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European Space Agency radar satellites ERS1, ERS2 and ENVISAT, image the Earth’s surface using Synthetic Aperture Radar (SAR). SAR images of the Earth's surface are produced by transmitting a radar signal to earth. This signal interacts with the surface of the Earth and some of the signal is scattered back to a receiver on the satellite. The return signal will have been modified by the surface interaction and will therefore contain information about the surface scatterer and dielectric conditions caused by such factors as the soil moisture, mineral and metal surfaces or vegetation cover. This can effectively produce a textural image of the surface (Figure 1). Information can be extracted using textural analysis and other techniques.

In addition to return signal amplitude, the terrain height or height change can be deduced from the (phase) difference in radar signal returns of two or more images of the same place. The images need to be acquired from slightly different orbit positions to produce this height information and the interference fringes formed by the two pahase data sets are known as SAR interferograms (InSAR).

1.3 Terrain Interferograms/DEMs

Data from a pair of satellite orbits preferably in close succession may be used to produce an interferogram from which a digital elevation model (DEM) can be generated by phase unwrapping. This interferogram and DEM is later used to subtract terrain from surface change in a differential interferogram.

  Figure 2 Interferogram showing terrain, centred on London. The Thames estuary seen on the right, the city area in the centre, the Chilterns to the North and the North Downs to the south ©EnviroScience

In phase unwrapping a processing starts from known reference points and follows points of the same phase value, then extending to greater or lesser phase values to generate digital elevation models (DEM) of either terrain or surface change.

Differential interferometry is aimed at the detection and measurement of small scale surface movements such as earthquakes, volcanoes, glaciers and subsidence such as by mining. The technique commonly involves the generation of two interferograms from three images, one interferogram spanning some surface change, the other a ‘reference’. The former is expected to contain fringes due to the terrain and the surface movement, whereas the latter contains terrain fringes only. By differencing the two interferograms, the terrain effects can be removed from the "surface" change measurement, leaving only the effects of the surface movement. InSAR fringe patterns are effectively sensitive radar derived contour lines of the Earth’s surface and it’s changes.

in 35 days.

SAR interferometry techniques may be able to provide subsidence information, for instance to the mining industry, with a potential elevation change resolution of millimtres and horizontal resolution of about 20m. The SAR interferogram (Figure 4) shows a number of distinctive concentric fringe features, which after geocoding to mine working drawings and Ordnance Survey 1:25,000 maps have been shown to align precisely with areas of mining activity. By comparison with ground data, it has been established that these do not correspond to terrain features. The circular features have been compared with areas of known mining activity and the locations correlate well, suggesting that they are probably due to ground subsidence.

Each fringe in a subsidence feature represents a surface movement of 28mm in the radar look direction. Some of the features have evidence of three or more fringes, suggesting a surface change of about 9cm

The close up phase image of the largest subsidence feature (Figure 5), geocoded to a 1:10,560 Ordnance Survey map and overlaid with a mine workings plan, shows two sets of fringes centred on mining activity between two satellite passes.

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 Back to EnviroScience - Home

 EnviroScience - Information and Contact details

 EnviroScience - Notes on Radar Remote Sensing and Interferometry

 EnviroScience - Mining Subsidence InSAR imaging - sample images

 EnviroScience - Other Remote Sensing Applications - sample images

 EnviroScience - Other Environmental Research and Consultancy

Richard J Stow BSc.ING. PhD
Environmental Scientist and Engineer