Land Ho!

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.

Panorama of the Sikuliaq’s back deck as we prepare to deployed SUESI and the surface towed receiver array.
Chris ready to start deploying the towed array.
A Vulcan three-axis EM receiver (front) being deployed along with two surface buoys containing GPS transponders (middle and read).

 

Pavlof and Pavlof Sister volcanoes viewed from 50 miles offshore.

Sun setting behind Pavlof and Pavlof Sister volcanoes.
The after glow of the sunset lasted for a few hours; even the night shift got to enjoy it when coming onto shift at midnight.

Un mes en el mar de Alaska trabajando en el proyecto EMAGE

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.

Jake attaching a mesh bag of styrofoam cups to a receiver being deployed in 4800 m water. Check back in a few days to see the shrunken cups.
Receiver being deployed in deep water.

Movie trailer

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:

! – WARNING *** LEAK DETECTED ***

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:

A clear deck means all the equipment is deployed and SUESI is transmitting current.
While collecting data, we saw SUESI’s output current drop from about 250 amps down to nearly zero, a telltale sign that we had either lost an electrode on the antenna or a leak had burned through one of the antenna’s cable terminations. Here’s Chris and Jake on the fan-tail with recovery hook poles as we bring SUESI back aboard.
Bummer – we did burn a cable termination. But luckily it happened right at the mounting point on SUESI, so we could relatively easily swap this with a spare antenna lead.
If the termination gets worked even a little bit it can increase its electrical resistance just a tad, but when you’re pumping 250 A through it, even a tiny increase is enough to cause it to heat up, which led to a runway process of the plastic insulation melting and electrical current leaking to seawater. We replaced the antenna lead and just as we were about to redeploy SUESI, Steve walks into the lab and says that Brandon spotted some loose screws on the A-frame joints! Thankfully the screws where just on protective plates covering maintenance access points, so there was no danger. One of the ship’s engineers came out and tightened them all up.
Away goes SUESI as we redeployed it  after looping the ship back up the tow profile a bit so that we wouldn’t have a data gap from where we had to pull it up due to the melted cable termination.
Here’s a look at the forearc receiver profile and grid of 3D stations. The bright cyan lines and ship icons show cargo vessels passing by along the great circle path that is the most efficient route from the US West Coast to Asian ports. Not exactly the best place to being deep-towing but they’ve been giving the Sikuliaq a wide berth.
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.
It was getting quite boring, meaning everything was working really well, and then I saw this on the terminal that displays the telemetered data from SUESI’s onboard computer. ! – WARNING *** LEAK DETECTED ***   Yikes!   Hearts racing, we powered SUESI down since we didn’t want 250 A output current if there was a leak, even a small one. Then Steve said it *might* be a glitch, and so we should power it back up to see if the warning comes up again. So we did, and the warning went away. Several minutes later it came back. Steve called his engineer back at Scripps and we learned the command we could use to toggle the leak detector on and off. We came up with a plan: we would toggle the detector off and then back on every time that message came up. If the warning came back on immediately then we definitely had a leak; if it was intermittent then we either didn’t have a leak and the detector was glitching, or the leak was just a tiny amount of water.  Over the course of the next few hours the message came on only sporadically so we decided the problem wasn’t significant and we carried on with the deep tow. We had already completed towing from the shallow continental shelf down into the trench and then had completed the two deepest crossing profiles on the 3D array, so we were feeling good about that, but we were still feeling delicate about the transmitter. I went down to the lab to check in with Eric while he was watching SUESI’s backup terminal screen. That’s when I heard a loud pop noise in the aft lab where SUESI’s  top side power supply is located.  I went to check it out and noticed its voltage had jumped up from about 1680 to 1704 V, and then we noticed SUESI was no longer telemetering data back to the ship. Arrgggh!  So I made the proverbial “phone call to wake up Steve” and he came down to the lab. Pretty quickly he decided it was a problem with the power supply and not related to SUESI’s leak. Regardless, we still had to bring SUESI back on the ship since it would take time to fix the power supply. We were already using the backup power supply since we had blown a transformer in the first one earlier in the cruise, so we decided to beach SUESI and start recovering receivers.
SUESI being brought back up.
Here you can see where the 120 V to 2000 V transformer blew earlier in the cruise.
Old transformer out, new better transformer in. After opening up the other power supply, we  also found out that the popping noise I had heard was from a big fuse blowing on the part of the circuit that puts FSK telemetry on top of the 1700 V 400 Hz signal we put down the deep-tow cable.
The next day with SUESI back on deck and the power supply fixed, we powered it back up and sure enough, we were getting constant LEAK DETECTED messages, so we decided that this was no longer a glitch and was likely a bonafide leak. Dang it. Steve and Jake decided to try drying out SUESI’s insides. SUESI’s pressure case is too bulky and heavy to safely deal with on the ship so instead they opened up some of the seal screws and used a vacuum pump to move desiccated air through it while aiming a heat gun at the base of the pressure case to help speed up the rate of evaporation on the inside. After doing that for a long time, we fired up SUESI and it was no longer issuing leak warnings, so case closed – SUESI did indeed leak. Crap. That means it is no longer safe to deploy into the deep depths of the abyssal plain. And further, we were already using the backup SUESI since we had some significant corrosion issues on the first one after towing the first two profiles, so we are now thinking about contingency plans. The current idea is to deploy have the receivers on to the abyssal plain so that we can at least get passive (and much lower frequency) MT data there to look at lithosphere-asthenosphere boundary structure. Then we will take the other half of the receivers and deploy them up in the 80-100 m depths of the shallow continental shelf to collect MT data to look at the deeper crust and possibly the mantle wedge; we will also surface-tow SUESI’s antenna across the profile of shelf receivers so we can get shallower sensing CSEM data there. We’ve seen lots of water current noise on the receivers in shallow water, as well their compass data showing some of those instruments being spun by the water currents, so there’s no guarantee we will get useful data on the shallow shelf, but we still have a week of ship time left and as long as we have anchors available we will keep on collecting any data we can. Also to put things in perspective, despite the setback with SUESI’s leak, we have already collected a TON of super awesome CSEM and MT data. We’ve been able to get CSEM and MT data along two profiles spanning the abyssal plain and forearc and we towed the forearc on a 3rd profile in addition to collecting a mini 3D survey on the forearc slope. So with all the data we have in hand already, the EMAGE project is a huge success. It’s a bummer that we won’t be able to get more abyssal plain data, but we have enough data in hand that we will be kept busy analyzing it for the next few years.

