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.
We finished deploying all 42 (err, now 39) receivers at 10:55 AM and then headed 10 nautical miles up the profile so that when we turned back around to tow SUESI down the line, we would be getting nice deep-sensing long offset CSEM data on the first receiver. SUESI was in the water and transmitting by just past 2 PM, and the rest of the day has quite peaceful and relaxing, including sunny skies and a pod of whales spouting about a kilometer from the ship most of the evening. Jupiter is visible just about the horizon again too. Photos and a video from today below.
SUESI being deployed for our second profile. We start deep-towing in the shallower water on the continental shelf and so unfortunately the visibility is a bit low compared to the deep ocean receiver deployment video I posted yesterday.
Around 3 AM last night we finished recovering the ocean bottom EM receivers deployed along our first profile and by 4:45 PM we were on the deep end of the second profile deploying receivers every with 30 minutes, with 20 already deployed as I write this at 1:45 AM. Some time after breakfast we should finish deploying the last receiver (39th) and then we will begin towing SUESI down the profile.
We have an awesome team of graduate students, postdocs, technicians and scientists that are doing a great job starting up data loggers, assembling and testing the receivers and making sure they get deployed safely down to the seafloor. BIG THANKS to all of you! Some photos and videos from today below.
Montage of video clips from receiver deployments in today's amazingly calm seas and clear skies. Stay to the end for the underwater view.
Two days ago I got the dreaded early morning phone-call wakeup, with Samer on the line hysterical that the RV Sikuliaq had just run over a second, yes SECOND!!!!$#@!!@#$, of our receivers, which was now stuck in the ship’s propeller and the data logger was dangling out of the receiver frame and about to fall away into the abyss. Uggh. I came out on deck just about when they freed the remains of the instrument and were raising it back onto the deck:
Miraculously we got both data loggers (and importantly the data they contained) but we lost an acoustic transponder unit, an induction coil magnetometer, two electronic compasses, and lots of cables to the sea. The instrument frames were destroyed, and we lost several electrode arms. The glass floatation floats where banged around enough that we can no longer trust their suitability for deep deployments. One of the induction coil magnetometers (a marine version of the EMI BF4 sensor) that made it back on board the ship was severely bent (and destroyed):
Ship handling for instrument recoveries isn’t easy but I’ve only seen a ship completely run over an instrument and destroy it a couple of times in over twenty years of going to sea. So two instruments destroyed in the same day was super upsetting, to say the least, especially since this happened on instrument recoveries #2 and #6 out of 42 total, which meant there were 36 more chances for instruments to get munched by the Sikuliaq. Morale was super low and that made tensions high for the rest of the instrument recoveries that day.
But all is not lost. We put the carnage of the morning behind us and the ship worked hard at more precise and careful handling. Now two days later we have only about 10 more receivers to recover. We’ve had a few hiccups with some of the ORE acoustic units on the receivers that were deployed for the first time in deep water on the incoming plate, and one stubborn receiver has refused to release from the seafloor, likely due to a malfunctioning ORE acoustic unit. But we now have lots of good data in hand and are cruising along, so things are looking up.
We started on the deep end of the profile and are now moving up the continental slope. We should finish picking up the last few receivers, located in the shallow waters of the continental slope, later tonight.
Here’s a video of a receiver recovery from yesterday afternoon in what could be called a text-book recovery with slow and careful positioning by the ship and efficient and careful handling by the deck crew.