Amanda Peretich: CTD and XBT – More Acronyms? July 8, 2012

NOAA Teacher at Sea
Amanda Peretich
Aboard Oscar Dyson
June 30 – July 18, 2012

Mission: Pollock Survey
Geographical area of cruise:
Bering Sea
July 8, 2012

Location Data
Latitude: 57ºN
Longitude: 172ºW
Ship speed: 11.2 knots (12.9 mph)

Weather Data from the Bridge
Air temperature: 6ºC (42.8ºF)
Surface water temperature: 7ºC (44.6ºF)
Wind speed: 2.5 knots (2.9 mph)
Wind direction: 156ºT
Barometric pressure: 1020 millibar (1.0 atm, 765 mmHg)

Science and Technology Log
Today’s post is going to be about two of the water profiling devices used on board the Oscar Dyson: the CTD and XBT.

CTD stands for Conductivity, Temperature, and Depth. It’s actually a device that is “dropped” over the starboard side of the ship at various points along the transect lines to take measurements of conductivity and temperature at various depths in the ocean. On this leg of the pollock survey, we will complete about 25-30 CTD drops by the end. The data can also be used to calculate salinity. Water samples are collected to measure dissolved oxygen (these samples are analyzed all together at the end of the cruise). Determining the amount of oxygen available in the water column can help provide information about not only the fish but also other phytoplankton and more. Although we are not doing it on this leg, fluorescence can also be measured to monitor chlorophyll levels.

From left to right: getting the CTD ready to deploy, the winch is used to put the CTD into the water, the CTD is lowered into the water – notice that the people are strapped in to the ship so they don’t fall overboard during deployment

DYK? (Did You Know?): What exactly are transect lines? Basically this is the path the ship is taking so they know what areas the ship has covered. Using NOAA’s Shiptracker, you can see in the photo where the Oscar Dyson has traveled on this pollock survey (both Leg 1 and Leg 2) up to this point in time.

Transect Lines
Using NOAA’s Shiptracker, you can see the transect lines that the Oscar Dyson has followed during the pollock cruise until July 8. The ship started in Dutch Harbor (DH), traveled to the point marked “Leg 1 start” and along the transect lines until “Leg 1 end” before returning to DH to exchange people. The ship then returned to the point marked “Leg 2 start” and followed transect lines to the current location. The Oscar Dyson will return to DH to exchange people before beginning Leg 3 of this survey and completing the transect lines.
Deploying the CTD
I was lucky enough to be able to operate the winch during a CTD deploy. The winch is basically what pulls in or lets out the cable attached to the CTD to raise and lower it in the water. Special thanks to the chief boatswain Willie for letting me do this!

The CTD can only be deployed when the ship is not moving, so if weather is nice, we should just stay mostly in one place. The officers on the bridge can also manually hold the ship steady. Or they can use DP, which is dynamic positioning. This computer system controls the rudder and propeller on the stern and the bowthruster at the front to maintain position.

Here is a video from a previous Teacher at Sea (TAS) about the CTD and showing its “drop” into the water: Story Miller – 2010. Another TAS also has a video on her blog showing the data being collected during a CTD drop: Kathleen Harrison – 2011.


The thermocline is the area where the upper isothermal (mixed) layer meets the deep water layer and there is a decline in temperature with increasing depth.

XBT is the acronym for the eXpendable Bathymetric Thermograph. It is used to quickly collect temperature data from the surface to the sea floor. A graph of depth (in meters) versus temperature (in ºC) is used to find the thermocline and determine the temperature on the sea floor.

DYK? Normally, temperature decreases as you go farther down in the sea because colder water is denser than warmer water so it sinks below. But this is not the case in polar regions such as the Bering Sea. Just below the surface is an isothermal layer caused by wind mixing and convective overturning where the temperature is approximately the same as on the surface. Below this layer is the thermocline where the temperature then rapidly decreases.

The MK-21IISA is a bathythermograph data acquisition system. This is a portable (moveable) system used to collect data including ocean temperature, conductivity, and sound velocity and various depths using expendable probes (ones you can lose overboard and not get back) that are launched from surface ships. The depth is determined using elapsed time from surface contact and a known sink rate.

There are three different probes that can be used with this data acquisition system:
1. XBT probe – this is the probe that is used on OD, which only measures water temperature at various depths
2. XSV probe – this probe can measure sound velocity versus depth
3. XCTD probe – this probe measures both temperature and conductivity versus depth

On the XBT probe, there is a thermistor (something used to measure temperature) that is connected to an insulated wire wound on two spools (one inside the probe and one outside the probe but inside the canister). The front, or nose, of the probe is a seawater electrode that is used to sense when the probe enters the water to begin data collection. There are different types of XBT probes depending on the maximum depth and vessel speed of the ship.

XBT Canister and Probe
This shows a sideview (left) and topview (middle) of the canister that houses the probe (right) released into the water during an XBT.

There are really four steps to launch the XBT probe using the LM-3A handheld launcher on board:
1. Raise contact lever.
2. Lay probe-containing canister into cradle (make sure to hold it upwards so the probe doesn’t fall out of the canister!).
3. Swing contact level down to lock in canister.
4. Pull release pin out of canister, aim into ocean, and drop probe.
Important: the wire should not come in contact with the ship!

Launching an XBT
“Launching” an XBT probe from starboard side on the Oscar Dyson. There is no actual trigger – you just make a little forward motion with the launcher to allow the probe to drop into the water.

Be sure to check out the video below, which shows what the data profile looks like as the probe is being dropped into the water. An XBT drop requires a minimum of two people, one at the computer inside and one outside launching the probe. I’ve been working with Scientist Bill and ENS Kevin to help out with the XBT launches, which also includes using the radios on board to mark the ship’s position when the probe hits the water.

