Follow the Leader – 13 – 15 August 2010

NOAA Teacher at Sea: Caroline Singler
Ship: USCGS Healy
Mission: Extended Continental Shelf Survey
Geographical area of cruise: Arctic Ocean north of Alaska in the Canada Basin
Date of Post: 16 August 2010

Location and Weather Data from the Bridge
Date: 13 August 2010 Time of Day: 2100 (9:00 p.m.) local time; 04:00 UTC
Latitude: 73º0’N Longitude: 145º3’W
Ship Speed: 3.9 knots Heading: 1.8º (north)
Air Temperature: 2.0ºC/35ºF
Barometric Pressure: 1018.9 millibars (mb) Humidity: 100%
Winds: 3-5 Knots SW
Sea Temperature: -0.4ºC Salinity: 25.37 PSU
Water Depth: ~3600 m

Date: 14 August 2010 Time of Day: 2105 (9:05 p.m.) local time; 04:05 UTC
Latitude: 73º36.4’N Longitude: 146º19.21’W
Ship Speed: 4.7 knots Heading: 223º (southwest)
Air Temperature: 2.15ºC/35.88ºF
Barometric Pressure: 1022.3 mb Humidity: 92.1%
Winds: 12.2 knots SE Wind Chill: -3.1ºC/26.5ºF
Sea Temperature: -0.7 ºC Salinity: 24.84 PSU
Water Depth: 3708.6 m

Date: 15 August 2010 Time of Day: 1500 (3:00 p.m.) local time; 22:00 UTC
Latitude: 72º56.4’N Longitude: 150º9.0’W
Ship speed: 11.8 knots Heading: 220º (southwest)
Air Temperature: 5.6ºC/42.2ºF
Barometric Pressure: 1015.6 mb Humidity: 98.1%
Winds: 17.7 knots E Wind Chill: 1.7ºC/35.1ºF
Sea Temperature: 3.9ºC Salinity: 24.5 PSU
Water Depth: 3691.1 m

Science and Technology Log

The Extended Continental Shelf Project is a multi-year effort between the United States and Canada. The two countries share knowledge, resources, and information to allow greater coverage of the region and more cost effective achievement of the mission objectives. For this mission, the USCGC Healy is working in tandem with the Canadian Coast Guard ice breaker Louis S. St. Laurent, called Louis (pronounced "Louie") for short. Healy is responsible for collecting bathymetric data and shallow subsurface imaging while Louis performs deeper subsurface imaging with her air-gun array. The instrumentation on Louis is towed behind the ship and requires a clear path through the ice; therefore, Healy’s primary responsibility when the ships are in ice is to lead and break ice for Louis. Healy opens a path and Louis follows, typically about one to two miles behind depending on ice and visibility conditions. It was foggy for most of the day on Friday as we led the way north along the first track line. The only way I knew that Louis was behind us was by watching the ship tracking chart and listening to occasional radio chatter between the two boats as the crews communicated about ice conditions. Skies cleared as we moved farther north and deeper into the ice on Saturday. Near midday, the fog lifted and there was Louis, first emerging like a ghostly image out of the fog and then, as we made the turn onto a new transect line, she was in full view. By Sunday afternoon we were heading south in open water, so Healy moved away from Louis to conduct other business while our ice breaking services were not needed.

While multibeam sonar allows us to “see the bottom”, subbottom profiling uses a different sound-producing system to see what is under the bottom. Geologists use the subbottom data both from Healy and Louis to estimate sediment thickness and make inferences about sediment types and structures beneath the seafloor. It makes me think of Superman’s x-ray vision! Like multibeam sonar, subbottom profilers are echosounding devices. They are active sonar systems – sound signals are transmitted and received by the instrument.

