18 February, 2000
Casting a CTD
It is three in the morning on the Research Vessel Nathaniel B. Palmer, and I'm standing "geophysics watch," which means monitoring the automatic data collection equipment to make sure that it continues running as it should. It's not a very tough job. Every 15 minutes I have to jot down readings on a clipboard. If anything goes wrong, I have to find or wake a technician to fix it, and I have to keep myself awake (that's probably the hardest part!). The readings include our position (78 degrees 10 minutes south, 168 degrees 4 minutes west), and such things as the sea surface temperature (-1.1 C), the salinity (33.9 p.p.t.), and the ships course and speed (84 degrees, 11.2 knots.) Research vessels are very expensive to operate, and to get the most value for the money work goes on 24 hours per day. So, even though it is the middle of the night, a group of scientists and crew are getting ready to do what is called a "CTD station." It's a complicated process which takes several people an hour (or more if the depth exceeds 1000 meters), but the result is simply a graph of salinity and temperature versus depth. The term "CTD", short for "conductivity, temperature and depth," is used for the graph, and the device which is lowered down into the ocean to give you that graph. Outside the ship the sky is brightening from its darkest at
midnight. The sun hasn't really set yet here, it just goes around the horizon and gets a little bit lower about midnight. Since we are within a half mile or so of the high edge of the Ross Ice shelf, the sun does disappear for an hour or so, behind the ice edge. But tonight it is very foggy, with the outline of the shelf barely visible. Waves lap against the hull. Ice and snow cover the decks, except in work areas where they are heated from below by the ship's boilers, to keep them safe for walking. When I go out on deck, I'm hit by an icy wind and horizontally driven snow flurries .
Inside, bright florescents light a room called the dry lab. The lab has numerous computers and TV monitor screens, all with multicolored displays of information. There are plotters and racks of other electronic gear all along the walls. Everything is firmly tied or bolted down, to keep it from moving around if the seas get rough. Spare wall space is taken up with large maps of parts of the Antarctic continent. The floor is covered with pale green tiles with bumps on them, to give better footing if the ship is rolling. An old Queen CD plays loudly, to keep tired scientists and technicians alert. In one corner of the dry lab, near the consoles for my geophysics watch, is a control station for monitoring the CTD.
The CTD instrument is kept in another compartment called the Baltic room, more like a big garage. The Baltic room has a large steel crane mounted to its ceiling, and a six by twelve foot power operated door opening in the side of the ship. The crane is able to extend out through the doorway, so heavy things can be hoisted safely up out of the water and brought inside.The door is then shut, to keep heat in and waves out. (The same door is used to board small inflatable boats called Zodiacs, when scientists need to use them.) People who run the winches and equipment in the Baltic room can communicate by telephone and radio with the people in the dry lab, and both can talk with the ship's bridge. Additionally, TV monitors allow the dry lab and the bridge to watch what is going on in the Baltic room.
The CTD itself looks like a space probe. It is a round framework made of steel tubing, about six feet high and about the same in diameter. Inside this framework are mounted 24 gray bottles for collecting water samples at various depths, and sensors for temperature, salinity,oxygen and other measurements. Each of the bottles holds 10 liters of sea water. When the CTD is lowered, the bottles are open at both ends, but on a signal from the dry lab both ends close. The whole thing is lowered down into the frigid ocean by a cable which not only supports it but has electrical connections to control it and send back data to the dry lab.
I feel the ship slow, and people stir to get ready for the CTD cast ahead. Dr. Stan Jacobs enters the room. He is the physical oceanographer who is the "PI" or principal investigator for this part of the ship's work. Anytime there is a CTD cast, he wants to be there, to watch the data come back, and decide at what depths the water sample bottles should be closed. Sarah Searson, the technician who will be controlling the test, sits at her console, watching instruments, and coordinating the bridge, the Baltic room, and Stan Jacobs.
