Kevin McMahon: Midnight Mapping! July 13, 2014


NOAA Teacher at Sea

Kevin McMahon

Aboard the NOAA ship Pisces

July 5 – July 18, 2014

 

Mission: Southeast Fisheries Independent Survey

Geographic area of the cruise: Atlantic Ocean, off the coast of North Carolina and South Carolina

Date: July 13, 2014

Weather Information from the Bridge

Air Temperature:            27.6 °C

Relative Humidity:         73%

Wind Speed:                  5.04 knots

 

Science and Technology Log

Someone is always working on the Pisces. When Nate Bacheler and the other fishery scientists have finished their work for the day collecting fish, it is show time for the hydrographers, the scientists who map and study the ocean floor. Their job is to map the ocean floor to help Nate find the best places to find fish for the next day.  Warren, Laura, David and Matt were kind enough to let me join them and explained how they map the ocean floor while on board the Pisces.

People have learned over the years that some fish like to hang out where there is a hard bottom, not a sandy bottom. These hard bottom areas are where coral and sponges can grow and it also happens to be where we usually find the most fish.

Instead of using a camera to find these hard bottom habitats, the mapping scientists use multibeam sonar. Here is a simple explanation on how sonar works. The ship sends a sound wave to the bottom of the ocean. When the sound wave hits the bottom, the sound bounces back up to the ship.

Since scientists know how fast sound travels in water, they can figure out how far it is to the ocean floor. If the sound wave bounces back quickly, we are close to the ocean floor. If the sound wave takes longer, the ocean floor is farther away. They can use this data to make a map of what the ocean floor looks like beneath the ship.

The neat thing about the Pisces is that it does not send down one sound wave only. It sends 70 waves at once. This is called multibeam sonar.

Single Beam versus Multibeam sonar.  Can you see why scientists like to use multibeam sonar?

Single Beam versus Multibeam sonar. Can you see why hydrographers like to use multibeam sonar? Credit: NOAA

So, now you know how sonar works in simple terms.

But it gets a little more complicated. Did you know that sound speed can be affected by the water temperature, by how salty the water is (the “salinity”), by tides, and by the motion of the ship?  Computers make corrections for all of these factors to help get a better picture of the ocean floor. But, computers don’t know the physical properties of our part of the ocean (because these properties change all the time) so we need to find this information and give it to the computer.

To find the temperature of the ocean water, the mapping scientists launch an “XBT” into the water.  XBT stands for “expendable bathythermograph.”  The XBT records the changes in water temperature as it travels to the ocean floor.  It looks like a missile.  It gets put into a launcher and it has a firing pin. It sounds pretty dangerous, doesn’t it!  I was excited to be able to fire it into the water.  But, when I pulled out the firing pin, the XBT just gently slid out of the launcher, softly plopped into the ocean, and quietly collected data all the way to the ocean floor.

 

Kevin McMahon nervously holding the XBT Launcher and waiting for the order to fire.

Kevin McMahon nervously holding the XBT Launcher and waiting for the order to fire.

                 

Kevin McMahon watches as the XBT gently plops out of the launcher.

Kevin McMahon watches as the XBT gently plops out of the launcher.

 

With the new data on water temperature, the hydrographers were able to create this map of the ocean floor.

Example of an Ocean Floor Map

Example of an Ocean Floor Map

 

In the map above, blue indicates that part of the ocean floor that is the deepest. The green color indicates the part of the map that is the next deepest. The red indicates the area that is most shallow.

Nate talks to the hydrographers early in the morning and then predicts where the hard bottom habitats might be. In particular, Nate looks for areas that have a sudden change in elevation, indicating a ledge feature.  If you had Nate’s job, where would you drop the 6 traps to find the most fish?  Look at the map below to see where Nate decided to deploy the traps.

 

The green dots are the spots where Nate dropped the traps in hopes of finding fish.

The green dots are the spots where Nate dropped the traps in hopes of finding fish.

 

To find out more about using sound to see the ocean floor and to see an animation of how this works, click on this link:

 NOAA: Seeing the Ocean Floor

               

Personal Log

 

We have now gotten into a regular routine on the ship.   The best part of the day for me is when we are retrieving the traps. We never know what we will see. Sometimes we catch nothing. Sometimes we find some really amazing things.

 

Here are a few of my favorites:

 

Closer view of sharksucker on my arm

Closer view of sharksucker on my arm

 

Somebody is crabby.

Somebody is crabby.

 

Sea stars with beautiful navy blue colors

Sea stars with beautiful navy blue colors

 

A pair of butterflyfish

A pair of butterflyfish

 

Did you know?

The ocean is largely unexplored.  Maybe someday you will discover something new about the ocean!

 

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