OBS assembly

Yesterday, Ernie and Mark, the techs from Scripps, gave us an overview of how all the pieces of the OBS “package” get put together. In the spirit of making you feel like you’re here with us, I’ll try to describe some of the nitty-gritty of the setup. Be warned: it gets both nitty and gritty.

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Artsy photo of our OBS bodies (without the sensor ball, which will eventually be attached to that white arm you see folded up in travel position) sitting strapped down to the deck. Ready.

The sensor balls that contain the sensitive seismometers are stored in their own special case, from which they need to be removed and then attached to a titanium tripod. The titanium won’t corrode, you see. The sensor balls themselves are quite heavy, ~165lb, and require two people to lift: one on either side. We practised lifting them in pairs, marvelling at our strength only after we’d safely got them up and down in once piece. Once the sensor ball is bolted to the tripod, we can then attach it to the rest of the package. The main physical body of the OBS includes the bright yellow plastic flotation device, an anchor, the transponder that we communicate with, batteries, the data logger, and cables connecting all the electronics. In order to help us find it when it comes back up (hopefully not for another 15 months!) there is a bright orange flag, a radio, and a strobe light (in case it is a night recovery), all of which also need to be attached and secured. Of all of the components, the thing that was most surprising was how many there were!

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Ernie adding some finishing touches.

It was also interesting to learn about how each of the different components work, and specifically how they get turned on or off. For example, we’d like the radio and strobe lights to only be turned on for deployment and recovery. We have two different models of these components: one has a switch that can be turned on, but another is activated by a magnet. Once either of them hits the salty, conductive seawater, it manages to turn itself off after a few minutes, so as to save battery during the long wait on the seafloor. When the package surfaces at recovery and the strobe/radio is out of the water for a bit, it will turn itself back on. Very clever.

Another clever trick used for recovery is the detaching of the anchor. When we wish to recover the OBS, 12-15 months from now, the recovery team will send a ‘burn’ signal to the transponder, which will pass a current though a wire, causing a crucial pin to corrode quickly in the salty seawater. When this particular component corrodes, it enables the anchor to detach from the rest of the package, allowing the package to freely float to the sea surface. Prior to each of our deployments, Mark or Ernie will test this signal to ensure that it is currently working and sending the appropriate voltage. If it doesn’t burn through the wire a year from now, then there will be no way to recover the instrument, or any of the data that was recorded by it! This tiny detail, along with many others (e.g. whether the washers are titanium or aluminum), could make the difference between a successful project and an unsuccessful one. Everything is important.

Seeing how much work and detail goes into the set-up of these instruments for deployments and knowing how expensive they are makes me glad that Mark and Ernie will be around for each deployment to double-check our work and make sure that everything goes as smoothly as possible.

Stay tuned for more updates as we begin the actual deployments!

Karen Godfrey (on behalf of the Pacific ORCA science team)

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