The most important physical properties of seawater for the hydrographer are conductivity (salinity), temperature and depth (pressure) - giving rise to the term CTD Probe. These measurements are presented as temperature and salinity from a series of depths to derive a sound velocity (SV) profile. The SV profile is used to convert the two-way travel time measurement of an echo sounder into depths. In addition, it is useful for refraction ray tracing in acoustic ranging techniques and swathe bathymetry where it also has an application for determining appropriate absorption corrections for back-scatter images.

Andrew Gerrard, contributing editor, Hydro International 

Sound velocity is also available as a direct measurement for some of the probes reviewed. A key feature of the probes is whether a real-time measurement can be made available - this has advantages in controlling the profile measurement close to seabed and can enable continuous real-time data to be used to improve both spatial and temporal modelling of SV. One system specialises in under-way profiling with self-recording measurements while freefalling to a predetermined depth. The requirement for accurate and frequent measurement of SV is a trade-off with vessel time lost while observing full ocean depth profile measurements.

Sound velocity is not the only physical ocean property of concern and many of the probes under review offer additional sensors to also measure turbidity, dissolved oxygen, pH, chlorophyll and other parameters. Oceanographic models based on these measurements are helping to map the composition, extent and movement or dispersion of bodies of water to learn their global impact on climate and biology. Real-time display and recording of all the parameters has many advantages in mapping specific properties.

Many small scale surveys of physical water properties monitor pollution and general water quality routinely in rivers and estuaries. By deploying these sensors on buoys or sub-sea frames in fixed locations, telemetry systems offer real-time data direct to the internet. Power consumption (battery life) and sampling intervals become more important considerations in these long-term deployments.