Hydrophone arrays, FTW!

Figure 1. Photo of Amy and Emily deploying the array off the back deck of the R/V Ocean Starr.
Figure 1. Photo of Amy and Emily pulling in the array at the end of the day.

We do a lot of things out here on the CalCurCEAS cruise – we play cribbage, we eat cookies, we ride the stationary bicycle – but mostly we do these two things, A LOT:

1) Look for whales and dolphins.
2) Listen for whales and dolphins.

Part 1, the “Look” part, is within the realm of the visual team, who stand bravely in wind and cold and rain (and sometimes gorgeous, balmy sunshine) and scan the surface of the ocean for animals that might pop up to say hi and or take a quick breath of air.

Part 2, the “Listen” part, exists because, well, it’s pretty difficult to see what animals are doing once they dip down beneath the waves. And we all know that’s where the fun stuff happens (synchronized swimming practice? underwater tea parties? you never know!).

Since I’m on the acoustics team, I thought I’d give a little overview of how our operation works.  We eavesdrop on the animals using a pair of hydrophones, which are basically underwater microphones. If we just used one hydrophone, it would be okay, we could hear the animals. But adding a second allows us to not only hear them, but figure out where they are.

Figure 1.  Top: Array geometry, showing how we can tell which direction a sound is coming from. Bottom: Signals measured on each of the hydrophones.
Figure 2. Top: Array geometry, showing how we can tell which direction a sound is coming from. (The poor creature pictured is my sad attempt at a striped dolphin, if you were wondering) Bottom: Signals measured on each of the hydrophones.

The pair of hydrophones are built into the end of a cable that is spooled onto a winch on the back deck. Every morning we head out there, don our life jackets, hard hats, and boots, and reel out the cable until the hydrophones are 300 meters behind us. The hydrophones are spaced one meter apart, and once the vessel is up to speed, they are roughly at the same depth.

The upper part of Figure 2 (blue) shows a cartoon schematic of the hydrophone setup. Here’s what it all means:

H_1 and H_2 – hydrophones #1 and #2

Known quantities:

d_h – distance between H_1 and H_2. For our array this distance is 1 meter

c_w – measured sound speed in water (approximately 1500 m/s, but depends on temperature and salinity)

Measured/derived quantities:

\Delta t – time delay between when the signal arrives at H_1 and H_2

d' – distance associated with the time delay \Delta t, derived using c_w

Unknown quantity:

\theta – angle between the incoming ray path and the array baseline. This is what we’re solving for!

OK, that seems complicated. But feel free to ignore the math, of course. The basic idea is that depending on where the sound comes from, it can arrive at the hydrophones at different times. For example, in the image above, it hits hydrophone 1 first, and after some amount of time, it hits hydrophone 2. The signals from each of the hydrophones are sent upstairs to the acoustics lab (see bottom part of Figure 2). The call shows up at slightly different times on each of the hydrophone channels, and we can measure that time delay \Delta t very precisely.

Using the time delay \delta t and the measured sound speed c_w, we can obtain distance d' using:

d' = c_w * \Delta t

So now we’ve got a right triangle where we know the hypotenuse and one other side, and you know what that means – trigonometry time!! Everyone’s favorite time! We finally have what we need to solve for the angle \theta.

\theta = acos( \frac{d'}{d_h})

We now know what angle the dolphin is at relative to the array. Huzzah! But wait. There are just a couple of little details that you need to remember (see Figure 3). First: you don’t know how far away the dolphin is. Second: there’s this pesky thing called “left-right ambiguity” *.

doge_angles3

From the perspective of the array, there’s no difference between an animal calling from an angle \theta to the left and an animal calling from an angle \theta to the right.

Figure 3. Left-right and range ambiguity.
Figure 3. Left-right and range ambiguity.

These are fundamental limitations of the method, but we can (sort of) overcome them. As the vessel moves along, and we estimate angles at different locations, we end up with a location where most of the bearing lines intersect. If the vessel is traveling in a straight line, we can get a good idea of range – how far the animal is from the trackline. We just won’t know which side of the trackline it’s on. But if the vessel makes a turn, the new bearings estimated after the turn will resolve which side of the line it’s on!

Resolving_ambiguities_600px
Figure 4. Bearing estimates (red lines) taken at different locations along the track line. Probable location is where most of the lines intersect.

At this point you might be wondering, Michelle, what assumptions are you making when you do these calculations? So here they are:

Assumptions**:

  • The array is horizontal
  • The animals are calling at the surface
  • The animals are staying in approximately the same location for the duration of the measurements

So there you have it. That’s how we locate animals underwater with a towed, 2-element hydrophone array.

 

* Yin, one of the amazing visual observers on the cruise, thinks “Left-right ambiguity” would be a great name for a band, and I agree.

** assumptions are made to be broken

Whales, dolphins, and seabirds, oh my!

