Figure 3. Institute of Acoustical Research 43nm (80 km) Florida to Bimini range.
One, transiting the Florida Straits from Fowey Rocks near the Florida coast to Bimini Island in the Bahamas, about 80 km distant, was installed in the early 1960's. A 420 Hz transmitter was located at Fowey Rocks, and receivers were placed on the bottom at ranges of 13, 77 and 80 km. The second range, not installed until the early 1970's, consisted of a 406 Hz sound source at 600m depth installed near Eleuthera Island, with receivers at Bermuda (1250 km). Ted Birdsall, from the University of Michigan, who would later become part of the team that developed acoustic tomography methods, was involved with the design of signals for the ranges. Experiments conducted on these ranges revealed
Figure 4. Phase variations (H42) compared with transport and tidal variations over the 80 km range (from Steinberg, et.el., Fixed system studies of underwater acoustic propagation, J.Acoust.Soc.Amer., 52,5 (2), 1521-1536, 1972.
both deterministic and stochastic variations in signal amplitude and phase. Concurrent observations of tides and transport through the Straits appeared to be highly correlated with acoustic phase fluctuations, and there was evidence of shorter period internal wave induced fluctuations. The Straits of Florida observations led to the conclusion that phase was relatively stable with variations that correlate with environmental effects, particularly the tides, although there was no direct attribution. Amplitude variations were much less stable and poorly understood. It was not appreciated at the outset that the observed statistics of amplitude and phase fluctuations were almost completely dominated by the effects of multipath propagation.
