大地水准面高对InSAR大范围地壳形变监测的影响分析

由于InSAR数据处理所用的WGS84参考椭球系统与通用的DEM高程系统(EGM96大地水准参考面)不一致,在InSAR形变监测分析中会引入大地水准面高导致的误差.本文利用覆盖青藏高原北部阿尔金断裂带西段的27景Envisat ASAR宽幅模式数据和44景条带模式数据,研究了大地水准面高与InSAR大范围形变测量不确定性的关系:(1)模拟分析表明对于100 m的垂直基线,8.8 m的DEM测量误差,若研究区域存在20 m的大地水准面高的变化,对宽幅或条带模式InSAR形变测量造成的影响将由3 mm增至10 mm左右;(2)实例验证表明对于不同的研究区域,大地水准面高与该地区地形变化存在较大相关性,对于同一研究区域,垂直基线的大小决定了大地水准面高对InSAR不确定性的影响程度;(3)对于大地水准面高有较大梯度变化的研究区域,组合短基线方法与去除轨道平面的方法难以消除大地水准面高的影响.使用基于WGS84高程系统的DEM,可以为InSAR形变测量分析提供统一的高程基准,有效避免大地水准面高误差的影响.

Impacts of geoid height on large-scale crustal deformation mapping with InSAR observations

An ellipsoidal height datum (e.g. WGS84) is required in SAR Interferometric processing, whilst DEMs are often referenced to a geoid datum (e.g. EMG96). Therefore, geoid height error is introduced due to the inconsistency of height datums in InSAR derived displacement maps. In this paper, the relationship between geoid heights and uncertainties in InSAR displacements is investigated using 27 scenes of Envisat ASAR ScanSAR images and 44 scenes of Image-mode images covering the western Altyn Tagh Fault, northern Tibetan Plateau. A typical error of 8.8 m in SRTM DEM leads to 3mm uncertainty in an InSAR interferogram with a perpendicular baseline of 100m, whilst a geoid height of 20m can result in 10mm uncertainty. Geoid height varies from one place to another, and is highly correlated with topography. The impacts of geoid height on InSAR displacements increase with perpendicular baselines, but SAR images with a small perpendicular baseline may not be available in some cases. In addition, it may also not always be feasible to use a best-fitting plane to remove the impacts of geoid height. It is thus highly recommended in this paper that DEMs with an ellipsoidal height datum should be employed in interferometric processing, particularly when long-wavelength crustal deformation is targeted.