About the author
More posts by celia
Last night on watch, we launched a total of 54 XBT and XSV probes in order to measure properties of the water column with depth. XBT (expendable bathy-thermograph) and XSV (expendable sound velocity) probes are, unsurprisingly, expendable instruments which remain on the seafloor once deployed. The probes drop through the water down to the bottom of the ocean, regularly measuring either temperature or sound velocity as they fall. Using these measurements it is possible to infer density as a function of depth in the locations where the probes are dropped. Density profiles derived from these measurements will be compared with reflections from within the ocean that are seen in the seismic data. “Seismic Oceanography,” an extremely new field, involves the determination of density or velocity structure in the ocean from seismic reflection profiles. Changes in velocity within the water should cause reflections that are measured by our hydrophones, just as changes in velocity in the solid earth also generate reflections. This method provides a two-dimensional image, as opposed to the one-dimensional density profiles derived using traditional oceanographic measurements. Comparing the reflection profiles with XBT and XSV data allows scientists to verify that the structures seen in the reflection profiles truly exist. In this study, we are primarily dropping the instruments over the continental rise, the part of the seafloor landward of the trench, where depths increase quickly from the shallow continental shelf to the deep oceanic crust.