Ji‐Xun Zhou

Resonant interaction of sound wave with internal solitons in the coastal zone

Naturally occurring internal solitary wave trains (solitons) have often been observed in the coastal zone, but no reported measurements of such solitary waves include low‐frequency long‐range sound propagation data. In this paper, the possibility that internal waves are responsible for the anomalous frequency response of shallow‐water sound propagation observed in the summer is investigated. The observed transmission loss is strongly time dependent, anisotropic and sometimes exhibits an abnormally large attenuation over some frequency range. The parabolic equation (PE) model is used to numerically simulate the effect of internal wave packets on low‐frequency sound propagation in shallow water when there is a strong thermocline. It is found that acoustic transmission loss is sensitive to the signal frequency and is a ‘‘resonancelike’’ function of the soliton wavelength and packet length. The strong interaction between acoustic waves and internal waves, together with the known characteristics of internal waves in the coastal zone, provides a plausible explanation for the observed anomalous sound propagation in the summer. By decomposing the acoustic field obtained from the PE code into normal modes, it is shown that the abnormally large transmission attenuation is caused by ‘‘acoustic mode‐coupling’’ loss due to the interaction with the internal waves. It is also shown that the ‘‘resonancelike’’ behavior of transmission loss predicted by the PE analysis is consistent with mode coupling theory. As an inverse problem, low‐frequency acoustic measurements could be a potential tool for remote‐sensing of internal wave activity in the coastal zone.

Geoacoustic parameters in a stratified sea bottom from shallow‐water acoustic propagation

Due to the difficulty of direct measurement, there is a need to develop inverse techniques for remote sensing bottom geoacoustic parameters in the lowan mode measurements are extended to extract acoustic attenuation and speed in a horizontally stratified bottom in shallow water as a function of frequency and depth. The computational and experimental results show that, for a limited frequency band, we can find an equivalent depth profile of sea‐bottom acoustic attenuation with a linear frequency dependence that simulates the effect of nonlinear frequency dependence (without depth structure) on some field characteristics, such as the attenuation rate of individual mode, the frequency response of long‐range sound propagation, and the amplitude ratio of mode 2 to mode 1. However, the resultant equivalent negative gradient for the sea‐bottom attenuation is too strong to be accepted in light of available data. The conclusion is that nonlinear frequency dependence of the acoustic attenuation in the upper sedimen...

Sea-bed acoustic parameters from dispersion analysis and transmission loss in the East China Sea

As a part of the Asian Seas International Acoustic Experiment (ASIAEX) in the East China Sea, sound propagation signals from wideband explosive sources were measured using a 32-element vertical line array. Measurements were made as a function of range in two perpendicular tracks. Sea-bed geoacoustic parameters based on a fluid half-space geoacoustic model (sound speed, density, and attenuation) are inverted from the sound propagation in the frequency range 100-500 Hz. The sea-bed sound speed and density were first derived from mode arrival time differences which were obtained using a spatial mode filtering technique. Sea-bed acoustic attenuation was subsequently estimated by comparing measured transmission loss with model results.

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.

Effect of frequency dependence of sea‐bottom attenuation on the optimum frequency for acoustic propagation in shallow water

The optimum frequency for acoustic propagation in shallow water is controlled by a number of physical effects and environmental parameters. This article concentrates on the effect of a nonlinear frequency dependence of the sea‐bottom attenuation on the optimum frequency. Experimental data on low‐frequency acoustic propagation in shallow water are presented, for which, over the frequency range where the optimum frequency should occur, no apparent optimum frequency is observed. If there is an optimum frequency for these sea areas, it is much lower than predicted by existing theories. It is demonstrated that the nonoccurrence, or lowering, of the optimum frequency can be easily explained if the sea bottom is assumed to have a nonlinear frequency dependence. Excellent agreement between theory and experiment is achieved for a site in the Yellow Sea for three different seasons using values for sediment attenuation (with nonlinear frequency dependence) and sediment sound speed which were measured independently a...

