POLENET project in Antarctica completes GPS/seismic array
Posted March 9, 2012
Talk to just about any scientist in Antarctica and eventually you’ll hear some variation on this refrain: We just don’t know that much about it. We need more data.
In part, the problem is related to the daunting logistics of doing research in Antarctica —the vast distances, the unpredictable, brutally cold weather. The sort of long-term datasets that give scientists a warm and fuzzy feeling about their conclusions are scarce and relegated to a few, regional locations.
This makes Terry Wilson’s efforts to instrument, in essence, all of West Antarctica in a bid to learn more about its ice sheet and the earth below, just that much more impressive.
Not to mention that much more difficult.
What had been projected to be an initial two-year effort to install an array of GPS and seismometer instruments across roughly a third of the continent is now in its fourth year. Naturally, the last four sites are the most difficult to reach, in a region known as Pine Island Bay, where most of West Antarctica appears to be losing ice, despite being an infamously stormy place.
“It’s been a big frustration,” Wilson said of the missing pieces of the West Antarctic array for the POLENET program. “Weather is the driver if we end up going somewhere.”
A geology professor at The Ohio State University, Wilson is the principal investigator for the National Science Foundation-funded project POLENET, for Polar Earth Observing Network. The project focuses on collecting data from GPS and seismometers at remote, autonomous sites in West Antarctica, as well as Greenland and elsewhere in the Arctic and Antarctic.
Up and down
POLENET has numerous goals; foremost among them is learning about how much ice is disappearing as the world prepares for what appears to be inevitable sea-level rise from shrinking glaciers and ice sheets.
But it’s not just about how much polar ice is discharging to the seas today. The scientists are also interested in calculating just how much ice once sat on the poles nearly 20,000 years ago when ice sheets had reached their maximum girth and extent during the last glacial maximum (LGM).
Antarctica’s mighty ice sheets were even more massive then, enough to depress the earth below. The ensuing loss of mass over the millennia has produced a post-glacial rebound, as the ground literally bounces back. Slowly.
That’s where the GPS instruments come in. These instruments can measure with millimeter-level accuracy the rate at which the bedrock of the continent moves vertically. Meanwhile, scientists can use the data from seismometers, which measure the seismic waves produced by earthquakes near and far, to determine the nature of the material that sits below the ice.
“The seismology gives us the earth properties. It tells us what’s under there, including what’s deep under there, which matters with the earth response,” Wilson explained.
“A really interesting part of the seismology is actually sensing the dynamics of the ice,” she added. “We’re finding some solid earth stuff, but it seems the majority of earthquakes that we’re recording are related to ice processes. It’s a different aspect of the project that’s going to be pretty exciting, because we’ve never had sensors in most of these places before.”
A model of exploration
While the nascent network represents one of the biggest efforts to emerge out of the International Polar Year — the 2007-08 global science campaign to study the polar regions in greater detail — the array of instruments isn’t just about monitoring Antarctica, Wilson insisted. It’s also about exploration, she said.
“Here we don’t have any idea of what we’re going to find, because nobody has measured it before,” she said. “We have no idea what the actual patterns and results are going to be. We have some predictions, we have some models, but those models are sometimes models of other models — things we had to resort to because we have no direct observations from these areas.”
Such as the post-glacial rebound effect. Scientists are counting on the POLENET data to fill in big data gaps that will help them create more realistic models of how the polar regions will respond to ice loss today. The data should prove particularly useful to a separate project involving a pair of satellites that measure gravity field changes on Earth.
The Gravity Recovery and Climate Experiment (GRACE) measures changes in gravity by making accurate measurements of the distance between the two satellites, using GPS and a microwave ranging system. GRACE is a collaborative effort between the Center for Space Research at the University of Texas, Austin, NASA’s Jet Propulsion Laboratory, the German Space Agency and Germany’s National Research Center for Geosciences.
Gravity is related to mass, so as mass grows or shrinks, such as with an ice sheet, the satellites can detect the changes. But one variable missing in its calculation is the rebound effect that POLENET measures, which entails mass flow in the solid earth beneath the ice sheets.
Scientists aren’t relying solely on space-based measurements of gravity. POLENET researchers are collaborating with French and Danish scientists who use ground-based instruments to measure gravitational changes over relatively small areas compared to the larger footprints covered by the satellites.
“It’s partly ground-truth for GRACE. It’s also an independent way of measuring uplift and mantle flow that’s related to rebound,” Wilson said.
The absolute gravity measurements are also important to separating the signals between the long-term post-glacial effect and the short-term “elastic” response of the earth from modern ice loss, according to Yves Rogister, an assistant professor at the University of Strasbourg in France.
“The GPS and gravity measurements are complementary,” he said.
Rogister and colleague Jean-Daniel Bernard brought down what’s called an FG5 absolute gravimeter, which looks a bit like a camera tripod on steroids, to McMurdo Station during the 2011-12 season to repeat a series of measurements they made two years ago.
Think about Italian scientist Galileo Galilei’s famous gravity experiment of dropping two balls of differing weights from the Tower of Pisa to get an idea of how the gravimeter works.
A vacuum is created in an upper chamber in which an object repeatedly free falls over a period of time. The acceleration of the object, measured using a laser interferometer and an atomic clock, is related to the pull of gravity, or the mass, of the earth below the site. By comparing this year’s measurements against those made two years ago, the scientists should be able to detect a subtle change in mass.
