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Name: Raafia J.
Status: Student
Age: 14
Location: N/A
Country: N/A
Date: May 2004


Question:
What is sonar mapping?



Replies:
Sonar mapping uses sound waves to map out the contours or the shape of the ocean bottom. Sonar is an acronym for Sound Navigation Ranging. Typically, a pulse of sound is generated using a kind of underwater loudspeaker towed behind a boat. The pulse of sound, or "ping," spreads out through the water and is reflected back again by objects in the water, such as the ocean bottom, a fish, or a submarine. Underwater microphones measure the reflected sound. The time that it takes the echo to return is roughly proportional to the distance to the reflecting object. Like an echo in the air.

Measuring the distance to a single object is relatively easy because it returns a simple echo. Making a map of the ocean floor is much more difficult because sound reflects back from many directions, from hills and valleys in the ocean floor. Complicated mathematics is needed to calculate a map of the ocean bottom.

Bob Erck


Sonar is when you make a sharp noise and listen for an echo off some large object at a distance. The farther away the object is, the longer it takes for the echo to come back to you.

I have read that some blind people are doing this now, using clicks they make with their tongues, and listening carefully with their hearing. They need a fair amount of training from someone who knows it well, but then they can, say, slowly ride a bike down the middle of a neighborhood street with parked cars on either side, or sense the wall of a house 20 feet away, or sense a quietly standing person about 6 feet away. Pretty good. I bet you can do a little of it, too, especially with practice.

Certainly this is what dolphins and whales and bats do so well.

But people usually do it with an electronic machine, and most often the sound is in water, instead of in air. The surface of the water makes a convenient reference plane for the boat to slide around on, and the echo return time is proportional to the distance to the nearest solid bottom beneath the boat. If a boat with a sonar sails all over an ocean, remembering the ocean depth at every place it goes, it eventually has a 3-D map of all the mountains and plains on the bottom of the ocean. This is sonar mapping.

True, it takes too long for one boat to sail over every spot of the ocean. Fortunately there are fancier sonars which shout and listen in specific directions, and figure out a real picture from a jumbled mess of echoes. These sonars can look sideways from the boat for miles, so the sonar boat only has to travel lines across the ocean about 50 miles apart. I think the earth's whole ocean bottom has been mapped this way. Most large lakes, too.

There should be lots of other things that can be sonar-mapped, but I can't think of too many examples. Any other planet with an ocean is a candidate. So far that would only be Europa, 2nd moon of Jupiter, which is thought to have a liquid ocean underneath it's ice crust. Seismography is related to sonar, and the earth's interior has been partly mapped this way. The ultrasound machine your doctor uses to look inside your body does a kind of sonar mapping, instantly.

In air, we can make our maps using sight or radar. But under water or ground, we need to see things with sonar. Then we can make maps.

Jim Swenson


This is mapping done using sound as a probe of the surroundings. It is most commonly used underwater. As with any radar, the distance to a surface which reflects the sound wave can be determined when one knows the time interval for the sound to make a round trip to the surface and the speed of sound in the water. It is possible to make detailed maps of ocean bottoms even if they are rugged with many peaks and other sharp changes in the bottom.

A rather beautiful and complete description of sonar mapping is provided by USGS (United States Geological Survey) at:

http://woodshole.er.usgs.gov/operations/sfmapping/dataacq.htm

Best, Dick Plano, Professor of Physics emeritus, Rutgers University


Raffia,

The structure and topography of the ocean floor are studied through the use of satellite mapping which measures the level of the ocean surface to estimate the shape of the ocean floor. Scientists also use sonar instrument to measure the depth of the oceans. Using sonar, depth measurements are made by measuring the time for a sound wave to travel from the surface of the ocean to the ocean floor and to return. Depth measurements are made by sonar from ships that travel slowly, so only a small fraction of the ocean's floor has been mapped from depth measurements. Even using the latest sonar techniques, it would take about 125 years to map the ocean floor with depth measurements. I hope that this helps.

Sincerely,

Bob Trach



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