Speed 8.4 knots (kts) Course 56° Location Canada Basin, 79.06° N, 144.18° W Depth 3811 meters
SPECIAL FEATURE DISCUSSION:
(see previous journal for the questions.)
The Canadian Coast Guard helicopter, a MBB BO 105, carries special gear for flying out alone over the Arctic ice. See which of your safety ideas match with the Canadian Coast Guard's!
First, this is a twin-engine helicopter so if one engine fails the aircraft can still fly (if not back to the ship then hopefully at least to a safe landing spot.) The skids have inflatable floats in case the ice isn't thick enough for an emergency landing. The pilot and passengers wear inflatable life jackets with a laser flare in one pocket and a personal locator beacon in the other. We wear exposure suits (aka Mustang Suits) as well. There is an inflatable life raft in the helicopter right in front of the rear center passenger, and the craft carries an Arctic survival kit including food, a satellite phone, extra arctic clothing & sleeping bags, and a shotgun for polar bear defense.
TODAY'S JOURNAL:
The ship's name sign on the Louis S. St. Laurent.
One of the primary reasons for my visit to the Louis yesterday was to see firsthand the science systems they have on board, particularly their seismic gear. One of the primary interests of this joint US-Canadian expedition is to characterize sub-seafloor geology in the Arctic Ocean north of both countries. But how can we 'see' into the sea floor without stopping to core or drill? A powerful technique called seismic reflection profiling is one way to peer beneath the sea bed. The concept is to send powerful sound waves down from the ship and to then record the waves' return as they reflect off the bottom and the different layers of sediment and/or rock underlying the sea floor. One difference between the Louis' gear and ours is that they produce lower frequency sound waves that can penetrate much farther into the sea bed then our subbottom profiler can. (Our subbottom profiler generally penetrates tens of meters below the sea floor while seismic reflection profiling can return data from many kilometers beneath the sea floor.)
The Canadian Coast Guard Cutter Louis S. St. Laurent and US Coast Guard Cutter Healy in the distance as our helicopter approached for landing on the Louis.
The sound waves are generated using compressed air that is sent to a weighted sled towed behind the ship about 10 meters deep. The sled distributes compressed air to three air guns at nearly 2000 psi. They release coordinated bursts of air that sound like a muffled boom when you are standing on deck. That impulse of acoustic energy travels down to the sea floor and into the layers below. Every time there is a change in material density (for example, between sea water and sea floor or between successively deeper layers), some of the sound energy is reflected back upwards and some continues to penetrate. The energy that continues through an interface changes direction, or refracts.
The Louis' seismic sled and three air guns (the silvery cylinders hanging from chains) are brought aboard for repair. A spare is also kept ready nearby under the red cover on the left.
Sending sound waves down does no good if they can't be detected upon their return to the surface, so a long streamer containing hydrophones (underwater microphones) is towed behind the sled. The Louis' streamer is actually fairly short by seismic system standards at about 200 meters long. Some seismic streamers are up to 8 kilometers long!
The Louis' seismic streamer is coiled in a figure 8 on the fantail while the sled and air guns are readied for re-deployment. The blue part on top contains the hydrophones and the yellow part below that is a leader that runs between the sled and the streamer.
The hydrophones digitally record the complex reflected and refracted sounds coming back to the surface, creating massive data files. It is then the job of computers to sort out this seismic data and put the information together into virtual cross-sections of the sea bed. The problem is that using the streamer data alone can only record sound return times. Scientists are interested in determining the thickness of the layers beneath the sea floor but sound travels at different speeds through different layers (going faster through more dense materials.) A way to determine sound speeds through the various layers is needed to determine their thickness.
Associate Team Chief Scientist Jonathan Childs shows the Louis' spare seismic streamer which is kept on a spool on the fantail.
There are several components within the clear streamer tubing including hydrophones for recording returning sound waves. The device in the black netting is a hydrophone. Everything is bathed in special oil inside the streamer to make it neutral buoyancy (it neither floats nor sinks in typical ocean water.)
To determine sound speeds through sub-sea floor layers a device called a sonobuoy is periodically deployed into the sea behind the Louis. It essentially stays in one place and also records sounds from the air guns as the Louis moves away. That data is sent back to the ship by FM radio for about 6 hours from each expendable sonobuoy. As the sound source (the ship) moves away from the sonobuoy the angle of the reflected sounds reaching the sonobuoy from sub-surface layers changes. By comparing the known ship-to-sonobuoy distance to the sonobuoy's changing return times from the sub-sea floor layers, the computer can calculate sound speeds through the various layers, allowing sediment and rock thicknesses to be accurately determined.
Associate Team Chief Scientist Jonathan Childs shows NOAA Teacher At Sea Caroline Singler an expendable sonobuoy which is used to determine sound speeds through sub-sea floor layers, allowing accurate interpretation of their thicknesses from reflected seismic data.
SPECIAL FEATURE:
What natural hazard generates seismic waves that in some cases are capable of damaging or destroying buildings?
How else can scientists find out about the layers found beneath the sea floor?
That's all for now! Best- Bill