Conductivity-Temperature-Depth

Likhi Ondov, a student on the Enlighten'10 expedition, collects water samples from a Niskin bottle on the CTD rosette. 

CTD Water Samples

The conductivity-temperature-depth (CTD) system takes many different kinds of ocean water measurements and transmits the data back to the ship. The system is mounted on a tall, round metal frame, which hosts 24, 10-liter bottles for sampling fluids. Other components of the system include oxygen, methane, and eH sensors, in addition to a transmissometer that measures the amount of particulates in the water. 

Each bottle that is mounted on the frame has two spring-loaded end caps that are fixed in the open position when the instrument is lowered off the ship into the water. When we see an interesting anomaly in the water column, or reach a target depth with the system, we can electronically snap the end caps closed, trapping fluids immediately. On this way, we can bring the instrument back onboard and retrieve the sampled water, which is later analyzed for its chemical and biological content. 

Transmitting Data to the Ship

The CTD is connected to the ship by a conducting wire, so that an electronic signal may be sent down and up the wire. When the instrument is in the water, data are continuously transmitted to the ship. Scientists onboard use the data to measure the chemical, thermal, and particulate character of the water that the instrument is in or passing through. 

Determining the Speed of Sound in Water

Data collected by the CTD help determine the speed of sound in the water, which varies with water temperature and salinity. This information is then used to correct the bathymetric maps created from acoustical data during the cruise. An alternate way to collect this data is by deploying an expendable bathythermograph (XBT). XBTs are probes that are dropped from the ship and measure the properties of the water as it falls towards the seafloor. Attached to the probe are two very small wires that transmit temperature data back to the ship which is then used to calculate the sound velocity profile.