Seeing the Seafloor Using Sound - Multibeam Sidescan Sonar
Steven Stevenoski Lincoln High School Wisconsin Rapids, Wisconsin firstname.lastname@example.org
Graphs made using a motion detector can be used to study mapping of the topography of a model seafloor. A motion detector measures the distance to the nearest object in front of it by emitting and receiving pulses of ultrasound. A calculator can use distance and time measurements to produce a graph that represents surface features in much the same way that sonar does. In this experiment, you will use a TI Graphing Calculator, a CBL System, and a motion detector to produce graphs of seafloor.
In this experiment, students will
You will need to have access to a computer (Mac or Windows) with a TI-Graphlink cable. This device connects to the communication port on your computer and allows you to add programs to the calculator for students to collect data with. The Graphlink program for the computer is supplied with the cable (Graphlink price = $55.00). Updates to the Graphlink program are available at the Texas Instruments Website (http://www.ti.com/calc/docs/downloads.htm)
The program PHYSCI will also have to be loaded onto each computer using the Graphlink and software. This program is also supplied with the Graphlink. Updates of the program are available at the same website. The program is also available at http://www.vernier.com/cbl/programs.html.
Plan on taking about 1 minute per calculator to load the program. You can also do just one from the computer and have the students use a link cable between calculators to transfer the program one to one. This is a preference that will be based on the background of your students.
Once the programs are on the calculator plan on about 5 to 10 minutes to test one of the units. Run through the program so that you are familiar with the program commands. The PHYSCI program is well written and will allow your students to do many other types of data collection as well as this activity. Occasionally students will get confused with selecting probes and picking the graph. If you read student procedure while checking through the program, you will recognize many of the potential problems your students might have.
You will have to collect about 5 objects per group. Boxes, cans, etc are suggested, but you can use anything you have in your classroom for your students to use. It is also possible to image a person sitting or laying down with the motion detector as well. The only limitation to working with the motion detector is that you must have the detector a minimum of 0.5 meters above any object. If the distance is smaller than this then your data will be noisy.
*** The graphs that the students generate on their calculators will be upside down relative to the objects. This occurs because the graph produced is Distance versus Time. The taller the object, the closer that it is to the motion detector. You can remedy this in one of two ways. 1. Simply explain to the students the idiosyncrasy of this graphing method and have them transpose their graphs 180 degrees when they draw them in their reports. 2. Have the students come up with a mathematical solution to get the data in the correct orientation. Both methods work well with the data. Choose which method works best for your class.
This activity can be conducted in two parts. The first part is student driven. It will give them an opportunity to test their problem solving and analysis skills. If you choose to do the final part of the activity you will need to prepare an "unknown seafloor". At the end of the student procedure, they are told that they need to check with the teacher to complete the activity. You will have to construct a "seafloor" just as the students had, but yours should be placed behind a screen of cardboard or a sheet draped over some ring stands. Students will use the motion detector just as they did during the first part of the activity, but the only way that they can "see" your seafloor is through their data.
Engagement and Exploration (Student Inquiry Activity
Discuss - What factors affect collecting sonar data? - Does the motion of the ship have any affect on the data? - How far can sonar pulses travel? A crewmember received an interesting email message while we were in the Bransfield Straight. A friend aboard a Navy submarine in transit sent a message that stated that our active sonar was so loud it was driving them crazy. They were many miles away from the ship. They never gave a precise position - Why it is important to know what's below the ice or the sea?
Do some hands-on modeling of the reflection of waves (Slinkies, etc.) A quick introduction to the fundamentals of sonar is a classic activity/demonstration where students measure the time for an echo. Have students measure a distance of about 50 meters away from a large solid two story or higher wall. Use a couple of boards slapped together to produce a loud sound wave. Have students record the length of time it takes from the moment the sound is made to the time that they hear the returned echo. Sonar works by measuring the echo time. This gives you an opportunity to discuss the importance of knowing temperature and density medium, because these affect the speed at which sound travels. (Distance = speed of sound x echo time/2)
When doing Antarctic research, the temperature and salt concentration are constantly measured. The water actually increases in temperature with depth. To measure distances under these conditions you have to use an average of temperature to make your measurements. Another problem about working in Antarctica is that the sound waves travel in all directions. Sound reflected from sea ice and icebergs creates noise that must be removed either manually or electronically to an accurate view of the seafloor can be made.
It is important that when you set up the hidden surface, have a parcticular feature those students need to find. This gives you an opportunity to assess their data interpretation skills and is a good practical evaluation tool.
Elaboration (Polar Applications)
Ask students how they think researchers know what type of topography is hidden by ice or water. Students might suggest the following:
Explain that researchers can use sound waves to explore hidden surfaces. Ask students how they think this is possible. Students may know that bats and dolphins use echolocation to figure out where they are. If they mention this, ask them to explain how echolocation works. Students should be able to say that these animals send out sound waves and these waves bounce off of objects and travel back to the animals.