Beautiful 3D map of the Great Barrier Reef

Led by geologist Robin Beaman, some of the clever folks at James Cook University in Townsville, in the northeastern Australian state of Queensland, have mapped in three dimensions and with unprecedented precision the seafloor off the Queensland coast.  In the process produced they’ve produced some spectacular imagery.

 Gorgeous isn’t it?  The map was made by melding together single beam and multi-beam sonar observations from ship-mounted equipment, with light detection and ranging (LIDAR) data, which are laser measurements taken from satellites.

Multi-beam sonar used to map the ocean floorBut it’s the scale thats really amazing here.  The path of the fly-through in that video is over 1,350 miles, or the distance from Miami to Provincetown (Cape Cod) or from the Straits of Gibraltar to the coast of Greece.  And they covered all the way from the Queensland coast to New Caledonia, encompassing an area of 6 million square kilometers, or about 2.3 million square miles.  Thats about the same area as the lower 48 states, except Texas, mapped - underwater mind you - to 100m resolution!  It was a mammoth job.  In the process they discovered some previously unknown features and, importantly, mapped the entire Great Barrier Reef, the worlds largest coral reef ecosystem.  These sorts of things will make the map an invaluable tool for folks at the Great Barrier Reef Marine Park Authority, who are charged with the management of this vast area.

You can read more about how they made their map here and a good story about the work in Australian Geographic here.

I'm surfing on the inside

I've always loved the idea of internal waves; the idea that gentle, rolling, and sometimes very large waves roll along, not on the surface of the sea, but deep below it.  How is that possible?  The best explanation is by analogy:  If you've ever swum in a lake in early summer, where your body was bathed in warm still surface waters but your legs were down in the icy deeper layer, then you've crossed the boundary that internal waves call home.  They travel along "density boundaries" and the most common of those is the bit where water goes from warm at the surface to cold at depths, called the thermocline; there are other types of density boundaries too, such as where fresh water overlies denser briny water.  Sometimes you can even see density boundaries near the surface; the change in density affects the way light passes through the water so you can sometime see a shimmery sort of distortion, even though the water is clear.  There's a good one shown here. The picture at right from the Institute of Hydromechanics at the University of Stuttgart shows an artificial internal wave produced as part of an experiment; they dyed the different densities of water to show it better.

The bigger the change from warm to cold in a thermocline, the bigger the density difference, with the colder water being more dense.  Really sharp thermoclines like this have some interesting properties, such as the ability to reflect some types of sonar.  In fact, Navy submarines have been known to "hide" below a good thermocline, and then be revealed for all to "see", by a passing internal wave.  The sub is below the thermocline one minute, then above it the next, exposed and vulnerable to the next sonar ping - oops. 

Internal waves can even "break", like a surf wave on a beach.  This image from Memorial University in Canada shows a model of how this happens, with the wave coming in from the left and breaking on the bottom as it gets shallower; all the while the water's surface is calm.  The internal wave doesn't look quite the same as a regular beach wave because the density difference isn't nearly as much as when you go from water to air and the internal wave pushes up against the heavy, viscous overlying water, but the principle is the same.

I don't know if its possible to somehow surf on an internal wave.  I doubt it, because the drag of the overlying water would be much more than you would experience in air, but its pretty cool to think about.  Its probably a good thing if you can't do it, because Al surfing is about the only thing scarier than Al dancing...

What do expectant parents and the Chilean earthquake have in common?

The recent Chilean earthquake was a disaster on a mind-boggling scale; one that had its genesis beneath the sea.  The temblor, and all those in Chile before it, including the biggest ever recorded anywhere, resulted from the Nazca plate sliding down under the South American plate, under the sea to the South West of Santiago.  Well, it doesn't exactly slide, I always imagined it would sound like a creaking door if you could speed up the process a few zillion times.  The upward pressure this collision puts on the South American plate is immense and produces the longest mountain range in the world, the Andes.

Anyway, this most recent slip, which shifted about 10 meters and registered 8.8 on the Richter scale, caused a small tsunami.  Now some researchers from Scripps and UCSD want to know whether it was because of the sea floor movement itself, or because the quake triggered undersea landslides ("slumping") that produced the wave.  They are going to do some nifty multi-beam sonar work to map the seafloor changes in unprecedented details.  Sonar technology has become a really cool tool these days; the same sorts of benefits that new parents reap when they ultrasound their new bundle of joy also give scientists a fantastic new view on the sea floor.  Just check out this example of a shipwreck revealed by NOAA's nautical survey side-scan sonar.