Sound Propagation In Shallow Water Research Articles

Multipath of sound propagation in shallow water and underwater spread spectrum communication

Due to the influence of the sea bottom and the sound speed profile, the propagation of sound in shallow water appears to be a complicated multipath. The time spread and the amplitude fluctuation of different arriving paths cause strong fading and intersymbol interference (ISI). At the receiving point the multipath signal can be expressed as r(t)=A0s(t−τ0)+∑i=1N-1Ais(t−τi)+n(t), where the first term is arriving signal directly, the second term is arriving signals by different paths, the third term is noise. When the difference of time delays is over the duration of code chip, a strong ISI occurs. As we know, the pseudo-random signal has high resolution in both time and frequency domains. It is often used in spread spectrum communication. In this paper a diversity technique with spread spectrum is used for realizing coherent accumulation of the arriving paths. The results of the lake experiment and the sea experiment are shown in this paper. The efficiency of coherent accumulation is obvious.

Finite difference time domain analysis of bottom effect on sound propagation in shallow water.

Finite difference time domain analysis of bottom effect on sound propagation in shallow water.

Time variability of acoustic signals in a benign shallow-water area

SACLANTCEN conducted the ADVENT’99 experiment in the Strait of Sicily, Mediterranean, to assess the time variability of broadband acoustic signals in a benign shallow-water region. Broadband acoustic signals (200–3800 Hz) were transmitted for up to 18 h over fixed paths of 2, 5, and 10 km. Dense sampling of the environment was performed including a sound-speed section every hour along the acoustic tracks. Band-averaged transmission loss is stable with time, which agrees well with modeling results. The Bartlett processor was applied to correlate the acoustic data. The correlation time varies from several hours at low frequency and short range to a few minutes at high frequency and longer ranges. Range- and time-dependent propagation modeling of the acoustic signals (200–800 Hz) shows behavior similar to the data. Particularly, the correlation time decreases abruptly from several hours to less than 1 hour at a frequency around 700 Hz for the 10-km data. This effect of the time-varying waveguide on the acoustic signals is correctly predicted by the propagation model. Severe problems using the acoustic signals coherently persist above a few hundred Hz and at ranges beyond a few kilometers, although the experiment was conducted under favorable conditions. The experimental data and the simulations illustrate the complexity of sound propagation in shallow water.

Observation of high-frequency sound propagation in shallow water with bubbles due to storm and surf

An experiment was performed off the shore of Panama City, FL, to measure the spatial and temporal coherence of high-frequency signals that were transmitted between fixed towers. Transmission was along paths at mid depths in about 10 m of water. During the time of the experiment, there were two stormy days with breaking waves and nearby high surf. It was observed that pulse-to-pulse variations (over seconds) in travel times, over a range of frequencies, increased dramatically from those observed on quiet days. Average travel times also increased by about 3%. Dispersion was also observed. The distance between the source and the receiver towers was approximately 60 m. By assuming that bubbles were either generated by breaking waves and advected down ward and/or generated by surf and advected outward, these results are explained. Estimates of the average bubble density and bubble-density variations are made.

The predictibility of broadband sound propagation in shallow water and geoacoustic inversion

An experiment was performed in a shallow-water region in the Gulf of Mexico to collect data for testing the performance of geoacoustic inversion algorithms using acoustic propagation measurements. Previous geophysical and acoustic measurements, along with inversions for seabed parameters in the general area, provide an excellent standard for validating several acoustic data inversion techniques. Continuous wave acoustic signals at multiple frequencies from a towed source, broadband impulsive source signals, and broadband signals generated by a moving surface ship were recorded on an advanced system that included a horizontal line array. Simulated annealing is used in conjunction with a normal-mode description of the propagation and various types of broadband matched-field cost functions to invert for a geoacoustic representation of the seabed. Consistent inversion results are obtained using all three types of acoustic data and are in general agreement with the reported physical measurements. The nature of the soft seabed results in a unique acoustic signature in the pressure time series from impulsive sources and the interference patterns in the transmission loss as a function of source-receiver range. The results indicate that the use of surface ships of opportunity can be successfully employed for geoacoustic inversion.

