V. A. Akulichev

Acoustic Cavitation Thresholds of Sea Water in Different Regions of the World Ocean

Results of experimental measurements of acoustic cavitation thresholds are presented for the waters of the Atlantic Ocean, Pacific Ocean, Indian Ocean, Arctic Ocean, and some other parts of the World Ocean, including the Arabian Sea, Baltic Sea, East Siberian Sea, North Sea, Philippine Sea, Black Sea, Sea of Japan, Sea of Okhotsk, and South China Sea. The measurements were carried out by many oceanic expeditions between 1963 and 1987. General laws governing the variations in the cavitation strength of sea water over the World Ocean are revealed.

Acoustic tomography of dynamic processes in a sea shelf zone with the use of complex signals

Experimental data are presented on the use of single receiving and transmitting systems in acoustic tomography of dynamic processes in a shallow sea. The experiments are based on the use of the transmission tomography and opposite-direction sounding with complex phase-manipulated signals. The original data are those obtained by the authors in 1990–2000 on the shelf of the Sea of Japan near the Gamov Peninsula, in the vicinity of the acoustical-hydrophysical experimental site of the Pacific Oceanological Institute. A possibility of using combined transmitting-receiving systems (transceivers) for monitoring the temperature and fields of currents in the ocean is demonstrated.

Application of Complex Acoustic Signals in the Long-Range Navigation of Underwater Objects

The creation of modern technical means of research and exploration of the ocean based on submersibles of different designations is a priority field of science and technology development. Creation of intellectual autonomous unmanned submersibles (AUS), which provide multidisciplinary wide-scale study of sea basins and the ocean bottom and new basic knowledge in oceanography, marine geology, biology, and power engineering, is a pressing issue [1]. Significant progress in the development of electric power systems promoted the creation of an AUS with an operating coverage range of hundreds of kilometers. It has become reasonable to equip them with systems of transmitting and receiving low-frequency acoustic signals with a propagation range not smaller than the operating range of the submersibles as means for remote control of the operation and mutual maneuvering during their group operation. Up to the present, the development of such systems has been hampered by a lack of technical solutions for the measurement of the time of signal propagation from a sound source to a receiver over a distance of hundreds of kilometers with the required accuracy. In this work, we present the results of experimental studies of the propagation of low-frequency broadband pulse signals in an underwater sound channel (USC) using state-of-the-art technologies for synchronizing the receiving and transmitting channels. This allowed us to acquire unique data on the stability of acoustic wave propagation velocity in the USC for developing the technology of measurement of the distance between the sound source and the receiver. The main objective of the experiment was to discriminate the part of the signal propagating closest to the USC axis and to measure the time of its propagation from the source to the receiver. In order to do this, the method developed by the authors for sounding the sea medium by complex phase-manipulated signals was used. This technology allows us to distinguish and identify the arrivals of acoustic energy over different beam trajectories [2, 3]. The experiment was carried out in August 2006 in the Sea of Japan. Figure 1 shows the schematic geometry and methodological peculiarities of the experiment. According to the commands of the receiving vessel, complex signals (M-sequences, 511 symbols, and 4 periods of the carrier frequency per symbol) were transmitted with an interval of 5 min at a frequency of 600 Hz by the sound source stationary located near the bottom at a depth of 40 m and a distance of 450 m from the coast. The receiving vessel was represented by a yacht, which was used to deploy the radio hydroacoustic buoy with a hydrophone. The yacht was maneuvering under sail in the zone of reliable recording of radio signals. The hydrophone was lowered approximately to the USC axis, whose location was found from the measurement of the vertical distribution of sound velocity using the hydrological profiler from the yacht (Fig. 2). Signals were received at six points of the path at a distance from the transmitter ranging from 55 km to 368 km (Table 1). The buoy with the hydrophone was drifting in this process, and the coordinates were measured from the yacht using a GPS navigator when the yacht was passing close to the buoy. The signal information and marks of the common time system were recorded on a PC. The systems of common time based on thermally stabilized generators were included into the transmitting and receiving systems and were started before the beginning of the experiment. This allowed us to measure the times of signal propagation between the corresponding points with an accuracy not less than 10 ‐8 s. Processing of the information consisted in the calculation of the cross-correlation function between the received signals and the mask of the transmitted signal preliminary recorded by the PC. From two to four arrivals of acoustic energy, which propagated over different beam trajectories, were recorded in the pulse characteristics obtained using this method. The latest and maximal (in amplitude) arrival was identified as the one that passed near the USC axis, because it propagated near the minimum of sound velocity along the shortest path

Acoustic remote sensing of currents at the shelf of the Sea of Japan

Results of experimental studies of the field of currents in the shelf zone of the Sea of Japan are discussed. The studies were carried out in 2001–2002 near the Gamov Peninsula, in the region of the acoustical-hydrophysical site of the Il’ichev Pacific Oceanological Institute (Far East Division, Russian Academy of Sciences). The purpose of the studies was related to the problems of developing the systems for long-term remote sensing of the climatic variability of the sea medium and for acoustic tomography of the dynamic processes in a shallow sea. In the experiment, combined acoustic transmitting and receiving systems (transceivers) and complex phase-manipulated signals with a carrier frequency of 2500 Hz were used. The velocities of currents calculated from the acoustic data agree satisfactorily with the velocities measured by standard methods of oceanography.

Experimental studies of seasonal variation for temperature fields on the Sea of Japan shelf by acoustical methods

The results of experimental studies are given for the pulsed characteristic of an underwater channel on the Sea of Japan shelf depending on the seasonal variation of a temperature field.

Acoustohydrophysical Complex for Marine Tomographic Research

An acoustohydrophysical complex for tomographic ocean surveillance is described. The complex has been developed within the framework of the US–Russia project JESAEX (The Japan / East Sea Acoustics Experiment). Technical characteristics of self-contained and mobile receiving and transmitting systems of the complex allow one to use them as building blocks in constructing tomographic systems of various configurations and complexities. Experimental tests of methods and facilities in September–October 1999 in the Japanese sea have shown that the range of reliable reception of composite phase–shift keyed signals, such as M—sequences, is 700 km at an acoustic pressure produced by a sound source of up to 7 000 Pa/m.

The First Discovery of Cryptotephra of the Catastrophic Eruptions of the Baitoushan Volcano in the Tenth Century A.D. in the Shelf Deposits of the Sea of Japan

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.

Peculiarities of the Formation of the Interference Structure of Scalar-Vector Acoustic Fields on the Shelf of the Sea of Japan

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.

Chukchi Sea Ice Conditions for the Last Few Centuries: Reconstruction from Sedimentation Records

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.

Acoustical Nonlinearity, Sound Absorption, and Scattering in Bubble-Saturated Seawater

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.