Breaking Ice—Like A Hot Knife Through Butter

Over the past couple days, there have been several cancellations and delays in helicopter flights between ships due to surrounding fog and potential for ice to build up on the helicopter blades. Also, both Louis and Healy have had a couple moments caught in the ice, but both ships have been working together to keep things moving and data rolling in. While I am enjoying the company onboard, I don’t think anyone wants to be stuck in the Arctic Ocean!

For both ships to collect accurate data, we have been alternating positions with Louis in leading and breaking through the ice. Seismic data needs to be collected at a slow speed, but for the ship to break ice, it needs to move at a faster pace. Therefore it is beneficial for one ship to lead at a slightly faster pace when going through ice while the other ship follows at a slower speed to collect accurate data. In addition, both ships use sound systems and energy waves for data collection and the sound of ice breaking can interfere and alter the data. When Louis leads the path, that interference is reduced for Healy and vice versa. As I mentioned in a previous blog, Louis is using streamers and buoys that trail behind their ship to collect data, so using Healy’s guided path helps prevent them from getting caught in the ice.

When I first heard of an ice breaking ship, I thought they plowed through the ice by ramming straightforward into it. My assumption was wrong.

The ship’s weight and generated momentum are essential characteristics that make it an effective icebreaker. Healy generates a maximum of 30,000 horsepower with two shafts and Louis produces 27,000 horsepower with three shafts. The bows on both Healy and Louis are designed with an arch, much like a spoon, so they can ride up on top of the ice, pushing it down and out the sides of the ship. Both ships also have what is called an ice knife. This isn’t the type of knife you might immediately imagine. It is basically a projection of solid steel (or wedge) located below the bow that helps prevent the ship from riding too far up onto heavy ice and effectively beaching itself or having ice travel under the hull to the propellers.

Low friction inertia paint, which provides a slick surface, is used to coat the entire underwater portion of ice breaking ships up to a few feet above the waterline. This allows for a smoother transition through the ice, helps keep buildup such as seaweed off the ship, and allows the ship to use less energy and fuel. In addition, ice breaking ships are constructed with thicker hull plating and heavier “web” frames in the bow area. The distance between the frames is reduced in the bow and stern areas for added strength. There is also an ice horn, which is basically a block of steel under the stern and behind the rudders and is there to protect the rudder when the ship is backing up in ice.

Some differences do exist between Louis and Healy. For example, Healy has a bow wash system that draws water from under the ship and distributes it onto the ice through nozzles along the ship’s hull. As a result, the ice is flooded and it is easy for the ship to move forward. This is especially helpful when snow, which offers greater friction on the hull, is present. The water helps reduce the friction that would otherwise slow down the ship and its momentum.

Louis also has a unique feature known as a bubbler system. High pressure air is discharged through underwater nozzles, thus providing a lubricating film of water and air between the ice and the ship’s hull. Louis also has a water ballast heeling system. Water in the vessel’s port (left) and/or starboard (right) ballast tanks can be quickly transferred from tank to tank to make the ship rock and break free if it becomes stuck in the ice. It is used only occasionally, and, so far, not at all this trip.

More from the beautiful Arctic Ocean soon!

Jessica Robertson

Making Bubbles in the Ocean with an Airgun

We saw a polar bear and cub today! First we saw the tracks, and finally we spotted them. They were about two miles away, so I couldn’t get a close-up picture. Those on the bridge were peering through their binoculars trying to tell the crowd gathered together on the bow where to look. It took a while to find them, and when we did, it was truly spectacular!

Now, on to science. I learned today that Louis is using airguns, which create an acoustic sound signal under water, to image the geologic structure of the sub-seafloor. Even more interesting, scientists used to use dynamite to generate this sound source before airguns were developed! The airgun pulse is heard by everyone onboard Louis and shakes the fantail when set off. These airguns are also called air hammers or pneumatic sound sources.

So how do the airguns work? The gun has two chambers—a release chamber with vents at the top and a pressure chamber underneath. The pressure chamber contains compressed air at a pressure of 1,800 pounds per square inch. A piston is located between the two chambers and when triggered, it shoots into the release chamber, letting air out of the pressure chamber and into the ocean through the vents. The piston slams down and closes before water can come in. This whole process takes about 10 milliseconds. There are three airguns towed behind the ship at one time and they are fired about every 20 seconds.

