## Teaching statistics, one comic at a time

Have you ever noticed that it’s easier to learn something complex if you already know a bit about the topic in general? There’s a word for this: elaboration [1]. It’s the process of linking the things you already know with the things that you’re learning, and one of the ways teachers can help that process along is by using analogies. Analogies and metaphors rely on the fact that you already have neural pathways setup that can be applied to a new situation [2].

With analogies, you can learn something totally new by comparing it to something you are familiar with. That’s what Dax and I are trying to do in a series of “statistical vignettes”. It’s all part of a larger education project called Project EDDIE. Our approach involves using comics and stories to give undergraduate students a framework to build upon when they’re learning about new things like correlation, p-values, and probability. Project EDDIE modules give students a chance to learn scientific concepts by analyzing real-world datasets, and the statistical vignettes are mini lessons that can be used as needed to support the main topic.

The sample comic at the top of this post shows a very abbreviated version of how we might build a statistical vignette on the concept of correlation. Of course, there’s a lot more to correlation than what’s shown here, but we’d just like to introduce Rita because we’re planning on including her as a character for many (all?) future vignettes.

I’ll probably keep posting ideas, sketches, and examples here. Feedback is welcome!

[1] Ancker, J. S., & Begg, M. D. (2017). Using Visual Analogies To Teach Introductory Statistical Concepts. Numeracy10(2), 7.

[2] Oakley, B. A. (2014). A mind for numbers: How to excel at math and science (even if you flunked algebra). Tarcher.

## Making a comic: Whale Scout edition

I’ve been working on a little comic for WhaleScout.org. It’s taken me a while, but it’s finally done, and will be up on the Whale Scout website soon! Woo!

I used the iPad app Procreate, which I’ve been a fan of for a while now. Years, maybe? I loved it from the get-go, and it really just keeps getting better. I’ve tried a lot of different drawing/painting apps, and this one is by far the best. Highly recommend!  As for ancilliary tools: I used an iPad 3, a Bamboo stylus, and Premiere Pro for some minor editing. I might as well also reveal that I got my best work done while sitting on the couch with Modern Family playing in the background. Don’t question the method people!

One of the features in the Procreate app allows you to export a video that shows the progression of your artwork. I’m actually not sure anyone other than me will be even vaguely interested in watching it, but here it is anyway.

Making a comic: Whale Scout! from Michelle Wray on Vimeo. Music: Something Elated (Broke For Free) / CC BY 3.0

## Sea level rise in Pohnpei

I finally posted my latest video a couple of weeks ago. You can check it out here:

This one was a lot of fun to make, and I learned a lot along the way. Unfortunately, there’s really not one single place to learn all of this, so I ended up watching tons of YouTube tutorials. At some point, I’d love to compile my own little set of tutorials to show others how to do this, but for now, here’s the overview:

1) Research the topic (interviews and lots of web searches mostly)

2) Storyboard: what order makes sense? What should I cover? This is a very rough draft and consists of words and some simple sketches

3) Write the script: Each paragraph is one, or maybe two “scenes”. My scripts have been about 1.5-2 pages long on MS Word (12 pt font)

4) Record script audio (I use a Blue Yeti mic)

5) Sketch out scenes in more detail, like this:

6) Adobe Illustrator: Draw things out using Illustrator + Wacom tablet (or mouse/trackpad). Save each section of the image as a separate layer. If I want to do the “drawing in” effect later, reverse the order of elements in each layer.

7) Adobe After Effects: Import illustrator file, along with audio. Use trim paths effect to make it look like it’s being drawn in. Synch audio and animation using keyframes.

8) Adobe Premiere Pro: Import scenes created in steps 6 & 7, put them in the right order, and add a soundtrack on top of it all.

That’s the gist of it anyway. Hope to give more details on specific sections at some point. For now, feel free to ask questions in the comments below.

## Argo Floats!!

If you have ever sat there and wondered to yourself, “What are Argo Floats? And why are they the coolest things ever?” – Look no further, my friend. I have just the (science-comics) video for you.  It’s based on an interview I did a while back with Rick Rupan, who runs the Argo Floats program at the University of Washington.

