Long-range sound propagation in the ocean
In the foreground of the theoretical analysis and experiments dealing with long-range propagation of sound in the ocean is the allowance for the statistical effects accumulated with distance, which are caused by the effect of random inhomogeneities of the depth and boundaries of the ocean waveguide. From the radiophysical point of view, these effects are quite general for the propagation of signals in randomly inhomogeneous multimode (super-dimensional) waveguides. Additional complexity and specificity of these problems are, firstly, due to the joint action of a regular (rather stable over time) depth stratification of the underwater channel and of fluctuations of the sound velocity field at its background, and secondly, due to a wide variety of spatio-temporal scales of these fluctuations, which are caused by essentially different physical factors typical of the real ocean. Besides, the horizontal variability of the regular (background) stratification that is usually significant and leads to a "restructuring" of the waveguide with distance, should be taken into account for long-range sound propagation (on scales of hundreds of kilometers or more).
To study the basic properties of sound fields in ocean waveguides and to perform large-scale field experiments on long-range sound propagation in the ocean, the IAP RAS team has statistically described the mode and beam structures of the LF sound field in the ocean, taking into account multiple scattering of sound by random volume inhomogeneities, primarily, by internal and surface wind waves. The basic peculiarities of the long-range LF sound propagation in ocean waveguides, and of the reverberation signal formation (sound backscattering) were determined analytically and numerically; the effect of the coherent properties of multimode signals on the operation efficiency of antenna systems were explored (L. S. Dolin, A. G. Nechaev, A. G. Sazontov, A. I. Malekhanov, A. L. Virovlyansky, and M. A. Raevsky). These results are important for developing the theoretical grounds of low-frequency acoustics of the ocean and analyzing the efficiency of sonar systems and signal processing techniques under conditions corresponding to those of long-range sound propagation in the actual ocean waveguides. All these results were reliably confirmed by sea mission investigations performed in various areas of the World Ocean. A series of experimental and theoretical studies on the peculiarities of the interference structure formation of tone and pulse signals was carried out (E. F. Orlov, G. A. Sharonov, V. N. Golubev, Yu. V. Petukhov, and E. L. Borodina).
In the 1990s, the IAP RAS participated in several joint US-Russian projects on the problem of acoustic thermometry of the ocean climate (ATOC) using extensive experience of theoretical and experimental work on long-range sound propagation in the deep ocean. The most important participation of the IAP RAS was in two pilot experiments carried out in the Arctic Ocean. To implement the second experiment called ACOUS (Arctic Climate Observations using Underwater Sound), a unique autonomous radiating complex was developed under the leadership of B. N. Bogolyubov at the IAP RAS and anchored in the area of the Franz Joseph Land at a depth of 60 m. An autonomous receiving antenna was installed by American experts in the Lincoln Sea near the north coast of Canada (1250 km from the source). According to the program of the experiment, the IAP RAS source radiated special-type signals at a carrier frequency of 20.5 Hz for the implementation of the scheme of temporal selection of pulses propagating in different modes of the Arctic channel along the signal propagation path. The ACOUS experiment yielded unique data on seasonal and climatic variations in the state of the deep Arctic waters and was the first experiment that demonstrated the principal possibility and layout of the system of long-term acoustic thermometry on ocean basin scales.
In close connection with a number of international projects under the ATOC program performed in the 1990s in various water areas of the deep ocean along the paths of up to several thousand kilometers, the IAP RAS started a relatively new exploration trend in low-frequency ocean acoustics, associated with studying the effects of beam and wave chaos in megameter acoustic paths in the deep ocean. The studies performed by the team headed by A. L. Virovlyansky statistically described the chaotic beam dynamics and its manifestations in the variations of the mode structure of the field in the underwater sound channel at distances of 1000 km or more. The IAP RAS scientists employed the developed approach to formulate a quantitative theory for the effect of clustering of the sound pulses arriving at an observation point along numerous chaotic beam paths. Warming (on the average) or, on the contrary, cooling of ocean water leads to the corresponding trend in changes of the arrival times of individual clusters, which can be recorded against relatively short-term fluctuations (primarily, seasonal ones).