We’re on our way back to Seward with an ETA of 0800 tomorrow. The ship is taking the scenic route via Shelikof Strait, which runs between Kodiak Island to the east and mainland Alaska to the west. Yesterday the crew had training drills planned so the ship pulled into the calm waters of Katmai Bay, and once again we got super lucky with the clouds parting as we neared the shore, giving us sunny skies and spectacular views of Katmai National Park. We saw several whales, lots of sea otters and a few volcanoes. After a month of working 24/7 on the ship for the EM survey, it was nice to have a day off while we steam back to port. In addition to enjoying the scenery, in the afternoon we had a round of 6 science talks in the ship’s lounge, including: analysis of melt inclusions from the western Aleutians (Janine Andrys), electromagnetic mapping of submarine groundwater off Hawaii (Eric Attias), EM exploration of the Middle America Trench (Samer Naif), EM mapping of seafloor gas hydrates (Steve Constable), active seismic imaging of subduction zones (Tanner Acquisto) and aquifer systems extending far offshore on the US Atlantic margin (me – Kerry). I’m looking forward to another five science talks today, as its nice to get to learn more about what everyone is researching, including the students from other universities who’ve volunteered for our cruise, especially since we’ve spent a month together plugging in electrodes, cabling instruments, throwing grapnels, driving deep tow winches and going through lots of scotch 3M electrical tape, cable ties and all the things required to get new seafloor EM data at a subduction zone. Now we get to spend time learning about everyone’s research and ongoing projects. Photos from Katmai bay below.
We pulled up to station 220 around 9 PM and sent a few release commands to receiver Steelhead but heard no replies. The Sikuliaq has a deployable centerboard that extends several feet below the hull and so we’ve been using the transducer on the end of the centerboard since that’s a less noisy environment for a transducer than up on the hull where more water turbulence and air bubbles can muddle the incoming pings. For almost all of the receiver recoveries that has been working really well. But alas, we couldn’t get any commands through to Steelhead, though we did sporadically get a reply when trying to range on it so we knew it was still alive (in the acoustical communication sense). Earlier in the cruise Jake noticed another receiver that was hard to communicate with, but which worked just fine when we switched to the ship’s hull mounted transducer (as opposed to the centerboard ducer). So we switched to the hull transducer and bam – it acknowledged its release command on the first try! About four hours later we landed it on deck. So new numbers for the cruise: 159 deployments and 159 successful recoveries. Coupled with the 168 deployments and recoveries we did on the HT-RESIST survey earlier this year, that’s makes for 327 total deployments without a single loss of instrument this year. To be fair, we did have two receiver frames and some sensors destroyed when the ship ran them over during their recovery this cruise, but we still got the data loggers (and the data) back.
We did it! The last receiver has been recovered, giving us a total of 158 seafloor EM stations occupied in just about four weeks. Actually, we’re hoping this is the penultimate receiver recovery since we’re steaming back to station 220 on our first survey profile to see if we can persuade receiver Steelhead to release from its anchor. We had trouble communicating with Steelhead’s acoustic system and gave up on it back a few weeks ago, but now we’ve got some extra time on hand so we can drive around it to see if it can hear its release command better when the ship is at a certain azimuth; there’s a big seamount nearby that may be giving some reflections that muddle the acoustic signals heard by Steelhead and hopefully we will find just the right angle that allows it to hear its release command. Fingers crossed…
In the mean time, below are some photos from the last few days.
Yesterday was an amazing day at sea. In the morning we got to see a pod of Orca (YES ORCAS!!!!) swimming past the ship. Check out Steve Constable’s photo of the orcas here. We made our way to within about 10 miles of Unga Island in Shumagin islands group just as the clouds were clearing, giving us a scenic view of the Shumagin islands as well as the nearby Alaska Peninsula coast, including Pavlof volcano and Pavlof Sister. The clouds kept clearing throughout the afternoon as we deployed SUESI for surface electromagnetic transmitter towing across the shallow waters of the continental shelf (similar to surface tows we did in a paper published today about mapping offshore groundwater on the US Atlantic coast). As the evening set it, the skyline lit up as the sun set just to the right of Pavlof, blazing the sky into a spectrum of yellow to orange to purple colors. Check out the photos below.
Julen Alvarez-Aramberri, a Lamont postdoc from Basque Country, an autonomous region of Spain, has been crafting a detailed blog about our cruise and life at sea. Check it out here: https://ciencilari.wordpress.com/
We’re nearly done redeploying all 39 receivers and will be towing SUESI on the continental shelf later this afternoon. At the moment there several small islands in view, which is nice sight after spending the last few weeks mostly out of sight of land. Some photos from yesterday below.
We finished recovering all 39 receivers and now are just about to redeploy them. We’re splitting the fleet in half, with twenty receivers to be deployed on the abyssal plain to collect MT data and 19 to be deployed on the continental shelf at 80 – 100 m water depths, where we plan to surface-tow SUESI so we can get shallow water CSEM data sensitive to the upper crust and MT data sensitive to the crust and the deeper mantle wedge. This will extend the forearc slope data so that in total we will have a 160 km profile of CSEM and MT data from the trench up onto the shelf (plus about 50 km of CSEM and MT data from two crossing profiles on the forearc slope). And with the abyssal plain data, the MT profile will be about 240 km long. Although SUESI has a high pressure leak somewhere, we tested that it can still hold a vacuum and so we feel safe lowering it to 10 m water depth on the shelf (i.e., only 1 additional atmosphere of pressure). The weather prediction is looking great for the next week so we’re all hoping we can successfully finish this last leg of the project.