This format is a bit experimental for me, but I LOVE this style, and wanted to give it a whirl. I sort of just jumped in, and because I was (am!) so very clueless, I asked basically everyone I know (and some people who I don’t know) to give me feedback. The suggestions started rolling in, and I tried to incorporate as many as I could.

I really hope you enjoy this, and if I’m lucky, you’ll even learn something along the way 🙂 I do plan to make more of these, so if you have any feedback for me after watching this, it would be a great help.

## Doodle videos, heck yeah!

My latest project is figuring out how to make a doodle video. Or a speed drawing animation. I don’t actually know what they’re called, but you’ll get the idea if you check out the videos below. I started off attempting to use my iphone. Setting the iphone up to record was… awkward:

I did my video this way, posted it to youtube, and got a bunch of feedback from my awesome Facebook and Twitter friends. I tried to take that feedback into account as best I could and eventually moved on to my Nikon D5100 (dSLR).  I even set it up on an actual tripod (Thanks, Andrew Shao!).

Recording on my SLR was pretty easy too, once I got it adjusted. I did have some issues with focusing – not that it was hard, I just kept forgetting to check. oops. Editing for the SLR video was done in iMovie, and the audio was recorded in Hindenburg Journalist. Here’s a 30-second sample from this experiment. I hope that you agree that it has improved, at least marginally, from my first attempt!

Next, I decided to try doing the video using Camtasia. Luckily they have a free trial, so I gave that a whirl. Camtasia is for screen recording and video editing, and was really easy to jump into and start using right away. I did the drawings in Adobe Photoshop using my Wacom Intuos4 tablet/stylus. Here’s the 30-second sample using the “all-digital” method:

Dear reader, if you have been patient and kind enough to actually read all this, and more importantly, watch the two videos, please feel free to give me feedback in the comments (below) or in email or on Twitter or Facebook!

## Harmful algal bloom forecasting: Chasing toxin producers

It started with “drunk” pelicans. While studying aquatic sciences at the University of Santa Barbara, Liz Tobin noticed that there was something wrong with the fish-gulping birds.  They exhibited unusual behavior, getting hit by cars, and sometimes simply falling out of the sky. The pelicans suffered from seizures induced by domoic acid poisoning, a particularly dangerous affliction when it hit mid-flight.  Biologists traced the source of the poisoning to rapid springtime growth (bloom) of Pseudo-nitzschia, a tiny, single-celled organism, distantly related to the giant kelps thriving in nearby coastal waters.  Small fish and shellfish consume these algae, and although they are not harmed, they store the toxins produced by algal cells and pass them on to their predators, including seabirds and marine mammals.

Scientists refer to these incidents as harmful algal blooms, or HABs.  They are not restricted to the waters of southern California, though.  And Pseudo-nitzschia cells are not the only culprits.  There are a number of other algal species that can cause trouble for local marine ecosystems. HABs occur in coastal waters around the globe, with larger and more frequent blooms being linked to a warming sea surface [1] and also to increased nutrient runoff from land [2], which is a common by-product of animal or plant agriculture.  Local economies suffer due to tourism losses and local residents can’t harvest shellfish from their beaches.

After wrapping up a bachelor’s degree at UCSB, Liz moved on to graduate school at the University of Washington so that she could study harmful algal species in Puget Sound.  One of the species she’s interested in is Alexandrium catanella, another single-celled marine alga that produces a suite of deadly neurotoxins causing paralytic shellfish poisoning, or PSP.  Alexandrium blooms can become so intense that they give the water a reddish-brown tinge, often called a “red tide” (although they don’t always cause water discoloration).

If you eat a shellfish that has accumulated Alexandrium toxins, a series of unpleasant events unfolds. First, your fingers and toes will tingle and your lips will feel numb.  Next, you may become nauseous and unsteady on your feet.  If the toxins hit you at full force, your diaphragm will become paralyzed, which means that you are unable to pull oxygen into your lungs.  Without prolonged artificial respiration, a severe case of PSP can lead to death in a matter of hours.

