by Alexander LUCHININ, Dr. Sc. (Phys. & Math.), Deputy Director for Scientific Work of the RAS Institute of Applied Physics (IAP), Director of the Department of Hydrophysics and Underwater Acoustics, Alexander MALEKHANOV, Cand. Sc. (Phys. & Math.), leading researcher of the same institute

The study of wave processes in the thick and on the surface of the ocean, search for radiophysical methods of its distance diagnostics are the key trends of research of the IAP Department of Hydrophysics and Underwater Acoustics created in 1988.

The World Ocean is the main exploration target, kind of habitat for its employees.

MULTIWAVE HYDROPHYSICS

Despite novelty of the problems which were set when establishing the Institute, hydrophysics and underwater acoustics didn't start developing here from scratch. The fundamentals of a number of their sections were laid in the 1960s-1970s by employees of the Scientific Research Radiophysical Institute Vladimir Talanov (academician since 1992), Vitaly Zverev (Corresponding Member of the USSR Academy of Sciences, then Russian Academy of Sciences, since 1979), Lev Ostrovsky, Dr. Sc. (Phys. & Math.), Lev Dolin, Cand. Sc. (Phys. & Math.) and their followers. A "cultural layer" of these pioneer works on wave physics in inhomogeneous media allowed the Institute of Applied Physics to rather quickly not only familiarize itself in a new area but take up the leading positions in several key directions. The fact that Academician Andrey Gaponov-Grekhov - the IAP founder and director during a quarter of the century - had undertaken scientific management of all works on perspective wave technologies of study and diagnostics of the ocean contributed to success to a considerable extent. In 1987, he replaced Academician Anatoly Alexandrov,* who had given very active support in his time at the position of chairman of

* From 1975 to 1986 - President of the USSR Academy of Sciences. - Ed.

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Breakup of along tide wave into a sequence of intensive solitary internal waves (solitons) on the shelf of the US Atlantic coast line. Further evolution of solitons at the distance of 20 km was reconstructed within the limits of the suggested model.

Research Council at the USSR RAS Presidium on complex problems of hydrophysics when setting up the IAP, which strengthened even more co-operation with the Council while carrying out corresponding large-scale studies in the Institute.

From the very first years of the Department's existence particular attention was paid to the creation of its own experimental base, first of all, for field tests in the ocean. Many technical means and complexes (unique by a number of positions) developed by our employees are actively used in joint researches together with other academic and field institutes.

The subject of the main interest of our specialists in the area of hydrophysics is wave physics in the upper ocean: its processes are different and have a significant effect on its dynamics. Great attention is paid to underwater acoustics though in this case we don't mean the ocean wave field proper but a sound "introduced" in the ocean environment to diagnose it or locate objects in it. Taking into consideration the fact that a low-frequency sound has no alternatives as a means of sea "X-raying" for hundreds and even thousands of kilometers, an exclusive role of low-frequency underwater acoustics as an independent section of hydrophysics becomes clear. We will come back to it later. Now we will briefly explain why we are interested in the upper ocean layer from the viewpoint of wave physics.

It is well-known that waves of different nature with various dispersing and non-linear properties can propagate in the ocean. The most important for the study are the waves emerging on the surface (wind waves which are from centimeters to tens of meters long, capillary waves - up to 1 cm, and tsunami) and in the very water thickness the so-called internal waves (typical of the ocean as vertically stratified environment). Not only these wave movements but their mutual interaction, with underwater flows and turbulence play a vital part in the dynamics of the upper ocean. Studies of the specified processes are also of great importance as the basis of remote sensing methods, above all, radar and optical ones.

Recently, these works have been carried out by our Department within the limits of national and international research programs, including the RAS Presidium and Department of Physical Sciences, Federal target and competitive programs of the Russian Fund of Fundamental Researches and European scientific funds.

For example, the results of study of the longest surface waves in the open ocean-tsunami - are well known. Being, as a rule, of seismic origin, when coming to the shore, they are able to give rise to a destructive sea accident. In the group headed by Yefim Pelinovsky, Dr. Sc. (Phys. & Math.), the methods of correct estimation of tsunami risk are developed (area of possible wave "splashing" to the shore) based on numerical model study of registered events within the limits of the theory of non-linear waves on water. Comparison with the data on a number of tsunami of recent years, including two catastrophic ones, which had emerged as a result of the Krakatau volcano eruption (August 27, 1883) in Indonesia and a recent earthquake there (December 26, 2004)* allowed to approve the suggested methods and assess tsunami risk for different water

* See: N. Laverov et al., "Sumatra Disaster: Lessons and Prognoses", Science in Russia, No. 1, 2007, "Seismic Strikes Prevention Technologies", Science in Russia, No. 4, 2005. - Ed.

