Speaker
Description
Microwave penetration into dry snow, firn and ice bodies leads to the fact that the backscattered signals received by SAR sensors depend on the characteristics of the subsurface of glaciers and ice sheets. This provides a potential link between SAR measurements and geophysical information, e.g. density, firn thickness, stratigraphy, accumulation rate, and melt-refreeze features.
While SAR polarimetry can provide information about the characteristics of the scatterers in the subsurface of glaciers, interferometric (InSAR) and tomographic techniques are mandatory to provide depth information. It was shown in previous (Pol-)InSAR studies that the InSAR coherence is affected by different subsurface characteristics and the estimation of the signal penetration was attempted by means of interferometric models under the assumption of a constant signal extinction with depth [1][2]. An approach to estimate accumulation rates with more physically based snow scattering models was shown in [3] with the drawback of relying on a priori information and assumptions about e.g. grain size and interface roughness. In recent studies multi-baseline data was exploited for SAR tomography and the imaging of subsurface features in both alpine glaciers [4] and the Greenland ice sheet [5] was demonstrated. This revealed that the subsurface of glaciers and ice sheets has a far more complex backscattering structure than what can be described by constant extinction models.
This contribution will give an overview on current InSAR modelling and its limitations and show research towards an improved model representation of the complexity of subsurface backscattering. Based on experimental airborne data acquired with DLR’s F-SAR sensor in Greenland, the influence of subsurface layers on InSAR coherence is demonstrated. Furthermore, SAR tomography is employed to investigate the vertical backscattering structure in the firn of the percolation zone of Greenland. This is utilized to find more flexible parameterizations of the vertical structure functions for InSAR modelling. These analyses are conducted at different polarizations and frequencies. The tomographic data indicates that e.g. X- and L-band signals are sensitive to complementary features in the subsurface of glaciers, due to their different penetration depths and scattering behavior. Therefore, there is great potential in multi-frequency approaches, but generally it is desirable to acquire data at longer wavelengths, e.g. L-band, to gain access to deeper parts of the subsurface. The presented tomographic experiment indicates that the co-polarization channels (HH, VV) are more sensitive to layered structures in the firn, which fits to the expected rough surface scattering mechanism at ice layers or interfaces, and that cross polarization channels (HV) are more sensitive to volume scattering from ice inclusions and firn grains. This shows that the combination of co- and crosspol data is crucial.
Despite the potential of glacier subsurface information retrieval by means of interferometric SAR data, the links to quantitative geophysical parameters, which are of value to the glaciological community, are not established yet.
[1] E. W. Hoen and H. Zebker, “Penetration depths inferred from interferometric volume decorrelation observed over the Greenland ice sheet,” IEEE Trans. Geosci. Remote Sens., vol. 38, no. 6, pp. 2572–2583, Nov. 2000.
[2] J.J. Sharma, I. Hajnsek, and K.P. Papathanassiou, “Estimation of glacier ice extinction using long-wavelength airborne Pol-InSAR,” IEEE Trans. Geosci. Remote Sens., vol. 51, no. 6, pp. 3715-3732, Jun. 2013.
[3] S. Oveisgharan and H. Zebker, “Estimating snow accumulation from InSAR correlation observations,” IEEE Trans. Geosci. Remote Sens., vol. 45, no. 1, pp. 10–20, Jan. 2007.
[4] S. Tebaldini, T. Nagler, H. Rott, and A. Heilig, “Imaging the Internal Structure of an Alpine Glacier via L-Band Airborne SAR Tomography,” IEEE Transactions on Geoscience and Remote Sensing, vol. 54, no. 12, pp. 7197–7209, Dec. 2016.
[5] F. Banda, J. Dall, and S. Tebaldini, “Single and Multipolarimetric P-Band SAR Tomography of Subsurface Ice Structure,” IEEE Transactions on Geoscience and Remote Sensing, vol. 54, no. 5, pp. 2832–2845, May 2016.