In recent years, poisoning cases in Washington State have been rare, but they do happen. Washington Department of Health aims to prevent outbreaks by regularly testing shellfish along Puget Sound beaches.  Commercially harvested shellfish are tested exhaustively before hitting the markets. Unfortunately, it’s tough to predict the severity, location, and timing of an Alexandrium bloom ahead of time. That’s where Liz’s work fits in: she’d like to improve our ability to forecast where those blooms will happen ahead of time so that public health officials, fisheries managers, and the shellfish industry have more time to react.

Alexandrium cells have a comfort zone, she explains.  They grow and divide furiously when the days are long and the surface waters are warm.  Once the spring bloom ramps up, zooplankton, fish, and shellfish feed on the algae, eventually decimating Alexandrium populations by early to mid summer.  Less Alexandrium cells means less food for their predators, and their predators begin to die off or find food elsewhere.  By late August, with newly lowered predation pressures and plenty of sunlight, a second bloom typically occurs.  As the season draws to a close, Alexandrium  algal cells feel the effects of shorter days and less sunlight.  They struggle to grow and divide and at some point cut their losses and transit down to the sediment to wait out winter.  -Settled into the muddy seafloor, they shift to survival mode, also known as the “cyst” stage.  The exact conditions that send them into their cyst stage are poorly understood, as are  the details of how they wake up and make their way to the surface come springtime.

Liz is particularly interested in how the Alexandrium cells swim, even though they are mostly at the whim of the currents.  Tides rush in and out twice each day, racing through narrows and washing languidly over mud flats.  Rivers pour into Puget Sound with irregular pulses of freshwater from rain and springtime snow melt. Alexandrium algal cells propel themselves through the water using a small whip-like appendage called a flagellum, but because of their tiny size – only about a third of the width of a single human hair – they can’t overcome even the weakest currents. However, if Liz knows their vertical swimming capabilities, she can better understand how quickly Alexandrium cells migrate from the seafloor to the surface (where blooms occur) and back again.

A lot of researchers, Liz included, study Alexandrium in the lab.  They blend solutions of water and various chemicals to imitate seawater, and observe how the cells behave and react to a variety of environmental triggers.  One of the labs she works in is a tiny bunker-like space on the second floor of the Ocean Teaching Building.  Two long black tables running along either side of the room are covered by a hodge-podge of beakers, elaborate video recording setups, and circuit boards sprouting nests of red and white wires.  At the back of the room, a giant insulated door opens to a fully programmable walk-in refrigerator, where algal cells can be subjected to controlled shifts in temperature and light.

Even at their best, however, lab experiments can’t possibly recreate the many complexities that exist in the natural world.  Liz knew that if she could monitor the emergence of the Alexandrium cells from the sediments she could combine those observations with detailed fluid flow models to better predict where the cells would eventually concentrate.  The only problem was that there were no “off-the-shelf” instruments that were capable of doing what she wanted.  So she set out to build her own.

When she starts describing the instrument she’s working on, I interrupt.  Pushing my notebook and pen across the table, I ask her to sketch her design for me.  She draws a tube a few inches in diameter, sitting upright on the seabed.  On one side of the tube a camera in waterproof housing points into a tiny window to capture any Alexandrium cells swimming past.  The end of a plankton trap sits on top, ready to catch any upward-swimming cells so they can be later compared with the video. She pauses her drawing to look up at me.  “It’s just like what we would do in the lab,” she says, “but instead of a camera looking at a tank, I have a camera looking into a chamber on the seafloor”.

As of mid-July 2013, Liz’s prototype of her seafloor camera was nearly ready to be deployed for testing.  If it works, she’ll try to capture Alexandrium cells emerging from the seafloor just prior to the late-summer bloom.  Her vision for the future, she tells me, is a network of these instruments deployed across Puget Sound, monitoring cyst emergence in real time.  Instead of only knowing about blooms once they are in full swing, they could be predicted days or weeks in advance, avoiding unnecessary closures and potentially saving lives.

