GNET measures Earth bouncing back By Marcy Davis
During the last ice age, ice sheets up to three kilometers thick covered the areas of northern Europe, Asia, and North America. The enormous weight of the ice down-warped the land surface, causing the material underlying these land masses to slowly flow away. The physics are similar to the reasons a balloon deforms when you press down on it. In the same way that the balloon reshapes itself when you remove your hand, the land actually pops back up over several thousand years when the weight of expansive ice sheets no longer weighs down the underlying area. In Greenland, where the ice sheet is currently receding, the island is actively rebounding.
And members of the GNET team are watching it happen.
GNET (Michael Bevis, The Ohio State University, PI) is part of POLENET (Polar Earth Observing Network), an IPY effort involving scientists, engineers, and students from 24 countries to create and maintain continuously recording land- and ocean-based geophysical laboratories in remote polar regions. POLENET’s autonomous sites run on solar power during summer months and wind power and batteries during winter months. An interdisciplinary instrumentation infrastructure will allow scientists to collect and exchange new data related to Earth’s structure, magnetic field, crustal deformation, and geochemistry, as well as to the interactions between the atmosphere, oceans, and polar ice-sheets.
“One of POLENET’s overall aims is to investigate systems-scale interactions between the polar Earth system and the cryosphere,” explains Mike Willis, a Postdoctoral researcher from The Ohio State University and one of the GNET field team leads. “It’s critical that we understand the contribution to rising sea-level due to changes in the mass balance of the major ice sheets of the world, most importantly the West Antarctic and Greenland ice sheets. GNET will provide that understanding for Greenland.”
Before 2007, Mike had never been to Greenland. He spent the previous eight winters working at the other end of the planet on the Trans-Antarctic Deformation (TAMDEF) Project, which studied bedrock motion in the Transantarctic Mountains of Southern Victoria Land using a network of Global Positioning System (GPS) instruments. Now, Willis is bringing his experience in remote-location GPS installation to Greenland.
In total, the GNET team hopes to cover most of the coastal areas of Greenland with some 40 new GPS stations installed approximately 100 km apart.
During the first GNET phase in the summer of 2007, three field teams of two to four people deployed instruments via helicopters during a series of day-trips over three weeks. The team focused on three areas: northwest Greenland between Upernavik and Petermann Glacier, south and central west Greenland, and southeast Greenland. To deploy the GPS instruments a team first stuffs as much gear as possible into a helicopter. If weather conditions prove favorable (GNET’s biggest challenge according to Willis), they take off to scout out viable installation locations.
“An ideal GPS site is on unfractured, strong, dense bedrock which has a good view of the sky. We really don’t want any blockage on the horizon, which obscures our satellite antennae or our solar panels (which must face south), so we go for the highest point we can find with good bedrock. It’s also good to be able to get a helicopter nearby (the sites are HEAVY). Furthermore you don't want a site which is going to be buried by snow,” explains Willis.
When an appropriate site is located, the helicopter lands and one person sinks four expansion bolts into the bedrock. Next, the team uses epoxy to secure the bolts and attaches an aluminum plate and tube, creating the GPS base, or monument. The GPS antenna is mounted atop the monument, facing northward. In the meantime, other team members build a modular frame which houses Iridium satellite modems (that will transmit data to UNAVCO every couple of days), Trimble NetRS GPS receivers (which record data at a 30-second sample rate) and associated instrumentation, battery banks, wind generators and 360 watts of solar panels that provide power to the unit. Following extensive testing, the team further secures the installation with rock pins and cargo chains. Scientists and engineers will return periodically for system maintenance as needed.
A team returned in 2008 to install further sites in the north; GNET hopes to complete installations this summer on the eastern coast. Seismic and gravity instrumentation eventually will be co-located.
GPS receiver figures its position by measuring the distance between itself and at least four satellites in a network of 24 Earth-orbiting satellites. It does this by measuring the time delay between microwave signals, which are also embedded with location information, sent by the satellites. Using the known locations of four satellites, and the measured distance between the GPS and each reference point, the GPS location is identified. In this way, each of the GNET stations will continuously pinpoint station location, and, therefore, the underlying bedrock location. Over time, a picture of Greenland bedrock motion will emerge.
“We can expect to start seeing answers to some questions, such as ‘what is the elastic response of the bedrock to ice mass changes?’ after a few months; but for big questions, like ‘how is the ice mass of the Greenland Ice Sheet changing on an annual to sub-annual time scale?’ or ‘are ice mass changes accelerating?’, we will need about 2.5 years of data collection,” says Willis.
GPS stations will measure Greenland’s post-glacial rebound – how the land moves in response to changes in the ice sheet’s weight. As the Greenland ice sheet melts, ice mass is lost and the land rises as a result. Collaboration with glaciologists like Kees van Der Veen (Kansas University) and Bea Csatho (The State University of New York at Buffalo) along with other scientists interested in using the data for seismology and gravity studies, will help Willis and his colleagues tease out bedrock movement due to changes in the ice sheet from movement due to tectonic processes.
“I couldn't have done this project without doing TAMDEF before. I have a greater appreciation of the logistics side of things and how to actually build the sites.” Willis says of his field experience in Antarctica. “I learned to be much more retentive and how to adapt to things when they go wrong! I also learned how to armor the sites against unexpected weather, which is good for Greenland. Make 'em bomb proof in other words.”