Penetrating the ice

Our main objective with this project is to trial the ground penetrating radar for crevasse detection. Numerous things can affect how well this technology works for such an application, from how fast we fly, to how wet the snow covering the glacier is. The radar system consists of an antenna (flown by Willy), which contains both a transmitter and receiver and has an inbuilt GPS on the top to give you the location of the data. The transmitter sends out the signal and each time this signal hits a boundary, for example between air and snow or snow and ice, part of the signal is reflected back toward the receiver and recorded.

A signal is sent from the transmitter and every time it hits a boundary, part of the signal is reflected back towards the antenna and recorded by the receiver.

As we fly along above the ice we send out 100 signals per second, all of which are reflecting back to the receiver which allows us to build up a detailed image of what is beneath the surface. We try to maintain a ground speed of 40 knots so the signal spacing is around 20 cm on the ground.

When we fly over ice that has no crevasses at the Totten glacier, the main thing we can see in the radar data are horizontal lines. These are layers in the firn. Firn is partially compacted snow, snow that has remained through the melt season and begun the transition to ice. As more and more snow falls on top the layers of snow underneath become more compacted and so have slightly different characteristics, different enough that we can pick them up with the radar. When we fly over a crevassed area we then see other structures in the radar data. Crevasses are all different shapes and sizes and so we have different patterns in the data. Very narrow, regular crevasses can look like straight, vertical lines.

GPR data from the Totten glacier, on the left the glacier has no crevasses and the only patterns are the horizontal lines. On the right there are some very narrow, straight crevasses features.

More often crevasse features in GPR data have a distinctive mushroom shape. These are known as hyperbola and from because we are sending out signals as we travel in a direction. This means that any one point in the glacier will reflect back numerous signal from different angles, giving the distinctive curved feature. However, when we fly over larger crevasses, particularly those that are open at the surface or have a thin snow bridge covering them, what actually happens is that the signal is reflected off all the different surfaces and angles in the crevasse. In this case we have much more messy looking signals in the GPR data.

On the left the pictures shows why we get hyperbolas in the GPR data. As we fly along and continuously send out radar signals, each point on the glacier receives (and reflects) signals from many different angles giving the distinctive mushroom shape in the data. The picture on the right shows why we have a much messier signal from large irregular crevasses with the radar signal reflecting many times at many different angles.
GPR data from the Totten glacier containing crevasse features with the distinctive mushroom shape. From left to right the crevasses are located further beneath the surface.
GPR data from the Totten glacier with large messy crevasse features. Here the crevasses are open at the surface or covered with a very thin layer of snow.



Leave a Reply

Your email address will not be published. Required fields are marked *