NOAA Teacher at Sea
Aboard NOAA Ship Rainier
May 6-16, 2013
Mission: Hydrographic surveys between Ketchikan and Petersburg, Alaska
Date: May 9, 2013
Weather on board. Taken at 1600 (4:00 in the afternoon)
Clear skies with a visibility of 10+ nautical miles
Light variable wind
Sea wave height – O
Air temperature 17.3° C
Water temperature 7.2° C
Science and Technology Log: Mapping the Ocean
The work we do on board the Rainier is all centered on the task of gathering data of the ocean bottom – shoreline to shoreline. These data are used to update the nautical charts (maps) used by sailors. The project we have been working on is a section of Behm Canal in SE Alaska.
Hydrographic data on parts of this stretch of water haven’t been updated for over 100 years. The tools and methods utilized have changed significantly during that time. Hydrographers of 1900 lowered a rope tied to a lead weight to the ocean bottom. Measurements were taken on the length of rope. The area we were surveying ranges from 150 to over 300 fathoms (one fathom = 6 feet) deep – that is a lot of rope. At each measure, they sighted a bearing to two or more locations on shore to locate where on the chart they could mark the depth. It’s surprising how closely their data matches what we found with the use of sophisticated modern techniques.
So how is it done? A good activity in the classroom is to make a sounding box with an ocean floor shaped on the bottom of the box. The top is covered and marked with a grid. Skewer sticks can be inserted at the grid corners, pulled out, measured, and transferred to another grid. A map is made. If only it were as easy. Simply put, modern hydrographers ping sound waves (sonar) from the bottom of the ship. The sound waves travel through the water to the ocean bottom and bounce back. We know how fast sound travels so measurements of time can be made and the distance calculated – just like the skewer sticks. If only it were as easy!
See the following website for information on hydrographic survey techniques. http://www.nauticalcharts.noaa.gov/mcd/learnnc_surveytechniques.html
My learning curve has been high as I have tried to understand all the moving variables that need to be taken into account before an accurate map can be made.
Here’s what I am beginning to understand:
- Starts with referencing benchmarks – both vertical and horizontal (see blog, May 7) to gain a standard of tidal variation (high and low tide can vary by as much as 20 feet) and GPS location.
- A measurement is made from the ship’s deck to the water surface. The twin sonar beams are located on the bottom of the ship. We know how far it is from the bottom of the ship to the deck – subtracting the deck to the water line gives the distance below the surface the sonar equipment is found at the time of measurement.
- The chart is marked off in rectangles. A line is marked for the ship to follow. Traveling at 10 knots, the multibeam equipment located on the bottom of the ship pings sound waves and measures how long they take to return from the bottom. A broad swath of ocean bottom can be measured at the same time. These data are transferred to a computer in the plotting lab where the computer archives it and generates visual images as they come in.
- The speed of sound varies in different water conditions, including temperature and salinity. Making it more complicated, temperature and salinity varies by depth in the water column beneath the ship. To capture these variables, we cast out a Moving Vessel Profiler (MVP) behind the ship while we travel along. The MVP looks like a small torpedo and is affectionately referred to as the fish. Attached is a sensor that reads temperature, conductivity (a measure for salinity), and depth. These data are transferred along a cable bound within the attached line to a computer on board the ship. “Casting” the fish means letting the line out until the fish approaches the bottom of the ocean – or 500 meters of line – whichever comes first. At that point the fish is retrieved. The data acquired as the fish makes its journey is transferred to the Plotting Lab computer.
- As the ship moves along the ocean surface it is subjected to constant movement. It pitches up and down from front to back (pitch), rolls side to side (roll), and rises up and down with the ocean swells (heave). As the survey data is collected, heave, roll, and pitch data is captured to allow for adjustments in the sonar data. All of this varies further with the tide level. All these data are captured and fed into the Plotting Lab computer.
- The ship travels its projected line, turns around and comes back on another.
- Small boats with similar beams are dispatched to capture the same measurements closer to the shoreline where it is too shallow for the ship (for tomorrow).
- This continues until the full ocean bottom in our project area is captured.
- Finally all these data sets are brought together and stored.
- During the off season, the data sets are utilized to generate the finished nautical charts ending a long, sophisticated process.
Personal Log: Life on the sea
I have to admit the living spaces on board a working ship are a bit tight. My “state room” measures approximately 10’ x 12’ and is shared with a roommate. In that space are our bunk beds, a sink, desk, and locker closets. I can’t sit up in bed without hitting my head on the bunk above. Shared between two rooms is a bathroom that is only 4’ x 8’ with a head (mariner’s term for a toilet) and shower. All this space rests on a floor that drops with the curve of the ship approximately 10” from one end to the other. The hallways in the ship are narrow and the stairways steep. Everything is bolted or tied to the floor or table to keep them from being tossed about in choppy waters.
While tight, I have yet to hear anyone wish for more. Perhaps the salt that runs in their mariner blood provides the sustenance they need to thrive in these close quarters at sea.
While my shipmates will call the Rainier home for the duration of the research season, I will be on board for only two weeks before I return to the comforts of my own home and spacious bed. I have to respect these hardy folk for who they are and all they do.