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.     

Spectral features of the water temperature and sound intensity variations measured on the shelf of the Sea of Japan

Results of a spectral analysis of the water temperature and sound intensity variations measured on stationary acoustic-hydrophysical tracks in the shelf zone of the Sea of Japan are presented. The measurements were carried out in different seasons with the use of equipment that included a vertical acoustic-hydrophysical measuring system, self-contained acoustic-hydrophysical radio buoys, and a self-contained electromagnetictype resonance (320 Hz) transmitter. Spectral features of temperature fluctuations caused by internal waves in a vertical water layer are studied, and their influence on the sound propagation is demonstrated on tracks of different lengths oriented along and across the shelf.     

Frequency shifts of the sound field interference pattern in shallow water because of second-mode soliton-like internal waves

Numerical simulation is carried out to analyze the effect of an internal soliton of the second gravity mode on low-frequency sound propagation in an oceanic shelf region. The simulation is performed using the data of a full-scale experiment performed on the shelf of the South China Sea near Dongsha atoll, where the aforementioned solitons had been detected by stationary vertical thermistor arrays. The calculations take into account the effect of horizontal refraction of sound waves. It is assumed that a stationary acoustic track is oriented across the predominant propagation direction of internal waves. The results of simulation show that, when the soliton crosses the stationary track, some of the sound field modes are focused, whereas other modes are defocused. It is demonstrated that the soliton parameters can be adequately determined from the frequency shifts of the sound field interference pattern. However, such an estimate of the soliton parameters is only possible for a limited length of the stationary track for which the effect of horizontal refraction is sufficiently weak.     

Effect of random hydrodynamic inhomogeneities on low frequency sound propagation loss in shallow water

Low frequency (100–500 Hz) sound propagation loss on the US Atlantic continental shelf and in the Barents Sea in the presence of stochastic surface waves, and for the US Atlantic shelf also in the presence of internal waves, is studied for the range of up to 150 km by means of numerical simulations. Qualitative difference between sound propagation loss behavior on the US Atlantic shelf and in the Barents Sea is demonstrated for summertime conditions even without random inhomogeneities. It is shown that whereas internal waves have a weak effect on propagation loss, surface waves result in its considerable increase in both areas under wintertime conditions with a wind speed of more than 9 m/s.     

Sound scattering by spatially localized inhomogeneities in a shallow-water waveguide in the presence of internal waves

A method is developed for solving the problem of sound scattering by concentrated inhomogeneities in a waveguide of small depth in the presence of internal waves (IW) typical of an oceanic shelf. The sound field fluctuations related to the motion of a model scatterer (a soft spheroid) and to the propagation of the IW are calculated and analyzed. It is shown that the field of internal waves considerably affects the scattered sound field even when the source-receiver and source-scatterer distances are relatively small (about several kilometers). This effect depends not only on the amplitude of the IW, but on their propagation direction as well.     

The sound speed in southern deepwater zone of the Caspian Sea,off Anzali Port

Sound velocity determination in seawater is a key component of modern hydrographic surveying; however, little data exists on sound velocity characteristics of the southern Caspian Sea. Hence, a study was undertaken in 2008 to examine the seasonal variability of sound speed in deep-waters of the South Caspian Sea near the Iranian coast. The seasonal cycle of seawater temperature and thermal stratification in the Caspian Sea water created a wide range of spatial and temporal changes of sound speed with relevant differences between shallow water (over the continental shelf) and deep-water area. The collected data showed that seasonal variations of the sound speed were most important in the upper 100 m water depth, while below this level that is in deepwater the changes were small. The maximum values of sound speed were observed at the surface in midsummer around 1517–1519 m s⁻¹ over the continental shelf while the speed of sound was about 1453 m s⁻¹ between 450–470 m depths with no major seasonal variations. Variations in vertical structure of the sound speed were in agreement with temperature changes, while effects of the salinity on the sound speed were little.     

Observationally constrained modeling of sound in curved ocean internal waves: examination of deep ducting and surface ducting at short range

A study of 400 Hz sound focusing and ducting effects in a packet of curved nonlinear internal waves in shallow water is presented. Sound propagation roughly along the crests of the waves is simulated with a three-dimensional parabolic equation computational code, and the results are compared to measured propagation along fixed 3 and 6 km source/receiver paths. The measurements were made on the shelf of the South China Sea northeast of Tung-Sha Island. Construction of the time-varying three-dimensional sound-speed fields used in the modeling simulations was guided by environmental data collected concurrently with the acoustic data. Computed three-dimensional propagation results compare well with field observations. The simulations allow identification of time-dependent sound forward scattering and ducting processes within the curved internal gravity waves. Strong acoustic intensity enhancement was observed during passage of high-amplitude nonlinear waves over the source/receiver paths, and is replicated in the model. The waves were typical of the region (35 m vertical displacement). Two types of ducting are found in the model, which occur asynchronously. One type is three-dimensional modal trapping in deep ducts within the wave crests (shallow thermocline zones). The second type is surface ducting within the wave troughs (deep thermocline zones).     

