Research on the radio wave propagation process in the ice sheets and internal structure of the ice sheets.

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The in situ high-frequency limit permittivity of Antarctic ice was measured by a radio-wave propagation experiment using a 2503-m-deep ice borehole. The pulse wave of a 179-MHz radar was scattered from a target moving vertically along the borehole to measure the average phase velocity that was used to determine the high-frequency dielectric permittivity at various depths. This allowed the high-frequency permittivity to be measured on ice at 215-232 K and at hydrostatic pressures of up to 14 MPa. This permittivity was always less than that predicted for ordinary hexagonal water ice (ice Ih). At depths of 300-400m, the permittivity was about 2.98 (±0.04), which is about 0.15 smaller than ordinary ice Ih at the temperature for this depth. The permittivity increased with depth and reached a maximum value of 3.11 (±0.04) at a depth of 800 m. Then it decreased slightly with depth to 3.06 (±0.08) at 1400 m. The effects of reduced density, low temperature, crystal orientation, and typical environmental impurities cannot explain these anomalously low values of permittivity. We suggest two causes for the low permittivity : (1) the suppressing effect by hydrostatic pressure in deep ice that had been proposed theoretically and (2) a change in crystal lattice structure at shallow depths. For (1), we used regression analysis to fit a theorized linearly decreasing permittivity with pressure to our data, and thus propose that the effect of hydrostatic pressure on the permittivity is -3.6 (±2.6) x 10-9 Pa-1. The largest anomalies in permittivity are near the surface ; therefore, a separate mechanism, mechanism (2), is required because the pressure is lowest near the surface.