Sunday, June 2, 2013

Deploying seismic equipment on the high seas

After steaming for twelve hours out of port, we started the long process of putting out all of the seismic gear needed for this program. The weather worsened as we headed towards the field area, and we have been deploying equipment in 3-4 meter swells for the last 18 hours. This ship can roll by up to 10-15 degrees in these conditions (and more than a couple of people are feeling sea sick as a result).  On the deck, ropes and cables connected to equipment towed off the back of the ship lurch and clank rhythmically, and water commonly washes over the lower deck. Down in the main lab, stray items that aren’t properly stowed or strapped down start to roll back and forth, the ship creaks and groans, and office chairs swivel with each swell.

For this program, we will be towing an enormous amount of gear behind the ship to enable us to image faults involved in rifting, the exposure of mantle rocks and continental breakup in 3D.  Four 6-km-long seismic ‘streamers’ filled with pressure sensors (which can detect returning sound waves) will be towed 200 m apart, for a full spread of 600 meters.  As we deploy the streamers, we add weights to ballast the streamer, acoustic units to determine the locations of the streamers, and ‘birds’ that enable us to control the depth of the streamer remotely.  We also swap out broken pieces. The streamers are held apart by two gigantic paravanes, which are like large metal kites that fly out from either side of the ship. Each one weighs an astonishing 7.2 tons and is ~7.5x6 meters in size.   There are also myriad floats, cables and ropes to maintain the correct geometry of the entire array.  The streamers will record returning sound waves generated by two arrays of air guns, which will be towed 100 m apart and fired separately.  We expect that it will take us 3 days to deploy this complicated array of equipment behind the ship. The weather is expected to start improving tomorrow afternoon, which will help us greatly!

Donna Shillington
2nd June
Looking forward on the Langseth as she takes a roll in the swell.
A streamer with a 'bird' being deployed off the Langseth's stern into the waves.