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MESA Archives: Aleutian Islands Deep Water Corals Cruise, July 30, 2004

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(text transcript)

(PLEASE NOTE: These web pages are for archival purposes only and are no longer maintained. For current information please refer to the MESA homepage.)

Guest journal by Gary Greene Marine Geologist, Moss Landing Marine Laboratories and the Monterey Bay Aquarium Research Institute
(Read how sound is used to explore the seafloor: Part I and II)

Part I:  Blind As A Bat: Sounds of Science at Sea

Gary Greene
In the darkened Deep Submergence "control van", scientist Gary Greene uses his computer to select images of the seafloor for later study. Behind him is one of the maps used to guide the expedition. Photo by Sonya Senkowsky.

Sound controls our lives at sea, especially if you are a geophysicist. It is what we use to tell us what lies in the water, how the sea floor is configured and what lies beneath the floor of the ocean. Onboard ship, it tells us when things go wrong or when things are going well. Sounds are all around us, and in this noisy environment we sort out the routine of seaboard life and scientific data collection.

Bells and whistles let us know if a fire has started or if the vessel is about to sink (or, more likely, that the crew is conducting a drill) so that we can reach for our survival suits and rush to the life rafts. The sounds of the constant hum of blowers, motors, thrusters and the thrashing of the large propellers tell us that things are normal, and whether the ship is underway surveying or keeping station for sampling and seafloor observations.

My day at sea on the Revelle generally starts with a gentle knock on the door of my cabin, located deep in the ship up against the starboard (right) side. It is 3:30 in the morning and time to relieve the watch in the Deep Submergence control van several decks above my cabin. In the dark of the cabin, I struggle to dress and collect all the things I will need to stand my four-hour-long watch. In this dark environment, I collect the sounds around me and assess the state of the vessel and its operation. Do the motors sound right? Is water rushing past the hull or is it gently lapping up against it? This assessment prepares me for what to expect when I reach the control van.

Sometimes the knock does not come. I awake anyway. In the dark, I can hear the sweep of sound and hard click against the hull from the multibeam echosounder, which sounds like a birdcall. This instrument is used to image the seafloor, using sound to give precise depth information based on the reflected and random scatter of sound that returns to the receiver after being bounced off the sediment or rock below. The duration between the sweep of sound and the click, along with the intensity of the click, tells me if we are in deep or shallow water. If the click taps the hull like a hammer would, and the duration between the sweep of initial noise and the click is short, I know that we are in shallow water. If the click is barely detectable and arrives long after the initial sweep of sound, I know we are in deep water.

So, this day that the knock does not come, I know that something has changed, either because of an instrument malfunction or from a change in plans. The echosounder noise tells me if we are in shallow water. The lapping or swishing of water along the hull outside of my cabin tells me if the vessel is swiftly moving through the water. And, the normal or interrupted background ship noise tells me if everything onboard the ship is operating normally. The ship's horn blasting away every 2 minutes to alert other vessels of our presence tells me that it is foggy on the calm waters. Armed with this sound assessment, I retreat from my dark cabin to the lighted world of the ship's interior. I grab a cup of coffee in the mess and slowly make my way to the control van, confirming my initial assessment of conditions based on sound.

So, like bats, we marine geologists rely upon sound to navigate and image the surroundings around us, the deep sea. Try it. The next time you get up in the dark, don't turn on the lights. Listen. Listen to sounds in your room. Listen to the sounds outside. Try to determine the activity in your house and in your neighborhood (you probably do this subconsciously anyway). Do you hear dishes rattling to indicate someone is up and in the kitchen? Do you hear the drip and patter of water, which may indicate rain, or a broken water pipe?

You have now collected and assessed these sounds, an assessment that helps you plan your day, answering questions such as: Do you need to fix breakfast or is it being fixed for you? And what you should wear today? Sound allows us to plan for our daily life, both scientifically and at home. So off I go for another day at sea using sound to plan and guide me through this very exciting scientific adventure aboard the Revelle.

Part II  >>>

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