Acoustical Physics. Vol. 44, No. 6, 1998. pp. 752-753. Translated from Akusticheskii Zhurnal.
Vol. 44, No. 6. 1998. pp. 861-S62. Original Russian Text Copyright © 199& by Andreeva.

INFORMATION

Seventh Advanced-Study School of Academician L.M. Brekhovskikh and the VII Session of the Russian Acoustical Society

The joint sittings of the Seventh Advanced-Study School of Academician L.M. Brekhovskikh and the VII Session of the Russian Acoustical Society (the president of the Society is Academician of the Russian Academy of Natural Sciences N.A. Dubrovskii) took place May 25-28, 1998, in Moscow.

About fifty years ago, Academician L.M. Brekhovskikh—a prominent theorist and an excellent organizer of marine studies—initiated the investigations into ocean acoustics, which was an entirely new direction of research at that time. The results obtained during oceanic expeditions were published in scientific journals and monographs, as well as reported and discussed at numerous All-Union conferences, seminars, and at the annual sessions of the Russian Acoustical Society. The participants of these events were scientists from different institutions dealing with ocean acoustics.

In the late 1970s, Brekhovskikh organized the Advanced-Study School on Ocean Acoustics. The school united young scientists and senior ones—those who worked in ocean acoustics since the 1960s. Beginning in 1980, the Advanced-Study School became a regular event, and its proceedings were published by the Nauka publishing company. From 1982 to 1993, six Advanced-Study Schools on Ocean Acoustics were held. The seventh Advanced-Study School was organized after a certain interval, and it was combined with the VII Session of the Russian Acoustical Society. The joint sittings were attended by more than 100 scientists from many Russian cities: Moscow, Nizhni Novgorod, St. Petersburg, Vladivostok, Irkutsk, Voronezh, Taganrog, etc.

More than 130 papers were submitted, but only 82 of them were accepted by the Organizing committee. The number of oral presentations was 34, and 48 papers were presented at the poster session. All papers accepted for the Advanced-Study School were published under the title Ocean Acoustics: Proceedings of the Advanced-Study School of Academician L.M. Brekhovskikh, Moscow: GEOS, 1998. The full program of the School and the papers can be found on the Internet at

http//rav. sio.rssi.ru/shkola.html.

The main directions of research represented at the seventh Advanced-Study School were as follows.

One of the promising directions of research discussed was acoustic tomography of the ocean and, in particular, the acoustic monitoring of the Arctic Ocean.

The problems of the "Dynamical Tomography of the Mediterranean" were considered by D.Yu. Mikhin, O.A. Godin, Yu.A. Chepurin, V.V. Goncharov, S.V. Burenkov, D.L. Aleinik, and V.V. Pislyakov. Their lecture contained a comparative analysis of different approaches to tomographic measurements in the ocean and the results of the full-scale experiment carried out by the authors in 1994 in the western Mediterranean. In addition, this lecture, as well as the lecture delivered by V.V. Goncharov entitled "Matched-Field Method in the Problems of Acoustic Ocean Tomography," contained a detailed analysis of the possibility of using the matched-field method in ocean tomography. The analysis was based on the data of the aforementioned full-scale measurements and some other experiments (both full-scale and model ones). The considerable improvement of the parabolic approximation proposed by O.A. Godin ("Three-Dimensional Parabolic Approximation with the Fulfillment of the Energy Conservation Law and the Reciprocity Principle") substantially extended the possibilities for solving tomographic problems. The problems of the acoustic monitoring of the Arctic basin were considered by K.D. Sabinin and V.T. Sokolov ("Acoustic Monitoring of the Arctic Ocean: Key Regions and Objects") and by A.N. Gavrilov ("On the Arctic Climate Observation Using Underwater Sound (the ACOUS Experiment)"). These authors discussed the choice of the geographical location of acoustic tracks for the acoustic monitoring from the viewpoint of obtaining maximum oceanographic information. Sabinin and Sokolov demonstrated the importance and the feasibility of the acoustic halinometry of the basin under study. Gavrilov analyzed the variability of the heat balance of the basin. He also reported on some technical details of the Russian-American ACOUS experiment planned for the years 1998 and 1999.

Several lectures (such as "Variability of the Interference Stmcture of the Sound Field in Shallow Sea" by Yu.A. Kravtsov, V.M. Kuz'kin, and V.G. Petnikov and "Acoustic-Oceanological Experiment on a Stationary Track in Shallow Sea" by A.I. Belov, A.N. Serebryanyi, and V.A. Zhuravlev) were concerned with the experimental observation and simulation of sound field fluctuations that appear in a shallow sea as a result of hydrodynamic perturbations of the medium (internal waves, tides, etc.). The possibilities of solving pertinent inverse problems were also considered.

