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InSAR—Satellite-based technique captures overall deformation “picture”
While ground deformation data provides data at a small number of points on the surface, the radar interferometry method has become a real revolution.
InSAR (Interferometric Synthetic Aperture Radar) is a technique for mapping ground deformation using radar images of the Earth’s surface collected by orbiting satellites. Unlike visible or infrared light, radar waves penetrate most weather clouds and are equally effective in the dark. With InSAR, it is therefore possible to monitor ground deformation even in bad weather and at night – two big advantages during a volcanic crisis.
It is possible to compare two radar images of the same area, collected at different times from similar vantage points in space. Any movement of the ground surface towards or away from the satellite can be measured and represented as an ‘image’ – not of the surface itself, but of the amount of movement (deformation) of the surface between two images. The images are provided by space agencies in Italy, Germany, Canada, Japan, Korea, Europe and the USA.
To create this radar deformation “image”, a pulse of radar energy is emitted by a satellite, scattered by the Earth’s surface and recorded by the satellite with two types of information: amplitude and phase. The amplitude is the strength of the return signal, influenced by the physical properties of the surface. The round-trip distance between the satellite and the ground is measured in units of radar wavelength, and changes in this distance between the time two radar images were collected appear as a phase difference. The combination of these two images is called “interference” because the combination of two waves has the effect of reinforcing or cancelling them.
InSAR greatly expands scientists’ ability to monitor volcanoes because, unlike other techniques that rely on measurements at a few points, InSAR produces a map of ground deformation that covers a very large spatial area with centimetre accuracy. This technique is particularly useful for remote and hard-to-reach volcanoes, as well as for locations where hazardous conditions prevent or limit ground-based volcano monitoring.
A satellite passes over an area and records data about it. Two or more passes are needed to create the InSAR images we use to examine changes in ground height.
Interferogram image made from InSAR monitoring, showing 1995-2001 ground-uplift pattern centered 5 km (3 mi) west of South Sister volcano, Oregon.
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