Improving InSAR geodesy using global atmospheric models

Spatial and temporal variations of pressure, temperature and water vapor content in the atmosphere introduce significant confounding delays in Interferometric Synthetic Aperture Radar (InSAR) observations of ground deformation and bias estimatesof regional strain rates. Producing robust estimates of tropospheric delays remains one of the key challenges in increasing the accuracy of ground deformation measurements using InSAR. Recent studies revealed the efficiency of global atmospheric reanalysis to mitigate the impact of tropospheric delays, motivating further exploration of their potential. Here, we explore the effectiveness of these models in several geographic and tectonic settings on both single interferograms and time series analysis products. Both hydrostatic and wet contributions to the phase delay are important to account for. We validate these path delay corrections by comparing with estimates of vertically integrated atmospheric water vapor content derived from the passive multi-spectral imager MERIS, onboard the ENVISAT satellite. Generally, the performance of the prediction depends on the vigor of atmospheric turbulence. We discuss (1) how separating atmospheric and orbital contributions allows one to better measure long wavelength deformation, (2) how atmospheric delays affect measurements of surface deformation following earthquakes and (3) we show that such a method allows us to reduce biases in multi-year strain rate estimates by reducing the influence of unevenly sampled seasonal oscillations of the tropospheric delay

Systematic InSAR tropospheric phase delay corrections from global meteorological reanalysis data

6p.International audienceDespite remarkable successes achieved by Differential InSAR, estimations of low tectonic strain rates remain challenging in areas where deformation and topography are correlated, mainly because of the topography‐related atmospheric phase screen (APS). In areas of high relief, empirical removal of the stratified component of the APS may lead to biased estimations of tectonic deformation rates. Here we describe a method to correct interferograms from the effects of the spatial and temporal variations in tropospheric stratification by computing tropospheric delay maps coincident with SAR acquisitions using the ERA‐ Interim global meteorological model. The modeled phase delay is integrated along vertical profiles at the ERA‐I grid nodes and interpolated at the spatial sampling of the interferograms above the elevation of each image pixel. This approach is validated on unwrapped interferograms. We show that the removal of the atmospheric signal before phase unwrapping reduces the risk of unwrapping errors in areas of rough topography

Mexico City Subsidence Measured by InSAR Time Series: Joint Analysis Using PS and SBAS Approaches

International audienceIn multi-temporal InSAR processing, both the Permanent Scatterer (PS) and Small BAseline Subset (SBAS) approaches are optimized to obtain ground displacement rates with a nominal accuracy of millimeters per year. In this paper, we investigate how applying both approaches to Mexico City subsidence validates the InSAR time series results and brings complementary information to the subsidence pattern. We apply the PS approach (Gamma-IPTA chain) and an ad-hoc SBAS approach on 38 ENVISAT images from November 2002 to March 2007 to map the Mexico City subsidence. The subsidence rate maps obtained by both approaches are compared quantitatively and analyzed at different steps of the PS processing. The inter-comparison is done separately for low-pass (LP) and high-pass (HP) filtered difference maps to take the complementarity of both approaches at different scales into account. The inter-comparison shows that the differential subsidence map obtained by the SBAS approach describes the local features associated with urban constructions and infrastructures, while the PS approach quantitatively characterizes the motion of individual targets. The latter information, once related to the type of building foundations, should be essential to quantify the relative importance of surface loads, surface drying and drying due to aquifer over-exploitation, in subsoil compaction

Heat transport in stagnant lid convection with temperature-and pressure-dependent Newtonian or non-Newtonian rheology

