SPATIAL DISTRIBUTION OF SUBSIDENCE IN HANOI DETECTED BY JERS-1 SAR INTERFEROMETRY

Abstract

Hanoi, the capital of Vietnam is located in a flood plain of the Red River with average elevation less than 20m. Water demand due to the rapid increase of population has led to too much groundwater exploitation, which has caused a remarkable land subsidence over the whole area of the city. The conventional surveying methods have been used to monitor the subsidence. However it requires a time consuming and intensive labor to survey densely over a large area. Therefore, differential radar interferometry was used here as a complementary technique. To detect the spatial extent of subsidence in the city area, we applied the so-call three-pass differential interferometry method that detects terrain change phase from three images acquired at different times. We used three JERS-1 SAR L-band images covering the area of Hanoi City in the period 1995-1998 to generate two interferometric pairs or interferograms. The data was analyzed focusing on the urban area and the suburbs with the area of about 400 km2. Although the differential interferometric phase was noisy in the Red River, the Tay Lake and the vegetated area outside of the city, signatures of ground movements were detected clearly within the urban area. The estimated rate of subsidence is about 2.7cm/year near the Tay Lake and the Red River, and 3.3 cm /year at Minh Khai near the south east part of the urban area. This result is consistent with the result of the leveling survey in 1994 and 1995 that indicated rapid subsidence of about 1.3-1.8 cm/year at benchmarks near the Tay Lake and the Red River. Because of low coherence at the southern part of the city, JERS-1 data could not determine subsidence near Phap Van and Van Dien where significant subsidence was observed by the leveling survey. Thus, we concluded that results derived from the area of high coherence confirm the capability of L-band images for monitoring of subsidence in Hanoi City.