multipath structure resolved by a spectrogram representation. As a first contribution of the communication, an inversion scheme to exploit this chorus of whistles is proposed to estimate the superficial sediment nature. A first part of our inversion scheme exploits one whistle a) to estimate the position of the emitter thanks to multipath (direct, bottom reflected, surface reflected) relative times of arrival, b) to estimate the elevation angle of bottom reflected path, c) to estimate relative strength between bottom reflected and direct paths and then d) to estimate a set {elevation angle, coefficient of reflexion} for this whistle. A second part of our inversion scheme consists in stacking the sets {elevation angle, coefficient of reflexion} obtained for many whistles to build the reflective properties of the bottom and to estimate a bottom structure that fits well with the measurements. The second contribution of the communication is to investigate and present the features of the whistles (level, directivity, bandwidth, frequency modulation law) and ambient noise (spectrum level) that conditioned the performance of our inversion scheme. The communication will ended by an attempt to apply our inversion scheme on the bay of Biscay and Ushant data.