An XBT is a small probe that is dropped over the side of a ship. As it falls through the water, it measures temperature. Small wires transmit the temperature data back to the ship where it is recorded for further analysis.
Because the probe falls through the water at a known rate, the depth of the probe can be inferred from the time of launch. Scientists then plot temperature as a function of depth to create a temperature profile of the water.
It may surprise you to learn that temperature profiles play an important role in multibeam mapping operations. In order to make accurate depth measurements using multibeam sonar, we need to know how fast the sound wave emitted from the sonar unit travels through seawater. The speed at which sound travels changes as the density of water varies through the water column. Temperature and salinity are the two main variables that affect the density of seawater.
Software allows scientists to combine temparture profiles from an XBT with surface salinity data to determine an overall sound speed profile. The sound speed values can then be applied to multibeam soundings in real time for an accurate measure of bathymetry.