基于GPS和GRACE数据的三维地表形变的比较及地球物理解释

GPS技术能以高空间和高时间分辨率监测地表形变.但由于测量原理的不同,GPS监测的地表形变与GRACE存在差异.本文比较了ITRF2008-GPS残差序列与基于CSR的RL05版本的GRACE球谐系数的地表形变序列的差异.结果表明,GPS和GRACE的周年变化在高程方向上具有较好的一致性,但水平方向的差异明显.重点分析了影响GPS/GRACE地表形变差异(尤其是水平方向)的三个因素:不同GPS站时间序列间的不确定性,热弹性形变和区域形变.GPS站地表形变本身的不确定度在一定程度上导致了GPS/GRACE间的差异(特别是水平方向).结合热弹性形变理论指出,由温度变化引起的热弹性形变也是导致GPS/GRACE的南北方向差异的主要原因之一.因此利用GPS数据研究地表质量负载时,必须消除热弹性形变的影响.区域负载对GPS/GRACE水平方向差异的影响也是不可忽略的,特别是对欧洲区域.

Annual variations of 3-D surface displacements observed by GPS and GRACE data: a comparison and explanation

Both GPS displacements and Gravity Recovery and Climate Experiment (GRACE) harmonics are important measurements for studying surface loading. The discrepancies of the annual variations of horizontal displacements observed by GPS and GRACE are much larger than in the verticals. So some previous studies only use GPS verticals to study surface loading. However, the precision of GPS horizontals is much higher than the verticals and GPS horizontals can be used to extend GPS studies of surface loading. Here we analyze and explain the discrepancies of the annual variations of horizontal displacements observed by GPS and GRACE. To investigate the possible origins of the discrepancies between GPS and GRACE (especially in the horizontals), we analyze the effects of GPS residuals uncertainty, thermoelastic deformation and region-scale loading variations. We compare ITRF2008-GPS residuals (232 IGS global stations) with surface displacements derived from GRACE gravity fields up to degree 60 (CSR RL05 version). All the ITRF2008-GPS residuals in the east, north and up components include only the non-linear component of station motion with respect to ITRF2008. Therefore, we do not need to remove linear terms. Outliers and offsets have also been detected and removed. For GRACE data, we firstly restore atmospheric and oceanic contributions, using a separate file (GAC) with monthly average numerical model values. Additionally an estimate of degree-1 gravity coefficients is added. Finally GRACE gravity coefficients are transformed to surface displacements according to. The difference of the annual phase is smaller than 60 days for about 50% stations and the annual amplitude of GPS is about 2~3 times larger than GRACE. Comparison between different GPS residuals (ITRF2008-GPS vs JPL) shows that some of the discrepancy between the GPS and GRACE is due to technique uncertainties in the GPS data processing. However further analyses show GPS-related uncertainties are not the major reason for the discrepancy. Firstly a large part of the discrepancies in the N-S direction between GPS and GRACE can be explained by GPS displacements induced by temperature variation, using the global thermoelastic solution from (Fang et al., 2014). Thermoelasitc displacements observed by GPS (not captured by GRACE) should be removed for loading study using GPS data. Secondly we find that the annual horizontal variations observed by GPS are not as coherent as GRACE, especially over Europe, although the uncertainty of the annual horizontal variations over Europe is better than other regions. Analyses demonstrate that the variability from station to station within Europe may be induced by small-scale loading variations. Agreements between GPS and GRACE in the horizontals are not as good as in the verticals. Firstly, it is difficult to exactly separate seasonal variations from GPS horizontals with large trends and smaller seasonal signals. Secondly, for the verticals, the annual phases caused by temperature variations and mass variations are almost in phase and the annual amplitude (~0.4 mm) caused by temperature variations is much smaller than that caused by mass variations. So even if the thermoelastic displacements are not removed from the verticals, GPS verticals still agree well with GRACE. The situation is more complex for the horizontals. Finally, the horizontals are more sensitive to the local loadings, as it is not only related to the mass variations (loss or gain), but also the location of the mass.