Tuesday, July 21, 2015

Meet the CTD!

Our cruise has 3 major sources of data collection: the Burkolator, the Bongo nets, and the CTD. You can find more information on the Burkolator in this post, and we’ll cover the Bongo nets later in the week. This post will be dedicated to the CTD, the workhorse of the ship. CTD stands for Conductivity, Temperature, and Depth, which are the three sensors that you’ll find on every CTD. In addition to those sensors, each CTD rosette (the metal frame which holds the CTD) gets customized with a variety of other sensors based on the purpose of the cruise. On our CTD rosette we have (in addition to the conductivity and temperature sensors), a dissolved oxygen sensor, a fluorometer, a backscatter sensor, an altimeter, a UVP, and a LISST. Also on the rosette are Niskin bottles to collect discrete water samples. We’re taking several different types of water samples, including carbon, chlorophyll, and nutrients from the Niskin bottles. All of the Niskin bottles are controlled by a single carousel, which gets connected to a central computer called the “Fish” along with all of the sensors. The “Fish” is connected to the deck unit by a sea cable that runs inside of the winch cable, so we can get the data in real time. At the bottom of this post are photos of all of the instruments describing what they do and how they work, and there are some short videos of how we deploy & retrieve it!

The short version is that the CTD gives us a physical profile of the water column and discrete water samples from the depths we program into it. We do a CTD cast at every station we visit, with the ultimate goal of getting a “snap shot” of the Gulf of Alaska at this time of year. We’ll be able to match the CTD profiles to the biological data collected by the Bongos (more on that later!). The CTD & Bongo nets are the lifeblood of the cruise—they cannot be deployed by drones or autonomous gliders, and they are the only way to collect the discrete chemical & biological samples we need to run the breadth of measurements that we need to understand the physical and biological systems of this area. 


Video 1- CTD Deployment (day). Morgan Ostendorf (PMEL/UW), Mark Bradley (NOAA OMAO) and Max Schoenfeld (UAF) deploy the CTD on the day shift. Taken by Kathryn Beaumont.

Video 2 –CTD Deployment (night). Julian Herndon (PMEL/UW) and Bill Potts (NOAA OMAO) deploy the CTD on the night shift. Taken by Kathryn Beaumont

Video 3 – CTD Retrieval. Morgan O. (PMEL/UW), Mark B. (NOAA OMAO) and Kathryn Beaumont (NMFS / UW) retrieve the CTD on the day shift. Taken by Kathryn Beaumont


Photo 1 – The CTD rosette being prepped for deployment by Mark B. (NOAA OMAO), Dan Naber (UAF) and Max S. (UAF). You can see that the Niskin bottles are cocked in this photo (the caps on the tops & bottoms of the bottles are pulled back. The bottles will be fired at certain depths (the caps will snap on) to collect discrete water samples. Photo by Kathryn Beaumont

The Niskin bottle carousel (top of the CTD) being prepped by Max S. (UAF). The caps are cocked by means of a nylon cord, which attaches to a firing pin on the carousel & keeps the caps open. The carousel is connected to the “Fish” computer, so the CTD operator can fire specific pins when the CTD reaches the correct depths. Generally the bottle samples are collected on the “upcast”, when the CTD is on its way back up to the surface. The “downcast” is reserved for getting a continuous physical profile of the water column, and then we stop at appropriate depths to collect water samples on the way up. Photo by Kathryn Beaumont 

Photo 3 – Whole CTD without cocked Niskin bottles. This is what the CTD looks like while we are in transit between stations. Photo by Kathryn Beaumont

Photo 4 – Carousel without cocked Niskin bottles. This is what the carousel looks like before the nylon cables get attached. Photo by Kathryn Beaumont

Photo 5 – Pump intakes, conductivity & DO probes. Our CTD has two different sets of pumps & conductivity, temperature and DO probes, a primary set and a secondary set (left and right respectively). The pump intakes are currently connected to syringes of DI water to keep the sensors from drying out and getting damaged. You can also see that the pumps and sensors are all connected to the top and bottom of the "Fish", the main brain of the CTD. Photo by Kathryn Beaumont

Photo 6 – Secondary pump, fluorimeter, and altimeter. The fluorimeter (black sensor) measures the amount of chlorophyll in the water as the CTD descends by reading certain wavelengths of light. The altimeter (silver sensor) pings the sea floor and tells us how close to the bottom the CTD is, which is very important so that we don’t crash the CTD into the sea floor! Photo by Kathryn Beaumont

Photo 7- Backscatter & primary pump. The backscatter (black sensor) reads the amount of reflected light as the CTD descends to get a measurement of turbidity.

Photo 8- The "Fish". This is the central computer which aggregates all of the sensor data and sends it back to the CTD operator via the sea cable, and also distributes the operator’s commands to the carousel and other instruments. Photo by Kathryn Beaumont

Photo 9 – Sea cable. The sea cable goes inside the winch cable to allow the CTD operator to control the instruments on the rosette as the CTD descends. Photo by Kathryn Beaumont

Photo 10 – UVP, which stands for Underwater Vision Profiler. This instrument takes greyscale photos of particles & organisms between 500 um and a few cm in diameter and also generates particle distribution for particles between 100um to ~3cm in diameter. Photo by Kathryn Beaumont

Photo 11 – LISST, which stands for Laser In Situ Scattering Transmission. This instrument uses laser backscattering to measure particle size distribution for sizes 2.5 um to 500 um.  Photo by Kathryn Beaumont

Photo 12 – Top down view of CTD. Photo by Kathryn Beaumont

No comments:

Post a Comment