Last night, at about 40 minutes past midnight, we arrived at the location of our first OBS deployment. After 9 days of steady transit and getting our sea legs, we were finally ready to do what we came here for! Mark and Ernie were down in the OBS lab preparing the instrument about an hour prior, and so with 20 minutes to go we had everything assembled and in place on the back deck. I suppose it’s not fair to crow about efficiency when we’ve had 9 days of preparation time, but we felt quite smug.
As we approached the pre-defined location, the team in the computer lab noticed some funny returns on the multi-beam (the instrument we use to sense a swath of sea floor bathymetry beneath us) that indicated we’d just sailed over a steep cliff. Not wanting to risk the instrument encountering that edge, or falling down a hole in the seafloor a la Jules Verne, they radioed up to the bridge to move us a little way off that spot in the direction of smoother seafloor. Safety first, here on the KM.
Once we got the “ok” from the bridge and the lab, the folks on the back of the ship put the final touches together. The ABs (ABle seamen) operating the A-frame and the winch got into place, the tag liners got into position, and lastly the safety chains on the back of the ship came down. Then, while once of the ship’s research techs (Julianna) grabbed the release rope (which does exactly what it sounds like it does), the other (Rob) initiated a complex set of hand motions to communicate with the AB in the winch room, where deafening motors make shouted commands inaudible.
Slowly, the OBS rose off the deck, looking like a bit like a drunken bird with its head (the dangling sensor ball, containing the actual seismometer) lolling off to one side. Ernie and Mark expertly kept it stable, despite the rocking ship, by carefully letting out and taking in rope on the tag lines. Once it was high enough to clear the back railing, Rob signalled and the A-frame lurched outward, moving the suspended instrument clear of the back of the ship. Now the winch spooled slowly out again, lowering the package towards the water line. Just before it touched the water, Mark and Ernie pulled off the tag lines in choreographed fashion so as to swing the sensor ball away from the dangerous side of the ship as it fell. At the last moment, Julianna yanked on the release hook, and the OBS was away!
Once in the water, the instrument quickly sank out of sight; the last glimpse we got was the submerged orange flag flying proudly in the ocean current. After tidying the deck away, we piled back into the computer lab to “watch” the instrument sink to the seafloor. Its downward progress was marked by longer and longer fractions of time between our ship sending a signal and receiving the OBS transponder’s reply. About two hours later (they sink at ~40m/min, and the ocean is deep here), the time between successive pings flat-lined: it had reached the bottom!
The next task was to find out where the OBS actually landed. To do this, we (well, the extremely capable ship’s crew) drove the ship around the drop location in a big circle, ranging periodically to the OBS transponder. These distances allow us to triangulate the location that the OBS landed. In case you’re wondering, we can’t just take the drop point as the station coordinates, as the OBS can drift a long way on its journey to the seafloor.
An hour or so later, once we were satisfied with our location survey, we sent one final command to the OBS, disabling the transponder. This way it will sit quietly on the sea floor, conserving battery for the long year ahead as it listens for earthquakes on the other side of the planet.
One down… twenty-nine to go!
Zach Eilon (on behalf of the Pacific ORCA team aboard the R/V Kilo Moana)