Methodological and technical possibilities of monitoring temperature fields along a 1000-km track in the Sea of Japan using acoustic thermometry are presented. The proposed tomographic method for monitoring the dynamics and structure of waters is based on emission and reception of complex phase-shift keyed signals on a diagnosed path with determination of the propagation time along various ray trajectories with further measurement of the speed of sound and temperature. The physical prerequisites for practical use of thermometric studies at large distances are based on the acoustic “mudslide” effect: the phenomenon of the transition of acoustic energy from the bottom shelf area to an underwater sound signal of the deep ocean. A high-precision system of acoustic thermometry on the basis of tomographic schemes with mobile and stationary hydroacoustic emitters and receiving systems is proposed and tested with the example of the Sea of Japan.

The results of experimental and theoretical studies on the propagation and reception of broadband pulsed signals based on pseudorandom sequences are discussed. The features of impulse response functions for reception of signals with different frequency bands and durations of symbols are investigated. Separation of acoustic energy arrivals in the cross-correlation function of the received and emitted signals is investigated in the framework of the normal mode theory for a deep-sea waveguides. The study concluded that a combination of navigation signals with different symbols duration must be used for solving practical problems of autonomous underwater vehicles positioning.

An experimental study aimed at the measurement of arrival times and effective velocities of acoustical pulses propagating approximately along the edge of the continental shelf was conducted in the Sea of Japan in early autumn. In a posteriori theoretical analysis and modeling of sound propagation in this experiment modal structure of acoustical field along the path was described and a number of general conclusions on the formation of such fields in course of sound propagation over distances of tens and hundreds of kilometers were formulated. An algorithm for predicting effective velocities of long-range propagation of acoustical pulses in shallow water was developed on the basis of these conclusions. The algorithm is based on the averaging of group velocities of the first modal component of the pulse over the entire track. It was shown that on the considered path of approximately 136 kilometers horizontal refraction is one of the major factors contributing to the dispersion of pulsed signals. This effect also causes additional delays as compared to the sound propagation along the respective geodesic path on the Earth’s surface. Implications of the importance of taking the horizontal refraction into account in estimation of arrival times in the modeling of long-range propagation and the solution of acoustical ranging problems are also discussed.

A hydroacoustic autonomous receiving system that uses a combined receiver has been developed for carrying out investigations of the vector−scalar structure of acoustic fields under shallow water conditions with a submersion depth as large as 100 m. The acoustic fields are investigated by measuring the acoustic pressure and three orthogonal components of oscillatory acceleration using sensors with a sensitivity of 150−180 μV/Pa. In addition, data on the spatial position of the system from the orientation and depth sensors are synchronously recorded and, together with the data from acoustic channels, are saved to an SD card.

A hydroacoustic towed emitting system has been developed based on an electromagnetic source with a system of hydrostatic pressure control and compensation. The frequency range of emitted signals is 140–270 Hz, the sound pressure is as high as 3700 Pa, and the depth of immersion is 1–25 m. The system has been designed for carrying out targeted routine investigations in different seasons of the year in shelf water area using a small vessel. The system has been tested successfully.

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 examines how hydrological conditions affect manifestation of the acoustic “landslide” effect, which consists in focusing of acoustic energy in the near-bottom layer on the shelf and its transition to the axis of an underwater sound channel in deep water. We compare the results of experiments performed in the Sea of Japan in April 2014 and August 2006 on the same acoustic track, where the distance between corresponding points was more than 100 km. In April, the hydrological conditions in the shelf region of the track and in the upper layer of the deep-water part of the sea were characterized by the presence of a relatively weak (~0.35 s –1 ) negative vertical sound velocity gradient, whereas in August 2006, it was ~1.5 s –1 . Experimental and numerical studies showed that the acoustic landslide effect also manifests itself under conditions of a weak negative sound velocity gradient, but the structure of the acoustic field trapped by the underwater sound channel has a more complex character with a time-expanded pulse characteristic. Nevertheless, its ordered, stable, and well-identified structure at all track points chosen for measurements make it possible to reliably create an efficient (with accuracies to hundredths of a percent) underwater navigation systems like GLONASS and GPS for the spring hydrology season.

We describe a technique and hardware-software implementation for remote monitoring of sea currents and temperature using data from pulsed hydroacoustic sounding on stationary horizontal tracks in shallow sea water areas. As sounding signals, we used complex phase-manipulated signals based on pseudorandom M-sequences. The complex was tested in Posiet Bay in the Korea Strait in the Sea of Japan.