“We have the same value as two years ago, which means that gravity isn’t changing here much,” Rogister said of the preliminary data around Ross Island.
A little lighter
Unfortunately, the FG5’s extreme sensitivity makes it less than an ideal field instrument. The French scientists are limited to doing the experiment in established shelters at research stations like McMurdo, New Zealand’s nearby Scott Base and Italy’s Mario Zucchelli Station at Terra Nova Bay to the north.
A more portable version of the instrument brought to the Ice from Denmark isn’t so limited, according to Larry Hothem, a scientist from the U.S. Geological Survey who oversees the absolute gravity component of the West Antarctic-POLENET (ANET) study.
“It’s not as accurate, but it’s not as sensitive,” he said.
Jens Emil Nielsen, a scientist with the Danish National Space Center at the Technical University of Denmark, hopes the portable A10 gravimeter will prove to be a valuable tool. It takes maybe 20 minutes to set up inside a tent and can make its measurements in less than an hour versus the 24 hours required by the FG5.
Nielsen made a number of trips aboard either a helicopter or Twin Otter with the A10 to ANET sites accessed from McMurdo Station to test how the instrument would handle field conditions. If deemed successful, it will be flown to ANET sites in West Antarctica next year. Ideally, each deployment should take about three hours.
“It’s supposed to be a hit and run,” Nielsen said.
The abbreviated measurements allow more “noise” from tides and other disturbances in the A10 data, which means there’s more work for the scientists using the portable gravimeter on the back end of the study.
“These effects are not eliminated by the measurements,” Nielsen noted. “In that case, you have to do some extra processing.”
Upgrading the hardware
Developing hardier instruments for the harsh Antarctic climate has been critical to the success of the ANET array. Ten years ago, scientists struggled to keep the autonomous sites running through the austral summer, let alone the winter when temperatures can plummet below minus 70 degrees Celsius.
Now the ANET sites operate year-round. Iridium satellites allow scientists to stream the GPS data to home institutions, though the data from the seismic sensors are too big and must be collected manually every two years.
Even if a communications modem fails, there’s a second one available at each GPS station for redundancy.
“If it fails, by a phone call we can activate the other one,” Wilson said.
The improved design of the observatories and the use of high-powered lithium batteries has truly been a game-changer for remote data collection, according to Patrick Shore, field team leader from Washington University in St. Louis who was at the South Pole Station this season to service seismometers in East Antarctica.
The eight-instrument array was part of the AGAP project, for Antarctica’s Gamburtsev Province Project, a field campaign in 2008-09 that studied a mountain range under the ice sheet. Scientists used the seismometers, along with airborne instruments, to image the Alps-sized mountain belt, making a series of well-publicized discoveries.
A handful of instruments were left in the high East Antarctica plateau to continue to collect data. Shore and a small team of students from Washington University and Penn State visit the array every year to collect the data, which requires swapping out instrument boxes at each site. The lithium batteries are replaced every two years.
“I can’t change the batteries out in the field. It’s just too cold. As soon as I open that box, everything freezes, cables are frozen, you can’t move anything,” said Shore, who works on similar seismic networks around the world, from Madagascar to Fiji.
Shore said the AGAP array would likely be folded into the POLENET program by next year.
Patience is obviously a virtue when it comes to getting results from processes that unfold over years, centuries, even millennia.
But results are coming. Slowly.
For parts of the ANET array that have been operating for several years, the post-glacial rebound effect appears to be smaller than first predicted, according to Wilson.
In Greenland, where a similar network exists, called GNET, researchers recently reported a strong signal emerged in 2010 from the elastic response of recently melted ice.
Some GPS stations around Greenland routinely detect uplift of about 15 millimeters or more year after year. But a temperature spike in 2010 lifted the bedrock a detectably higher amount over a short five-month period — as high as 20 millimeters in some locations.
“It’s kind of incredible but true,” Wilson said. “You’ll see a trend in the uplift rate that will suddenly accelerate. That acceleration is a signal of the fast ice-loss that is going on today.”
It’s the sort of trend Antarctic scientists expect to see around fast-flowing ice areas like Pine Island Bay. If they can ever get there.
Wilson points to a map where four red dots stand out. The final ANET sites.
“Maybe after the end of this season it will be nice not to have any more red dots on this map. That will be a happy moment,” she said.
(Editor’s Note: As of Feb. 9, the ANET team had installed three of the four Pine Island Bay region sites, completing, for all intents and purposes, the array.)
NSF-funded research in this story: Terry Wilson and Michael Bevis, The Ohio State University, Award No. 0632322; Doug Wiens, Washington University, Award No. 0632209; Robert Smalley, University of Memphis, Award No. 0632339; Carol Raymond, Alberto Behar, Andrea Donnellan and Erik Ivins, NASA, Award No. 0632335; Andy Nyblade, Sridhar Anandakrishnan, Audrey Huerta, Penn State University, Award No. 0632136; Ian Dalziel, University of Texas at Austin, Award No. 0632330; Richard Aster, New Mexico Institute of Mining and Technology, Award No. 0632185; and Larry Hothem, U.S. Geological Survey.
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