Methodology for the Practical Implementation of Monitoring Temperature Conditions over Vast Sea Areas Using Acoustic Thermometry

: The methodological and technical possibilities of monitoring temperature fields in the Sea of Japan by acoustic thermometry methods are presented. The proposed tomographic method for monitoring the dynamics and structure of water is based on the transmission and reception of complex phase-shift keyed acoustic signals on a diagnosed track with the determination of the travel time along various ray trajectories, followed by the sound speed (temperature) determination. The physical prerequisites for the practical implementation of thermometric studies at large distances are based on the acoustic “mudslide” effect—the phenomenon of the acoustic energy “injection“ from the near-bottom shelf area to the underwater sound channel of the deep ocean. Based on the Sea of Japan example, an acoustic thermometry system based on tomographic schemes with mobile and stationary hydroacoustic sources for promising work in the field of oceanic climatology is proposed. For numerical calculations of the signal propagation channels' impulse responses between sources and receivers, a specialized database of oceanological information was formed for the north-western part of the Sea of Japan. The database includes all available data from organizations in Russia, Japan, North Korea, the Republic of Korea, and the United States (23,247 stations completed from 1925 to 2017). In the example of the Sea of Japan, a high-precision acoustic thermometry system based on tomographic schemes with developed mobile and stationary hydroacoustic transmitting and receiving systems was proposed and experimentally tested.


Introduction
Acoustic tomography (thermometry), used to measure the structure and dynamics of deep and shallow water areas, has been demonstrated in several experiments over the past decades [1][2][3].They showed that it was possible to measure temperatures along acoustic paths up to 18,000 km [4] with an accuracy of several millidegrees per year [5][6][7][8] and that it was possible to establish the origin of the characteristic features of eddies, fronts, flows, tides and internal waves.
The current article discusses the methodological (Sections 2.5-2.6) and technical (Sections 2.1-2.5)possibilities of monitoring temperature fields in the Sea of Japan using acoustic thermometry methods.These methods are especially preferred due to the presence of a constant and dynamic polar front, the features of which are often not detected by satellite telemetry sensors.
The Sea of Japan is a semi-closed basin with an average depth of 1500 m (maximum-4049 m), connected with the Pacific Ocean by narrow and shallow straits.In the south is the Tsushima Strait between Korea and Japan, which allows the Tsushima Current (Kuroshio Arm) to enter this sea and rush northward with an exit through the Tsugaru Strait into the subpolar northern zone of the Pacific Ocean.
This warm (the temperature difference can be more than 5 °C) saline water forms an upper layer in the basin's southern part, separated from the northern region by a polar front at about 40 ° N. The maximum horizontal temperature gradient of the front is about 8 °C/100 km and is strongest near the east coast of Korea, at about 100-m depth.Warmcore eddies are shed to the north, and cold-core eddies to the south [9].
From September to October 1999, together with specialists from the United States and the Republic of Korea, a 40-day experiment was carried out in the Sea of Japan to study the spatial dynamics of warm water masses of the polar front [9].Sound sources installed at the bottom in the shelf zone of Posyet Bay were emitting acoustic signals towards receiving systems located at a distance of 558 km south of the northern coast of Ulleung-do Island (Republic of Korea).An analysis of the impulse responses of the waveguide showed that the position of the polar front on October 20 th was shifted 20 km south of its average position during that month.The results of this work marked the beginning of a series of studies aimed at studying the variability of the temperature and sound velocity fields, which are directly related to temperature in the Sea of Japan for various applications (underwater navigation, communications, acoustic ranging).
Section 2.6 presents an example of the practical implementation of the application of the information and reference system [10] in terms of the analysis and forecast of longterm climatic data of hydrological measurements in the northwestern part of the Sea of Japan.The analysis is given in comparison with the experimental data obtained from 2006 to 2022.
In Section 3, an experiment in August 2022 on an acoustic path 1073 km long is discussed as an illustration of the methodological and technical implementation of the method of acoustic thermometry.