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Radar imaging of the internal wave packet on the ocean surface (according to the data of the Russian-American experiment in north-west Atlantic).

areas of the World Ocean, including a number of peripheral seas of Russia. Lately, the results of this group have allowed to approach the understanding of physical nature of another catastrophic phenomenon - the so-called anomalous surface waves. These unexpectedly large and, therefore, dangerous waves often appear as if "from nothing", and their peculiarities (height, profile, lifetime) are such that they are figuratively called "killers-waves".

The dynamics of the upper ocean-at the depths of up to several hundreds of meters - are, in many ways, defined by internal waves. They appear in the so-called pycnocline - a layer in the thickness of water where the water density changes with depth much quicker than in the neighboring layers. Our employees carried out a wide cycle of researches, having found out mechanisms of generation and unsteadiness of these waves, specific character of their interaction with turbulent currents and evolution on oceanic shelf. These works, which had been started under the direction of Lev Ostrovsky when setting up the Institute of Applied Physics, played a significant role in the understanding of proper physics of internal waves and building of prospective models of their dynamics in specific water areas.

The interaction of waves of different spatial and time scales is a classical problem of wave physics. One of the most interesting and abounding in phenomena examples of this kind in natural environment is interaction of surface wind waves with the internal ones in the ocean. And, at the same time it is one of the most important examples because here the key applied aspect is connected with creation of "transparent" or through-the-surface models of reflection of internal waves field (generated, for instance, by a moving underwater object) in the wind wave pattern which is observed from above by radar or optical devices. The Department employees defined physical mechanisms of influence of internal waves and underwater currents created by them on wind wave, and, based on this, methods of remote sensing of underground processes were suggested. Recent laboratory and field tests have pointed at significant role of cascade effects of internal waves on the wind waves when the impact is sent up by spectrum of the latter from meter and decimeter to centimeter ripple. Moreover, an inhomogeneous field of currents gives rise to changes in the field of wind speed over the surface, which also results in increment modulation of the short wind waves.

Another topical problem is interaction of waves with inhomogeneous currents and flows. As the upper ocean represents a typical example of open non-equilibrium environment, both growth of wave disturbances (caused by underwater currents and near-surface wind) and their natural dying out (determined by energy transmission in small-scale turbulence) are possible in it. It is important that with such a "contact" characteristics of both a wave field and the current itself change due to exchange of energy and by impulse. Such effects are well-known and applied in different areas (for example, in ultra-high frequency electronics and physics of plasma). However, as to the real ocean, building of correct physical models and their experimental confirmation were required. These works were carried out under the direction of Yulia Troitskaya, Dr. Sc. (Phys. & Math.).

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Coming back to formation of pycnocline in the ocean, it is worth mentioning that stratification of the depth-dependent water temperature plays the main role in this process. This circumstance was assumed as a basis of the large-scale laboratory model of the upper ocean implemented in the Large Thermostratified Tank of the RAS Institute of Applied Physics (was commissioned in 1991). The efficient and ecologically safe (without additional reagents) method suggested by Vladimir Talanov to support steady temperature stratification was laid in its construction. Its depth-dependent profile reproduces the profile typical of real ocean on full scale of ≈ 1:100. Due to these parameters, rather big sizes (20 · 4 · 2 m³) and modern instrumentation, the Tank gives large opportunities for modeling hydrophysical processes (Head of the laboratory is Viktor Bakhanov, Cand. Sc. (Phys. & Math.)). The unique facility is actively used when carrying out joint researches with a number of academic institutes: Institute of Space Research, Institute of Aerophysics named after A. Obukhov, Institute of Geochemistry and Analytical Chemistry named after V Vernadsky; on its basis long-term international projects are implemented.

Our second experimental facility to model wave processes in the upper ocean is the Circular Wind Wave Tank (commissioned in 1992). It is a wind-water channel of closed oval form. It envisages creation of stratified salt layers with assigned profile of density and generation of wind flow and, therefore, wind waves on the surface. Here the methods and facilities of diagnostics of wave-covered sea at different stages of development are approved (Head of the laboratory is Stanislav Yermakov, Cand. Sc. (Phys. & Math.)). This facility is actively used in joint experiments both with the Russian colleagues, and foreign partners from the European research centers.

WHAT CAN ONE SEE IN MUDDY WATER?

We have mentioned above an important diagnostic aspect of our researches in the area of hydrophysics relating to improvement of methods and means of remote probing of the ocean. Let us dwell on it in detail.

