The Waveguide Invariant (WI) theory has been introduced to quantify the orientation of the intensity interference patterns in a range-frequency domain. When the sound speed is constant over the water column, the WI is a scalar with the canonical value of 1. But, when considering shallow waters with a stratified sound speed profile, the WI ceases to be constant and is more appropriately described by a distribution, which is mainly sensitive to source/receiver depths. Such configurations have been widely investigated, with practical applications including passive source localization. However, in deep waters, the interference pattern is much more complex and variable. In fact the observed WI varies with source/receiver depth but it also varies very quickly with source-array range. In this paper, the authors investigate two phenomena responsible for this variability, namely the dominance of the acoustic field by groups of modes and the frequency dependence of the eigenmodes. Using a ray-mode approach, these two features are integrated in a WI distribution derivation. Their importance in deep-water is validated by testing the calculated WI distribution against a reference distribution directly measured on synthetic data. The proposed WI derivation provides a thorough way to predict and understand the striation patterns in deep-water context.
Passive source localization is a crucial issue in underwater acoustics. In this paper, we focus on shallow water environment (0 to 400 m) and broadband Ultra-Low Frequency acoustic sources (1 to 100 Hz). In this configuration and at a long range, the acoustic propagation can be described by normal mode theory. The propagating signal breaks up into a series of depth-dependent modes. These modes carry information about the source position. Mode excitation factors and mode phases analysis allow, respectively, localization in depth and distance. We propose two different approaches to achieve the localization: multidimensional approach (using a horizontal array of hydrophones) based on frequency-wavenumber transform (F-K method) and monodimensional approach (using a single hydrophone) based on adapted spectral representation (FT a method). For both approaches, we propose first complete tools for modal filtering, and then depth and distance estimators. We show that adding mode sign and source spectrum informations improves considerably the localization performance in depth. The reference acoustic field needed for depth localization is simulated with the new realistic propagation modelMoctesuma. The feasibility of both approaches, F-K and FT a , are validated on data simulated in shallow water for different configurations. The performance of localization, in depth and distance, is very satisfactory.
The fundamental and practical problem of passive localization in range and depth, of an acoustic underwater source is addressed, with application to an at-sea experiment. We propose and try a new matching method based on a metric called as Hausdorff distance as a cost-function to be minimized, in order to perform the localization inversion. The data set analyzed here was collected during the DGA campaign ALMA 2015, which took place in a shallow water environment of the southern coast of France. Acoustic data were measured over a 10m-high vertical linear array (VLA), composed of 64 hydrophones. The 2-D localization, in range and depth, is performed by matching the patterns of time difference of arrival (TDOA), between respectively observed and modeled sequences. Several variants of the Hausdorff Distance are applied, firstly separately in each single hydrophone, and then combined in order to improve the localization accuracy, reducing the ambiguity either is depth and in range. The performance is evaluated in terms of the localization accuracy of the proposed method, in a context of passive localization with a cooperative system considering a motionless target. Very satisfactory performance and accuracy are obtained.
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