NOAA Teacher at Sea
Aboard NOAA Ship Fairweather
August 12-23, 2019
Mission: Cascadia Mapping Project
Geographic Area of Cruise: Northwest Pacific
Weather Data from the Bridge
August 17th 2019
Latitude & Longitude: 43◦ 53.055’ N 124◦ 47.003’W
Windspeed: 13 knots
Geographic Area: @10-15 miles off of the Oregon/California coast
Cruise Speed: 12 knots
Sea Temperature 20◦Celsius
Air Temperature 68◦Fahrenheit
Navigation is how Fairweather knows its position and how the crew plans and follows a safe route. (Remember navigation from the last post?) But what “drives” where the ship goes is Hydrographic survey mission. There is a stunning amount of sea floor that remains unmapped, as well as seafloor that has not been mapped following a major geological event like an earthquake of underwater volcano.
Why is Hydrography important? As we talked about in the previous post, the data is used for nautical safety, creating detailed maps of the ocean floor, setting aside areas are likely abundant undersea wildlife as conservation areas, looking at the sea floor to determine if areas are good for wind turbine placement, and most importantly to the residents off the Pacific coast, locating fault lines — especially subduction zones which can generate the largest earthquakes and cause dangerous tsunamis.
In addition to generating the data needed to update nautical charts, hydrographic surveys support a variety of activities such as port and harbor maintenance (dredging), coastal engineering (beach erosion and replenishment studies), coastal zone management, and offshore resource development. Detailed depth information and seafloor characterization is also useful in determining fisheries habitat and understanding marine geologic processes.
The history of hydrographic surveys dates back to the days of Thomas Jefferson, who ordered a survey of our young nation’s coast. This began the practice and accompanying sciences of the coastal surveys. The practice of surveys birthed the science of Hydrography (which we are actively conducting now) and the accompanying science of Bathymetry (which we will go into on the next post.) This practice continues of providing nautical charts to the maritime community to ensure safe passage into American ports and safe marine travels along the 95,000 miles of U.S. Coastline.
Want to learn more about Hydrographic Survey history? Click on THIS LINK for the full history by the NOAA.
Scientists have tools or equipment that they use to successfully carry out their research. Let’s take a look at a few of the tools hydrographic survey techs use:
Want to learn more about the science of SONAR? Watch the video below.
On board Fairweather (actually underneath it) is the survey tool call a TRANSDUCER which sends out the sonar pulses.
The transducer on Fairweather is an EM 710- multibeam echo sounder which you can learn more about HERE.
The Transducer is located on the bottom of the ship and sends out 256 sonar beams at a time to the bottom of the ocean. The frequency of the 256 beams is determined by the depth from roughly 50 pings per second to 1 ping every 10 seconds. The active elements of the EM 710 transducers are based upon composite ceramics, a design which has several advantages, which include increased bandwidth and more precise measurements. The transducers are fully watertight units which should give many years of trouble-free operation. This comes in handy since the device in on the bottom of Fairweather’s hull!
Here is the transducer on one of the launches:
The 256 sonar beams are sent out by the transducer simultaneously to the ocean floor, and the rate of return is how the depth of the ocean floor is determined. The rate of pulses and width of the “swath” or sonar beam array is affected by the depth of the water. The deeper the water, the larger the “swath” or array of sonar beams because they travel a greater distance. The shallower the water, the “swath” or array of sonar beams becomes narrower due to lesser distance traveled by the sonar beams.
The minimum depth that this transducer can map the sea floor is less than 3 meters and the maximum depth is approximately 2000 meters (which is somewhat dependent upon array size). Across track coverage (swath width) is up to 5.5 times water depth, to a maximum of more than 2000 meters. This echo sounder is capable of reaching deeper depths because of the lower frequency array of beams.
The transmission beams from the EM 710 multibeam echo sonar are electronically stabilized for roll, pitch and yaw, while they receive beams are stabilized for movements. (The movement of the ship) What is roll, pitch, and yaw? See below – these are ways the Fairweather is constantly moving!
Since the sonar is sent through water, the variable of the water that the sonar beams are sent through must be taken into account in the data.
Some of the variables of salt water include: conductivity (or salinity) temperature, depth, and density.
Hydrographic scientists must use tools to measure these factors in sea water, other tools are built into the hydrographic survey computer programs.
One of the tools used by the hydrographic techs is the XBT or Expendable Bathy Thermograph that takes a measurement of temperature and depth. The salinity of the area being tested is retrieved from the World Ocean Atlas which is data base of world oceanographic data. All of this data is transmitted back to a laptop for the hydrographers. The XBT is an external device that is launched off of the ship to take immediate readings of the water.
Launching the XBT: There is a launcher which has electrodes on it, then you plug the XBT probe to the launcher and then XBT is launched into the ocean off of the back of the ship. The electrodes transmit data through the probe via the 750-meter copper wire. The information then passes through the copper wire, through the electrodes, along the black wire, straight to the computer where the data is collected. This data is then loaded onto a USB then taken and loaded into the Hydrographic data processing software. Then the data collected by the XBT is used to generate the sound speed profile, which is sent to the sonar to correct for the sound speed changes through the water column that the sonar pulses are sent through. The water column is all of the water between the surface and seafloor. Hydrographers must understand how the sound moves through the water columns which may have different densities that will bend the sound waves. By taking the casts, you are getting a cross section “view” of the water column on how sound waves will behave at different densities, the REFRACTION (or bending of the sound waves) effects the data.
See how the XBT is launched and data is collected below!
Videos coming soon!
The other tool is the MVP or moving vessel profiler which takes measurements of conductivity, temperature, and depth. These are all calculated to determine the density of the water. This is a constant fixture on the aft deck (the back of the ship) and is towed behind the Fairweather and constantly transmits data to determine the speed of sound through water. (Since sonar waves are sound waves.)
The sonar software uses this data to adjust the calculation of the depth, correcting for the speed of sound through water due to the changes in the density of the ocean. The final product? A detailed 3d model of the seafloor!
All of this data is run through the survey software. See screen shots below of all the screens the hydrographers utilize in the course of their work with explanations. (Thanks Sam!) It’s a lot of information to take in, but hydrographic survey techs get it done 24 hours a day while we are at sea. Amazing! See below:
Did You Know? An interesting fact about sonar: When the depth is deeper, a lower frequency of sonar is utilized. In shallower depths, a higher sonar frequency. (Up to 900 meters, then this rule changes.)
Question of the Day: Interested in becoming a hydrographic survey tech? See the job description HERE.
Challenge yourself — see if you can learn and apply the new terms and phrases below and add new terms from this blog or from your research to the list!
Roll, Pitch, and Yaw
Animals seen today:
Bathymetry and USGS friends coming soon!