海潮负荷对沿海地区宽幅InSAR形变监测的影响

Impacts of ocean tidal loading on coastal deformation mapping with wide-swath InSAR observations

海岸带地区是全球自然生态环境最为复杂和脆弱的地域之一,合成孔径雷达干涉测量(InSAR)技术可以为全球人类活动、气候变暖和俯冲带剧烈构造运动等背景下的大范围海岸带地理环境变化研究提供重要观测资料.海洋潮汐导致固体地球长周期形变,波长尺度为102~103km的海潮负荷引入mm级至cm级的形变梯度,此类非构造信号对海岸带InSAR精密形变分析(如:大范围、微小、缓慢且非稳态构造过程等)造成显著影响.本文以宽幅模式SAR数据为例,基于多种海潮模型研究了全球典型海岸带地区(福建、智利和阿拉斯加湾)海潮负荷效应对宽幅InSAR形变监测的影响,给出了宽幅InSAR海潮负荷三维分量估计与差分相位提取方法,并进一步讨论了基于不同海潮模型估计海潮负荷位移的差异.海潮负荷影响不仅与研究范围大小有关,其形变梯度变化与研究区域地形特征存在强相关,对于长波长形变分析而言,传统平面或者曲面拟合方法难以有效分离海潮负荷位移.

Coastal zone is one of the most complex and fragile natural ecological environments in the world.Synthetic Aperture Radar Interferometry(InSAR)techniques can provide vital information on surface topography and deformation in the ocean-land transition zones under the background of global human activities,climate warming and violent tectonic movements over subduction zones.Ocean tides cause a temporal variation of the ocean mass distribution and the associated load on the crust and produce time-varying deformations of the Earth that can reach 100 mm with a wavelength scale of 10 2~10 3 km and deformation gradients of mm level to cm level.This kind of non-tectonic signals will have a significant impact on InSAR precision deformation analysis(e.g.large-scale slow and non-steady tectonic deformation with small magnitude).In this study,we used various ocean tidal models to investigate the ocean tidal load(OTL)effects on wide-swath(WS)mode InSAR(e.g.ALOS-2 PALSAR-2 and Sentinel-1A)deformation monitoring in three typical coastal regions including China,Chile and Gulf of Alaska.We detailedly demonstrated the method of three-dimensional component estimation of OTL and differential OTL phase for WS InSAR and further discussed the differences of ocean tidal models in the estimation of OTL displacements.We find that(1)the magnitude of the OTL effects relates with the spatial range of the study regions,and(2)there are strong correlations between the OTL deformation gradient changes and costal topography.Therefore,traditional flat or curved surface fitting methods are difficult to effectively separate the tidal load displacement for the long wavelength deformation monitoring.It is thus highly recommended in this paper that they should be estimated and corrected carefully in interferometric processing,particularly when long wavelength crustal deformation is targeted.For long time series of deformation monitoring over a wide range of coastal zone(>100 km),if image acquisition time was not away from the astronomical tide and tidal changes in the peak time especially for those Sentinel SAR images with shorter temporal baselines(e.g.12 days),one should consider how to effectively estimate and correct the OTL effects.Besides,InSAR OTL estimation should keep up to date with the latest global ocean models at high spatial resolution or region tidal models integrated with long-term tide gauge observation.