NOAA Teacher at Sea
Aboard NOAA Ship Fairweather
June 5 – 26, 2017
Mission: Hydrographic Survey
Geographic Area of Cruise: Southeast Alaska – West Prince of Wales Island
Date: June 16, 2017
Wind: 3 knots from the east (272° true)
Visibility: 6 nautical miles
Barometer: 997.6 hPa
Air temperature: 9 °C
Cloud: 100% cover, 1000’
Science and Technology Log
It would be easy to assume that once the small boat surveys are conducted and data from the larger sonar equipment on Fairweather is also acquired, that the hydrographers’ work is done and the data can be used to create navigational charts. As I have learned, pretty quickly, there are many parameters that affect the raw data, and many checks and balances that need to be conducted before the data can be used to create a chart. There are also a significant amount of hurdles that the crew of Fairweather deals with in order to get to their end goal of having valid, accurate data. Some of the parameters that affect the data include tides, salinity of the water, temperature of the water, and the density of the data.
Tides play a huge role in data accuracy. But how do tides work and how do they influence navigational chart making? Tides on our planet are the effect on water due to forces exerted by the moon and the sun. The mass and the distance from the Earth to these celestial bodies play significant roles in tidal forces. While the sun has a much greater mass than the moon, the moon is much closer to the Earth and it is distance that plays a more critical role. Gravity is the major force responsible for creating tides. The gravitational pull of the moon moves the water towards the moon and creates a ‘bulge’. There is a corresponding bulge on the other side of the Earth at the same time from inertia, the counterbalance to gravity. The moon travels in an elliptical orbit around the planet and the Earth travels in an elliptical orbit around the sun. As a result, the positions of the moon to the Earth and the Earth to the sun change and as a result, tide height changes. The tides also work on a lunar day, the time it takes the moon to orbit the Earth, which is 24 hours and 50 minutes. So high tide is not at the same time in one area each solar day (Earth’s 24 hour day). There are three basic tidal patterns on our planet. Here is southeast Alaska, the tides generally are what is called ‘semi-diurnal’, meaning that there are two high tides a day and two low tides a day of about the same height. Other areas of the world may have ‘mixed semi-diurnal’ tides, where there are differences in height between the two high and two low tides, or ‘diurnal’ tides, meaning there is only one high and one low tide in a lunar day. The shape of shorelines, local wind and weather patterns and the distance of an area from the equator also affect the tide levels. How does this affect the hydrographers’ data? If data is being collected about water depth, obviously tide levels need to be factored in. Hydrographers factor this in when collecting the raw data, using predicted tide tables. However, later on they receive verified tide tables from NOAA and the new tables will be applied to the data.
Sound Speed Profiles:
Traveling down through the water column from the surface to the seafloor, several factors can change, sometimes significantly. These factors include temperature, pressure and salinity. These variables affect the accuracy of the sonar readings of the MBES (Multibeam Echo Sounders), so have to be factored in to account with the raw data analysis. What complicates matters further is that these factors can vary from location to location, and so one set of readings of salinity, for example, is not be valid for the whole dataset. Many fresh water streams end up in the waters off the islands of southeast Alaska. While this introduction of freshwater has effects on the community of organisms that live there, it also has impacts on the hydrographers’ data. To support accurate data collection the hydrographers conduct sound speed casts in each polygon they visit before they use the MBES. The data is downloaded on to computers on the boat and factored in to the data acquisition. The casts are also re-applied in post processing, typically on a nearest distance basis so that multiple casts in an area can be used. In the picture below, the CTD cast is the device that measures conductivity (for salinity), temperature and depth. It is suspended in the water for several minutes to calibrate and then lowered down through the water column to collect data. It is then retrieved and the data is downloaded in to the computers on board.
Hydrographers also need to make sure that they are collecting enough sonar data, something referred to as data density. There are minimum amounts of data that need to be collected per square meter, dependent on the depth of the sea floor in any given area. Having a minimum requirement of sonar data allows any submerged features to be identified and not missed. For example, at 0-20 meters, there need to be a minimum of five ‘pings’ per square meter. The deeper the sea floor, the more the beam will scatter and the ‘pings’ will be further apart, so the minimum of five pings occupy a greater surface area. Hydrographers need to make sure that the majority of their data meets the data density requirements.
After much of the initial raw data has been collected, and many of the polygons ‘filled in’, the hydrographers will also conduct crossline surveys. In these surveys they will drive the small boat at an angle across the tracklines of the original polygon surveys. The goal here is basically quality control. The new crossline data will be checked against the original MBES data to make sure that consistent results are be acquired. CTD casts have to be re-done for the crossline surveys and different boats may be used so that a different MBES is used, to again, assure quality control. At least 4% of the original data needs to be covered by these crossline surveys.
Low tides are taken advantage of by the hydrographers. If the research is being conducted in an area where the low tide times correlate with the small boat survey times, then a vessel mounted LIDAR system will be used to acquire measurements of the shoreline. Accurate height readings can be extracted from this data of different rocks that could prove hazardous to navigation. Notes are made about particular hazards and photos are taken of them. Data on man-made objects are also often acquired. Below are pictures produced by the laser technology, and the object in real life. (for more on LIDAT: http://oceanservice.noaa.gov/facts/lidar.html)
Each evening once the launches (the small boats) return, the data from that day has to be ‘cleaned’. This involves a hydrographer taking an initial look at the raw data and seeing if there were any places in the data acquisition that are erroneous. None of the data collected is deleted but places where the sonar did not register properly will become more apparent. This process is called night processing as it happens after the survey day. After night processing, the sheet managers will take a look at remaining areas that need to be surveyed and make a plan for the following day. By 6 a.m. the next day, the Chief Scientist will review the priorities made by the managers and let the HIC (Hydrographer In Charge) know what the plan in for their survey boat that day.
Throughout the Science and Technology log in this blog post, I keep referring to technology and computer programs. What stands out to me more and more each day is the role that technology plays in acquiring accurate data. It is an essential component of this project in so many ways, and is a constant challenge for all of the crew of Fairweather. Daily on Fairweather, at mealtimes, in the post survey meetings, or on the survey boats themselves, there is discussion about the technology. Many different programs are required to collect and verify the data and ‘hiccups’ (or headaches) with making this technology work seamlessly in this aquatic environment are a regular occurrence. I am in awe of the hydrographers’ abilities, not only in knowing how to use all the different programs, but also to problem solve significant issues that come up, seemingly on a regular basis. Staff turnover and annual updates in software and new equipment on the ship also factor significantly in to technology being constantly in the foreground. It often eats in to a large amount of an individual’s day as they figure out how to make programs work in less than forgiving circumstances. Tied to all of this is the fact that there is a colossal amount of data being collected, stored and analyzed each field season. This data needs to be ‘filed’ in ways that allow it to be found, and so the tremendous ‘filing system’ also needs to be learned and used by everyone.
Word of the day: Fathom
Fathom is a nautical unit of measurement, and is the equivalent of 6 feet. It is used in measuring depth.
Fact of the day:
Prince of Wales Island, west of which this research leg is being conducted is the fourth largest island in the United States. 4,000 people live on the island, that is 2,577sq mi.
What is this?
(Previous post: a zoomed in photo of ‘otter trash’ (Clam shell)
Acronym of the day:
LIDAR: Light Detecting and Ranging