International audienceA numerical model of two‐dimensional Rayleigh‐Bénard convection is used to study the relationship between the surface heat flow (or Nusselt number) and the viscosity at the base of the lithosphere. Newtonian or non‐Newtonian, temperature‐ and pressure‐dependent rheologies are considered. In the high Rayleigh number time‐dependent regime, calculations yield Nu ∝ RaBL1/3beff−4/3 where beff is the effective dependence of viscosity with temperature at the base of the upper thermal boundary layer and RaBL is the Rayleigh number calculated with the viscosity νBL (or the effective viscosity) at the base of the upper thermal boundary layer. The heat flow is the same for Newtonian and non‐Newtonian rheologies if the activation energy in the non‐Newtonian case is twice the activation energy in the Newtonian case. In this chaotic regime the heat transfer appears to be controlled by secondary instabilities developing in thermal boundary layers. These thermals are advected along the large‐scale flow. The above relationship is not valid at low heat flow where a stationary regime prevails and for simulations forced into steady state. In these cases the Nusselt number follows a trend Nu ∝ RaBL1/5beff−1 for a Newtonian rheology, as predicted by the boundary layer theory. We argue that the equilibrium lithospheric thickness beneath old oceans or continents is controlled by the development of thermals detaching from the thermal boundary layers. Assuming this, we can estimate the viscosity at the base of the stable oceanic lithosphere. If the contribution of secondary convection to the surface heat flux amounts to 40 to 50 mW m−2, the asthenospheric viscosity is predicted to be between 1018 and 2×l019 Pa s

Influence de l'eau sur les interactions lithosphère-asthénosphère dans les zones de subduction

Les zones de subduction sont le siège d interactions thermique, mécanique et chimique entre plaque plongeante, manteau supérieur et plaque chevauchante. Les conditions de ce couplage sont étudiées à l aide de simulations numériques à deux dimensions. La rhéologie simulée est pseudo-cassante ou ductile, selon la pression, température, taux de déformation et composition. Les transferts d eau sont calculés dynamiquement en s appuyant sur des diagrammes de phase. La déshydratation se produit lors de l éclogitisation de la croûte océanique et de la déstabilisation de la serpentine présente dans la plaque avant subduction. L hydratation générée est continue, sur environ 150 km de large et plus de 80 km d épaisseur. Deux modèles de dépendances en eau de la rhéologie sont testés. Suivant le premier, la chute de résistance des roches hydratées dépend du monde d assimilation de l eau (dissolution ou formation de phases hydratées). L influence de l eau est étudiée en faisant varier l amplitude de la perte de résistance (facteur fnu). Si fnu 20, des cellules convectives apparaissent et affinent la lithosphère sur plus de 70 km d épaisseur en moins de 15 Ma. Le mécanisme érosif est contrôlé par un niveau de découplage associé au changement de mode d absorption de l a=eau. Dans le second modèle d effet mécanique de l eau, la résistance du manteau hydraté ne dépend plus du mode d assimilation du fluide. L amincissement de la plaque supérieure se produit alors dès que fnu > 3 par convection secondaire.NICE-BU Sciences (060882101) / SudocSudocFranceF

Numerical simulations of the cooling of an oceanic lithosphere above a convective mantle

International audienceNumerical simulations of two-dimensional Rayleigh-Bénard convection are designed to study lithospheric cooling above a convective mantle. A strongly temperature-and pressure-dependent viscosity fluid is heated from below or from within. An imposed velocity at the surface of the box mimicks the plate motion between the ridge on one side and the subduction zone on the other side. As the lithosphere cools, its upper part remains rigid and therefore conductive, while its bottom part is convectively unstable. Dripping instabilities are not observed close to the ridge. Nevertheless, the material flows along the slope defined by the lower part of the lithosphere and feeds the first descending drip. Afterwards, cold downgoing instabilities develop continuously and randomly at the base of the lithosphere and are replaced by hot material from the convecting core of the box. The lithosphere continues to thicken even after the onset of the first instability. Surface heat flow, subsidence and lithospheric temperature structure obtained by the convective simulations are compared to the predictions of three conductive models: the Plate, Chablis, and modified Chablis models. These models differ by their applied bottom boundary condition which represents the lithosphere/asthenosphere convective coupling, i.e. by the presence or absence of instabilities developing at the base of the lithosphere. The conductive model which best explains the lithospheric cooling obtained by convective simulations is the modified Chablis model. In this model, a variable heat flow (depending upon the viscosity at the base of the lithosphere) is applied along an isotherm located in the lower unstable part of the lithosphere

