Measurement of the Alps deformation by radar interferometry : large-scale inventory of deep seated gravitational slope deformations

Période : Février – Juillet 2020

Summary :
A first InSAR velocity map along the satellite line of sight (LOS) across the Alps shows surprisingly widespread DSGSD (deep seated gravitational slope deformation) (Hippolyte, 2009, Agliardi et al., 2013 Solari et al., 2019) along numerous mountain flanks, with velocities of the order of 5 to 10 mm/year. The goal of this master 2 internship is to complete this first measurement using another set of descending Sentinel-1 SAR data covering the Alps from the Mediterranean sea to lake Leman. The goal will be to produce an inventory of the numerous DSGSD or sackungs, and to analyse systematically their relationships to terrain slope, geological nature of rocks, and already published morphotectonic features.

Detailed description :
The Alps present a general uplift of a few mm/yr, more important in certain areas, that has been clearly observed by GNSS and levelling (Calais et al., 2002, Nocquet et al., 2016, Serpelloni et al., 2016, Nguyen et al., 2016, Houlié et al., 2018). This active vertical deformation is only associated with horizontal deformation of moderate amplitude, mainly extensive. This suggests that the present-day dynamics of the Alps is dominated by a lithospheric rebound, with some possible localization along weakness features, in response to gravitational unloading, of deep or superficial origins, whose relative contribution remain unknown (Chéry et al., 2016, Nocquet et al., 2016). These forces include, at the surface, erosional and deglaciation unloading, as well as, at depth, a dynamic topgraphic effect due for example to slab breakoff.

InSAR time series processing of the whole SAR Sentinel-1 archive, with an acquisition every 6 days, allows to recover the ground deformation at large-scale across the western Alpine arc. The processing requires to manipulate massive data (a few To of raw data developing into 15To of intermediate and final products) with a complex processing chain in a difficult context. Snow, vegetation, abrupt relief and strong atmospheric heterogeneities in the Alps make the deformation extraction particularly challenging. However, a first study led by M. Mathey during her PhD using data in ascending geometry allowed to test improvements of the NSBAS processing chain to adapt it to this challenging environment. As a result, we obtained a very interesting velocity map along the satellite line of sight (LOS) across the Alps. The map shows surprisingly widespread DSGSD (deep seated gravitational slope deformation) (Hippolyte, 2009, Agliardi et al., 2013 Solari et al., 2019) along numerous mountain flanks, with velocities of the order of 5 to 10 mm/year. However, in order to separate the contribution of the general Alps uplift from DSGSD that should have large horizontal displacement rates, it is necessary to obtain a new velocity map using the ascending satellite acquisition geometry.

The goal of this master 2 internship is to process the whole data archive on a descending track geometry using Sentinel-1 SAR data (about 220 SAR acquisitions). About 1000 interferograms covering the Alps from the Mediterranean sea to lake Leman, will be computed and unwrapped. They will then be inverted into time series. The methodology will follow that developed for the ascending track. This processing, although mostly automatized, will require a large investment from the master student, that will allow him to get familiarize with multi-temporal InSAR techniques, their products, and their limitations. Then, the student will associate both ascending and descending LOS velocity maps to obtain maps of vertical and E-W velocities.

The second goal of the master 2 internship will then be to produce an inventory of the numerous DSGSD or sackungs, and to analyse systematically their relationships to terrain slope, geological nature of rocks, and already published morphotectonic features. A temporal analysis between slip rates and precipitation will also be possible on certain DSGSD.