Sound Field In Shallow Water Research Articles

Abnormal interference of very low frequency sound field in shallow water

In a multi-channel seismic exploration experiment in the South Yellow Sea, a tuned air gun array with a shooting interval of 37.5m was used as broadband pulse source, and a horizontal line array with 336 elements is used to receive transmitted signals. The received single shot records in the range-time domain are transformed into an interference pattern in the range-frequency domain using Fourier transform. It is found that there is an abnormal interference pattern of the very low frequency (VLF) sound field in the stratified shallow water environment. It is a complex nonlinear structure, no longer regular striations in the range-frequency domain. The mechanism of this particular interference pattern is interpreted according to normal mode theory. In shallow water stratified leaky waveguide, VLF sound propagates as waterborne modes and bottom-trapped modes. These two kinds of normal mode interfere with each other and form the particular stable interference pattern. In the experimental environment, the group speeds for these two type normal modes change differently with frequency. And it causes slope of interference striations change from positive to negative with increase of frequency. The theoretical simulation results are in good agreement with the experimental data. It is shown that the structure of interference pattern is sensitive to bottom structure and geoacoustic parameters.

The frequency choice of continuous-wave signal in low frequency active detection

In active sonar, continuous-wave (CW) pulses are used to evaluate the velocity of a target because of its high-resolution of velocity. Unfortunately, due to multipath interface in shallow water, the received CW signals are fluctuating. This phenomenon degrades the performance of velocity measurement. The interference structure of low frequency sound field in shallow water is relatively stable and the intensity striations are regular in range-frequency graph, which makes it possible to reduce the influence of amplitude fluctuations by reasonable frequency design. In this paper, we obtain the separations of striations caused by different modes interfering. According to the separations, appropriate frequencies of multi-frequency CW pulses were selected to compensate the fluctuations. The simulation and experimental results show that this method can mitigate the influence of frequency selective fading in shallow water channel.

Spatial attenuation of different sound field components in a water layer and shallow-water sediments

The paper presents the results of an experimental study of spatial attenuation of low-frequency vector-scalar sound fields in shallow water. The experiments employed a towed pneumatic cannon and spatially separated four-component vector-scalar receiver modules. Narrowband analysis of received signals made it possible to estimate the attenuation coefficients of the first three modes in the frequency of range of 26–182 Hz and calculate the frequency dependences of the sound absorption coefficients in the upper part of bottom sediments. We analyze the experimental and calculated (using acoustic calibration of the waveguide) laws of the drop in sound pressure and orthogonal vector projections of the oscillation velocity. It is shown that the vertical projection of the oscillation velocity vector decreases significantly faster than the sound pressure field.

Effect of Source's Height on Air-to-Water Sound Transmission with a Small-Scale Rough Sea Surface

A self-consistent perturbation approach is presented for analyzing the effect of the airborne source's height on the air-to-water sound transmission in shallow water with a randomly rough sea surface. It is shown in early researches that, in shallow water with a smooth sea surface, the airborne source's height mostly affects the phase of the sound field and barely influences the amplitude. However, in shallow water with a rough sea surface, few researches about such a problem have been published. In this work, the sound fields in shallow water with a randomly rough sea surface induced by an airborne source at different heights are calculated by a self-consistent perturbation approach. The numerical simulation results show that the fluctuation of the scattered field decreases as the source's height increases, in contrast, the averaged energy of the total field is hardly influenced by the source's height in the statistical sense.

Phase fluctuations of focused low-frequency sound fields in shallow water

Numerical experiments are carried out to study the phase fluctuations of a focused low-frequency sound field on an oceanic shelf. The focusing of sound at a distance of several kilometers is simulated using the phase conjugation of sound waves. Perturbations of the medium are represented by high-frequency (>1 cph) background internal waves and by the wind waves on the ocean surface. It is shown that, for a focused sound field at frequencies of several hundreds of hertz, the phase fluctuations do not exceed π and can be measured against the background of acoustic noise typical of shallow-water regions of the ocean. The fluctuation magnitude can be reduced approximately by half through the optimal choice of the mode composition. In the presence of such fluctuations, it is possible to measure the relative variations of the length of a stationary acoustic path with an accuracy of 1 m or better at a wind speed no greater than 10 m/s and a typical intensity of background internal waves.