The resulting air bubble emits energy waves into the seafloor. A signal bounces back, much like an echo sounder, helping image the underlying geologic structure. So, how are the resulting signals recorded? Louis has a streamer about 100 meters long trailing behind in the ocean. Within the streamer are 16 hydrophone channels, which listen for the waves’ return signals. Scientists also deploy sonobuoys behind the ship, each with an attached hydrophone. The hydrophone transmits the signal from the airguns back to the ship by radio. The sonobuoys drift freely behind the ship for several hours before they self-scuttle and sink to the bottom.

In a previous blog, I discussed how Healy is also imaging the geologic structure of the Arctic sub-seafloor. What’s the difference between the instruments on the two ships? The sub-bottom seismic reflection profiler, which is used on Healy, emits lower energy waves and can reach at most 100 meters into the sediment. The airguns, however, can penetrate 100 times further, reaching through sediment up to 10 kilometers thick.

At one point yesterday, Louis was stuck in the ice, and one would think they could just back up and ram forward to break through, right? In this situation, however, that would not be the most productive solution. Since the streamer is behind the ship, backward movement would tangle it with the propellers and disrupt data collection. In the end, Healy altered their track, made a circle around Louis, and set it free. This shows one of the many benefits of this joint expedition, as they will help us if put in a similar situation.

From the cold,

Jessica Robertson

Canada In … Daylight Out

The Louis is here and our joint expedition is underway! On Tuesday afternoon, everyone gathered outside to see them slowly appear through the fog. Our ship was filled with excitement as this was a change from the sight of endless sea ice.

Healy is cutting through the ice in front of Louis so they have a clear path to collect data. Louis has already been at sea since August 21, so I can only imagine their anticipation to see another ship in this remote area. As the flight crew prepared for the Canadian helicopter to join our ship, many waited on the flight deck anxious to see our visitors.

USGS scientist Deborah Hutchinson, Captain of Louis Mark Rothwell, and Canadian Geological Survey scientist Ruth Jackson, also Louis chief scientist, were greeted with smiling faces and lots of handshakes as they arrived on Healy. They were quickly whisked away and briefly met with Frederick Sommer, Captain of Healy, the rest of the operations crew, and USGS scientist Jonathan Childs, also Healy chief scientist, to discuss procedures from this point forward.

Currently we are traveling around 4 knots as Louis follows anywhere from 1 to 3 miles behind us. One knot is 1 nautical mile per hour and that is equal to approximately 1.15 miles per hour. We plan to have several more helicopter trips between ships during the joint expedition, and will hopefully have our internal network and phone systems linked together soon.

On a side note, I have been collecting video footage of our journey so far in collaboration with Michael Anderson, public affairs with the U.S. Coast Guard. When we aren’t shivering from the cold outside, we are running—well walking since running isn’t allowed—around the ship to make sure we don’t miss anything. At one point I was leaning over the bow capturing video, but don’t worry: Mike was right there for security. So that shows our dedication to document every aspect of this expedition!

Many of you have been asking me about the extent of daylight while we are here, so I thought I would mention it in my journal today. Last night the sun set at 11:33 p.m. MDT and rose this morning at 6:38 a.m. MDT. We are losing a little over 20 minutes of sunlight a day and starting September 21, the sun will be below the horizon longer than above.

Our science team is separated into two twelve hour monitoring shifts, and the extended periods of daylight have definitely altered their perception of time. One group monitors data from midnight to noon and the other group is on duty from noon to midnight. After some sound sleep—well, with interruptions from the ice breaking—I usually go to the computer room to visit the night crew toward the end of their shift and I can tell they have a little ways to go before they completely adjust.

If you want to follow our journey in near real time, you can view the ship’s aloft camera. It is updated every hour and can be found at http://mgds.ldeo.columbia.edu/healy/photos/aloftcon/2008/. You can also view a slideshow of the images at anytime from the menu on the right.

There is still a lot to write about, including ongoing sea ice observations, updates on data collection, life onboard Healy, and more! So, don’t forget to check back for later posts!

Until next time,

Jessica Robertson

Breaking Ice and Mapping New Ground

I had to catch my balance when climbing down my bunk ladder yesterday morning as our ship was swaying back and forth due to the wind and ocean waves. I peeked out my window to see what was going on and there was my first glimpse of bright blue sea ice. As the day progressed, we traveled through thicker ice, bringing colder weather and a bit more shaking. This made my run on the treadmill quite the adventure, but I was up for the challenge!

