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U.S. Navy Museum Cold War Gallery Lesson Plan
How Does Sonar Work?  Mapping the Ocean Floor
Developed by Kenneth A. Nagel, Apex High School, Academy of Information Technology, Apex NC
2011 Naval Historical Foundation STEM Teacher Fellowship
 
 
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This lesson provides students a basic understanding of how sonar (SOund NAvigation and Ranging) is used to map the ocean floor. Students use a simple calculation to determine the depth of the ocean floor at 15 points from Miami Beach eastward to the wreck of the SS Sapona in the Bahama Islands. This is a total distance of 53 miles.

It is important for students to understand that in actuality, a ship using sonar to map the same 53 miles of ocean floor would be processing millions of echoes as it travels resulting in a much more detailed visual of the ocean floor.

Background

Sonar technology has made giant leaps forward since its invention and first uses during World War I. Development of this technology has been an invaluable tool vital to national security and the success of the United States Navy particularly during the Cold War.

As the former Soviet Union and the United States raced for nuclear superiority, each country strategically targeted the other's supply of land-based nuclear warheads. The solution to these relative "sitting ducks" was a launching method that couldn't be tracked from the air, wouldn't remain in the same location, but could be deployed quickly; the solution was submarines.

With both countries moving to this mobile solution, finding the enemies mobile warheads became an important endeavor which lead to a race in the advancement of submarine technology. Submarines are blind in that they do not have windows, windshields, or portholes to see your surroundings. Submarines operate at depths where there is very little if any light. It would be pointless to have a camera or porthole in the darkness of the deep sea, not to mention that considerations for creating weak points in the hull, increasing drag and disturbances in the water that would likely give away the submarines presence to enemy ships. Instead, submarines rely on a type of echo-location, Sonar, to "hear" their way around underwater obstacles or find an enemy ship.

While starting out as a military technology, sonar is readily accessible today. Sonar is one of three methods (along with satellites and submersibles) used to explore the ocean floor. In fact the technology is so accessible that even private pleasure craft and fishing boats utilize sonar technology to avoid underwater obstacles and to find fish.

Sonar can be either active or passive. Passive Sonar arrays simply collect any sound travelling through water. During WW I and WWII ships used hydrophones (underwater microphones) to listen for the sound of submarines. Active sonar arrays actually emit sound which travels through the water until it bounces off of an object and echoes back to the array's receiver. Knowing the speed of sound in water (saltwater: 5,000 ft/sec) the distance to the object can easily be calculated.

Mapping the global ocean floor remains one of the grand challenges of oceanography. Humankind has made more progress during the past 20 years of mapping the surface of our moon, Venus and Mars than during the past 500 years of mapping the oceans (Wright). Sandwell et al. (2003) estimate that it will take another 125 years to fully map the ocean floor at a resolution of 200 m, the level of detail needed for most geological studies and deep nautical charts. An even higher resolution is needed for global near-shore ocean resource management, conservation, and spatial planning. Much of this mapping is done by the National Oceanic and Atmospheric Administration (NOAA). You can follow the NOAA Research Ship Okeanos Explorer searches live at:

oceanexplorer.noaa.gov/okeanos/explorations/acumen12/welcome.html (Click on "live feed")

Similarly, you can follow Dr. Robert Ballard's Research Ship Nautilus undersea searches live at:  www.nautiluslive.org. S.S. Sapona can be located on Google Earth at:  25°39'2.22"N  79°17'36.17"W. For more history on how the ocean has been mapped check out:  www.msnucleus.org/membership/html/jh/earth/oceanography/index.html

Video Resources

play videoGraphic Display of Sound:  This sample of the visual display of underwater sound is part of the Covert Submarine Operations Exhibit, located in the Cold War Gallery, Washington Navy Yard.

play videoSonar: "The Submariners" (1967):  This segment from the 1967 Navy documentary "The Submariners" explains the basics behind the use of sonar. Source: Naval History and Heritage Command, UMO-41.

play videoIndependent Variable Depth Sonar (1971):  The basic technology and concepts behind sonar are explained in this 1971 Navy training film, which was used to train operators of the towed array's hoist mechanism. Source: Naval History and Heritage Command, Photographic Section UMO-11.

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Ocean Floor Mapping Worksheet

Objective:  Provide students a basic understanding of how sonar (SOund NAvigation and Ranging) is used to map the ocean floor. Students use a simple calculation to determine the depth of the ocean floor at 15 points from Miami Beach eastward to the wreck of the SS Sapona in the Bahama Islands. This is a total distance of 53 miles.

Materials:  Data Map with sonar duration times listed (below), Graph Paper (example below)

Instructions - Ocean Floor Mapping Worksheet:

In 1926, the SS Sapona, a ship used to transport alcohol during prohibition, ran aground during a hurricane. Today the ship sits almost 54 miles from the beaches of Miami, yet even at this distance the water is so shallow that much of Sapona's remains are well above the surface. You can actually see the remains of the ship on Google Earth at these latitude and longitude coordinates: 25°39'2.22"N 79°17'36.17"W.

How is it that fifty-four miles out from the beach this ship is sitting in such shallow water? How deep is the water where the ship lies if it can still be seen above the waves? Does the water stay shallow all the way from Miami Beach to the wreck? With sonar sounding data it's possible for us to sketch a profile of what the ocean floor looks like between Key Biscayne Beach and the wreck of the Sapona.

The map below shows the Atlantic Ocean from Miami Beach eastward to the Bahamas. Imagine, you are on a boat traveling out to scuba dive the wreck of the Sapona. Along the way the boat you are traveling on is collecting sonar data. The sonar is emitting a soundwave and tells you how long it took the sound to travel to the bottom and bounce back. Using the times listed on the map, construct a cross-section showing the slope of the ocean. Remember, in saltwater sound travels at 5000 ft/sec.
Click here for a sample graph paper layout.

Data Map:  The 17 positions on the map shown here give the length of time that a sonar pulse took to travel to the ocean floor and back to the sonar array onboard your boat. Use the formula discussed in the article to determine the depth at each location. Transfer the data to the Ocean floor worksheet to create an ocean floor profile. (click the image below for a larger map image)



 
 

 

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