Internal wave effects on acoustic propagation

Underwater Acoustics Measurements (UAM) 1st International Conference “Underwater Acoustic Measurements: Technologies & Results,” 28 June- 1 July 2005, Heraklion, Crete, Greece,Internal gravity wave induced acoustic fluctuations are reviewed. It is well known that internal waves cause temporal and spatial variability above and beyond that caused by mesoscale and larger ocean heterogeneity. Increasingly detailed work over many decades has shown that deep-ocean internal waves, which have often been parameterized using the Garrett-Munk spectrum as a guide, are responsible for rapid acoustic field variability at all propagation ranges. Numerous experiments have shown that various sections of wavefronts from impulsive sources have fluctuation qualities well-described by theory, simulation, or both. In contrast, fluctuations in shallow water experiments, although known to be consistent with those expected from internal waves via theoretical and simulation arguments, are incompletely described by theories for a number of reasons. These reasons include nonstationary, inhomogeneous or anisotropic wave environments, unknown geoacoustic properties, and rapidly changing background currents, all of which prevent detailed comparison of observation and prediction. At this time, many different shallow-water internal wave scenarios give rise to similar field fluctuations, within reasonable confidence intervals for the predictions. This may simplify order-of magnitude fluctuation prediction, while simultaneously making inversion and highly-detailed prediction problematic.This work was supported by the Office of Naval Research

Grid-free compressive beamforming

The direction-of-arrival (DOA) estimation problem involves the localization of a few sources from a limited number of observations on an array of sensors, thus it can be formulated as a sparse signal reconstruction problem and solved efficiently with compressive sensing (CS) to achieve high-resolution imaging. On a discrete angular grid, the CS reconstruction degrades due to basis mismatch when the DOAs do not coincide with the angular directions on the grid. To overcome this limitation, a continuous formulation of the DOA problem is employed and an optimization procedure is introduced, which promotes sparsity on a continuous optimization variable. The DOA estimation problem with infinitely many unknowns, i.e., source locations and amplitudes, is solved over a few optimization variables with semidefinite programming. The grid-free CS reconstruction provides high-resolution imaging even with non-uniform arrays, single-snapshot data and under noisy conditions as demonstrated on experimental towed array data.Comment: 14 pages, 8 figures, journal pape

Investigation of mode filtering as a preprocessing method for shallow-water acoustic communications

Author Posting. © IEEE, 2010. This article is posted here by permission of IEEE for personal use, not for redistribution. The definitive version was published in IEEE Journal of Oceanic Engineering 35 (2010): 744-755, doi:10.1109/JOE.2010.2045444.Acoustical array data from the 2006 Shallow Water Experiment (SW06) was analyzed to show the feasibility of broadband mode decomposition as a preprocessing method to reduce the effective channel delay spread and concentrate received signal energy in a small number of independent channels. The data were collected by a vertical array, which spans the water column from 12-m depth to the bottom in shallow water 80 m in depth. Binary-sequence data were used to phase-shift-keyed (PSK) modulate signals with different carrier frequencies. No error correction coding was used. The received signals were processed by a system that does not use training or pilot signals. Signals received both during periods of ordinary internal wave activity and during a period with unusually strong internal wave solitons were processed and analyzed. Different broadband mode-filtering methods were analyzed and tested. Broadband mode filtering decomposed the received signal into a number of independent signals with a reduced delay spread. The analysis of signals from the output of mode filters shows that even a simple demodulator can achieve a low bit error rate (BER) at a distance 19.2 km.This work was supported by the U.S. Office of Naval Research (ONR)