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30. Differential interferometry

Differential interferometry Differential interferometry involves taking at least two images (with addition of DEM), normally three images, of the same ground area. Passes 1 and 2 are used to form an interferogram of the terrain topography using the basic interferometric technique. Similarly, passes 2 and 3 produce a further interferogram of the same area.
The two interferograms are then themselves differenced to reveal any changes that have occured in the Earth's surface. Such changes could be the result of shifting geological faults or the buckling of the surface due to volcanic activity.

In principle, the ERS SAR is sensitive to changes of the Earth's surface topography on a scale comparable to the radar wavelength, i.e. 5.6 cmm.

31. The Bonn experiment

Phase difference
Taking advantage of the 3-day repeat orbit during the first ice phase of ERS-1, 19 corner reflectors (CR) were installed along an approximately straight line of 15 Km length in a relatively flat and homogeneous area west of Bonn.

The ERS-1 SAR image shows the position of some corner reflectors placed in the area. The corner reflectors were constructed in such a way as to allow a vertical lifting.

From the 19 CRs only 2 had been lifted during the period in question. A vertical lifting of 1 cm was actually done at each, and this movement has been estimated using the SAR differential interferometry technique with an error of 1-2 mm.

This figure shows the spatially averaged phase difference measured for the corner reflector N. 11 (left side) and N. 13 (right side).

The corner reflectors have been moved between the satellite passages of 23 March and the one of 26 March, and this is visible in the phase difference, comparing the phase differences between 23 and 29 and between 26 and 29 of March.

The experiment proved that the sensitivity of differential interferometry is in the sub-cm range, at least for point targets.

32. Landers Earthquake in South California

Landers earthquake
Ground movement
The Landers earthquake (south California) of 28 June 1992 (magnitude 7.3) presented a surface rupture over 75 km. It was followed three hours later by the Big Bear earthquake (magnitude 6.4). No surface rupture was reported for this later event.
From several images, acquired before and after the earthquake, a differential interferogram was computed (see figure), which clearly shows the seismic movements due to the earthquake. The banana-shaped fault is clearly visible. Each fringe corresponds to a co-seismic movement of 28.3 mm. The measured precision is 9 mm.
The differential interferogramme was compared with the effects of the movement predicted by an elastic dislocation model (computed by geophysicists of the French Groupe de Recherche an Géodésie Spatiale, GRGS), and the figure shows a very good correspondance between the modelled and the observed pattern of the fringes.

(Massonnet, CNES, Toulouse)