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