NOAA Teacher at Sea
Tom Savage
Aboard NOAA Ship Fairweather
August 6 – 23, 2018
Mission: Arctic Access Hydrographic Survey
Geographic Area of Cruise: Point Hope, northwest Alaska
Date: August 12, 2018
Weather data from the Bridge
Wind speed 8 knots
Visibility: 10 nautical miles
Barometer: 1010.5 mB
Temp: 8.5 C 47 F
Dry bulb 8 Wet bulb 6.5
Cloud Height: 5,000 ft
Type: Alto Stratus
Sea Height 2 feet
Science and Technology
Why is NOAA taking on this challenging task of mapping the ocean floor? As mentioned in an earlier blog, the ocean temperatures worldwide are warming and thus the ice in the polar regions are melting. As the ice melts, it provides mariners with an option to sail north of Canada, avoiding the Panama Canal. The following sequence of maps illustrates a historical perspective of receding ice sheet off the coast of Alaska since August 1857. The red reference point on the map indicates the Point Hope region of Alaska we are mapping.
This data was compiled by NOAA using 10 different sources. For further information as how this data was compiled visit https://oceanservice.noaa.gov/news/mar14/alaska-sea-ice.html.
The light grey indicates 0-30% Open Water – Very Open Drift. The medium grey indicates 30 – 90 % Open drift – Close Pack. The black indicates 90 – 100% very close compact.
Ships that sail this region today rely on their own ships sonar for navigating around nautical hazards and this may not be as reliable especially if the ships sonar is not properly working (it’s also problematic because it only tells you how deep it is at the ship’s current location – a sonar won’t tell you if an uncharted hazard is just in front of the ship). Prior to mapping the ocean floor in any coastal region, it requires a year of planning in identifying the exact corridors to be mapped. Hydrographers plot areas to be mapped using reference polygons overlaid on existing nautical charts. Nautical charts present a wealth of existing information such as ocean depth, measured in fathoms(one fathom is equal to six feet) and other known navigation hazards.
As mariners sail closer to the shorelines, the depth of the ocean becomes increasingly important. Because of this uncertainty in the depth, the Fairweather herself cannot safely navigate safely (or survey) close to shore. In order to capture this data, small boats called “launches” are used. There are a total of four launch boats that are housed on the boat deck of the Fairweather. Each boat can collect data for up to twelve hours with a crew of 2-5. Depending on the complexity of the area, each daily assignment will be adjusted to reasonably reflect what can be accomplished in one day by a single launch. Weather is a huge factor in the team’s ability to safely collect data. Prior to deployment, a mission and safety briefing is presented on the stern of the ship by the Operations Officer. During this time, each boat coxswain generates and reports back to the operations officer their GAR score (safety rating) based on weather, crew skills and mission complexity (GAR stands for Green-Amber-Red … green means low risk, so go ahead, amber means medium risk, proceed with caution; red means high risk, stop what you’re doing). In addition, a mission briefing is discussed outlining the exact area in which data will be collected and identified goals.
The sonar equipment that transmits from the launch boats is called EM2040 multi beam sonar. A multi beam sonar is a device that transmits sound waves to determine the depth of the ocean. It is bolted to the hull that runs parallel to the boat, yet emits sound perpendicular to the orientation of the sonar. In the beginning of the season, hydrographers perform a patch test where they measure the offsets from the sonar to the boat’s GPS antenna, as well as calculating any angular misalignments in pitch, roll or yaw. These measurements are then entered in to software that automatically corrects for these offsets.
The first measurement to collect is the ocean’s conductivity, temperature and depth. From this information, the scientists can determine the depths in which the density of the water changes. This data is used to calculate and correct for the change in speed of sound in a given water column and thus provide clean data. The boats travel in pre-defined set lines within a defined polygon showing the identified corridor to be collected. Just like mowing a lawn, the boat will travel back and forth traveling along these lines. The pilot of the boat called the Coxswain, uses a computer aided mapping in which they can see these set lines in real time while the boat moves. This is an extremely valuable piece of information while driving the boat especially when the seas are rough.
The coxswain will navigate the boat to the position where data collection will begin inside a defined polygon. Since the multibeam echosounder transmits sound waves to travel through a deep column of water, the area covered by the beam is wide and takes longer to collect. In such stretches of water, the boat is crawling forward to get the desired amount of pings from the bottom needed to produce quality hydrographic data. The reverse is true when the boat is traveling in shallow water. The beam is very narrow, and the boat is able to move at a relatively fast pace. The boat is constantly rolling and pitching as it travels along the area.
As the boat is moving and collecting data, the hydrographer checks the course and quality of the data in real time. The depth and soundings comes back in different colors indicating depth. There is at least four different software programs all talking to one another at the same time. If at any point one component stops working, the boat is stopped and the problem is corrected. The technology driving this collection effort is truly state of the art and it all has to operate correctly, not an easy feat. Every day is different and provides different challenges making this line of work interesting. Troubleshooting problems and the ability to work as a team is crucial for mission success!
Personal Log
I have found the work on the Fairweather to be extremely interesting. The crew onboard has been exceptional in offering their insights and knowledge regarding everything from ship operations to their responsibilities. Today’s blog marks my first week aboard and everyday something new and different is occurring. I look forward in developing new lesson plans and activities for my elementary outreach program. Prior to arriving, I was expecting the weather to be mostly overcast and rainy most of the time. However, this has not been the case. Clear blue skies has prevailed most days; in fact I have seen more sun while on the Fairweather than back home in Hendersonville in the entire month of July! For my earth science students, can you make a hypothesis as to why clear skies has prevailed here? Hint, what are the five lifting mechanisms that generate instability in the atmosphere and which one(s) are dominant in this region of Alaska?
Question of the day. Can you calculate the relative humidity based on the dry and wet bulb readings above? Data table below…… Answer in the next blog
Until next time, happy sailing !
Tom
