An echo sounder is an instrument on a ship that measures water depth by sending a pulse of sound down and timing how long the echo takes to come back from the seabed.
Key takeaways
- An echo sounder measures depth from the time a sound pulse takes to go down to the seabed and back.
- A single-beam echo sounder reads one spot under the ship. A multibeam system sends many beams in a fan and reads a wide strip.
- The speed of sound in water varies, so surveyors measure it before turning travel time into depth.
- The GEBCO grid behind this map is built partly from this kind of ship data, with satellite estimates filling the gaps.
How an echo sounder works
An echo sounder is a simple idea. A device on the hull of a ship, called a transducer, sends a pulse of sound straight down. The sound travels to the bottom, bounces, and returns as an echo. The instrument times the gap between sending and receiving. If it knows how fast sound moves through the water, it can turn that time into a distance [1].
Active sonar of this kind works by measuring the time between the emission of the pulse and its reception, from which the range of whatever the pulse hit can be found [1]. When the target is the seabed, that range is the depth.
The speed of sound is the awkward part. It changes with the temperature, salt content and pressure of the water. Surveyors take measurements of the speed of sound where they work so that they can translate the two-way travel time into a depth [2]. Without it, the same echo would give a slightly different depth on a different day.
A single-beam echo sounder reads one spot under the ship. Drive in a line and you get a profile of the seabed along your track. A multibeam system uses several simultaneous beams in a fan shape instead, covering the space under the ship and out to each side [2], which is why it maps a strip rather than a line. Our page on the multibeam echosounder covers that, and side-scan sonar is a different tool again.
Historically, depth was measured by lowering a weighted line (see depth sounding). An echo sounder replaced that with a few seconds of sound,.
An example from our data
Ship soundings are the sharp end of the map. The deepest place in our grid, Challenger Deep, reads 10,927 m, and the deepest single cell, in the Mariana Trench, reads 10,931 m. Published figures for the same place differ by tens or hundreds of metres, because different ships used different instruments and different sound-speed corrections.
The GEBCO_2026 grid combines ship data with satellite estimates. Where a ship has passed with a modern echo sounder, the grid can show detail. Between the ship tracks, the satellites supply the broad shape. The homepage tells that story in short.
The thing people get wrong
People imagine a ship sweeping the ocean like a scanner. An echo sounder only sees what is below it. A research ship moves slowly, so even with a fan of beams it covers a narrow strip of the ocean. This is why mapping all of it takes so long, and why the grid still relies on satellite estimates.
Another mistake is to take an echo sounder reading as exact. The echo is a pulse that spreads out, the speed of sound has to be estimated, and the ship rocks. A depth is an excellent measurement, not a perfect one.
Echo sounder vs sonar
| Echo sounder | Sonar | |
|---|---|---|
| What it is | A sonar pointed down to measure depth | The whole family of sound-based detection |
| Gives | A depth under the ship | Depth, objects, images, or listening, depending on type |
| Also includes | Single-beam and multibeam | Passive sonar, side-scan sonar and more |
Frequently asked questions
What is the difference between an echo sounder and sonar?
An echo sounder is one kind of sonar, aimed straight down to measure depth. Sonar is the wider family and includes systems that look sideways, listen passively or search for objects.
How does an echo sounder measure depth?
It sends a pulse of sound down, times the echo from the bottom and converts that time to depth using the speed of sound in the water.
Do echo sounders map the whole ocean?
No. They only see the water under and beside the ship, so the seabed is mapped one ship track at a time.
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).