by Noel Jackson
Our Cascadia expedition this year is not just for the Near Trench Community Experiment. We are also working on a second seafloor geodesy project in Cascadia using Seafloor Optical Fiber Strainmeters (SOFS) instruments to look for shallow slow slip in Cascadia. We have two such fibers placed at a roughly 90 degree angle that have been on the seafloor since summer 2022. These projects are usually combined into one expedition because they are both in Cascadia (the SOFS is right next to one of the community experiment GNSS-A sites), both require the use of Jason, and both Principal Investigators (PIs) on the SOFS experiment are also PIs on the community GNSS-A experiment. One of those shared PIs is me!
The SOFS instruments are both completely amazing and sometimes quite finicky. Amazing because they can record changes in length smaller than the width of a human hair on a 250 meter fiber optic cable. They are the only instruments of their kind capable of this type of measurement. But they are sometimes finicky because they require a lot of complex electronics that need to operate at high pressure in seawater, which is highly conductive to electricity. SOFS instruments are still fairly new, and we are still discovering new ways to improve their design.
At this time we only have batteries for one fiber. Our main goal for today was to take a frame with batteries, a data recorder, and a computer to the seafloor, plug in one of our fibers that hasn't been running for the last year and turn it on to verify functionality, then take the frame to the other fiber and turn it on to record data. We call this frame the Replaceable Electronics Module, or REM, a.k.a. REMy because it's cute. We also planned to bring the old REMy from the working fiber back on board the ship to retrieve the data. Because these instruments are not connected to the outside world while they are on the seafloor, we have no way of checking on them or retrieving the data without Jason physically going to visit the instruments.
The recovery of the old REMy worked as planned, and we plan to extract the data tomorrow. I am excited to have a second year of data from these instruments to add to the preliminary results from the first year of data that we retrieved last year. We also did bungee pull tests on both fibers - this is a test that makes sure the fibers are still under tension and not broken. Both fibers passed this test with flying colors.
However, we ran into issues with the new REMy. The new REMy works perfectly on the deck of the ship, but has problems once in the water. It responds only intermittently to attempts to connect to the onboard computer. The issue appears to be repeated rebooting of the onboard computer, likely related to an electrical short in seawater.
After a long and frustrating day, we pulled the new REMy back on deck for additional testing to attempt to identify the issue. In the meantime, we are transiting to our second GNSS-A site to be re-occupied with Fetch sonar transponders, NGH1 which is to the north of us. After replacing the transponders at NGH1, we plan to return to the SOFS instruments and attempt to send the REMy down again to record a third year of data.
The seafloor is a harsh environment for electronics, and issues happen. But the data we can get is critically important for studying earthquake hazards, because the locked zone where the next big earthquake rupture will take place is entirely offshore. There is a good chance we can fix the issue before we return to this site the day after tomorrow. Luckily, we planned in contingency time to allow for an issue such as this.
Interesting! I didn't realize you could put strainmeters on the ocean floor. I'm curious about how they are secured in place. I know the ones on land are grouted to the bedrock. Thanks for sharing.
ReplyDeleteThe anchors at either end are secured with metal pipes that jason drives into the sediment through holes in the anchor plates. The fiber itself is not secured in any special way, but it is under tension and buried in the sediment a few inches. The seafloor is actually a fairly quiet environment, there isn't much down there that would shift the cable, so it doesn't need anything more. We do see a log(t) decaying settling signal in our strain time series, which we don't totally understand, but it's similar to the onshore borehole instruments in that way. We simply fit and remove the log(t) decay, and the instruments work great!
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