Despite nine years of working with seismic data from Alaska and six years of working with data from ocean bottom seismometers (OBS), this is my first time in Alaska and my first OBS research cruise. One of the reasons why I’ve been able to do research in these areas without physically being involved in the data collection is the thorough record of open seismic data. Which brings us to why we’re here, on the R/V Sikuliaq south of the Alaska Peninsula for three weeks: to recover more than half of the OBS that were deployed last year as part of the Alaska Amphibious Seismic Community Experiment, one of the next large-scale, open-data seismic experiments… just like the data I used for my PhD research offshore Cascadia that I never actually went to sea to collect. But this time I have the opportunity to be one of the people on the boat, and I couldn’t be more amazed and impressed.
The process of getting OBS data has always seemed a bit magical – you drop these instruments off the side of the ship, they settle on the seafloor; you come back about a year later, acoustically ping to them, and they float back up to the surface (flag waving) waiting to be hooked and hauled back onto the ship, their data downloaded and used to learn something new about the earth. About half of the OBS we are retrieving are this type, but we’re also retrieving 20 trawl-resistant-mount (TRM) OBS. Watching a TRM OBS recovery, I saw a lot less magic, and a lot more incredibly hard and dedicated work.
The TRMs are specially designed instruments to withstand trawl fishing nets, like what takes place off Alaska. This allows the instruments to be deployed in very shallow coastal waters, which is a critical region for studying subduction zones (like here in Alaska). It is where the oceanic plate begins to dive beneath the continent, and where large megathrust earthquakes (like the 1964 M9.2 Good Friday earthquake) occur. From my point of view, these instruments really provide the amphibious part of the array, maintaining continuous instrument coverage and thus resolution from offshore to onshore in the seismic data.
Unlike normal OBS, TRMs don’t float to the surface when released. They have a giant shield protecting them and weigh more than 1000 lbs, so they need a little help getting back up. Some have a pop-up buoy that is attached to the instrument that then allows it to be connected to a heave compensated winch and then hauled up from the sea floor. Otherwise, we have the ROV Jason aboard to bring the instruments back up to the surface. About ten of the instruments have to be recovered with Jason dives. Five days into the cruise we’ve successfully retrieved five of the TRMs, including three with Jason dives. The fourth is underway right now! It’s a bit tense…
The first Jason dive did not go well. Everything seemed to be going according to plan, but at the surface the line connecting the TRM to Jason broke. The TRM sunk back to the seafloor, landing upside down. Even when these retrievals go well, they involve the cooperation of three different crews – the Lamont OBS crew (the designers/engineers of these instruments), the WHOI Jason crew, and the R/V Sikuliaq crew. From onsite to on deck, it takes about three hours if everything goes smoothly. That first Jason retrieval took about twelve hours, and all three crews worked to try to figure out how to get the instrument back and fix the system for the next recoveries. I’ve used data from TRMs that were deployed off of Cascadia for the last six years, I’ve seen the pictures, heard the stories about their deployment and recovery process. I will never look at data from them the same way again after seeing first-hand just how much coordinated effort goes into making this data possible. I don’t think I could give enough thanks to all of the crews involved from what I’ve seen so far, and we’re less than halfway done. Fingers crossed for more smooth retrievals!
I do have one suggestion though… put some smiley faces on the buoys for the pop-up TRMs.
Cheers! – Helen Janiszewski, postdoc, DTM Carnegie Science