I find it so interesting that the object of my research is mostly invisible. No one, sighted or not, can “see” ocean currents below the surface. In the same way, no one can “see” the wind—we can see and hear the effects of wind—blowing leaves, horizontal snow, rattling windows. But the human eye can’t actually detect the movement of the air. We can’t see water moving below the surface either because light only penetrates down from the surface to about 100 meters, or about 300 feet. This is only 2.5% of the average depth of the ocean, leaving 97.5% of the depths completely in the dark.
So how do we find out what’s going on down there? We use sound. It turns out that sound penetrates through the water very well. Many marine animals – most famously, whales and dolphins – use sound, not sight, to find food, locate their family group, and choose a mate. I’ve counted more than a half dozen ways that sound is used on this research vessel or on the moorings to collect data. In my last post, I described one of these—the multi-beam echo sounder system for mapping out the shape of the sea floor. There are several other echo sounders for measuring the depth of the ocean under the ship. But in addition, we have:
- Sonic anemometers: mounted on the OOI surface buoy and on the ship to measure wind speed. They do this by very precisely measuring the difference in the time it takes for sound to travel around a group of sensors in one direction versus the other direction. This difference in travel time is proportional to the wind speed.
- Acoustic current meters: operate in the same way as the anemometer, but they attached to the mooring line to measure ocean current speed
- Acoustic Doppler Current Profilers: two different models, one mounted on the hull of the ship to measure currents from the surface to about 1000 meters depth, and another attached to the mooring line to do the same over about 300 meters depth. Both operate on the Doppler principle by sending out sound at one frequency and recording the frequency that the reflected sound has. The sound is reflecting off tiny particles drifting with the currents, and the difference in frequency is proportional to the drifting speed of the particles.
- Acoustic altimeter: mounted on the package of sensors that we lower on a wire to measure temperature, salinity, dissolved oxygen and other water properties, to tell us when the package is getting close to the sea floor (just like an altimeter in an airplane)
- Acoustic modem: this is really fancy technology that works much like a computer modem except that information is encoded in sound signals instead of electrical signals. This is used a lot to send information from one sensor to another underwater, or from a sensor to the ship.
- EK-80: this is a very sophisticated echo sounder that is used to see things that reflect sound in the water column in fine detail. This can be layers of plankton, or sensors on a mooring. We have been using it on this cruise to check the depth of sensors on moorings that we have just deployed. Similar devices can be attached to a mooring to track the size and depth of plankton.
- Acoustic mooring release: this device is attached to the very bottom of the mooring, right above the anchor. For the entire two years that the mooring is in the water, this release device will be carefully listening for a special signal, sent by us from the ship, that tells the release that it is time to let go of the anchor. At that point, the glass balls and foam spheres will cause the whole mooring, including all the wire and sensors, to rise to the surface so we can recover them. the anchor gets left behind on the sea floor.
And a few other sounds we hear now and then at sea:
- The ship’s horn, which we usually don’t blow except when leaving the dock.
- Emergency alarm, which is just like a fire alarm in a building. We have a drill about once a week to review safety procedures in case of fire, man overboard or abandon ship—all very unlikely events, but it’s important to be prepared.
Sound is also used extensively for military purposes. Some of the earliest research carried out at the Woods Hole Oceanographic Institution was sponsored by the U.S. Navy, and focused on the details of how sound travels in the ocean. For example, submarine propellers make noise which can travel a long distance underwater, and anyone listening with hydrophones (underwater microphones) can sometimes figure out where exactly the submarine is. So over the years, engineers have worked hard to make submarines as quiet as possible, while other researchers have been working equally hard to hear the faintest of sound signals. Just read the book or watch the movie “Hunt for Red October” to see this cat and mouse game in action.
Audio Postcard: Acoustic Mooring Release Test
In this audio clip below, you will hear us in the lab testing three acoustic mooring releases. These devices are being checked out before we attach them to the moorings. To test them in realistic conditions, we lower them on a wire to near the sea floor, and then send test acoustic signals and listen for signals back from the device that tell us it is working properly.
Audio Postcard: Acoustic Modem
This audio clip below demonstrates the sounds of an acoustic modem. This recording is a real data transfer from sensors on a mooring we just deployed a couple of days ago. The acoustic modem is on the mooring, and packages data from the sensors in special sound patterns that a hydrophone on our hull picks up. Later, technicians will de-code the sound signals to look at the data. There is low-level background noise in this recording, which is the sound of the ship’s propeller, heard underwater.