Transcript Mapping the Seafloor - THS Aquatic Science

Mapping the Seafloor
Mapping the Seafloor
Depth Soundings
The earliest soundings were made with a hand
line of rope weighted at one end.
 Sticky tallow was often smeared on the weight
to pick up sand and other sediments from the
seafloor. The weight was then dropped
overboard and the rope allowed to run free
until the weight reached the bottom.
 The length of the line let out was an
approximate measurement of the water
depth.
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This type of sounding is only reliable only in calm,
shallow areas
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Near continental shelves, in inland seas, and near med
ocean islands.
The first successful deep water soundings were
made with large balls of twine.
A heavy weight was attached and tossed overboard,
pulling twine from the ball until it hit bottom.
 Then the ball of twine was cut from the weight and the
twine that had run out.
 Because the weight of a length of twine was known, the
depth of the ocean at that point could be determined.
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Sonar Soundings
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Modern oceanographers use sophisticated
remote-sensing techniques to gather data.
The data is then plotted on charts and maps and
used to create models that help us understand
what seafloor features are like.
In 1930 the echo-sounding sonar devices
(fathometers) replaced sounding lines.
The term SONAR is formed from the term
sound navigation ranging.
Sonar Soundings cont.
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Sonar works by sending out pulses of sound waves from a ship.
 Instruments record the time it takes for the sound waves to travel
to the bottom, reflect, and return to the ship.
 Because the velocity of sound in seawater is known to be about
1,460 meters per second, the depth can be calculated.
 The speed does vary with the temperature and salinity in different
regions of the ocean.
Sonar Soundings cont.
The advantage of using sonar is that a vessel can
keep moving at normal speed while soundings are
made.
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Recording a series of soundings on a strip of
paper or making a computer printout produces a
two-dimensional visual profile
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also called an echogram or sonograph
Research vessels cross the ocean making sonar
profiles along navigated parallel course called
transect lines.
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Only one beam is used.
Swath Mapping
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Developed in 1970’s
Instead of a single line of sounding, this makes many measures of
depth within a two-dimensional area of the seafloor.
With swath mapping one transect the
area sounded may be 10-60 km wide.
Details are so clear that small scale features
can be clearly identified.
Features 10m across can be detected.
Scientists use detailed swath mapping and
image mapping to learn more about the
processes that formed the sea floor.
There 2 types of swath
mapping:
1. multibeam sonar –
sends out and tracks up
to 16 closely spaced
sonar beams at a time
2. side-scanning sonar –
uses computers to
translate the multiple
echoes into detailed
three dimensional
images of the seafloor
features
Bathymetry
Multibeam
systems can
provide more
accurate
measurements than
echo sounders.
Multibeam systems
collect data from as
many as 121
beams to measure
the contours of the
ocean floor.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Claims to the Ocean
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Nations considered the ocean open to everyone (Freedom
of the Sea)
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Ocean resources were available on a first come, first served basis.
In an effort to find fuel, food and minerals to be used by 6 billion
people, 40% of the oceans have been claimed.
The claimed regions are called Zones.
Coastal nations claimed a territorial zone or sea of 3 nautical miles
from shore.
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They had full control of the zone but allowed nonmilitary commercial
ships to pass through.
Many nations claim 12 nautical miles. This was complicated by
straits.
Claims to the Ocean
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1958 – United Nations agreed upon a “Law of the Sea”
which provided for passage provisions for ships, subs and
airplanes
Today we have “Exclusive Economic Zones”
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Nations claim sovereign rights to all resources, living or nonliving
up to 200 nautical miles from shore.
The U.S. made their claim in 1958.
The U.S. EEZ is larger than the land area of the U.S.
The world’s EEZ zones, shown as a white extension
of land territory.
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By 1990 The two federal agencies, the U.S. Geological
Survey and the National Oceanic and Atmospheric
Administration, planned to complete swath mapping and
assessment of the living and nonliving resources of the
U.S.EEZ.
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It is not finished.
In 1982 the new Law of the Sea Convention (LOSC) was
written by the United Nations
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acknowledged the 12 mile territorial sea, making provisions for
international straits and allows for 299 mile EEZ.
All waters beyond the EEZ’s of nations are the High Seas and
the seabed and subsoil beneath are called “the Area”.
LOSC proposes that the U.N. administer the High Seas and the
Area and that all nations share their ocean technologies and the
wealth obtained from those technologies.
Today’s bathymetric maps of the ocean
accurately show ocean depths determined by
seismographic techniques.
Importance of Accurate Maps
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Detailed, accurate maps and models
of geological features are used by
scientists to explain how the features
of the earth form and how they
change over time.
Bathymetric Maps
•Data typically comes from a sonar mounted beneath or over the side of a
boat, "pinging" a beam of sound downward at the seafloor.
•The amount of time it takes for the sound or light to travel through the
water, bounce off the seafloor, and return to the sounder tells the
equipment what the distance to the seafloor is.
•Data is supplied on the elevations and depressions of the ocean floor.
•By drawing contour lines connecting points of equal depth, oceanographers have
found that:
- the floor of the ocean has features like land-form features
•undersea mountains, ridges and other masses
Contour Maps
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Use colors to represent different elevations
Use shades of blue to show changes in depth.
Features above sea level are positive numbers
Features below sea level are negative
numbers.
http://academic.brooklyn.cuny.edu/geology/leveson/core/linksa/elevation.html
Contour Map
Practice with Contour Maps
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Three-Dimensional Maps
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The most realistic maps are contour maps
and three-dimensional raised relief maps.
Both use colors and shades to convey
information about features.
Landform models can also be sculpted to
show the effects of erosion, etc.
Three-Dimensional
Raised Relief Map
3-D Contour Map
Contour Map
2 Dimensional vs. 3 Dimensional