Anomalous sound propagation in shallow water due to internal wave solitons

At-sea experimental data and numerical simulation results are given to show that, acoustic normal-mode coupling induced by internal solitons could be an important loss mechanism for shallow water sound propagation.<<ETX>>

Seabottom Acoustic Parameters from Inversion of Yellow Sea Experimental Data

The direct measurement of seabed parameters over a wide frequency range for a large sea area is often very difficult and always costly and time consuming. Inversion techniques for obtaining seabed acoustic parameters are often an attractive alternative. In this paper, seabottom acoustic parameters obtained from inversion of experimental data from the Yellow Sea are reported. The experiments were conducted over many years by using explosive signals at three different flat-bottom sites with water depths of 28.5 m, 36.5 m and 76.2 m. Several different inversion techniques, all based on normal mode models, were employed. Depending on the site, geoacoustic models of varying complexity were required. Using these techniques, the sound speed ratio of seabottom to water for the three different Yellow Sea sites is found to be about 1.056 – 1.072. The seabottom attenuation in the Yellow Sea for all of the sites exhibits a strong nonlinear frequency dependence in the frequency range of 80–1500 Hz. The acoustic attenuation coefficient is found to be proportional to frequency to the 1.6 – 1.9 power. This result differs from the widely used Hamilton’s seabed geoacoustic model, which has a linear frequency dependence.

Low-frequency sound speed and attenuation in sandy seabottom from long-range broadband acoustic measurements

A joint China-U.S. underwater acoustics experiment was conducted in the Yellow Sea with a very flat bottom and a strong and sharp thermocline. Broadband explosive sources were deployed both above and below the thermocline along two radial lines up to 57.2 km and a quarter circle with a radius of 34 km. Two inversion schemes are used to obtain the seabottom sound speed. One is based on extracting normal mode depth functions from the cross-spectral density matrix. The other is based on the best match between the calculated and measured modal arrival times for different frequencies. The inverted seabottom sound speed is used as a constraint condition to extract the seabottom sound attenuation by three methods. The first method involves measuring the attenuation coefficients of normal modes. In the second method, the seabottom sound attenuation is estimated by minimizing the difference between the theoretical and measured modal amplitude ratios. The third method is based on finding the best match between the measured and modeled transmission losses (TLs). The resultant seabottom attenuation, averaged over three independent methods, can be expressed as alpha=(0.33+/-0.02)f(1.86+/-0.04)(dB/m kHz) over a frequency range of 80-1000 Hz.

Sea surface effect on shallow-water reverberation

Wideband reverberation measurements were made at a fixed location in the East China Sea on 3 and 5 June 2001 using the same measurement system. Sound-speed profiles were similar during both measurements. Wind speed (W) and rms surface-wave height (σ) changed from 2.74m∕s and 0.10m on 3 June to 7.45m∕s and 0.33m on 5 June. Thus, these measurements offer an opportunity to evaluate sea-surface effects on reverberation vertical coherence (RVC), RVC-inverted bottom acoustic parameters, and reverberation level (RL) in shallow water. The two sets of RVC and RL data, in a frequency range of 100–2500Hz, show differences that are the apparent effects of the surface roughness. With increasing sea state, the RVC increases and the RL decreases. The effective bottom losses, inverted from the RVC data, correspond to the variation of sea state. This additional loss gives a physical explanation of the characteristics of both the measured RVC and RL. These preliminary findings show the importance of surface effects in shal...

Shallow-water reverberation level: measurement technique and initial reference values

A quality database of reverberation is absolutely essential if one is to understand the shallow-water reverberation problem. However, to get wideband reverberation levels (RL) simultaneously for both short and long ranges at low- and mid-frequencies is a delicate task that can be subject to errors. This paper introduces a simple method to get RL for the Asian Sea International Acoustics Experiment in the East China Sea (ASIAEX01). Special attention is paid to the measurements of the RL at short- and mid-ranges. With this method, one does not need to accurately calibrate hydrophones and measurement systems, or to measure absolute source level (SL). It can avoid signal overflow and saturation problems caused by powerful sound sources. The RL (relative to SL) at 1 s (or at 2 s) after an explosive source is detonated is defined as the initial reference reverberation level (IRRL). The IRRLs from four sites with different sandy sediments and different water depths have been given as a function of frequency in the 150-2500 Hz range. A mathematical model gives a physical explanation of the measured IRRL data. The resultant RL and IRRL may offer some reference values for the design of reverberation measurements or numerical simulations of shallow-water reverberation and bottom scattering