Sea Beam The multi-beam survey system is a tool to measure the depth of the seafloor by using sonar technologies. During the evenings when Alvin is not deployed, Dr. Lonsdale has been using the Atlantis and the tool Sea Beam to map the seafloor. At night, Atlantis has been moving back-and-forth over the region, pinging away at the ground. Figure 1: Click on the image to see how sonar works.
Like all sonar tools, the multibeam device sends an audible signal (ping) from projectors mounted along the keel of the ship to the sea floor. Receivers on the ship listen for the time it takes for the reflected signal to return to the ship. Like an echo, the farther it is to the ground, the longer it takes for the echo to return.
In the basic sonar animation, it seems that the system is only mapping a thin line of depth data at a time. The multibeam device allows us to map a wider line or swath of data at a time. The wider the swath, the fewer times we'll have to go back-and-forth over an area to map it.
Figure 2: Multibeam survey of a swath. The colors show the resulting contour map of the seafloor.
If you think about sound, the capabilities of multibeam seem natural. Sound does not move in one line away from a source. Unimpeded, it radiates outward in all directions and reflects off the ground across a wide area. When you hear a singer, the voice is heard not only by you, but also by everyone around. The reflections from the ground similarly radiate in many directions, sending the ship many echoes from even one initial ping.As complex and messy as it can get, analysis of multiple rather than single reflections can allow estimation of the shape of the seafloor across a swath. Putting it all together creates a map of the regional seafloor created by Dr. Lonsdale. Look in the Activities column for the map.
• Activity 3.2a Sea beam map The result of the multi-beam sonar survey is a map of the topology of the sea floor. Click on the thumbnail below to view this map.
• Activity 3.2b Hardware and signal processing For more detail about hardware and signal processing, check out what fellow SIO scientist Dr. Christian DeMoustier has to say about multibeam bathymetric surveys. [TBA]
• Activity 3.2c Subduction How long do you think it will take for Kodiak Seamount to disappear below Alaska? Click on the map in Activity 3.2a to take a closer look at the multibeam map.
Hint: Use your knowledge of the relationship between distance, velocity, and time to solve this problem. Use the map, classroom and web resources to make this estimate. You be the scientist!
A C T I V I T I E S
