Interferometry

In Episode 3 of IAM News, Iphigenia told us all about Synthetic Aperture Radar, what it is, and how it works. SAR data analysis can be made even more valuable by applying interferometry to monitor ground movement and generate elevation models.

Displacement Measurement and Phase Differences

Interferometry is the comparison of two SAR images taken at different times. These images are processed to create an interferogram which maps the phase difference between the two images. Phase differences are caused by changes in distance between the SAR antenna and the ground due to variations in terrain which could include instances such as landslides, earthquakes, or subsidence. Rather than direct displacement, the output is fractions of the phase. If all displacements are less than half a wavelength (for C-band wavelength = 5.6cm), they are easy to determine, but if they are greater than half a wavelength, this leads to ambiguity.

Elevation Models

Accurate displacement output is obtained by correcting for topography using a digital elevation model (DEM) in a process called Differential Interferometric Synthetic Aperture Radar (DInSAR). With a high-resolution DEM that accurately represents the topography of an area, a digital terrain model can be used to remove topographic effects from the SAR data, allowing for more precise displacement measures. Another approach is to use a DEM to calculate the topographic component of the interferogram. The topographic phase component is caused by the differences in the radar signal path length due to topography. By removing the topographic phase component from the interferogram, the remaining phase differences can be attributed to surface displacement.

Practical Application

We have applied DInSAR to many projects at Geofem, including this case in Cyprus. DInSAR analysis was carried out using historical and present data to assess mass movement on hilly terrain that could be the precursors to landslides. Using these results in combination with assessment of hydrogeological and topographical data, an adaptive, high resolution susceptibility map was created that could be updated to reflect changes in landslide risk every season.

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