M. B. Salin

Calculation of the reverberation spectrum for Doppler-based sonar

For monostatic sonar using long pulsed tone signals, the problem of evaluating the spectrum of reverberation due to sound wave scattering by a rough sea surface is solved. Relatively simple computational schemes are proposed, which make it possible (i) to transform the three-dimensional spectra of surface waves to the frequency-angular characteristics of reverberation and (ii) to choose the optimal operating frequency band for a Doppler sonar from the point of view of reverberation. For typical wind wave characteristics measured in shallow water areas, the spectral levels of reverberation are estimated in the frequency band of acoustic signals within 0.4–2 kHz.

Methods for calculating low-frequency surface reverberation at known sea roughness characteristics

The problem of estimating the parameters of a reverberation signal generated due to scattering of acoustic waves on a rough surface is considered for a bistatic localization scheme using the example of tone and tone-pulse signals. Relatively simple calculation schemes that make it possible to recalculate three-dimensional space-time roughness spectra into angular-frequency reverberation characteristics and determine the positions of the surface regions that mainly contribute to the integral reverberation level are found. A reverberation spectrum is calculated for the wind waves typical for shallow-water closed basins, and the positions of the scattering regions forming the components of this spectrum are determined.

COMBINED METHOD FOR MEASURING 3D WAVE SPECTRA. I. ALGORITHMS TO TRANSFORM THE OPTICAL-BRIGHTNESS FIELD INTO THE WAVE-HEIGHT DISTRIBUTION

Optical instruments for measuring surface-wave characteristics provide a better spatial and temporal resolution than other methods, but they face difficulties while converting the results of indirect measurements into absolute levels of the waves. We have solved this problem to some extent. In this paper, we propose an optical method for measuring the 3D power spectral density of the surface waves and spatio-temporal samples of the wave profiles. The method involves, first, synchronous recording of the brightness field over a patch of a rough surface and measurement of surface oscillations at one or more points and, second, filtering of the spatial image spectrum. Filter parameters are chosen to maximize the correlation of the surface oscillations recovered and measured at one or two points. In addition to the measurement procedure, the paper provides experimental results of measuring multidimensional spectra of roughness, which generally agree with theoretical expectations and the results of other authors. Index terms – directional spectrum, geophysical signal processing, image processing, optical measurement of surface roughness, video processing, wave number spectrum, wind-generated waves.

Near-field measurements of the scattering characteristics of a moving object with doppler filtering of the signal

The scattering characteristics of a moving object can be measured by insonifying it with a tone source and receiving the scattered signal with a linear array positioned in the near zone of the object. Possible algorithms of scattered signal processing aimed at determining the angular dependence of scatterer’s target strength are considered. On the basis of experimental data on the angular and frequency characteristics of reverberation caused by insonification of the water area with a tone source, the possibility of the aforementioned measurements is estimated and the minimal measurable values in the angular dependence of scatterer’s target strength are determined for a water area perturbed by weak wind waves.

Numerical simulation of Bragg scattering of sound by surface roughness for different values of the Rayleigh parameter

Numerical simulation methods are described for the spectral characteristics of an acoustic signal scattered by multiscale surface waves. The methods include the algorithms for calculating the scattered field by the Kirchhoff method and with the use of an integral equation, as well as the algorithms of surface waves generation with allowance for nonlinear hydrodynamic effects. The paper focuses on studying the spectrum of Bragg scattering caused by surface waves whose frequency exceeds the fundamental low-frequency component of the surface waves by several octaves. The spectrum broadening of the backscattered signal is estimated. The possibility of extending the range of applicability of the computing method developed under small perturbation conditions to cases characterized by a Rayleigh parameter of ≥1 is estimated.

Calculation of the bistatic target strength of a complex multiresonant shell by the finite element method

A method for calculating the bistatic target strength of a complex elastic structure with characteristic linear dimensions of one to ten wavelengths is developed and tested. The proposed method is based on the finite-element approach to modeling the mechanoacoustic properties of the object under consideration, which is complemented with the algorithms of incident plane wave generation and far-field calculation using the Green function of free space. The results of testing the accuracy of the proposed computational method in application to bodies with a simple shape and a known analytical solution are presented. Specifically, a deviation of less than 3 dB from the theoretical value is obtained for the main lobe of the directional pattern of the target strength. The results of calculations are compared with experimental data obtained for a thinwalled reinforced shell used as an example.

Reconstruction of the time dependence and signal parameters of far-field extended wideband sources: Part 1. Reconstruction techniques and technical instruments

The problem of reconstructing the time dependence of an extended source in the far zone is considered. It is shown that the time dependence of the source signal is reconstructable in the far zone during acoustic signal recording with a far-field array and in further processing. In this work, possible signal disturbances due to the finiteness of the receiving aperture are estimated and optimal parameters of the signal processing system are determined. A linear array is suggested in the form of digital receiving elements that synchronously record the acoustic field in a band of up to 10 kHz and transfer the received signal through four twisted pairs directly to a PC network card. The technique described in the work and its technical implementation can be used in hydroacoustic measurements and in designing sound-recording systems for orchestras and large choirs.

Study of the mechanism for broadening of the spectrum of a low-frequency reverberation signal for sound scattering by near-surface inhomogeneities under conditions of intense wind waves

The paper considers the problem of backscattering of sound waves by near-surface volumetric inhomogeneities under conditions of intense wind waves. We calculate the expected share of the scattered signal spectrum based on the given wind-wave intensity and the depth distribution of volumetric inhomogeneities. For deep ocean conditions in the frequency range of 500–1000 Hz for a pulse duration of 10 s, we measure the levels and shape of the reverberation spectrum for time delays from 20 to 100 s. Comparison of the measured and calculated reverberation spectra has shown their good coincidence.

Methods for measuring bistatic characteristics of sound scattering by the ocean bottom and surface

It is very interesting to measure the bistatic characteristics of sound scattering by the ocean bottom and surface for the development of present-day hydrolocation net-centric schemes. Possible methods for measuring scattering bistatic characteristics are evaluated. The angular characteristics of reverberation related to illumination of a water area by active linearly frequency-modulated signals are studied using linear passive arrays to test a measuring scheme in a shallow water area with a depth of ~20 m in the 1–3 kHz frequency range. The bistatic sound-scattering characteristics in the water area are calculated based on the measurements. The obtained characteristics are compared with the known data on the backscattering of sound.

Algorithms for converting 3D surface roughness spectra when calculating frequency-angle reverberation characteristics

On the basis of an earlier developed model of sound scattering by surface disturbances characterized by 3D roughness spectra, we propose and test a sufficiently fast algorithm for calculating the spectral power density of surface reverberation that takes into account the propagation conditions, distance, and beam pattern of the receiver system. For a distance of up to 1000 m, a water area depth of ∼20 m, and actually measured 3D roughness spectra in the Doppler frequency range of ±5 Hz, we have calculated the frequency-angle reverberation spectra for an acoustic background-radiation signal of 1.5 kHz. The obtained angle and frequency reverberation characteristics are compared with the results of acoustic measurements conducted using linear horizontal receiver antennas.