I ventured toward the labs yesterday afternoon and sat with some of our science team asking them for a rundown of each of the data collection systems. I was informed by USGS scientist Bill Danforth, who was on watch duty during my inquiring state, that we are using three major systems to collect data. So, here’s an overview of what I learned.

To map the seafloor, scientists are using the multibeam echo sounder. Some of you may be wondering, as I was too, what that means exactly. This system emits a sound pulse or “ping” into the water beneath the ship and onto the seafloor. That signal bounces off the seafloor and transmits an energy wave back, providing data on the seafloor depth. The deeper we are, the longer it takes for the signal to reach the floor and come back. Unlike other “single beam” echo sounders, the multibeam system’s sound pulse is received back at the system as a series of “beams” that are translated into depth data points across a wide but narrow swath of the ocean floor.  This allows scientists to collect anywhere from 61 to 121 depth points at a time along a narrow line perpendicular to the ship’s direction of travel for each ping. This process repeats every 10 seconds in the depths we are working in (3500 - 4000 meters), and the result is compiled into a map that shows the depths of the ocean floor along the ship’s track in a swath up to 8 kilometers wide in some areas.

Another system is the sub-bottom seismic reflection profiler, which essentially measures the geologic structure of the sub-seafloor. To do this, a sound pulse is transmitted that penetrates through the seafloor. A strong return, or “reflector”, shows up as a black line in the data profile, and indicates a change in composition of the material beneath the seafloor as the sound travels through the sediment layers.  Many of these reflectors can be present; the orientation of these reflectors to each other help the scientists to interpret the geologic history of the area are studying.

So, what do these pulses sound like? There are two sounds that can be heard by the human ear. The multibeam sends out a soft ping (12 kilohertz) and the sub-bottom profiler emits a chirp sound (3.5 kilohertz).  While I can hear both these sounds faintly throughout the ship-whether in my room, the science lounge or the mess hall-some people can’t hear the ping at all.

My roommate USGS scientist Ellyn Montgomery and I stood in our room for awhile as she closed her eyes attempting to hear the sound. She was not successful, but I am sure she will keep trying!

The third thing we are measuring is the surrounding gravitational field using an instrument called a “gravimeter”. To us, the gravity field of the earth feels constant, but the composition, structure, and density of subsurface materials affect the gravitational field of the earth very subtly. When a change in the gravity field is detected by the instrument, it suggests a change in the geologic subsurface beneath us and provides insight on the type of rock and structure of the seafloor that may be present.  A higher density material, such as volcanic lava or basalt, will have a higher gravitational force than a lower density material such as sedimentary rocks like shale or limestone. The gravitational field can also impact the ocean’s surface level. For example, a stronger gravity field will lower the water surface slightly, like a dimple on the sea.

So that’s the brief rundown. Thanks to everyone for your kind emails and messages on the blog site! I look forward to hearing more of your comments and updates of life back on land.

More from the Arctic coming soon!

Jessica Robertson

And We’re Off!

The deployment of Healy is now officially underway! As I write, we are headed north and expect to be in the ice tomorrow.

Most of our science team spent the night at the Barrow Arctic Science Consortium Thursday and woke up bright and early the next morning to head to the hangar. We were picked up from our huts and piled our bags in the van, said farewell to Barrow, and anxiously waited our turn to board the helicopter and arrive on Healy.

The fog almost left half of us behind to board the next day, but thankfully it let up. As this was my first time in a helicopter, I was a little scared beforehand. I sat in the front seat next to the pilot and I have to admit the ride was a lot smoother than I expected.

As the view of Barrow drifted away and the sight of our boat slowly approached, it finally sank in that the journey was about to begin. We spent the night at sea, and three more people arrived in Barrow and boarded the ship today.

The U.S. Coast Guard has been welcoming in every way. We ran through man overboard drills this afternoon and were briefed on the protocols of the ship. Our science team met throughout the day to discuss the science plan, schedules for monitoring stations and expectations. There have been several introductions, lots of card games, and we are ready to start collecting data.

If you want to send me a question or comment, you can email me at jessica.robertson (at) healy.polarscience.net. That address will only work while I am on Healy though.

Until next time

Jessica

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