Acoustics: Substitute for Superman Vision?
Acoustic reflection profiling
It would be nice if we could see inside the Earth, but lacking Superman's x-ray vision, we have to rely on remote sensing instead. On land, geophysicists send sound pulses into the Earth to see what's there. We do that in the oceans too, except that we also have to shoot the acoustic waves through the water. Oceanographers use sound waves of different frequencies to probe at different depths and resolutions. High frequency sound waves are used to get a high resolution picture of the near surface. The short wavelengths of these waves give high vertical resolution, but they quickly die out with distance, so their penetration into the bottom is small. Low frequency sound waves, called seismics because they literally make the ground shake, don't give very high resolution because the wavelengths are long, but they don't die out as fast, so they penetrate deeper.
Acoustic profiling works because the medium in which the waves travel is not uniform, so when the sound wave hits an interface where there is a change in density or rigidity properties, part of the sound will pass through, but part will be bounced back. If we point the acoustic source straight down, then the wave will bounce back to the source, where it can be recorded.
Typically, we see a big interface at the ocean bottom (the change from water to sediment or rock) and often we see layers within the sediments that have different properties, and therefore they cause a reflection. (as in the the lighter sediment areas at the top of the image above)
High frequency acoustic profiling
We already discussed high frequency profiling in the section on bathymetry.Onboard the Revelle, we have a chirp echosounder that emits a sound pulse with frequencies between 2-10 kHz. Inside the ship, it sounds like a high-pitched chirp. It goes off about once per second, and in between pulses it listens for the sound waves returning from the ocean bottom. These sound waves mainly bounce off the seafloor and they may penetrate about 50 meters into the bottom if there are soft sediments. This technique is called profiling because as the ship moves along, sequential pulses are sent out and the return echoes are plotted vertically to make a picture that looks like a profile of the ocean bottom.
We use low frequency sound to see deeper below the ocean bottom. This requires large noise sources and special equipment to listen for the returning sound waves. On this cruise, we are using two air gun seismic sources. Each air gun is a high-pressure air chamber. It is pumped with high pressure air from compressors and then once every 10 seconds, a trigger activates a port, which allows the air to escape. The explosive release of air causes a "pop" which is the sound wave in the water. From the ship, we can hear the air guns go off as a "thud."
Those air guns are towed about 20 m behind the ship on floats. (known as "fish", shown being moved into position by Masako, Amy and Lee)
To record the sound waves bouncing back from the seafloor and below, we have a device called a hydrophone array, a.k.a. "streamer" or "eel" after what it looks like. The hydrophone array is a plastic tube about 800 meters long that is filled with cables and microphone-like devices spaced at regular intervals. There are hundreds of hydrophones in the cable, and groups are linked together to form "channels" that can be recorded separately. Our eel has 48 separate channels. The reason for a long eel and so many channels is that with multiple separate channels, computer processing can be used to increase the resolution of the seismic imaging as well as to increase the signal while decreasing the noise.
In the images below, the metal spool (left image) on the back deck is about 2 meters in diameter. (6 feet) In the image to the right, Brandi, Lee and James feed out the "eel", with one of the floats attached.
As it travels through the water, the "eel" is constantly shaken and twisted by the waves. This creates noise that we have to remove to see the signal from the bottom. On our cruise, the acoustic profiles are showing us the sediments up to about 500 to 1000 meters below the seafloor.
Even Superman can't see that deep!
Usually the deepest we can see is the volcanic underpinnings of the Ninetyeast Ridge, an interface that marine geophysicists usually call "basement."
updated June 30, 2007 from the Indian Ocean