[1] Climate change and harmful algal blooms, NCCOS http://www.cop.noaa.gov/stressors/extremeevents/hab/current/CC_habs.aspx

[2] Anderson, Donald M., Patricia M. Glibert, and Joann M. Burkholder. “Harmful algal blooms and eutrophication: nutrient sources, composition, and consequences.” Estuaries 25, no. 4 (2002): 704-726.
http://www.whoi.edu/fileserver.do?id=47044&pt=2&p=28251

## Whales in a Noisy Ocean

In the ocean, sound rules.  Unlike on land, where animals (just like us humans!) get a lot of information from light, animals in the ocean have evolved to take advantage of sound, which is much more effective than light under water.  Light gets absorbed quickly, but some sounds, like the low-frequency calls of blue whales, can travel hundreds of kilometers under water, under the right conditions.

Whales have developed really fascinating ways of using moans and clicks and songs to help them get the things they need in life – friends, mates, food… what more could a whale want?  Some whales find food by making sounds and then listening for echoes that bounce off of their prey.  Using those echoes, they can figure out where their food is (mmm, delicious fish!).  Whales can also use sound to communicate with each other.  A mother and calf might need to keep track of each other as they swim.  Or a male might show off his sweet singing voice to attract a female.

Unfortunately, noise in the ocean is on the rise, and it’s making life tougher for whales. Some of the sources of human-caused noise include ship traffic, seismic exploration for oil & gas and sonar testing. It’s worth noting, though, that there is noise in the ocean that is not caused by humans. Earthquakes, volcanoes, breaking waves, rain and even lightning are some of the things that add to the background noise. The thing is, whales have spent thousands of years evolving to deal with these natural noise sources – but in the last few decades they have suddenly had to get used to the growing din caused by human activities. And, as Chris Clark from the Bioacoustics Research Program at Cornell University explains, “it’s not just one ship. It’s ten thousand ships”. It’s the same as if only one person litters on the side of the road, it might not be such a big deal. The problem arises when everyone does it, all at the same time.

Scientists are still not sure exactly how noise affects whales – different whales might be more or less sensitive to particular sounds, and might respond in different ways. For example, right whales in the Bay of Fundy showed lower levels of stress hormones when ship traffic stopped briefly following the 9-11 terrorist attacks [1]. Killer whales off the coast of Washington state and British Columbia have increased the volume of their calls so that they can be heard above vessel traffic [2]. And in the most severe cases, some beaked whale strandings have been linked to mid-frequency naval sonar operations [3].

Excessive noise in the ocean causes a sort of masking effect – meaning that the noise is loud enough that the whales can’t hear what they normally would need to hear in their environment, whether it’s echoes from fish, or a signal from another whale. Not being able to find a mate or find food or find each other is a serious problem, especially for species that are already endangered.

But there is still hope. Now that we’re starting to realize how harmful noise can be, we’re finally in a position to actually do something about it. Small steps can make a big difference. Even slowing ships down can substantially reduce the overall noise. In recent years, the US Navy has funded a lot of basic research that has taught us a huge amount about how marine mammals hear and use and produce sounds. We’ve still got a long way to go, but now that we’re aware that there is a problem, we can work on ways to fix it.

This post was done with help from Chris Clark and Kevin White from the Bioacoustics Research Program at Cornell University.  The PDF version can be downloaded here.

References

[1] http://news.sciencemag.org/sciencenow/2012/02/shhh-ocean-noises-stress-out-wha.html
[2] Holt, Marla M., et al. “Speaking up: Killer whales (Orcinus orca) increase their call amplitude in response to vessel noise.” The Journal of the Acoustical Society of America 125.1 (2008): EL27-EL32.
[3] D’Amico, Angela, et al. Beaked whale strandings and naval exercises. Space and Naval Warfare Systems Center, San Diego CA, 2009.