Personal Log

Quickest Route?
We’ve been taught in school that the quickest way from point A to point B is a straight line, so you’d think that the red voyage would be the fastest way to get from Seattle, Washington across the Pacific Ocean to Japan. But it’s actually a path up through Alaska!

It’s been a little slow on the trawling during my shift recently, so I’ve had some extra time to wander around the ship and talk to various people amidst researching and writing more blog posts. I think one of my favorite parts so far has been all of the great information I’ve been learning up on the bridge from the field operations officer, LT Matt Davis.

DYK? When looking at the map, you’d think the quickest route from Seattle, Washington to Japan would be a straight line across the Pacific Ocean. But it’s not! Actually, ships will travel by way of Alaska and it is a shorter distance (and thus faster).

View from the Bow
View from the bow of the Oscar Dyson.

Vessels  use gnomonic ocean tracking charts to determine the shortest path. Basically a straight line drawn on the gnomonic projection corresponds to a great circle, or geodesic curve, that shows the minimum path from any two points on the surface of the Earth as a straight line. So on the way to Japan from Seattle, you would travel up through Alaskan waters, using computer software to help determine the proper pathway.

I’ve also had some time to explore a few other areas of the ship I hadn’t been to before. I’ve learned some new lingo (look for this in an upcoming post) and plenty of random facts. One of the places I checked out is the true bow of the ship where, if I was standing a bit higher (and wearing a PFD, or personal flotation device), I’d look like I was Rose Dawson in one of the scenes from Titanic.

Animal Love
All of the time I spend on the bridge also allows for those random mammal sightings and I was able to see a few whales from afar on July 7!

Whale Sighting
Whale sighting from the bridge! You have to look really closely to see their blow spouts in the middle of the photo.

Thomas Ward, September 15, 2010

NOAA Teacher At Sea: Thomas Ward
Aboard NOAA Ship Miller Freeman

Mission: Fisheries Surveys
Geographical Area of Cruise: Eastern Bering Sea
Date: September 15, 2010

At Sea

King Crab

The science is going forward with rigor here on the Miller Freeman.  If you get a chance you should go back to this link   so that you can see the area that we have covered. I also made an error in reporting that the seas that made me sick were 9 foot seas when they were actually 12 foot seas.  The forecast calls for flat seas, 2 feet, through Friday. I have received a few questions through the blog and I will try to address them here.

The first one is about the marine mammals that we have encountered while out at sea.  On board with us is a bird observer and his secondary function is to identify and count any marine mammals.  He reported to me the following list; Killer Whale, Humpback Whale, Harbor Porpoise, Dall’s Porpoise, Fin Whale, Minke Whale, Northern Fur Seal and Steller Sea Lion.  I was lucky enough to see the Humpbacks and even saw one breech, jump out of the water and land on its side. An interesting fact about the fur seal is that they will stay at sea for up to 8 months and only come to land to breed.

Another question that I received is regarding a picture that I have posted on my blog.  It was a picture of a volcanic mountain, Mount Shishaldin.   A description of this volcano is sufficient in understanding the characteristics of it but its majesty is truly appreciated viewing it in person.

Someone asked if the jellyfish could be petted?  We do handle them with gloves on.  They are not significant in our study at all.  We pull them out of our catch and throw them overboard.  They are relatively difficult to pick up and their tentacles are very stringy.  They are surprisingly heavy and of course jelly like.  While we have gear down and we are moving very slowly, 1-3 knots, at certain locations you can look down and see them swim by, pretty cool. E

We have been blessed here with good weather.  The website for the agency that operated my program can be found by going to this link  If you were to look around this site you may notice a function of NOAA is to forecast the weather.  I believe it is one of the most important factors in people’s lives.  When you have a dependable agency predicting weather people can make better plans for what they may want to do.  The site that I personally frequent is with in this link

To find Central New York’s radar, which shows precipitation, click on the link and mouse over Central New York and click.  The Montague radar should come up.  Montague New York, the town that received 8 feet of snow in one storm a few years ago.  It is no surprise though seeing that it is in the Tug Hill Plateau and orographic lifting happens to air masses coming off Lake Ontario here.  We call it lake effect snow. When on this site in the upper left corner is a grid with adjacent radars.  Most weather moves across our country with the southwest prevailing winds.  So if you click on the grid to the left, Buffalo radar for example you can see what is coming your way.

Kimberly Lewis, July 1, 2010

NOAA Teacher at Sea Kimberly Lewis
NOAA Ship: Oregon II
July 1 -July  16 2010

Mission: SEAMAP Summer Groundfish Survey
Geographical Area of Cruise: Gulf of Mexico
Date: Sunday, July 1, 2010

July 1, 2010 in port

Me in front of the Oregon II
Me in front of the Oregon II

Hey fans (LOL),

My official day 1 aboard the Oregon II is nice and relaxing, but that will soon change. Due to Hurricane Alex creating large waves out in the sea (21 feet yesterday and about 11 feet today) our captain has delayed our departure for July 2, 2010. That is fine with me as I have never been aboard this size of vessel for extended period of time, so large waves while sleeping can wait. We should have waves slowly declining once we depart.

Everyone on board is extremely nice, from the scientist, to crew, to officers. Bruce (TAS from NJ) and I have been exploring the ship to get our ‘bearings’. We have seen the wetlab, from where we understand will be our main location for the next 17 days, to the dry lab, chem lab, the lounge, the various heads, galley, and misc. workrooms. The captain showed us the Oregon II’s newest toy, which I agree is very cool. He can watch the weather, click on buoys in the gulf with weather and sea data, and many other options.

Although Bruce and I are relaxing today, the crew is busy preparing for our voyage.

You can see our current location by clicking here – Oregon II location.