Subsidence Map of the City of Tianjin (2005-2006)
Land subsidence is a well-recognized environmental problem in some provinces in China, for example in the city of Tianjin in province Hebei and Jiangshu. The principal cause of the subsidence is continues and excessive extraction of groundwater to fulfill the demands of fast development of industry and agriculture in the both regions. For the urban development and protection, it is necessary to monitor and measure the subsidence with very high vertical accuracy and spatial continuity at regular time intervals. Tianjin city in province Hebei and Wuxi, Suzhou, Changzhou region are chosen as the working area. The Tianjin municipality is situated on the coast of China on 117°15' East, 39°10' about 200 km South East of Beijing. It covers an area of about 3000 square km. Tianjin is a major Chinese city ("city province") with 9 million inhabitants. It is the main harbor for Beijing and the north of China. After the Shanghai and Beijing municipalities, Tianjin is economically the third most important center of China. Subsidence due to groundwater extraction has become a major problem in this municipality. Governmental regulation has resulted in a decreased subsidence. In the coastal areas outside Tianjin the subsidence rate is still high. The leveling data consist of point measurements with a density of approximately one point per four square kilometer. Continuous subsidence maps are produced by interpolation between the points. The collection of this information puts very high demands on financial resources: in the order of 1 million US dollar per year. For this reason the Tianjin Control Land subsidence Office has actively been considering the potential of new technologies to improve the cost to benefit ratio of their work. One of these new techniques is the use of the Global Positioning System (GPS). In 1998 a GPS measuring network has been established in part of the area. Another relatively new technique with a high potential in this respect is SAR interferometry (InSAR). We used the Envisat ASAR data and D-INSAR technique to measure the subsidence in this region. The following figure shows the subsidence map of Tianjin from 2005 to 2006.