| Interferometric synthetic aperture radar (InSAR) phase observations have greatly increased our understanding of the topography and motion of ice sheets, but yield little information on the sub-surface structure, a needed description for mass-balance estimates. Inversion of a diffuse volume scatter model shows that InSAR correlation values, ρ, can be related to radiowave penetration depths, d, which depend on characteristics of the snow/ice volume. Application to European Research Satellite (ERS) images (VV, 5.6 cm, 23° incidence angle) of the Greenland ice sheet imply C-band d of 0 m along the rocky coast, 10–20 m in the bare ice zone, and 20–35 m in the percolation zone and dry snow zone, consistent with in situ results. Moreover, volume scattering reduces the ERS critical baseline from about 1100 m to 300 m.; Correlation and backscatter power (σ0) observations can be combined for further understanding of the snow/ice volume. In particular, ρ and σ0 data of 15 km-long, 50 m-high topographic undulations in the dry snow zone are minimum on the windward side and maximum on the lee side, with 1 to 3 dB variation typical. These spatial variations in the scattering medium appear to follow from differences in snow accumulation due to prevailing winds. Assuming that snow-grains are the dominant source of backscatter, the classical independent-scatterer model is physically implausible at firn densities; a second-order dense-medium radiative transfer model also is unable to explain both the observed d and σ0. A modified Born approach provides a better match to σ0 and ρ separately, but leads to different grain size solutions for each measurement type. A buried layer model based on the incoherent addition of echoes from hoar layer interfaces, in which scattering from a single layer is derived from small-perturbation methods, reconciles the ERS σ0 and ρ data, with variations in hoar layer spacing of 12–17 cm providing the needed structural fluctuations for the observed range of σ 0 and ρ. Translation of layer spacing into accumulation rates predicts a 40% variability in accumulation rate from the windward to lee side and, more importantly, addresses high-resolution mapping of continental accumulation rates. |
