Expendable Bathythermograph (XBT)

Each day aboard the R/V Marcus Langseth we launch an Expendable Bathythermograph (XBT) off the side of the ship. The XBT itself is a small, torpedo-like probe that is used to record the temperature of the water. Because the XBT is shaped in a particular, hydrodynamic fashion with a specific weight, it is known at which speed the probe will fall through the water column and we can infer the depth of the probe based on the time of its entry into the ocean. This allows us to gain a profile of temperature versus depth. When we are ready to launch our probe, we prepare by putting the XBT in a launcher. The launcher itself is what is used to send the data to the computers onboard the ship. It has a series of wires within that are used to communicate the data once it receives it from the XBT. The launcher that we use is handheld, so it is easier to move into position and get the XBT into the ocean. We use a plastic pipe to help guide the XBT into the ocean. The pipe hangs off the side of the ship and is tied to the vessel through a series of ropes. When we are ready to launch the XBT, we aim the launcher down the pipe so that we are more certain of a clean entry. We then simply unlatch the pin holding back the probe, and we watch the XBT slide down the pipe and into the water.

The XBT is connected to the launcher through a spool of copper wire that unravels as it travels through the water column. The data it records is transmitted to the launcher via the copper wire. The deepest we have recorded with an XBT so far is 1830 meters. Other attempts are less successful, as sometimes the probes get snared in the propellers or in the streamers pulled by the vessel. Once we are done collecting data, we simply snap the copper wire off. This is why the devices are called “expandable.” These XBT’s play in important role during many research operations. We often want to accurately map the ocean bottom and doing so requires multibeam operations that utilize sound waves. Bathymetry requires that we know the velocity of the sound waves in the water, so as to get accurate measurements of depth. The velocity of these sound waves through the water column are affected by the density of the water, and the density in turn is affected by the temperature and the salinity of the water. Thus, the temperature versus depth measurements that the XBT’s provide are useful in inferring the velocity through the water.

Mitchell Spangler

University of Indiana

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