September 12, 2020: XBT Casts
Image courtesy of NOAA Office of Ocean Exploration and Research, Tropical Exploration 2015. Download larger version (jpg, 518 KB).
NOAA Office of Ocean Exploration and Research Explorer-in-Training Abigail Casavant prepares for an Expendable Bathythermograph – or XBT – cast during the Tropical Exploration 2015 expedition. XBT casts are used to measure temperature throughout the water column.
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.
On NOAA Ship Okeanos Explorer, XBT casts are conducted every four to six hours. Collected temperature data are combined with salinity data to generate sound speed profiles that can be applied to multibeam soundings in real time to ensure an accurate measure of bathymetry.
From: Training to Be an Ocean Explorer.