Duda Timothy F.

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Timothy F.

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  • Article
    Effects of pacific summer water layer variations and ice cover on Beaufort Sea underwater sound ducting
    (Acoustical Society of America, 2021-04-01) Duda, Timothy F. ; Zhang, Weifeng G. ; Lin, Ying-Tsong
    A one-year fixed-path observation of seasonally varying subsurface ducted sound propagation in the Beaufort Sea is presented. The ducted and surface-interacting sounds have different time behaviors. To understand this, a surface-forced computational model of the Chukchi and Beaufort Seas with ice cover is used to simulate local conditions, which are then used to computationally simulate sound propagation. A sea ice module is employed to grow/melt ice and to transfer heat and momentum through the ice. The model produces a time- and space-variable duct as observed, with Pacific Winter Water (PWW) beneath a layer of Pacific Summer Water (PSW) and above warm Atlantic water. In the model, PSW moves northward from the Alaskan coastal area in late summer to strengthen the sound duct, and then mean PSW temperature decreases during winter and spring, reducing the duct effectiveness, one cause of a duct annual cycle. Spatially, the modeled PSW is strained and filamentary, with horizontally structured temperature. Sound simulations (order 200 Hz) suggest that ducting is interrupted by the intermittency of the PSW (duct gaps), with gaps enabling loss from ice cover (set constant in the sound model). The gaps and ducted sound show seasonal tendencies but also exhibit random process behavior.
  • Article
    Predicted scattering of sound by diffuse hydrothermal vent plumes at mid-ocean ridges
    (Acoustical Society of America, 1998-01) Duda, Timothy F. ; Trivett, D. Andrew
    Amplitude and phase fluctuations of monochromatic acoustic signals traveling through diffuse mid-ocean ridge hydrothermal vent plumes are modeled using existing theory in an attempt to find suitable frequencies and path lengths for plume monitoring. Weak-scattering solutions are evaluated numerically, with model parameters adjusted to match observed plume characteristics. Constraints required for weak-scattering solutions to be valid can be met for transmission ranges of 500–2000 m and frequencies of 20–80 kHz. Therefore, because fluid structure and scattering strength are more closely linked for weak scattering than for stronger scattering, inversion for fluid statistical properties may be possible, enabling diffuse vent monitoring. Such monitoring would be subject to geometric assumptions such as transmission entirely within a statistically homogeneous plume. Performance-limiting phase fluctuations have also been computed for a 13–17 kHz geodetic survey system.
  • Article
    A modeling study of acoustic propagation through moving shallow-water solitary wave packets
    (IEEE, 1999-01) Duda, Timothy F. ; Preisig, James C.
    Propagation of 400-Hz sound through continental-shelf internal solitary wave packets is shown by numerical simulation to be strongly influenced by coupling of normal modes. Coupling in a packet is controlled by the mode coefficients at the point where sound enters the packet, the dimensions of the waves and packet, and the ambient depth structures of temperature and salinity. In the case of a moving packet, changes of phases of the incident modes with respect to each other dominate over the other factors, altering the coupling over time and thus inducing signal fluctuations. The phasing within a moving packet varies with time scales of minutes, causing coupling and signal fluctuations with comparable time scales. The directionality of energy flux between high-order acoustic modes and (less attenuated) low-order modes determines a gain factor for long-range propagation. A significant finding is that energy flux toward low-order modes through the effect of a packet near a source favoring high-order modes will give net amplification at distant ranges. Conversely, a packet far from a source sends energy into otherwise quiet higher modes. The intermittency of the coupling and of high-mode attenuation via bottom interaction means that signal energy fluctuations and modal diversity fluctuations at a distant receiver are complementary, with energy fluctuations suggesting a source-region packet and mode fluctuations suggesting a receiver-region packet. Simulations entailing 33-km propagation are used in the analyses, imitating the SWARM experiment geometry, allowing comparison with observations
  • Article
    Acoustic signal and noise changes in the Beaufort Sea Pacific Water duct under anticipated future acidification of Arctic Ocean waters
    (Acoustical Society of America, 2017-10-09) Duda, Timothy F.
