Conductivity (C) is typically measured with Temperature (T) and Depth (D) as the CTD parameter triplet and housed in a combined sensor unit that allows direct output of C and T values with D as a means of referencing the data in the vertical plane. CTD data is required to calculate the speed of sound through water (used in sonar systems such as echo sounders) using well-established equations, as well as being critical in understanding the mixing of saline and fresh waters in estuaries or near the coast, as well as wider ocean circulation studies.
The most common form of CTD are units that are deployed from a vessel via a cable with data being recorded on both the descent and during recovery for download onboard. CTD can also be integrated into gliding and powered Autonomous Underwater Vehicles (AUV), giving the benefit of providing almost continuous data collection along both horizontal and vertical planes as well as being able to access areas that the vessels may not be able to reach such as under ice. This high-resolution sampling can result in the capturing of details of circulation such as eddy currents that may be less visible to a lower-resolution, vessel-based approach. CTD can also be deployed in a static arrangement as part of an instrument package on moorings,
It is now possible to gather CTD (and other measurements) from a moving vessel using systems that repeatedly release, allow to descend and then recover a probe unit connected by telemetry cable to the vessel. This approach allows continuous updates of changing temperature and salinity conditions throughout an operation, which is invaluable for hydrographic surveys using multibeam echo sounders as it removes the need for the vessel to come to a stop to conduct a CTD profile and so calculate the speed of sound. The more accurately the speed of sound value used in multibeam echosounder software matches the real conditions, the better quality the output of the survey. As the tidal conditions change, so does the speed of sound, and since surveys typically consist of multiple parallel lines with the “swath” of the multibeam overlapping on each successive pass, there will be differences in the calculated depths due to the change (in speed of sound) between when the lines were collected. The more frequent the collection of CTD or sound velocity data, the more effectively the changes in sound speed can be accommodated in processing.