The standard tool for profiling the basic physical properties of the water column is the CTD—an instrument package that measures conductivity, temperature, and depth. Lowered from a research ship on a conductive cable, it sends real-time data to the surface while a rosette of water-sampling bottles can be closed at chosen depths.

Conductivity is converted to salinity; pressure is converted to depth. The resulting profiles reveal the layered structure of the ocean, the boundaries between water masses, and the intensity of mixing. CTDs are calibrated against laboratory standards so that measurements from different ships and years remain comparable.

Nearly every oceanographic expedition begins with CTD stations. The data feed models, calibrate satellite observations, and provide ground truth for autonomous sensors. Modern CTDs often carry additional sensors for oxygen, fluorescence, turbidity, or pH, turning a single cast into a multi-parameter snapshot of the ocean’s interior.

• Instrument measuring conductivity, temperature, and depth
• Primary tool for water-column profiling
• Often paired with water-sampling bottles
• Foundation of physical oceanographic data sets