Sonar

The ocean floor is full of many incredible features, from natural structures like underwater mountains and deep canyons to human-introduced objects, like shipwrecks. If the location of these features, as well as others beneath the surface, are not properly marked on a nautical chart, mariners can quickly find themselves in danger by accidentally hitting something underwater, damaging their vessel, or even running aground. Knowing where these features are located and what they look like is essential for safe navigation. Since the ocean floor can’t easily be seen, scientists rely on sophisticated tools to explore and document what is down there. One important tool they use is sonar.

An illustration showing how sonar works, with a transmitter aboard a ship sending sound waves downward in a cone shape toward the seafloor. Those sound waves then bounce back t up to a receiver. Labeled arrows and text indicate the transmitter, receiver, sound waves, return echo, and the seafloor.

On sonar systems, a transmitter emits pulses of sound energy through the water, which are reflected back from the seafloor or other objects, including living organisms! The return signals (echoes) are detected by a receiver.

Sonar, which stands for Sound Navigation and Ranging, uses sound waves to detect features underwater. Unlike light or radar, which travel shorter distances underwater, sound waves travel much farther, at about 1,500 meters per second. The sonar system will send a sound pulse, or ping, into the water. The emitted sound waves travel until they hit something, like the seafloor or another structure, and then bounce back as echoes. By measuring how long it takes for an echo to return, scientists can figure out how deep an object is and even learn about its shape, position, and texture. You may be familiar with the process of echolocation, where animals, like bats and dolphins, produce sounds to help navigate and find food within their environment. Sonar works similarly but relies on technology instead of biology to create the sound and measure the returning echoes.

In the ocean, the density of the seawater is not always the same. Saltier or colder seawater is denser, and therefore heavier, than warmer, less salty water. These differences in water density can impact how sonar sound waves travel, sometimes causing them to bend or refract. To adjust for this, scientists use tools known as sound speed profilers to measure salinity and temperature at various depths. Thanks to these tools, scientists can calculate the speed of sound for water with different densities to ensure that sonar data collected is accurate.

A gray sonar depiction showing underwater acoustic data, with irregular shapes indicating variations in the water column. Grid lines and measurements overlay the sonar readings, and a label at the bottom indicates that this data was collected aboard the NOAA Ship Rude in 1991. — Changes in characteristics, such as temperature, throughout the water column can distort sonar signals. This side scan sonar image is corrupted by a strong thermocline, a layer in the water column where temperature drops quickly with depth as the warmer surface water transitions to colder, denser deep water.

Sonar systems use sound sensors called transducers, which are made up of a transmitter to send the sound and a receiver to listen for the returning echoes. Some systems use a single transducer, called a single-beam sonar system, while others use a group of transducers, known as multibeam sonar. Multibeam sonar sends out sound waves in broad sweeps, allowing scientists to scan wide areas of the seafloor and take many detailed measurements at once.

Three images illustrating sonar mapping and detection. The image on the left is an illustration of a boat using sonar to map the seafloor, with sound waves spreading downward and reflecting off underwater features. The top right image shows a colorful 3D sonar map of a shipwreck on the seafloor, with surrounding terrain using color to show shallow and deeper areas. The bottom right image shows a sonar scan with a whale-shaped silhouette to the left of a central track line. — Hydrographic survey vessels utilize both a multibeam sonar and a towed side scan sonar to map the sea floor. The image on the top right shows a multibeam 3D point cloud visualization of a shipwreck resting on the seafloor, with colors representing depth from red (shallow) to blue (deep). The image on the bottom right shows a humpback whale detected using side scan sonar.

Once the data is collected, scientists can use it to create bathymetric maps, which show the shape and depth of the ocean floor. This information can then be added to nautical charts to provide more details that will help navigators travel safely. Thanks to tools like sonar, scientists are able to better explore and understand the world beneath the waves — one ping at a time!