ABSTRACT. Crystal-orientation fabric (COF) has a large influence on ice-sheet flow. Earlier radar studies have shown that COF-based birefringence occurs within ice sheets. Radio-wave scattering in polar ice results from changing physical properties of permittivity and conductivity that arise from differing values of density, acidity and COF. We present an improved mathematical model that can handle all these phenomena together. We use this matrix-based model to study the two-way propagation of depolarized radio waves that scatter at both isotropic and anisotropic boundaries. Based on numerical simulations, we demonstrate how COF affects the radar signals in terms of radar polarization and frequency. We then compare the simulated features with VHF radar data obtained at two contrasting inland sites in East Antarctica, where COF is known from ice-core studies. These two sites are Dome Fuji, located near a dome summit, and Mizuho, located in a converging ice-flow region. Data at Dome Fuji are dominated by typical features resulting from birefringence. In contrast, both birefringence and anisotropic scattering affect the radar data at Mizuho. We argue that radar methods can be used to determine principal axes and strength of birefringence in the ice sheets.

Abstract. 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 "PD" zone, changes in dielectric permittivity are mainly due to density fluctuations; in the "PcoF" zone, changes in dielectric permittivity are mainly due to changes in crystal-orientation fabrics; and in the "CA" 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 PCOF zone. These four zones and their distribution suggested variations of the physical conditions within the ice sheet.

[1] To investigate the viscosity structure of ice sheets induced by crystal orientation fabric (COF), we carried out a multipolarization plane and dual-frequency radar survey in East Antarctica. Radar surveys were done along a 670-km-long flow line from Dome Fuji toward the coast and two transverse lines of 300-km and 20-km length, respectively. The radar echoes were highly dependent on the polarization plane for ice depths between about 40 and 60% of the ice thickness in the lower reaches of the convergent ice flow sector approaching the outlet glacier. When the polarization was perpendicular to the ice flow, echoes were about 10 dB stronger than when the polarization was parallel to the ice flow. This feature was not clear in the upper part of this convergent flow sector. Farther inland, where ice flow is divergent or parallel, the radar echo varied by several decibels because of changes of the radar polarization and had maxima in two orientations. Dual-frequency data showed that the cause of the reflections was changes in COF. Multipolarization data identified anisotropic reflectivities and birefringence as causes of the anisotropic radar echoes in the lower and upper reaches, respectively. With the aid of ice-core-based studies on COF, we show that ice is composed of stacked layers of single-pole and vertical girdle fabrics in the lower reaches. In contrast, we argue that changes of single-pole clustering cause isotropic reflectivities in the upper reaches. We also discuss on the development of COF along ice flow and its implication to ice sheet dynamics.INDEX TERMS: 0669 Electromagnetics: Scattering and diffraction; 0933 Exploration Geophysics: Remote sensing; 1827 Hydrology: Glaciology (1863); 6969 Radio Science: Remote sensing; KEYWORDS: ice-penetrating radar, internal layers Citation: Matsuoka, K., T. Furukawa, S. Fujita, H. Maeno, S. Uratsuka, R. Naruse, and O. Watanabe, Crystal orientation fabrics within the Antarctic ice sheet revealed by a multipolarization plane and dual-frequency radar survey,

detected at Syowa Station, Antarctica, by means of differential-Doppler measurements of the 150 and 400 MHz beacon waves from six NNSS satellites. It is found from statistical analysis that (1) the medium-scale TID's at high-latitudes appear quite often during geomagnetically quiet and moderately disturbed conditions, and their occurrence seems not to increase with increasing geomagnetic activity, (2) they attain the maximum activity in winter and the minimum in summer, (3) diurnal variation shows the maximum occurrence around 1400-1600 LT with a second maximum around midnight, and (4) most of the medium-scale TID's propagate from south toward the cquator. These findings are compared with TID observations using NNSS

We studied the scattering of radio waves off strata within the ice sheet at Mizuho station, Antarctica, to determine the most plausible scattering mechanisms at this location. We measured the effects of birefringence and anisotropic scattering boundaries on the return signal using the following set of experimental conditions: (1) co-polarization and cross-polarization antenna arrangements, (2) all orientations of the antenna system, (3) 60 and 179 MHz frequencies, and (4) pulse lengths of 150–1000 ns. Analyses of the propagated radio waves suggested that the signal is dominated by anisotropic scatter-ingboundaries at 179 MHz, but effects from birefringence also occurred. At depths of 250– 750 m, the scattering was stronger when the polarization plane was along the flowline. In contrast, at depths of about 900–1500 m, scattering was stronger when the polarization plane was perpendicular to the flowline. We suggest that the scattering below about 250 m is related to a layered ice stratum of crystal-orientation fabrics with different types or different cluster strengths. Although our study was at a single site, similar remote measurements over wider regions should provide valuable information about the physical structure of this vast ice sheet.