## The mystery of the shifting tropical rain belt

This week’s science+comics interview is brought to you by Yen-Ting Hwang, a UW Atmospheric Sciences student in the final year of her PhD. Ting’s work is focused on large-scale climate dynamics.

The Sahel region in Africa is a semi-arid boundary along the southern edge of the Sahara desert. The people living in this region are balanced on the very edge of habitable terrain, and rely on a short annual rainy season for their crops to grow. The slightest shift in annual climate can result in devastating crop failures and ultimately widespread famine. During the second half of the twentieth century, the region was hit by year after year of drought.

For her final thesis chapter, Ting has tried to understand why that drought might have happened when it did. The life-sustaining annual rainfall in the region occurs as part of a seasonally migrating tropical rainfall pattern. Scientists call this the ITCZ, or the inter-tropical convergence zone, and it’s a rain band that circles the globe like a belt. Solar radiation near the equator is far stronger than at the poles, and when the moist air near the equator is heated, it rises. The rising air eventually reaches an altitude where it’s cooled, and the moisture that was carried aloft condenses to form clouds and rain.

This air is transported poleward at high altitude until it reaches about 30 degrees latitude either north or south, at which point it sinks down toward the earth’s surface. The air then moves back toward the equator, picking up moisture along the way. This cycle is called Hadley Cell circulation.

By piecing together climate observations, scientists know that the drought in the Sahel region between about 1950-1990 was related to a very slight southward shift of the tropical rain belt. This type of shift happens when the temperature difference between the northern and southern hemispheres changes. If the northern hemisphere is cooler, the northern Hadley cell will strengthen as it tries to pull warm air up from the south, and the tropical rainfall band will shift southward.

To answer this question, Ting compared results from twenty different IPCC models (IPCC = Intergovernmental Panel on Climate Change). She looked at a variety of different possible factors that could result in an uneven heating or cooling of the planet between the northern and southern hemispheres – things like clouds and ocean circulation. The factor that showed the strongest correlation was surprising: it was the aerosols!

Aerosols are basically tiny particles that are suspended in the air. Naturally occurring aerosols are always floating around our atmosphere – dust from deserts, smoke from forest fires, and even sea salt. But there are also anthropogenic aerosols – the ones that occur as a result of pollution. There are different types, but Ting found that the strong correlation was with sulfate aerosols, which are white in color. These aerosols have two main effects:

a) They reflect sunlight, which means less sunlight reaches the earth’s surface.
b) Aerosols in the air tend to cause clouds to persist for longer than they would other wise – you might imagine droplets of water staying aloft when they have a handy bit of sulfate to stick to.

Overall, both of these lead to a cooling effect.

Aerosols don’t stick around in the air for long though, and they don’t travel far from their source. It’s a bit counterintuitive to imagine that the smog from an American or European factory could be affecting tropical precipitation. But the slightest shift in the temperature balance between the northern and southern hemispheres is all it takes. Here’s a cartoon showing the basics of how this works:

While the Sahel region was being decimated by drought in the twentieth century, industrialization in developed countries was on the rise.   Since most of the industrialization happening at the time was in the northern hemisphere, that hemisphere was cooler than it otherwise would have been.  And model results indicate that this change was enough to shift the tropical rain band southward.  By the 1980s and 1990s, air pollution was widely recognized as being harmful, and various environmental regulations were put into place.  And, lo and behold: that’s about the same time that the ITCZ shifted back to its previous position.

