CTD eh?

Time: 2032 UTC
Longitude: 59 05.1 W
Latitude: 66 19.5 N
Conditions: wicked good (5-10 kts wind, fog)
Upcoming Conditons: ughhhhhh (Monday - 40 kts wind)

The SSSG Technician manages the deployment of the CTD
aboard the R/V Atlantis (photo by Lorenza Raimondi)

What is a CTD, you say?  Right now, it seems like a self induced water torture device designed to make the victim experience piercing pain in each finger as you purposefully collect bone numbing water sample after bone numbing water sample, but I digress.  The acronym stands for Conductivity, Temperature and Depth; each parameter represented by a different sensor which collects these data and either stores them internally or transmits them through a cable back to the ship where they are viewed live and stored by operators.  OK, Coles Notes version (you can stop here if you want).

The name, "CTD", vastly understates the complexity of this bundle of oceanographic wizardry. It was originally branded the STD (Salinity, Temperature, Depth - Wow, poor acronym choice!) in 1960 but was later sold as the CTD in 1970 (Phew, that was close!  Find out more by reading Instrumentation and Metrology in Oceanography by Marc Le Menn). Since that time, the CTD has gained momentum, with new sensors occasionally being developed to add to the ever growing arsenal of this "Swiss Army Tool" of water sampling. As such, a world wide community of CTD users and data providers are established and require high levels of instrument precision and accuracy based on a generally agreed upon set of international standards.  Data generated by CTD's are sent by organizations the world over to central data repositories like the World Ocean Database (formerly, the National Oceanographic Data Center).  

Niskin bottles with tops open connected to the rosette carousel.
Photo by Lorenza Raimondi.

A modern CTD, like the one aboard the Atlantis, is truly amazing!  When combined, the bundle of probes which measure water column characteristics, is about the size of a large microwave oven.  The list of sensors on the CTD aboard the Atlantis is extensive and includes a primary and secondary set of temperature and conductivity (salinity) probes, oxygen, fluorometer, pH, Photosynthetically Active Radiation (PAR), pressure,(depth) and an altimeter. Whoa, that's a lot going on at the same time! Each instrument requires it's own form of baby sitting that could involve: soaking in fresh water between casts, bathing in a standard solution, covering a sensor head, etc... To further complicate the issue (or compliment probe measurements I suppose), this hyper-complicated mass of temperamental electronics is housed in a large cylindrical frame, upon which mounts 24 - 20L Niskin bottles.  These vertically positioned bottles employ an ingenious series of high tension bands that run through the center of the bottle to caps on either end.  Small but strong plastic wires at the top of the bottles radiate to ridged hooks on a central "carousel", holding the bottles open.  

A large electro-magnetic cable is "terminated" to the "CTD Rosette".  Many kilometers of cable (~6 km in our case), snakes its way through a "block" or wheel on the crane which protrudes from the starboard side of the ship.  The cable is wound tightly on a winch drum, which pays out the desired amount of cable required for the depth of deployment. The data from the CTD makes it's way up the cable during the cast and is visualized using software at the "Command and Control Center" in the Computer Room on the Atlantis. The bottles are fired at pre-determined depths, and each bottle contains enough water (hopefully) to meet the sampling demands of our scientists in the lab.

Note the large CTD cable attached to the central
part of the rosette.

Underway information provided by the ship.  Note the Time
Until Station used in the lab to determine when to prep
for deployment.

Alrighty then, here is our typical order of events.  You might want to grab a drink or get up and stretch a little before reading. Long before the cast, a spreadsheet of required sampling is produced along with the depths samples are required from. Depending on the area being sampled, different questions are being asked and require a unique subset of our analytical suite.  Each bottle on the rosette is then labelled with a unique sequential numerical identifier (a little sticker with numbers). All samples drawn from a bottle inherit this identifier for analysis.  As we transit towards our stations, we are watching a screen which provides us with an approximate arrival time.  We typically use this time to prepare our sample bottles and label them with stickers. Approximately 20 minutes from station, we make sure the rosette bottles are labelled with stickers, cock the bottles, remove sensor coverings and tubing and tie on tag lines to rings on the rosette frame. Once on station, the ship's SSSG (Shipboard Scientific Support Group) technicians correspond with the bridge to acquire approval for launching the gear over the starboard side of the ship.  The technician coordinates with the winch operator (housed in the next level up from the working deck) and the deck crew to deploy the CTD over the rail of the ship and into the water (science staff BTW - we don't sit at computers or stand in labs with coffee cups all day discussing the latest pocket protectors and episodes of Big Bang Theory.  OK, that happens sometimes, but who doesn't love a good pocket protector, right!) 

The Computer Room CTD Command Center aboard the R/V Atlantis

An operator in the Computer Room at the CTD control center (me) turns the CTD on via the "Deck Unit". Some time is required for pumps to engage that will move water through the CTD sensor package, and then the system is brought to the surface. The CTD acquisition software is turned on to log the data and the descent is initiated by a command to the winch operator to descend to 100 m at 30 m/min.  At 100 m, control of the winch is assumed by the SSSG technician in the Computer Room and descent to the final depth is initiated at 60 m/min until near the bottom where the speed of descent is reduced to minimize the likelihood of impact (and making lots of people very agitated).  The SSSG technician and the CTD computer operator can acquire the water depth from a sounder or on board multibeam system.  At 5 m off bottom, the CTD is stopped and the sensors are given time (~1 min) to come to equilibrium prior to firing the first bottle.  The SSSG technician then brings the CTD up to the next required depth and the process is repeated at predefined intervals until the CTD is at the surface.  At the surface, the science crew and SSSG tech prepare for receiving the CTD on the starboard side of the ship.  The CTD deck unit is shut off and the deck crew use long poles with hooks on the end to latch on to the CTD and use small tugger winches in coordination with the crane to bring the CTD back up over the rail and into position on the deck.  Once on deck, the CTD is ratcheted down and the science staff assume their positions to acquire samples from the rosette. OMG, I'm tired just thinking about it!

Water sampling order posted on white board in the lab for
a reminder when we start to lose our marbles.

The rosette sampling order is completed the same way each time based on the volatility of the parameter being investigated.  I'll have another post describing laboratory analysis, so I won't beat the point to death right now. Anyway, we all play "ring around the rosette" until samples are collected. This is the torture part I mentioned above.  The water is below zero at some depths and really makes you question your choice of career as it cascades over your increasingly useless and painful digits. Coupled with the wind and freezing temperatures on the open deck, it can be a less than pleasant experience.  Once the water has reduced your hands to frozen stumps, water sampling is complete. The water is then paraded back to the lab (just feet from the CTD for those of you on the Hudson) for sample preparation and/or storage.  The torture now enters it's second stage as feeling returns to your fingers. Joking aside, time at the rosette can be a pleasant experience.  Scientists emerge bleary eyed from the lab to take samples and usually return to the lab full of energy after getting some much needed arctic air and great conversation as we make our way around the rosette.  

Scenes of torture portrayed - parental guidance is recommended!  Steve rinses a flask in preparation for taking an oxygen sample.

The CTD is then cleaned and prepped for the next cast and the process repeats itself.  This is not meant to be an in depth review.  I've simplified a great deal and left out some information for the sake of brevity (OK, that did not work - sorry).  If you'd like to see a time lapse of the deck operations then click here for the YouTube video.  It's a messy business.  Water sampling at the end of the cast gets the lens wet, but you'll more than get the idea by then.

Thanks again for tuning in.  I'd like to have Steve Punshon go through the laboratory analysis for the next post, but he does not know this yet. Now you know how we get the water and monitor the environment, but the next question is, "Why Bother?".