Estimating temperature and current using a pair of transceivers in a harbor environment

Obtaining the horizontal variation of temperature and current fields of a water column usually requires travel-time measurements of acoustic signals traveling along different paths between several horizontally distributed transceivers. This study explores the possibility of using a pair of transceivers deployed in a highly-reverberant harbor environment to extract spatial information of the water. Multipath acoustic propagation of two main arrival groups, i.e., direct arrivals and arrivals reflecting off the harbor side, was observed in the pulse responses measured in the harbor environment during the flood tide. Compared with the direct point measurements of temperature and current, the path-averaged measurements show a similar temporal variation during the experiment, demonstrating the possibility of estimating the spatial variation of the currents and temperatures using the multipath acoustic propagation.

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Sediment sound speed inversion with time-frequency analysis and modal arrival time probability density functions

The dispersion pattern of a received signal is critical for understanding physical properties of the propagation medium. The objective of this work is to estimate accurately sediment sound speed using modal arrival times obtained from dispersion curves extracted via time-frequency analysis of acoustic signals. A particle filter is used that estimates probability density functions of modal frequencies arriving at specific times. Employing this information, probability density functions of arrival times for modal frequencies are constructed. Samples of arrival time differences are then obtained and are propagated backwards through an inverse acoustic model. As a result, probability density functions of sediment sound speed are estimated. Maximum a posteriori estimates indicate that inversion is successful. It is also demonstrated that multiple frequency processing offers an advantage over inversion at a single frequency, producing results with reduced variance.

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Passive fathometer reflector identification with phase shift modeling

In passive fathometer processing, the presence of wavelets in the estimate of the medium's Green's function corresponds to the location of reflectors in the seabed; amplitudes are related to seabed properties. Bayesian methods have been successful in identifying reflectors that define layer interfaces. Further work, however, revealed that phase shifts are occasionally present in the wavelets and hinder accurate layer identification for some reflectors. With a Gibbs sampler that computes probability densities of reflector depths, strengths of the reflections, and wavelet phase shifts, the significance of phase shift modeling in successful estimation of reflectors and their strengths is demonstrated.

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Hear Nature

We recently marveled at an audacious attempt to create sound maps from photographs. Let us go to the other end of the spectrum and examine an attempt by professional audio geeks to capture the sounds of our world.

The Nature Sound Map is a collaboration by a group of professional sound recordists, sound designers, and other audio professionals. They combine satellite imagery with professional grade recordings to bring us an interactive map of the natural sounds of our wonderful planet.

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Memory cost of absorbing conditions for the finite-difference time-domain method

Three absorbing layers are investigated using standard rectilinear finite-difference schemes. The perfectly matched layer (PML) is compared with basic lossy layers terminated by two types of absorbing boundary conditions, all simulated using equivalent memory consumption. Lossy layers present the advantage of being scalar schemes, whereas the PML relies on a staggered scheme where both velocity and pressure are split. Although the PML gives the lowest reflection magnitudes over all frequencies and incidence angles, the most efficient lossy layer gives reflection magnitudes of the same order as the PML from mid- to high-frequency and for restricted incidence angles.

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