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CTD Introduction

One of the most versatile and useful oceanographic instruments carried aboard the R/V THOMAS G. THOMPSON is the CTD. The CTD gets its name from the three conditions measured by the main sensors on the instrument: Conductivity, Temperature, and Depth. Electrical conductivity of seawater is measured as a way to calculate salinity, temperature is measured directly, and depth is computed from water pressure. The basic sensors and associated electronics are mounted on a metal frame, which also carries 24 10-liter capacity sampling bottles. Additional sensors may also be mounted on the frame: oxygen, light transmission, fluorescence, and optical backscatter are the most common.

When scientists are interested in mapping the properties of the ocean water -- near the surface where the thermocline may be detected, or close to the sea floor where hydrothermal vents may alter temperature and salinity, the CTD package is lowered from the ship on a cable. The cable carries signals from the ship to the instrument and data from the CTD back to computers aboard ship. The sensors are operated continuously and provide high-resolution profiles of the water column. The sample bottles are controlled from the ship, and may be closed individually at different depths to sample water from those depths.

The CTD can be used in a variety of ways: strictly vertical hydrocasts, towed casts where the package is raised and lowered as the ship steams slowly along a predetermined line, or cycles between intermediate depths. The versatility and accuracy of the CTD make it an extremely useful tool for oceanographic exploration.

Often an acoustic transponder is attached to the CTD frame, allowing its location to be monitored during a hydrocast. Other instruments may be added to the frame to investigate a variety of water conditions. When searching for hydrothermal vents, for instance, a transmissometer is added so that particle-rich clouds of vent fluid can be detected.