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Canadian Navel Acoustics
sources to provide acoustic measurements over a wide fre- quency band (400 Hz to 16 kHz). Chapman was unable to participate in sea trials, and he thus relied on collaboration with others (Merklinger and Osler, 2015). In March 1961, Jim Harris organized a sea trial to measure surface rever- beration using explosive sources north of Bermuda, which had a deep isothermal surface layer in the wintertime. Wind conditions varied from 0 to 30 knots (0-15 m/s) during the experiment, covering all the wind speeds of interest.
Chapman’s initial analysis showed that time of day and wind speed were perfectly correlated, and thus it could not be de- termined which was the cause of the observed changes in scattering strength. However, Harris had collected an initial “test” dataset on a day with completely different wind condi- tions, but he had omitted it from the original plots because it was not part of the “official” dataset (he had used only two hydrophone depths). When the test dataset was included, it became clear that the surface scattering did primarily vary with wind speed. When Chapman presented the paper at the 1961 Fall US Navy Symposium on Underwater Sound, the data were plotted in several octave frequency bands, and an audience member suggested plotting it all together to elicit greater understanding. On his return to the NRE, Chap- man did just that and then asked Anne Robison to calculate the nonlinear curve fit that is now known as the Chapman- Harris equation that relates the surface-scattering strength to wind speed and frequency (Chapman and Harris, 1962).
The same Journal of the Acoustical Society of America (JASA) paper by Chapman and Harris noted the time-of-day varia- tion in scattering strength that was ultimately attributed to volume reverberation. In later years, using a 3.5- to 6-kHz echo sounder on the HMCS New Liskeard, they were able to identify several volume-scattering layers, some that migrat- ed and some that did not. These “deep scattering layers,” now known to be found worldwide, are predominantly biological in origin, and most of the scattering was due to fish swim bladders (Chapman et al., 1974).
Early Arctic Acoustics
Arctic underwater acoustics in Canada began at the PNL in 1958 when Al Milne was inspired by stories of the Canadian Army’s Operation Muskox and tales of the Beaufort Sea tri- als. In 1959, Milne organized Paclabar (Pacific Laboratory Arctic), the first of what were to be numerous Arctic trials. The trials (Icepack 1-8 and Polarpack 1-3) took place in lo- cations across the Arctic Ocean, including Barrow Strait, Prince Gustav Adolph Sea, M’Clure Strait, Prince Regent In- let, Viscount Melville Sound, and Mould Bay (Milne, 1998). 30 | Acoustics Today | Summer 2018
The trials followed the same general formula. First, by some combination of airlift, ship, sled, and helicopter, the team would arrive at a suitable location on the ice with appropri- ate scientific and survival equipment, a nontrivial task. Next, they drilled a hole through the ice through which they low- ered a hydrophone. A small “shooting party” traveled by sled or helicopter 1-2 km away, drilling or blasting a second hole through the ice through which to make oceanographic mea- surements and then set off explosive charges at some depth in the water while at the first hole, the hydrophone signal was being recorded. Then the shooting party would navigate farther away and repeat the drilling-oceanography-blasting process (Milne, 1998). Through this series of experiments, the teams (including Milne, Tom Hughes, John O’Malia, and John Ganton among many others) made measurements of noise and reverberation under the Arctic sea ice (e.g., Milne, 1964; Milne and Ganton, 1964; Brown and Milne, 1967) and the sound speed in the bottom (Milne, 1966). One inter- esting series of measurements explored the stability of the sound transmission medium under land-fast ice over short and long ranges (Ganton et al., 1969).
The PNL team developed numerous innovations for trans- portation and survival in the Arctic (Ganton, 1968). The drill used for ice 5-7 feet (1.5-2.1 m) thick had a triangu- lar bit to drill holes 9 inches (23 cm) in diameter; for larg- er holes, several “dry” holes were drilled in a circle and a central “wet” hole was filled with explosives that were then detonated. They then faced the problem of how to keep the holes ice free; an inflatable neoprene balloon was a simple solution that worked for up to a week. For longer deploy- ments, they used the “rope trick.” A 7-inch (18-cm)-diame- ter plastic cylinder was wrapped with nylon rope embedded in silicon rubber. Instrument cables were threaded through the cylinder and the whole apparatus was allowed to freeze into the 9-inch (23-cm) hole. To recover the instruments, the rope was easily unspooled from the outside of the cyl- inder, and the freed cylinder was recovered, with the cables frozen inside.
Survival and navigation also required innovative solutions. Regular compasses were useless near the magnetic pole, so wayfinding was achieved with a combination of a sextant, radio fixes, and bamboo poles with black flags marking their route. The Army-issued Arctic clothing used for the first trial was insufficient; on subsequent trials, it was enhanced by additional liners and newer materials such as L19 Ventile cloth (Milne, 1998). Heavy steel sleds and wooden wanni- gans (insulated sheds) were replaced with lightweight alu-