CD 157: Investigating the submarine canyons off Portugal


 


All about the echo sounder

Using sound to look at the seafloor


How can we find out what the deep sea looks like?

Because air is transparent it’s easy to view things through it. But because water is an opaque medium it’s much harder to get an idea of what the surface beneath it looks like.

Sound has been one of the most useful tools used in geological oceanography. Echo sounding has been used since the fifties to measure the depth of the ocean. Before the use of this technique the depth of the ocean was measured by reeling a weighted rope into the water and when it reached the bottom it gave the depth measurement.

Echo sounding is a geophysical technique of finding the approximate depth of water and therefore the topography of the deep sea surface. It works on the principle that sound waves are transmitted through water; when they strike an object they are reflected back as echoes.

The quantity measured by an echo-sounder is the elapsed time between the emission of a downward- directed pulse of high frequency sound and the return of part of the same pulse as it’s reflected from the seabed and reaches the transducer.

The equipment displays and records digitally the approximate depth of water. It does this by halving the 2-way pulse travel time to give a one-way travel time. Using the formula speed equals distance divided by time it finds the depth by multiplying the time by the speed of sound in water which is around 1500m/s.

On board Charles Darwin we use two conventional echo sounders that use different frequencies of sound; the 3.5KHz and the 10KHz. The difference between them is that the 10KHz uses a higher frequency. It has a lower penetration but has a good resolution giving a good picture of what the sea surface looks like. The 3.5KHz uses a lower frequency; the penetration is higher giving a better idea of what the deeper layers of sediment are like but the resolution isn’t that good. Because each one gives slightly different information they are both used on board.

Screens of the 3.5KHz and 10KHz echo sounders

Conventional echo sounders have two main limitations; one is that they produce only a single line of depth measurements beneath the ship. Secondly the measurements are not very sharp. The sound the sonar emits spreads out over a large area of sea surface, so the resulting echo comes from different places creating a depth measurement that comes from different points.

On board we also have a multibeam sonar. Rather than using a single transmitter it has a line of sound transmitters, this way the waves are fanned in a wide swath. The waves actually interfere with each other but this creates an insonified fan of beams. To receive the waves you have a line of receivers, so you collect information from a whole insonified stripe rather than just from a single line.

Most whales, dolphins and porpoises also use sound to picture their surroundings. They make sounds that bounce off nearby objects and alert them if there is something near in the water.



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© Challenger Division for Seafloor Processes
April 2004
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