This Topics volume is devoted to a study of sound propagation in the ocean. The effect of the interior of the ocean on underwater sound is analogous to the effect of a lens on light. The oceanic lens is related, as in light propagation, to the index of refraction of the medium. The latter is giv~n by the ratio of the sound frequency to the speed of sound in water, typi ca lly about 1500 m s -1. It is the vari­ ation of the sound speed due to changing temperature, density, salinity, and pres­ sure in the complex ocean environment which creates the lens effect. Many oceanic processes such as currents, tides, eddies (circulating, translating regions of wa­ ter), and internal waves (the wave-like structure of the oceanic density variabil­ ity) contri bute in turn to the changes in sound speed'. The net effect of the ocean lens is to trap and guide sound waves in a channel created by the lens. The trapped sound can then propagate thousands of miles in this oceanic waveguide. In addition to the propagation in the interior of the ocean, sound can propagate into and back out of the ocean bottom as well as scatter from the ocean surface. Just as the sound produced by a loudspeaker in a room is affected by the walls of the room, so the ocean boundaries and the material properties below the ocean bottom are essential ingredients in the problem.

1. Introduction.- 1.1 A Brief History.- 1.2 Outline of the Book.- References.- 2. Theoretical Methods in Ocean Acoustics.- 2.1 Conservation Laws, Fluid and Acoustic Equations.- 2.2 Propagation in Deterministic Media.- 2.3 Wave Propagation in a Random Medium.- 2.4 Scattering from Rough Surfaces.- References.- 3. Numerical Models of Underwater Acoustic Propagation.- 3.1 Range-Independent Models.- 3.2 Range-Dependent Models.- References.- 4. Physical Modeling of Underwater Acoustics.- 4.1 Background Information.- 4.2 Water Facilities.- 4.3 Targets.- 4.4 Instrumentation and Signal Processing.- 4.5 Results of Physical Model Experimentation.- 4.6 Remarks.- References.- 5. Oceanography in Underwater Acoustics.- 5.1 Properties of Seawater.- 5.2 Ocean Variability.- References.- 6. Inverse Methods for Reflector Mapping and Sound Speed Profiling.- 6.1 The POFFIS Identity.- 6.2 An Inverse Method for Determining Small Inhomogeneities in a Medium.- References.- 7. Acoustic Probing of Space-Time Scales in the Ocean.- 7.1 Sound Probes of Ocean Currents.- 7.2 Acoustic Fluctuations as a Measure of Ocean Dynamics.- 7.3 Sound Speed Variations and Internal Gravity Waves.- 7.4 Acoustic Fluctuation Theories and Their Relationship to Ocean Dynamics.- 7.5 Implications for Ocean Probing.- References.