Methods and Equipment for Studying the Temperature Dynamics
The main issues for performing studies of the climatic variability of temperature in the northwestern part of the Sea of Japan using acoustic thermometry are associated with the selection of the necessary receiving and transmitting equipment and its optimal deployment at the acoustic-hydrophysical marine research station of the Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences (POI FEB RAS), located at Cape Schultz.The favorable geographical location and developed technical infrastructure of the station made it possible to plan and carry out unique experiments in the field of hydroacoustics and oceanology.The bathymetric characteristics of the shelf zone allow the use of the "acoustic mudslide" effect (by deploying acoustic sources near the coast at shallow depths) for the implementation of the method of acoustic thermometry along propagation paths of various lengths and over necessary time intervals, including year-round.Below are the methods and technical solutions selected for solving the identified problems.

Transmitting Systems. Hydroacoustic Sources
A technically and economically efficient solution to the problem of assessing climate variability is to equip the water areas of interest with an acoustic thermometry system based on tomographic schemes with mobile and stationary hydroacoustic transmitting and receiving complexes.The results of research in the field of acoustic thermometry show sufficient sensitivity of the method for recording the variability of large-scale hydrophysical and hydrological processes in the ocean.Below several examples of hydroacoustic sources used for carrying out marine experimental work are described.
A broadband hydroacoustic transducer (Figure 1a) is an unloaded type structure (freely filled with water when immersed) of two docked coaxial ring piezoceramic transmitters supplemented at the ends with rigid cylindrical baffles.A single piezoceramic transducer has the following dimensions: the ring outer diameter is 1100 mm, the inner diameter is 970 mm, and the ring height equals 225 mm.Baffles made of rigid material serve to reduce the effect of acoustic closure and, together with the rings, form a total volume of the medium inside the transducer, which results in the resonant frequencies of the latter within the band from 300 to 800 Hz.Piezoceramic transducers and screens are interconnected by means of intermediate steel ring flanges on studs, which provide stability and rigidity to the structure.The emitter is mounted on a power tubular support frame with dimensions of 2 m × 2 m with the estimated position of the acoustic center at a level of 1 m above the ground.The total height of the assembled structure is 1.5 m, and the diameter is 1.2 m.The piezoceramic transducer, in terms of the ratio of the cylinder thickness to the radius, belongs to the category of "thin" and is characterized by a broadband spectrum.This was confirmed during the calibration of the source in natural conditions.The high reliability of piezoelectric transducers and the structural elements' applied materials ensure the source's long-term operation (Figure 1b).In the course of the development of the acoustic thermometry studies, a modified source consisting of three piezoceramic rings was also created.It was used for stationary setting with the output of cable lines connecting it to the shore in the 2020s experiments (Figure 2).The dimensions of the source support frame ensure its stability and the position of the acoustic center at a level of 1.5 m above the seafloor.With the existing variety of geographic, hydrophysical, and hydrological conditions of the Far Eastern seas, authors solved the problems of constructing tomographic schemes using broadband sources of smaller weight and size characteristics built into mobile and autonomous devices.The latter made it possible to increase the efficiency of deploying equipment in the water area, to carry out short-term transmissions by drifting equipment, and to carry out research in a wider range of depths, not limited to the depths of ship anchorages.For this purpose, an autonomous version of the "loaded" type source based on two piezoceramic rings with an outer diameter of 1100 mm was used.The source (Figure 3a) was supplemented with means for controlling the depth of immersion and undocking of the ballast.The transducer is suspended on a separate buoy and is connected by a cable line to a surface beacon transceiver (Figure 3b), which has an autonomous power supply for two days of energy-intensive operation.The radio transceiver allows one to control the operation of the source and use it to transmit its own coordinates and service information to the receiving post via a hydroacoustic channel to a signal reception point hundreds of kilometers away.Another type of transducer used for both long-term stationary and short-term settings belongs to the type of "loaded" piston bicone structures [11].Due to the presence of a powerful rod transducer and design features, it produces high sound pressure levels (over 10 kPa at a distance of 1 m), which is required for long acoustic paths (Figure 4).In the last decade, authors experimental studies in the Sea of Japan using stationary, mobile, and autonomous transducers (Table 1) allowed them to acquire significant methodological and technological experience in the organization of several variants of tomographic schemes and their adaptation to water areas characterized by the different spatial scale and the bottom topography.The method of acoustic thermometry on the scale of the Far Eastern seas can be implemented using any combination of pairs of the presented sources.The tomographic method for monitoring the dynamics and structure of waters is based on the transmission and reception of complex phase-shift keyed signals on diagnosed tracks with the determination of the propagation time along various ray trajectories, followed by the determination of the speed of sound (temperature).To implement the method, an improved receiving system in the form of a beacon has been developed and tested to transmit hydroacoustic information from a hydrophone located at a given depth over a distance of up to 20 km to a support vessel or to a coastal post for processing.
When carrying out measurements, a stationary or drifting scheme (with precise fixation of coordinates) for setting up a beacon can be used.The beacon equipment makes it possible to record acoustic data on an SD card, and the results of correlation processing (the time of detection of correlation maxima and their amplitudes), the depth of penetration of the hydrophone, and own coordinates are transmitted via a radio channel to the receiving unit.The receiving unit is designed to receive data from the beacon and calculate the propagation time and sound speed (temperature).All the information received with the help of specialized software is recorded on a personal computer.
The radio beacon consists of surface and submersible parts connected by a cable line.The appearance and block diagram of the beacon is shown in Figures 5-7.The submersible part is a pyramid-shaped truss made of stainless steel, with a hydrophone fixed with rubber braces in the center.The pre-amplifier, the circuit for analogto-digital conversion of acoustic signals from the hydrophone, and the electronic board of the depth sensor module are located in a sealed container with a connector for the power cable and digital communication with the surface part.The surface part of the beacon consists of two spaced hermetic containers fixed on the mast.The containers contain a power supply unit, a correlation processing module, an acoustic signal recording circuit on an SD card, a GPS receiver, and a radio modem with a Very High Frequency (VHF) transmitting antenna.Correlation processing is carried out on the basis of the Cyclone V FPGA.
The receiving unit consists of a radio modem with a VHF receiving antenna and a personal computer with specialized software connected via an RS-232 interface.This development's advantage is mobility, as it allows deployment and retrieval at sea in the shortest possible time.When carrying out acoustic research, removing the receiving system from the side of the vessel reduces noise and minimizes its drift.The presence of GPS data allows one to track the current location of the buoy and greatly facilitates the identification of its location at sea.In addition, the ability to measure temperature at a distance of up to 20 km from the receiving post makes it possible to implement year-round monitoring in the shelf zone with data reception onshore (Figure 8).

