On acoustic transmission in ocean-surface waveguides

• Published in: Philosophical Transactions of the Royal Society of London. Series A: Physical and Engineering Sciences
• Format: Article in Journal/Newspaper
• Language: English
• Published: The Royal Society 1991
• Subjects: Arctic Ocean
• Online Access: http://dx.doi.org/10.1098/rsta.1991.0059; https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.1991.0059

A sound speed profile which increases monotonically with depth below the ocean surface is upward-refractive, acting as a duct in which sound may be transmitted to long ranges with little attenuation. A well-known example is the mixed layer, in which the temperature is uniform and the sound speed approximately scales with the hydrostatic pressure, increasing linearly with depth. The depth of the mixed layer depends on surface conditions, but is of the order of 100 m. Deeper channels are found in ice-covered polar waters, where the temperature and sound speed profiles both show a minimum at the surface. A typical surface duct in the Arctic Ocean may extend to depths of 1000 m or more and is capable of supporting very-low-frequency (VLF) (1-50 Hz) acoustic transmissions with no bottom interactions. On a depth scale that is smaller by several orders of magnitude, wave-breaking events create a bubbly layer one or two metres thick below the sea surface, with the highest concentration of bubbles, and correspondingly the lowest sound speed, at the surface. The bubble layer acts as a waveguide for sound in the audio frequency range, above 2 kHz, although transmission may be severely attenuated due to absorption and scattering by the bubbles, as well as by the irregular geometry of the sea surface and the bubble clouds. Most ocean-surface waveguides can be accurately represented by an inverse-square sound speed profile, which may be monotonic increasing (upward refracting) or decreasing (downward refracting) with depth, and whose detailed shape is governed by just three parameters. An analysis of the sound field below the sea surface in the presence of such a profile shows that it consists of a near-field component, given by a branch-line integral, plus a sum of uncoupled normal modes representing the trapped radiation which propagates to longer ranges. The modal contribution is identically zero in the case of the downward refracting profiles. The properties of the modes emerge from a straightforward theoretical ...