Acoustic Track Research Articles

Effect of background internal waves on the interference structure of the sound field in a shallow sea

The results of a theoretical analysis of the effect produced by an anisotropic field of background internal waves on the localization of the interference pattern in a shallow sea are presented. The space-time variability of the interference invariant and the smearing of the observation direction of interference fringes are considered in a wide frequency range. The stability of the interference pattern formed by both the superposition of the fields of different mode groups and separate mode groups is analyzed in comparison with the unperturbed waveguide. Numerical calculations are performed for longitudinal and transverse orientations of the acoustic track relative to the propagation direction of internal waves.

Effect produced on sound propagation by an internal temperature front moving over the shelf

Results of observing the changes that occur in the vertical distribution of water temperature under the effect of an intense atmospheric cyclone and the influence of these changes on sound propagation in the shelf region of the Sea of Japan are presented. The measurement results refer to the autumn conditions. The measuring equipment includes a vertical acoustic-hydrophysical measuring system, a broadband transmitter (both of them being connected with the shore station by cable lines), and a self-contained resonance (320 Hz) transmitter of the electromagnetic type. The sound (tone signals) propagation is studied on a 510-m-long constant-depth (38 m) track (TON-310 Hz) and a 10.6-km-long track (TON-320 Hz), which is set up by placing the self-contained transmitter at the bottom (at a depth of 65 m). Results of field experiments are presented along with those of numerical simulation of the effect produced by an internal temperature front moving toward the coast and formed by the seasonal thermocline on the propagation of 320-Hz sound signals through it. It is shown that refraction and scattering of sound waves propagating through the temperature front moving along the acoustic track may cause intensity variations of acoustic field at the reception point, which occur synchronously at different depths and have amplitudes of up to 14 dB and a period of about 40 min.

Surface Acoustic Wave Based Pressure Sensor with Ground Shielding over Cavity on 41° YX LiNbO3

A surface acoustic wave (SAW)-based pressure sensor was fabricated for stable mechanical compression force measurement. A single phase unidirectional transducer (SPUDT) and two acoustic tracks were employed to minimize inherent insertion loss and improve reflectivity from the reflectors. The coupling of modes (COM) theory and finite element methods (FEMs) were used to determine optimal design parameters. A LiNbO3 diaphragm was bonded to a heavily doped silicon substrate with a cavity of ∼250 µm deep, in which gold was lined all over the inner cavity to reduce the coupling loss of SAW energy to the surrounding atmosphere. As a mechanical compression force was applied to the diaphragm, the diaphragm bent, resulting in phase shifts of the reflected peaks. The phase shifts were modulated depending on the amount of mechanical compression applied. The measured reflection coefficient S11 showed good agreement with simulated results.

Perturbation of arrival times in sound pulses propagating in shallow water through moving internal solitons

Sound pulse propagation in shallow water is studied. We assume a geometry where an acoustic track crosses the wave-front of a moving internal soliton (IS) or train of solitons. During propagation, sound pulses undergo perturbation due to mode coupling. For each individual mode selected at the receiver, we observe a set of incoming pulses, each having different arrival times. Time-frequency analysis is carried out for the modal arrival times for different frequencies, emphasizing the dependence on position of the moving solitons. It is shown that the maximum amplitude of the incoming pulses created at the ISs due to mode coupling has approximately the same arrival time, independent of frequency. This invariant arrival time is determined by waveguide parameters and corresponds to maximal coupling of different pairs of modes. Analysis is carried out both for a model shallow water channel and for real experiment data (SWARM95, NRL acoustic track). Satisfactory agreement is demonstrated, and physical interpretation of observed effect is discussed. [Work is supported by ONR and RFBR.]