Receiving Systems. Vertical Autonomous Receiving System
For deep-sea research, a distributed vertical autonomous receiving system was specially designed, manufactured, and tested in natural conditions.Its design assumes an arbitrary depth distribution of hydroacoustic signal reception points, both for verifying the numerical simulation of sound propagation and for simulating the position of the submersible hydroacoustic antenna at these points.The system is an array of four hydroacoustic digital autonomous recorders suspended through elastic shock-absorbing lounges to the buoyancy group and the surface buoy.Each self-contained recorder is a sealed stainless-steel container with a hydrophone and a depth sensor.The container contains a preamplifier with bandpass filtering, a circuit for analog-to-digital conversion and registration of acoustic signals from a hydrophone and a depth sensor on an SD card, and a battery pack (Figure 9).The system includes the following elements: a nylon halyard with a diameter of 8 mm, a set of buoyancy, a selective end, a landmark pole, shock-absorbing lounges, a set of quick-release clamps, an end weight or an anchor device (Figure 10).The landmark pole is equipped with a GPS receiver and a radio transmitter for realtime tracking of the location of the system by the accompanying research vessel.With the help of quick-release clamps, autonomous registrars, lounges, and buoyancy are attached to the halyard.At the same time, the deployment depth of each of them is set according to the markings on the halyard.Changing the depth of the recorders is carried out after retrieving the entire system on board the ship with their subsequent reinstallation to a new position, according to the marking.Depth-separated shock-absorbing longs provide protection for the halyard and hydrophones from vibration caused by surface waves and currents.All elements of the system are promptly dismantled during sampling.An electromechanical winch is used for setting and hauling the system.

