Half-way point

It’s day 7 of the cruise. We deployed 15 OBS stations (out of 30 total) and are about to turn the corner to start sailing back towards Seward, deploying remaining 15 stations along the way.

Although I listened to many presentations describing OBS deployments and data, what was a big surprise for me on this cruise is how much time it takes to deploy each station. So, in my blog I would like to describe a typical site deployment sequence.

R/V Sikuliaq is ready for departure from Seward.

While each station is already packaged on it’s own platform and sitting on the deck ready to be deployed, there are still quite a few tasks that the OBS engineers have to complete before sending the package away. OBS engineers start with positioning preassembled package on the aft of the vessel. This can be done hours ahead of time. About 30-45 minutes before we reach the site (we can keep track of estimated arrival time through the ship’s navigation system), they start working on final assembly tasks: attaching the broadband sensor onto the ready rack, mounting release rods, securing ballast to the station frame, attaching and activating strobe lights, radio beacons, performing last minute tests of the equipment, making sure that all components are securely attached with zip-ties, bolts, etc. This can take up to an hour, depending on the weather and time of the day. Usually, all preps are completed by the time we reach the site.

WHOI OBS Engineers and R/V Sikuliaq crew members working on last preparations for the package deployment.

While OBS engineers are getting the package ready, R/V Sikuliaq crew members are on stand-by to assist with A-frame operations. Once the package is ready and the bridge gives a go-ahead, the package is carefully lifted with the crane, guided with the tag lines and gently lowered into the water.

WHOI OBS Engineers and R/V Sikuliaq crew members guiding package into the water.

In a mean time, the science watch party takes pictures and detailed notes on the location, time, depth and weather conditions during the deployment, as well as other pertinent information. As soon as the OBS crew reports that the package is away, the science crew activates transponders on the instrument and starts an algorithm to track the package as it sinks toward the ocean bottom. We use Edgetech Deck box to send pinging signal to the package and record pings that come back to the boat. Depending on the depth of ocean floor, it can take up to 2 hours for the package to come to rest on the bottom (it sinks with an average rate of 44 m/sec). All this time the vessel is being maneuvered to remain stationary on top of the deployment site. We don’t want to drift away too far before the package reaches the ocean floor.

Science crew is monitoring the package on its way to the bottom of the ocean.

While the science crew listens to the pings in the lab and keeps an eye on the transponder signals flashing across the screens, the OBS engineers bring out the next sensor package and secure it on the launching pad. They have to make sure that they use correct package since configurations vary depending on the depth of deployment (it ranges from 1,000 to 5,000 meters). They have packages labeled as green, orange and pink for easy identification. Once the engineers finish their initial prep tasks, they can leave and rest until we get to the next site.

Science party is monitoring M-Cal survey in computer lab.

In a mean time, the science crew keeps waiting for the package to get to the bottom, checking the computer screens and counting seconds between the pings. Once it gets there, the science crew springs into action once again. It’s time to do the survey! They activate another pinging sequence on the Edgetech box and fire up M-Cal program on the laptop that will triangulate accurate location of the sensor. The Chief Scientist will give directions to the bridge on how to navigate for the survey. Depending on the conditions, we may do a circle around the sensor’s resting place, or a square, or some other pattern to make sure we get enough pings from all 4 quadrants around the site. It’s a careful balance between frequency of the pings (not too many, not too few), ship speed (somewhere between 4-5 knots), distance to the sensor (about half-ocean depth, but not more than 1.5 nm) and wave action (works best in calm seas). And again, the science party listens attentively to the pings and tracks location of the sensor, adjusting parameters along the way. Once we get enough pings, we wrap up M-Cal sequence, record final metrics and call the bridge to start navigating to the next site. All in all, we may spend 3-4 hours at each site working through the deployment sequence.

And we repeat this sequence at each site, no matter time of the day. There is always someone on watch. Prep, drop, track, survey, sail, repeat…

Sunrise

By Natalia Ruppert

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