With global warming, glaciers are melting, and mountain ranges in the mid-latitudes such as the Swiss Alps are showing significant glacier retreat. For decades researchers have measured the length and area of glaciers to see if they are shrinking or not— a key symptom of disequilibrium— which can be done using photographs and satellites.
But a key indicator of a glacier’s health is the volume of the ice, and that’s impossible to calculate without knowing its thickness. To measure this, scientists can take advantage of advanced tools involving helicopters and radar, according to a recent study conducted in the Swiss Alps by Anja Rutishauser, Hansruedi Maurer, and Andreas Bauder and published in the journal Geophysics.
To map the ice-bedrock interface, researchers use ground-penetrating radar to go through the air and ice and then down to the rock so they can determine how far down the rock is. While it’s easy to measure the where the top of the glacial ice is, figuring out where it meets the rock below, and thus calculating its thickness, requires instrumentation. However, this is tricky because glaciers are in narrow valleys. So how do you get the equipment above the glacier? It’s possible to place radar directly on the glacier surface; this system produces high-quality images, but there are many places where it is difficult or impossible to gain access to the surface. And it’s cumbersome and expensive to move the equipment from one spot to another on the surface. As the paper states,
A major challenge in conducting ground-based surveys arises from the logistical and accessibility problems posed by rough and potentially dangerous terrain (e.g., crevasses). In contrast, airborne GPR systems are less affected by terrain challenges and have a high potential for rapidly investigating large areas. Most such systems used to investigate valley glaciers have been mounted beneath helicopters.
Because they can fly, helicopters can soar over tough terrain and cover a lot of ground, and offer a solution to the limits of surveys with radar equipment placed directly on the glacier surface. The authors discuss three different helicopter-borne ground-penetrating radar (GPR) systems. The first system, developed at the University of Münster in Germany, is a low-frequency pulsed system (BGR), the second system is a stepped frequency system, produced by a commercial firm (RST), and the third, with a frequency profile closer to the second, is also a commercial system (GSSI).
The BGR system uses two shielded broadband antennae mounted on a frame structure. This structure is attached to a rope, and when in operation hangs 20 meters below the helicopter in flight. The RST system is similar to the BGR system, and differs only in the frequency of the radar pulses that it emits. The GSSI system uses a distinct technique, in which the antennae are mounted directly on the helicopter skids. This GSSI system seemed attractive, since the first two systems, in which the helicopter carried a weight suspended below it, could interfere with the stability and efficiency of the helicopter. Moreover, the GSSI system might allow the helicopter to fly more steadily, producing a smoother image that required less processing to compensate for fluctuations in velocity.
The researchers conducted a number of repeat flights to assess the three systems. They used different systems on individual sections of the glacier, and compared the images for two features: the clarity of the images which they produced and the depth of ice that they could penetrate. The RST system proved to be the most effective on both features. Though the GSSI system was more favorable in terms of its effects on the flight performance of the helicopter, the images it produced were inferior, perhaps because of interference between the radar and the body of the helicopter itself. The authors note that these results reflect specific characteristics of the glaciers: the ice is relatively warm, in comparison to glaciers at higher elevations and latitudes, and it includes some sections of liquid water. So they suggest that the relative performance of the systems might differ under other conditions, and propose that other frequencies might perform better in these circumstances as well.
Helicopter-borne ground-penetrating radar systems are a good approach to mapping bedrock on temperate alpine glaciers. It’s a technical challenge to figure out whether the glaciers are growing or shrinking and by how much, but scientists have to do it by improving analytical methods and measurement tools because tracking what is going on with glaciers is an important tool in climate science. These comparisons of techniques in the Swiss Alps point to similar experiments that could be conducted in other mountain regions of the world.