Breaking Ice and Mapping New Ground
Jessica Robertson, U.S. Geological Survey Public Affairs Specialist
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
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Wow – running the treadmill while the ship is swaying? Pretty brave! Hope you are enduring the cold… stay bundled up – and please be careful and don’t go overboard!
So now you are hearing pings and chirps? And they let you keep your job? And dimples in the sea? Sure, Jessica. Sure.
I guess if the scientists say so.
Its fun to experience your learning experiences with you, tho. When you put your skates on and get out on the ice, make sure someone gets pictures.
Dad
Be sure to mention the GPS which will help all the measurements get placed on a map correctly.
@T. Mathiasmeier: The coordinates are actually on the “Follow the Journey” map at the top of the page. Clicking on the markers give you the coords and the dates are on each one. Very cool to watch.
What a great way to learn about the Healy Jessica! Thank you and good job!
Good morning, Jess! This is quite exciting – - I’m enjoying the blog. Keep up the great work!
Jess this is really great. I feel like I am there with you. Keep the exciting travel log going and have fun. We miss you. Be safe. Barbara
Dear Ms Robertson,
Could you include an occasional description of the sea ice that is being encountered (thickness, % of ocean surface covered)? Thank you,
“For example, a weaker gravity field will lower the water surface slightly, like a dimple on the sea.”
Actually a weaker gravity field locally creates a higher water elevation, more like a pimple than a dimple. The weaker gravity force at sealevel means that there is more depth of water or other low-density material below, and shows up as a higher local variation in sea surface height. The presence of shallows therefore little water underneath the observation area, or high density material appears as a dimple; the sea surface is locally pulled down.
Variations can range approx 100 meters +/- from the geoid or “where the water geometrically should be on a globe.” These highs and lows are not noticeable on the ocean surface due to the very gradual nature of the slopes.
See David Sandwell, Scripps Institute; & Walter Smith, NOAA.