Temperature Measurement Accuracy
The acoustic method for measuring the integral temperature in the Underwater Sound Channel (USC) axis is based on determining the propagation time of acoustic signals at a known distance between the source and the receiver.From the obtained mean sound speed along the path, the known values of the depth of the USC axis and salinity, according to the algorithm by Chen and Millero [12], widely accepted in oceanology, the temperature is calculated.Thus, the error of the result is determined by the error in measuring time and distance.The developed equipment calculates the arrival time of signals (maxima of correlation function peaks) with an error of ∆τ = ± 0.001 s.The receivers used as part of the GPS devices provide position determination with an error of ∆d = ±5 m.For the acoustic track with a length of 1073 km and propagation time of 736.19 s: Using the mentioned algorithm for calculating the temperature depending on salinity, which practically does not change along the track during the year and has 34.0-34.04‰ (see below), pressure (depth) and sound speed, the temperature measurement error on the USC axis will be ∆t = ±0.002°C.
When calculating the temperature (from the surface to the deepening of the USC axis) with multipath propagation, the error is determined by the time resolution of the applied acoustic signal [13].It is equal to the duration of the M-sequence symbol.For the M1023 signal (Section 3) with a symbol duration of 0.01 s, the temperature measurement error will be ∆t = ±0.007°C.For an M127 signal with a symbol duration of 0.1 s, the temperature measurement error is ∆t = ±0.044°C.

Practical Implementation of the Information and Reference System Application on the Example of the Sea of Japan. Long-Term Climatic Data of Hydrological Measurements in the Sea of Japan
This section provides an example of the practical implementation of the information and reference system application [10].This system analyzes and predicts long-term climatic data of hydrological measurements in the northwestern part of the Sea of Japan compared with the experimental data obtained from 2006 to 2022.
For a more accurate representation and prediction of the hydrological situation, including the characteristics of the USC (temperature, salinity, sound speed) in the Sea of Japan, the results of long-term oceanological observations available at POI FEB RAS were used.A specialized array of oceanological information was formed for the northwestern part of the Sea of Japan to obtain the initial data for numerical calculations of the signal propagation channel characteristics between sources and receivers.The array includes all available data from organizations in Russia, Japan, North Korea, the Republic of Korea, and the United States.These are stations with bathometric observations (with a sparse vertical resolution, which is equal to or close to standard horizons) and CTD observations, which have a higher vertical resolution.This also includes observations of drifting buoys (Profiling Autonomous Lagrangian Circulation Explorer-PALACE).Only oceanological stations with simultaneous temperature and salinity observations were left in the formed array, which makes it possible to calculate the sound speed, which is the main characteristic in solving thermometry problems.
In the first stage, the procedure for eliminating duplicate stations was carried out, which is inevitable when summarizing the massive observational material taken from various sources.Then, unreliable temperature and salinity values were rejected using statistical methods and regional features of the selected polygon.After eliminating duplicate and inaccurate information, the resulting oceanographic array has 23,247 stations completed from 1925 to 2017.
At each oceanographic station, temperature and salinity values were interpolated to horizons multiple of 5 m.When processing bathometric observations, the linear interpolation of values was used.CTD observations were brought to horizons multiple of 5 m using the median procedure (capturing above and below observations at a distance of 2.5 m from the calculated one).Then, at all horizons, the speed of sound was calculated using the Chen-Millero formula.Using these values, vertical sections of sound speed, water temperature, and salinity were plotted.
Then, at all stations, the horizon with the minimum values of the sound speed (USC axis) was determined.Moreover, this horizon could not be the last one at a particular station.At this horizon, sound speed, water temperature, and salinity values were chosen.Further, for the selected period of time, the calculation of the average long-term values of all parameters was carried out within the limits of trapezoids 0.25° × 0.25° in the direction of the meridians and parallels.Based on these data, the construction of spatial distributions of the USC depth and the sound speed in it, water temperature, and salinity along the selected routes were carried out.
Figure 11 shows the sound speed field on the section from the outer edge of the shelf to the Kita-Yamato bank during the period of maximum surface water warming (from 15 August to 15 September).This illustrates the spatial changes in the USC depth and its sound speed (the small dotted line indicates an approximate finding of the USC axis according to long-term measurements, and the dotted line with circles denotes the path with the measurement points).The analysis shows that from the outer edge of the shelf to the Kita-Yamato bank (at a distance of up to 300 km from Cape Gamov), the sound speed in the USC changes little (varying about 1455.5 ± 0.5 m/s).Moreover, on this part of the route, the USC depth is slowly deepening towards the sea from 150-200 m to 250 m.Above the Kita-Yamato bank, there is a significant increase in the sound speed (up to 1458 m/s) and the USC depth (up to 400 m).Then, in the zone of the Tsushima current, the USC depth and the sound speed in it stabilize, amounting to 400 m and 1458 m/s, respectively.
Figure 12 shows the average long-term spatial distributions of the USC parameters within the entire polygon in August.Comparison of the sound velocity field in the vertical section (Figure 11) with the USC characteristics, shown in Figure 12, shows that the section data well reflect the regularities of the USC parameters throughout the entire poly.To assess the adequacy of the reflection of the acoustic situation by climatic data, a comparison was made of the USC characteristics, measured during experiments in different years and months, and the long-term data obtained.In Figure 12a,b, the dots indicate the places where the authors of the project carried out measurements at oceanographic stations of water temperature and salinity (sound speed) in different years in the summerautumn period.Figure 13 in the upper part shows the dependences of changes in the sound speed on the USC axis on distance, obtained experimentally (stars) and from longterm data (brown dots).A comparison of the given data (Figures 12a and 13) shows that the sound speed values on the USC axis on both dependencies are mainly located within 1455-1456 m/s (points on the white field of Figure 12a were not considered).Experiments on the same track from 2019 to 2022 showed that the sound speeds at distances from 300 to 330 km were about 1457 m/s with a slight deepening of the USC axis, which was located at the horizons of 200-250 m.This fact may indicate that the polar front zone in 2019-2022 was moved to the north.Therefore, the part of the acoustic track at a distance of 300-330 km was in the zone of warmer transformed Pacific waters.
Let us consider the possibility of monitoring temperature fields in the Sea of Japan by acoustic thermometry using the example of an acoustic track in the Sea of Japan (from the shelf near Cape Schultz to the Kita-Yamato bank (see below).The length of the acoustic track is 317.8 km.The beginning of the track was at the point with coordinates 42.3745° N and 131.2352°E, and the end was at the point 39.9666° N and 133.2813°E. Average longterm calculations were performed at 32 points, with a distance of 10.25 km between them.All currently available data of oceanographic observations were used (23,247 stations performed from 1925 to 2017).Based on these data, with a discreteness of 5 days, the average weighted values of the USC parameters along the acoustic track (sound speed, temperature, and salinity) were calculated.Figure 14 shows seasonal changes in sound speed and water temperature along the track.Noteworthy is the good correspondence between intra-annual changes in sound speed and water temperature, with minima in early February and maxima in mid-November.They correspond to the period of maximum development of winter convection and the peak of maximum heating of intermediate waters in the northwestern part of the Sea of Japan.The correlation coefficient between these series is 0.96 (at Rkrit.= 0.23).The sound speed data also correlate quite well with the water salinity on the track, but the correlation coefficient between these series, although significant, is significantly lower-0.47.Here it should be noted the scale of intra-annual changes in the average weighted values of salinity along the path, which are in the range from 33.99 to 34.05 ‰.The obtained results showed that in the presence of a long-term series of effective sound speed values on this track, it is possible to obtain an estimate of climate changes in the Sea of Japan region south of Peter the Great Bay.To do this, it is sufficient to use linear dependences of the sound speed with temperature and salinity even for fixed periods (for example, for January-February, and November).As a result, we can (as a first approximation) attribute a particular year to cold or warm.Moreover, taking into account the positive correlation coefficients of the sound speed with temperature and salinity (only by high values of the effective sound speed along the path), one can assume a northward shift of the polar front and transformed Pacific waters (with increased values of temperature and salinity).A very important circumstance is that, against the background of stable values of sound speed on the USC axis, significant (from 100 to 250 m) changes in the USC depth axis with distance are observed on both dependences (lower part of Figure 13, green stars-experiment, brown dots-long-term data).This means that, on the whole, longterm climate data correctly describe the patterns of formation and quantitative USC characteristics.Therefore, they can be used in the technical and methodological implementation of acoustic thermometry in a given area.