Sound field temporal fluctuations due to mode coupling in shallow water with internal soliton

Sound field temporal fluctuations due to mode coupling, resulting from traveling internal solitons (IS) approximately along the acoustic track is addressed. The spectrum of received signal amplitude fluctuations in the range of very low frequency (∼ F = 0.001–0.01 Hz) is obtained. Maximums in this spectrum, correspond to the times which IS passes the sound field ray cycles. A relationship between F‐spectrum and acoustic frequency is illustrated by showing Frequency‐frequency (F−f) diagram. It is shown that depending on the sound field frequency (f), different mode pairs give the most pronounced contribution to the sound intensity fluctuations. At the same time invariant frequency, F, of maximal fluctuations remains approximately the same for all sound frequency (f) bands. For example in the case of the SWARM95 experiment, F is about 0.0015 Hz for sound frequency, f in the range of 30–150 Hz. Generally, F depends on waveguide parameters and velocity of IS. In this paper, the physical meaning of the invariant frequency and its relationship to the sound frequency is discussed. In addition, the experimental setup for observation of F in future shallow‐water acoustic experiments is proposed. [Work supported by CRDF, PFBR and ONR‐321OA.]

Measurement and modeling of three-dimensional sound intensity variations due to shallow-water internal waves

Broadband acoustic data (30-160 Hz) from the SWARM'95 experiment are analyzed to investigate acoustic signal variability in the presence of ocean internal waves. Temporal variations in the intensity of the received signals were observed over periods of 10 to 15 min. These fluctuations are synchronous in depth and are dependent upon the water column variability. They can be explained by significant horizontal refraction taking place when the orientation of the acoustic track is nearly parallel to the fronts of the internal waves. Analyses based on the equations of vertical modes and horizontal rays and on a parabolic equation in the horizontal plane are carried out and show interesting frequency-dependent behavior of the intensity. Good agreement is obtained between theoretical calculations and experimental data.

The Effect of Internal Waves on the Sound Propagation in the Shelf Zone of the Sea of Japan in Different Seasons

Results of the observation of seasonal variations in the vertical distribution of water temperature in the shelf zone of the Sea of Japan are presented, and the effect of this variability on the parameters of internal waves and on sound propagation is studied. The measurements were carried out in different seasons using a vertical acoustical-hydrophysical measuring system. The propagation of sound (tone and noise signals) was studied on a 510-m-long track at a constant depth of 38 m. Using a self-contained resonance (320 Hz) transmitter of the electromagnetic type, which was bottom-moored at a depth of 65 m, a 10.6-km-long stationary acoustic track crossing the shelf was set up. During the in-sea experiments, the spatial characteristics of internal waves were measured along with the distributions of temperature, salinity, sound velocity, and sea level variations.

Acoustic effects in presence of internal solitons in shallow water

The given lecture (mini-tutorial) is dedicated to description of recent efforts (last 10–15 years) in experimental observation and theoretical modeling of the sound propagation in shallow water in the presence of traveling internal solitonlike waves (ISW). It contains a review of the main experimental observation of acoustical effects (Rubinstein and Rubinstein and Brill, Zhou et al., Badiey et al., etc.) The main theoretical approaches can be divided in dependence on orientation of ISW wave front: (1) Modes coupling due to ISW, traveling along acoustic track. Resonance effects in sound propagation, resonance coupling by packets and separate solitons (J-X Zhou, Preisig and Duda, etc.) are considered. (2) Horizontal refraction (HR) caused by ISW crossing acoustic track. Technique of vertical modes and horizontal rays, vertical modes and PE in horizontal plane and modeling of acoustic effects in typical conditions of Barents sea are shown. Analysis of specific peculiarities due to HR (synchronicity, periodicity, and depth dependence of intensity fluctuations) is given. Experimental observations of acoustic effects in the SWARM’95 (broadband sound propagation, shot and LFM signals) are presented. New specific features of signals, passing through ISW space-frequency horizontal structure, and fluctuations of modal spectrum are demonstrated, and experimental setup to register these is discussed. [Work was supported by RFBR and CRDF.]

