Radioglaciology

Radioglaciology is the study of glaciers and ice sheets using radar. It employs a geophysical method similar to ground-penetrating radar that operates at frequencies in the MF, HF and VHF portions of the radio spectrum. Radioglaciology is sometimes referred to as "ice-penetrating radar" or "radio-echo sounding".

Glaciers are well suited to investigation by radar because the imaginary part of the permittivity of ice is small relative to its real part; this ratio is called the loss tangent. The conductivity of ice is small at radio frequencies, so its dielectric absorption is also small.

The primary goal of many radioglaciological surveys is to measure the thickness of a body of ice, which is an important boundary condition for ice-flow models. Ice thicknesses greater than 4 km have been measured in East Antarctica. Internal reflections have also been detected in many alpine glaciers and all modern ice sheets. These layers represent the internal stratigraphy and can also be used to constrain ice-flow models. The shapes of these internal reflections generally follow the bedrock topography and they are often assumed to be isochronous. Disturbances in these reflections that are unrelated to bedrock topography can be used to understand past ice flow, for example the anticlines arising from the Raymond Effect.

The cause of the observed internal reflections partly depends on the frequency of the radar system used to detect them. There are three primary types of reflections:

• In the firn and at depths where densification is occurring, small changes in density alter the real part of the permittivity, which can cause reflections. Once densification is complete, changes in density in an ice column are not expected to be large enough to cause radar reflections.
• High concentrations of volcanic acids, e.g., sulfuric acid or hydrochloric acid, increase the conductivity of the surface snow over which they are deposited. Acidity increases the conductivity, which produces a reflection. Reflections due to volcanic layers are possible at any depth.
• Individual crystals of ice display dielectric anisotropy. Layers that have a preferred crystal fabric direction different from that above it can therefore also cause reflections.

Ice-penetrating radar systems, particularly the antennae, are often homemade systems made of commercially available components. However, commercial ground-penetrating radar systems are sometimes used.

There are currently two ice-penetrating radars orbiting Mars: MARSIS and SHARAD. An ice-penetrating radar system was planned for the canceled Jupiter Icy Moons Orbiter, and such systems are also being planned for orbiters that are part of the Europa Jupiter System Mission.

References

Bogorodsky, V., C. Bentley and P. Gudmandsen (1985), Radioglaciology, D. Reidel Publishing Co., ISBN 90-277-1893-8

Fujita, S. and S. Mae (1994), Causes and nature of ice-sheet radio-echo internal reflections estimated from the dielectric properties of ice, Annals of Glaciology, 20, 80-86.