Abstract
Classical ocean acoustic experiments involve the use of synchronized arrays of sensors. However, the need to cover large areas and/or the use of small robotic platforms has evoked interest in single-hydrophone processing methods for localizing a source or characterizing the propagation environment. One such processing method is "warping," a non-linear, physics-based signal processing tool dedicated to decomposing multipath features of low-frequency transient signals (frequency f < 500 Hz), after their propagation through shallow water (depth D < 200 m) and their reception on a distant single hydrophone (range r > 1 km). Since its introduction to the underwater acoustics community in 2010, warping has been adopted in the ocean acoustics literature, mostly as a pre-processing method for single receiver geoacoustic inversion. Warping also has potential applications in other specialties, including bioacoustics; however, the technique can be daunting to many potential users unfamiliar with its intricacies. Consequently, this tutorial article covers basic warping theory, presents simulation examples, and provides practical experimental strategies. Accompanying supplementary material provides matlab code and simulated and experimental datasets for easy implementation of warping on both impulsive and frequency-modulated signals from both biotic and man-made sources. This combined material should provide interested readers with user-friendly resources for implementing warping methods into their own research.
Highlights
The need to cover large areas and/or the use of small robotic platforms has evoked interest in single-hydrophone processing methods for localizing a source or characterizing the propagation environment. One such processing method is “warping,” a non-linear, physics-based signal processing tool dedicated to decomposing multipath features of low-frequency transient signals, after their propagation through shallow water and their reception on a distant single hydrophone
The development of underwater acoustic signal processing was originally driven by military applications that require advanced sonar processing (Ainslie, 2010) to detect and localize quiet sources in an uncertain environment (Dosso and Wilmut, 2011)
We describe a relatively recent nonlinear signal processing method—termed warping—that is dedicated to the study of low-frequency (f < 500 Hz) transient sounds recorded in coastal environments
Summary
The development of underwater acoustic signal processing was originally driven by military applications that require advanced sonar processing (Ainslie, 2010) to detect and localize quiet sources in an uncertain environment (Dosso and Wilmut, 2011). Most advanced signal processing methods require the use of extended time-synchronized array hydrophones to perform spatial and temporal filtering The deployment of such systems is awkward and expensive. Subsequent localization of animal sounds from these data sets is often desirable, because source localization is a key step in establishing population density estimates and evaluating subtle responses to anthropogenic activities This is rarely implemented, because most traditional source localization methods require the deployment of multiple hydrophones over wide spatial regions, and complex measurements of relative arrival times between sensors. In this tutorial, we demonstrate how localization information can be extracted from both baleen whale impulsive and frequency-modulated sounds from single-hydrophone recordings in shallow water. The aim is to provide the reader with the opportunity to try warping, to confirm they are using the technique properly, and to facilitate their application of the method on their own datasets
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