In the mean time, here’s a funny EMAGE movie trailer made by Bern:
The last few days have been quite eventful. Let’s walk through the sequence with a series of photos and videos. You can also read more about the events on Steve Constable’s blog at: https://marineemlab.ucsd.edu/Projects/Megathrust/index.html
First we deployed all 39 seafloor receivers along a forearc-crossing profile (aka up the continental slope) and got SUESI in the water and transmitting, all in less than a day. Since some rough weather and large swell was in the forecast, we decided to deploy all the receivers in the shallower depths of the forearc; that would pose less of a problem for deep towing since we’d need to let out less tow wire at those depths and hence would have lower wire tension spikes when the ship started heaving in the big swell. Then next week when the weather is better, we could move all the receivers out to the ~4.8 km depths of the abyssal plain, and hence we would be able to let out more deep tow wire in the gentler seas without worrying about exceeding the wire’s tension limits. An upside of this is that we decided to deployed a mini 3D array of 24 receivers (4 x 6) deployed every 4 km, which would give us the first 3D CSEM survey of forearc structure. Here is the night shift deploying SUESI after breakfast:
After an overnight transit we arrived at the start of our third survey profile and we’ve been deploying receivers every 25 to 30 minutes. At this pace we will be maneuvering to deploy SUESI around 9 or 10 am. I better make this post quick so I can get some rest before then. Photos from today below.
The night shift just pulled the 39th (and last) receiver back on the ship and that completes our second EM survey profile. Woohoo! Now we have a 140 nautical mile transit to the southwest to our next survey area, corresponding to ALEUT Line 5 (our first two profiles corresponded to ALEUT Lines 2 and 3). The 2011 ALEUT project used seismic refraction and reflection data to image the subduction zone structure along this section offshore the Alaska peninsula; we’re now collecting EM data along a few of their profiles so that we can jointly interpret electrical conductivity and fluid content with geologic layering and structure inferred from the seismic images. Photos from today below.
We’re cruising along picking up the receivers deployed along our second profile the past day, with 19 on deck, 20 more to go, and an expected completion tomorrow night. Three receivers are currently in the middle of their four-hour journey floating back up to the sea surface. They rise at only about 20 meters per minute, so we stagger their releases about half-hour apart since that’s about how long it takes to bring them back aboard the ship and then drive the 4 km to the next station. Yesterday we released six of them in a row, timed to be about three hours between the first and last one reaching the surface, which may be a new record (or tie) for most number of instruments we’ve had in the water column at the same time. Ship time is expensive and we only get one shot at this experiment, so we’re always trying to be as efficient as possible so that there’s more time for getting more data.
To navigate and release the instruments, we have three acoustic ranging systems at our disposal, as shown in the photo below. The yellow ORE deck box on the left sends the frequency modulated acoustic codes for the transponder units on each receiver by making acoustic pings with the transducer mounted on the ship’s hull. The ORE transponders on the seafloor receivers are always listening for their own special frequency code pings to do some action (enable, disable or release). After using the ORE box to enable the seafloor transponder unit, we then perform a brief navigation survey using the Benthos digital ranging system (blue box on the right). Technically we could do this with the newer yellow ORE box, but we haven’t had a chance to update our ranging software for that system, so for now we stick with the Benthos unit. We have the ship drive in a cross pattern over the deployment location for the receiver and we collect acoustic travel time ranges, which are the time it takes for a ping to travel from the ship to the receiver, and then back to the ship. We also collect the ship’s position and heading data for each ping. Since we know the speed the ping travels in seawater (about 1500 m/s), we can then use the range measurements and ship’s position data to triangulate the receiver’s position and depth on the seafloor.
Here’s an example in the figure below. The bottom plot has the acoustic ranges we measured as the ship drove in a pattern over the receiver. The y-axis is the two-way travel time for the ping to go from the ship to the receiver and back. The horizontal axis is time of day the ping was sent. The upper plot shows the ship’s track and is color coded by the two-way travel time for the range measurements at each location. When the ship is directly over the receiver its at the closest range and that’s when the ranges are colored red below, whereas the blue dots show when the ship was farther away. So x marks the spot in this example. However, we’ve seen the receivers drift up to a few hundred meters from their drop locations, so sometimes the shortest ranges are offset from the locus of the navigation survey’s X pattern.
After we’ve collected enough navigation data we switch back to the ORE box and send a release code to the seafloor receiver. Once confirmed, the receiver will electrolyze a small wire holding a spring gate on its release, a process that takes about four minutes. When the release gate opens, the anchor strap is released and slips through loops on the concrete anchor, allowing the receiver to begin floating up to the sea surface. We confirm liftoff by using the Scripps analog ranging system attached to an old school dot-matrix printer. In the video below you can “see” the ping for the receiver at a constant distance from the left side, which corresponds to its acoustic range. When it starts to lift off from the seafloor, we get two pings, one from the receiver and one from the ping bouncing off the seafloor, which you can see in the section printed in the video below. That’s the quickest way to confirm liftoff. Alternatively we could collect digital ranges with the ORE unit and wait around until the ranges are shallower than the water depth, but that could take 10 minutes or more, whereas the more detailed analog print possible with the Scripps system gives you that nice seafloor bounce echo to confirm liftoff immediately. Again, we’re always looking for ways to save time.
A video and some photos from yesterday: