We are all done picking up the marine MT receivers and the RV Sikuliaq is heading back to Dutch Harbor! Its been a fun cruise and the time has gone by fast. We recovered 53 out of the 54 receivers deployed, which makes our success rate 98%. That’s really good as far as deploying sensors to the seafloor goes, but i’m still feeling sore about losing one of them.
The one we lost was deployed in the middle of Umnak Pass and we knew this was a risky proposition due to the strong currents, but we had to test whether this sort of deployment would provide feasible data (nothing ventured nothing gained). We started the recoveries at the northern end of the survey profile in the Bering Sea and worked our way south. Everything was going good in the Bering Sea, but I was still a bit nervous about what would happen when we got to the strong currents of Umnak Pass. Station R08 at the northern end of the pass came up fine (other than being filled with tephra), so I thought we were in the clear for the rest, but when we got to station R09, it was nowhere to be found. We tried to get the ship to work a pattern around the location where the receiver was deployed, but they were having trouble with the engines (new ship syndrome I think) as well as dealing with the strong current in the pass, and our search wasn’t very effective. No sign of R09 was found on our acoustic system. So we suspended the search and moved on to station F25 further to the southwest, knowing that we would get a second chance when we headed back through the pass on the way back to Dutch Harbor. By midnight last night we recovered the final receiver at the southern end of the profile, station F01, which was deployed in 5800 m water depth near the base of the Aleutian trench. We then got a few winks of sleep while the ship steamed back up to Umnak Pass.
Around 630 this morning we were back at the entrance to the pass and so we fired up our acoustic system and started pinging for R09. I had instructed the bridge to follow an expanding rectangular spiral pattern around the waypoint for R09, with the hope that it had probably been pushed by the currents only a little bit off station and we would eventually find it. However, before we even got to the search pattern we started getting replies from R09 at a distance of 2.5 km to the southwest of where we deployed it! Those of us at the acoustic station let out a loud cheer at this great luck. That’s the largest distance i’ve seen one of our receivers be pushed, especially given the 345 lb concrete anchor that holds them to the seafloor.
The receiver was at a slant range of only about 800 m from the ship, so we immediately fired its release command and then everyone started looking for it from up high on the bridge. It soon disappeared from the acoustic system, which is typical when the receivers arrive at the sea surface. We were so close to having 100% success, once we finally got this one back on deck, but then our victory all started to crumble. This instrument was one of the few we deployed that didn’t have a GPS unit in its stray line buoy, so the only way we could find it was by making visual contact; given the strong currents that pushed it, it was likely that its fiberglass flag pole had been sheared off making it difficult to see above the waterline. It was also daylight out by then and so we wouldn’t be able to see its strobe light flashing. To make matters worse, the current was pushing strong to the southwest and the rising sun ahead of the ship made it hard to see the water’s surface. After about 30 minutes holding station we decided that it must have drifted past us so we turned around and started racing through the current with everyone scanning about for the receiver. We searched and searched and searched and after about 1.5 hours we gave up and drove back up current towards the location where we first saw it. We tried to reach it on the acoustics but it was gone. So long instrument Wobby, may you have a good life floating in the Pacific Ocean, and may a passing ship or islander find you some day and call or email the contact information engraved onto the side of you!
So we’re sort of feeling like we made it to the championship game only to lose in overtime. Well, that’s a bit too melodramatic, especially given that the 53 stations we did recover have some great looking MT signals that will make for good imaging of the crust and upper mantle beneath the volcanic arc. Below is an example of data from four stations. Each instrument records two components of the horizontal magnetic field and two components of the horizontal electric field. So each group of 4 traces shown below is from an individual station. The top two stations are F15 and F10 on the forearc slope (south of Umnak Island) and the bottom two stations are B10 and B04 from the back-arc (north of Umnak). This is only 15 minutes of data out of the three weeks of continuous recordings at each station. The very coherent wiggles you see on all channels are from naturally occurring MT signals diffusing into the seafloor. We will take this data and form seafloor impedance transfer functions for each station, and then those will be the input data for a Maxwell’s equation simulation code that will solve for the electrical conductivity structure of the crust and mantle beneath the survey profile.
Here are a few more photos from this cruise. This one shows Adrian Doran, Dallas Sherman, Georgie Zelenak and Dan Bassett trying on their gumby suits during the safety drill two days ago:
Georgie and Dan securing the last instrument around midnight last night:
The sunrise this morning presumably with instrument Wobby floating away in it somewhere:
Here’s a screen shot showing our ship track for the RV Sikuliaq cruise. Only a couple more hours and we will be doing a victory lap celebration at the Norwegian Rat in Dutch Harbor! Tomorrow we will unload the ship and pack our gear into shipping containers and then we fly home Thursday. I’m looking forward to finally spending some time with my family after being away for a month, but I will also miss the great team of people that helped make this project a success. We will also be continuing to update the blog with our after-that-fact posts about the onshore part of the field work inside Okmok caldera, so stay tuned.