Bénédicte Fruneau

Support Vector Machine for Multifrequency SAR Polarimetric Data Classification

The objective of this paper is twofold: first, to assess the potential of radar data for tropical vegetation cartography and, second, to evaluate the contribution of different polarimetric indicators that can be derived from a fully polarimetric data set. Because of its ability to take numerous and heterogeneous parameters into account, such as the various polarimetric indicators under consideration, a support vector machine (SVM) algorithm is used in the classification step. The contribution of the different polarimetric indicators is estimated through a greedy forward and backward method. Results have been assessed with AIRSAR polarimetric data polarimetric data acquired over a dense tropical environment. The results are compared to those obtained with the standard Wishart approach, for single frequency and multifrequency bands. It is shown that, when radar data do not satisfy the Wishart distribution, the SVM algorithm performs much better than the Wishart approach, when applied to an optimized set of polarimetric indicators.

Detection of ground subsidence in the city of Paris using radar interferometry: Isolation of deformation from atmospheric artifacts using correlation

This paper presents a new method for the isolation of displacement from atmospheric fringes. This novel approach is based on complex correlation of interferograms. Compared to other methods, this method has the advantage of requiring only a few interferograms (two at least). Its main limitation is the hypothesis of temporally varying atmospheric conditions between acquisitions (not the same heterogeneities on different interferograms). In the city of Paris, this method reveals 2 subsiding zones. Both have the same location as an important underground working site, which took place from 1995 to 1997. The existence of subsidence in the area was known previously from ground truth data. Their spatial extent can now be mapped by interferometry, and the temporal evolution of the subsidence is also examined here.

Coseismic displacements of the footwall of the Chelungpu fault caused by the 1999, Taiwan, Chi-Chi earthquake from InSAR and GPS data

Abstract The differential SAR interferometry technique (interferometric synthetic aperture radar, InSAR) is applied on the Chelungpu fault surface rupture zone of the September 20, 1999, Taiwan, Chi-Chi earthquake using six ERS-2 images covering the period from February 1999 to January 2000. As compared with available geodetic data, InSAR measurements result in more extensive analysis because of high spatial sampling and centimetric accuracy. However, coseismic displacements can be evaluated only on the footwall of the fault. The analysis of interferograms shows the existence of a linear trend in phase difference mainly caused by orbital errors, which we removed from interferograms using GPS data. The corrected interferograms provide a precise map of the InSAR component of the coseismic displacement, showing a continuous decrease over the footwall from a maximum of 36.7 cm at the fault east of Taichung city to a value of about 5 cm at the coastline 30 km further west. The map analysis reveals that the Changhua fault (whose surface trace is located about 20 km west of the Chelungpu one) and the Tuntzuchio fault influence the displacement field. We interpret this in terms of minor reactivation of these faults triggered by the earthquake. A 1.7 cm uncertainty, estimated from the GPS data, is proposed to quantify the precision of the map. Beyond this single value, we highlight the interest of having several coseismic interferograms to evaluate the reliability of the map in a more comprehensive way. Comparisons with displacements inferred from models of slip distribution inverted without InSAR data highlight the advantage of carrying out a joint inversion including our results as new constraints.

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

In 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.

Uplift of Tainan Tableland (SW Taiwan) revealed by SAR Interferometry

Interferometric processing of five SAR-ERS images reveals uplift of the Tainan Tableland (SW of Taiwan) during the period 1996–1998. The maximum measured ground motion for these two years is 2.8 cm along the radar line of sight towards the satellite, indicating for the displacement vector a vertical component of 3.2 cm, and a horizontal component of 1.6 cm towards the WSW considering additional information from GPS data. The reconstructed displacement field is consistent with the geological interpretation of the Tainan Tableland as an actively growing anticline connected to the Taiwan fold-and-thrust belt. This implies that the deformation front is located farther west than usually assumed in the Tainan area. The large Tainan city is thus located in an active deformation zone. Seismic hazard assessment is however difficult because the mechanisms and kinematics are not known in detail.

Classification of Tropical Vegetation Using Multifrequency Partial SAR Polarimetry

This letter presents a case study addressing the comparison between different synthetic aperture radar (SAR) partial polarimetric options for tropical-vegetation cartography. These options include compact polarization (CP), dual polarization (DP), and alternating polarization (AP). They are all derived from fully polarimetric (FP) SAR data acquired by the airborne SAR (AIRSAR) sensor over the French Polynesian Tubuai Island. The classification approach is based on the support vector machine algorithm and is further validated by several ground surveys. For a single frequency band, FP data give significantly better results than any other partial polarimetric configuration. Among the partial polarimetric architectures, the CP mode performs best. In addition, the DP mode shows better performance than the AP mode, highlighting the value of the polarimetric differential phase. The combination of different frequency bands (P-, L-, and C-bands) holds the most significant improvement: The multifrequency diversity adds generally more information than the multipolarization diversity. A noticeable result is the major contribution of the C-band at VV polarization (the only polarization available at C-band with the AIRSAR data set used in this letter) to the classification performance, due to its ability to discriminate between Pinus and Falcata.

Contributions of InSAR to study active tectonics of Taiwan

Presents four case examples of contribution on InSAR to active tectonics issues in Taiwan Island that is one of the most seismically active regions in the world. For these studies, differential InSAR technique is used in a 2-pass approach, the resulting interferograms combine images of the ERS-1/2 satellites from 1993 to 2001. The results illustrate the different tectonic processes that InSAR can investigate in Taiwan: (1) regarding the 1999 Chi-Chi earthquake event, InSAR allows to capture coseismic displacements, and to detect displacements at nearby faults triggered by the earthquake, (2) in the Tainan area (SW Taiwan), InSAR can measure the interseismic crustal deformation field (uplift of an anticline) over eight years, (3) for the Fengshan fault and Longitudinal Valley fault, InSAR is able to monitor fault creep. This study makes it possible to consider InSAR as a tool (in complement to the GPS network) for monitoring several active faults in Taiwan that have the potential to produce earthquakes.

Phase unwrapping for DEM generation as an inverse problem

A traditional way to add short wavelength information on an existing Digital Elevation Model (DEM) is to produce a geocoded residue interferogram, i.e. removing the synthetic fringes calculated from the input DEM. The phase unwrapped product is then simply added to the original DEM in its geographic geometry. Our different approach consists in four steps: (1) we first build a topographic interferogram in the Synthetic Aperture Radar (SAR) natural geometry. (2) We then compute the SAR orthorectification consisting in calculating independently each pixel location on the ellipsoid (latitude, longitude and elevation). In this step, we use an iterative search algorithm to retrieve the correct elevation within an SRTM DEM. (3) The elevation matrix is used as a priori information in the least-squares phase unwrapping which is performed in the SAR geometry. (4) At last, the geometry matrices are used to resample the elevations in a cartographic reference system. The outcome forms the fusion of the interferogram and the information available in the DEM and sparse GPS measures. This approach is a classical least-squares inversion with a priori information. We finally discuss results over the Athens-Corinth region (Greece) comparing them with a high resolution SPOT DEM.