    It is predicted that Arctic Ocean acidity will increase during the next century as a result of carbon dioxide accumulation in the atmosphere and migration into ocean waters. This change has implications for sound transmission because low-pH seawater absorbs less sound than high-pH water. Altered pH will affect sound in the 0.3−10 kHz range if the criterion is met that absorption is the primary cause of attenuation, rather than the alternatives of loss in the ice or seabed. Recent work has exploited sound that meets the criterion, sound trapped in a Beaufort Sea duct composed of Pacific Winter Water underlying Pacific Summer Water. Arctic pH is expected to drop from 8.1 to 7.9 (approximately) over the next 30−50 yr, and effects of this chemical alteration on the intensity levels of this ducted sound, and on noise, are examined here. Sound near 900 Hz is predicted to undergo the greatest change, traveling up to 38% further. At ranges of 100−300 km, sound levels from a source in the duct may increase by 7 dB or more. Noise would also increase, but noise is ducted less efficiently, with the result that 1 kHz noise is predicted to rise approximately 0.5 dB.
  • Article
    Acoustic intensity fluctuations induced by South China Sea internal tides and solitons
    (IEEE, 2004-10) Chiu, Ching-Sang ; Ramp, Steven R. ; Miller, Christopher W. ; Lynch, James F. ; Duda, Timothy F. ; Tang, Tswen Yung
    Between late April and May 23, 2001, a suite of acoustic and oceanographic sensors was deployed by a team of U.S., Taiwan, and Singapore scientists in the northeastern South China Sea to study the effects of ocean variability on low-frequency sound propagation in a shelfbreak environment. The primary acoustic receiver was an L-shaped hydrophone array moored on the continental shelf that monitored a variety of signals transmitted along and across the shelfbreak by moored sources. This paper discusses and contrasts the fluctuations in the 400-Hz signals transmitted across the shelfbreak and measured by the vertical segment of the listening array on two different days, one with the passage of several huge solitons that depressed the shallow isotherms to near the sea bottom and one with a much less energetic internal wavefield. In addition to exhibiting large and rapid temporal changes, the acoustic data show a much more vertically diffused sound intensity field as the huge solitons occupied and passed through the transmission path. Using a space-time continuous empirical sound-speed model based on the moored temperature records, the observed acoustic intensity fluctuations are explained using coupled-mode physics.
  • Article
    Are any coastal internal tides predictable?
    (The Oceanography Society, 2012-06) Nash, Jonathan D. ; Shroyer, Emily L. ; Kelly, Samuel M. ; Inall, Mark E. ; Duda, Timothy F. ; Levine, Murray D. ; Jones, Nicole L. ; Musgrave, Ruth C.
    Surface tides are the heartbeat of the ocean. Because they are controlled by Earth's motion relative to other astronomical objects in our solar system, surface tides act like clockwork and generate highly deterministic ebb and flow familiar to all mariners. In contrast, baroclinic motions at tidal frequencies are much more stochastic, owing to complexities in how these internal motions are generated and propagate. Here, we present analysis of current records from continental margins worldwide to illustrate that coastal internal tides are largely unpredictable. This conclusion has numerous implications for coastal processes, as across-shelf exchange and vertical mixing are, in many cases, strongly influenced by the internal wave field.
  • Article
    Piecewise coherent mode processing of acoustic data recorded on two horizontally separated vertical line arrays
    (Acoustical Society of America, 2012-05-25) Udovydchenkov, Ilya A. ; Brown, Michael G. ; Duda, Timothy F.
    Motivated by measurements made in the 2004 Long-Range Ocean Acoustic Propagation Experiment (LOAPEX), the problem of mode processing transient acoustic signals collected on two nearby vertical line arrays is considered. The first three moments (centroid, variance, and skewness) of broadband distributions of acoustic energy with fixed mode number (referred to as modal group arrivals) are estimated. It is shown that despite the absence of signal coherence between the two arrays and poor high mode number energy resolution, the centroid and variance of these distributions can be estimated with tolerable errors using piecewise coherent mode processing as described in this paper.
