Fine Structure and Stability of the Sound Channel in the Ocean

Abstract

The sound-channel region in the ocean is traditionally assumed to be stable and uniform over large areas, although indications of rapid and appreciable variations have been observed. To improve understanding of the true situation, a study of the fine structure and short-term temporal and spatial variations in this region has been undertaken. The instrumentation and method developed for this purpose are described briefly. An underwater package containing two independent velocimeters, an FM pressure gauge, and auxiliary circuitry is repeatedly lowered and raised on a single-conductor armored cable through the channel region. Sensor information is telemetered to the ship and recorded. Results from three stations south of Bermuda in the Fall of 1963 are described and analyzed. A surprising amount of activity is found, proving that the sound-channel region, at the base of the main thermocline, is subject to much larger rapid changes than generally recognized. Considerable fine structure, evidence of internal waves, layers, and cells of “strange” waters have been observed. Sound-velocity changes of several meters per second in a few hours, at the same depth, are common. From August to October, the average sound velocity on the axis has decreased from 1494.5 to 1491 m/sec, the axis depth from 1300 to 1150 m.