On Ocean Waveguide Acoustics

BOOKREVIEW ___________________________________________________ GEORGE V. FRISK ON OCEAN WAVEGUIDE ACOUSTICS acoustic waves with multilayered media is also an ac ACOUSTIC WAVEGUIDES: APPLICATIONS TO OCEANIC SCIENCE tive area of research in underwater acoustics. By C. Allen Boyles, Principal Professional Staff, In recent years, the inverse problem of determining The Johns Hopkins University Applied Physics Laboratory oceanographic properties from acoustic measurements Published by John Wiley & Son, New York, 1984. 321 pp. $46.95 has become increasingly important and has given rise to the term "acoustical oceanography," a variant of "ocean acoustics" that emphasizes the oceanograph ic implications of acoustic experiments. A major de The field of ocean acoustics is an active area of the velopment in this area is time-of-flight acoustic oretical and experimental research, with a continual tomography in which front and eddy intensity and ly expanding body of literature in research journals variability over hundreds of kilometers are measured and textbooks. Boyles' book is a welcome addition to acoustically. In ocean-bottom acoustics, direct inverse the literature and provides a useful text for both stu methods are being developed that utilize some mea dents and practitioners in the field. surement of the acoustic field, such as the plane wave Although electromagnetic waves are strongly ab reflection coefficient of the bottom, as direct input to sorbed by water, acoustic waves can, under the prop algorithms for determining the acoustic properties of er conditions, propagate over hundreds, even thou the bottom. This approach is to be contrasted with sands, of miles through the ocean. As a result, sound conventional techniques in which forward models for waves and sonar assume the major role in the ocean computing the acoustic field are run for different bot that electromagnetic waves and radar play in the at tom properties until best fits to the data are obtained. mosphere. Sound is used for navigation, communica In addition to wave physics and geophysics, electri tion, and underwater detection and as a tool for cal engineering plays a major role in underwater acous studying the oceanography of the water column and tics. State-of-the-art instrumentation often involves its boundaries. Active and passive sonar systems are microprocessor-controlled devices deployed under the most important methods used by the Navy in de water, sometimes for extended periods of time. The tecting and tracking submarines in antisubmarine processing of these signals usually requires the use of warfare. sophisticated signal processing techniques and numer For the physicist specializing in wave propagation, ical methods in order to extract the measurement of the ocean provides a medium in which virtually every interest. These techniques contribute to an improved conceivable basic problem in wave propagation can be understanding of the physics of acoustic wave propa studied. The ocean is an inhomogeneous medium with gation, and the fields of wave physics and signal pro an acoustic index of refraction that varies with depth cessing combine to yield an exciting area of research. as a result of changes in temperature and hydrostatic Ocean acoustics has produced a vast research and pressure and, to a lesser extent, in salinity. On a finer textbook literature that reflects the diversity of prob spatial scale, thermal microstructure introduces a ran lems described above. Although many of the basic dom component to the sound velocity structure. Wave problems in underwater acoustics are similar to those propagation through inhomogeneous and random me in electromagnetic theory, seismics, and quantum dia is, therefore, an important part of ocean acoustic mechanics, the specific details of ocean acoustic prob research. The ocean surface and bottom are, in gener lems are sufficiently different to warrant a distinct sub al, rough boundaries, thus requiring the study of ject area. Perhaps the best all-around graduate level acoustic scattering from rough surfaces. The ocean text in underwater acoustics is the one by Clay and bottom is a multilayered structure composed of sedi Medwin, 1 who present a well balanced view of most ments and rocks, which, in general, supports both of the important acoustics problems, including signal compressional and shear waves, in addition to contain processing, and their relationship to oceanography. ing velocity gradients. Therefore, the interaction of They also provide a set of problems for the student at the end of each chapter. On the other hand, their George V. Frisk is an associate scientist at the Woods Hole Oceano theoretical development is somewhat elementary and graphic Institution, Woods Hole, Massachusetts. lacks the depth that one would hope for in teaching 390 Johns Hopkins APL Technical Digest students who are about to embark on research work guide problem that consists of a homogeneous layer in acoustical oceanography. In a similar vein, but more overlying a homogeneous half-space. Next, he con sophisticated theoretically, are the books by Tolstoy siders the normal mode solution for a model consist and Clay2 and Officer. 