Summoning ocean bottom seismometers from the deep
After leaving our seismometers on the seafloor offshore Alaska for a few days to record sound waves generated by the air guns of the R/V Langseth, we returned to collect them. The recovery of OBS always involves a certain amount of suspense. Despite all of the advanced engineering and planning that goes into these instruments, it is an endeavor with inherent risk, and things can and do go wrong sometimes: one or more of the glass balls that make the OBS float could implode; the acoustic communication with the instrument could fail; it might be stuck on the seafloor for one reason or another; it could have been accidentally dragged off by trawlers. All of these thoughts ran through my mind at each site as we waited for the instrument to come to the surface.
To recover the OBS, we return to the place where we deployed it and communicate with it acoustically. We send it a command to release from its anchor and float back to the surface. The OBS rises through the water at 45 meters per minute, so the wait can be long if the water is deep. Some of ours were 5500 m below the surface! The instruments can also drift away from their original deployment location on the way down or the way back up due to ocean currents. When they arrive at the surface, we can spot their orange flags and strobe lights; they also send out radio signals.
Despite all the technology required to place a seismometer many miles below the ocean on the seafloor and summon it back to the surface, many aspects of actually plucking an OBS out of the ocean and pulling it on deck are remarkably low tech (yet still very impressive). Once we have spotted the OBS floating on the surface, the ship drives alongside. It is akin to driving your car up next to a ping-pong ball. People lean over the starboard side of the Langseth and attempt to attach a hook with rope to the bars on top of the OBS using a long pole. Its not always easy since the OBS is bobbing up and down in the waves. Once we hook it, we can attach a rope to the winch and haul the OBS onboard. Sometimes, OBS’s bring back surprises – an octopus returned with one of our OBS’s! He was alive and healthy, so we returned him to the sea (though some lobbied to keep him for lunch…)
Happily, we recovered 100% of our OBS’s and have started to (briefly!) pore over the data they recorded while they were on the seafloor. We can see the arrivals of sound waves from our air guns as well as lots of earthquakes, some very close and others far away. It would be delightful to dig into the analysis of these data immediately, but it must wait – there is more data to collect! We’re currently deploying OBS’s along our second profile.