InSAR on unstable rock slopes
Figure 1: Osmundneset, located on the east side of Hyenfjord, is an example of a large unstable rock slope that was discovered with the help of InSAR. The mountainside is highly fractured over a length of approximately one kilometre. The light blue points indicate insignificant or no movement, while the yellow and red points are moving up to five millimetres per year.
Since the early 1990’s satellite-based radar interferometry has been used to identify large ground movements due to earthquakes and volcanic activity. Data stacking methods that take advantage of a growing archive of radar images, as well as increasing computing power, have led to a large increase in the precision of the technique.
A radar satellite emits pulses of radar energy, which are scattered by the Earth's surface. When such a pulse of radar energy is reflected back to the satellite, two types of information are recorded. The first information recorded is the amplitude of the signal. The amplitude is influenced by factors such as the surface material, the slope of the surface and surface moisture content. The second information recorded is the phase of the wave. The phase of the wave upon return depends primarily on the distance between the satellite and the surface. It is also affected by changes in the atmosphere, but this is a relatively small effect. Differences in phase between two images are easily viewed by combining (or interfering) the two phase-images. In the resulting image, the waves will either reinforce or cancel one another, depending upon the relative phases. The resulting image is called an interferogram and contains concentric bands of colour, which are related to topography and/or surface deformation.
Radar interferometry is a field that is evolving rapidly. Early radar satellites had relatively low spatial resolution, infrequent and irregular data acquisitions and relatively poorly known orbit trajectories. Today’s radar satellites can acquire images every few days with spatial resolutions of one metre or less. Onboard navigation systems allow us to determine the orbit trajectory with centimetre accuracy. The results is that we can now measure deformation of the Earth’s surface and infrastructure with an accuracy of better than one millimetre per year.
Radar interferometry is being used at NGU to identify and help monitor landslides (Figure 1). Since 2009, this has been done using images from the Canadian Radarsat-2 satellite, obtained through an agreement between the Norwegian Space Centre and Canada. Beginning in 2014, NGU will be using images from the European Sentinel-1a satellite.
Urban subsidence, due to both man-made and geological processes, is also an important application InSAR. NGU has studied subsidence in the major Norwegian cities using InSAR for more than ten years (Figure 2).
InSAR processing at NGU is done using software developed by Norut, in Tromsø, with the support of the Norwegian Space Centre.