To investigate whether it is possible to use SAR interferometry to monitor land subsidence over a wide area. For this purpose, we tried to delineate a ground subsidence region over a comparatively long period of time, including the remarkable period of land subsidence in the summer of 1994, the period also called as the Great Heisei Water Shortage.

A second objective of investigation was to simultaneously examine whether the techniques are also effective or not in vegetation-covered area such as in Japan, using C-band SAR data.

1) Image processing software used for the analysis
APP (Advanced Precision Processor) and EarthView InSAR softwares from Atlantis Scientific Inc.

2) Data used in the analysis
Satellite data: ERS-1/AMI data (Level 0)
Ancillary data: 50m mesh digital elevation data
Reference material: Land subsidence (isopleth) map

3) Summary of analysis method
We examined the change distribution in the slant direction of microwave irradiation by image reproduction processing (1 look Range Doppler processing) the ERS-1/AMI data (level 0) and SAR interferometry processing (including orbit fringe removal and topographic fringe removal using an SAR simulation created from DEM data).

By using C-band SAR data on-board the ERS satellite (data acquisition dates were November 8, 1992 and March 5, 1996 respectively) and interferometric SAR technique, we clarified the existence of ground subsidence believed to be the influence of a water shortage in the northern part of Chiba Prefecture during the period of approximately three years and four months, including the dry season in 1994.

As a result, we ascertained that ground subsidence regions existed inland along the Kuju-kuri plain and at a number of locations on the north shore region of Tokyo Bay. In addition, for vegetated regions we obtained results as a positive example showing it is also possible to use C-band SAR data, with the appropriate correlation values which was thought to decrease.

Land subsidence monitoring is currently conducted by municipal governments throughout Japan, using either leveling surveys or subterranean water level observation wells. For ground subsidence in particular, leveling surveys are carried out every year on the designated reference dates, which requires several months. Although the use of SAR interferometry does not mean it is always possible at present to detect the amount of subsidence during a specified time period, it is possible to conduct various surveys of actual conditions and provide data that supplements leveling because SAR interferometry can be used to estimate the amount of subsidence over a period of several months or even several years.