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RADAR INTERFEROMETRY
Differential
interferometry (InSAR) is a technique that can be used for the
detection of crustal deformation using suitable pairs of SAR images.
Current
activites
Conventional
InSAR
Conventional InSAR can deliver spectacular measurements of the
large-scale ground deformations associated with main earthquake events,
volcano deformation, landaslides etc (Massonnet and Rabaute 1993,
Zebker et al., 1994, Wright et al., 2001), provided the temporal
separation and horizontal baseline between the two SAR scenes used are
kept within appropriate limits. Many examples exist and such results on
their own offer unique input to strain models and support the
understanding of fault mechanisms and volcanic deformation modelling
and have even been successfully used for the verification of insurance
claims. Though usually applicable to the main co-seismic event, and so
is perhaps a 'response' technique, the deformation information can
provide valuable understanding of fault mechanisms and thus input to
forecast models in the mitigation phase. However, conventional InSAR is
not considered a tool for the measurement of the millimetre-scale
motions associated with interseismic activity; the displacement
resolution of the technique becomes degraded by temporal decorrelation
and/or atmospheric heterogeneity resulting in phase ambiguites of
similar orders of magnitude as the ground displacements anticipated.
PS
InSAR
Permanent Scatterer InSAR technique (Ferretti et al., 1999) involves
the processing of more than 30 interferograms over the same place to
identify a network of temporally-stable, highly reflective ground
features – permanent scatterers. The phase history of each scatterer is
then extracted to provide interpolated maps of average annual ground
motions, or more importantly, the motion history, up to 9 years (length
of SAR data archive), of each individual scatterer, thus providing a
'virtual' GPS network with 'instant' history. Due to the relatively
high density of scatterers that occur in built environments (a few
hundred per square kilometre) and the large number of atmosphere
samples (SAR scenes) used, the heterogeneity of the atmosphere can be
accurately modelled so that measurements of sub-millimetre accuracy can
be calculated. A limitation of PSInSAR is the lack of control over
precise scatterer location, but with the densities obtained in built
environments this is not considered an issue for the mapping of
interseismic ground motions.
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