Experimental Validation of the Developed Method and Equipment for Acoustic Thermometry
This section discusses the results of a validation experiment.The experiment illustrates the technical implementation possibilities of acoustic thermometry using the method and the equipment described in Section 2. The experiment was conducted using the experience of the authors' previous works [14][15][16][17][18][19].
Source deployed near the coast near Chekhov (Sakhalin Island) at a depth of 34 m transmitted every 5 min at a pulse signal consisting of several phase-shift keyed pseudorandom M-sequences of signals: 1023 symbols long with a filling of 4 carrier frequency periods per symbol (hereinafter M1023) and 127 symbols with 40 periods per symbol (hereinafter M127).Both signals had the carrier frequency of 400 Hz but different effective frequency bands: for M1023, it was the band 300-500 Hz (signal duration 10.23 s), while for M127, the band was 390-410 Hz (12.7 s).The acoustic pressure reached the value of 198 dB re 1 µPa at 1 m from the source.Based on the sonar buoy, the receiving system drifted near the support vessel at a distance of 1073 km from the transmitter (Figure 15).The drift track coordinates were recorded every minute and considered when calculating the distance between the transmitting and receiving systems.The vertical sound speed and temperature profiles were measured close to the receiving system and at distances of 271.3, 404.3, and 652.5 km from the source (Figure 16).An analysis of these dependencies shows that the USC axis at all points was at a depth of about 200 m.In layers above the axis, the sound speed and the temperature increased when approaching the receiving system.The receiving system hydrophone was located at the USC axis, and the received signal information was communicated to the surface buoy and transmitted via radio to the receiving ship.Figure 17   It is very important that this value and the corresponding temperature (1.31 °C) are equal to the sound speed and the temperature on the USC axis, measured by the CTD probe.This indicates the method's accuracy and the quality of the measuring instruments.
It should be noted that the impulse response estimated using M127 signals differs significantly from the one obtained using M1023 signals.The former allows the detection of early arrivals of acoustic energy due to the longer duration of the symbols, which makes it possible to integrate the energy over rays with large arrival angles at the receiving point.This is due to the better stability of the M127 signal, compared to M1023, to inter-symbol interference, which worsens the possibility of correct demodulation of the M-sequence symbols and, as a result, the identification of weaker early arrivals of acoustic energy.It can be argued that the shift of the impulse response of M127 signals relative to the maximum arrival towards shorter time values confirms the above-mentioned fact of warming of the upper layers, noted in the CTD measurements (Figure 15).Thus, the presented results demonstrate the capabilities of the methodology discussed in the article for monitoring temperature regimes and their variability over extended water areas in various layers (from the surface to the deepening of the USC axis) of the studied waveguides.

