An Expendable Bathythermograph (XBT) is a probe used to measure temperature throughout the water column.

Mapping Team Lead Meme Lobecker conducts an XBT cast to measure temperature down to 760 meters in order to correct mapping data for water column variability. Image courtesy of the NOAA Okeanos Explorer Program.

Okeanos Explorer Mapping Team Lead Meme Lobecker conducts an XBT cast to measure temperature down to 760 meters in order to correct mapping data for water column variability. Image courtesy of the NOAA Okeanos Explorer Program.

Did You Know?

On NOAA Ship Okeanos Explorer, water column temperatures are measured using XBT casts and surface salinity values are determined using a thermo-salino-graph.

Data collected with these tools are then used together to determine an overall sound speed profile for the water column. This information is in turn used to adjust the multibeam sonar mapping system to ensure accurate data collection.

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 temperate 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.

 

For More Information:

Exploration of the Gulf of Mexico 2014: Meet Explorer in Training Marah Dahn

Watching in 3-D (pdf), How Do We Explore? Education Materials Collection

Introduction to Water Column Investigations (pdf), How Do We Explore? Education Materials Collection

 

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