File:Interferogram of the southern Rhine Rift Valley near Freiburg by TerraSAR-X satellite.jpg

English: The image, obtained by TerraSAR-X shows what is known as an interferogram that covers a part of the southern Rhine Rift Valley near Freiburg. In the centre of the image, the former Bremgarten Airbase can be seen; the Rhine is visible as a dark ribbon running vertically through the image. At the right of the image, marked by a red arrow, is the city of Staufen. The image shows an area of approximately 20 by 30 kilometres. An interferogram is formed when two radar images of the same area are combined, where the differences in the travel time of the radar signals to and from the target can be measured to a high degree of accuracy. These differences are shown in colour in this image. The two images from 2008 were taken approximately six months apart. Several effects can lead to a difference in travel time such as this. Firstly, the dielectric characteristics of the target may have changed. This effect is often observed with fields and meadows and is largely influenced by the water content of the ground. Secondly, changes in the atmosphere could cause differences in travel time. Fluctuations in the water vapour content of the air lead to characteristic bubble-shaped structures in an interferogram. Thirdly, ground movement could be the cause. If ground movement occurs between the times when the two images were acquired, this leads to a change in the distance between the satellite and the Earth that can be recognised in an interferogram. The various colours assigned to the settlement areas in the picture show atmospheric effects. In the built-up area of Staufen (see red arrow), however, a small-scale but clear pattern of deformation can be seen that is the result of an elevation of the substrata.