Sound fluctuations caused by internal waves in a shallow sea

Estimates are presented for the fluctuations of the parameters of low-frequency sound fields in shallow-water regions of the Barents Sea, in the presence of seasonal internal gravity waves. The objective of the experiments is to reveal the main mechanisms that govern the sound fluctuations and their statistical parameters on paths of moderate lengths (50–60 to 100–120 km). Another objective is to determine the features of the sound interaction with internal waves for the sound speed profile of the summer—autumn type for which the water stratification is most pronounced. As the probing signals, continuous tonal ones produced by bottommoored sources at the frequencies about 100 and 300 Hz are used along with the 1/3-octave noise signals with the central frequency 1000 Hz, which are generated by a source deployed from a vessel. For the signal reception, both fixed bottom-moored hydrophones and a vertical chain of hydrophones are used, the chain also being deployed from the vessel. The water temperature, the salinity, and the thermocline displacements are monitored with standard hydrographic sensors. The following main results are presented: the estimate of the degree of correlation between the sound fluctuations and the parameters of the water layer, the comparison of the fluctuations in the signal amplitude envelope with the data obtained in other regions, and the estimate of the statistical parameters of the signal amplitude fluctuations, including their dependence on the path length. One more result consists in the proof of the wave nature of the interaction of sound and internal waves, which manifests itself in a strong dependence of the sound interaction with internal waves of discrete frequencies on the frequency of the probing signal and on the angle at which these wave beams intersect. An attempt is made to explain the observed phenomena by the synchronism in the interacting sound and gravity waves. The data obtained can be used to analyze and compare the fluctuations of the sound fields in the ocean, especially in shallow-water regions.     

Sound field phase front variations in shallow water in the presence of intense internal waves

The method of vertical modes and horizontal rays and the method of parabolic equation in the horizontal plane are used to study space-time fluctuations of the phase front of the sound field in the presence of a train of internal waves passing through the acoustic path. The possibility of measuring the distortions of the phase front with a simultaneous filtering of modes by a horizontal array is analyzed. A numerical simulation is carried out for the conditions corresponding to the Eastern coast of United States and the Barents Sea.     

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.     

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.     

The fluctuation of acoustic field due to internal waveson the thermocline in typical shallow water

1IntroductionPartialwaterhasverticaldisplacementwhentheexternalforces,suchastheblockingeffectoftide,thefluctuationofatmospherepressureandthesurgesonthesurfacearepushit.Duetothelayeredcharacteristicsofdensity,itproducesarestoringforceindirectiontothebalanceplaceandthewavespreadingaround,whicharetheinternalwaves.Theinternalwavesarethedynamicprocessmovinginsidetheocean.Theyappearattheinferiorsurfaceofstructure-layeredarisenfromthedensitydifference[1],whichisduetotemperatureandsalinityintheocean.…     

An experimental study of acoustic scattering from thermocline in shallow sea

I.IntroductionThetemperature,salinityandstaticprcssureofscawaterchangewithdepth,whichformsthcsoundvelocityvertica1distributioninthesea.Duringsummcrscason,thercaren0n-uniformthermoclinelaycrsinthedepths0fshallowseaal0ngChinacoast.Wearenowabletodeducctheimpedanccproflle(pc)byechoesofacousticwavesfr0mvarioustherm0c1ine1ayers.Thewaterdcnsityvaricsonlyslight1y,hencetheac0usticve1ocityde-Pendsmainlyonthetemperatureofseawatcr,andthercfore,itispossiblctorecordthcdepthofthermoclinelaycrsbyusingacoust…     

Effect of intense internal waves on the sound field interference structure

The effect of a train of internal-wave solitons on the formation of space-frequency interference pattern of sound field in an oceanic waveguide has been analyzed. Numerical calculations have been performed for the shallow-water channel parameters corresponding to the conditions of the SWARM’95 natural experiment.     

Effect of internal waves on intrusions in a thermocline and on upwelling

Results of the investigation into the interaction of internal waves and flows and the elaboration of a relevant mathematical model are reported. It is found in the field experiments that internal waves develop against the background of upwelling, which results in formation of jets causing intrusion velocity fluctuations. The mechanism and mathematical model of this process are proposed. A mathematical model of the suspended matter transport by jets with a wave-modulated trajectory is developed and verified.     