Deploy, deploy, deploy

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.

We have several students and other volunteers on board who are new to marine geophysics, let alone seafloor electromagnetic imaging, so this morning Samer gave a lecture on how the seafloor EM receivers, the SUESI transmitter and other pieces of hardware work. Here he is explaining how we use acoustic ranging to triangulate the position of the receivers on the seafloor (see the post from two days ago for some data examples).
This little blue pressure case attached to the frame on the EM receiver contains a recording electronic compass, which we use to determine the instrument’s orientation on the seafloor (side note: the 316 stainless steel used on the frame behind it is non-magnetic so it does not distort the compass reading).
The receiver named Dogfish about to be deployed to the seafloor.
Another good sunset tonight.

Second profile done? Check.

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.

Day shift: Eric Attias, Li Wei, Christine Chesley, Janine Andrys and Peter Miller.
Day shift landing a receiver on deck.
Night shift landing a receiver on deck.
Night Shift: Tanner Acquisto (back left), Julen Alvarez-Aramberri, Samer Naif, Chris Armerding, Goran  Boren (blue hard hat), Brandon Chase and Jasmine Zhu.
LDEO graduate student Tanner Acquisto.

 

 

X marks the spot

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.

Three acoustic systems: ORE (yellow), Scripps (gray) and Benthos (blue).

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.

Acoustic ranging data for navigating the position of the seafloor EM receiver deployed at station 307.

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:

Christine nails the grapnel throw!
Bern trained Janine how to use the ship’s air tugger winch and here she is helping recover a receiver. The air tugger pulls in the line that goes through a block on the crane and then down to the recovery hook attached to the EM receiver being recovered (see one of our previous photos for a more complete instrument recovery scene).
Nice sunset tonight

Unintended seafloor sampling

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.

Goran watching one of SUESI’s copper electrodes being brought back on deck.
Reeling in the final section of SUESI’s antenna which is terminated with a 50 foot long copper electrode.
Some of the white corrosion on the aft half of SUESI’s pressure case on both the edges of the seal screws and the gap in the main cylinder.
The Vulcan towed EM receiver back on deck. Note the mud inside the nose electrode’s tube.
Plant that was scraped off the seafloor in about 1700 m water depth during the Vulcan’s low altitude event.

New world record!

We did it! Today we deep-towed SUESI to 5100 m deep in the Alaskan subduction zone, making this the deepest ever deep-tow of a controlled-source electromagnetic transmitter.  Woohoo!  Edit: the night team just pushed SUESI down to 5150 m! This beats our previous record of 5000 m set in 2010 at the Middle America Trench offshore Nicaragua.

We celebrated 5100 m earlier tonight but as I write this the night shift pushed the record to 5150 m!
We maxed out the amount of cable we’re allowed to deploy at 7600 m. We were told the RV Sikuliaq would have 9000 m of 0.680 cable for us to use, but when we were loading the ship in San Diego at the start of May, they informed us that the cable was damaged at around 7800 m and so we could only let out 7600 m. So we’re not able to go as deep as the trench is here (about 6000 m for this profile), but at least 5150 m deep still beats our previous depth record of 5000 m.
Eric, Steve, Janine and Jake watching the SUESI monitor as we approach 5100 m.
Janine and Peter stoked on the new world record for deepest CSEM deep-tow!
Jake and Li on watch while we tow across the deepest part of the trench.