Let us start with optics of the ocean and underwater seeing. The Institute of Applied Physics inherited this theme from the Scientific Research Radiophysical Institute where the system of underwater laser observation had been created for the first time in the USSR in the 1960s. Pioneer experiments on the location and shaping the images of objects had been carried out with its help. The foundation for imaging in muddy (closely diffusing) media was laid then. The experience of this exploration was used in the Department to build a theory of laser location and imaging of underwater objects from flying apparatuses, to implement a remote-sensing optical method to measure spectral characteristics of wind wave and make an automated naval instrument package. This equipment proved to be irreplaceable in many our sea expeditions. Recently, Lev Dolin and Alexander Luchinin have developed a principally new approach to the solution for the problem of underwater imaging through wavy sea surface. Its subject matter is to correct distortions of a probing signal based on the information about sea-surface relief, which is directly

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Optic images of a test object when observing through sea surface: A - through plane boundary of the section air-water; B - at wind speed of 5m/sec; C - restored image. Object depth is 10m, size of resolution element is 2 cm.

included in its image. Once adjustable optics* passed a similar way as a result of appearance of large telescopes: from calculation of distortions of images of astronomic objects caused by deformation of mirrors to correction of these images based on relevant algorithms.

Sea water is not the only example of optically muddy media. Among them there are biological tissues the diagnostics of which is possible only in superficial layers due to rapid fadeout of optical radiation in them. The results relating to study of the ocean which had been obtained earlier underlay the method of optical coherent tomography of biological tissues - a promising direction in the area of medicine; relevant works are concentrated in the Department of Non-linear Dynamics and Optics of the Institute of Applied Physics.

In panoramic means of sea surface survey "all weather" UHF signals are applicable to a greater extent than optical and, therefore, radar probing. Significant attention is paid to this direction, too.

An important cycle of researches relating to remote probing of organic and oil slicks on sea surface were carried out under the direction of Stanislav Yermakov, including joint field experiments with the European Space Agency. In the course of them a degree of dying out of small-scale wind waves in specially created slicks was measured together with radar mapping of sea surface by ERS-2 satellite. The obtained results open the door for development of remote technology of selective ecological monitoring of water areas letting not only register the very fact of appearance of contaminating slicks but identify their origin.

Another promising trend is diagnostics of tsunami under the conditions of open ocean (where these waves are practically invisible). As a result of analysis of the data of satellite observation of the Indian Ocean water area during the disaster in the Southeastern Asia in 2004, there was discovered an effect of change in scattering cross-section of the radar signal reflected from sea surface when tsunami passed in the open ocean, and its theoretical interpretation was given. Therefore, satellite registration and effective forecast of destructive wave transmission before its emergence in the coast are possible.

LOW-FREQUENCY ACOUSTICS OF THE OCEAN

By the time the Institute of Applied Physics was set up, it was well-known that a low-frequency sound is transmitted in the ocean practically without dying-out due to forming of an underwater sound channel in the upper ocean - a natural acoustic wave-guide. Therefore, low-frequency acoustics has an evident advantage in diagnostics of water areas of hundreds and thousands of kilometers long. However, to implement it, one had to study a wide range of interrelated problems of forming sound fields in underwater sound channels at so long distances, diagnostics of ocean environment based on the data of acoustic "X-raying", setting up of technical base for long-range field tests.

Close interaction with Navy organizations and branch industrial institutions plays a key role in development of this direction. The role of academic program "Coherent acoustic fields and signals" is significant in support of the fundamental component of researches.

We also mention the experience of long-standing cooperation in carrying out field tests with colleagues from academic institutes: Institute of Oceanology named after P. Shirshov, Institute of General Physics named after A. Prokhorov, Pacific Oceanologic Institute named after V. Ilyichev, Acoustic Institute named after N. Andreyev, and others. The works were carried out in different underwater sound channels: subsurface channels of Arctic and north-west of the Pacific; deep-water channels of middle and tropic latitudes of Atlantic; shallow-water channels of adjacent seas and

* Adjustable optics is a division of optics engaged in development of optical systems with dynamic control of wave surface front to compensate random disturbances and rise resolution limit of observation equipment, concentration level of study with receiver or target. - Ed.

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self zones. We emphasize that all mentioned channels have a noticeable specific character of low-frequency sound transmission, and due to such variety, in the past years the Institute of Applied Physics accumulated a considerable volume of actual data on long-range underwater sound. The most important obtained result was demonstration of the possibility of long charting (up to 500 km) of underwater highlands and finding out of local inhomogeneities in deep ocean.