Rearrangement of the horizontal space-time structure of the sound field in shallow water in the presence of moving internal waves

The rearrangement of the space-time structure of the sound field in a shallow-water waveguide with a moving intense internal wave packet is considered. The analysis is performed in terms of an approach characterized by space-time horizontal rays and vertical modes. It is shown that, within the time the packet travels over the acoustic track (about an hour), considerable spatial and temporal fluctuations occur in the field intensity and interference structure.

Focusing of low-frequency sound fields in shallow water

A numerical experiment is carried out to study the focusing of a low-frequency (100–300 Hz) sound field in a shallow-water acoustic waveguide typical of an oceanic shelf. Focusing with the use of time reversal of broadband acoustic signals, which is called time reversal mirror (TRM) of waves, is considered along with focusing by phase conjugation (PC) of a monochromatic sound field. It is demonstrated that, in the case of focusing by the TRM method in the waveguide of interest, it is sufficient to have a single source-receiving element. The use of a vertical array improves the quality of focusing. The quality achieved in the latter case proves to be approximately the same as that achieved in the case of focusing by phase conjugation of a monochromatic field at a frequency identical to the carrier frequency of the broadband signals. It is also shown that, in a range-independent waveguide, intense surface waves considerably reduce the quality of focusing. This effect is most pronounced in the case of using phase conjugation.

Frequency shifts of the spatial interference structure of the sound field in shallow water

A fluctuation model of the frequency shifts of the spatial interference structure formed by the sound field in a randomly inhomogeneous oceanic waveguide is constructed. The relation between the random fields of the perturbation-caused variation of the waveguide’s dispersion characteristic and the frequency deviation of a local interference maximum is analyzed. The applicability of the results to the cases of the waveguide perturbation by the background internal waves and by the roughness of the bottom relief is considered.

Fluctuations of high-frequency sound field in shallow water in the presence of internal waves

The structure of space-time fluctuations of a high-frequency (several kHz) sound field in a shallow water waveguide is considered. The sound field fluctuations due to internal and surface waves show different temporal variability. The perturbation theory within the framework of ray description is applied in order to derive some analytical estimation of fluctuations of arrival time and vertical angle distribution of high frequency broadband acoustics signals. The parabolic equation is used for numerical modeling of space-time fluctuations and vertical sound field intensity distribution. The experimental measurements of ocean dynamics from recent acoustic experiments are used as input data in numerical simulation of sound field fluctuations. [Work was supported by ONR and CRDF].

Horizontal refraction and sound fluctuations caused by internal waves packets in shallow water

The subject of the paper is to consider the propagation of a low-frequency sound field in shallow water in the presence of solitonlike internal waves (SLIW). It is assumed that SLIW spread approximately across the stationary acoustic trace. The problem of sound propagation is considered within the framework of 3-D model of the shallow water. The theory of ‘‘horizontal rays and vertical modes’’ is applied to take into account influence of horizontal refraction due to SLIW. As shown in the paper SLIW can lead (periodically in time) to ‘‘focusing’’ and ‘‘defocusing’’ of the horizontal rays or, in other words, to the periodic appearance of ‘‘horizontal waveguide.’’ Estimations of the sound fluctuations due to SLIW demonstrate that the amplitude of fluctuations can reach up to 10–20 dB (between moments of the ‘‘focusing’’ and ‘‘defocusing’’). The next effect conditioned by internal waves is horizontal filtering of vertical modes, where we have separate focusing and defocusing rays, corresponding to different vertical modes at different points of horizontal waveguide. The results presented in the paper can be used as the basis of remote sensing of the internal waves packets in shallow water. [Work supported by RFBR, Grant 97-05-64878.]

Sound waves attenuation in shallow water with rough boundaries

Here the object is to estimate the effect of scattering by randomly rough seabed and surface waves on attenuation of low-frequency sound waves in shallow water. The experiment was pursued with CW signals (100 and 230 Hz) in Barents Sea. The transmission loss and interference pattern of a sound field were measured within 100 km of the acoustic sources. Mesoscale relief of the bottom with spatial resolution 10 m and wind parameters were recorded as well. The experimental results (the average sound intensity) were compared with theoretical ones calculated with and without taking into account seabed and surface roughness. The calculation was carried out on the base of modal theory of sound scattering by rough boundaries in a waveguide. It was demonstrated that the random roughness substantially affects the transmission loss for long distances. In this paper the method is suggested for bottom parameter estimation using the statistical attributes of sound field in shallow water with random rough boundaries as well. [The work was supported by Russian Foundation of Basic Research (Grant 96-02-17194).]