More recently, Ting has been looking at how we can use climate models to predict how the tropical rainfall band might shift or change given different climate scenarios that are tested by IPCC models.  Of course, the effect of aerosols is only one piece of the puzzle.  The northern hemisphere cooling that Ting was investigating was overlaid on an overall warming trend.  The northern hemisphere was actually warming between 1950-1990, just not as quickly as the southern hemisphere.  In fact, recent studies indicate that the northern hemisphere appears to be warming more quickly, which may have significant consequences for tropical precipitation patterns.

http://www.atmos.washington.edu/~yting/

http://www.atmos.washington.edu/~dargan/

Press releases about some of their other work:

http://www.washington.edu/news/2013/03/11/remote-clouds-responsible-for-climate-models-glitch-in-tropical-rainfall/

http://www.theverge.com/2013/3/25/4129026/clouds-are-hiding-the-the-truth-of-how-much-earths-climate-will-change

http://newscenter.berkeley.edu/2013/04/02/shifting-rainfall-patterns-in-tropics/

Also, this paper is in press, so you can read all the details soon!

Hwang, Y.-T., Frierson, D. M. W., and S. M. Kang. Anthropogenic sulfate aerosol and the southward shift of tropical precipitation in the late 20th century. in press, Geophysical Research Letters

## Stars in the Andromeda Galaxy find the fountain of youth

This science+comics post takes us where this blog has never gone before:  to outer space.

Phil Rosenfield is a PhD candidate at UW in the department of Astronomy, and studies stellar and extra-galactic astrophysics (woah).  He uses data collected on the Hubble Space Telescope to look at stars in distant galaxies and is particularly interested in how stars change over time.

One of Phil’s interests is in the Andromeda Galaxy.  Here’s a photo – breathtaking, isn’t it?

The Andromeda Galaxy, like other galaxies, is a cluster of stars and dust and gas and dark matter.  And get this – it’s the farthest thing you can see with just your eyes from Earth.  Wow, now that’s crazy!  If you go out on a clear night in late summer or early fall, you can find it for yourself, using these steps from Phil’s blog.  And while you’re gazing at that tiny speck in the night sky, know that the light hitting your eyeballs from this particular galaxy actually left it 2.5 million years ago – back when our earliest ancestors were just figuring out how to use primitive tools to help them scavenge for food.

Phil and his lab group are working with data collected on the Hubble Space Telescope (a.k.a the HST, if you’re hip with the Hubble lingo).  The HST orbits around the earth about every 97 minutes.  As a scientist, if you want to get the HST folks to take a particular set of images, you need to put together a proposal – usually with a large group of scientists who are interested in the same thing.  Julianne Dalcanton, the head of Phil’s lab group, and several other scientists, successfully convinced the folks at Hubble to capture super-detailed images of the Andromeda Galaxy.  These images are being collected over a total of 828 HST orbits over the course of four years.

The Andromeda Galaxy is shaped sort of like a spiral-y pancake, a flattened disk with a bright bulge in the middle.  New stars tend to form in the disk part of a galaxy, and closer to the bulge you’d normally expect to see older stars. So how do you know whether a star is young or old, just by looking at it?  The main way to tell the difference is by looking at the frequency of light that is emitted.  Young, massive stars are mostly emitting light in the ultra-violet (UV) part of the light spectrum – if you picture the violet part of a rainbow, ultra-violet would be the next color past violet, which our eyes can’t even pick up. Older, not as massive stars, on the other hand, emit most of their light in redder parts of the spectrum, like our sun, which emits most of its like in optical frequencies.

The first zoomed-in image that the HST collected of the Andromeda Galaxy was near the central bulge.  As an astronomer, you would not expect to find stars emitting UV light from so near the bulge.  But when Phil’s group started doing analysis on the image, they were surprised to find that there were LOTS of stars near the bulge with the characteristics of young stars.  But they couldn’t be so young so near the bulge – star formation doesn’t happen there.

To explain this discrepancy, astronomers have figured out that there must be a completely different type of star.   Here’s a figure describing the “normal” life history of a star, and then the life history for the “new” type of star.  Here’s a figure comparing the two paths that a star might take, if it started out about the same mass as our sun.

Learning about what is happening in a nearby galaxy like Andromeda can help astronomers understand where the light is coming from in more distant galaxies where even the HST can’t make out individual stars. If Phil’s group can confidently say how old stars like these are, then they can use that knowledge to help them interpret light from other galaxies that have this strange amount of UV light in their centers. They can then begin to address some big questions, like: how and when did galaxies form? And how old is our universe? The results of this analysis give astronomers a way to independently test other methods that are used to answer these kinds of questions.

