Sonar, short for Sound Navigation and Ranging, uses sound waves to detect and measure things underwater. It is used to map the seafloor, find wrecks and spot hazards, because sound travels farther in water than light or radar.
Key takeaways
- Sonar uses sound because sound waves travel farther in water than radar and light waves.
- Active sonar sends a pulse and times the echo. Passive sonar only listens.
- Seafloor mapping, nautical charts, wrecks and hazards are among sonar's main uses.
- Sonar surveys supply the sharp part of the GEBCO_2026 grid behind this map, and satellites fill the gaps.
How sonar works
Sonar is short for Sound Navigation and Ranging. It is useful for exploring and mapping the ocean because sound waves travel farther in water than radar and light waves do [1]. Scientists primarily use it to make nautical charts, locate underwater hazards, search for and map objects on the seafloor such as shipwrecks, and map the seafloor itself [1].
There are two types, active and passive [1].
Active sonar sends out a pulse of sound from a device called a transducer. If something is in the way, the sound bounces off and comes back as an echo. The time between sending the pulse and receiving the echo gives the range of the object, and the strength of the signal says something about it [1].
Passive sonar sends nothing. It listens for noise from ships, submarines and animals such as whales, and it only detects sound waves coming towards it. It is useful when you do not want to be heard, or want to listen quietly to the ocean [1].
Seafloor mapping uses active sonar, and it comes in several designs. A single beam looking straight down is an echo sounder. A multibeam system sends many simultaneous beams in a fan, covering the space under the ship and out to each side, and collects both depth and a measure of the strength of the returning echo, called backscatter [2]. Backscatter can give clues about the make-up of the seabed [2]. Side-scan sonar looks sideways rather than down and is used to make images of the bottom, as in the glossary entry on side-scan sonar.
Together these tools give the sharp parts of the map. The homepage explains how the sonar surveys are blended with satellite estimates.
An example from our data
Sonar is why we know anything about the deepest places. The deepest point in the GEBCO_2026 grid is Challenger Deep, at 10,927 m. The grid has 950 named features, such as trenches (39), seamounts (420) and ridges (112), and most of their shapes were first drawn by ships pinging the bottom.
The limits of sonar show up in the map too. A ship sees a narrow strip. In the open Pacific Ocean, a huge area with a mean depth of 4,315 m, large parts are known only through satellite estimates, so the shading is smooth.
The thing people get wrong
Sonar is often thought of as a military device for finding submarines. Passive sonar does suit that job, but active sonar is also the main tool for mapping the seafloor [1]. Another confusion is that sonar is a single instrument. It is a method, and echo sounders, multibeam systems and side-scan sonar are different tools that all use it.
Sonar vs echo sounder
| Sonar | Echo sounder | |
|---|---|---|
| Scope | The method: sound to detect and measure | One tool that uses it |
| Direction | Down, sideways or listening | Straight down |
| Gives | Depth, images, objects, noise | Depth under the ship |
Frequently asked questions
What does sonar stand for?
Sound Navigation and Ranging.
Why is sonar used instead of cameras or radar?
Because light and radar waves fade quickly in water, while sound travels much farther.
Can sonar see the whole seabed?
Only where a ship or vehicle with sonar has been. Most of the deep ocean has not been surveyed that way.
Sources
- What is sonar?, NOAA Ocean Service.
- Multibeam sonar, NOAA Ocean Exploration.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against GEBCO_2026 grid).