Jeffrey Simmen

Signal variability in shallow-water sound channels

Coherence of broad-band acoustic waves for mid-to-high frequencies (0.6-18 kHz) is obtained for a very shallow-water (15-m-deep) waveguide over a wide band of environmental conditions and for a source-receiver range of 387 m. Temporal behavior is sampled at two different rates: one that resolves at fractions of a second over intermittent periods of 40 s and another that resolves at 10 min over periods of several days. Spatial behavior is sampled in the vertical by hydrophones with spacings of the order of meters. The effect of environmental variability on coherence, in particular, soundspeed fluctuations in the water column and wind-induced modulations of the air-sea interface, is noted as a function of acoustic frequency and ray path. Analysis of the acoustic fluctuations over short time scales more accurately resolves the temporal decorrelation of the received signal due to sea surface waves. The vertical sampling of the received signal permits an analysis of arrival-angle fluctuations. The dependence of coherence on the number of surface bounces is studied by comparing arrivals that have zero, one, two, and three surface bounces.

Research highlights from the Asian Seas International Acoustics Experiment in the South China Sea

The Asian Seas International Acoustics Experiment (ASIAEX) included two major field programs, one in the South China Sea (SCS) and the other in the East China Sea (ECS). This paper summarizes results from the work conducted during April and May 2000 and 2001 over the continental shelf and slope in the northeastern South China Sea, just east of Dongsha Island (Pratis Reef). The primary emphasis of the field program was on water-column variability and its impact on acoustic propagation loss. The reader is steered to the appropriate paper within this Special Issue when more information on a specific topic is desired.

Reverberation vertical coherence and sea-bottom geoacoustic inversion in shallow water

Optimal array-processing techniques in the ocean often require knowledge of the spatial coherence of the reverberation. A mathematical model is derived for the reverberation vertical coherence (RVC) in shallow water (SW). A method for analysis of RVC data is introduced. Measured reverberation cross-correlation coefficients as a function of time and frequency, obtained during the Asian Seas International Acoustic Experiment (ASIAEX) in the East China Sea, are reported. SW reverberation from a single shot provides a continuous spatial sampling of the surrounding sound field up to several tens of kilometers and holds valuable information on the geoacoustic properties of the sea floor over this distance. SW reverberation data can, therefore, be used as the basis for a quick and inexpensive method for geoacoustic inversion and has the obvious advantage that acquiring the data in situ requires only a single platform. This paper considers the use of the vertical coherence of the reverberation as the starting point for such an inversion. Sound speed and attenuation in the sea bottom at the ASIAEX site are obtained over a frequency range of 100-1500 Hz by finding values that provide the best match between the measured and predicted RVC.

Frequency dependence of broadband propagation in coastal regions

To study the frequency-dependent spatial and temporal variabilities of sound propagation in coastal regions two experiments were conducted by transmitting sound impulses in the form of M sequences centered from 0.6 to 22 kHz. The site of the first experiment was the Atlantic Generating Station (AGS) where the source–receiver range was 214 m. The site of the second experiment was Delaware Bay where the range was approximately 760 m. The mean water depth was 14–15 m in both locations. Oceanographic data and acoustic data were collected simultaneously in both experiments. It was found that the temporal coherence of the propagated broadband signal changes significantly with pulse center frequency, as well as varying with geographic location and time. Trends of increasing signal decorrelation (between consecutive pulses) with increasing center frequency, and increasing signal decorrelation for paths with increasing number of interface interactions, were observed. It was found that for lower center frequencies there is hardly any decorrelation in signal over several hours, while for signals with center frequencies only a few kHz higher there was substantial decorrelation over times as short as 10 to 20 min. No significant spatial decorrelation of the signal was observed over the hydrophone spacing of a few meters for these experiments.