Frequency dependence of the sound fluctuations in the presence of internal solitary waves in SWARM’95

In this paper the propagation of sound pulses in a shallow-water region in the presence of internal soliton train (IS) traveling approximately across an acoustic track of 15 km is studied. Broadband sound pulses were produced by an airgun (shots with frequency band about 30–120 Hz) and an electromechanical (J15) sound source (LFM sweeps with frequency band 50–200 Hz). Experimental data show significant correlation of sound fluctuations with IS motion. In addition, difference in the excitation source causes different modal energy partitioning in the waveguide. Modeling is carried out within the framework of the theory of vertical modes and horizontal rays (or PE in the horizontal plane). Calculations give a good agreement for temporal fluctuations of the sound energy for the sources used. It is shown that frequency dependence of intensity fluctuations is connected with the frequency dependence of refraction index of horizontal rays due to the 3D nature of these fluctuations. [Work supported by ONR and RFBR, Grant 03-05-64568.]

Narrowband signals propagation in randomly inhomogeneous shallow water waveguide

In the presented work results of experiment and theoretical modeling are considered for the narrowband (ratio of frequency band to frequency ∼0.1) sound propagation at the long range (up to 200 km) acoustic track in Barents sea. Modes selection for these conditions can be used for acoustical tomography of large-scale perturbations. Characteristics of the sound signals passing through this area (and in turn feasibility of tomographic methods) are determined by mutually competing mechanisms: waveguide dispersion, providing modes filtering, modes attenuation and modes coupling due to random inhomogeneities, masking separation of modes. Experimental results presented in this work show significant influence of noise (or modes coupling). For interpretation of experimental results (frequency and time dependencies of arriving pulses) background internal waves are taken as a main reason of the modes coupling. It is shown that for the modes separation it is necessary to use vertical array along with the measuring difference in arrival time, and correlation processing of received signals. [Work supported by RFBR, grants Nos. 03-05-64568 and 02-02-16509.]

Sound fluctuations due to horizontal refraction in the SWARM-95 experiment

Fluctuations of the sound field measured in the SWARM-95 experiment are interpreted as manifestations of three-dimensional effects in shallow-water sound propagation. It is shown that features of the received sound signal, along an acoustic track placed at 5° to the crests of propagating internal soliton trains, can be explained by significant horizontal refraction of the sound waves. This refraction leads to a remarkable redistribution of the energy in the horizontal plane as has been recently remarked by other investigators. These features are specifically: (1) a high level of fluctuations of the sound energy (up to 6 dB) which are synchronous in time and have same amplitude over depth of waveguide and (2) anomalies in the arrival times and amplitudes of separate waveguide modes. A theoretical analysis based on the theory of vertical modes and horizontal rays is presented, along with computer modeling of the experiment. Analytical estimations of the observed effects as well as results from numerical modeling are in a good agreement with experimental observations and are consistent with simulations. [Work supported by ONR and US CRDF Award No. VZ-010-00.]

Determination of the absorbing and scattering properties of shallow-water sediment using wideband signals

Sound propagation in a shallow-water region is studied for bottom parameter estimation within the framework of MFP (matched field processing) method. The experiment was conducted in the Barents Sea at the stationary acoustic track (about 14 km length). Basic bottom parameters are determined on the basis of comparison between experimental and theoretical frequency spectra of a signal received from a wideband source (20–100 Hz). A shallow-water waveguide model is developed on the basis of measured bathimetry, sound-speed profile, and two-component model of the bottom sediment. Porosity and coefficients of absorption and volume scattering are considered as the parameters, characterizing bottom sediment. The connection between porosity and mentioned coefficients is taken from Hamilton’s data [J. Acoust. Soc. Am. 59, 528–535 (1976)]. Porosity (correspondingly, density and sound speed in sediment) is obtained, and a conclusion is made about relative contribution of different mechanisms of losses (absorption and volume scattering) in the seafloor at different sound frequencies. [Work supported by CRDF and ME RF, Award No. VZ-010-0.]

