NOAA Teacher at Sea
Aboard NOAA Ship Fairweather
April 16-27, 2018
Mission: Southeast Alaska Hydrographic Survey
Geographic Area of Cruise: Southeast Alaska
Date: April 20, 2018
Weather Data from the Bridge
Latitude: 47° 44.116′ N
Longitude: 122° 32.070′ W
Sea Wave Height: 1 foot or less
Wind Speed: 5-8 knots in the AM, then less than 5 knots in PM
Wind Direction: SSE, variable
Visibility: 16.1 km
Air Temperature: 8oC
Sky: Scattered Clouds
Science and Technology Log
For the past two days, NOAA Ship Fairweather has been anchored in Port Madison, part of Puget Sound off the coast of Seattle, Washington. The crew is currently stopped for a few days in Puget Sound before heading north to Alaska in order to complete the yearly Hydrographic Systems Readiness Review (HSRR). During HSRR, the survey techs test all of the hydrographic survey equipment that will be used during the field season. It’s essential to test and calibrate the equipment at the start of the season in order to ensure the data accuracy for upcoming projects.
The first part of HSRR began Thursday morning. Because NOAA Ship Fairweather spent winter at dock in Yaquina Bay, barnacles and algae were able to grow plentifully on the ship’s bottom, making it their home. The dive team deployed to check the Fairweather‘s hull and clean off the sonar transducers, removing any biofouling (sea life that had built up on the ship’s bottom) from the winter in port.
On Thursday afternoon and Friday, the next phase of HSRR began. On Friday, I was able to spend most of the day on the survey launches as a few of the survey techs conducted patch testing (a process for precisely determining an orientation of the launch’s sonar). NOAA Ship Fairweather has four 28-foot launches, and I spent the morning on 2808, and then the afternoon on 2806. When working on projects in relatively shallow waters, the Fairweather deploys these launches to collect data more efficiently as four launches can work on a project simultaneously.
The launches are driven by a coxswain, often a NOAA officer or deck hand, while a Hydrographer-in-Charge (HIC) plans track lines for the vessel to run. Sometimes, a coxswain-in-training or HIC-in training will also join the launch. As part of HSRR, the HIC chose a few track lines for the launch to run, and the coxswain, drove the launch back and forth on the lines at various speeds. While we ran the track lines, the HIC was able to gather data by sending an acoustic ping from the sonar which reflects off the seafloor and is then recorded when it returns to the sonar. The two-way travel time of the pin is measured, which (when coupled with the speed of sound through the water) can be used to calculate the water depth.
While in Port Madison, the crew will send all four of the Fairweather‘s launches out to run the same track lines and to ensure the data collected by each launch matches. At night, after the HIC’s have gathered data, the survey techs spend hours in the plot room, looking at the day’s data and checking for any discrepancies. The survey techs correct any errors in the data and the saved changes are sent back to each launch’s computing system. This is known as calibrating. By running patch tests and calibrating the launches to one another, survey techs are able to guarantee that data collected throughout the season is precise, no matter which launch is used for a given area.
Data Being Collected from the CTD on the Launch Monitor: Conductivity (Salinity), Temperature, and Depth (Pressure)
Before and after running the patch tests, the crew deploys a CTD The CTD measures the conductivity, temperature, and depth of the water. The survey techs are interested in the CTD readings because this information helps them assess the speed of sound (or the sonar waves) in a given body of water. In turn, knowing the speed of sound and the amount of time the CTD takes to reach the ocean floor, allows survey techs to calculate ocean depths. (The classic distance equation, d=rt!)
Conductivity refers to the ability of the given water sample to pass an electrical current. Survey techs are interested in the conductivity, because the conductivity is another way to gauge the salinity (or “saltiness” of the water). The more salt in a sample of ocean water, the greater the ocean water’s conductivity and the faster the sound waves travel. Next is temperature. Water closer to the surface is warmer, and thus, sound will travel faster closer to the surface. Conversely, the cooler the temperature, the slower the sound waves travel. The final measurement is depth, or pressure. The deeper the water, the greater the pressure. Greater depths increase the speed of the sonar waves. The average speed of sound in the water is 1,500 m/s. By comparison, the average speed of sound in air is about 340m/s.
After dinner, survey techs are assigned to night data processing. I joined one of the survey techs, Ali, who was kind enough to explain how the launch data is analyzed. One interesting note is the red light in the plot room. The red light is used because the plot room is next to the bridge, where the officers and deck crew keep watch. The red lights help the crew keep their eyes ready for night watch, so those processing data also work under red lights.
In the above photograph, notice the various colors representing the differing ocean depths. In this case, red is shallower and purple is deeper. Notice that as the survey tech, hovers over a datapoint with her mouse, the data collected by Fairweather launch 2807 is shown as a coordinate with a depth of 168.3 meters. Creating a color “painting” of the data points is helpful because the changing colors help the survey techs understand the slope of the ocean floor; closer together colors mean a steeper slope or a sharp increase in depth, whereas larger swatches of the same color mean a flatter seafloor.
The green lines in the picture represent the “lines” that the launch ran, meaning the area where the coxswain drove back and forth in the boat at varying speeds. Notice that there are two lines as the launches always run two lines to ensure accuracy. As the launch is driven back and forth in the water, the transducers on the bottom of the launch emits multi-beam sonar, and sound waves ping off the ocean floor several times per second, sending sound waves back to the launch which are translated into millions of data points by the survey techs.
The survey techs use various computer programs and imaging software to analyze the data. Above, the survey techs can look at a 3D cross-section of the data, which essentially looks like a virtual map of the sea floor. In the bottom right corner, the survey tech compares two lines for accuracy, one with data points colored red, the other green. When the lines line up exactly, precision is ensured. The survey techs analyze the data to make sure the rocking of the boat in any direction (front/back, side-to-side, etc.) won’t interfere with mapping accuracy later in the season. Finally, survey techs compare their work with each other to ensure precise calibration.
One of my favorite things about being onboard NOAA Ship Fairweather are the tremendous views every time I look outside. Sunrises and sunsets are spectacular. We’ve had some really great weather over the last few days, and though it has been a bit chilly, the skies have been fairly clear.
Did You Know?
On nautical charts (or maps), units of measurement vary. Ocean depths can be marked in feet, meters, or fathoms. Fathoms, like knots, is another term steeped in nautical history. When sailors used to measure ocean depths by hanging rope over the side of a vessel, they would pull in the line, looping the rope from hand to hand. The distance of the rope from one outstretched hand to another (a sailor’s wingspan) became known as a fathom.
Challenge #2 – Devotion 7th Graders: Measure your wingspan, the distance from one outstretched hand to another. Then measure four other friends, classmates, or family members’ wingspans. What is the median wingspan for you and your friends? What is the mean wingspan for you and your friends? What is the mean absolute deviation for your collective wingspans? One fathom is equal to 1.8288 meters or 6 feet. If one fathom is the average sailor’s wingspan, how do your wingspans compare? Present your findings on a 8.5x11inch paper as a mini-poster. Include illustrations and calculations.