4 April 2015 Multi-beam Bathymetry

We’ve made it to the continental shelf of Antarctica! It’s freezing
down here, literally. There’s a light snow falling outside. But no sea
ice yet.
Now that we're here we can get started with all the work! We’ve started
working shifts (usually 12a-12pm or 12p-12a) to monitor the equipment
around the clock. As we approached the continental shelf we turned on
the multi-beam bathymeter to begin taking measurements of and mapping
the ocean floor. I’ll be spending a lot of time working on this
equipment over the next several weeks, so here’s a little more
information about what it is and how it works.

A Closer Look at Multi-beam bathymetry

BathymetryBathymetry is the study of the depth of water in the ocean. is the study of the underwater depth of ocean floors; it’s
like underwater topography. Bathymetric charts show the terrain of the
sea floor and can be used for navigation, science, and exploration.

In the past, ropes or cables were lowered off the side the boat to
measure depth. Now scientists use “multi-beam” or “swath” echo-sounders.
These machines send thousands of beams of sound out at different angles.
This sound, usually 0.1 to 50 Hz depending on the water depth, bounces
off the ground and returns to sensors on the boat. The time it takes for
the sound to return can be used to calculate the depth of the ground.
Other sensors on the boat correct for the boats movement to ensure
accurate measurements. This data and the exact GPSA Global Positioning System (GPS) is a satellite-based navigation system used to track the location or position of objects on the Earth’s surface. locations of the
soundings are compiled into a map.

I can’t actually take a picture of the equipment as it’s under the
ship, but here’s what the workstation where we manage the equipment
looks like.

There is someone at the station 24 hours a day to ensure the multi-beam
is working properly, making sure it is bouncing sound off the ocean
floor (and not a piece of stray ice or something) and to make any
adjustments to the sensors as needed. As you sit at the station, you can
actually hear the “ping” made as the equipment sends sound out to the
ocean floor echo around the room—it sounds like a dolphin trying to
speak “baby bird.” During shifts we also “clean up” the data and get rid
of sound distortion or sound points that bounced off incorrect things.

Right now I’m fairly new to all this, so I have a shorter shift, about
6pm-12am, with someone supervising me part of the time (thank goodness…
I can ask lots of questions). I’m told I’ll get the hang of it very
quickly. It’s nice because I get the “morning” of 12p-6p to help out
with other data collection we’ll be starting soon.

Try it at home

Although scientists now use sound to measure depth, you can try your
hand at bathymetry the old fashioned way. You’ll need: a friend or
parent, play dough, a small box, tin foil, and a thin skewer longer than
the box.

• Have a friend or parent make a simple shape out of play dough. Place
  this at the bottom of a small box and cover it tin-foil or a dark fabric
  (something you can’t see through, but you can poke a skewer through).
• Use the skewer to take “soundings”. Poke the skewer through the cloth
  or foil and measure how deep it goes. Be careful not to poke a hole in
  the dough. Try to take your soundings in a grid pattern for best
  results. You may wish to draw out a grid on your foil to help you decide
  where to take your soundings.
• Record your data and then draw a map of your “soundings” on paper,
  graph paper will work best. See if you can determine what shape was in
  the box (or create an accurate map of your “seafloor”).

For a more detailed version of this activity, check out one my fellow
PolarTREC teacher’s lesson on
bathymentry.