Welcome to You Ask Andy

Roderick MacBain, age 10, of Fredericton, New Brunswick, Canada, for his question:

How do they measure the ocean depths?

They use a clever system called sonar, which is related to radar. Both sonar and radar depend on echoes that bounce back from there to here. This seems like a complicated way to measure the depth of the ocean. But actually the idea is very ancient, at least in the animal world. Scien¬tists suspect that dolphins use a sort of sonar system to detect solid objects in the water. Electric eels use a similar idea.

In olden days, many a ship was wrecked on a reef and many a river boat ran aground on a sandbar. After all, when you are on top of the water, it’s hard to see way down to the bottom. Chances are, you can see only a few feet below the surface. This is where rocks and reefs lurk, waiting to wreck a passing ship.

Until modern times, many ships carried a heavy weight on a long long line. When a pilot suspected submerged rocks or sandbars, the weight was dropped overboard and the length of the line told how far it was to the bottom. This system sometimes saved a ship, but it was clumsy. Then a system called “Sound Navigation And Ranging” was invented. The only clumsy thing about it is its name, but this has been shortened to SONAR.

There are several sonar systems designed for different underwater jobs. But all of them work on the same basic idea. An energy source is used to send sound waves through the water. They travel on and on at an average speed of 4,700 feet per second—until they bump into something solid, then they bounce off and echo back the way they came.

The echoing sound waves travel back at the same, speed and the sonar system on the ship has a receiving set to catch them the moment they return. It also has a clever timer that measures exactly how long it took them to make the trip there and back.

With this information, a computer can figure out just how far the returning echoes have traveled. Suppose the departing sound waves returned after two seconds. Obviously they spent the first second on the way there and the second on the way back. The solid under water object must be 4,700 feet from the ship, because this is how far the sound waves travel in one second.

When a sonar system is set up to measure the ocean depths, its sound waves are directed straight down to the solid sea bed. If the trip there and back takes six seconds, the sound waves and their echoes travel a total distance of 28,200 feet. The solid sea bed is half this distance, or 14,100 feet, below. This is over three miles—and we must have been measuring a very deep part of the ocean.

Some sonar systems are set up to detect solid submarines, prowling below the surface of the sea. Some are set up to detect and trace other ships sailing far and near. These systems also detect deadly icebergs, long before they have a chance to strike a ship.

 

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