Passive acoustical survey of finless porpoises in the Yangtze River, China

Finless porpoises (Neophocaena phocaenoides) are distributed in Asian waters. The narrow-band and high-frequency pulse sonar signals produced by this species are distinctive from background noises. Underwater sound monitoring by a hydrophone (BK8103) along board sides of a research vessel concurrent with visual observation were conducted in the Yangtze River from Wuhan to Poyang Lake in 1998. The peak-to-peak detection threshold level was 133 dB re: 1 μPa, which could be roughly converted to the theoretical detection range of 316 m, hypothesizing the porpoise directing to the hydrophone and 168-dB source level of the sonar signal under a shallow water sound propagation (combination of spherical and cylindrical spreading). In a total of 1064-km cruise, 717 finless porpoises were observed. The acoustical monitoring system could detect the sonar signals from finless porpoises, found within 200 m from the research vessel. The sonar signals could also be detected at night and under windy weather conditions. Basically, the acoustical observation system was operated automatically and free from the individual difference of observers. The high-frequency acoustical monitoring seems to be an effective method for the survey of small cetaceans which produce sonar signals.

Beam-displacement ray-mode theory of sound propagation in shallow water

A normal mode method for propagation modeling in common horizontally stratified shallow water, which is called beam-displacement ray-mode (BDRM) theory, is introduced. The peculiarity of this method is that the boundary effects on the sound field can be expressed by the equivalent boundary reflection coefficient, so BDRM theory can be extended to elastic bottom easily. Theoretical calculations of shallow-water sound field show that BDRM has high accuracy and fast speed. The pulse propagation in shallow water is also calculated by BDRM, and the calculated waveforms are in good agreement with the measured waveforms.

Effects of the time-varying ocean on broadband propagation in shallow water

Prediction of underwater sound propagation is generally performed by assuming a time-invariant ocean. In May 1997 SACLANTCEN conducted an experiment south of Elba in the Mediterranean Sea with the aim of investigating the time-dependent effects of the ocean on sound propagation in shallow water. Broadband LFM signals (300 Hz to 7.5 kHz) were transmitted every 1 min for 36 h over a fixed 15-km propagation path. Extensive measurements of the environmental parameters were done during the transmissions in order to perform a meaningful correlation between the time-varying environmental and acoustic data. Previous analysis of the low-frequency part of the acoustic data [Jensen et al., J. Acoust. Soc. Am. 103, 2856(A) (1998)] show high stability in the first 6 h of propagation, but at longer transmitting time and higher frequencies a strong variability in the received acoustic signals is seen. The main features in the time-varying environmental parameters causing fluctuations in the acoustic signals are identified. These features are then used in a numerical propagation model in order to assess to what degree it is possible to predict the time dependence of the received acoustic data in this particular shallow-water area. [This work was supported by the EC under the MAST-III project PROSIM, Contract No. MAS3-CT95-0008.]

High-resolution modal mapping in a complex shallow-water environment

During March 1997, a modal mapping experiment (MOMAX) was conducted to measure the spatial variability of low-frequency sound propagation in shallow water [G. V. Frisk et al., J. Acoust. Soc. Am. 103, 3028(A) (1998)]. The fields created by cw sources in the frequency range 50–300 Hz were measured using an array of freely drifting hydrophone buoys equipped with GPS navigation. The resulting data consist of complex pressure mapped onto a high-resolution geospatial grid. Based on the tenets of normal mode theory, the wave number content of the field was extracted using a high-resolution implementation of the Hankel tranform technique [K. M. Becker et al., J. Acoust. Soc. Am. 103, 3029(A) (1998)]. Neglecting horizontal refraction, the resulting spectra can be mapped onto a corresponding spatial grid. Several modal maps are presented for various source/receiver configurations and frequencies with emphasis placed on the variablility of the wave numbers in space. These modal maps can then be used as the basis for inferring the local geoacoustic properties of spatially varying shallow-water waveguides. [Work supported by ONR.]

High-resolution, three-dimensional measurements of low-frequency sound propagation in shallow water

An overview is presented of the modal mapping experiment (MOMAX), which was conducted in March 1997 in the vicinity of the East Coast STRATAFORM site. Both fixed and moving source configurations were used to transmit several pure tones in the frequency range 50–300 Hz. The magnitudes and phases of these signals were recorded on several freely drifting buoys, each containing a hydrophone, GPS and acoustic navigation, and radio telemetry. High-resolution, three-dimensional measurements of the sound field were made out to ranges of 10 km and illustrate the influence of the laterally varying seabed, measured with a chirp sonar system. The precision navigation also enabled the creation of a two-dimensional, synthetic aperture planar array, parallel to the ocean surface. The pressure field data measured on the array were transformed into the wave number domain, where the lateral variability of the waveguide manifests itself in the spatially evolving spectral content of the modal field. Finally, the phases of the measured signals show remarkable stability and regularity, even in the context of complex, multimodal fields. This behavior can be exploited to make accurate estimates of the relative source/receiver speed from measurements of the time rate-of-change of the phase. [Work supported by ONR.]

