Measurements of deformation

Measurements of movement in unstable rock slopes are crucial to assess how active deformation is today. NGU maps the current deformation by satellite based InSAR and GPS.
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.

InSAR as a method uses radar signals from satellites to measure changes in height in the surface. Advanced processing algorithms makes it possible to measure millimetre scale movements in the surface. The method works best on surfaces with little or sparse vegetation, because the radar signals are reflected by solid surfaces, such as rocks. Regions with optimal surface may provide full covering measurements.

No InSAR measurements can be obtained when the surface is covered by snow. To get measurements during winter also, NGU and NVE install reflectors in certain unstable rock slopes. The reflectors provide frequent measurements all year round at the locations they are installed.

The InSAR-reflector is mounted on a two-meter high mast to make sure that it is kept free of snow during winter. The installation behind the reflector is a permanent GPS instrument.

In addition to InSAR, measurements of deformation in unstable rock slopes are done using a precise GPS method. GPS can be applied in all accessible places and provides the full three-dimensional vector of movement. The measurements are done either on permanent GPS installations that measure throughout the year or by episodic measurements of bolts mounted in rock. The episodic measurements are typically done every one or two years together with mapping fieldwork.

GPS instrument measuring on a bolt in an unstable rock slope near Kåfjord, Troms county.

The combination of InSAR and GPS with structural mapping and other measurements in the field provides the geologists with detailed knowledge about where and how the deformation occurs.

Detailed map showing InSAR data and GPS vectors in the unstable rock slope Gammanjunni 3 in Manndalen, Troms county. The red colours in InSAR corresponds to movements of about 6 cm/yr. Illustration: Martina Böhme, NGU.