THÈSE - Recherche - IGN

Abstract
This exploratory thesis prepares the U.S. DESDynI (Deformation, Ecosystem Structure
and Dynamics of Ice) mission. To better understand and quantify the main errors affecting
repeat-pass InSAR measurements, we studied the synergy between ALOS-PALSAR images
and airborne LiDAR data acquired over the Piton de la Fournaise volcano (Reunion Island,
France) in 2008 and 2009. We investigated how microwaves are affected by several types of
volcanic terrains: some are covered by a vegetation canopy of variable density, others are bare
surfaces that display a wide range of physical properties (a'a, pahoehoe or slabby pahoehoe
lava flows, and lapilli).
LiDAR data allowed us to build a high resolution digital terrain model of the volcano.
We used it to remove the topographic contribution from the interferometric phase. The height
anomaly map has been used to analyze the topographic variations of the volcano which may
explain the loss of coherence observed over the Dolomieu crater between 2008 and 2009. We
also calculated a canopy height model and showed that the higher the vegetation, the lower
the coherence. Finally, the LiDAR intensity has been used to segment the image on the basis
of the surface optical properties. It provides ancillary information on volcanic terrains, which
allows to better evaluate the coherence loss terms.
Additionally, the geometrical and physical properties of these surfaces have been
measured in situ in October 2011. From digital photos, we have generated small digital terrain
models at 1.2 mm spatial resolution using an automatic image matching method. We studied
the surface roughness of the lava flows and lapilli thanks to five statistical parameters: the
standard deviation of height, the correlation length, the Z s parameter, the tortuosity index, and
the fractal dimension. This analysis helped us better understand how electromagnetic waves
interact with such media: rough and porous surfaces, such as the a’a lava flows, produce
multiple scattering and then a loss of coherence whereas smoother surfaces, such as the
pahoehoe lava flows, are more coherent. Decorrelation over the lapilli is mainly caused by
volumetric effects but it also depends on the dielectric constant of the medium. To assess the
penetration depth of the radar waves into the medium, we measured the complex relative
permittivity of volcanic rock samples in the lab. Results suggest that the penetration depth is
higher in lapilli than in lava flows. Finally, we studied vegetation density: we produced a map
of LAI (Leaf Area Index) from a SPOT 5 images and showed that the LAI negatively
correlated with the radar coherence. For LAI values higher than 7, the coherence is very low
which means that InSAR measurements become difficult to do or even impossible.
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