Sound scattering in shallow water in presence of internal solitons

Sound scattering by localized inhomogeneity (object) in a shallow-water waveguide is studied. Influence of internal solitons (IS) traveling in the sea, on the scattered field is considered. The problem is analyzed within the framework of theory of the sound scattering in the waveguide developed by the authors, where the scattering matrix is expressed through modal decomposition including the scattering amplitude of the object in free space. The peculiarity of the given work is the taking into account of additional modes conversion due to IS. The waveguide and IS are modeled on the basis of experimental data (sound-speed profile, bottom relief, IS parameters, etc.) collected in the Japan Sea. The inhomogeneity is selected in the form of soft spheroids with dimensions characteristic of gray whales (eschrichtius). Calculations of amplitude fluctuations of the low-frequency sound field at the receiving point are carried out for moving spheroid and different orientations of acoustic track with respect to direction of propagation of IS. It is shown that sound fluctuations have essentially different characteristics for longitudinal and transversal propagation of IS. [Work supported by RFBR, Grant No. 00-05-64752.]

FOOD AVAILABILITY AND TIGER SHARK PREDATION RISK INFLUENCE BOTTLENOSE DOLPHIN HABITAT USE

Although both food availability and predation risk have been hypothesized to affect dolphin habitat use and group size, no study has measured both factors concurrently to determine their relative influences. From 1997 to 1999, we investigated the effect of food availability and tiger shark (Galeocerdo cuvier) predation risk on bottlenose dolphin (Tursiops aduncus) habitat use and group size in Shark Bay, Western Australia. Food availability was measured by fish trapping, while predation risk was assessed by shark catch rates, acoustic tracks, and Crittercam deployments. Dolphin habitat use was determined using belt transects. The biomass of dolphin prey did not vary seasonally and was significantly greater in shallow habitats than in deeper ones. Tiger sharks were virtually absent during cold months of 1997 and 1998, abundant in warm months of all years, and found at an intermediate density during cold months of 1999. When present, shark density was highest in shallow habitats. Decreased echolocation efficiency in very shallow water and poor visual detection of tiger sharks (camouflaged over seagrass) probably further enhance the riskiness of such habitats, and the relative riskiness of shallow habitats is supported by the observation that dolphins select deep waters in which to rest. The observed dolphin group sizes were consistent with a food–safety trade-off. Groups were larger in more dangerous shallow habitats and larger during resting than during foraging. Foraging dolphins matched the distribution of their food when sharks were absent. However, during warm months, the distribution of foraging dolphins significantly deviated from that of their food, with fewer dolphins foraging in the productive (but dangerous) shallow habitats than expected on the basis of food alone. When shark density was intermediate, habitat use by foraging dolphins was more similar to the high-shark-density seasons than periods of low shark density. These results suggest that foraging dolphin distributions reflect a trade-off between predation risk and food availability. Because the distribution and abundance of tiger sharks are influenced by species other than dolphins, the distribution of the tiger sharks' primary prey may indirectly influence dolphin habitat use, suggesting that it is important to consider the community context in studies of habitat use.

Predicting the distribution of sublittoral benthic biotopes using acoustic remote sensing

To implement the European Directive on conservation of natural habitats (92/43/EEC), government conservation agencies, in this case Scottish Natural Heritage (SNH), must identify areas as possible Special Areas of Conservation (pSACs). To inform the future management and monitoring of these sites, SNH need to know the distribution of habitats and species in Scottish marine SACs. Distribution of marine benthic biotopes (habitats and their associated species) was predicted for two physiographically different pSACs in Scotland: Papa Stour in Shetland, and Lochs Duich, Alsh and Long, NW Scotland. The methodology involved acoustic remote sensing, ground validated by remote video and grab sampling, with analysis and display of results using a PC based GIS. Field surveys were carried out in 1996. A coustic track point data obtained were interpolated to obtain a continuous polygon coverage of the acoustic properties of the seabed within each pSAC area. An acoustic signature for each biotope type, or biotope complex was determined by buffering the ground validation sites, intersecting buffers with acoustic track data, and deriving descriptive statistics for each acoustic variable. Spatial analysis of acoustic and ground validation data within the GIS was then used to produce a continuous coverage polygon map of predicted biotope types for each pSAC.

