The results of experimental studies on the features of propagation and reception of broadband pulsed signals in an underwater sound channel (USC) at a distance of 300 km from the source are discussed. The characteristics of the formation of the pulse responses by reception of the phase-shift keyed signals with different frequency bands and durations of symbols are studied. From the point of view of the ray theory of sound propagation in a deep-ocean waveguide, a physical interpretation of the results is presented and practical conclusions are drawn for solving the problems of positioning of autonomous underwater platforms.

The results of an experiment conducted in September 2017 are considered with regard to the applicability of the acoustic "mudslide" effect for positioning autonomous underwater vehicles (AUV) operating substantially deeper than the axis of the deep sound channel (DSC). The results of the experimental study and numerical analysis of the effect of focusing acoustic energy in the bottom layer on the Sea of Japan shelf and transitional zone with the deep water (up to 500 m deep) for summer-autumn hydrological conditions are presented. Mathematical modeling of the propagation of acoustic waves in a waveguide reproducing the experimental conditions using the parabolic equation method has been performed. The effective propagation velocities of the signal are explained in terms of the modal-analysis method.

The results of an experiment conducted in September 2017 to substantiate the applicability of the “acoustic mudslide” effect when solving the positioning problems of autonomous underwater vehicles in cases of their operation at depths substantially exceeding the depth of the axis of the underwater sound channel are discussed. The results of the experimental studies and numerical analysis of the effect of an acoustic energy focusing in the near-bottom layer of the shelf and its transition into deep-water (up to 500 m) layers of the Sea of Japan for summer-autumn hydrological conditions are presented. Mathematical modelling of the propagation of acoustic waves in a model waveguide reproducing the experimental conditions using the parabolic equation method has been performed. The observed effective signal propagation speeds are explained using the normal wave method.

There are contradictory opinions on the contribution of the near-surface layer of bubbles to the attenuation of low-frequency sound in the ocean. Taking into account the new experimental data on the distribution of bubbles in seawater, it is shown that the influence of the near-surface layer of bubbles on the structure of the spatial decay in sound propagation can be significant at rather typical concentrations of bubbles in the near-surface layers of the ocean. A possible explanation for the contradictions is the spatial restructuring of the field in which the main effect of the bubbles is focused on the near distance; sound attenuation at a great distance is not affected.

Taking into account new experimental data on the distribution of bubbles in sea water, it is shown that the influence of a near-surface layer of bubbles on the structure of the spatial decay in the propagation of sound can be significant at fairly typical concentrations of bubbles in the near-surface layers of the ocean. In the presence of bubble clouds, a spatial restructuring of the field energy structure is observed, in which the main effect of the bubbles is concentrated at small distances, at the same time not affecting the sound attenuation at large distances.

For the first time, the reconstruction of variations in the duration over several hundred years was conducted on the basis of models developed by comparing hydrometeorological observations and detailed scanning of the chemical composition of bottom sediments accumulated during the corresponding period. The gauge models for the reconstruction of air temperature and ice cover of the water area for three sites in the northern part of the Chukchi Sea have been developed. During the reconstruction, an insignificant increase in the duration of the ice-free period during the Little Ice Age was established, which does not coincide with the changes in the temperature regime that took place. It is concluded that the dynamics of the inflow of warm Pacific waters through the Bering Strait had the predominant influence on the ice regime of the Chukchi Sea.

A correlation between acoustical nonlinearity, sound absorption, and scattering in a subsurface bubble-saturated layer is established. A model of effective parameters of a bubbly liquid is developed that allows one to obtain results coinciding with experimental field studies. It is shown that "bubbly clouds" under the sea surface increase substantially the sound scattering and the nonlinear acoustic parameter of the seawater.

The results of experimental study of the spatial structure of the scalar-vector acoustic field formed during towing of a tone low-frequency emitter over the shelf of the Sea of Japan are discussed. The experiment was accomplished by towing the source of a tone signal with a frequency of 134 Hz at a depth of 20 m over various acoustic tracks at distances up to 10 km from an integrated receiving system consisting of a receiver of acoustic pressure and three orthogonal components of the acoustic pressure gradient. Special attention has been focused on study of the interference structure of the scalar and vector fields with provision of the technical reliability of the method and the results of the experiment under controlled hydrological conditions. We discuss the quantitative characteristics and peculiarities of the interference formation along tracks that differ in depth. The unique results of comparing the horizontal and vertical components of the fields are most interesting of all. They allowed us to reveal the existence of eddy structures in the acoustic field of the source over several tracks. We analyze the possibility of practical application of the results of our research.

The paper discusses the results of an experiment conducted in the Sea of Japan in March 2016 on an acoustic track 194 km long in winter hydrological conditions. The most complex case of propagation of pseudorandom pulse signals from the shelf to deep water in the presence of gyre formation on the acoustic track. An analysis of the experimentally obtained pulse characteristics show that at all points, a maximum, in terms of amplitude, first arrival of acoustic energy is recorded. This is evidence that at a given depth horizon, pulses that have passed the shortest distance through a near-surface sound channel at small angles close to zero are received first. The calculation method of mean sound velocity on the track, based on the satellite data of surface temperature monitoring, is proposed. We expect that the results obtained with this method can be successfully used for the purposes of acoustic range finding and navigation.

The interlayers of the cryptotephra of different episodes of the catastrophic eruption of the Baitoushan volcano (Paektu-san, Changbaishan-Tianchi) in the 10th century were discovered in the sedimentary cover of Amur Bay in the Sea of Japan by the geochemical and paleomagnetic characteristics. The petrochemistry of the volcanic glass indicates the possible occurrence of pyroclastic material in the B–Tm layer and more recent episodes that have not been identified before in the sediments of the Sea of Japan. The impact of the eruption on the bay environment is noted. It is shown that the medieval state of Balhae occupying vast areas and adjacent to the volcano no longer existed after the more earlier episodes of eruption.

We present the results of sound scattering studies to estimate the distribution and dynamics of zooplankton in the upper sea layer under various conditions. The measurements of the sound scattering coefficients were carried out over different pathways during the ship motion and at individual stations at frequencies of 100–250 kHz. The studies were carried out in 2004–2014 in the Sea of Japan and in 2013 in the Eastern Arctic seas. The studies of sound scattering were performed simultaneously with net catches of plankton in situ. These data allowed us to study the details of correlation between the sound scattering coefficient and zooplankton concentration. The studies revealed significantly stronger sound scattering in the Eastern Arctic, which is related to the greater concentration of zooplankton, the migration of which differs strongly from the migration of plankton in the warm seas.