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CONCEPTS
- Sound energy traveling through the water is used to measure the depth
of the water. From this you can determine ocean floor topography.
- TOPEX/Poseidon uses radar to measure the height of the satellite above
the sea. From this you can determine sea surface
topography. The radar signal also measures wind speed and wave height.
- Click here to see an animation
of how an altimeter works (323 K).
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MATERIALS (per group of four students)
- Shoe box with lid
- Stiff wire or 1/4" wooden dowels
- Ruler
- Newspaper, clay, plaster, or rocks to use for the simulated ocean
bottom (or you can use wood scraps of varying thickness and shapes and
glue them to the bottom of the box).
- 2 sheets of graph paper
- Ice pick (or other tool to punch holes in the top of a shoe box)
- (optional) QuickTime animation viewer installed on your computer
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PREPARATION
- Set up one work station for each group. Use an ice pick to punch holes
in the top of the box every two centimeters (about 1 in) along one or
more lines running from one end of the box to the other. Having only
one line of holes simplifies the activity, but produces only one profile
across the simulated sea floor. Label holes from one end of the box
to the other as 0 cm, 2 cm, 4 cm, etc. Label them with 2 coordinates
if you have more than one row of holes. For a more elaborate project,
use plaster of Paris to form the simulated sea floor.
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PROCEDURE
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Engagement
- How is ocean depth measured? What different instruments can
we use? In this activity you will see how a fathometer uses sound
to measure ocean depth and helps to provide a picture of ocean
floor topography. When you are finished, you can use the information
you learned about a fathometer to understand how scientists use
an altimeter to measure sea surface topography.
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Activity
- Gather materials to create a model of the ocean floor. Open
the shoe box and use the materials provided to shape an ocean
bottom in your box. You might stack rocks, wad wet newspaper,
or layer clay. Make sure that there are at least two different
major topographic features (mountains, slopes, shelves, trenches)
in your design. Label one end of the box "X" and the
other "Y".
- When finished, sketch an approximate side view picture of the
ocean bottom on graph paper starting at "X" and ending
at "Y". Tape this picture on the underside of the lid,
and write your group number on your box.
- The teacher will collect and redistribute all of the boxes.
Each group should get a box other than the one that they built.
- The wire or wooden dowel represents the sound wave that is bounced
off the ocean floor. Slide the wire or wooden dowel vertically
into each hole in the box. Keep track of the length that fit into
the box with either a pencil mark or your fingers. Use the ruler
to measure how deep the "sound wave" penetrated the
box at each hole.
- On the graph paper, plot depth versus location for each row
of holes. The graph should be a fairly accurate representation
of the underwater features along that row of holes.
- After the graph (or graphs) have been made, open the box and
compare your graph to the "ocean floor" in the box and
to the drawing on the lid of the box.
- Compare results with the class.
- You may want to repeat the procedure after redistributing boxes
again.
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Explanation
- Sailors have long used ropes with weights on the end to measure
depths in shallow water. Often they tied knots 6 feet (1 fathom)
apart in the rope for ease of measurement. The number of knots
let out gave the depth in fathoms once the weight hit the bottom.
The first accurate physical measurements of the deep ocean bottom
were made by Sir J. Clark Ross in 1840 who measured a depth of
4435 meters off Antarctica. Later, extensive depth recordings
were taken by scientists on the H.M.S. Challenger using steam-driven
winches with one inch hemp rope that did not tangle. Scientists
lowered the rope until it hit the bottom, recorded how much rope
had been let out, and pulled the rope back. Letting out 3,000
meters of rope and reeling it back in could take as long as twelve
hours. Today, scientists can use sonar to make the same measurement.
First, a sound or "ping" is sent toward the bottom.
The"ping" reflects off the bottom, and an instrument
(called a transducer) mounted in the ship's hull receives the
reflected ping. A timer records the time from transmission to
reception. The depth of the water is calculated by multiplying
the time by the speed of sound through water. The distance traveled
by the ping is twice the depth from the vessel to the bottom.
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EXTENSION
- The TOPEX/Poseidon satellite's primary instrument for measuring ocean
topography is a radar altimeter. The altimeter bounces a microwave pulse
off the sea surface. By measuring how long the signal takes to return,
the altimeter determines the distance between the satellite and the
sea surface. Water vapor in the atmosphere slows down the return of
the microwave signal. So an instrument called a radiometer is used to
adjust for the influence of water in the atmosphere. Altimeter and radiometer
data, combined with precise knowledge of the satellite's position, are
used to obtain a detailed map of sea surface topography. From this,
scientists calculate ocean current patterns and speeds.
- Radar altimeters have also been used to determine the topography of
Venus, most notably from the Magellan spacecraft. Its radio waves were
able to penetrate Venus' thick cloud cover (click
here to see an image of Venus' cloud cover). Have the students research
methods and uses of altimeters on Earth and other planets. If you were
planning a mission to Mars or Jupiter's moon, Europa, would you explore
using sound waves or radio waves? Why? What type of scientific studies
are aided by topography data?
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VOCABULARY
- altimeter: an instrument that is used to measure altitude.
The altimeters on TOPEX/Poseidon measure the distance between the spacecraft
and ocean surface.
- fathom: a unit of water depth, 6 feet or 1.83 meters. It was
originally derived from the distance be-tween the hands of a large man
with his arms outstretched. It is also sometimes used in expressing
horizontal distances, in which case 120 fathoms make one cable, or very
nearly 0.1 nautical mile.
- fathometer: instrument used for a sonic (sound waves) depth
finder.
- ocean floor topography: the hills and valleys of the ocean
floor. Maps of the ocean floor topography are called bathymetric maps.
- radar: from "RAdio Detection And Ranging," a method
for detecting the position, velocity, and other characteristics of a
distant object by analyzing the high frequency radio waves reflected
from the object's surface.
- radio wave: electromagnetic wave with frequencies lower than
microwave.
- sea surface topography: the very subtle hills and valleys on
the surface of the ocean caused by gravity and wind patterns. The pressure
differences within these hills and valleys can drive ocean currents.
- sonar: an acronym for SOund NAvigation Ranging. It refers
to method or equipment for determining by underwater sound techniques
the presence, location, or nature of objects in the sea.
- sound wave: longitudinal pressure waves in any material medium
regardless of whether they constitute audible sound. Earthquake waves
and ultrasonic waves are sometimes called sound waves.
- topography: the configuration of a surface, including its relief.
In oceanography, it refers to the ocean bottom or the surface of a mass
of water.
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SOURCE
- "Visit to an Ocean Planet" educational CD-ROM, Copyright
Caltech and NASA/Jet Propulsion Laboratory
- Adapted from Orange County Marine Institute Science TV Curriculum
Series.
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