When the ship stops the bridge tells her that we are "on station", and says "deploy at your leisure," in other words, we are at the proper location, the ship is stopped, and it's OK to start lowering. Sarah relays the message to the crew in the Baltic room, and a hydraulic cylinder opens the big door, letting in light, cold and blowing snow. The winches whine, the CTD rises, and then as the crane is extended outwards, it hangs above the water ready to descend. It is lowered and disappears into the steely gray water, pushing some floating slush out of the way as it goes. It is first lowered to thirty meters below the surface, to make sure all its instruments are working properly, and then brought up to just below the surface again. Sarah watches her indicators, and when she is satisfied everything is working OK, she says "down at 60 meters per minute," and the winches wind out cable. As she does so, three lines begin to appear on the computer screen in front of her. A white line gives temperature, a red one gives salinity, and a green one gives the amount of dissolved oxygen in the water. She is careful not to lower too fast, because the instruments do not instantaneously react to changes in salinity and dissolved oxygen, and she needs to give them time to adjust.
Stan Jacobs quietly watches the traces on the graph. He knows that the seawater isn't the same everywhere, and that each mass of water has characteristic salinity and temperature. He has spent a great deal of time trying to figure out how seawater circulates near Antarctica, and how it interacts with the glaciers and ice shelves of the continent. As he watches, he is deciding at what depths he wants to take water samples, which will be done as the CTD is being pulled back up to the surface. On a pad, he lists the depths for samples.
As the colored traces get longer and approach the bottom of the computer screen, the CTD is approaching the ocean bottom, way below the ship. It is important for it to get near the bottom to sample the "boundary layer" of the bottom few meters. Because the cable is so long, it's difficult to tell by the way the winch acts exactly when the CTD hits the bottom. If you've ever fished in deep water with a light sinker, you know the problem. In fact Sarah would rather the CTD didn't actually touch bottom, because mud and debris might be stirred up, cling to the instrument, and contaminate the water samples, or the instrument could even be damaged or lost. She has several ways of knowing when it is near the bottom. One is a string with a weight that hangs a few feet below the probe. When the weight touches bottom, it activates a switch, which gives her a message on her computer screen.
Another way works with sound. A pinger mounted on the CTD chirps like a spring peeper in Maine, once a second or so. I heard the chirps when I was in the Baltic room watching the CTD launched, and wondered about them. Special microphones on the bottom of the ship can hear the chirp. They can also hear the sound reflected off the bottom, although the second sound is much weaker, and arrives later. When the CTD is half way down, the chirp will arrive at the ship much earlier than the reflection, because it has a shorter distance to travel. When the CTD is near bottom, though, the sounds get closer and closer together. If the CTD were actually sitting on the bottom, the sound and its reflection would arrive at the same time. Up in the dry lab the original chirp and its reflection are recorded every second on an advancing roll of paper. As the CTD approaches bottom the lines get closer and closer together. Sarah can measure the distance between the lines with a special ruler, and know how far the CTD is above bottom.
Eventually, the colored traces are very near the bottom of Sarah's computer screen, and the chirp tell her that the CTD is very near the bottom. She picks up the phone to the Baltic room and says "all
stop." Stan Jacobs hands her the list of depths for samples. She tells the winch operator "up 50 meters," and watches as the traces start back upwards on her screen. When she is sure the CTD is at the correct depth, she presses control F 6 on her computer keyboard, a red light flashes on a nearby panel, and half a mile down in the cold dark ocean, a bottle closes. She keeps calling depths into the phone and tapping on the keyboard, taking sample after sample. Soon the CTD is getting near the surface. She takes a TV remote and points it at one of the nearby monitors . She switches channels until she has an image of the side of the ship, the crane, and the door to the Baltic room. As we watch, the CTD breaks the surface, water pouring off. She switches channels again, and we watch it being brought inside and tied down.
There is lots of work left to do. Samples will be analyzed for traces of CFCs, helium, tritium, and oxygen isotopes. The bottles need to be prepared for their next trip to the bottom. But for now, some people can go back to bed, and I can go back to filling out the clipboard for my geophysics watch.
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