breaking the waves

Too_cooljenluc_5Is it just me, or did Bloglines go completely bonkers over the weekend, posting and re-posting all the most recent entries on several different blogs, including Cocktail Party Physics and my occasional weekend getaway, 3 Quarks Daily? Perhaps the system is rebelling against the cold snap that enveloped the Northeast this weekend. It certainly came as a shock to my system, after 10 days in Los Angeles and Hawaii. I comfort myself by recalling the warm sunshine, refreshing breeze and  sounds of the waves breaking on Waikiki beach from my hotel balcony. Just think, a little over a week ago, I was basking in the sun, watching surfers frolic in the ocean — and would-be surfers practice the basic mechanics on the shore, under the watchful eye of a very peppy, bikini-clad instructor.

Sadly, I didn't have the chance to take any surfing lessons myself, which is a shame, because I've always wanted to learn. (Note to self: must stop living in frigid climes.) Among other things, there's an awful lot of fundamental physics involved: potential and kinetic energy, surface tension, friction, buoyancy, hydrodynamics, and good ol' Newtonian gravity/laws of motion. The Exploratorium has an excellent summation of all that basic physics here, including all the factors that conspire to make the "perfect wave."

There's also some less fundamental science associated with surfing, in the form of acoustic waves. The bigger the wave, the more sound you get — a statement that seems, like, totally obvious to any diehard surfer. And it's not just the waves we can hear breaking along the shore, although those are scientifically interesting in their own right. Human hearing is rather limited in range, from 20 Hz to 22 kHz, but sound waves exist far beyond that. We can't hear ultrasonic pulses, like bats use for echolocation, and we can't hear the ultra-low-frequency waves of acoustic energy (infrasound) employed by elephants or tigers, for example. Wind, water, earthquakes, avalanches, tornadoes and hurricanes all produce infrasound as well. Most of us aren't aware that Nature has an entire palette of sounds that play constantly, just beyond our ken (although psychologist Richard Wiseman of the University of Hertfordshire has speculated that the odd sensations people believe are caused by ghosts are actually a subconscious "sensing" of infrasonic vibrations). To an acoustician, there's no such thing as perfect silence; to them, what we think of as "silence" can be downright loud.

Volcanoes, for example, exhibit seismic rumblings — low-frequency audio waves vibrating through the medium of the earth's crust — and make a lot of noise when they erupt (the shock wave phenomenon). But they also produce infrasonic waves via combinations of any number of underlying physical mechanisms, depending on the type of volcano. Scientists have known this since the late 19th century, when the Indonesian volcano, Krakatoa, erupted on August 27, 1883. Krakatoa The eruption produced a flurry of infrasonic waves, the acoustical equivalent of "an earthquake in the air," per Simon Winchester, author of the 2003 book, Krakatoa: The Day the World Exploded.

This makes the infrasound emissions — or "vocalizations" — of active volcanoes a fascinating subject of study for Milton Garces, an oceanographer at the University of Hawaii, Manoa. Garces is a colorful guy. Most acousticians have a touch of the maverick in them, almost by necessity: if you're trying to study the propagation of acoustic waves, you've got to go where the waves are happening, even if that leads you to remote Mayan ruins or the foot of a very-much-active volcano. Garces is no exception. When he's not exploding missiles at the White Sands Missile Range in New Mexico (to better study the infrasonic waves that result from the explosion), he's setting up infrasound sensor arrays around volcanoes in Ecuador, or Japan's Kyushu Island. And yes, he has been caught napping — literally! in a Toyota Corolla! — in the vicinity of a volcanic eruption, resulting in some harrowing, ash-choked moments before he was able to drive to safety.

In short, Garces is the adventurous type, and a bit of a risk-taker — traits that have served him well in his research, since all the best science requires a certain assumption of risk. Plus, he's got this whole "Antonio Banderas with a PhD" thing going. It was fascinating to watch the audience's fluttery reactions at the ASA meeting as he waxed poetic about giving his beloved volcanoes a "voice" while playing sound clips of recorded infrasonic waves (with the pitch shifted by a factor of 400 or so to make it detectable to the human ear). I mean, if those sound files are anything to go by, a volcano's "voice" resembles severe gastric distress, and yet Garces had us all marveling at these vocalizations as if they were the Song of Songs. (Rest assured, I am utterly devoted to my betrothed, that Fourier-transform-spouting cosmologist, who can certainly hold his own against Garces on the Charmingly Attractive Scientist front. But Jen-Luc Piquant remains a free agent, and is a sucker for a Spanish accent. So is Wired, apparently; in a recent article, reporter John Geirland raved about Garces' "trim" physique and "glowing hazel eyes." Um, drool much, Mr. Geirland?)

So Garces is seriously mediagenic. (Jen-Luc calls him the Volcano Whisperer, lovingly coaxing the desired infrasonic data from the groaning depths of a coy volcanic mountain reluctant to reveal her darkest secrets.) Frankly, he knows it, and isn't afraid to "work it" a little in order to drum up some much-needed media exposure for his area of expertise. And kudos to him for doing so. Infrasound was all the rage from World War I through the 1950s, when it was used to monitor Soviet nuclear testing on the other side of the globe. But the technology was abandoned in the late 1960s in favor of satellite monitoring. It languished as an academic backwater for the next 30 years, and only now is it coming back into its own — thanks in part to engaging, media-friendly scientists like Garces.