RVOceanStarr-1

Hi all! I’m on a ship called the R/V Ocean Starr, and we’re out on the 2014 California Current Cetacean and Ecosystem Assessment Survey, or CalCurCEAS for short. We are collecting data that will allow NOAA scientists to estimate the abundance of whales and dolphins off the west coast of the U.S. They’ll also be able to use these data to better understand what affects the distribution of marine mammals – where do they like to hang out, and why? We’re gathering this data using two primary methods: visual and acoustic, and are also conducting photo-ID and biopsy sampling of species of special interest.

In addition to the marine mammal portion of the survey, we’re looking at the pelagic ocean ecosystem in the region.  This means taking measurements of various properties of the water, doing net tows, and using acoustic backscatter to look at biomass in the upper part of the water column. There are also two observers onboard to survey seabirds.

I’m out here with an awesome science team and a great crew. There are two other acousticians besides me: Emily T. Griffiths and Amy Van Cise. Emily has been onboard for the first two legs. She makes sure we stay on track and don’t get into (too much) trouble. Amy is a PhD student at Scripps Institution of Oceanography studying pilot whales, and she’s here for the third leg of the cruise, just like me. The three of us all love ice cream and get along famously.

I have one or two shiny new blog posts that I’m hoping to share soon (with comics! woo!), and I might even have a couple of surprise guest posts! Stay tuned…

Eavesdropping on dolphins

Please welcome my next science+comics interview victim, Alexis!

Name: Alexis Rudd
Job: PhD student in Zoology at the University of Hawaii
Research: Alexis uses passive acoustic monitoring to study whales and dolphins off Hawaii

News Flash!! Whales and dolphins, a.k.a. cetaceans, hang out near the Hawaiian Islands! Okay, not a news flash at all, everyone knows that. But you may be surprised to learn that we don’t actually know much about them – what they’re doing, where they’re going, and why – especially once you get out to the deeper rougher waters further from shore. So why is it important to know about their hangout spots and behaviors? It’s because that kind of information can help us design and implement effective management and conservation strategies. Yes, cetaceans hang out near Hawaii, and we want to keep it that way!

Here’s the thing. Whales and dolphins (and loads of other animals) like to spend time where food is available. In the ocean, the big driver behind food abundance is primary productivity – phytoplankton, or plants that live in the sunlit upper layers of the ocean. And just like on land, different places in the ocean are more productive than others: some places are lush and green with plants and all the life they support, and other places are sort of like deserts – yes, animals live there, but it’s on a whole different scale. Here’s an image of productivity (actually, it’s an indicator of productivity – chlorophyll concentration, which satellites pick up by its color).

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This picture shows green where there is a lot of productivity, and blue where there’s not much. So the deepest blues show where the ocean “deserts” are. The green places tend to be where nutrients are available – things like nitrogen and phosphate (yup, same stuff that’s in fertilizers for your garden), which might run off the land, or be brought up to the surface in upwelling zones.

But – not to worry! – it’s not exactly a dead zone around Hawaii. It just means that cetaceans might need to look for an oasis sometimes. One theory is that cold, nutrient-rich water gets pulled to the surface by cyclonic (counter-clockwise) eddies, triggering a cascade of activity up the food web – a very enticing fish/squid/zooplankton buffet indeed.

To address these kinds of theories, Alexis needs to figure out whether cetaceans are indeed seeking out certain environmental and oceanographic conditions. Often, studying cetaceans means getting on a boat and looking for them visually. This works reasonably well in calm waters, but it gets pretty tough to pick out a dolphin or a whale when the water is choppy. The prevailing winds are westerlies, and they come sweeping across the Pacific from California. Calm zones form on the leeward side of the islands, but the wind speeds up as it squeezes through the gaps, creating regions of high wind and rough seas.

windpatterns_600px

Instead of looking for cetaceans visually, Alexis listens for the sounds they make. And here’s where the super creative part of her research comes in: she tags along on a tugboat that brings supplies to the different islands. Being the persistent scientist that she is, Alexis went out every two weeks for a year and a half, installing her underwater recording package (hydrophone) on the barge behind the tugboat. This setup was ideal in a lot of ways: first of all, people need their supplies all year round, so this vessel does regular trips. Second, having the hydrophone on a barge – separated from the loud tug engines – makes it easier to pick out cetacean sounds. Here’s what the basic setup looks like:

tugboat_600px

As the tugboat moves from island to island, it provides a great acoustic “view” of a very large area – the calm leeward side of the islands and also the windier regions between the islands. So Alexis is able to get a great “big-picture” idea of what’s going on.

She’s now collected hundreds of hours (!!) of recordings, and is going through the process of checking and documenting all of the identifiable cetacean calls. She links each call up to the location where the barge was at that time so that she can gather up all the data at the end and figure out whether there are correlations between the animals’ presence and environmental or oceanographic conditions.

Alexis has an excellent and very informative blog, and I encourage you to head on over and check it out: http://bioacoustics.blogspot.com/

She also did a guest blog post over at Scientific American Blogs, where she talks about the experience of partnering with commercial shipping to do her research: Towing my weight: partnering with commercial shipping for whale and dolphin research.

By the way, this is how an ocean themed interview should be conducted:

pirate_interview

… with pirate hats all around.