Student Activity - Multibeam

Activity idea

How can we make a map of the sea floor that is kilometres below the surface of the water and that we can not see? Scientists on board the Tangaroa are tackling this challenge using technology called the ‘multibeam’. The multibeam measures the time it takes for a sound signal to reach the bottom of the sea floor and come back again, and from this it measures the water depth.

This activity allows you to build your own ‘sea floor’ and then simulate being in a boat and using the principle behind the multi-beam to determine if you can create a map of the sea floor.

Instructions

You have just been asked to join the IPY (International Polar Year) Ross Sea voyage on board the Tangaroa. You will be working with the multibeam team, using the beam to create a map of the sea floor. By measuring the time it takes for a signal to be sent from your boat to the sea floor and back, you will be able to determine the various depths. But first you need to build your equipment.

What you need

• cardboard box (such as a photocopy paper box)
• two sheets of cardboard cut the same size as the box (or two box lids)
• polystyrene or similar foam like structure
• dowel wood (height of the box plus 5 centimetres)
• ruler
• permanent marker pen
• paper

What to do

 1. Cut the polystyrene into several square pieces and glue them to the cardboard. Make sure that you ‘build’ an uneven seafloor with mountains. 2. On one of the cardboard sheets, draw up a grid of 5x5cm squares and punch holes big enough to allow the dowel to pass through in each square corner. 3. On the second cardboard sheet, draw up a grid of 2x2 cm squares and again punch holes in each corner. 4. Prepare a template for each box lid. Use 2 sheets of paper and draw the 2 grids (5x5cm and 2x2 cm). These templates will be used to map your readings. You may find it easier to label each box lid with a number, name or symbol. 5. Using a ruler and pen, mark and label the doweling rod every 1cm from the bottom of the rod. Each centimetre represents the time it takes to reach the seafloor and back to the surface, with 1cm equalling 0.7 seconds (round trip). Later, you will use this information to work out the sea floor depth – 0.7 seconds is equal to 500m for the multibeam. Now that you have built your equipment, you can begin to survey the sea floor: 6. Using the box and the larger (5x5cm) cardboard grid, insert the dowel into the first hole until it hits the sea floor. 7. Record the mark (representing the time) from the dowel in a table.

Hole Number

Time (s)

Depth (m)

1

3

2.1

1500

2

3

2.1

1500

3

1

0.7

500

...

...

...

...

 8 Repeat until all holes have been measured. 9 Change box lids (cardboard grid) to the smaller 2x2cm grid and again measure the sea floor depth. 10 Using the data table of your recorded data, convert centimetres into seconds and into meters (1cm dowelling rod = 0.7 seconds round trip = 500m depth to seafloor using the multibeam).

Questions for consideration

• Was there much difference in the readings between the larger and smaller grid?
• Which one do you think gives you a better reading?
• How do you think you can use your data to make a map?

Possible solution

This picture is from NIWA and shows how the multibeam is used to create maps of the seafloor. You can see that a 3-D image is created with different depths appearing to be different colours. You can do something similar by making a depth colour table; it is possible to create a 2-D map that gives you 3-D information. A depth colour table assigns a range of depths to a colour.

 11 Looking at your data table and template, colour in each square according to the table depth table you created.

Depth

Colour

0-500

White

500-1000

Red

1000-1500

Blue

1500-2000

Green