3 Unfortunately, both books ing of N inhomogeneous layers (terminated by a rigid are out of print. On the more practical side, Urick's boundary) where the sound velocity, attenuation, and book4 emphasizes the solution of practical engineer density can be discontinuous from layer to layer. ing and sonar system problems at an elementary In Chapter 5, Boyles discusses approximate solu mathematical level. At the other end of the spectrum, tions to the wave equation that include ray theory, ray at a high theoretical level, are the books by Brekhov theory with asymptotic corrections for smooth caus skikh and Lysanov, 5 Brekhovskikh,6 Keller and tics, and the Wentzel, Kramers, Brillouin (WKB) ap Papadakis, 7 and DeSanto.8 The Brekhovskikh and proximation. He shows that ray theory can be obtained Lysanov work has a scope that approaches that of Clay through a combination of the WKB method and the and Medwin (though no signal processing is includ method of steepest descents. He also discusses cusp ed) and includes additional modern theoretiCal topics. caustics and the physical basis for the formation of However, the information is presented in a condensed smooth caustics. In Chapter 6, Boyles applies ray the fashion and may present a challenge to the mathemat ory and normal mode theory to velocity profiles asso ically unprepared reader. The book by Brekhovskikh ciated with three realistic ocean environments. In two is an excellent advanced text that emphasizes the top of these cases, the North Pacific and North Atlantic ic of acoustic interaction with layered fluid and elas summer profiles, the propagation characteristics are tic media. Keller and Papadakis and DeSanto focus dominated by the formation of convergence zones, on special advanced topics and are most useful for ac while in the third, the North Atlantic winter profile, tive researchers in the field. the dominant feature is surface duct propagation. Into this spectrum of textbooks, Boyles has in Finally, in Chapter 7, Boyles presents his original troduced a book with narrower scope than Clay and work on propagation in a waveguide where the sound Medwin but with substantially greater theoretical speed varies both horizontally and vertically and the sophistication. The mathematical level is comparable ocean surface is modeled as a randomly rough inter to that of the advanced texts described above, but face. This is recent work, some of which has appeared Boyles' exposition typically contains considerably in journals, while some appears here for the first time more background and detail. This detailed approach in published form. Because separation of variables is useful not only in understanding his book but also cannot be used in this case, he uses the method of cou as background for reading some of the other advanced pled, local normal modes. The resulting sets of cou texts. Herein lies the major strength of Boyles' treat pled, second-order differential equations do not lend ment: he has written a self-contained book in which themselves to the use of known numerical integration 9 ll he derives the wave equation from first principles, algorithms. But Boyles ,10 and Dozier have shown presents the required mathematical methods, and uses that if the equations are reformulated, they can be these methods to solve the wave equation for wave solved using known numerical techniques. guides of increasing complexity. Boyles states in the Boyles presents a detailed, comprehensive treatment preface that this was in fact his intent in writing the of the topics covered, although certain major areas are book, which originated as a set of lecture notes for conspicuously absent. These include propagation in a course he taught at The Johns Hopkins University random media, inverse problems, signal processing, Applied Physics Laboratory. and the parabolic equation method, another frequently In Chapter 1, Boyles begins with the basic equations used technique for computing acoustic fields in range of fluid mechanics and thermodynamics and uses them dependent environments. Although the ocean bottom to derive the acoustic wave equation, the energy con is included in some of his models, he does not discuss servation equations, and the boundary conditions as the bottom interaction problem per se. His discussion sociated with the

Load more