Out of the water and back in again

Map of progress. White dots show ocean bottom seismometers from two transects. Solid black line shows profiles were we have finished OBS data collection. Dotted lines show transects where we plan to acquire data.

Over a four day period, we picked up nearly all of the ocean-bottom seismometers from our first transect along the Aleutian volcano chain between Seguam and Gareloi Islands and put them back out again on a transect across the chain near Atka. 

To summon the ocean bottom seismometers back from the seafloor, we steam to the location where we deployed them and send them an acoustic command to release from their anchor.  Without the anchor, the instruments are buoyant because they have glass spheres (and sometimes syntactic foam ) attached to them. We wait as they rise up to the surface at a speed of ~120-150 ft per minute. On our first transect, water depths were generally less than 1500 ft, so we did not have to wait very long for them to make the trip to the sea surface.  Once they reach the surface, the crew on the bridge can spot them because each OBS is outfitted with a flag and light.  Remarkably, the bridge crew maneuver the ship so that the OBS is right next to the starboard side of the vessel.  This is very impressive when considering how small the OBS is compared to the ship; imagine driving your car up next to a ping pong ball.   Our OBS teams hook them using long poles with ropes attached and then pull them out of the water using a crane. It takes careful coordination and skill by everyone involved, and I never cease to be impressed by the entire operation. 

WHOI OBS Team and Langseth crew deploy an OBS

Equally impressive, this bundle of electronics that we dropped into the ocean a few days ago usually comes back having recorded a trove of data.  We are happily pouring over some of the data now to get a first impression, and they look very promising! I will write a future post with some examples.  

Less than a day after we finished retrieving the OBS from the first line, we started putting them back into the water on our second line.  Sadly, we could not deploy as many seismometers as we had hoped because a storm is forecast to arrive in our field area in a few days, and we want to finish with the OBS work before then.  So it goes, but we are still excited about what will be recorded by the OBS on their second trip to the seafloor.

Donna Shillington, NAU

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