An algorithm allowing simultaneous detection and localization of multiple submerged targets crossing an acoustic tripwire based on forward scattering is described and then evaluated based upon data collected at sea. This paper quantifies the agreement between the theoretical performance and the results obtained from processing data gathered at sea for crossings at several depths and ranges. Targets crossing the acoustic field produce shadows on each side of the barrier, for specific sensors and for specific acoustic paths. In post-processing, a model is invoked to associate expected paths with the observed shadows. This process allows triangulation of the target's position inside the acoustic field. Precise localization is achieved by taking advantage of the multipath propagation structure of the received signal, together with the diversity of the source and receiver locations. Environmental robustness is demonstrated using simulations and can be explained by the use of an array of sources spatially distributed through the water column.
Today's minimum requirements for ocean acoustic models are to be able to simulate broadband signal transmissions in 2D varying environments with an acceptable computational effort. Standard approaches comprise ray, normal mode and parabolic equation techniques. In this paper we compare the performance of four broadband models [Formula: see text] on a set of shallow-water test environments with propagation out to 10 km and a maximum signal bandwidth of 10–1000 Hz. It is shown that a computationally efficient modal approach as implemented in the [Formula: see text] model is much faster than standard, less optimized models such as [Formula: see text] and [Formula: see text]. However, the handling of range dependency in the adiabatic approximation is not always sufficiently accurate, and it is suggested that a mode coupling approach be adopted in [Formula: see text]. Moreover, the interpolation of modal properties in range could lead to a further significant speed-up of mode calculations in range-dependent environments. It is concluded that coupled modes with wavenumber interpolation in both frequency and range remain the most promising wave modeling approach for broadband signal simulations in range-dependent shallow water environments. At higher frequencies (> 1 kHz) there is currently no alternative to rays as a practical signal simulation tool.
The standard parabolic approximation to the acoustic wave equation is known to have intrinsic phase errors, which will degrade the accuracy of any PE solution for long-range propagation in the ocean. Pierce recently suggested a remedy to minimize these phase errors by simply choosing an appropriate mean phase speed (c0), eventually updated with range, representing a weighted average of all phase speeds involved in a particular propagation problem. We have now extended Pierce's formalism to include the wide-angle parabolic equation of Thomson and Chapman [J. Acoust. Soc. Am. 74, 1848 (1983)], which inherently has smaller phase errors than the standard parabolic equation. The importance of using c0 updates in PE calculations for range-dependent environments is demonstrated through numerical results for a wedge-shaped ocean. Comparison with alternative solution techniques (coupled modes, intrinsic modes) shows that accurate PE solutions for the wedge problem can be obtained only for single-mode situations, even when using c0 updates with range.
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