Mermaids File: How sonar works and its uses

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Sonar
The word "sonar" comes from the first letters of "sound navigation ranging." Sonar can detect and locate objects under the sea by echoes, much as porpoises and other marine animals navigate using their natural sonar systems.

How Sonar Works
There are two types of sonar sets: active and passive. An active sonar set sends out sound pulses called pings, then receives the returning sound echo. Passive sonar sets receive sound echoes without transmitting their own sound signals.

In active sonar sets, the sound signals are very powerful compared with ordinary sounds. Most sonar sets send out sounds that are millions of times more powerful than a shout. Each ping lasts a fraction of a second.

Some sonar sets emit sounds you can hear. Other sonar signals are pitched so high that the human ear cannot hear them. These signals are called ultrasonic waves. ("Ultra" means "beyond," and "sonic" means "sound.") The sonar set has a special receiver that can pick up the returning echoes. The location of underwater objects can then be determined by the length of time that elapses between sending the signal and hearing the returning echo.

Uses of Sonar
Sonar has many uses. Submarines use sonar to detect other vessels. Sonar is also used to measure the depth of water, by means of a device called a Fathometer. (One fathom equals 6 feet, or about 1.8 meters.) The Fathometer measures the time it takes for a sound pulse to reach the bottom of the sea and return to the ship. Fishing boats use Fathometers to locate schools of fish.

Oceanographers use sonar to map the contours of the ocean floor. Sound signals can also be sent into the mud or sand on the ocean floor and strike a layer of rock underneath. An echo then comes back, giving the distance to the rock layer.

The same principle is used in searching for oil on land. A sonar pulse is sent into the ground. Echoes come back from the different layers of soil and rock and tell geologists what kinds of soils and rocks are present. This helps them identify areas for drilling that are most likely to contain oil or gas. This subterranean mapping is called seismic exploration.