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Where When How N/D 2017 - J/F 2018 : Page 93

Relax | DIVE | Chill a uniform speed in seawater; its speed in-creases with temperature, salinity and pressure. So, sound travels faster in the warmer surface layer of the ocean than it does in deeper, cooler water. This decrease in speed continues with depth, as the water gets colder, eventually reaching a minimum at about 3,300 feet (1,000 m) (the point at which the ocean reaches a constant temperature of about 36 F [2 C]). Below that depth, the effect of increasing pressure offsets the effect of decreasing temperature, so the speed of sound begins to once again increase. As a result, the speed of sound may actually be higher near the seafloor than at the surface. Salin-ity has the smallest effect on the speed of sound because salinity changes in the open ocean are also minimal. While these speed variations amount to only 2 per cent or 3 per cent of the average speed of sound in seawater, they are enormously important in how sound behaves in the sea. Figure 3 shows the relationship between depth and the speed of sound. The low-velocity zone of sound varies with conditions and location, but in midlat-itudes it typically occurs at around 3,900 feet (1,181 m) in the North Atlantic and about 2,000 feet (606 m) in the North Pa-cific. Although the speed of sound at this depth is relatively slow, the transmission of sound within this zone is very efficient be-cause refraction tends to confine sound energy to within this zone. The outer edges of sound waves escaping from this zone will enter water in which the speed of sound is higher. This causes the wave to speed up but then pivot back into the mini-mum-velocity layer. Unless aimed at an acute-enough angle to escape, upward-traveling sound waves will tend to be re-fracted downward, and downward-traveling waves will tend to be refracted upward, as depicted in Figure 4. As the sound waves bend toward layers of lower sound velocity, they tend to stay within the SOFAR channel. Yet there is one other anomaly regard-ing sound in the sea. As you now know, sound travels at a relatively slow speed in the SOFAR layer. However, it moves rapidly near the bottom of the well-mixed surface layer near the permanent thermocline in temperate oceans (a depth of about 200 feet [61 m]). Because temperature and salinity conditions are homogeneous at that depth, sound waves are not refracted. But pressure still increases with depth, causing a thin, high-velocity layer, as de-picted in Figure 5. This phenomenon can produce a prob-lem or an advantage, depending on your perspective, when yet another very familiar sound technology is used — SONAR (Sound Navigation And Ranging). It turns out that when vessels project sonar pulses into the water to search for marine ani-mals, submarines or hazards to navigation, the target can be obscured or “shadowed.” Depending on the angle at which they ar-rive at the high-velocity layer, sound waves will sometimes split and refract to the sur-face or bend into the depths. Therefore, any object beyond the area of divergence may be undetectable by sonar. This is a well-known technique used by submariners to hide their location; and Tom Clancy fans may remember this phenomenon featured in an episode of the novel and movie, “The Hunt for Red October.” The best possible diving in the Turks & Caicos. We provide a high level of personal service, professionalism and fun to a small group, on our fast, comfortable dive boat. Private charters and private guiding -our speciality. Instruction, rental equipment and NITROX available. Complimentary Grace Bay pick up. Call us at 649-432-2782 aquatci@live.com | www.aquatci.com Out of Harbour Club Villas & Marina Visit the Turks & Caicos Islands at www.WhereWhenHow.com NOV/DEC/JAN/FEB 2017/2018 • • • • • 93

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