Water And Oceanography
Echo sounding

How the Depth of the Ocean Floor is Determined

Echo sounding
Jose Juan Gutierrez's image for:
"How the Depth of the Ocean Floor is Determined"
Caption: Echo sounding
Image by: Hannes Grobe
© CC-BY-SA-2.5 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Sediment_echo-sounder_hg.png

Depth sounding is the word used to refer to the task of measuring the depth of a body of water. The sounding of a body of water was initially done by using sounding sticks or weighted sounding lines of rope with spaced marks along it. This was done by early sailors and fisherman who wanted to know the depths of particular regions in a body of water. Sounding lines were commonly used until the development of other techniques, such as echo sounding. With the advent of modern technology, the measuring of the depth of the ocean has changed. The measurement of the ocean is typically done in two distinct ways: The first one is by using echo-sounding instruments and the second consists in using data provided from satellite altimeters.

The first transatlantic measurements of the ocean were made in 1922 by the ship USS Stewart. Following this, other measurements of the ocean were performed by the German research ship called Meteor during a two year expedition to the South Atlantic in 1925. After this, the echo sounding technique has been utilized by oceanographic and naval ships to map much of the ocean floor. Although the echo sounders perform some of the most accurate measurements of the ocean floor, the paths in which echo sounding measurements have been done are not well distributed, and there have been areas in the ocean floor on which the depths have not been measured from aboard a ship.

Echo sounding

Echo-sounding instruments have been utilized to map most of the ocean’s floor. Multibeam sonar is used to measure the time it takes for a pulse of sound to travel from a ship to the bottom of the ocean and back. The interval from the time the sound wave leaves the ship to the time it returns back indicates the ocean’s depth. Echo sounding has allowed oceanographers to map much of the sea floor, revealing its physical features. Echo sounding technique is very effective in areas of rocky and rough terrain, giving an ample picture of its characteristics.

Satellite altimetry

Reliable measurement of the ocean depths is provided by satellite altimetry. Satellite altimeter systems use radar to measure the altitude of the satellite from the sea surface. A tracking system is used to determine the altitude of the satellite in relation to the Earth’s center of mass. This measurement determines the shape of the surface of the ocean. Satellite altimetry instrumentation relies on the variations of local gravity due to the differences of mass along the sea floor which produces sea level undulations. For example, since rocks are denser than water, a sea mount will increase local gravity, attracting water toward the sea mount, affecting the shape of the ocean’s surface.

The geoid

The geoid is a feature that corresponds to the ocean’s surface at rest. The shape of the ocean’s surface is due to gravity variations, which generate the geoid undulations. The undulations of sea level from the geoid are the ocean’s topography. The geoid undulations are produced by the local differences in gravity due to the irregular distribution of mass at the bottom of the ocean.  Deep ocean rocky bodies, such as sea mounts, are denser than water; their excess of mass causes an upward bulge in the geoid. Trenches, which have less mass, generate a downward deflection of the geoid. Oceanic maps of the geoid closely resemble the topography of the ocean floor.

The combined digitized data from echo sounders and the information gathered by satellite altimeters has produced maps of the ocean floor with horizontal resolution of 3 kilometers and an overall average depth accuracy of plus or minus one hundred meters. According to oceanworld.tamu.edu, a number of altimetric satellites analyze the influence of sea-floor on the geoid. GEOSAT, ERS-1, Envisat, ERS-2, Topex/Poseidon and Jason are some of the altimeters that generate the most precise data of the ocean floor.

More about this author: Jose Juan Gutierrez

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