Where the ocean influences the impulse response and its effect on synchronous changes of acoustic travel time

• Published in: The Journal of the Acoustical Society of America Vol. 130; no. 6; pp. 3642 - 3650
• Main Author: Spiesberger, John L.
• Format: Journal Article
• Language: English
• Published: Melville, NY Acoustical Society of America 01-12-2011; American Institute of Physics
• Subjects: Acoustical measurements and instrumentation; Acoustics; Exact sciences and technology; Fundamental areas of phenomenology (including applications); Marine; Physics; Underwater sound; I, Pacific; Pulse response; Underwater acoustics; Travel time
• ISSN: 0001-4966; 1520-8524
• DOI: 10.1121/1.3652864

In 1983, sounds at 133Hz, 0.06s resolution were transmitted in the Pacific for five days at 2min intervals over 3709km between bottom-mounted instruments maintained with atomic clocks. In 1989, a technique was developed to measure changes in acoustic travel time with an accuracy of 135 microseconds at 2min intervals for selected windows of travel time within the impulse response. The data have short-lived 1 to 10ms oscillations of travel time with periods less than a few days. Excluding tidal effects, different windows exhibited significant synchronized changes in travel time for periods shorter than 10h. In the 1980s, this phenomenon was not understood because internal waves have correlation lengths of a few kilometers which are smaller than the way sound was thought to sample the ocean along well-separated and distinct rays corresponding to different windows. The paradox's resolution comes from modern theories that replace the ray-picture with finite wavelength representations that predict sound can be influenced in the upper ocean over horizontal scales such as 20km or more. Thus, different windows are influenced by the same short-scale fluctuations of sound speed. This conclusion is supported by the data and numerical simulations of the impulse response.