NOAA Teacher at Sea
Aboard NOAA Ship Fairweather
August 28 – September 8, 2017
Mission: Hydrographic Survey leg IV
Geographic Area of Cruise: Alaska
Date: Wednesday, August 30, 2017
Location: Port Clarence: 65o14.034N 166o43.072W
Weather on the bridge:
30+ knot winds, 42o F, 4ft seas, heavy stratocumulus clouds (9/10 coverage)
Science & Technology Log
Over the past two days I have been introduced to tremendous amounts of the science of hydrography. In this blog post I will focus on the hardware used and the process of surveying. There are two types of sonar that are being employed. The first is side scan sonar and the second is multibeam sonar.
Side scan array sonar housed underneath one of the small launch vessels
Side scan is shorter range and performs better in shallower water. Side scan is used in conjunction with multibeam, however, as side scan does not give true depth values. The function of side scan is to show features evident on the ocean floor. For this reason, multibeam is run in conjunction with side scan in order to keep an accurate record of depths.
Multibeam sonar housed underneath another of the small launch vessels
Multibeam shows an exact depth. Due to the fact that it is an angular spreading band from the center of the underside of the launch, at shallow depths it will only show a very narrow strip of ocean floor.
Stop and imagine…a lit flashlight shining on a wall from only a few centimeters away. What happens to the image on the wall as you pull the flashlight back? The area of coverage of the image will become larger. The concept is similar for the multibeam in shallow versus deeper water.
Using multibeam in shallow water then would create a need for more passes closer together in order to cover an area. There are instances where using this technology even in shallow water would make sense, but for a full coverage survey, this would not be the case.
A CTD; it contains sensors for conductivity, temperature and density of the water column
The third piece of hardware used for the standard small boat launch hydrographic surveys is the CTD device. The CTD will measure conductivity of the water and also give both a temperature and density profile. The CTD is deployed multiple times during a survey as a tool to calibrate the data that is coming in via the sonar. Conductivity of the water gives an estimate of the total dissolved solids in the water. This information, along with the temperature and density will give an estimate of sound speed through the water column.
Stop and try this one for better understanding… knock on a door normally with your head roughly arm’s distance from the point where you are knocking. Now repeat the process of knocking, but with your ear pressed against the door approximately an arm’s length away from the knock. What is different? You should have noticed that a more precise (and typically louder) sound reached your ear. If you pay close attention, you will also notice that the sound reaches your ear more quickly. This is roughly analogous to how changes in the water column will affect sound speed.
The final piece of equipment used regularly for surveys is a HorCon (horizontal control) station. This is a land-based station that will help to define accurate position in the water. It allows for greater precision with global positioning data. The signals of satellites responsible for global position are affected daily by changing atmospheric conditions. Moreover, the precise positions of the satellites themselves are actually not well known in advance. This may result in a GPS location moving a few centimeters in one direction or another. While this is not going to heavily impact your ability to find a Starbucks in a strip mall, it can have a definite impact on the accuracy of charts for navigation. The HorCon station always remains in the same place on land, and can therefore be used to calibrate the measurements being read in the survey waters nearby and that information can be used along with corrected satellite positions since it is coming after the fact.
A nautical chart of the Port Clarence and Grantley Harbor area where we were surveying
Today we worked in Port Clarence, Alaska, both outside and inside of Grantley Harbor. Most of the depths being surveyed are in the 4-6 meter range. The particular area being surveyed had been previously surveyed in the 1950s by the US Coast and Geodetic Survey, likely using a single beam sonar system. The current survey is intended to note changes that have occurred since that prior survey and to accurately update all of the charts. The area of western Alaska is expected to increase in boat traffic over the coming years due to the opening of the Northwest Passage from the Pacific to the Atlantic via the Arctic. This route is significantly shorter for most shipping traffic than the route through the Panama Canal. Because of this expected increase in traffic, there is a need to identify areas for sheltering during heavy seas. Port Clarence is a natural inlet that offers some protection and holds potential for this purpose.
