Experimental study of mode selection in shallow-water sea

Abstract

on how well the transmission and reception of probing signals are matched to the variability of the underwater (UW) waveguide [1][2][3][4][5][6][7][8]. Solving such a problem involves the optimal selection of the positions and sizes of antenna arrays and adaptive tuning of their aperture distributions to the waveguide characteris tics,which vary in time; reverberation disturbances and noise; and variations in the shape of the transmit ting and receiving arrays under the influence of under water currents [9][10][11][12][13][14][15][16]. Theoretical and experimental studies show that, in systems for long range acoustic monitoring of shallow water shelf regions of the ocean, pulsed signals produced by a small number of modes with frequencies of 100-400 Hz propagate well. The approach based on such a method can be called few mode pulsed tomography (FMPT) [9][10][11][12][13][14][15][16][17][18][19]. Efficiency in selectively exiting and receiving few mode signals can be achieved by optimizing the com plex factors along the apertures of vertically extended transmitting and receiving arrays [30][31][32][33][34][35]. According to theoretical estimates, successful matching of UW signals in FMPT requires transmission of highly coherent pulses, which cannot diffuse due to geomet rical dispersion and strongly attenuate in their propa gation. This makes it possible to use relatively weak, environmentally friendly signals, while maintaining high tomographic measurement accuracy. It is known that signals in shallow water UW waveguides suffer from a substantial geometrical dispersion that leads to low frequency underwater (LFUW) signals with a cumbersome interference structure, which results from coherent summation of a large number of indi vidual waveguide modes [38, 39]. The resulting speck lelike interference structure of the pulse, which ham pers sensing of inhomogeneities, can change signifi cantly as the phases of frequency components of individual modes vary. Thus, the pulse structure depends both on the variability of the medium and on variations in parameters of the transmitting and receiving systems; such a situation can lead to a decreased accuracy in tomographic reconstruction of inhomogeneities of the medium. In addition, the high number modes that form the specklelike struc ture of the UW pulse rapidly attenuate, and a substan tial part of the transmitted energy is absorbed by the bottom. To dampen the aforementioned interference noise, one should select individual modes of the waveguide or excite groups of modes, which form weakly dispersing wave beams in the waveguide [38, 39]. Here, we attempt to choose the optimal parame ters of the LFUW pulses basing on an analysis of cal Abstract-The possibilities of matching low frequency underwater sound pulses to the parameters of an oce anic waveguide are considered. The objective is to optimize the system of few mode tomographic observation in a shallow water sea. Experimental data are analyzed for two methods of selecting the low frequency few mode pulses propagating i...

Target strength in oceanic waveguides

“…As our analysis has shown, this quantity most frequently has a complex interference structure, which noticeably Because multistatic underwater observations are initially designed to adapt to the propagation condi tions, the use of the BTS in engineering calculations and estimates of such systems seems important. The possibilities of selecting energy carrying waveguide components, including the use of matched filtering methods, were also investigated in marine experiments [6,11,29,30]. In the acoustic designing of underwater observation systems, which is especially important for a multistatic underwater acoustic observation system, it is necessary to use multiparameter numerical BTS calculation models [3][4][5][6].…”

“…The diffracted signals of individual partial components are separated via spatiotemporal fil tering. This procedure implies the use of complexly modulated narrowband pulses and extended transmit ting and receiving arrays [1,2,8,11,12]. Among other components, there are additional waveguide compo nents that are repeatedly excited by complexly con structed bodies, e.g., by a set of reflective regions on the surface of a body with a complex shape, or components that are formed upon excitation of eigenmodes of elas tic vibrations in this body [7,8,13].…”

The spatio-temporal structure of wideband acoustic pulses in shallow sea

“…However, the creation of radiating arrays is a complex engineering problem, so that it is interesting to study the possibilities of using point wideband hydroacoustic sources for the generation of wideband pulses with complex modulation that are successfully used in radar to achieve high sensitivity and resolution. In shallow sea, the use of wideband pulses is impeded by the presence of the modal and intermodal dispersion, owing to which the pulse structure is distorted with increasing distance between the source and the observation point [4][5][6][7][8][9][10]. In particular, experiments show that the pulses exhibit time spreading and form complex spatio-temporal interference structures that are unstable with respect to even relatively weak variations in the waveguide parameters, the radiating and receiving elements, and random variations in the medium properties [4].…”

“…In shallow sea, the use of wideband pulses is impeded by the presence of the modal and intermodal dispersion, owing to which the pulse structure is distorted with increasing distance between the source and the observation point [4][5][6][7][8][9][10]. In particular, experiments show that the pulses exhibit time spreading and form complex spatio-temporal interference structures that are unstable with respect to even relatively weak variations in the waveguide parameters, the radiating and receiving elements, and random variations in the medium properties [4]. In the case of radiation of lowfrequency pulses (in the range 10 − 20 Hz) under typical shallow-sea conditions, the structure of the pulses is determined by a relatively small number of energy-carrying modes, as well as the Rayleigh-Stoneley waves and the lateral waves [5][6][7][8][9][10].…”

“…The results of theoretical and experimental studies show that such pulses can be formed by using vertically oriented arrays of radiators, which makes it possible to excite weakly-attenuated high-coherence low-mode pulses matched to the hydroacoustic waveguide [2,3]. It is also important that such low-mode pulses are distorted due to the influence of the geometric dispersion relatively slowly when propagating under the shallow-sea conditions [2][3][4]. However, the creation of radiating arrays is a complex engineering problem, so that it is interesting to study the possibilities of using point wideband hydroacoustic sources for the generation of wideband pulses with complex modulation that are successfully used in radar to achieve high sensitivity and resolution.…”