Effect of the Refraction of Sound Waves in the Sediment Layer on the Sound Field Levels in the Ocean at Short Ranges

On the basis of the results of an acoustic experiment, which was carried out in the Indian Ocean with the use of explosive sources of sound, the effect of the refraction of sound waves in the sediment layer on the levels and the structure of the sound field formed in the water column at short ranges, namely, in the first geometric shadow zone and in the insonified zone below the channel axis, is considered. The length of the acoustic track under study is 45.0 km, and the frequency range is 10–500 Hz.

An evaluation of the accuracy of shallow water matched field inversion results.

In this article the accuracy of geo-acoustic and geometric parameter estimates obtained through matched field inversion (MFI) was assessed. Multi-frequency MFI was applied to multi-tone data (200-600 Hz) received at a 2-km source/receiver range. The acoustic source was fixed and the signals were received at a vertical array. Simultaneously with the acoustic transmissions, a CTD (conductivity, temperature and depth)-chain was towed along the acoustic track. A genetic algorithm was used for the global optimization, whereas a normal mode model was applied for the forward acoustic calculations. Acoustic data received at consecutive times were inverted and the stability of the inverted parameters was determined. Also, the parameter estimates were compared with independent measurements, such as multi-channel seismic surveys (for geo-acoustic parameters). The obtained uncertainty in the inversion results was assumed to have two distinct origins. The first origin is the inversion method itself, since each optimization will come up with some solution close to the exact optimum. Parameter coupling and the fact that some parameters hardly influence the acoustic propagation further contribute to this uncertainty. The second is due to oceanographic variability. Both contributions were evaluated through simulation. The contribution of oceanographic variability was evaluated through synthetic inversions that account for the actual sound speed variations as measured by the towed CTD-chain.

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.

Low-frequency horizontal acoustic refraction caused by internal wave solitons in a shallow sea

The effect of internal wave solitons on the sound field generated by a point source in a shallow sea is considered. In the framework of the theory of “horizontal rays and vertical modes,” the sound field pattern governed by the aforementioned hydrodynamic effect is investigated. It is shown that solitons can induce time-periodic focusing and defocusing of horizontal rays propagating at shallow angles to the internal wave front. This may result in the formation of “dynamical” horizontal sound channels, which, in its turn, results in considerable temporal fluctuations of the field along the acoustic track oriented along the internal wave front. For the sound field calculations, an approach is developed on the basis of the parabolic approximation in the horizontal plane and the mode representation in the vertical direction. The results obtained can be used for remote monitoring of internal wave packets in a shallow sea.

Matched-field processing, geoacoustic inversion, and source signature recovery of blue whale vocalizations.

Matched-field processing (MFP) and global inversion techniques have been applied to vocalizations from four whales recorded on a 48-element tilted vertical array off the Channel Islands in 1996. Global inversions from selected whale calls using as few as eight elements extracted information about the surrounding ocean bottom composition, array shape, and the animal's position. These inversion results were then used to conduct straightforward MFP on other calls. The sediment sound-speed inversion estimates are consistent with those derived from sediment samples collected in the area. In general, most animals swam from the east to west, but one animal remained within approximately 500 m of its original position over 45 min. All whales vocalized between 10 and 40 m depth. Three acoustic sequences are discussed in detail: the first illustrating a match between an acoustic track and visual sighting, the second tracking two whales to ranges out to 8 km, and the final sequence demonstrating high-resolution dive profiles from an animal that changed its course to avoid the research platform FLIP (floating instrument platform). This last whale displayed an unusual diversity of signals that include three strong frequency-modulated (FM) downsweeps which contain possible signs of an internal resonance. The arrival of this same whale coincided with a sudden change in oceanographic conditions.