21 January, 2004
At midnight we deployed the mutichannel seismic streamer. The streamer is a complex bundle of wires, microphones and electrical components all bundled into a 1500-meter long cable. With the aid of "birds" that connect to the streamer and have controllable wings, the scientists are able to level the streamer at a depth of twenty feet below the surface. The streamer stretches out behind the ship like a yellow and orange stripped snake. It is essential that the streamer stay as straight as possible during the data collection.
The streamer is really a series of microphones (hydrophones) positioned a known distance apart. They record the echoes produced by the surface and subsurface features of the Earth's crust. From these echoes the geoscientists can peer into the layers of the crust looking for faults, volcanoes, and other structures. The echoes reflect back to the microphones as different rates the deeper that they penetrate into the crust. These differences in time represent a difference in depth and in the physical characteristics of the rocks and sediments that make up the crust in that area.
To produce the reflected sounds, high-pressure air cannons operate at 2000 lbs per square inch. When the cannons are fired the produce a low frequency pulse that travels down through the seawater and is reflected by the different layers in the crust back to the microphones in the streamer. There are six air cannons in an array that are towed behind the ship below the surface. They are connected to the ship by cables, electrical wires and hoses. They are attached to six buoys so that they do not sink too low below the surface.
We are in normal watch standing mode for the first time. Each team of three rotates between recording watch data, beam editing and Marine Mammal Observing. No whales or seals were observed today, only a few penguins on icebergs that floated by the ship. Changing duties makes the time pass by very quickly, and the interludes are filled with discussions about the maps and charts that are being made from the initial data.
Ashley Lowe (MPC) gave us a briefing on working on the back deck. The marine techs are very willing to show us how the equipment operates and have invited us to help them in retrieval and deployment of the equipment during the course of the cruise. There is a cooperative spirit between the science team, the marine support and the crew. Everyone is made to feel welcome and important to the success of the cruise.
The size of B15A is amazing. We transit for hours and the berg is always in our portholes. At over sixty miles it is like a small state floating in the ocean here. Clouds form locally over the iceberg, which is apparently massive enough to create its own weather. For us the seas have been calm and relatively ice free.
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