Propagation of pulse pseudorandom signals from a shelf into shallow water in winter hydrological conditions of the Sea of Japan

The paper discusses the results of an experiment conducted in the Sea of Japan in March 2016 on an acoustic track 194 km long under winter hydrological conditions. We have studied the most complex case of propagation of pulse pseudorandom signals from the shelf into shallow water during vortex generation on the acoustic track. Analysis of the experimentally obtained pulse characteristics have shown that the maximum first approach of acoustic energy recorded at all points agrees well with the calculation. This testifies to the fact that at a given reception depth, the first to arrive are pulses that have passed in the near-surface sound channel over the shortest distance and at small angles close to zero. We propose a technique for calculating the mean sound velocity on the track from satellite monitoring data on the surface temperature, which makes it possible to rely on the successful application of the results obtained in acoustic ranging and navigation problems.     

The influence of internal waves on losses during sound propagation on a shelf

We discuss the results of investigations into the influence of internal waves on sound losses during propagation along stationary lanes on shelves of the Sea of Japan and the Sea of Okhotsk. Measurements were conducted with a Mollusk-07 autonomous vertical acoustic-hydrophysical measuring system and TONE-320 and FM-290–330 autonomous electromagnetic-type emitters generating a tonal acoustic signal and a frequency modulated signal, respectively. It is established that sound losses at 290–330 Hz as a result of internal waves do not exceed 5 dB. At the same time, intensive nonlinear internal wave packets that are relatively rare but characteristic of a shelf can increase losses up to 15 dB.     

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.     

The coherence of low-frequency sound in shallow water in the presence of internal waves

The coherence time and transverse coherence length of a low-frequency (100–300 Hz) sound field that is formed by an omnidirectional point source at a distance of 10–30 km in a shallow-water acoustic waveguide, which is characteristic of an open ocean shelf, were estimated analytically and in a numerical experiment. An anisotropic field of background internal waves is considered as a source of spatiotemporal fluctuations. It is shown that the coherence time decreases as the frequency increases, and strongly depends on the perturbation-movement direction. The transverse coherence length is primarily determined by phase incursions that are related to the cylindrical shape of the acoustic-wave front. In the case of transverse propagation, background internal waves may lead to significant variations in this length. The introduction of compensating phase corrections during processing provides a considerable increase in the average transverse coherence length.     

Effect of tide on sound propagation in the shelf zone of the Sea of Japan

Experimental and numerical studies of the effect of surface and internal tides on 315-Hz sound waves propagating along fixed paths, 260 m to 23 km in lengths, oriented across the shelf of the Sea of Japan, are discussed. The measurements are performed using self-contained radio-hydroacoustic receiving stations, which are equipped with hydrophones and scalar-vector receivers, and two vertical acoustic-hydrophysical measuring systems. For the sound signals propagating along the longer paths, the intensity fluctuations are shown to loose their linear relation to the tide-caused changes in the waveguide parameters because of the refraction by the sound speed inhomogeneities induced by different hydrodynamic processes. However, it is established that the phase variations can serve as quantitative indicators of the integral changes in the waveguide parameters.     

Horizontal coherence of low-frequency fixed-path sound in a continental shelf region with internal-wave activity

Sound at 85 to 450 Hz propagating in approximately 80-m depth water from fixed sources to a joint horizontal/vertical line array (HLA/VLA) is analyzed. The data are from a continental shelf area east of Delaware Bay (USA) populated with tidally generated long- and short-wavelength internal waves. Sound paths are 19 km in the along-shore (along internal-wave crest) direction and 30 km in the cross-shore direction. Spatial statistics of HLA arrivals are computed as functions of beam steering angle and time. These include array gain, horizontally lagged spatial correlation function, and coherent beam power. These quantities vary widely in magnitude, and vary over a broad range of time scales. For example, correlation scale can change rapidly from forty to five wavelengths, and correlation-scale behavior is anisotropic. In addition, the vertical array can be used to predict correlation expected for adiabatic propagation with cylindrical symmetry, forming a benchmark. Observed variations are in concert with internal-wave activity. Temporal variations of three coherence measures, horizontal correlation length, array gain, and ratio of actual correlation length to predicted adiabatic-mode correlation length, are very strong, varying by almost a factor of ten as internal waves pass.     

Sound attenuation in a random waveguide when using a radiating system with different patterns of directivity

Using numerical simulations of sound propagation on Russia’s shallow Arctic shelf, low frequency sound attenuation is analyzed for acoustic sources with different patterns of directivity, i.e., vertical discrete radiating arrays of different length. It is shown that sound attenuation depends largely on the parameters of patterns of directivity and the intensity of surface waves even at short ranges r from a source (r < 250H, where H is the waveguide depth).     

Frequency dependence and intensity fluctuations due to shallow water internal waves

A theory and experimental results for sound propagation through an anisotropic shallow water environment are presented to examine the frequency dependence of the scintillation index in the presence of internal waves. The theory of horizontal rays and vertical modes is used to establish the azimutal and frequency behavior of the sound intensity fluctuations, specifically for shallow water broadband acoustic signals propagating through internal waves. This theory is then used to examine the frequency dependent, anisotropic acoustic field measured during the SWARM'95 experiment. The frequency dependent modal scintillation index is described for the frequency range of 30-200 Hz on the New Jersey continental shelf.     

