B. Nicolas

Single-receiver geoacoustic inversion using modal reversal

This paper introduces a single-receiver geoacoustic-inversion method based on dispersion analysis and adapted to low-frequency impulsive sources in shallow-water environments. In this context, most existing methods take advantage of the modal dispersion curves in the time-frequency domain. Inversion is usually performed by matching estimated dispersion curves with simulated replicas. The method proposed here is different. It considers the received modes in the frequency domain. The modes are transformed using an operator called modal reversal, which is parameterized using environmental parameters. When modal reversal is applied using parameters that match the real environment, dispersion is compensated for in all of the modes. In this case, the reversed modes are in phase and add up constructively, which is not the case when modal reversal is ill-parameterized. To use this phenomenon, a criterion that adds up the reversed modes has been defined. The geoacoustic inversion is finally performed by maximizing this criterion. The proposed method is benchmarked against simulated data, and it is applied to experimental data recorded during the Shallow Water 2006 experiment.

Modal depth function estimation using time-frequency analysis

Acoustic propagation in shallow water is characterized by a set of depth-dependent modes, the modal depth functions, which propagate in range according to their horizontal wavenumbers. For inversion purposes, modal depth function estimation in shallow water is an issue when the environment is not known. Classical methods that provide blind mode estimation rely on the singular value decomposition of the received field at different frequencies over a vertical array of transducers. These methods require that the vertical array spans the full water column. This is obviously a strong limitation for the application of such methods in an operational context. To overcome these shortcomings, this study proposes to replace the spatial diversity constraint by a frequency diversity condition, and thus considers the case of a field emanating from an impulsive source. Indeed, because of the discrete nature of the wavenumber spectrum and due to their dispersive behavior, the modes are separated in the time-frequency domain. This phenomenon enables the design of a modal filtering scheme for signals received on a single receiver. In the case of a vertical receiver array, the modal contributions can be isolated for each receiver even when using a partial water column spanning array. This method thus eliminates the receiving constraints of classical methods of modal depth function estimation, although it imposes the use of an impulsive source. The developed algorithm is benchmarked on numerical simulations and validated on laboratory experimental data recorded in an ultrasonic waveguide. Practical applications are also discussed.

Automatic and passive whale localization in shallow water using gunshots

This paper presents an automatic and passive localization algorithm for low frequency impulsive sources in shallow water. This algorithm is based on the normal mode theory which characterizes propagation in this configuration. It uses specific signal processing tools and time-frequency representations to automatically extract features of the propagation. Then, it uses the dispersive properties of the oceanic waveguide as an advantage to perform the localization. Only few hydrophones are needed and neither knowledge of the oceanic environment nor simulation of the propagation is required. The proposed method is successfully applied on North Atlantic Whale gunshots in the Bay of Fundy recorded with a network of three hydrophones.

Source localization on a single hydrophone

Source localization in shallow water is a crucial issue in underwater acoustics. The aim of this work is to propose efficient methods to perform source localization in depth and range using a single hydrophone. As a filtering method matched to single hydrophone configuration has been developed, the normal mode propagation model is considered. Several methods are exposed: incoherent matched mode processing (MMP), coherent MMP, a source depth estimation proposed by Nicolas et al. and a range estimation that we have developed. Methods are applied and compared on a real dataset.

Mode sign estimation to improve source depth estimation

This paper describes source depth estimation in shallow water environments using a Horizontal Line Array of hydrophones on the sea bottom. A method based on modal propagation, using mode amplitude modulus, has been proposed by Nicolas, B., et al, (2006). As knowledge of the sign of mode amplitudes can improve source depth estimation, we present a estimator of these signs and integrate them in the source depth estimation method. Results on real data are shown.