Interpretation of polarimetric radar waves transmitted through Antarctic ice shelves
British Antarctic Survey,
CAMBRIDGE CB3 0ET,
A network analyser has been operated on the surface of several Antarctic ice shelves as a step-frequency ice penetrating radar. Being a wideband phase sensitive instrument, the radar is capable of allowing the vectorial nature of the interaction between radio waves and the ice and reflecting surface to be explored. Using separate linearly polarised dipole antenna for transmitting and receiving, four independent measurements with parallel and orthogonal orientations allow the (Sinclair) scattering matrix to be determined. Single crystals of ice are birefringent at radar frequencies, so in an ice sheet the crystal orientation fabric determines the overall level of birefringence. The polarisation of radio waves is changed not only by the birefringent nature of the ice but also by the reflecting surface, whether an internal layer or the basal boundary. Thus, the scattering matrix is modelled by a part depending on the birefringence and a part depending on the reflecting surface. Parameters related to the orientation of the effective optic axis, the overall birefringence and the reflection coefficients in orthogonal directions can be found by a semi-analytic method. An additional advantage of the network analyser is that its wide bandwidth allows the frequency dependence of the polarisation behaviour to be explored. Separating out the contributions made by the ice and the reflection process to the depolarisation behaviour is an inverse problem with a non-unique answer. However, comparison of the results from different ice shelf environments allows some broad principles to be established. Where there is a 'smooth' isotropic reflecting surface the effective birefringence of the ice sheet can be readily determined, while a highly anisotropic reflecting surface, such as regular grooves that might be formed by sub-ice shelf currents, can dominate the polarisation behaviour. Analysis of data from two sites on Ronne Ice Shelf, one near the grounding line of Rutford Ice Stream and the other close to the shear margin with Korff Ice Rise, shows the fluted type of reflecting surface seen on George VI Ice Shelf but not on Brunt Ice Shelf.