Christophe Laplanche

Male sperm whale acoustic behavior observed from multipaths at a single hydrophone

Sperm whales generate transient sounds (clicks) when foraging. These clicks have been described as echolocation sounds, a result of having measured the source level and the directionality of these signals and having extrapolated results from biosonar tests made on some small odontocetes. The authors propose a passive acoustic technique requiring only one hydrophone to investigate the acoustic behavior of free-ranging sperm whales. They estimate whale pitch angles from the multipath distribution of click energy. They emphasize the close bond between the sperm whales physical and acoustic activity, leading to the hypothesis that sperm whales might, like some small odontocetes, control click level and rhythm. An echolocation model estimating the range of the sperm whales targets from the interclick interval is computed and tested during different stages of the whales dive. Such a hypothesis on the echolocation process would indicate that sperm whales echolocate their prey layer when initiating their dives and follow a methodic technique when foraging.

Non-Stationary Time-Series Segmentation Based on the Schur Prediction Error Analysis

This paper proposes a non-stationary time-series segmentation method based on the analysis of the forward prediction error issued from the adaptive Schur orthogonal signal parameterisation. There is no a priori information about the analysed signal thus this method can be easily adapted to a large family of different types of signals for which two different stochastic processes are present. In this paper we set out some of the advantages of the adaptive Schur filter in deducing the presence of different non-stationary transient or long-term events leading to the signal segmentation. For each sample, the adaptive Schur algorithm calculates the optimal second-order solution for the signal prediction resulting in a set of time-varying model parameters (inter alia forward prediction error). We define the likelihood ratio (LR) test based on the Schur forward prediction error that is evaluated at each sample, thus giving excellent time-reaction properties. The LR test allows us to effectively partition the analysed time-series into homogeneous segments by considering its second-order statistics which are tracked adaptively by the Schur filter. The results performed by applying the proposed method to simulated signals are shown to verify its high performance

A Bayesian method to estimate the depth and the range of phonating sperm whales using a single hydrophone

Some bioacousticians have used a single hydrophone to calculate the depth/range of phonating diving animals. The standard one-hydrophone localization method uses multipath transmissions (direct path, sea surface, and seafloor reflections) of the animal phonations as a substitute for a vertical hydrophone array. The standard method requires three multipath transmissions per phonation. Bioacousticians who study foraging sperm whales usually do not have the required amount of multipath transmissions. However, they usually detect accurately (using shallow hydrophones towed by research vessels) direct path transmissions and sea surface reflections of sperm whale phonations (clicks). Sperm whales emit a few thousand clicks per foraging dive, therefore researchers have this number of direct path transmissions and this number of sea surface reflections per dive. The author describes a Bayesian method to combine the information contained in those acoustic data plus visual observations. The authors tests using synthetic data show that the accurate estimation of the depth/range of sperm whales is possible using a single hydrophone and without using any seafloor reflections. This method could be used to study the behavior of sperm whales using a single hydrophone in any location no matter what the depth, the relief, or the constitution of the seafloor might be.

New effective analytic representation based on the time-varying Schur coefficients for underwater signals analysis

The paper proposes a new effective analytic signal representation dedicated to underwater signal analysis. The proposed approach is based on the recursive normalized exact least-square ladder estimation algorithm because of its excellent convergence behaviour, extremely fast start-up performance and its capability to quickly track parameter changes. The linear orthogonal parameterization procedure considered in this paper is numerically efficient and stable and follows from the celebrated Schur algorithm. The generalized Schur filter at each time-step calculates optimal orthogonal signal representation using second-order statistics which results in a set of time-varying Schur coefficients. The filter transforms a one dimensional signal (time-series) to a multidimensional sequence of the time-varying Schur coefficients. The second-order signal description based on the generalized Schur filter is efficient and robust. The model parameters such as the forward prediction error or the Schur coefficients can be used for detection and segmentation or for deducing parametric joint time-frequency signal representation. These are common stages in signal estimation and analysis. Moreover, in the future we envisage using the time-varying Schur coefficients to classify and identify different events in the analyzed time-series. The results performed by applying the proposed method to simulated and real-world signals are shown to verify its high performance.

Bayesian three-dimensional reconstruction of toothed whale trajectories: Passive acoustics assisted with visual and tagging measurements

The author describes and evaluates a Bayesian method to reconstruct three-dimensional toothed whale trajectories from a series of echolocation signals. Localization by using passive acoustic data (time of arrival of source signals at receptors) is assisted by using visual data (coordinates of the whale when diving and resurfacing) and tag information (movement statistics). The efficiency of the Bayesian method is compared to the standard minimum mean squared error statistical approach by comparing the reconstruction results of 48 simulated sperm whale (Physeter macrocephalus) trajectories. The use of the advanced Bayesian method reduces bias (standard deviation) with respect to the standard method up to a factor of 8.9 (13.6). The author provides open-source software which is functional with acoustic data which would be collected in the field from any three-dimensional receptor array design. This approach renews passive acoustics as a valuable tool to study the underwater behavior of toothed whales.

