Crystal orientation fabrics within the Antarctic ice sheet revealed by a multipolarization plane and dual-frequency radar survey

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.