Abstract
This paper presents a novel method for extracting the land vertical displacement rates (VDRs) in Shanghai (China) using the multi-platform SAR images collected between 2009 and 2010, i.e., the ascending/descending COSMO-SkyMed (CSK) X-band images and the descending TerraSAR (TSX) X-band images. Different from the conventional approach the proposed method estimates the VDRs by multi-temporal interferometric SAR processing (i.e., temporarily coherent point InSAR, TCPInSAR) and the localized least square (LS) solution, through which the horizontal displacement rates (HDRs) can be also derived as the by-product. The three types of displacement rates in the radar line of sight (LOS) are first estimated by TCPInSAR with use of the ascending/descending CSK and descending TSX data. Both the VDRs and HDRs are then estimated by a localized LS method on the basis of the three types of LOS displacement rates derived by TCPInSAR. For comparison purposes, the VDRs for the single-platform SAR data are also obtained through dividing the LOS displacement rates by the cosine of the radar incidence angle. The analysis with the aid of GPS data shows the horizontal motion due to the geological settings in Shanghai cannot be ignored, and the VDRs derived from the single-platform data are not accurate and reliable without taking into account the horizontal motion. The experimental results indicate the TCPInSAR-LS method can be used to efficiently improve the accuracy in the VDRs by integrating the multi-platform SAR data and considering both horizontal and vertical motion, thus overcoming the disadvantages in the conventional approach. The improved VDRs in Shanghai range between −22.8 and 9.6 mm/year, while the relative east-west motion rates range between −7.2 and 6.2 mm/year, and the relative north–south motion is subtle and can be ignored. Analysis of the improved VDRs shows that a slightly uplifting area of about 70 km2 appears in the west downtown of Shanghai, and a subsiding bowl appears in the Hongkou District of Shanghai. The validation indicates that the uplifting trend can be ascribed to the measures conducted by the Shanghai municipal government, i.e., decreasing the groundwater pumping volume and increasing the groundwater recharge volume.
Highlights
Shanghai, the largest city in China with a population of more than 24 million as of 2013, is located in the Yangtze River Delta in Eastern China
We propose to extract the vertical displacement rates (VDRs) through a localized least square (LS) method in a moving window by utilizing the measurements derived from the TCPInSAR solution
The VDRs at the temporarily coherent points (TCP) were calculated by dividing the line of sight (LOS) displacement rates by the cosine of the radar incidence angle
Summary
The largest city in China with a population of more than 24 million as of 2013, is located in the Yangtze River Delta in Eastern China. This city has experienced land subsidence since the 1920s, and the cumulative subsidence reached up to 2.63 m in 1965 with an affected area of 400 km2 [1]. Various control measures, such as limiting groundwater pumping and recharging groundwater, have been implemented since by the Shanghai municipal government to mitigate the land subsidence.
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