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Image Distortion Effects in SAR Subsurface Imaging and a New Iterative Approach for Refocusing and Coregistration

High-resolution subsurface imaging and topography estimation in deserts is very useful in many applications such as oil-field and ground-water explorations and archaeological surveys. To address this problem, we previously developed a subsurface imaging interferometric synthetic aperture radar (InSAR) concept that can estimate the subsurface topography. However, the image resolution of such a system is rather limited by the current techniques available for SAR focusing and InSAR image coregistration as the propagation effects and phase-front distortion caused by the top layer are not accounted for. In this paper, we discuss different image aberrations that result from the top-surface topography, including geometric and defocusing distortion. The issue created by subsurface caustics and their effect on SAR imaging are discussed. We then present a new approach to estimating and correcting such aberrations. By using simulations and measurements, it is shown that up to an order of magnitude improvement in the subsurface image resolution as well as significant improvement on subsurface interferogram coherence can be achieved. The proposed approach is based on the application of a previously developed subsurface inversion algorithm and a newly developed fast subsurface InSAR simulator. The results are verified numerically using 3-D simulations of different sand-dune geometries and an experiment using scaled-model measurements under laboratory conditions.