High-frequency sound propagation in shallow water with rough surface scattering

Sound propagation at mid to high frequencies (1–20 kHz) in shallow-water environments is extremely complicated, variable, and poorly understood due to multiple processes that cover a wide range of temporal and spatial scales. Numerical simulations using a full-wave PE model are carried out to interpret the acoustic data taken during very carefully designed experiments at a shallow-water site inside the Delaware Bay. Records of broadband acoustic wave time series (from hours to days) are numerically reproduced by using the collected environmental data as input to the PE model. The statistics of both measured and simulated multipath signals are calculated and compared in the presence of rough sea-surface scattering and surface bubble layer attenuation. The surface wind effects can be separated from the slowly varying volume fluctuations of sound speed, by comparing the temporal variations; while it can be separated from the bottom effects by examining different multipath arrivals. The dependence of coherence on the parameters of wind strength and acoustic frequency is examined.

4-D modeling of sound propagation in shallow water with anisotropic sediment layers

A highly efficient 4-D PE model is developed to study broadband sound propagation at the Atlantic Generating Station (AGS) site. Extensive core data has shown that the sediment at this AGS site consists of multiple layers which vary in both range and azimuth. Consequently, sound propagation in the water column in this area is significantly affected by the complex sediment structure due to the fact that the layered sediment acts like a bandpass filter in which broadband acoustic energy is selectively broken into narrow-band components propagating in different layers, usually referred as sediment mode trapping. The details of the range, azimuth, frequency, and bandwidth dependences as well as their relationships to the sediment structure of such mode trapping effect is numerically explored by performing the 4-D PE model. The empirical orthogonal function representation method is employed to transfer the geoacoustic parameters of the sediment core data into PE inputs. The broadband travel time results at eight different bearings are compared with the existing experimental data. The importance of the 3-D effects is also examined by comparing the numerical results of azimuthal coupled and uncoupled models.

Modeling of broadband time signals in shallow water using environmental inversion

A numerical modeling scheme is applied to perform broadband environmental inversion by using explosive data acquired in the Mediterranean Sea in May 1997. The area shows a moderate range dependency, and the received time signals from explosive charges cover a frequency band of several kilohertz. The acoustic propagation model is based on a layered normal mode approach, which is capable of efficiently predicting broadband sound propagation in shallow water [Westwood et al., ‘‘A normal mode model for acoustic–elastic ocean environments,’’ J. Acoust. Soc. Am. 100, 3631–3645 (1996)]. This model has been extended to handle range-dependent environments using the adiabatic approximation. Thus calculation of broadband transfer functions from 0 to 10 kHz of range-dependent, shallow-water waveguides can now be done within a few minutes. The combination of the above propagation model and a state-of-the-art global inversion scheme [Gerstoft, ‘‘SAGA user manual 2.0: An inversion software package,’’ SACLANTCEN SM-333 (1997)], makes it possible to perform environmental focusing by optimizing the correlation between the numerical and experimental received time signal at one or more receiver locations. The broadband inversion scheme is introduced to extract the uncertain acoustic parameters in the waveguide, and to assess how accurate broadband signals can be modeled in complex shallow-water regions.

Modeling for ocean bottom and analyses of sound propagation in shallow water

Modeling for ocean bottom and analyses of sound propagation in shallow water

Possibility of reconstruction layered bottom parameters in the shallow sea by the range-frequency dependencies of losses

In this work the opportunity to use range-frequency dependence of losses (RFDL) for restoration of the acoustic characteristics of the marine bottom is discussed. RFDL and dependence of optimum propagation frequency (OPF) (the frequency, at which losses are minimum) from a sound-speed profile, and characteristics of the layered bottom (impedance, attenuation of longitudinal and share waves in the bottom, sediment layers thickness) are analyzed. For numerical simulation of sound propagation in shallow water, a normal-mode method was used. Numerical simulation has shown that the optimum frequency of sound propagation in shallow water is rather sensitive to the acoustic characteristics of the upper sediment layers. In this work the opportunity to use RFDL and OPF for reconstruction of characteristics of the sea bottom and creation of acoustic model of the bottom in a wide range of frequencies is discussed. For experimental measurement of RFDL in shallow water, a method of broadband pulsing probing was used. The signal was received by autonomous bottom modules. The satisfactory agreement of experimental measurements with the results of numerical simulation confirms this opportunity of reconstruction of the bottom in shallow water by RFDL. [Work supported by RFFI Project N 96-02-18944.]