Conclusions
1.The results of the studies on acoustic thermometry show the sensitivity of the method for recording large-scale hydrophysical and hydrological processes that affect climate variability in the Sea of Japan.For example, in Section 2.6, based on the results of long-term instrumental sound speed (temperature) measurements on the USC axis along the acoustic track from the shelf of the Peter the Great Bay to the Kita-Yamato Bank, it can be argued that the polar front zone in 2019-2022 shifted to the north.Therefore, the part of the acoustic track at a distance of 300-330 km is located in the zone of warmer transformed Pacific waters.2. A specialized database of oceanological information for the northwestern part of the Sea of Japan has been formed, which includes all available data from organizations in Russia, Japan, the DPRK, the Republic of Korea, and the USA (23,247 stations completed from 1925 to 2017).Calculations using it showed that on the acoustic track (shelf of Peter the Great Bay-Kita-Yamato Bank) in the northwestern part of the Sea of Japan, a good agreement was obtained between intra-annual changes in sound speed and water temperature with minima in early February and maxima in mid-November (Figure 14).The correlation coefficient between these series is 0.96.3. The results of the demonstration experiment confirmed the correctness of the methodological and technical approaches to the practical implementation of the acoustic thermometry method of extended marine areas.Quantitative estimates of average temperatures in the active layer of the marine environment of the Sea of Japan along a thousand-kilometer acoustic path have been obtained and confirmed by instrumental measurements.The advantages of using phase-shift keyed signals with long symbol durations are shown to ensure the reception and analysis of acoustic energy along ray paths with steeper angles to obtain information about the temperature in the entire active layer of the waveguide.

Funding:
The work was financially supported by the FSTP "Substantiation of the system for climate monitoring of the Far Eastern seas and development of methods for monitoring extreme weather and climate events associated with ocean, based on stationary and mobile measuring systems and multisensory sounding".
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.
Data Availability Statement: Not applicable.

Figure 1 .
Figure 1.Stationary broadband hydroacoustic source: (a)-appearance, (b)-after 12 years of operation at a stationary position at sea.

Figure 3 .
Figure 3. Autonomous broadband hydroacoustic source (a) and its surface buoy with power supply and radio transceiver (b).

Figure 5 .
Figure 5.View of the deployed beacon.

Figure 6 .
Figure 6.Block diagram of the surface part of the beacon.

Figure 7 .
Figure 7. Block diagram of the submersible part of the beacon.

Figure 8 .
Figure 8.The scheme of the shelf zone monitoring.

Figure 9 .
Figure 9. Block diagram of a stand-alone digital recorder.

Figure 10 .
Figure 10.Scheme of setting up a distributed receiving system.

Figure 11 .
Figure 11.The average long-term sound speed distribution at the acoustic track Cape Gamova-Kita-Yamato bank.

Figure 12 .
Figure 12.Map of depths and sound speeds on the USC axis, built on the basis of long-term observations for August: (a) Average sound speed (m/s) on the USC; (b) the USC depth of occurrence (m) based on the results of long-term data and the position of the control CTD measurements in the summer-autumn period.

Figure 13 .
Figure 13.Dependences of changes in the sound speed on the USC axis and the depth of the axis from a distance were obtained experimentally and from long-term data.

Figure 14 .
Figure 14.Seasonal changes in the average weighted values of sound speed (a), temperature (b), and water salinity (c) in the USC on the shelf route near Cape Schultz-Kita-Yamato Bank.The horizontal axis is months.

Figure 16 .
Figure 16.Vertical sound speed (left) and temperature (right) distributions (measured by CTDprobe) at given points at distances of 271.3, 404.3, 652.5, and 1073 km from the source.
shows the waveguide impulse responses, which were obtained as a result of the convolution of the received signals with the transmitted ones.On all impulse responses at the same time (736.19s), the last maximum arrival of acoustic energy is recorded.The respective energy propagates along ray trajectories near the USC axis with minimal horizontal velocity (1,073,144 m/736.19s = 1457.7 m/s).

Author
Contributions: G.D.-problem statement, discussion, and writing the article, Y.M.-problem statement, discussion, and writing the article.A.B.-data processing, discussion, and writing the article.V.L.-data processing, discussion, and writing the article.V.B.-equipment development, discussion, and writing the article.A.G.-data processing, discussion, and writing the article.A.T.-equipment development, discussion, and writing the article.All authors have read and agreed to the published version of the manuscript.
2.3.Receiving Systems.Equipment for Monitoring the Variability of Temperature in Offshore Marine Zones