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Even though we are on our way to Manzanillo, Mexico, we are continuing to collect scientific data, and in particular, multibeam bathymetry data. As you know, the multibeam bathymetry system uses sound to determine the depth of the seafloor. But the speed of sound changes as it travels through the water column! So we need a way to correct our sound data for the differences in the speed of sound as it travels through the water column. We can determine how fast sound travels at different depths in the ocean if we know the temperature at different water depths. We can measure this using an XBT (which stands for “Expendable Bathythermograph”), being launched here by Jim Charters, the Scripps Computer Technician. The expendable part of the XBT (shown below Jim's picture) contains a temperature sensor and a very fine copper wire. When Jim shoots the expendable part into the ocean, it sinks and the seawater temperature at different depths is transmitted up the copper wire to a recorder on the ship.

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