The Mariana Trench

The Mariana Trench - Exploration (cont.)

Exploring the deep - A Brief History

In the days of wooden sailing ships, little was known about the depth of the open oceans other than that the water was deep enough to allow safe passage of a ship's keel. Even less was known about the incredible structures and processes that shaped the oceans and continents. The only direct measurements of the water's actual depth were obtained by lowering a heavy weight on the end of a long line (rope or cable) marked in fathoms and trying to tell when it hit bottom. Although the length of the line from the ship to the weight could theoretically derive accurate water depth measurements, inaccuracies due to the effects of currents, ship motion, and stretching on the line were inevitable. Furthermore, this technique allowed only single point measurements that may (or may not) be representative of the entire region being sampled. Therefore, a complete and accurate map of the submerged ocean floor would have required millions of such observations. Without modern technology, sailors would still be at sea dropping weighted long lines overboard, while the truth about the world beneath the waves would still be buried under theory, conjecture and fantasy.

Today's oceanographers use sonar instruments to generate a sound signal that is bounced or "echoed" off the sea floor and then recorded on board the ship. The speed of sound in water is approximately 1,500 meters per second (four times faster than the speed of sound in air). By carefully measuring the round-trip time of the sound waves (taking into account the effects of temperature, pressure and salinity on sound speed), the depth of the water and/or the distance to another object can be accurately measured. When pulses are sent out and received in quick succession, an almost continuous recording of the ocean depth (i.e., a bottom profile) may be obtained. Furthermore, multiple sonar beams arrayed perpendicular to the survey ship's track can produce a swath of data points that contains far more useful information than a "simple straight-down look." In recent years, radar measurements of the sea surface from an orbiting satellite have been interpreted to measure the effects of variations in the Earth's gravitational field on the shape of the sea surface. Stronger gravity from shallow masses (like sea mounts and ridges) attracts water toward the object and creates a "hump." Weaker gravity where the dense sea floor is farther from the surface attracts less water and leaves a "depression" over trenches and abyssal plains. Although these satellite data yield no absolute depth measurements, they can be used to fill gaps between trackline data by providing relative "ups and downs."

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The information appearing on this page was taken from an original article published by NOAA (National Oceanic and Atmospheric Administration) Magazine Online.

Fact Finder



In the days of wooden sailing ships, depth was measured by lowering a heavy weight on the end of a long line (rope or cable).

 

Today's oceanographers use sonar instruments to generate a sound signal that is bounced or "echoed" off the sea floor and then recorded on board the ship.

 

The speed of sound in water is approximately 1,500 meters per second


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