Several lectures were concerned with the acoustics of the ocean bottom. For example, A.N. Ivakin ("Models of Sound Scattering by the Ocean Bottom: the Current State of the Art") proposed new geoacoustic models of sound scattering by the ocean bottom. His models were based on the assumption that the boundaries of the bottom layers were rough and, on the average, horizontal. The calculation of the characteristics of the scattered field was based on the solution of a unified integral equation taking into account the sound scattering from both the rough boundaries of the layers and the inhomogeneities in the layer bulk. The elastic properties of the bottom were also taken into account. In other lectures concerned with acoustics of the ocean bottom, the possibilities of solving the inverse problems were considered, i.e., the determination of the parameters of the bottom structures from the characteristics of the acoustic field scattered or reflected by these structures (e.g., "Reconstruction of the Parameters of a Multilayer Bottom of a Shallow Sea" by V.P. Antonov, V.V. Borodin, G.N. Kuznetsov, A.A. Kuz'menko, and V.P. Tebyakin and "On the Reconstruction of the Characteristics of an Elastic Layered Bottom from the Behavior of the Reflection Coefficient in the Plane of the Bottom Parameters" by V.M. Fokin and M.S. Fokina).

Sound scattering in the water column was also discussed. Here, we mention two lectures on this subject. One of them ("Acoustic Inhomogeneities of Biological Origin in the Ocean" by I.B. Andreeva) presented the possible limits for the variations of the main acoustic characteristics of sound-scattering layers in different regions of the ocean in a relatively wide frequency range. The criterion of the applicability of the single-scattering approximation for describing the sound scattering from dense biological accumulations was formulated. In the lecture delivered by V.A. Bulanov on "Sound Scattering, Bubble Distribution, and Acoustic Nonlinearity of the Sea Surface Layer," the surface layer containing air bubbles was considered, the corresponding full-scale measurements performed in the ocean were reviewed, and the results obtained by the author by using the method of nonstationary scattering of light were presented. Bulanov described the full-scale measurements of the parameter of nonlinearity of the surface water layer in the ocean. Of other lectures concerned with this subject, one should mention the lecture given by V.S. Gostev and R.F. Shvachko on "Prereverberation in the Ocean with a Thin Stratification." The authors described the effect of the specific type of ocean structure on the propagation of acoustic waves in the underwater sound channel. E.A. Kopyl reported on the "Sounding of the Perturbations of the Wind-Wave Field Formed at the Water Surface." He proposed hydrodynamic models of the perturbations of a wavy water surface under rain, currents, and surface-active films and presented experimental results obtained by sounding.

One of the sessions was devoted to the problem of acoustic noise in the ocean. In the review presented by B.F. Kur'yanov ("Development of the Notion about the Low-Frequency Noise in the Ocean through 50 Years"), the author described the history of the studies of the average characteristics of ambient noise in the ocean. A.V. Furduev ("Underwater Noise of Spatially-Inho-mogeneous Wind over the Sea") considered one of the nonstationary factors of noise, namely, the effect of squalls and gusts of wind on the space-time fluctuations of the noise field spectra. Of the lectures devoted to special problems, one should mention the lecture entitled "Thunderstorm Noise in the Ocean" by N.A. Dubrovskii and V.M. Frolov. They considered the contribution of atmospheric thunderstorm sources to the formation of noise in the ocean at infrasonic and low sonic frequencies.

A.L. Matveev, V.V. Mityagov, and V.M. Salin reported on the "Experimental Study of Acoustic Diffraction." They discussed methods for the optimization of acoustic signal processing for the purposes of selecting acoustic shadows of moving underwater diffracting objects against the noise background. They presented experimental results obtained in half-simulated conditions. The lecture delivered by V.A. Gordienko was entitled "Vector-Phase Methods in Acoustics: Problems and Prospects of Their Use." He evaluated the advantages offered by the vector-phase methods, or, more precisely, by the vector receivers of underwater sound, for different purposes of ocean acoustics.

The closing session was devoted to general discussion. It was concluded that, in the following years, the Advanced Study School of Academician L.M. Brekhovskikh on Ocean Acoustics should continue to be held on a regular basis.

I.B. Andreeva

Translated by E.M. Golyamina