How Sonar Maps the Seafloor for Nautical Charts: Pings, Echoes and Multibeam

The seafloor is full of hazards: underwater mountains, deep canyons, and objects people left there, such as shipwrecks. If they are not marked on a nautical chart, a ship can hit one, damage its hull or run aground. Because nobody can simply look at the ocean floor, surveyors rely on an instrument that listens instead: sonar.

Sonar surveys run from the coast out to offshore waters. Image from NOAA.

Timing an echo

Sonar stands for Sound Navigation and Ranging. Light and radar do not go far underwater, but sound does, at about 1,500 meters per second. A sonar system sends a pulse of sound, a ping, into the water. It travels until it hits the seafloor or another object and bounces back as an echo. The time the echo takes to return gives the depth, and the echo can also reveal an object's shape, position and texture. Bats and dolphins find their way the same way by echolocation; sonar does it with technology.

A transmitter sends pulses of sound; the seafloor, objects and even animals reflect them back to a receiver. Image from NOAA.

When the water bends the sound

Seawater is not the same everywhere. Saltier or colder water is denser than warmer, fresher water, and those differences can bend, or refract, the sound. Surveyors therefore use sound speed profilers to measure salinity and temperature at different depths and work out the speed of sound through each layer, so that the depths they record are accurate.

A side scan sonar image corrupted by a strong thermocline, a layer where the temperature drops quickly with depth. Image from NOAA.

One beam or many

The part of a sonar system that makes and hears the sound is the transducer, a transmitter and a receiver together.

• Single-beam sonar uses one transducer and measures the depth straight below.
• Multibeam sonar uses a group of them, sending sound out in broad sweeps to measure a wide strip of seafloor in detail at once.
• Side scan sonar, towed behind the ship, builds an image of the seafloor and anything on it.

A survey vessel using multibeam and towed side scan sonar. Top right: a multibeam 3D view of a shipwreck, red shallow to blue deep. Bottom right: a humpback whale caught by side scan sonar. Image from NOAA.

From echoes to charts

The measurements become bathymetric maps, which show the depth and shape of the ocean floor, and those details go onto nautical charts to help mariners navigate safely. Earlier charts had far fewer soundings to work with.

A 1939 chart in the Caribbean showing water depths. Image from NOAA.

Sources

• NOAA National Ocean Service, "Sonar," Nautical Charts tutorial; rewritten in hubnx's own words. Images from NOAA.