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:
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.