Séries temporelles de la subsidence de la ville de Mexico obtenues par interférométrie radar

Dans la ville de Mexico, les taux de subsidence atteignent 40 cm/an, principalement à cause de la compaction du sol provoquée par la surexploitation de l'aquifère du Bassin de Mexico. Au cours de cette thèse nous cartographions de manière précise en temps et en espace la subsidence de la ville de Mexico en utilisant l'interférométrie radar. Nous présentons les problèmes de déroulement dûs à la perte de cohérence et au grand nombre de franges. Nous avons surmonté ces difficultés en utilisant une nouvelle méthodologie aidant l'étape de déroulement. Elle est basée sur le fait que la forme de la déformation est similaire tout a long de la période étudiée. Cela nous a permis de construire un stack d'interférogrammes représentant le taux de déformation pour une période fixée. Pour faciliter le déroulement des interférogrammes, le nombre de franges est réduit grâce à l'utilisation d'une version pondérée du stack. Les interférogrammes construits avec de petites lignes de base sont inversés pour obtenir les délais de phase entre les acquisitions successives. La redondance de notre base de données interférométrique nous a permis de quantifier les erreurs de déroulement et montrer qu'ils sont fortement réduits après application de notre méthode. De plus, nous présentons un nouvel algorithme pour analyser les séries temporelles, qui diffère de l'algorithme classique SVD et qui est mieux adapté à notre base de données. Les séries de la déformation sont alors présentés pour chaque pixel imageant la zone métropolitaine de la ville de Mexico avec une résolution spatiale de 20 x 20 m. Finalement, les composantes non linéaires de la déformation sont modélisées et analysées.In Mexico city, subsidence rates reach up to 40 cm/yr mainly due to soil compaction led by the over exploitation of the Mexico Basin aquifer. In this work we map the spatial and temporal patterns of the Mexico city subsidence by differential radar interferometry. We present the severe interferogram unwrapping problems partly due to the coherence loss but mostly due to the high fringe rates. This difficulties are overcome by designing a new methodology that help the unwrapping step. Our approach is based on the fact that the deformation shape is stable for similar time intervals during the studied period. As a result, a stack of interferograms can be used to compute an average deformation rate for a fixed time interval. The number of fringes is then decreased in wrapped interferograms using a scaled version of the stack to facilitate their unwrapping. The small baseline unwrapped interferograms are inverted to obtain increments of radar propagation delays between the acquisition dates. Based on the redundancy of the interferometric data base, we quantify the unwrapping errors and show that they are strongly decreased after the application of our method. Moreover, we present a new algorithm for time series analysis that differs from classical SVD decomposition and is best suited to the present data base. Accurate deformation time series are then derived over the metropolitan area of the city with a spatial resolution of 20 x 20 m. Finally, the nonlinear components of the deformation are modelised and analysed.PARIS-Télécom ParisTech (751132302) / SudocSudocFranceF

Sparsity Optimization Method for Slow-Moving Landslides Detection in Satellite Image Time-Series

International audiencehis paper presents a new method based on recent optimization technique to detect slow-moving landslides (<150m/year) in time series of displacement field generated by satellite images. Sparse optimization is applied simultaneously on the 3-D data set in space as well as in time. The proposed method takes into account the distinctive signal physical properties in space and time, by enforcing a sparse representation by blocks in space, but a continuing and monotonous representation in time of the landslides. As a result, we show that a mixed ℓ1,2-norm is the most suitable norm for this detection problem, compared to pure ℓ₁-norm or ℓ₂-norm. Moreover, an outlier estimation step is included that sets apart the Gaussian noise from locally sparse processing errors in the data. The performance of this approach is tested by applying it both on synthetic data and on a time series of displacements fields over 16 dates in the Colca Valley, Peru. This detection presents commission and omission errors for landslides of 29% and 14%, respectively, using a medium resolution (10 m) data set of optical satellite images. It detects all important landslides, already known from field investigations. Moreover, it also points out other smaller or unknown landslides, increasing the existing slow-moving landslide inventory by +50%