Ocean Discoveries: 2012 Nancy Foster Mapping Mission/Day 8 – Seafloor Mapping with Multibeam SONAR

By Will Sautter, Acoustic Mapping Specialist, Center for Coastal Monitoring and AssessmentNational Centers for Coastal Ocean Science

This is Will Sautter, the acoustic mapping specialist for the Nancy Foster 2012 Seafloor Characterization mission of North East Puerto Rico. I am very excited to be back on board this year in the Caribbean with Chief Scientist, Tim Battista and the whole crew. This is my second year on this type of project. My primary goals are to assist in collecting the multibeam SONAR (SOund Navigation And Ranging) data and to process that data and create maps that highlight the different features on the seafloor, which will ultimately be made into maps that we will provide for researchers, fisheries managers, and the general public. It is a rewarding experience being able to collect the data on the ship’s SONARs and process it all the way in a few hours to an actual map, whereas back at the office, I get data from other oceanographers and NOAA ships that were collected years ago with missing information or outdated technology. With collecting SONAR data, what you see is what you get, well actually what you hear is what you get.  You’ll see what I mean.

NOS Biogeography survey technicians Gustav Kagesten (left) and Will Sautter (right) are collecting and monitoring the multibeam data of the seafloor of the new marine reserve in the dry lab of the Nancy Foster. Kagesten keeps a diligent eye on the raw soundings that are are detecting the bottom, while Sautter communicates with the officers on the bridge about selecting the next area for a sound velocity cast. Credit: NOAA/CCMA/Will Sautter

Like most remote-sensing technology, SONAR was pioneered by the US Navy during World War II. During the Battle of the Atlantic, the Navy relied on SONARs, albeit primitive ones compared to today’s technology, for protecting valuable supply ships from enemy submarines and sea mines. SONAR is used to map the seafloor by broadcasting sound from a sensor on the ship and then listening back for the echo to return off of the bottom. The frequency of the SONAR determines how deep it can penetrate into the ocean (higher frequency shorter distances and longer frequency further). This beats the old way in which a line with a chunk of lead at the end is dropped to the seafloor and measuring how much line went out. This lead line technique was used by mariners and to make nautical charts for hundreds of years. SONARs can accurately estimate how deep the water is by measuring how long and fast the sound took to hit the bottom and return to the sensor. This is the same basic principle that whales and bats use called echo location. But instead of using clicks or chirping noises, our sensors create pings at different frequencies that can very accurately detect the bottom up to 1000 meters and at high resolutions.

The biggest problem with sonar data is that the ocean is very noisy. Not that we hear fish chatting with each other, although it is possible to detect whale songs, but sound spreads in every direction hitting bubbles, different water densities, and the marine life as well. The noise has to be filtered using CARIS sea floor mapping software to produce a smooth sonar image. The random noise can create a false reading or an alias, which I have to tried to edit out by using noise filters, complex algorithms, and by manually going through all of the swaths and deleting the stray pings.

NF Survey lines of some patch reefs from a Reson 7125 multibeam echo sounder are being manually cleaned in CARIS “Subset Editer”. To the left is the multibeam bathymetry showing the depth, to the right is the editor window where survey technicians remove false surfaces, or aliases from the bathymetry. The editer window displays the multibeam imagery as individual pings that are colored by the individual survey line. Can you guess which features are real and which are false? Credit: NOAA/CCMA/Will Sautter

The final result is a smooth image of the bottom that can show depth changes, the steepness of the slopes, and a habitat model used to understand the dynamics of the marine environment. I will write more about all of the different kinds of SONARs that we are using for this mission and the different kinds of maps that we can create of the seafloor.

To see the the Nancy Foster throughout the 2012 mapping mission, visit the NOAA ship tracker site and click on “Enter NOAA’s Ship Tracker link, then scroll down to “NF – Nancy Foster” in the box on the upper right of the screen to see where she is at any given time!

Be sure to visit this blog often for field updates, pictures and videos posted by members of the science team.

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This entry was posted in Benthic Mapping, Biogeography Branch, Caribbean, Caribbean Research, Center for Coastal Monitoring and Assessment, Marine Regional Planning, Nancy Foster Exploration, National Centers for Coastal Ocean Science, NOAA's National Ocean Service, Ocean Exploration, Ocean Field Work, Ocean Research and tagged , , , , , , , , , , , , , . Bookmark the permalink.

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