Efficient theoretical and numerical models of low-frequency sound fields in the ocean were built together with carrying out field tests. The main regularities of sound long transmission in different underwater channels and forming of singing signals (sound back-scattering) were established, influence of statistic features of receivable signals on efficient work of aerial systems was studied.

From the end of the 1980s when joint researches were developed together with foreign colleagues, Russian-American projects of acoustic thermometry of the oceanic climate were of great importance. It is a question of nearly the most ambitious program in the area of acoustics in its whole history-the program of long-term registration with the help of low-frequency signals of weak climatic trends of global temperature of oceanic waters (within the accuracy of a tenth of degree a year!). A number of international experiments with the assistance of the IAP were carried out at stationary acoustic paths in the Pacific (Hawaiian Islands-Kamchatka peninsula, 4,700 km long) and Arctic Ocean (from 1,000 to 2,600 km long). As a result, an important conclusion was drawn on the possibility of practical implementation of acoustic system

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prototype of long-term monitoring of the oceanic waters temperature.

In close connection with these works a new direction in the ocean acoustics which had appeared in the mid-1990s, was reflected in the subject matter of our Department. It turned out that multipath structure of an acoustic field in paths of thousand kilometers long shows chaotic behavior. However, the most surprising thing is that time of arrival of impulse signal on separate rays is grouped in a certain way, and such groups (they are called radial clusters) are rather steady. These important results of ray chaos theory which is developed by Anatoly Virovlyansky, Dr. Sc. (Phys. & Math.), let acoustically diagnose deep ocean in megameter rangers.

In the last decade significant progress was achieved in small sea acoustics. From the viewpoint of underwater acoustics, the key specific character of the small sea is in a relatively small number of modes along which a sound signal is transmitted. Their composition (mode range of signal) is responsive to availability of different inhomogeneities in the channel. Therefore, registration of changes of the specified range determined by diffusion on inhomogeneities of the channel can give information about the latter. This consideration underlay development by the Department of a new method of acoustic tomography of small sea which is called low-mode impulse tomography (Head of laboratory is Alexander Khilko, Cand. Sc. (Phys. & Math.)). To test it, we created unique radiating and receiving antenna complexes capable to ensure selective excitation and probing signal receipt, accordingly, at different modes. The results of a series of experiments in the Baltic and Barents seas carried out in 2002 - 2005 confirm an opportunity of building a relevant system of monitoring of small sea and shelf zones.

Other important results to develop the means of acoustic diagnostics of heterogeneities in the small sea relate to the use of the method of "dark field" well-known in optics to locate small-scale objects dissipating a direct intensity signal. The method of acoustic "dark field" developed by Vitaly Zverev was successfully approved by his employees.

The technical means of low-frequency underwater acoustics deserve special mentioning. An extensive program of field experiments would not have been implemented without them. The Institute became one of the world leading centers in the area of ocean acoustics due to those experiments.

Powerful hydroacoustic radiating elements which had been created in the Institute of Applied Physics became well known. Among them there are unique ones, which, in particular ensured success of the above mentioned Russian-American experiments relating to ocean heat flow measurements in Arctic. Among last developments in this area broadband piezoelectric ceramic radiating elements of a new generation are worth mentioning, proposed by Boris Bogolyubov, Cand. Sc. (Technology). They have record parameters for a frequency band from hundreds of hertz to units of kilohertz and allow to use different types of modulation signals to enhance resolution of the location system.

Importance of receiving antennas is also evident for experimental underwater acoustics which can be compared to contribution of optical and radar telescopes to astronomy. In the Department there were created autonomous measuring complexes, which have no analogues, for experiments in the water areas of small sea and on the shelf, aimed at researching hydroacoustic fields of surface and submarine ships in a broad band of frequencies. A digital antenna complex was developed and commissioned under the direction of Pavel Korotin, Cand. Sc. (Phys. & Math.) to measure parameters of weak hydroacoustic signals against the background of navigation noises.

Thus, in the past three decades hydrophysics within the precincts of the Institute of Applied Physics became a productive division of general science about vibrations and waves. Not only interesting phenomena were understood and outstanding regularities of many processes in the World Ocean were established, but efficient methods of their diagnostics were also developed, tools of large-scale experimental research both under the conditions of laboratory and open ocean were created. At the same time, the ocean is not the only object of our research. In the last decade a number of new promising directions appeared and developed fast in the Department. Among them there are coherent seismo-acoustics, acoustic diagnostics of complicated vibro-active systems, nonlinear acoustic nondestructive testing. Together with physics of wave processes in the upper ocean and radiophysical methods of its exploration, wave sensing of "other" inhomogeneous media and systems also becomes a point of rest for the development of the Department.