No abstract

Underwater acoustic echosounding for surface roughness parameters retrieval is studied in a frequency band that is relatively new for such purposes. During the described 2-weeks sea experiment, 1–3 kHz tonal pulses were emitted from an oceanographic platform, located on the northern Black Sea shelf. Doppler spectra of the resulting reverberation were studied. The frequency band of the acoustic system, selected for this study, is chosen due to the fact that the sound propagation range is large enough for remote sensing in a coastal zone, and the resolution cell size does not limit the research. Backscattering of acoustical signals was received for distances around two nautical miles. However, it turned to be quite difficult to interpret the obtained data since backscattering spectrum shape was influenced by a series of effects, resulting in a complicated link to wind waves and currents’ parameters. Significant wave height and dominant wave frequency were estimated as the result of such signals processed with the use of machine learning tools. A decision-tree-based mathematical regression model was trained to solve the inverse problem. Wind waves prediction is in a good agreement with direct measurements, made on the platform, and machine learning results allow physical interpretation.

220When the monostatic sonar is used with long tone pulses the duration of which is 5-10 s, the main mech anism responsible for the broadening of the received signal spectrum is reflection from a rough sea surface. The computational schemes used for calculating the parameters of reverberation are usually based on the Bragg scattering of an acoustic field by the spatial structure of wind waves [1][2][3][4][5]. In [6,7], for a homoge neous waveguide with a constant depth, we derived and analyzed an expression that, in application to insonification of a shallow water area with tone sig nals, allows calculation of the normalized reverbera tion spectrum in the form (1)Here is the ratio of the power spectral den sity of reverberation at the point r 0 to the direct signal level p(r 0 ) measured at the same point,(2) k is the wave number of the insonifying tone signal with a frequency f; H is the characteristic depth of the waveguide;is the ray capture angle in the waveguide; χ m and χ n are the grazing angles corresponding to the modes of the homogeneous waveguide with the numbers m and n and the longitudinal wave numbers k m and k n , respectively; and is the power spectral density of surface waves, which is mea sured at a frequency Ω with the wave vector K deter mined by the formula ,where r and r 0 are the vectors determining the coordi nates of the scattering point and the reception point with respect to the source position (see Fig. 1). Here and below, the wave vector of surface waves is denoted by K and the wave vector of acoustic waves by k. From the product of quantity (1) and the square of quantity (2), we easily obtain the expression for the nonnormalized reverberation spectrum,which is convenient for use, e.g., in the case of monostatic sonar (r 0 = 0) or in calculating the rever beration for pulsed radiation modes.The reverberation spectrum for frequencies Hz is mainly formed by scattering by a rough sea surface; therefore, it is convenient (see para graph 8.12 in [8]), for the given frequency band, to ( ) 2 , G Ω K sgn , ( ) m n = − Ω K K r 0 0 , ( ) , m n m n k k r − = + − r r r K r r r ( ) ( ) ( ) ( ) ( ) ( ) 0 0 sgn 2 2 2 2 2 2 0 4 , 2 , sin sin 4 , cos cos ( ), , m n rev m n m n m n S k p p H G ds r0.01 Ω π ≥ Abstract-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 mea sured in shallow water areas, the spectral levels of reverberation are estimated in the frequency band of acous tic signals within 0.4-2 kHz.Keywords: scattering by the rough sea surface, scattered signal spectrum, acoustic reverberation, Bragg scat tering, shallow wate...

No abstract

No abstract