Long-range sound propagation in the region of the subarctic front

The data of two experiments performed in the northwestern Pacific are presented. The propagation path crosses the northern boundary of the subarctic frontal zone. The influence of the frontal zone on the time structure and intensity of the sound field is studied. This influence most clearly manifests itself in the range dependence of the level of the normalized sound field at frequencies of 63–800 Hz. In the region of crossing the boundary of the frontal zone, a change of 1.5–2 dB in the sound field level is observed with localization in distance. In this region, a pronounced increase in the frequency-independent component of the exponential attenuation is also observed (by 0.015 dB/km for explosion-generated signals received at a depth of 600 m). At depths of 150–800 m, a zone of weak variations of the propagation loss is present in the vertical structure of the sound field at the 100-km part of the path in the region of crossing the front. In the experiment with explosion-generated signals, phenomena that are unrelated to the frontal zone are observed as well, namely, the appearance of reverberation forerunners (prereverberation) on separate parts of the path and the presence of bottom-reflected signals on one of the path fractions with a local bottom rise.     

The effect of geoacoustic characteristics of the bottom on the long-range sound propagation in an inhomogeneous ocean

The long-range sound propagation from a deep ocean to a receiving system located on the shelf is modeled. The waveguide model is constructed on the basis of the data of an acoustic-oceanographic experiment carried out in the northwestern Pacific. The sensitivity and the frequency dependence of the difference in the sound field levels at the crossing of the frontal zone on the geoacoustic characteristics of the bottom of the shelf and the continental slope are investigated. It is shown that the level difference decreases by 8.2 dB as the velocity of longitudinal waves increases by 100 m/s in the range within 1490–1820 m/s.     

Effect of internal tides on slow energy fluctuations of impulse signals in an experiment on a long-distance stationary path

Temporal energy fluctuations of impuse signals generated by powerful explosive sources of sound are considered by using the experimental data obtained on a long-distance stationary path in the northwestern Pacific. The signals were received on the Kamchatka shelf (a characteristic ocean depth of about 200 m) at two points spaced by 115 km in the latitudinal direction, one of the reception points being located on the shelf far from its edge, and the other, on the shelf edge. The signal radiation was performed in the deep ocean, at a distance of about 1000 km from the shelf edge. Charges were exploded every hour for more than five days. The frequency-dependent energy fluctuations of the signals and those of the first mode alone were measured in the frequency range 10–80 Hz. It is shown that the amplitudes of energy fluctuations of the first mode in narrow bands can exceed 10 dB and make the main contribution to the energy fluctuations of the total signal. The maximum energy of the first mode is reached on the high tide. It is hypothesized that internal tide-induced variations of the sound velocity field on the shelf affect the degree of energy interaction between the first mode and the bottom.     

Long-range sound propagation in the northeastern Atlantic

Experimental data on the long-range propagation of explosion-generated signals are analyzed. The experiments were performed in the northeastern Atlantic under the conditions of a two-axis underwater sound channel. The sound field in the upper channel was governed by the vertical redistribution of the ray structure and sound energy under the influence of a smooth increase in the depth of the channel’s axis along the propagation path. The explosions were produced in the upper sound channel at a depth of 200 m, which was constant along the path. The time structure of the sound field is analyzed for the upper channel (a reception depth of 200 m) and for deeper layers lying somewhat below the boundary between the upper and lower sound channels (a reception depth of 1200 m). The deviation of the decay law obtained for the sound field level in the upper channel from the cylindrical law is used to estimate the attenuation coefficient. The low-frequency (several hundreds of hertz) attenuation coefficients experimentally determined with allowance for the sound field redistribution agree well with the calculated sound absorption in seawater. The attenuation coefficients determined by the differential method also agree well with the absorption calculated by the formulas proposed earlier. The analysis of the time structure of the sound field near the boundary between the upper and lower channels reveals a permanent insonification of this horizon by weak water-path signals propagating with the velocity typical of the signals traveling in the upper channel.     

Energy fluctuations of high-frequency sound signals in a shallow water in the presence of nonlinear internal waves

The paper presents an analysis of energy fluctuations of high-frequency (2–4.5 kHz) sound signals propagating in a shallow water in the presence of nonlinear (soliton-like) internal waves (2006 Shallow Water experiment, US Atlantic shelf). Signals were received by three single hydrophones in different directions at distances of ～4, ～12, and ～5 km from the source. The angle between the first two acoustic tracks was ～15°. The third track was almost an extension of the first and was on the other side of the source. A relatively short (one to two solitons) nonlinear internal wave packet first moved approximately along the first two tracks and then along the third track. It is demonstrated that in the presence of solitons on the track in the frequency spectrum of energy fluctuations, there is an isolated frequency that depends, in particular, on the angle between the soliton front and the acoustic track. The experimental results agree well with the theory previously proposed by the authors, where the occurrence mechanism of fluctuations is explained using the ray approach.