Frequency shifts of sound field maxima under the effect of intense internal waves

Numerical simulation is carried out to study frequency shifts of a low-frequency sound field maxima under the effect of solitary internal waves (solitons) propagating along an acoustic track in the presence of mode coupling. The frequency shifts are measured by the correlation method. Simulation data obtained with allowance for mode coupling and data obtained in the adiabatic approximation are compared and analyzed.     

Space-frequency dependence of the Horizontal structure of a sound field in the presence of intense internal waves

Sound propagation in a shallow sea is considered in the presence of a packet of intense internal waves. It is shown that horizontal refraction caused by these packets can lead to noticeable changes in the spectrum and spatial intensity distribution of a signal propagating approximately parallel to the wave fronts of internal wave packets.     

Acoustic effects caused by high-intensity internal waves in a shelf zone

The sound field fluctuations caused by high-intensity, solitonlike, quasi-plane internal waves crossing a fixed acoustic path at different angles are numerically modeled for natural conditions of the shelf zone of the Sea of Japan. The horizontal refraction of sound is considered for the case of an acoustic path parallel to the internal wave front.     

Modelling of shallow sea waves in non-linear transmission lines

Summary We have modelled gravity waves in shallow water by means of non-linear transmission lines working in the radiofrequency range. The analogy between a one-dimensional shallow-water medium and a transmission line is demonstrated by the similarities of both their non-linear and dispersive properties. We discuss the results of our experiments on specially constructed radiofrequency models, and compare them with the observations of other authors on shallow-water sea swell. The formation of the shallow-water swell spectrum may be explained, in a whole variety of cases, by the non-linear mixing of the spectral components of waves approaching the shore. This is accompanied by both up- and down-conversion of the wave energy.     

Interaction of intense laser radiation with weak electromagnetic waves in an evacuated section of a ring laser

A laboratory experiment for the study of nonlinear interaction of electromagnetic waves in vacuum is proposed. The basic idea of the experiment is as follows. If one separates a certain region of the circuit in a ring laser from the remaining part with gas-impenetrable partitions, evacuates it, and forms external electromagnetic fields there, the oscillation frequencies and the polarization states of the electromagnetic waves traveling in this ring laser in counter directions are different. A calculation is made for the oscillation frequencies of electromagnetic waves traveling in a ring laser in opposite directions in the case when they interact in an evacuated section of the ring laser circuit with intense laser radiation. It is shown that the effect can be observed in experiments using presently available pulsed lasers of ultrahigh power.     

Two-level system in the field of two intense waves

The problem of a two-level system in the field of two intense monochromatic waves is reduced to the problem of a two-level system in the field of a nonmonochromatic but periodic (in time) wave, which makes it possible to introduce quasienergy states. An investigation is made by the method proposed in [1, 2]. For the quasienergy an analytic expression is obtained that is valid in the complete range of variation of the frequencies at all values of one of the intensity parameters and moderate values of the other. The quasienergy is also calculated in the case of asymptotically large values of the parameters of both intensities when one of the frequencies of the external field is equal to the resonance frequency. The Fourier components of the density matrix at the frequencies of the external field are also calculated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 23–25, August, 1980.We thank M. L. Ter-Mikaelyan for interest in the work.     

Dislocation-induced internal friction of a material with vacancies in weak magnetic field pulses

It is found that the form of the amplitude dependence of low-frequency internal friction in a quenched and aged aluminum sample (99.999% pure) changes under the effect of weak magnetic field pulses (H≥10⁵ A/m): the general level of internal friction increases. This effect is attributed to the influence of a magnetic field on the structural complex formed by a dislocation and point defects (the role of point defects is played by vacancies).     

Dispersion dependences of elastic waves in an ice-covered shallow sea

The dispersion dependences of acoustic waves propagating under conditions close to an ice-covered shallow sea are investigated. A dispersion equation is derived for a layered medium consisting of an elastic layer (the ice cover), a liquid layer (the water column), and a homogeneous elastic half-space (the bottom). The possibilities of estimating certain parameters of the studied layered structure from an analysis of the dispersion curves are discussed. The results of numerical calculations are compared with the data of a full-scale experiment on detecting seismoacoustic signals in an ice-covered sea region.