Selected Topics of the Past Thirty Years in Ocean Acoustics

This paper reviews some of the highlights of selected topics in ocean acoustics during the thirty years that have passed since the founding of the Journal of Theoretical and Computational Acoustics. Advances in computational methods and computers helped to make computational ocean acoustics a vibrant area of research during that period. The parabolic equation method provides an unrivaled combination of accuracy and efficiency for propagation problems in which the bathymetry, sound speed, and other environmental parameters vary in the horizontal directions. The extension of this approach to cases involving layers that support shear waves has been an active area of research throughout the thirty year period. Interest in basin-scale and global-scale propagation was stimulated by the Heard Island Feasibility Test for monitoring climate change in terms of changes in travel time that occur as the temperature of the ocean rises. Diminishing ice cover in the Arctic, which is one of the consequences of climate change, has stimulated renewed interest in Arctic acoustics during the past decade. Reverberation is a challenging problem that was the topic of a major research program during the beginning of the thirty year period. An innovative approach for making it feasible to solve such problems was applied to data for reverberation from the seafloor and from schools of fish, and some of the findings were featured in Science and Nature. Source localization is one of the core problems in ocean acoustics. When applied on a 2-D array of receivers, an approach based on the eigenvectors of the covariance matrix is capable of separating the signals from different sources from each other, determining when this partitioning step is successful, and tracking sources that cross each other in bearing; one of the advantages of this approach is that it does not require environmental information or solutions of the wave equation. Geoacoustic inversion for estimating the layer structure, wave speeds, density, and other parameters of ocean bottoms has also been a topic of interest throughout the thirty year period

Hope, Belonging, and Catharsis: Critical Urban Pedagogies in Istanbul

Co-written by bachelor students and their lecturer, this commentary is a critical reflection on the Materiality and Urban Politics (SOC387) course taught at Boğaziçi University in Istanbul in the summer of 2022. The course unfolded during a time of political unrest at Boğaziçi following the appointment of a new president, which brought the campus under a state of police siege. In this context, SOC387 explored relations between the material and the urban/political through democratic and inclusive pedagogical approaches. Bringing together reflections on the sociopolitical context in which the course took place, classroom pedagogies, and students’ commentaries, we reflect on how the course helped participants redefine their sense of belonging to, and engagement with, Istanbul’s urban/political environment during a time of perceived disempowerment and “crisis of democracy” in Turkey. By exploring the productive tensions between urban space, politics, and democratic pedagogy, this commentary argues that teaching and learning in and about the city can be cathartic in reinforcing participants’ will to act on and contribute to urban politics.&nbsp

Estimate of the bottom compressional wave speed profile in the northeastern South China Sea using "Sources of Opportunity"

Author Posting. © IEEE, 2004. 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 29 (2004): 1231-1248, doi:10.1109/JOE.2004.834681.The inversion of a broad-band "source of opportunity" signal for bottom geoacoustic parameters in the northeastern South China Sea (SCS) is presented, which supplements the towed source and chirp sonar bottom inversions that were performed as part of the Asian Seas International Acoustics Experiment (ASIAEX). This source of opportunity was most likely a "dynamite fishing" signal, which has sufficient low-frequency content (5-500 Hz) to make it complimentary to the somewhat higher frequency J-15-3 towed source (50-260 Hz) signals and the much higher frequency (1-10 kHz) chirp signals. This low frequency content will penetrate deeper into the bottom, thus extending the other inverse results. Localization of the source is discussed, using both a horizontal array for azimuthal steering and the "water wave" part of the pulse arrival for distance estimation. A linear broad-band inverse is performed, and three new variants of the broad-band inverse, based on: 1) the Airy phase; 2) the cutoff frequency; and 3) a range-dependent medium are presented. A multilayer model of the bottom compressional wave speed is obtained, and error estimates for this model are shown, both for the range-independent approximation to the waveguide and for the range-dependent waveguide. Directions for future research are discussed.This work was supported by the Office of Naval Research under Grant N0 001 498-1-0413, Grant N00014-00-0931, and Grant N00014-01-0772 and by the National Science Council, Taiwan, R.O.C. under Grant NSC92-2611-E-002-005-CCS

Temporal and spatial dependence of a yearlong record of sound propagation from the Canada Basin to the Chukchi Shelf

