SONAR, short for SOund NAvigation and Ranging, is a tool that uses sound waves to explore the ocean. Scientists primarily use sonar to develop nautical charts, locate underwater hazards to navigation, search for and identify objects in the water column and on the seafloor such as shipwrecks, and map the seafloor itself. Sonar is used for oceanography because sound waves travel farther in the water than radar and light waves can.

A side-scan sonar image of the passenger freighter Robert E. Lee, collected by the HUGIN 3000 AUV in 2001.

A side-scan sonar image of the passenger freighter Robert E. Lee, collected by the HUGIN 3000 AUV in 2001. Image courtesy of NOAA. Download image (jpg, 117 KB).

The sonar system consists of physical sound sensors called transducers. Scientists may opt to use a single transducer or a group of them, called a transducer array. An array can be attached to a variety of platforms, including a remotely operated vehicle, an autonomous underwater vehicle, a ship, or a platform like a towfish or glider. There are two types of sonar—active and passive.

A three-minute video about multibeam and side scan sonar, including a visualization that shows how sonar data is used to make products like nautical charts. Video courtesy of National Ocean Service, NOAA. Download largest version (mp4, 76.5 MB).

Active Sonar

With active sonar, the transducer array emits an acoustic signal or pulse of sound into the water. If an object is in the path of the sound pulse, the sound bounces off the object and returns an “echo” to the array. If the array is equipped with the ability to receive signals, it measures the strength of the signal. By determining the time between the emission of the sound pulse and its reception, the transducer can determine the range and orientation of the object.

Passive Sonar

Passive sonar systems are used primarily to detect noise from marine objects (such as submarines or ships) and marine animals like whales. Unlike active sonar, passive sonar does not emit its own signal, which is an advantage for military vessels that do not want to be found or for scientific missions that concentrate on quietly “listening” to the ocean. Instead, passive sonar only detects sound waves coming towards it. An individual passive sonar instrument cannot measure the range of an object unless it is used in conjunction with other passive listening devices. Multiple passive sonar devices may allow for triangulation of a sound source by looking at the time delay for the sound to reach the instruments.

Types of Sonar Systems

There are several different types of sonar systems. Examples of passive sonar systems could include an individual or an array of hydrophones, either towed behind a vessel or fixed to a platform. Multibeam, side-scan, split-beam, sub-bottom profiling, and synthetic aperture sonar are all examples of active sonar systems.

Scientists choose the type of sonar based on the goals of an expedition. Some sonar systems can image a small area in very high resolution, which is useful for details like artifacts at a cultural heritage site; on the other hand, scientists may want to use a sonar system that is lower in resolution but can map a much larger area, such as a seamount or other geological feature. Learn about the different kinds of sonar and the role each plays in ocean exploration below.

Text from National Ocean Service Ocean Facts: Sonar (originally adapted from the NOAA Ocean Exploration Sonar page).