Spatial coherence in range-dependent shallow water environments

In this presentation, effects of small random fluctuations on the spatial coherence of the sound field in shallow water with multilayered sediments will be examined. These fluctuations occur within the water volume and the sediment layers as well as at the layer interfaces. Most previous work has dealt with either range-independent or weakly range-dependent environments with random variabilities. However, strong range dependence often occurs in sound-speed profiles and at layer interfaces. For example, the region of the ACT III experimental series in the Straits of Korea shows range variability arising from sloping bathymetry and a coastal front. Consequently, spatial coherence for range-dependent waveguides with small random fluctuations will be treated. Numerical simulations of the coherence function will be presented. Results will be compared with coherence lengths of the field extracted from ACT III data.

Spatial horizontal coherence of sound in shallow water

A theory for the problems of spatial horizontal coherence of a sound field in shallow water was developed by using the concept of horizontal rays and vertical modes, and by adiabatic approximation. Both analytical and numerical results were presented for transverse coherence of sound in a shallow sea under summer and winter conditions, and the results were compared with those of measurements. Analysis showed that the higher-ordered modes are more sensitive to the disturbance of boundaries, but the sound-speed disturbance has similar influence on all modes; and that the absorption of the bottom has a significant influence on the range-dependent properties of coherence of sound in winter conditions.

Ray calculations with beam displacement

Ray theory with beam displacement is used to calculate the sound field in shallow water as a function of range. Results for the Pekeris model are compared with normal mode calculations and agreement is good even when the water depth is only a few wavelengths. The inclusion of beam displacement leads to the formation of ray caustics. Caustics are important at all ranges and occur even in isovelocity layers. Eventual failure of the ray method at short ranges and very low frequencies is traced to difficulties with the approximate evaluation of integrals by the method of stationary phase.

Frequency‐dependent influence of the sea bottom on the near‐surface sound field in shallow water

The influence of the sea bottom on shallow‐water sound propagation can be especially pronounced for the sound field near the boundaries. Comparison of the near‐surface sound field with the mid‐depth field, for example, can serve as a useful diagnostic tool for understanding the frequency‐dependent influence of the sea bottom on sound propagation in shallow water. Broad‐band shallow‐water sound‐propagation data analyzed in one‐third octave bands are compared for a very shallow receiver (2 m) and a mid‐depth receiver (50 m). The data are also compared with results from modal calculations. The interplay between the behavior of the modal depth functions and modal attenuations, as frequency is varied, provides a convenient model for interpreting the results.

Fluctuation Statistics of Sound in Shallow Water

Many different stochastic measures, second-order in the pressure amplitude, can and have been made of sound fields in shallow water: variance of pure-tone amplitude and phase, spatial and frequency coherence intervals, impulse response, etc. The experimental literature is summarized, and a conceptually simple stochastic model is proposed based on the physical processes involved. The stochastic field is described as a superposition of locally plane waves, including a spatially variable “coherent” component and a statistically independent surface-scattered component, variable in space and time. Simple estimation formulas for stochastic measures are found in the canonical problem of constant sound speed. Comparisons of predictions with reported data show few contradictions. However, the experiments and their reports seldom provide a sufficient basis for a critical test of theory.

Asymptotic Solution for the Sound Field in Shallow Water with a Negative Velocity Gradient

A solution, valid at high frequencies, for the sound field in shallow water having a negative velocity gradient is obtained by normal-mode theory. Using this solution, relatively simple expressions for the decay of intensity over intermediate and long ranges can be obtained if it is assumed that propagation is basically governed by the mode with phase velocity equal to the surface sound velocity. The transmission loss as a function of range follows a three-halves-power law at intermediate ranges and a cylindrical spreading law at long ranges. In both regions, attenuation due to bottom losses is assumed to be present, and it is shown that the value of this attenuation is approximately the same as that derived by a ray-theory model. In addition, the results of this analysis are in accord with other semiempirical predictions for the transmission loss in shallow water.