CTD 101: Remote Sensing Basics
Just relax; there will not be a test. The CTD is one of the robots used to explore the depths of the ocean. Unlike the landers, it is not usually a long-term probe. It is often lowered to near the bottom, and then hauled back up to the top. There is an altimeter on the CTD to tell how close it is to the bottom. Along the way down and up, some sophisticated sensors capture many kinds of data about the water. At any point on the dive, an electric signal to the CTD can cause one or more of the Niskin bottles to close, capturing seawater at specific depths. The signal is sent through the cable, which to outside appearance looks simply like a twisted steel cable. But inside the steel are copper wires to signal the Niskin bottles and to receive the data from the sensors.
CTDs come in different sizes. There are larger Niskin bottles. Bigger frames, known as rosettes, can have more of them mounted. The kind of data collected varies depending on the desires of the science team. The main component is the white metal Rosette. The central unit that holds the Niskin bottles open, and releases to close them and capture water, is called the Cake or more technically the Carousel. The metal tube on the bottom, sometimes called the fish, holds the electronics, meters, and circuit boards that collect the data from the probes and sends it up the cable to the ship’s computers.
CTD is an acronym for Conductivity, Temperature, and Depth. These are traditionally the sensors used most often. On the CTD being used by the Deepwater Canyons Expedition, there are these three plus turbidity, oxygen, density and fluorescence, for a total of seven. The probes sample the water many times a second, so that even on maximum speed descents, which for us is 60 meters/minute, layers of water that have different characteristics are not missed.
Depth is measured by pressure. All the other readings are plotted versus depth. Temperature is measured by a digital thermometer. Conductivity is determined by how fast an electric current goes through the water, and is directly related to salinity. A few more details may be in order:
- Density can be calculated if you know the conductivity, temperature and pressure. With increasing depth, temperature decreases and pressure increases, both causing the density to increase. Beyond the surface waters, conductivity or salinity is fairly constant, but there are changes in the amount of dissolved nutrients such as phosphate and nitrate.
- Turbidity is the amount of undissolved solids, usually bacteria, plankton, marine snow, or small invertebrates like copepods or jellyfish.
- Oxygen is measured directly using specially designed probes that measure a difference in current between the sample and a reference.
- Fluorescence is a measure of the amount of chlorophyll and colored dissolved organic materials (CDOM) in the water. Fluorescence correlates well with nutrients (NO3−, PO43−) in surface to upper-depth waters (surface–1000 m). The fluorometer on the CTD is calibrated to only measure chlorophyll.
On the ship, the data are collected by computers and plotted in real time. While there is a lot of information, it is humbling to realize that this is just a snapshot of some of the water conditions in one thin column of water. In the vastness of the ocean, conditions change with the seasons and the locations. Our knowledge of the oceans, while increasing, is by no means complete.