Thursday, February 5, 2009

Ocean-Bottom Seismometers

One of the best parts of my time on the boat (so far) was deploying the 59 ocean-bottom seismometers (OBSs). These instruments measure vibrations in the Earth, which are created by "air-guns" towed behind the boat. The bursts from the guns shake the crust beneath the water and the vibration moves outwards through the crust in all directions to the array of seismometers. The signals recorded by our seismometers can show hotter regions, where seismic waves travel slower, and colder regions, where the waves move faster. For this reason, our work out here in the back-arc basin is meant to "see" any hot regions beneath the surface, as we would expect to see in an active back-arc basin.

In order to get all of this data, we first had to send all of these seismometers (OBSs) to the seafloor. Each OBS has a sensor (the actual seismometer part), a data recorder (which records the data as a digital signal), a heavy weight, and empty glass spheres surrounded by yellow plastic. When the instrument is deployed, the weight drags it down to the seafloor. Then when the experiment is over, a signal is sent to the OBS, telling it to release the weight. After that, the OBS rises back to the surface, where we collect it. The OBSs were built mostly ahead of time, so as we cruised over the sites for the instruments, we were able to carefully drop it into the water and continue onward. We finished this stage in about 2-3 days - fortunately before Cyclone "Hettie" came our way!

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