Echo-sounding instrument

From OceanWiki_EN

Echosounder - an electrical depth sounder that uses sound echoes to determine water depth. It does so by timing how long it takes a sound pulse to leave the instrument travel to the seafloor and return to the receiver on the ship.

The speed of sound in the ocean

Soundings are now made by sending a sound pulse into the water. To use sound to measure water depth we need to know how sound travels through the ocean. The speed of sound depends on the temperature of the water, its salinity, and the pressure (which is equivalent to depth below the sea surface). The speed of sound ranges between 1400 and 1570 m/sec (4593 and 5151 ft/sec). This is roughly 1.5 km/sec (just under 1 mile/sec) or about 4 times faster that sound travel through air. The graph below shows how sound speed varies as a function of water depth.

Since World War II echosounders have been used to determine water depth in the oceans. Echosounders are attached to the hull of a ship or a towed vehicle. An echosounder sends an outgoing sound pulse into the water. The sound energy travels through the water to the ocean bottom where it is reflected back towards the source, received, and recorded.

The time that it takes for sound to make the round trip to the seafloor and back is accurately measured. Water depth is determined from the travel time and the speed of sound in water.

Water depth can be estimated simply by using an average sound speed and the following relationship: Distance = speed multiplied by time/2

The time is divided by 2 to account for the two-way trip to the sea floor and back.

[edit] References

• www.punaridge.org/doc/factoids, the content and illustrations in the Science Factoids were motivated by subjects covered in two excellent textbooks: Introduction to Oceanography, David A. Ross, HarperCollins College Publishers, 1995 und Essentials of Oceanography, Tom Garrison, Wadsworth Publishing Company, 1995