Sound-velocity perturbations due to low-frequency motions in the ocean

Abstract

The properties of low- and intermediate-frequency external and internal waves in the open ocean, and the nature of the acoustic velocity perturbations they produce, are examined. The basic principles are illustrated with theoretical calculations based on representative oceanic cases. Such calculations provide a rational estimate for the vertical and horizontal dependence of the perturbation of the acoustic velocity field due to external and internal waves. External waves can cause travel-time fluctuations by their horizontal velocity fluctuations (or fluctuations in the total sound velocity). Internal waves can cause acoustic travel-time fluctuations both by their vertical displacement of isotherms (or isolines of sound speed) and by their velocity fluctuations (or fluctuations in total sound velocity). Both external and internal waves can have horizontal wavelengths ranging from a fraction of a kilometer to hundreds of kilometers. The longer waves can be effective in producing travel-time fluctuations over long propagation paths; the shorter waves are probably most influential in the scattering of acoustic energy. Subject Classification: 30.20.