dc.description.abstract | Radar interferometry has been widely applied in measuring the surface height and deformation which may be derived from phase interferogram. However, the inherent phase noise reduces the accuracy and reliability of that information. Hence, the minimization of phase noise is essential to the retrieval of surface height information at acceptable accuracy.
This work proposes a refined filter that is based on the revised Lee InSAR filter, the revised sigma filter and the Minimun Mean Square Error estimator. The basic idea is to use 16 directional windows to counter the difficulty of preserving the fringe patterns, to select proper pixels within a reasonable range, and to filter adaptively the interferometric phase according to the local noise level, especially in such extreme cases as involve broken fringes. The goals are to preserve the fringe pattern, to reduce the number of residues, and to minimize the phase error and deviation at a same time. The proposed filter was validated using both simulated data and real interferometer data. Results confirm that the filter performance is better than that of commonly used filters.
Furthermore, we also show a practical application of the proposed filter in area of dense vegetation of low coherence to demonstrate its performance. By improving the interferometric quality, two actual sets of observations of surface deformation in the area of low coherence are further analyzed in what follows.
(1) Observations of the Ailao-Red River Shear Zone (A-RRSZ) with discontinuous geologies: the A-RRSZ is at the junction between Indochina and south China. The geological structure that exhibits remarkable continuous activity is a focus of concern for the scientific community. This investigation applies Differential Interferometric Synthetic Aperture Radar (DInSAR) to observe surface deformation in the vicinity of near Hanoi in the A-RRSZ. Both C-band data of ENVISAT and L-band data of ALOS/PALSAR were used to analyze the mechanisms of deformation. The preliminary results reveal that the tectonic activity varied from the northwest to the southeast along the A-RRSZ near Hanoi. The crustal surface clearly underwent an uplift of 1.3 cm/year, caused by squeezing movement in the period 2007~2009. Surface deformation of the 10 km-wide A-RRSZ is observed not only to differ from that in adjacent areas but also to be relatively stable.
(2) Observations of land subsidence in Hanoi, northern Vietnam: human activity of groundwater extraction is one of the factors that cause surface deformation in the region. The interferometric results, which compared the groundwater levels at five monitoring wells, suggest a land subsidence in the northern part of Hanoi city in the period 1996~1999. Later, land subsidence stopped changing in the northern region and occurred at the rate of around 3 cm/year in the southern region in the period 2004~2010.
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