Monday, May 14, 2012

Erin Hunter's Internship, Spring 2012 - Part 2

Week 2: Multibeam Sonar

A multibeam echosounder is a device used by hydrographic surveyors to determine the depth of water and the basic geomorphology of the seafloor.
The rhythmic pitch from bow to stern, the steady roll from side to side wake me from my mid-shift sleep. It's time for watch. The seas are about four feet and the ship is steady with its anti-roll stabilizers. We are making a 180 degree turn easily and swiftly with the dual shaft rudder system accompanied by two stern thrusters and combination hydrualic bow thrusters. We stay on our mapping line via GPS coordinates while the internal motion unit (IMU) keeps track of the sonar transducers' position under the hull at a rate of a several times per second. Our every move is recorded and used to record the multibeam soundings accurately.

Down the hall and into the control room, the two watchstanders turn over their shift. Computer screens fill the room. The multibeam sonar readings are up on the big screen, and are used to determine the depth of the seafloor. We are cruising 105 km (168 miles) due south of the Pensacola pass. Our water depth is 2,906 m or the equivalent of 29 football fields deep.

View of the control room on the Okeanos.
This multibeam screen shows an underwater sediment river.

As the ship holds its course, from under the ships' hull a sound pulse, or ping, is transmitted through the water column to the seafloor. After the ping bounces off the seafloor the receiver, positioned next to the transmitter. The time it takes for the ping to go to the bottom and return, translates to the depth of the seafloor at that position. When a ping takes longer to return from the seafloor, the depth is greater. A multibeam acoustic sensor can cover a large area, or swath. It has several transmitters and receivers called transducers that change electric pulses to acoustic pulses and vice versa. This allows a large area to be mapped on one heading, or line. The E302 multibeam used aboard the NOAA Okeanos Explorer can generate 486 depth sounding per swath at a rate of 10 swaths per second. This seafloor depth information, or bathymetric data, is used to create the maps produced today. All of this data is publicly available on the NOAA website. The Okeanos-google digital atlas is also a good place to look at survey sites.

http://www.ncddc.noaa.gov/website/google_maps/OkeanosExplorer/mapsOkeanos.htm

http://oceanexplorer.noaa.gov/explorations/09bermuda/background/multibeam/multibeam.html

Personal Log for Week 2

All is well. There was a movie night outside on the fantail, or aft deck, and they watched The Grey, about Arctic wolves. We have been eating an array of cookies and cakes. My favorite spot to hang out is on a wooden bench on the bow. It is nice to sit up there, get fresh air and read a good book. We had safety drills again today (it was my shower time), luckily we only had to put on life jackets and not the big survival suits, aka gumby suits.

Post Author:
Erin Hunter

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