Depth/range localization of diving sperm whales using passive acoustics on a single hydrophone disregarding seafloor reflections

Depth and range of sound sources can be estimated using a single hydrophone. Such a passive acoustic technique requires the detection of direct path transmitted, sea surface, and seafloor reflected source signals, so as to measure their time of arrival differences (TOADs). Sperm whales almost continuously emit powerful, directional echolocation sounds (usual clicks) when diving. Sperm whales often dive in deep water, and click seafloor reflections are usually well detected only at the beginning of the dive. Surface echoes may be detected during the entire dive. If the measurement of the surface reflection TOAD of a single click is not enough for estimating the depth/range of the sperm whale at the time when this click was emitted, the joint consideration of delays emitted during the whole dive may provide this estimation. Such delays are indeed the measurements of a single phenomenon: the underwater movements of a single‐clicking sperm whale. One can merge such data using a Bayesian technique, as well as ...

Real time sperm whale depth estimation using passive acoustics

Sperm whales (Physeter macrocephalus) make series of transient echolocation sounds (clicks), when foraging. Clicks reflect to the sea surface and the seafloor when propagating towards a receiver. The detection of the once reflected surface and seafloor echoes of a click, and the measurement of the delays of both echoes to the direct path transmitted signal, make possible the localization in depth and in range of the sperm whale which emitted the click, by using a single receiver. Repeating this process click after click then leads to the plotting of the whale depth/range variations while diving. The main difficulty when automating this process is usually to correctly identify direct path signals and echoes. The authors compute the a priori probability density functions of both surface/floor echo delays, improving this click echo identification process. The authors then use an adaptive filter, detecting simultaneously direct path signals with surface echoes. A visual estimation of the range of the whale when starting a dive is used to initiate the detection process. The detection of seafloor echoes then leads to the depth/range estimation of the whale while diving. The detection process works fine (84%, considering a 45-minute dive recording) assuming that a single sperm whale is clicking. The localization process works correctly when seafloor echoes are detected. This method is non-invasive, as it uses neither tags nor active acoustics, and easy to set, as requiring a single hydrophone and a CTD meter, and makes possible an unbiased, inexpensive, automated survey of the diving behavior of single sperm whales in a given area.

Adaptative Schur algorithm dedicated to underwater transient signal processing

The algorithm proposed by Lee and Morf [IEEE Transactions on Circuits and Systems 28(6) (1981)] which stems from the method defined by Schur [Operator Theory: Advances & Application, Vol. 18 (Birk‐Verlag, 1986)] has acquired a new significance [Zarzycki, Journal of Multidimensional Systems & Signal Processing (Kluwer Academic, 2004)], due to its performances and particularly due to its applications in real time, made possible by the speed of processors available nowadays. Based on the innovations filter principle, Schur’s proposal models the signal by calculating reflection coefficients, describing entirely the second‐order signal. The reflection coefficients can be simply transformed to the AR coefficients, from which one derives the time‐frequency representation. We compare performances of this approach with other time‐frequency representations commonly used in the signal processing (spectrogram, AR, wavelet transform); we subsequently present the results obtained for transitory underwater acoustic signals, which our laboratory is investigating. The Lee and Morf algorithm offers an excellent tracking of the signal’s characteristics and allows us to systematically detect transitory signals. This is particularly pertinent to segmentation problems relating to the application of underwater acoustics. The robustness of the Schur detector and a resolution of the Schur time‐frequency representation support the resurgence of the Schur algorithm.

Broken line 3‐dimensional sperm whale diving trajectory reconstruction using passive acoustics on a single hydrophone

Sperm whales make deep dives to hunt. A dive, lasting 45 minutes on average, is composed of a vertical descent to the prey layer depth, the properly so called hunt (at a quasi constant depth) inside the prey layer, and a vertical ascent back to the sea surface. Sperm whales make series of echolocation signals (clicks) during the two first stages. The sea surface/bottom click echo detection and delay measurements then make possible the sperm whale range/depth estimation during these stages, by passively using a single hydrophone. The vertical, rectilinear sperm whale trajectory during the first stage is unambiguously estimated from the echo delays. The sperm whale trajectory can also be reconstructed during the second stage, from the sperm whale range variations only, even when not detecting sea bottom click echoes. These range variations strongly suggest the sperm whale trajectory to be a broken line (e.g., a 2‐piece line, 600 m straight ahead, 85 degree bend, 1000 m straight ahead). Assuming a vertical s...