Healy’s profiler is a “chirp” system mounted inside the bottom of the ship’s hull – so called because it sounds like a bird chirping, a sound that one hears in the background throughout the ship. It releases high frequency pulses of acoustic energy that travel through the water column and (in theory) hit the seafloor and penetrate into subsurface materials to depths of tens of meters. Signals are reflected at the seafloor and at interfaces between different subsurface layers within the seafloor. The reflection of acoustic energy depends on the “acoustic impedance” of the material encountered. Acoustic impedance is related to the density of the material and the velocity of sound in that medium. Different materials have different acoustic impedance and therefore different reflectivity. The concept is similar to that of albedo when one considers the reflection of solar energy from different surfaces. A smooth, light-colored surface like a field of snow reflects a high percentage of incoming solar rays and therefore has a high albedo– hence the glare that hurts your eyes on a sunny day. Dark-colored surfaces reflect much lower percentages of incident light and therefore have low albedo. (They also absorb more energy which is why they get hotter on a sunny day.)
With subbottom profiling, sands typically reflect sound differently than mud, and layers or other structures in the subsurface result in different signal strengths returning to the receivers on the ship. The picture on the right shows an image of the raw chirp data displayed on the computer screen at the watch stander station. It does not show a lot in this state, but after processing the data will provide important information about the subsurface in the Arctic Ocean.

Subbottom surveying on Louis is performed with a multi-channel air gun system that is towed behind the ship. Three air guns, powered by air compressors on the ship's deck, provide the acoustic energy source. A streamer with an array of 16 hydrophones trails behind the air guns; the hydrophones receive the return signals reflected by the seafloor and subsurface sediments. In open water, the air guns are attached to a float and hang about three to five meters below the surface, at a distance of about 100 meters behind the ship. In ice, the air guns are attached to a metal sled (depressor) that hangs below the sea surface (and hence the ice) to a depth of about 10 meters and at a distance of about 10 meters behind the ship. When fired, the air guns simultaneously emit large air bubbles into the water column. As the bubbles collapse, an acoustic pulse is produced that moves through the water. It is similar to what happens in the atmosphere when air rapidly expands and contracts as a lightning bolt passes through, creating the sound we know as thunder. The air guns generate sound at a lower frequency than the chirp system; sound at these lower frequencies penetrates deeper into the subsurface but produces lower resolution than the higher frequency chirp system. Such air gun systems can provide images to depths of several kilometers below the seafloor.
Image source: USGS Woods Hole Science Center

References:
USGS Woods Hole Science Center http://woodshole.er.usgs.gov/operations/sfmapping/seismic.htm
NOAA Coastal Services Center http://www.csc.noaa.gov/benthic/mapping/techniques/sensors/subbottom.htm

Personal Log
Saturdays are “Field Days” on Healy. No, we did not all get into boats and take a trip away from the ship or get out onto the ice. Field Day is a fancy way of saying that it is time for cleanup and inspection of common areas and personal berthing areas. All personnel on board are responsible for trash removal and cleaning of staterooms, restrooms and common living and working spaces. Anyone who is not on duty pitches in to clean the Science lounge and labs – vacuuming, sweeping, washing floors and generally putting things in order. The “trash vans” are open twice a week; everyone brings trash and recycling to two large blue bins on the port side of the 02 deck (the same deck as the science staterooms). Coast Guard volunteers work the trash vans. Healy will be at sea for another long mission after this one, so efficient trash removal and storage is critical. Healy personnel are dedicated to recycling and have an award winning recycling program on board – no small feat when it is necessary to haul it all around for months at sea. Think about that when you are tempted to complain about separating recyclables from trash at home or at school.

Since everything was neat and tidy, I decided it was a good time to show you my living space on Healy. Science staterooms are set up for three occupants, but on this trip we have two people per room. I share a room with Sarah Ashworth, a marine mammal observer; she is currently on Louis, so for now I have my own room. The room is more spacious than I expected on a ship, similar in size to a lot of college dorm rooms.
Space is used very efficiently. There are bunk beds; Sarah has more experience at sea than I, so she has the top bunk or “rack”.








Each person has a good sized locker for clothes and since there are only two of us, we each have a desk and filing cabinet, so there is plenty of storage space – more than we need for our personal belongings.

There’s nothing like a room with a view, even if they left the tape on the window the last time they painted the ship.

Each room has its own sink, and shares a bathroom with the adjoining room. Okay, they call it a “head” on a ship; don’t ask me why! The bathroom is small, but one does not linger when taking a “sea shower”, and there is always plenty of hot water. In case you ever wondered what a marine toilet looked like, here it is.

We headed towards Barrow on Sunday to pick up a crew member and some supplies for the Louis. There was a steady wind from the east for most of the afternoon, and the boat was rolling a little, but I was more prepared for it this time than I was the first time it happened, but I still stumble when I walk down the hall.

We have had beautiful views of ice, sea, and sky for the last few days.



Caroline