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
Date: 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
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.

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.

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.

XBT

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.

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!

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

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).

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!