Radio Wave Propagation in Temperate Ice

Background Information

As most people know, both water and ice are transparent to the visible light portion of the Electro-Magnetic (EM) spectrum. At the much lower frequencies (and longer wavelengths) of radio waves, liquid water is opaque while ice is still relatively transparent. This is why radio-echo sounding is used in the sub-freezing regions of the Arctic and Antarctic glaciers and ice sheets. There is little water present within these cold ice masses to scatter or block the radio signals. The lack of water has allowed researchers to use frequencies ranging from a few MHz for subglacial mapping, up to 200-500 MHz for crevasse detection near the ice surface. Frequencies in the GHz range are used for studies of snow structure and stratigraphy.

By definition, temperate ice exists at the pressure-melting point. This means that both ice and water phases coexist. The presence of liquid water presents a problem when trying to use radio waves in temperate glaciers because the water scatters the radio signals making it difficult to receive coherent reflections that can later be interpreted.

In the late 1960s through the mid-1970s, a number of researchers experimented with various frequencies and transmitter designs. Their findings concluded that frequencies between ~2 and ~10 MHz are best for temperate glaciers. 5 MHz pulse-transmitters are the most common used today.

The basic reason that a 5 MHz signal works in most temperate ice is that the resulting 34 m wavelength is far larger than the size of the majority of the englacial water bodies that scatter the signal. Unfortunately, the long wavelength of the signal seriously limits the resolution of the radio-echo sounding survey.

EM Wave Propagation Through a Dielectric Material

Radio waves travel through ice due to its dielectric properties. The dielectric constant of a given material is a complex number describing the comparison of the electrical permittivity of a material and that of a vacuum. As a complex number, the dielectric constant contains both real and imaginary portions. The imaginary part of the number represents the polarization of atoms in the material as the EM energy passes through it (Feynman, 1964). The EM wave propagation velocity is determined by its entire complex dielectric constant.

The propagation velocity of a radio wave in ice is determined by the dielectric properties of ice. Liquid water and various types of bedrock have unique dielectric constants. Since the dieliectric properties of a material are related to conductivity, concentrations of dissolved ions in liquid water will affect the dielectric constant (more free ions increase the conductivity of water). The dielectric constants of some materials are listed below:

Material Dielectric Constant Reference
Air ~1.0 Serway (1990)
Ice (at 0șC) 3.2 ± 0.03 Paren (Unpublished)
Water ~80 Hasted (1961)
Quartz ~4.3 Gregg (1980)

Related Topics


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Updated: August 03, 2011
© 1998, Brian C. Welch, Univ. of Wyoming