Effects of sound on the marine environment (ESME): an underwater noise risk model

Effects of Sound on the Marine Environment (ESME) is a computer model of the effects of underwater sound on marine life. The modular design behind the ESME model is motivated by the sonar equation, with subcomponent models for characterization of the source, for modeling of sound transmission through a medium, and for receiver properties (e.g., hearing abilities, behavioral responses to sound, and receiver distribution and abundance). Each subcomponent of ESME is intended to capture the current state of understanding in the relevant scientific field and to be capable of being updated as the understanding in the field advances. ESME is envisioned to have three primary applications: 1) retrospective studies of historical data, 2) predictive modeling of anticipated outcomes from a given scenario of sound in a marine environment, and 3) prescriptive guidance for research investments and efforts that will likely have the greatest effect on increasing confidence in decisions about underwater sound use and its effects

Program fosters advances in shallow‐water acoustics in southeastern Asia

Two impressive field programs were recently completed in the South and East China Seas as part of the Asian Seas International Acoustics Experiment (ASIAEX). Under the direction of the U.S. Office of Naval Research (ONR), scientists from several countries joined together to collect unique and high-quality acoustical, oceanographic, and geophysical data at the same place and time, enabling underwater acoustic fluctuations to be understood and modeled at the space and time scales of interest. A comprehensive field effort such as this one is required to decipher the cause and effect between environmental and acoustic variability which exist on a wide range of time and space scales. Groundwork for the two largest experiments of ASIAEX was laid with preliminary survey and studies of spring in 2000. In 2000 in the East China Sea (ECS), a 21-day cruise on the R/V Revelle allowed extensive geophysical surveys of the upper sea floor using chirp sonars and a low-frequency air gun, as well as allowing some sampling of the physical oceanography. In the same year in the South China Sea (SCS), a SeaSoar survey aboard the R/V Ocean Researcher 1 delineated the major meso-scale features of the northeastern SCS, while a limited moored array obtained a first look at the dramatic non-linear internal waves. These pilot field studies proved indispensable when designing the two main field programs that were executed in spring 2001.

Nonlinear behavior of acoustic rays in underwater sound channels

Abstract The nonlinear behavior of acoustic rays in underwater sound channels is examined using a set of parabolic ray equations. Two sound speed profiles, a Munk canonical and a double-duct profile, are investigated. For range-independent but stratified ocean sound speed profiles, analytical results concerning the sound ray trajectory and wave length are obtained. The stability of the system is found to be marginal stable, which leads to the phenomenon of wave front folding. Next, a perturbed system attributed to a single-mode internal wave is examined. The stability, bifurcation and other nonlinear dynamic issues of the nearly integrable system are explored using a combination of phase plane trajectories, Poincare maps, bifurcation diagrams, Lyapunov exponents and Floquet multipliers.

Sea surface effects on reverberation vertical coherence and inverted bottom acoustic parameters in the East China Sea

Wideband reverberation measurements were repeatedly conducted at the center of the ASAEX site in the East China Sea, on 3 and 5 June, 2001. Between these two measurement days, wind speed and rms wave height changed significantly, going from about 1 m/s and 0.1 m to 10 m/s and 0.35 m, respectively. This paper will compare reverberation vertical coherence (RVC) and RVC‐inverted equivalent bottom acoustic parameters in frequency range of 100–1200 Hz for the two measurements [J. X. Zhou and X. Z. Zhang, J. Acoust. Soc. Am. 113, 2204 (2003)]. The difference of equivalent bottom reflection losses, obtained from two measurements, is well explained by the supporting sea surface data. The results show that an inversion of seabottom acoustic parameters from shallow‐water long‐range reverberation (or sound propagation) should take the surface condition into account, especially for higher sea states and higher frequencies. [Work supported by ONR and NNSF of China.]

Graduate studies in underwater acoustics at the University of Washington

The University of Washington through its Departments of Mechanical and Electrical Engineering (College of Engineering), Department of Earth and Space Sciences, and School of Oceanography (College of the Environment), and by way of its Applied Physics Laboratory, which links all four of these academic units, offers a diverse graduate education experience in underwater acoustics. A summary is provided of the research infrastructure, primarily made available through Applied Physics Laboratory, which allows for ocean going and arctic field opportunities, and course options offered through the four units that provide the multi-disciplinary background essential for graduate training in the field of underwater acoustics. Students in underwater acoustics can also mingle in or extend their interests into medical acoustics research. Degrees granted include both the M.S and Ph.D.

Fifteen Years of U.S.—China Cooperative Research in Underwater Acoustics: 1995–2010

The Office of Naval Research and the Chinese Academy of Sciences established and continue to encourage an unprecedented collaboration between the United States and China in the basic research field of ocean acoustics. This paper describes the progression of events in the scientific relationship. This cooperative research effort, primarily focused on shallow‐water acoustics, has varied over fifteen years. It has involved numerous researchers and institutions from the U.S., China, and other countries, and it has resulted in productive workshops, conferences, field experiments, research collaborations, and scientific publications.