Sonar: seeing under water

I, Polar Bear BS, have been back in the northern hemisphere for several weeks now.  However, this does not mean my work is done!  As the rest of the polar bears continue to gather information about the life under the sea ice in McMurdo Sound, my job is to receive the data back here and try to figure out how much krill and fish are present.  The main tool I am using is a sensitive sonar, called a biological echosounder.

The echosounder works by making a very high pitched chirp noise called a ‘ping,’ and then it listens for echos of that ping from things in the seawater. The both the echosounder’s ping and where it listens from are very directional.  This means that only a narrow cone of seawater outward from the echosounder is sampled.  This helps us to take independent, non-overlapping ‘pictures’ of the water below.  The picture below shows an echosounder on the bottom of a boat (a), and the cone in which it listens.  The resolution of the picture is dependent on several things, including the length of the chirp (b), and the width of the beam (c).  In this case, three fish would be combined into a single data point.

echosounder_temp

On the SCINI-Penguin project, the SCINI robot drags the sonar behind.  You may wonder why we need the echosounder since SCINI itself carries two cameras.  The answer is range!  It is dark under the ice, and in the clearest water the lights on SCINI allow us to take pictures up to 4 meters away.  The echosounder can see more than 500 meters!

The echosounder does not make pictures like you expect from a camera.  Instead it produces ‘echograms.’  Here is an echogram from November:

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The echosounder pings once every 2 seconds.  Each chirp makes one vertical line of dots.  Every vertical black bar is ten minutes of pings, and there is a horizontal line for every 50 meters of depth.  The echogram above is then showing ping echoes from about the length of a full-size soccer field.  In this echogram, there are several blue-grey streaks moving downward.  These are likely small fish swimming slowly down, but it is hard to tell!

It turns out the hardest part of working with echograms is understanding what animals are making the echoes we see.  We have learned to recognize the shapes and behaviors or different animals make different shapes and echo strengths, but in many cases we are still not sure what it is making the echoes. However we have SCINI that can dive down to where we see the echoes so we can go take a picture with the cameras to help figure it out.

Two days later, the echograms looked like this:

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It seems that more fish were moving into our study area, and they were swimming in tighter schools.  This may be a different species of fish than before, or may be an older, bigger version of the same fish type since the echoes are louder/stronger (warmer colors mean stronger echoes).  In the first week of December we started seeing very different shapes in the echograms:

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In the upper 50 meters of water there were large blobs of echoes. They look small on the screen, but considering water movement, these blobs are giant groups of tiny animals that are about the size of a house!  These echoes are likely created by a small animal called a pteropod.  Pteropods are small snails smaller than 1 cm that fly through the water with tiny wings:

antarctic-snails-sea-butterflies

(http://econews.com.au/news-to-sustain-our-world/rising-acidity-threat-to-ocean-food-chain/)

The grey/blue specs below 50 meters depth are a combination of lots of different kinds of very tiny sea creatures.  This layer is sometimes called the ‘deep scattering layer’ by oceanographers.  Three days later the echograms look different again:

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The blobs above 50 meters depth have changed shape – they are thinner and longer.  This is a bit tricky, but they are likely the same animals as before, the pteropods, even though the shapes are very different.  This is because the water currents were moving slower in this echogram, making the groups of animals pass through the listening cone more slowly and creating longer blobs.  The deep scattering layer is stronger here as well.  Finally in mid-December we started seeing one of the animals we are most interested in – krill!

krill

Krill are stronger reflectors than some of the animals, and are found in more irregular-shaped groups.  They are the top-most blobs in the echogram below:

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My ongoing job is to classify all the animals that I see in the echograms.  At the end of the project, all the echograms will tell us when the fish, terrapods, krill and other animals arrived in McMurdo Sound, how long they spent there, how deep they were, and how much often they spent time around each other.  With our other data, such as penguin and whale diving records and and ice and water sampling, we will be able to better understand how the ecosystem is working and fits together.

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3 Responses to Sonar: seeing under water

  1. Steve H says:

    Interesting post!
    It is Pteropod (silent ‘p’, like pterodactyl) not Terrapod :^)

  2. Betsy Wilkening says:

    Great explanation and visuals!

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