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Topic: Properties of the Ocean

Theme: Systems and Structures

In the summer of '99 the news focused for several days on the search for the downed plane of John F. Kennedy, Jr. and his wife and sister-in-law. Students who heard the news know that SONAR was used in the search. Since the ocean is so large and the ship can cover only a very small part of the ocean each time it passes over an area, many trips-back and forth-are required for the ship's sonar to get a more complete picture of the ocean floor. This was the case with the missing plane. Even though radar readings gave them an approximate idea of where the plane was when it disappeared, it took almost five days to find it on the ocean floor. In February of 2000 an Alaskan Airline when down off the coast of California. Unlike the Kennedy scenario we had radio contact early with the doomed aircraft and radar readings that somewhat pinpointed the spot where the ship went down. It did not take as long to find the Alaskan Airline.

Your teams task will be to discover the difficulties encountered when trying to locate objects or to map the ocean floor using "mock" sonar set-ups.

You will need the three mystery "bathymetry shoeboxes" (created earlier by your Admiral [teacher] from the Ocean World Web Site on the Activities page), three wooden dowels, graph paper (or access to Chart Wizard/Microsoft Excel) and three diferent colored pencils

Mission:

Team members will need:

  • Graph paper
  • Three different colors of map pencils (crayons or ink pens will do)
  • A completed shoebox
  • A wooden dowel
  • Ruler

What to do:

  1. Crew members of the lab team need to assign a different color for each of the three rows of holes on their shoebox. For example, Row 1 = red, Row 2 = yellow, Row 3 = blue.
  2. Select one row to do first (Again, it doesn't matter which row--just do one.)
  3. Insert the wooden dowel in the first hole of their chosen row and measure the depth from the surface (lid) by marking it with their finger, pulling the dowel out and measuring it on the ruler. Record this data on the graph paper in the appropriate color.
  4. Continue doing step three until all the holes in their first row have been completed. Connect the dots on the graph.
  5. Based on this one row of measurements, predict what the seafloor in you shoebox bathymetry activity looks like. Record predictions.
  6. Repeat step three on the next row of holes using a different colored pencil. Record data on the same graph paper on the same lines as for row one. Examine your results. Are they the same? What does this new information reveal to you? Predict what you now think the seafloor bathymetry is like.
  7. Repeat step three for the last row of holes using a different colored pencil. Record data on the same graph paper. Has you idea of what the seafloor bathymetry for your area of the ocean changed from your original prediction?
  8. When all three bathymetry boxes have been measured and graphed, open your shoebox and examine the bottom. Compare the revealed bottom to your lab teams graphs.

Assessment:

You will be assessed on the completition of your graphs.

It has been said that sound is the "eyes" of the ocean. What might this mean?

In all but the shallowest areas, the seafloor cannot be seen by the naked eye. In the clearest, shallow water we see at best a murky or hazy image of the seafloor, but only where the depth is a few tens of meters at most. Our ability to study the deep ocean is limited because light only reaches down abut 100m and we can see less than 50m underwater. Satellites cannot measure the seafloor directly because most electromagnetic radiation (light or radio waves) cannot penetrate the depths of the ocean.

What does SONAR stand for?

SONAR is an acronym for sound navigation and ranging. It is an instrument used to locate objects underwater by reflecting sound waves.

How did early oceanographers explore the bathymetry of the ocean before echo-sounding/Sonar?

They lowered a line with a lead weight attached into the water and counted the the number of lengths of line hauled back onboard after the weight hit the bottom.

Why do we still know less in some ways about the floor of the oceans than we do about the surface of the moon?

It is hard to study the ocean and its floor because:

  • Even though the moon is a greater distance from Earth than the ocean bottom is from the sea's surface, we can't see the ocean's bottom, but we can see the moon's surface.
  • The oceans are nearly 4,000m (on average) deep and sea water is not very transparent.
  • Pressure is extremely high in the deep parts of the ocean.
  • Seawater is corrosive so instruments must be kept from getting wet.
  • The sea surface is dynamic so it's hard to deploy (place or launch) instruments over the side of a ship.
  • We cannot breathe in water so we can't easily swim down to the bottom.
  • Water absorbs light and other electromagnetic radiation.
  • We must tow sonars back and forth across the ocean to map the sea floor.
  • Most important reason: There just aren't enough ships available with sonar to map the seafloor. Areas the size of Texas have never been surveyed.

Extensions:

  • Find out about theTOPEX/Poseidon satellite's (and the soon-to-be launched JASON) contributions to knowledge of seafloor topography. It doesn't use light and it doesn't use sonar. So how does it work?
  • Find out what side-scan SONAR is?
  • Find out more about each of the factors listed above as reasons why we know less than we would like about the floor of the ocean.

Lesson Ideas created/adapted by Margaret Hammer (Graduate Research Assistant) and Judith Kenworthy (Technology Mentor Fellowship Associate) Texas A&M University. All comments and questions can be directed to stewart@ocean.tamu.edu