Author Posting. © Acoustical Society of America, 2020. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 148(3),(2020): 1663, doi:10.1121/10.0001970.The Pacific Arctic Region has experienced decadal changes in atmospheric conditions, seasonal sea-ice coverage, and thermohaline structure that have consequences for underwater sound propagation. To better understand Arctic acoustics, a set of experiments known as the deep-water Canada Basin acoustic propagation experiment and the shallow-water Canada Basin acoustic propagation experiment was conducted in the Canada Basin and on the Chukchi Shelf from summer 2016 to summer 2017. During the experiments, low-frequency signals from five tomographic sources located in the deep basin were recorded by an array of hydrophones located on the shelf. Over the course of the yearlong experiment, the surface conditions transitioned from completely open water to fully ice-covered. The propagation conditions in the deep basin were dominated by a subsurface duct; however, over the slope and shelf, the duct was seen to significantly weaken during the winter and spring. The combination of these surface and subsurface conditions led to changes in the received level of the sources that exceeded 60 dB and showed a distinct spacio-temporal dependence, which was correlated with the locations of the sources in the basin. This paper seeks to quantify the observed variability in the received signals through propagation modeling using spatially sparse environmental measurements.This work was supported by the Office of Naval Research Ocean Acoustics Program (ONR OA322) under Grant Nos. N00014-15-1-2144, N00014-15-1-2119, N00014-15-1-2017, N00014-15-1-2068, N00014-15-1-2110, N00014-19-1-2721, N00014-15-1-2898, N00014-15-1-2806, and N00014-18-1-2140. The basin moored environmental data were supported by the ONR Arctic and Global Prediction Program (ONR AG322) under Grant No. N00014-15-1-2782. Mooring and hydrographic data were collected and made available by the Beaufort Gyre Exploration Program based at the Woods Hole Oceanographic Institution (http://www.whoi.edu/beaufortgyre) in collaboration with researchers from Fisheries and Oceans Canada at the Institute of Ocean Sciences. The ITP data were collected and made available by the ITP Program (Krishfield et al., 2008; Toole et al., 2011) based at the Woods Hole Oceanographic Institution (http://www.whoi.edu/itp). We acknowledge the use of imagery from the Worldview Snapshots application (https://wvs.earthdata.nasa.gov), part of the Earth Observing System Data and Information System (EOSDIS).2021-03-2

Acoustic wave propagation in porous media; velocity, attenuation, permeability, and porosity inversion

Acoustic wave propagation in porous media is formulated according to the Biot theory. The theory is extended for causal wave propagation in a dissipative porous medium with arbitrary pore size distribution. Using Kramers-Kronig relations, it is demonstrated that previous Biot models with slightly inelastic frame assumption is not causal.A one-dimensional forward model is developed for the acoustic wave propagation in layered marine sediments. An iterative inverse method is also introduced to determine frequency dependent compressional wave velocity and attenuation in a layered medium. Approximate relations are proposed to determine porosity and permeability from the velocity and attenuation data.Low frequency (300 Hz-30 kHz) measurements of compressional wave velocity and attenuation are performed in saturated beach sand. Biot formulation of acoustic wave propagation and prediction of viscous losses for saturated sandy sediments are confirmed experimentally. Consequently, it is demonstrated that acoustic remote sensing of porosity and permeability is possible.</p

Acoustic wave propagation in porous media; velocity, attenuation, permeability, and porosity inversion

Acoustic wave propagation in porous media is formulated according to the Biot theory. The theory is extended for causal wave propagation in a dissipative porous medium with arbitrary pore size distribution. Using Kramers-Kronig relations, it is demonstrated that previous Biot models with slightly inelastic frame assumption is not causal.A one-dimensional forward model is developed for the acoustic wave propagation in layered marine sediments. An iterative inverse method is also introduced to determine frequency dependent compressional wave velocity and attenuation in a layered medium. Approximate relations are proposed to determine porosity and permeability from the velocity and attenuation data.Low frequency (300 Hz-30 kHz) measurements of compressional wave velocity and attenuation are performed in saturated beach sand. Biot formulation of acoustic wave propagation and prediction of viscous losses for saturated sandy sediments are confirmed experimentally. Consequently, it is demonstrated that acoustic remote sensing of porosity and permeability is possible