Application of radar for imaging the snow, firn and ice column usually utilizes a transmitter and receiver moved at a certain distance from each other across the surface along the survey profile. The device is either towed by hand, a snowmobile or tractor, or carried on-board an airplane or helicopter.
At defined intervals, either at equal temporal or spatial increments, the transmitter emits an electromagnetic pulse into the snow column. Distances between consecutive measurements vary, depending on the system performance, between about 0.1 and 10 m. The pulse penetrates into the snow column and is partly reflected where the complex dielectric permittivity changes. The reflected signals travel back to the receiving antenna at the surface. The complete signal is recorded as a function of traveltime of the transmitted radar pulse.
Three factors are known to change the dielectric constant in firn and ice:
- Gradients in the real part, the permittivity, are mostly related to density; they dominate reflections in the upper 100 s of meters.
- Variations in the imaginary part are proportional to conductivity, related to acidity, and depend on frequency. They are the governing reflection cause in deeper ice.
- A third mechanism involves dielectric anisotropy of the crystal fabric of ice, but it becomes significant only at the deeper levels (> 500–1000 m) of the ice sheet, where changes in anisotropic crystal fabrics could develop.
Other radar techniques are based on frequency-modulated continuous wave (FMCW) transmissions or stepped-frequency radars. Although the technical details on data acquisition and processing are different, the results are the same – an image of subsurface reflections along a profile.