How does Antarctic ice flow?

Terra Daily: Currently the yearly General Assembly of the European Geological Union takes place in Vienna, Austria. Dr. Olaf Eisen from the German Alfred Wegener Institute presents results from an environmentally friendly measurement method that he and his colleagues used on an Antarctic ice-shelf for the first time in early 2010. It supplies data that are input to models for the ice mass balance and thus permit better forecasting of future changes in the sea level.

The quality of scientific models depends to a decisive degree on the available database. Therefore members of a young investigators group supported by the German Research Foundation (DFG) now applied a special geophysical measurement method, vibroseismics, for data collection in the Antarctic for the first time.

"By means of vibroseismic measurements, we would like to find out more about the structure of the ice and thus about the flow characteristics of the Antarctic ice sheet," explains Dr. Olaf Eisen from the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association. He is head of the LIMPICS young investigators group (Linking micro-physical properties to macro features in ice sheets with geophysical techniques).

….One of the problems involved in the application of seismic methods on ice sheets is the very porous firn layer, which may be 50 to 100 metres thick. Explosive seismics involves drilling a hole, approximately 10 to 20 metres deep, into the firn to achieve a better coupling between the explosive charge and the surrounding firn or ice. Drilling takes a lot of time and permits only slow progress along the seismic profiles.

Vibroseismics entails the generation of seismic waves directly on the surface. For this purpose the vibrator pad of a 16-ton vibroseis truck of the University of Bergen is pressed onto the precompressed firn and set into operation at a defined vibration rate. In contrast to explosive seismic methods, the excited seismic signal is known and can be repeatedly generated as frequently as desired, leading in the end to improved data quality. However, the loss of seismic energy in the porous firn is a disadvantage….

The Beardmore Glacier, Antarctica