Seeing in the dark

This is an essay I wrote for my writing class with Stacey Solie. It was our first assigment and we were instructed to write about why we do what we do. I wasn’t going to post this, but I guess I just figured – hey, why not? If I don’t post it here, it will just languish in a dusty Dropbox folder and be forgotten.  At least here it can languish in a hidden “about” page, and be forgotten, huzzah!

Marine animals had it figured out long before we did:  the ocean is a dark place, and sound, rather than light, makes survival in such an environment possible.  Snapping shrimp use a large claw to produce a sound that can be used to stun their prey.  Several species of fish use sound to defend their territory or attract mates.  Dolphins find food by producing clicks and then listening for echoes.  We can learn a lot from these ocean-dwellers about using sound to explore and understand their world.

In the spring of 2010, I visited William Wilcock’s lab at the University of Washington.  Dax, a graduate student in the lab and outgoing ex-car salesman from Texas, described his research project. In a nutshell, he looked for fin whale calls recorded on seafloor seismometers off the coast of Vancouver Island. The instruments were designed to detect earthquakes, but happened to pick up the deep, booming calls of fin whales as well.  “It’s piggyback science,” he explained over coffees at the nearby Rotunda Café, “someone else’s noise is our data”.

Fin whales are the second largest animals in the world, after blue whales, and yet they choose to dine on tiny crustaceans, often gulping swarms of them in a single mouthful.  They are fast swimmers, sometimes called the greyhounds of the sea, and they spend a lot of their time underwater, two characteristics that make them particularly difficult to observe from ships or airplanes.  Because of this, their migratory patterns are still a mystery.  Seafloor seismometers give researchers an opportunity to listen as fin whales swim past, dropping vocal clues that tell us where they are.  Fascinated by the mystery and by the opportunistic nature of the project, I joined William’s lab group.  I now spend my days scanning the rhythmic call sequences of fin whales for clues about where these elusive animals hang out and how they move around in the northeast Pacific.

Underwater acoustics can be divided into two sub-categories.  Our lab group’s fin whale research sits squarely in the realm of “passive acoustics”, which means that our instruments listen quietly for whatever might be out there.  The other category is “active acoustics”, where you generate your own sounds and use them to “light up” the darkness, not unlike how you might shine a flashlight to see at night.

One fascinating application of active acoustics is seafloor mapping.  Techniques for seafloor mapping have evolved significantly over the past one hundred years or so.  Up until the early 20th century, water depth measurements for nautical charts were still obtained by lowering a weighted line over the side of a ship.  Mariners would carefully measure the length of line that was let out to get  an estimate of depth.  In 1923, the first US coast survey vessel was fitted with a single-beam echosounder.  It was essentially the acoustic version of the weighted-line measurement, estimating depth directly beneath the ship.  The single beam echosounder was faster and more accurate, allowing continuous depth measurements as the ship moved. The state of the art in seafloor mapping today is the multibeam sonar.  It consists of several acoustic elements wired together in a configuration that produces a fan of acoustic “beams” that trace wide swaths beneath a moving vessel.

Multibeam sonars aren’t always used for making charts.  For two summers, I worked with nautical archaeologists on a ship in the Mediterranean Sea using multibeam sonars to scan the seafloor for the remains of Byzantine era shipwrecks.  Usually all that’s left of ancient wooden shipwrecks after several hundred years are amorphous piles of ballast stones or the ubiquitous clay pots used at the time to transport everything from olive oil to wine.  I’m not an archaeologist, but when you’re huddled around a computer monitor with scientists and crew, staring at something that no human eyes have seen for five hundred years, it’s hard not to feel the excitement of discovery.

One of the problems with ship-based multibeam surveys, for charting, archaeology, or anything else, is that they are very expensive.  They are also sensitive to sea surface conditions; rough seas usually translate to less-accurate measurements – and seasick scientists. One solution that has garnered a lot of interest is to mount multibeam sonars to underwater robots (also known as autonomous underwater vehicles), which are potentially cheaper and less affected by a choppy sea surface.  The challenge? Knowing where they are. Ships at the surface can get their position using GPS, but GPS doesn’t work underwater.  For my master’s thesis at the University of New Hampshire, I looked at Portsmouth Harbor as a test location for acoustically positioning these underwater robots.  Portsmouth Harbor is a shallow estuary at the coastal boundary between New Hampshire and Maine.  Strong tides churn things up, but at slack tide, the fresh river water separates, forming a distinct surface layer and trapping dense seawater into what’s known as a “salt wedge”.  Locating an AUV acoustically in an environment like this may be possible, but it’s complicated because the sharp gradients in temperature and salinity cause acoustic rays to bend and bounce in ways that are difficult to predict.

Whether locating an underwater robot or eavesdropping on whales, underwater acoustics gives us the ability to peek into a mysterious world that we’re only just beginning to understand.  The oceans may well be our planet’s last unexplored frontier, and sound is one of the most important tools we have to reveal its secrets.  Sound lets us lift the blindfold, even for a moment, and see in the dark.

 

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