Ｔhis discovery led to a series of oceanographic experiments that gradually revealed a much more complex ocean,one in which highly variable currents played a major role. These experiments culminated in the Mid Ocean Dynamics Experiment (MODE), conducted in the Atlantic in the mid-1970s, which revealed conclusively the exrstence of mesoscale eddies. It was apparent that the pre-1960 picture of the ocean was not only incomplete, but wrong, and that a complete picture could not be developed without further understanding the properties of the mesoscale field. Deducing the statictics of the mesoscale field, with eddies having spatial scales of 100's km and temporal scales of months, implied continuous observations over large ocean basins for long periods of time. Unfortunately, such an observational and measurement capability did not exist.
Fusion - Acoustics and Oceanography
Although there were a few ocean acousticians who were talking about acoustical oceanography, the much larger community of underwater acousticians representing Navy interests was focused more on understanding the statistics of fluctuations than on their underlying causes. Their goal was to improve detection performance in a noisy and fluctuating background by applying the methods of detection and estimation theory developed for radar. Examples of this research are books by Ol'shevski(15) in the USSR, and Van Trees in the US (16). On the other hand oceanographers, focused on physical processes, were uninterested in underwater acoustics.
It was fortuitous that in the early 1970's the US Defense Department's Advanced Research Project Agency (ARPA) convened a series of studies focused on understanding fluctuations of low frequency acoustic signal in order to improve the performance of passive Anti-Submarine Warfare (ASW) surveillance systems. Members of the study group came mostly from the ranks of physicists, not underwater acousticians, and they approached the problem of interpreting observed fluctuations from a different perspective. The group also included a few oceanographers, notably Walter Munk.
This group developed theories for acoustic-internal wave interactions, and recognized the consequences of multipath propagation on amplitude and phase fluctuations, especially in CW signals where random phase paths combine. Their results went a long way toward explaining experimental observations along long paths such as 1250 km, 406Hz Eleuthera to Bermuda transmissions (17), as well as fluctuations along single paths that were observed in (a few) much shorter range and higher frequency single path experiments (18). In general, their work showed direct, quantifiable connections between ocean processes and acoustic fluctuations, and thus put acoustical oceanography on a firm footing.
During this period Worcester, who was one of Munk's graduate students at the Sripps Institution of Oceanography, was developing equipment to test the idea of using reciprocal acoustic transmissions to measure ocean currents. Spindel, at the Woods Hole Oceanographic Institution, was working on measuring acoustic fluctuations at low frequencies using fixed (moored), autonomous, acoustic sources and receivers, as well as moving hydrophones to sweep out large synthetic spatial apertures (19,20). He developed precision acoustic navigation methods to accurately track the drifting receivers (21), and
