Figure 5. Phase fluctuations show evidence of internal waves (from Clark, J.G.and J.R. Yamall, Long range ocean acoustics and synoptic oceanography:Straits of Florida results, Pro. 4th US Navy Symp. on Military Oceanography, I, 309-365,1967)
From these and other experiments it was obvious there was a strong connection between ocean environmental processes and acoustic variability, even though most of the relationships were not yet quantified. This gave rise to the idea that the acoustic signals themselves could be used to measure ocean processes, and the phrase‘acoustical oceanography' was coined. One of the first applications was Worcester's measurement of currents by reciprocal acoustic transmissions (11). The notion of using sound to measure ocean temperature and eddy fields began to appear in publications by Munk and Worcester(12), Porter(13), Spindel(14) and others.
Figure 6. An early suggested scheme for acoustic observation of the ocean eddy field using a single acoustic source and multiple receivers (from ref. 13).
The Beginnings- Oceanography
The early 1960's witnessed the beginning of radical change in our understanding of the oceans. The picture that had developed up to that time was based on relatively sparse current measurements, taken with crude instruments such as drifiing bottles. The result was an ocean that seemed to be composed of large, steady, and generally slow moving currents such as the Gulf Stream. There was little hint of the energetic and ubiquitous meso and gyre-scale variability we know today. The first glimpse of a different ocean was revealed by John Swallow's acoustically tracked neutrally buoyant floats, which were launched from the R/V Aires in 1959-60 in an experiment to detect and track the deep Gulf Stream countercurrent predicted by Henry Stommel. Instead of seeing a stately flowing current, the floats dispersed rapidly and widely, implying the existence of swift, deep flows, in many directions.
