Poseidon Visits (and Seismic Oceanography)

One of the secondary activities on the cruise has been the deployment of XBTs off the stern. XBTs are a standard oceanographic tool designed to measure the variation of water temperature with depth, providing information on mixing processes within the water column. As temperature is one of two primary controls on velocity of sound in water (the other being salinity), it is also of interest in the processing of our bathymetric data.

Poseidon's Zodiak on the way over to exchange supplies.

A few years ago, it was realised that seismic provides a method of directly observing the mixing processes, as the different water layers have sufficiently different seismic velocity and salinity for reflections to be generated at their boundaries: we have already seen reflections in the water column of our data, probably from boundaries between North Atlantic water and warmer, more saline Mediterranean water. However there have been relatively few studies of these processes using traditional oceanographic and seismic techniques, a deficiency being rectified by the deployment of XBTs at regular intervals during our cruise.

A successful exchange on medium-high seas!!

In addition to deploying ocean bottom seismometers to record our seismic shots, the German research vessel F.S. Poseidon has been carrying out oceanographic measurements, mainly using CTD casts (conductivity-temperature-depth), which provide more information than XBTs. As a result they had several XBTs left over. These they transferred to us this morning: Poseidon came within about 1 km of the Langseth and sent the XBTs over in a small boat. A real bumpy ride!

Goodbye, until we meet in Vigo!

Tim Reston
University of Birmingham