Nature of radio echo layering in the Antarctic Ice Sheet detected by a two-frequency experiment

A two-frequency radio echo sounding experiment was carried out at Dome Fuji, the second highest dome in East Antarctica, and along a 1150-km-long traverse line from the dome to the coast. The goal was to determine the dominant causes of the radio echo internal reflections and to investigate their possible changes with depth ranges and regions. From the two-frequency (60 MHz and 179 MHz) radio echo responses at various sites, we distinguished four zones. Each of the zones is characterized by a dominant cause of radio echo internal reflection as follows. In the "P_D" zone, changes in dielectric permittivity are mainly due to density fluctuations; in the "P_COF" zone, changes in dielectric permittivity are mainly due to changes in crystal-orientation fabrics; and in the "C_A" zone, changes in electrical conductivity are mainly due to changes in acidity induced by past volcanic eruptions. In each of these three zones, the changes occur commonly along isochrones. In addition, a basal echo-free zone, the fourth zone, was found to appear always below the P_COF zone. These four zones and their distribution suggested variations of the physical conditions within the ice sheet.