Sure, the research is way cool, dude, but there's a practical side as well. Infrasound is potentially a more accurate barometer of volcanic activity than traditional seismography. Ash clouds, for instance, are a serious aviation hazard. In the last 20 years, more than 200 aircraft have flown into clouds of ash from unexpected volcanic eruptions. This is very dangerous because the silicon-based particles from the eruption can enter jet engines and melt, impairing or even destroying the engines. Per the aforementioned Wired article, catastrophe was narrowly averted in 1982, when a Boeing 747 with 240 passengers on board flew through a plume of ash from Indonesia's Galunggung volcano at 37,000 feet. All four engines shut down and the craft plummeted 25,000 feet before three of those engines finally restarted.

Combining seismic data with infrasound monitoring could make it easier for scientists to tell when a given volcano is about to blow, whether ash will be ejected, and hopefully avoid such near-catastrophes in the future. Garces and his cohorts have developed a prototype system called Acoustical Surveillance for Hazardous Eruptions (ASHE), deployed in Ecuador in January 2005, because the region has numerous active volcanoes in a relatively small geographical area. They've used it to identify specific infrasonic signals associated with explosions, seismic activity, and flows of debris. The good news: there may be distinctly different infrasound signals for volcanic eruptions that produce ash, and those that do not.

You're probably wondering at this point what Garces and volcanoes have to do with surfing, which is where we started out. No, I'm not aimlessly rambling — not much, anyway. It's all about the infrasound, baby! For Garces, it's a natural connection, since he is also an avid surfer. (He's probably one of those daring sorts who jet-ski out to the oceanic wilds so they can catch the "big waves.") It just so happens that breaking waves also produce infrasonic signals. Garces' new work exploits this feature to (hopefully) achieve Real-Time Surf Infrasonic Monitoring, or, as he prefers to phrase it, "the deep sound of one wave plunging."

Garces is specifically studying breaking waves along Oahu's North Shore, widely deemed to be a surfer's Mecca. There are three types of wave "breaks" that produce infrasound: plunging breaks, cliff breaks, and reef breaks, and Garces' latest work focuses on the latter. He is attempting to isolate the sound of a single wave in the process of breaking — essentially, he's tracking progressive wavefronts — with acoustically sensitive pressure sensors deployed along the ocean floor, augmented with conventional seismography. He hopes to use the collected raw data to extract useful information about wave height, for example, to better identify potential hazards to surfers (and, one assumes, swimmers as well). 

It's trickier than it seems: such predictions currently rely on the observations of surfers themselves to determine wave heights. True, there are sensor-equipped buoys in the cove designed to collect that information, but the data are insufficient to make accurate predictions.

I initially found this quite surprising, since a similar system works quite well along the coastline of San Diego, where the Scripps Institute deploys a similar set of buoys, and crunches the raw data using clever algorithms to separate the meaningful signals from background noise. This enables them to plot the direction, speed and curvature of incoming waves to determine the location of the sound source, and to make more accurate predictions. So why wouldn't it work on Oahu? Fortunately, I chanced to strike up a passing conversation with Geoffrey Edelmann, an acoustician at the Naval Research Laboratory, after the infrasound session, who explained that it's easier to establish directionality along San Diego's far more sheltered coastline than it is in Hawaii, where wave directionality isn't clear at all — they're literally coming in from all directions at once. So the San Diego algorithms just don't apply; scientists can't make the same set of underlying assumptions.

Anyway, Garces' latest project is still in its earliest stages, and his team will continue collecting and analyzing data throughout this winter. But if his hunch turns out to be right, infrasound could end up being a very useful tool for oceanographic monitoring as well. He reported some intriguing preliminary results: there appear to be seasonal changes, with certain areas becoming more acoustically active than others at certain times of year.

Perhaps surfing does have its seasons, even in Hawaii; those of us living in more variable climes have a far different concept of "seasonal." Except for certain people in Cleveland, apparently, according to an article in today's New York Times.  These people love to surf so much not even frigid temperatures and (occasionally) raw sewage can keep them away from Lake Erie. They don wet suits, wear goggles, and learn to avoid ice chunks the size of bowling balls. Ouch. (Hat tip to Orac at Respectful Insolence, who observes, "There are no depths of craziness they won't plumb.") Those unwilling to face such hardship can always avail themselves of the growing number of indoor wave pools/water park resorts sprouting up all over the nation, like Wisconsin Dells.

But really, it makes far more sense to switch to something like snowboarding, a la Olympic gold medalist Shaun "The Flying Tomato" White, who skateboards in the summers when snowboarding is out of season. Both skateboarding and snowboarding have their own underlying physics principles (not identical, but similar); check out discussions of the topic here and here, and marvel all the more the next time you see White performing those jaw-dropping airborne flips and 360-degree rotations. Athletes like White have an intuitive grasp of the physics behind their sports. They wouldn't excel  so spectacularly otherwise. But what is the sound of a single snowflake falling onto the slick, icy surface of the half-pipe? Scientists like Garces who study infrasound could probably figure out the answer.

5 thoughts on “breaking the waves”

  1. This guy Garces sounds great, does he go to AGU meetings? An active volcano is endlessly fascinating, I just went hiking there and already want to go back. I hope you were able to see the turtles from your lanai at the Sheraton.

  2. I live in the woods where we have some perfectly quiet days, no ambient noise, and on one such day I heard the quake of 2001- I think it’s called the Nisqually Quake. Totally awesome sound, like nothing else in the world. We shook pretty good, but for once it was the sound, not the shaking, that made you think.

  3. Hey Jennifer, nice job in making deep vibrations more palpable. And yes, the jet-ski is fueled and prepped for tomorrow’s NW swell.
    Hope BuffyVerse is killing it!
    Aloha, Milton

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