Laura Guertin: Pre- and post-hydrographic surveys on the Thomas Jefferson, September 12, 2014

NOAA Teacher at Sea
Laura Guertin
Onboard NOAA Ship Thomas Jefferson
September 2 – September 19, 2014

Mission: Hydrographic Survey
Geographical area of cruise: North Atlantic Ocean
Date: September 12, 2014
Location of ship: 41o 21.217′ N, 72o 05.508′ W (docked at City Pier in New Haven, CT)

That's me, getting ready to handle the bow line for the HSL 3101 launch
That’s me, getting ready to handle the bow line for the HSL 3101 deployment. (photo taken by R. Bayliss)

This post will summarize some of what happens before hydrographic research vessels such as the Thomas Jefferson head out to collect data; a little more information and some history on the tools utilized to collect the data; and then where the data are used once the ship has accomplished its mission.


Science and Technology Log

You may recall in my third post that there are three questions the NOAA’s Office of Coast Survey asks and answers several years in advance to prioritize survey plans:

  • Is it considered a critical area? If so, how old are the most current survey data?
  • Have local pilots or port authorities submitted reports of shoaling, obstructions or other concerns?
  • Does the U.S. Coast Guard or other stakeholders from the maritime community (e.g., fisheries, energy, pipelines) need surveys for economic development or ecological protection?

Once the NOAA Coast Survey tells the ships in their hydrographic fleet where to survey, an initial chart is created to break down the region into pieces (termed polygons) for mapping.

Boat sheet for Long Island Sound survey (provided by T. Walsh)
Boat sheet for Long Island Sound survey (provided by T. Walsh)

Once the region is set and defined, it is now time to get the equipment ready to generate an image and/or record the depth of the ocean floor. The technology for collecting this data has certainly come a long way over time!  The image below shows the “technologies” over time. You may also want to review the History of Hydrographic Surveying and Using Lead Lines to Collect Hydrographic Data.  Remember that you can go back and visit NOAA’s site to review What is sonar? and the different hydrographic survey equipment NOAA uses, specifically side scan sonar and the multibeam echo sounder.  Remember that side scan sonar is good for getting an overview of features on the seafloor, while multibeam data are needed to obtain an absolute depth measurement at a location.

Over 50 percent of the depth information found on NOAA charts is based on hydrographic surveys conducted before 1940. Surveys conducted with lead lines or single-beam echo sounders sampled a small percentage of the ocean bottom. Due to technological constraints, hydrographers were unable to see between the sounding lines. Depending on the water depth, these lines may have been spaced at 50, 100, 200 or 400 meters. Today, as NOAA and its contractors re-survey areas and obtain full-bottom coverage, uncharted features (some that are dangers to navigation) are routinely discovered. These features were either: 1) not detected on prior surveys, 2) manmade objects, like wrecks and obstructions, that have appeared on the ocean bottom since the prior survey or 3) the result of natural changes that have occurred since the prior survey.
Over 50 percent of the depth information found on NOAA charts is based on hydrographic surveys conducted before 1940. Surveys conducted with lead lines or single-beam echo sounders sampled a small percentage of the ocean bottom. Due to technological constraints, hydrographers were unable to see between the sounding lines. Depending on the water depth, these lines may have been spaced at 50, 100, 200 or 400 meters. Today, as NOAA and its contractors re-survey areas and obtain full-bottom coverage, uncharted features (some that are dangers to navigation) are routinely discovered. These features were either: 1) not detected on prior surveys, 2) manmade objects, like wrecks and obstructions, that have appeared on the ocean bottom since the prior survey or 3) the result of natural changes that have occurred since the prior survey. (Text for this caption from NOAA Hydrographic Survey Techniques webpage)

Here is a photo of the side scan sonar device from the Thomas Jefferson launch HSL 3101.

Side scan sonar recording device being removed from the HSL 3101, as the launch was going to be surveying in shallow/rocky waters that could damage the instrument.
Side scan sonar recording device being removed from the HSL 3101, as the launch was going to be surveying in shallow/rocky waters that could damage the instrument.

Here is a photo from underside of the Thomas Jefferson of the dual-frequency projector to capture multibeam data.

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If we go back to the map above that shows the regions to be charted, NOAA’s hydrographic crew will first run some multibeam lines to get a general overview of what to expect in terms of depth variations across the survey area.

Boat sheet with initial lines of multibeam data (provided by T. Walsh)
Boat sheet with initial lines of multibeam data (provided by T. Walsh)

Finally, the multibeam data are collected to produce a detailed map (red is for shallow depths, purple is for the deepest depths).

Initial multibeam data for a region, collected by one of the launches of the Thomas Jefferson (provided by T. Walsh)
Initial multibeam data for a region, collected by one of the launches of the Thomas Jefferson (provided by T. Walsh)

But collecting the side scan and multibeam data is just one half of the story – the other half includes knowing where you are when you collect the data.  Please listen to this important audio file from NOAA’s Diving Deeper podcast series, titled Accurate Positions: Know Your Location (from August 2012, 14:01 minutes, transcript).  If the audio player does not appear for you below, click here.


Personal Log

So we have the data collected on the water so we can add the water depths to the nautical charts.  And we have the locations where we collected that data.  But we still have a missing piece…  I have added the next part of this story to my Personal Log, as this information I can provide from my prior experiences during two summer internships while I was an undergraduate student.  The coast itself must be mapped with land surveys, aerial photographs, and remote sensing (see What is remote sensing?).  In addition to the shoreline, NOAA’s cartographers must plot any manmade structures such as docks and jetties that would be an obstruction to navigation, and any objects along the shoreline that would be visible to boaters such as radio and water towers.


Back to the Science and Technology Log

Finally, we have all the pieces to our puzzle, now it is time to put together the nautical chart!  I know I have been throwing around the term “nautical chart,” but let’s make sure you have this in your vocabulary. Please listen to this audio file from NOAA’s podcast series Diving Deeper, titled What is a Nautical Chart? (from March 2009, 15:04 minutes, transcript).  If the audio player does not appear for you below, click here.

Wondering how long it takes to create a nautical chart?  View NOAA’s page on The time needed to make a new nautical chart depends on how many pieces of the puzzle are in the box.


OK GEOSC 040 students at Penn State Brandywine, here is your next round of questions.  Please answer these questions online in ANGEL in the folder “Dr. G at Sea” in the link for Post #7.

  1. Why might hydrographers use side scan sonar rather than multibeam echo sounding?  Give two examples.
  2. For oceanographers, especially for a hydrographic survey, why is it important to get accurate positions while collecting survey data?
  3. How and why are nautical charts updated?

Random Ship Fact!

The NOAA Ship Thomas Jefferson started its life as the US Naval Ship Littlehales.  From January 1992 to January 2003, the Littlehales recorded 85,018 hydrographic survey miles along the coast of Africa and in the Red Sea and Mediterranean Sea.  The Littlehales even assisted local authorities in halting a piracy incident against another ship at a West African port in 2001 (see article).  At the end of her Navy career, the number of survey operations personnel reached 660.  The Littlehales ended its time with the Navy but then became the Thomas Jefferson and officially entered the NOAA fleet on July 8, 2003 (see article).  It is pretty amazing to be on a ship that has traveled and contributed so much to ocean navigation and safety.

US Navy Ship LIttlehales
US Navy Ship Littlehales. (image from Navsource)
NOAA Ship Thomas Jefferson (image from NOAA)
NOAA Ship Thomas Jefferson (image from NOAA)

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