What Are Sea Measurements?

Sea measurements are the observational techniques and instrumentation used to quantify the physical, chemical, and acoustic properties of the ocean and its boundaries. They encompass in-situ measurements taken directly in the water column by sensors deployed on ships, moorings, floats, and autonomous vehicles, as well as remote sensing observations made from satellites and aircraft. The parameters of primary interest include temperature, salinity, pressure, current velocity, wave height, sea surface topography, and acoustic backscatter. Together, these measurements underpin physical oceanography, marine navigation, climate science, and ocean engineering.

Ocean measurement programs date to the Challenger expedition of 1872 to 1876, which systematically sampled water properties across ocean basins for the first time. Modern observational systems combine continuous satellite coverage with the Argo profiling float network, which deploys more than 3,900 autonomous instruments worldwide to measure temperature and salinity profiles throughout the upper 2,000 meters of the ocean.

Remote Sensing of Ocean Properties

Satellite remote sensing provides synoptic coverage of ocean surface properties at spatial and temporal scales impossible to achieve with ship-based sampling alone. Radar altimeters measure sea surface height with centimeter precision, yielding data on ocean circulation, geostrophic currents, and tides. Microwave radiometers operating in the L-band (1.4 GHz) measure sea surface salinity by detecting the passive emission of the ocean surface, whose brightness temperature is a function of salinity and temperature. The NASA Salinity program describes how the Aquarius and SMAP satellite missions have together provided a continuous record of sea surface salinity since 2011, enabling monitoring of the global water cycle and ocean-atmosphere freshwater exchange. Synthetic aperture radar instruments image ocean surface roughness, wave fields, and the signatures of surface currents and internal waves at meter-scale resolution.

Sonar Measurements

Acoustic measurement systems exploit the efficient propagation of sound through seawater to sense a broad range of ocean properties. Single-beam and multibeam echosounders emit short acoustic pulses and measure the two-way travel time to the seafloor, yielding depth measurements used to produce bathymetric charts. Acoustic Doppler current profilers (ADCPs) transmit sound at a fixed frequency and measure the frequency shift of backscatter from particles drifting with the current, converting the Doppler shift to current velocity profiles across depth. Sub-bottom profilers use lower frequencies (0.5 to 24 kHz) to penetrate sediment layers, imaging stratigraphic structure below the seafloor. The NOAA Ocean Exploration multibeam sonar technology overview describes how multibeam systems simultaneously collect seafloor depth and backscatter intensity data, producing co-registered maps of bathymetry and substrate type in a single survey pass.

Salinity and Hydrographic Measurements

Salinity is among the most fundamental ocean parameters, jointly determining seawater density with temperature and governing thermohaline circulation. In-situ salinity is measured using conductivity-temperature-depth (CTD) instruments, which determine salinity from the electrical conductivity of seawater at known temperature and pressure. Salinity values are reported on the Practical Salinity Scale (PSS-78), where typical ocean values range from 33 to 37 practical salinity units. Argo profiling floats repeat CTD profiles autonomously, descending to 2,000 meters on programmed cycles, drifting for ten days at a parking depth, and then rising while collecting profiles before transmitting data via satellite. The Frontiers in Marine Science satellite salinity observing review provides a detailed account of how satellite and in-situ salinity observations are merged into global ocean analyses used by climate models.

Applications

Sea measurements have applications across a wide range of scientific and operational fields, including:

• Ocean weather forecasting and hurricane intensity prediction
• Navigation safety through real-time bathymetric and current data
• Climate monitoring and coupled ocean-atmosphere model initialization
• Fisheries management through surveys of ocean temperature and productivity
• Offshore energy installation and pipeline engineering