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:
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:
As predicted, the rough weather arrived and the Sikuliaq started pitching and rolling a lot. Luckily the wire tension on the deep tow cable stayed low, so we were feeling good about deciding to survey at shallower depths of the forearc first.
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.
We completed deep-towing our second profile today and brought SUESI back on deck around lunch time. Samer told me that two days ago during the tow down the continental slope they had an unintended low altitude event on the Vulcan receiver that is towed on a tether 500 m behind SUESI, and that it likely had scraped the sea bottom. Sure enough, the Vulcan receiver came back up today with its nose electrode pushed in about three centimeters with the pipe full of seafloor mud. The acoustic relay transponder towed behind it also had a small fan shaped plant attached it. We haven’t seen any animal or plant life on the instruments yet other than jelly fish tentacles so it was nice to finally see evidence of life on the seafloor. We also discovered some corrosion on the back half of SUESI’s pressure case on the unpainted but anodized sections. Steve thinks something may have come loose inside and is shorting current to the pressure case; that current travels out the easiest paths, which are the non-painted parts of the pressure case, where it electrochemically corrodes the aluminum. This is the first time we’ve seen this on SUESI in its ten years of service, so we’re planning to switch to the spare SUESI for the final two planned deep-tows. Photos from today below.