  • Working Paper
    Theory and observation of anisotropic and episodic internal wave effects on 100-400 Hz sound
    (Institute of Applied and Computational Mathematics, 2011-06) Duda, Timothy F.
    Propagation of sound through shallow-water internal waves of various types is discussed. The anisotropy of the waves imparts an anisotropy to their effects on sound. The internal waves are of two types: Long-wavelength internal tides and short-wavelength high-frequency waves. On the continental shelf both types of waves tend to move shoreward from deep water (i.e. have anisotropic motion and anisotropic correlation scales). The internal tides are less predictable than the surface tides that generate them. The short-wavelength nonlinear internal waves are also somewhat unpredictable, and also have anisotropic correlation scales, having crests of tens of kilometres in length but wavelengths of order 300 to 1000 m. Coupled-mode propagation dominates across-shelf sound propagation, which in the direction of short internal wave correlation scale. Refracted-mode propagation dominates along-shelf propagation. Data from two sea exercises illustrate the character of the waves and their effects on sound.
  • Technical Report
    Preliminary acoustic and oceanographic observations from the ASIAEX 2001 South China Sea Experiment
    (Woods Hole Oceanographic Institution, 2001-09) Newhall, Arthur E. ; Costello, Lawrence ; Duda, Timothy F. ; Dunn, James M. ; Gawarkiewicz, Glen G. ; Irish, James D. ; Kemp, John N. ; McPhee, Neil M. ; Liberatore, Stephen P. ; Lynch, James F. ; Ostrom, William M. ; Schroeder, Ted ; Trask, Richard P. ; von der Heydt, Keith
    The Asian Seas International Experiment (ASIAEX) was a very successful scientific collaboration between the United States of America (USA), the People’s Republic of China (PRC), Taiwan (ROC), the Republic of Korea (ROK), Japan, Russia, and Singapore. Preliminary field experiments associated with ASIAEX began in spring of 2000. The main experiments were performed in April-August, 2001. The scientific plan called for two major acoustics experiments, the first a bottom interaction experiment in the East China Sea (ECS) and the second a volume interaction experiment in the South China Sea (SCS). In addition to the acoustics efforts, there were also extremely strong physical oceanography and geology and geophysics components to the experiments. This report will concentrate on describing the moored component of the South China Sea portion of ASIAEX 2001 performed from the Taiwan Fisheries research vessel FR1 (Fisheries Researcher 1). Information on the environmental moorings deployed from the Taiwanese oceanographic research vessel OR1 (Oceanographic Researcher 1) will also be listed here for completeness, so that the reader can pursue later analyses of the data. This report does not pursue any data analyses per se.
  • Preprint
    Validation of genetic algorithm-based optimal sampling for ocean data assimilation
    ( 2016-08) Heaney, Kevin D. ; Lermusiaux, Pierre F. J. ; Duda, Timothy F. ; Haley, Patrick J.
    Regional ocean models are capable of forecasting conditions for usefully long intervals of time (days) provided that initial and ongoing conditions can be measured. In resource-limited circumstances, the placement of sensors in optimal locations is essential. Here, a nonlinear optimization approach to determine optimal adaptive sampling that uses the Genetic Algorithm (GA) method is presented. The method determines sampling strategies that minimize a user-defined physics-based cost function. The method is evaluated using identical twin experiments, comparing hindcasts from an ensemble of simulations that assimilate data selected using the GA adaptive sampling and other methods. For skill metrics, we employ the reduction of the ensemble root-mean-square-error (RMSE) between the “true” data-assimilative ocean simulation and the different ensembles of data-assimilative hindcasts. A 5-glider optimal sampling study is set up for a 400 km x 400 km domain in the Middle Atlantic Bight region, along the New Jersey shelf-break. Results are compared for several ocean and atmospheric forcing conditions.
  • Article
    Modal analysis of the range evolution of broadband wavefields in the North Pacific Ocean : low mode numbers
    (Acoustical Society of America, 2012-06) Udovydchenkov, Ilya A. ; Brown, Michael G. ; Duda, Timothy F. ; Mercer, James A. ; Andrew, Rex K. ; Worcester, Peter F. ; Dzieciuch, Matthew A. ; Howe, Bruce M. ; Colosi, John A.
    The results of mode-processing measurements of broadband acoustic wavefields made in the fall of 2004 as part of the Long-Range Ocean Acoustic Propagation Experiment (LOAPEX) in the eastern North Pacific Ocean are reported here. Transient wavefields in the 50–90 Hz band that were recorded on a 1400 -m long 40 element vertical array centered near the sound channel axis are analyzed. This array was designed to resolve low-order modes. The wavefields were excited by a ship-suspended source at seven ranges, between approximately 50 and 3200 km, from the receiving array. The range evolution of broadband modal arrival patterns corresponding to fixed mode numbers (“modal group arrivals”) is analyzed with an emphasis on the second (variance) and third (skewness) moments. A theory of modal group time spreads is described, emphasizing complexities associated with energy scattering among low-order modes. The temporal structure of measured modal group arrivals is compared to theoretical predictions and numerical simulations. Theory, simulations, and observations generally agree. In cases where disagreement is observed, the reasons for the disagreement are discussed in terms of the underlying physical processes and data limitations.
  • Article
    Revisiting experimental methods for studies of acidity-dependent ocean sound absorption
    (Acoustical Society of America, 2009-04) Duda, Timothy F.
    The practical usefulness of long-range acoustic measurements of ocean acidity-linked sound absorption is analyzed. There are two applications: Determining spatially-averaged pH via absorption measurement and verifying absorption effects in an area of known pH. The method is a differential-attenuation technique, with the difference taken across frequency. Measurement performance versus mean frequency and range is examined. It is found that frequencies below 500 Hz are optimal. These are lower than the frequency where the measurement would be most sensitive in the absence of noise and signal fluctuation (scintillation). However, attenuation serves to reduce signal-to-noise ratio with increasing distance and frequency, improving performance potential at lower frequencies. Use of low frequency allows longer paths to be used, with potentially better spatial averaging. Averaging intervals required for detection of fluctuations or trends with the required precision are computed.
  • Article
    Acoustic ducting, reflection, refraction, and dispersion by curved nonlinear internal waves in shallow water
    (IEEE Oceanic Engineering Society, 2010-02-08) Lynch, James F. ; Lin, Ying-Tsong ; Duda, Timothy F. ; Newhall, Arthur E.
    Nonlinear internal waves in shallow water have been shown to be effective ducts of acoustic energy, through theory, numerical modeling, and experiment. To date, most work on such ducting has concentrated on rectilinear internal wave ducts or those with very slight curvature. In this paper, we examine the acoustic effects of significant curvature of these internal waves. (By significant curvature, we mean lateral deviation of the internal wave duct by more than half the spacing between internal waves over an acoustic path, giving a transition from ducting to antiducting.) We develop basic analytical models of these effects, employ fully 3-D numerical models of sound propagation and scattering, and examine simultaneous acoustical and oceanographic data from the 2006 Shallow Water Experiment (SW06). It will be seen that the effects of curvature should be evident in the mode amplitudes and arrival angles, and that observations are consistent with curvature, though with some possible ambiguity with other scattering mechanisms.
  • Technical Report
    Initial results from a cartesian three-dimensional parabolic equation acoustical propagation code
    (Woods Hole Oceanographic Institution, 2006-12) Duda, Timothy F.
    A three-dimensional (3D) parabolic equation acoustical propagation code has been developed and run successfully. The code is written in the MATLAB language and runs in the MATLAB environment. The code has been implemented in two versions, applied to (1) Horizontal low-frequency (100 to 500 Hz) propagation through the shallow water waveguide environment; (2) Vertical high-frequency propagation (6 to 15 kHz) to study normal-incidence reflection from the lower side of the ocean surface. The first edition of the code reported on here does not implement refinements that are often found in 2D propagation models, such as allowing density to vary, optimally smoothing soundspeed discontinuities at the water/seabed interface, and allowing an omni-directional source. The code is part of a development effort to test the applicability of 2D (and N by 2D) models, which have more refinements than this model, to the study of fully 3D propagation problems, such as sound transiting steep nonlinear coastal-area internal waves and/or sloping terrain, and to provide a numerical tool when the full 3D solution is needed.
  • Article
    Experimental and numerical studies of sound propagation over a submarine canyon northeast of Taiwan
    (IEEE, 2015-01-09) Lin, Ying-Tsong ; Duda, Timothy F. ; Emerson, Chris ; Gawarkiewicz, Glen G. ; Newhall, Arthur E. ; Calder, Brian ; Lynch, James F. ; Abbot, Philip A. ; Yang, Yiing-Jang ; Jan, Sen
    A study of sound propagation over a submarine canyon northeast of Taiwan was made using mobile acoustic sources during a joint ocean acoustic and physical oceanographic experiment in 2009. The acoustic signal levels (equivalently, transmission losses) are reported here, and numerical models of 3-D sound propagation are employed to explain the underlying physics. The data show a significant decrease in sound intensity as the source crossed over the canyon, and the numerical model provides a physical insight into this effect. In addition, the model also suggests that reflection from the canyon seabed causes 3-D sound focusing when the direction of propagation is along the canyon axis, which remains to be validated in a future experiment. Environmental uncertainties of water sound speed, bottom geoacoustic properties, and bathymetry are addressed, and the implications for sound propagation prediction in a complex submarine canyon environment are also discussed.
  • Article
    Modeling weak fluctuations of undersea telemetry signals
    (IEEE, 1991-01) Duda, Timothy F.
    Numerical calculation of acoustic field perturbation expressions can be used to predict fluctuations after propagation through ocean sound-speed structures, but before the onset of multipath. The general form of the expressions for signal spectra or correlation functions allow numerical evaluation for an unlimited quantity of vector wave-number spectral models of refractive index. In order to help define the bounds of applicability of the theory, log-intensity fluctuation variances have been calculated for three major situations: ocean internal waves, ocean turbulence, and continuous strong large-scale turbulence. Propagation through ocean thermocline internal waves, realistically weak thermocline turbulence, and unrealistically strong turbulence show that scintillations of intensity can be predicted and understood to first order up to ranges of tens of kilometers, given the proper transmission geometry. Internal wave effects dominate over any effects from expected microstructure. Nonhorizontal transmission yields small fluctuations, but eventually refractive effects of the sound channel will contribute some additional spatial variability and multipath, complicating the use of the theory. Multipath due to the sound channel can exist at ranges where the random small-scale structures would contribute only small perturbations (no multipath from small structures)
  • Article
    Observed limiting cases of horizontal field coherence and array performance in a time-varying internal wavefield
    (Acoustical Society of America, 2008-08-28) Collis, Jon M. ; Duda, Timothy F. ; Lynch, James F. ; DeFerrari, Harry A.
    Using a moored source and horizontal/vertical line array combination, horizontal coherence properties of high signal to noise ratio (>=20 dB) 100–1600 Hz signals have been measured. Internal waves in the area of the measurement created moving episodic sound-speed anomaly structures, influencing coherence length. Measured horizontal coherence scales for 100 Hz ranged from 5 to 20 acoustic wavelengths, and were inversely related to the sound-speed anomaly strength. Horizontal field properties were compared with fields computed using modal decompositions of the vertical signals. The comparison allows azimuthal field coherence properties to be studied apart from normal-mode interference effects.
  • Article
    Comparison of shear measurements and mixing predictions with a direct observation of diapycnal mixing in the Atlantic thermocline
    (American Geophysical Union, 1995-07-15) Duda, Timothy F. ; Jacobs, David C.
