An Airborne Look Through the Ice (cont.)
Posted February 24, 2015
“The premise behind it is that there are different instrument bays and they are interchangeable so you can put different instruments in for different projects,” Tinto said. “We developed a suite of instruments that would be good for ice surveying.”
Photo Credit: Kirsty Tinto
The Rosetta team flew 17 successful flights over the Ross Ice Shelf with the IcePod this season. When the grid is filled in, researchers will have an unprecedented cross section of the ice shelf and sea floor.
To get a complete picture of the Ross Ice Shelf, the team installed two ice-penetrating radars, a LIDAR, a system that measures distance with lasers, and a magnetometer inside the pod. Inside the plane two gravimeters map the gravitational pull of the ocean floor. Using these instruments the team can chart the surface of the ice, profile a cross section through it, and map the seafloor.
The pod’s antennae transmit radio pulses down into the ice, and the researchers can produce a cross-section of the ice based on how the signals reflect back. The two radars are calibrated to penetrate through to different levels of the ice. The pod’s shallow ice-penetrating radar yields information about ice nearest to the surface, while the deeper-penetrating radar reveals what the base of the ice looks like.
LIDAR works much the same way, except instead of using radio waves to penetrate through the ice, it shoots lasers down to record the contours of its surface.
“It sends pulses to the ground, [and] it gets back a range,” said Sarah Starke, a research assistant at LDEO. “We get about a point per square meter in LIDAR.”
In addition to mapping the surface, these LIDAR data add an extra dimension to the radar profiles.
“When you start to see interesting features within the radar, its nice to be able to go back and look at what the surface looks like,” Starke said.
To look past the ice to the rocky seabed, the team is using a pair of gravimeters that measure the subtle variations in the pull of gravity from the changing features below.
Photo Credit: Michael Lucibella
While on the ground, the IcePod is retracted close to the side of the LC-130. Once the plane is in the air, the arm extends and the pod starts collecting data.
“The change in the shape of the seafloor will create a gravity anomaly because when you have a mountain on the seafloor you will have an excess of mass,” Caratori Tontini said. “There’s more gravity basically.”
The machines are sensitive enough to tell when gravity is pulling harder because of an undersea hill, or weaker from a trench. But the effect is subtle, so much so that the signal the team is looking for could easily be lost amidst the noise of the constantly moving airplane.
“The accelerations that the plane is subject to are so strong, that they are several orders of magnitude larger than the gravity anomaly that you are looking for,” Caratori Tontini said. “Here is where we really have a very accurate knowledge of the motion of the plane so that we can subtract the motion of the plane from the gravitational data.”
Throughout the entire flight, a GPS unit records the plane’s location ten times every second. The measurements are so precise that the team can extrapolate from the GPS’s location data exactly when the plane turned, changed speeds or bounced around from turbulence. The team can then subtract those changes from what the gravimeter recorded, leaving only the gravitational anomalies created by the changing undersea landscape. The project’s two gravimeters are too large to be housed inside the pod, and are carried inside the plane instead.
Caratori Tontini works for GNS Science based out of Wellington New Zealand, which supplied one of the two gravimeters used in the project. New Zealand researchers have used his detector for a quarter century, mostly for surveys of the ocean floor aboard ships at sea.
“About five years ago we decided to try the gravimeter on an airborne platform, so we did a full survey of New Zealand,” Caratori Tontini said. “We collected a pretty accurate and high resolution survey, like what we’re doing here at the moment, but over New Zealand.”
The one owned by Columbia is newer and hasn’t been field tested nearly as much. ROSETTA is, in part, a chance to test its performance.
Photo Credit: Michael Lucibella
Fabio Caratori Tontini monitors the two gravimeters on board the LC-130 flying over the Ross Ice Shelf while Chris Bertinato adjusts a switch to the IcePod control panel.
“Because gravity is such a fundamental part of the project, we wanted to measure it as well as we possibly could, so we had the two gravimeters that fly side by side each other,” Tinto said. “So we have really good confidence in the tried and true meter, and the new one working together at the same time.”
Inside the pod is a magnetometer registering the strength of the Earth’s magnetic field directly below the plane. It effectively measures how much iron is imbedded in the rock, which shows when the rock type changes. These readings act as a check to make sure that the gravitational pull the gravimeters record isn’t being thrown off by different densities of rock.
Much of the project is about setting up ways to recheck and recalibrate data as they are collected. The dozens of flight paths that the ROSETTA team planned out line up with the locations of the RIGGS measurement taken in the 1970s. This way the team can compare their flyover data with the existing measurements.
“Even though they’re sparse, they provide ground truth tie-points every 55 kilometers along the track,” Tinto said. “You can’t go very far out of line before you know to pull yourself back into being true.”
This year the team completed 17 successful flyovers of the ice shelf. They’re hoping that they’ll be able to finish the remaining flights next season.
Though the team has only begun to process the many terabytes of data they collected over the season, they’ve already started finding features below the ice that had never been seen before.
“We’ve started seeing the smaller, finer picture in the big picture… In between the 55 kilometer points that we knew before, there’s definitely some local smaller scale things that will be pretty interesting to map,” Caratori Tontini said. “There’s definitely more than meets the eye.”
NSF-funded research in this article: Robin Bell, Christopher Zappa, Nicholas Frearson, Kirsteen Tinto, Indrani Das, Award no. 1443534 ; Robin Bell, Nicholas Frearson, Christopher Zappa and Michael Studinger, Columbia University, Award No. 0958658 ; and Robin Bell, Nicholas Frearson and Christopher Zappa, Columbia University, Award No. 1444690 .