Neat Stuff on the Intertubes: Ocean Edition
Some interesting things I found on our favorite series of tubes.
Recording the sounds of the deep ocean
The hydrophones themselves are interesting; they consist of an anchor chain sunk to the ocean floor, and a long (several thousand meter) nylon/polyester rope that holds the actual recording equipment floating (buoyed by a giant foam ball) at a depth near 800 meters, in the middle of what's known as the SOFAR channel (explained below.) The recording equipment looks like something your crazy uncle would have cobbled together - consisting mostly of batteries, but also with an embedded CPU, some home brewed software, and a giant (!) 32 GB hard drive, all sealed in a watertight container. Periodically, researchers need to send out a ship, locate the precise spot where the hydrophone was dropped, and try to retrieve the thing from a half a mile below the surface. They don't have much detail about the process, but I can't imagine it's easy.
Now about that SOFAR channel: sound propagation in the ocean is more complicated than in the atmosphere. One of the major differences has to do with the fact that sound velocity in water is affected by the temperature, pressure, and salinity. Under normal stratification of the open-ocean water column, there's a minimum in sound speed at a depth of 500-1000 meters. Because the sound speed increases no matter whether you go up or down in the water column, sound waves near this depth will be refracted downward as they propagate towards shallower water, and refracted upward if they propagate toward deeper water. The net effect is to loosely confine the waves to this particular layer of water, and attenuate leakage either up or down.
(original image at Vents Program: source)
This means that sounds produced at this depth don't spread out as much as sounds at other depths, and don't get quieter with distance as quickly. In fact, it's well documented that marine mammals can communicate over distance reaching thousands of miles; and the ability of this layer to boost sounds horizontally may be a large part of the reason (although whales generally reside in much shallower water). The Vents hydrophones often record whale calls in the open ocean. Here's a sample of a blue whale calling in the Pacific, off the western coast of South America. The recording is actually sped up 10 times to move the frequencies into the range of human hearing (and the same for all other sounds below.) The image on the right is a spectrogram; time is on the vertical axis, frequency on the horizontal axis, and the strength of a particular frequency is represented by brightness.
And here's the sound of humpback whales off the coast of Alaska.
Most hydrophones are placed in known positions as part of an array - anywhere from three to ten scattered about with a separation of several hundred kilometers. This serves a purpose - by comparing data from different locations, researchers can roughly triangulate the source of a sound. This is useful because the main reason for dumping this equipment in the water in the first place is to monitor seismicity and volcanism on mid-ocean ridges, volcanic island/seamount arcs, or subduction trenches. Those kinds of events have a distinguishable audio signature, and if there's an earthquake swarm or eruption area, it's useful to know approximately where it's occurring. Here's the sound of a harmonic tremor in the Volcano Island seamount chain south of Japan:
These hydrophones are recording 24-7, and you wouldn't be surprised to find that there's a lot of ambient noise in the ocean. Which leads us to "the Bloop."
This is a sound recorded by an array in the eastern equatorial Pacific multiple times during the summer of 1997. It was triangulated to roughly 50°S, 100°W, still strong and easily recordable at a distance of more than 5000 km. It's also a unique sound, and nothing like it has been recorded since. The Vents program flatly states "The origin of the sound is unknown."
And here's another mystery sound they've dubbed "The Slowdown", originating near 15°S, 115° W, also unique.
There's still a lot we don't know about the world.