    Four sets of velocity and density profiles have been measured with an autonomous profiler during an upper ocean intentional‐tracer (SF6) diapycnal diffusivity measurement, the North Atlantic Tracer Release Experiment (NATRE). The tracer was injected near 310 m depth in the Canary Basin. Two profile sets were collected 6 months after tracer release, and two were collected 1 year after release, all within the horizontal boundaries of the SF6 patch. Shear and strain can be combined with turbulent kinetic energy dissipation and diffusivity measurements (published elsewhere) to test existing expressions for dissipation and diffusivity due to shear‐induced turbulence. These expressions arise from internal‐wave decay modeling. One expression of dissipation parameterized in terms of shear, based on stochastic nonlinear internal‐wave interaction, has fared well empirically; its extension to estimate diffusivity is evaluated. Shear variance of the first two data sets was about 1.6 times GM76, and 2.5 to 3.0 times GM76 in the later sets. The average parameterized mixing estimate computed using all of the temporally limited shear measurements overestimates annual mean NATRE diffusivity, 1.5 × 10−5 m2 s−1, by a factor of 1.2. A modified parameterization gives an underestimate. To first order, this supports the present understanding of open‐ocean diffusivity in terms of fine‐scale shear and internal‐wave decay, that is, the slow diapycnal mixing was not a consequence of unusually low shear. Adjustment of the shear‐induced mixing models to better fit the data is not warranted because of the lack of direct comparability between the various measurements, the expected natural variability of the shear, and sampling errors.
  • Article
    Modeling and analysis of internal-tide generation and beamlike onshore propagation in the vicinity of shelfbreak canyons
    (American Meteorological Society, 2014-03) Zhang, Weifeng G. ; Duda, Timothy F. ; Udovydchenkov, Ilya A.
    A hydrostatic numerical model with alongshore-uniform barotropic M2 tidal boundary forcing and idealized shelfbreak canyon bathymetries is used to study internal-tide generation and onshore propagation. A control simulation with Mid-Atlantic Bight representative bathymetry is supported by other simulations that serve to identify specific processes. The canyons and adjacent slopes are transcritical in steepness with respect to M2 internal wave characteristics. Although the various canyons are symmetrical in structure, barotropic-to-baroclinic energy conversion rates Cυ are typically asymmetrical within them. The resulting onshore-propagating internal waves are the strongest along beams in the horizontal plane, with the stronger beam in the control simulation lying on the side with higher Cυ. Analysis of the simulation results suggests that the cross-canyon asymmetrical Cυ distributions are caused by multiple-scattering effects on one canyon side slope, because the phase variation in the spatially distributed internal-tide sources, governed by variations in the orientation of the bathymetry gradient vector, allows resonant internal-tide generation. A less complex, semianalytical, modal internal wave propagation model with sources placed along the critical-slope locus (where the M2 internal wave characteristic is tangent to the seabed) and variable source phasing is used to diagnose the physics of the horizontal beams of onshore internal wave radiation. Model analysis explains how the cross-canyon phase and amplitude variations in the locally generated internal tides affect parameters of the internal-tide beams. Under the assumption that strong internal tides on continental shelves evolve to include nonlinear wave trains, the asymmetrical internal-tide generation and beam radiation effects may lead to nonlinear internal waves and enhanced mixing occurring preferentially on one side of shelfbreak canyons, in the absence of other influencing factors.
  • Article
    Weakly dispersive modal pulse propagation in the North Pacific Ocean
    (Acoustical Society of America, 2013-10) Udovydchenkov, Ilya A. ; Brown, Michael G. ; Duda, Timothy F. ; Worcester, Peter F. ; Dzieciuch, Matthew A. ; Mercer, James A. ; Andrew, Rex K. ; Howe, Bruce M. ; Colosi, John A.
    The propagation of weakly dispersive modal pulses is investigated using data collected during the 2004 long-range ocean acoustic propagation experiment (LOAPEX). Weakly dispersive modal pulses are characterized by weak dispersion- and scattering-induced pulse broadening; such modal pulses experience minimal propagation-induced distortion and are thus well suited to communications applications. In the LOAPEX environment modes 1, 2, and 3 are approximately weakly dispersive. Using LOAPEX observations it is shown that, by extracting the energy carried by a weakly dispersive modal pulse, a transmitted communications signal can be recovered without performing channel equalization at ranges as long as 500 km; at that range a majority of mode 1 receptions have bit error rates (BERs) less than 10%, and 6.5% of mode 1 receptions have no errors. BERs are estimated for low order modes and compared with measurements of signal-to-noise ratio (SNR) and modal pulse spread. Generally, it is observed that larger modal pulse spread and lower SNR result in larger BERs.