Inversion Techniques and the Variability of Sound Propagation in Shallow Water

Inversion Techniques and the Variability of Sound Propagation in Shallow Water

Frequency dependence of sound propagation in shallow water: Theory and experiments

Experimental observations of broadband acoustic propagation in a known geological region of the Atlantic Generating Station (AGS) site [Badiey etal., J. Acoust. Soc. Am. 96, 3593–3604 (1994)] has prompted new approaches to understanding frequency-dependent behavior in shallow-water regions. First, recent acoustic observations and detailed geological borehole measurements are reviewed, along with a three-dimensional model of the geoacoustic data that have been developed using the kriging method. Parabolic equation modeling, including range-dependent sound speed and attenuation, is performed for both cw and broadband signals in this region. This is accompanied by normal mode investigations in which trapped modes in the layered media and range-dependent mode coupling are examined. A modal-based theory is presented to explain quantitatively the interference patterns observed in the experimental data (transmission loss versus frequency) in terms of waveguide parameters. It is shown how layered shallow-water waveguides act as bandpass filters in which broadband acoustic energy is selectively broken into narrow-band components, thereby providing new insights applicable to existing inverse techniques.

High-frequency sound variation in a coastal environment

Despite the proliferation of high-frequency sonar imaging systems, the variability of high-frequency sound propagation in shallow water has only recently been measured. In a series of experiments adjacent to the Scripps Institution of Oceanography pier, the variability of sound at a frequency of 450 kHz has been measured. The experimental measurement consisted of deploying a multibeam imaging system on a tripod in approximately 20 ft of water. Repeated insonification and subsequent measurement of the backscattered waveforms indicate that a large degree of variability can exist in the backscattered data. In order to define whether the source of the variability in the backscattered data was originating from the bottom or the volume, a corner reflector was deployed. Assuming that the variability in the sound reflected from the bottom consists of the superposition of both volume and the bottom variability, and that the sound from the corner reflector consists of variability only from the volume, assignment of the relative importance between the two forms of variability can be made. The importance of these two sources of variability as a function of environmental conditions such as tide and surface conditions will be presented. [Work supported by ONR.]

Average acoustic field in shallow water I: Active sonar performance prediction

The seabed dominates the shallow-water acoustics problem, but bottom properties and bottom scattering mechanism are usually poorly known. Detailed interference patterns in shallow water are thus not always physically meaningful for engineering applications. A simple analytical expression averaged over frequency or space can sometimes give better insight into some physical problems. In this presentation, an average angular power spectrum method for calculating shallow-water sound propagation, reverberation, noise, and spatial coherence is briefly introduced. The method is based on normal-mode and ray-mode analogies and originally appeared in Chinese papers which are not available in English [Zhou, Acta Acoust. Sin. 5, 86–99 (1980); Acta Ocean. Sin. 1, 212–217 (1979)]. Taking active sonar performance prediction as an example, analytical expressions for range/depth dependences of the sound propagation, average reverberation intensity, and echo-reverberation ratio will be given for several typical cases, including Pekeris shallow water, wedged continental shelf, and negative gradient waveguide. [Work supported by ONR.]

The effective depth approximation for sound propagation in shallow water over a sediment layer and a hard rock basement

This paper describes an effective depth approximation for a constant depth ocean waveguide with a layered bottom. It is shown that a good approximation to the acoustic field can be obtained by using an effective depth approximation based on the bottom impedance, and that attenuation of the modes propagating in the waveguide can be included if the complex form of the impedance is utilized. It has also been shown that the approximation of the field should include both effective depth modes, which are similar to those in a pressure release waveguide and rigid bottom modes which are similar to those in a wave guide with a rigid bottom. A simple criteria has been given to distinguish between these two types of modes depending on the bottom impedance, and the attenuation factors for each type of mode has been shown to be of a different form. Finally, a formula is given for the calculation of the distance between nulls in the long-range transmission loss in terms of the effective depth of the water channel.