If you want to know more about Phil’s work, check out his blog: https://sites.google.com/a/uw.edu/philrose/blog, especially his post on Bright UV stars in the bulge of M31 (Andromeda Galaxy).

## Seismic tomography: the easy way

When I was an undergrad, I had to take a year of geophysics, and on the first day, the professor explained that geophysics meant finding things underground without digging.  The rest of my lab group here at the UW are all marine geophysicists, and they routinely have to find things underground without digging, but with the added complication of having to get through a couple of kilometers of water, too.

You may remember my interview with Rob Weekly a few weeks back. This week’s interview is with my office-mate, Dax Soule. Dax studies marine geology and geophysics in the School of Oceanography at the University of Washington. Like Rob, he focuses on active source seismology. In fact, he’s looking at some of the same data as Rob, just different parts. And since we already went over a lot of the basic background in Rob’s post, I’m going to jump right into some fun tomography stuff. In a nutshell, tomography means building up a 3D picture of what’s underground by measuring how long it takes for seismic waves to travel through. In tomography, you’re not sampling the material directly, but you’re measuring how the seismic velocity varies. Seismic velocity is a measure of how quickly a seismic wave can travel through a certain type of rock, so if you know the velocity structure you can make inferences about the geological structure.

Imaging the earth’s crust

When you’re doing seismic tomography, it’s a bit like peeling back the layers of an onion: you have to figure out the shallow stuff before the deep. Rob’s work focused on obtaining a three-dimensional seismic velocity structure from the seafloor down to about 2.5-3 km depth. Dax is taking Rob’s results and extending downward to include the entire depth of the crust. He’s looking between the seafloor and the Mohorovicic discontinuity, or the “Moho”, which is the boundary between the earth’s crust and the mantle.

In seafloor maps of the region, you can see that there’s a bathymetric high – a plateau where the water depth is shallower than the surrounding seafloor. A seismic reflection study a few years back [1] showed that this plateau likely corresponds to a thickening of the oceanic crust at this location. Dax’s tomography work will help to clarify exactly what’s going on, and will give a more detailed picture of how the crustal thickness varies in the area.

Dax explained that measuring crustal thickness variations near a mid-ocean spreading center can tell you about how that crust was produced – was a episodic or constant? And where was the source of new crust – was it on the axis of the ridge, or off-axis?

How the experiment works

Here’s a little cartoon showing the basic geometry of the seismic experiment.

A seismic source is generated near the sea surface, and the energy travels through the water column and into the sea floor. Once in the earth’s crust, the energy is converted into to types of waves: primary waves (p-waves) and secondary waves (s-waves). There are many different types of paths that these waves can take as they travel through the earth, and each type of path is known as a “phase”. In the above figure, the two phases that are shown are Pg and PmP. Rob used the Pg phase arrivals to image the upper portion of the crust. Dax looks at the PmP phases, which are the ones that bounce off the Moho.

Dax goes through the data and manually picks out the PmP phase arrival times. This part of the job is not the most exciting, but someone’s got to do it! Fortunately, all that work does pay off, and once Dax has his picks, he can start digging into the tomography part of his work.

Five easy steps
Here’s a handy-dandy summary of the steps that Dax goes through to build up a tomographic inversion.

So that’s what my office-mate does… good to know!  Dax is still working through the data and the very complex inversion code, so stay tuned for a future post on what he finds deep in the crust, and what the crustal thickness can tell us about seafloor production at the Endeavour Ridge.

Reference
[1] Carbotte, Suzanne M., Mladen R. Nedimović, Juan Pablo Canales, Graham M. Kent, Alistair J. Harding, and Milena Marjanović. “Variable crustal structure along the Juan de Fuca Ridge: Influence of on‐axis hot spots and absolute plate motions.” Geochemistry, Geophysics, Geosystems 9, no. 8 (2008).