The process of surveying:
Two launches were deployed. I was on launch 2808, the second described here. The first was equipped with only multibeam sonar and the second had both multibeam and side scan. The plans for the two launches were different. The launch with only multibeam was working in an area of Grantley Harbor and covering an area that had previously been mapped to insure that the values were acceptably accurate. This focus existed primarily because of extra time available up in this area. The launch running the side scan was completing some unfinished work in Port Clarence and then did further work inside of Grantley Harbor. These areas, or “sheets” are described below. As a side note, small boat deployment is a fascinating and involved activity that I will discuss in a later blog.
Survey areas are broken up into sections known as “sheets” – each sheet has a manager. This person will be from either the NOAA Corps or a civilian member of the scientific survey team. The sheet manager will be responsible for setting up the plan for survey and doing all of the final checks after data has been gathered, cleaned and examined to determine if there are areas that should be rechecked or run again before it is completed and undergoes final processing.
A sheet manager will need to consider several questions prior to setting up the initial parameters for the survey. What is the depth being surveyed? What type of bottom is it? What type of coverage is needed? All of these factors will come into play when determining how the lines will be run – how long, how far apart, which sonar type, etc.
Once the plan is determined, it will be the job of the Operations Officer, LT Damian Manda, to parse out the duties and create a daily work plan to cover all of the areas. Each day, multiple launches will be sent out to gather data as described above. As the fieldwork finishes for the day, data will be transferred to a drive and then brought into the ship’s mapping room where night processers will begin the lengthy work of checking and cleaning the data so that it can all be ready for the final processing step prior to being sent to the client.
Senior surveyor Hannah Marshburn at the computer terminal in launch 2808
How good are those data?
There are several checks built into the data collection process. First, the survey team members on the launches are watching in real time. With three screens to work from, they are able to see what the sonars are seeing and can also set certain limits for the data that will alarm when something appears to be contrary to what’s expected. Night processors look for anomalies in the data like sudden inexplicable drops in depth in an otherwise flat surface or an extremely “noisy” area with little good data. Any area with a former survey will also be compared to the previous values with large differences signaling possible issues. Many trained eyes look at the data before it is accepted for charting and there will commonly be at least one return to an area to check and recheck prior to completion. One area in the current survey has continued to show odd results, so trained NOAA divers will dive the area to find out what is really going on.
So far this has been an amazing experience. I fully enjoy being among the crew of the Fairweather and living on the ship. It’s hard to say what my favorite part has been so far because I have honestly enjoyed all of it! Since we didn’t get underway until Monday, I had the opportunity on Sunday to roam around Nome with a couple of the other folks that are just here for two weeks, LT Joe Phillips and LCDR Ryan Toliver. I learned a lot more about both the NOAA Corps and the Public Health Service of which they are respectively a part. (These are two of the seven uniformed services – can you name the other five?) NOAA Corps officers are in command on all of the active NOAA commissioned ships and aircraft and you will learn a lot more about them in future posts. The PHS is an organization made up primarily of medical professionals. These folks serve in various medical and medical research positions around the nation. There are many who will work for the National Institutes of Health in research, or the Bureau of Prisons or commissioned vessels like Fairweather as practitioners. Unlike NOAA Corps, PHS is not on a billet cycle where every two to three years you will be moved to a new position in a different office or location. Similar to all of the other uniformed services, though, promotion through the ranks is both encouraged and desired.
As we walked all around Nome, this was one of the sights – the frame of a traditional fishing boat.
We also saw the marker for the end of the Iditarod race. I was able to see the historic beginning in Seward, Alaska back in 2010, so seeing the end in Nome was an unexpected treat. Nome also has Cold War-era missile early warning system arrays at the top of a mountain nearby. We had a chance to hike around them and see some of the interesting geologic features of the area. There’s so much more to talk about, but I think I’ll stop here and save shipboard life for my next post.
Did You Know…
… that the Iditarod has its historic beginnings with the Public Health Service? There were many children in interior and western Alaska dying of diphtheria in the early 1920s. When it reached epidemic proportions, the only doctor in Nome reached out to the PHS in the lower 48 to ask for help. Vials of serum were found and sent north to Seward, but then because of heavy ice and storming, dog sled teams were used to get the vials to the interior towns and to Nome. The original race along the Iditarod Trail was run as a memorial to the “Serum Run” and eventually evolved into the highly competitive race it is today.