Figure 1. Spectra of short period fluctuations at long and short ranges (from ref. 8).
Both used CW signals and both featured transmission paths wlth fixed endpoints in order to eliminate fluctuation effects due to source and receiver motion. Nichols and Young speculated that internal waves were the likely source of observed fluctuations with periods of cycles/day to cycles/hour, and that surface waves were the source of short period fluctuations whose spectra peaked at 0.12 Hz. A different kind of experiment was conducted by Hamilton (9) during the period 1961-4 to support missile test and development. In order to use underwater hydrophones to detect the impact point of a missile on the ocean surface, and then to localize it by triangulation, it was necessary to know accurately the average speed of
Figure 2. Sound speed axis speed variations over 2.5 years. Explosive shots were detonated near Antigua Island and received on hydrophones at Eleuthera. Bermuda and Ascension Islands (from ref. 9).
sound over long distances. Hamilton measured repeatedly, for several years, the travel time of explosively generated signals over several fixed paths roughly 2000 km long. He observed much longer period variations, consistent with mesoscale eddies, but he could not attribute them to the oceanic mesoscale as it had not yet been discovered. He simply concluded that the observed observations were due to something happening in the water between the source and receiver.
The real start of systematic research to gain understanding of long range, low frequency acoustic fluctuations began in 1960 with a remarkable set of observations which lasted into the mid-1970's. They were made mainly by the Institute for Acoustical Research in Miami using two fixed rangaes (10).
