Hélène Vadon

Land subsidence caused by the East Mesa geothermal field, California, observed using SAR interferometry

Interferometric combination of pairs of synthetic aperture radar (SAR) images acquired by the ERS-1 satellite maps the deformation field associated with the activity of the East Mesa geothermal plant, located in southern California. SAR interferometry is applied to this flat area without the need of a digital terrain model. Several combinations are used to ascertain the nature of the phenomenon. Short term interferograms reveal surface phase changes on agricultural fields similar to what had been observed previously with SEASAT radar data. Long term (2 years) interferograms allow the study of land subsidence and improve prior knowledge of the displacement field, and agree with existing, sparse levelling data. This example illustrates the power of the interferometric technique for deriving accurate industrial intelligence as well as its potential for legal action, in cases involving environmental damages.

Readjustment of the Krafla Spreading Segment to crustal rifting measured by satellite radar interferometry

Readjustment of the Krafla spreading segment on the Mid-Atlantic Ridge in Iceland, after a rifting episode from 1975 to 1984, is detected by radar interferometry. Crustal deformation from 1992 to 1995 is dominated by ∼24 mm/year subsidence above a shallow magma chamber at Krafla, superimposed on ∼7 mm/year along-axis subsidence of the spreading segment relative to its flanks. The deformation is caused by cooling contraction at ∼3 km depth and ductile flow of material away from the spreading axis, at a rate decreasing with time.

Reduction of the need for phase unwrapping in radar interferometry

Monitoring of a small change by synthetic aperture radar (SAR) has previously been demonstrated for several sites, We derive two methods for detecting small displacements in the general case of differential interferometry: (1) the topographic elimination method and (2) the three-pass method. We explain the reasons which make us favor the former method. Validation and calibration of both methods are described. The topographic elimination method is preferred because of the limited need for phase unwrapping. Although phase unwrapping is usually considered to be a key component of radar interferometry, we show that it can be avoided most of the time by using a rough digital elevation model available throughout the world and a new technique, the integer interferometric combination (IIC). Several examples validating the IIC technique are presented as well as considerations about the probability and cost of having a successful study on a given site, which is not a site of opportunity. We conclude with some examples of topography-free, map-registered interferograms, which were produced automatically from SAR raw data and digital elevation models of various quality using the above methods. This validates these methods from an operational point of view.

Coseismic deformation field of the M=6.7 Northridge, California Earthquake of January 17, 1994 recorded by two radar satellites using interferometry

Interferometric combination of pairs of synthetic aperture radar (SAR) images acquired by the ERS-1 and JERS-1 satellites before and after the Northridge, California earthquake of January 17, 1994 provide two maps of the coseismic deformation field. The fringes corresponding to contours of equal change in satellite-to-ground distance show the coseismic displacement of the mainshock. The fringe patterns remain clear for over a year, even in urban areas with structural damage. Elastic dislocation fault models fitting the two radar interferograms are substantially equivalent despite the different radar wavelengths, 56 and 235 mm, for ERS-1 and JERS-1, respectively. A single-fault model based on surveying measurements using the Global Positioning System (GPS) fails to account for parts of the fringe patterns observed by radar satellite, particularly the effects of several aftershocks and localized ground motion.

ERS-1 internal clock drift measured by interferometry

Radar interferometry opens a very wide field of new applications to synthetic aperture radar data. Although some of them could be satisfied with relative calibration of the phase signal over a short range, many of the applications linked to geophysics would require a global calibration of the signal. This calibration should ideally be valid over several hundred kilometers. Among the potential artefacts to be taken into account to fulfill this requirement are atmospheric thickness variation, improper orbit modeling, spacecraft vibrations. In order to eliminate such artefacts, the orbits must be properly tuned. This can be done by taking advantage of the limited number of the parameters which govern orbital positions. In this paper, we show evidence that the internal clock aboard the EM-1 satellite can cause phase artefacts. The evidence is obtained through the analysis of a very long segment of ERS-1 data, in which the fringe patterns show features which can be caused only by a drift of the internal clock. A hear drift with time is sacient to explain the phenomenon: the total frequency change over a five minutes orbital segment is of the order of 5000 Hz. The drift is characterized and its consequences are detailed, in particular on the image geometry. Some new specifications are given which should avoid such an artefact in future SAR systems: with the features of ERS-1, the clock should stay within 200 Hz of its nominal value during the largest interferometric segment envisaged for the mission.

Automatic cloud detection on high resolution images

Remote sensing optical images are often cloudy and then partially unusable. Thus, cloud detection can optimize the image acquisition loop and the end-user image selection. The current pre-processing of SPOT images includes an automatic cloud and snow detection algorithm based on neural networks and fuzzy logic, which globally provides correct cloud masks but with a perfectible confidence. This process must be improved in order to avoid systematic manual re-notation. For Ple/spl acute/iades high resolution (HR) satellite, CNES experiments an innovative cloud detection method based on correlation and stereoscopic effect (B/H ratio) between images. Thanks to the quasi-simultaneous acquisition of the five spectral bands (panchromatic, red, blue, green and near infrared), this new method can be systematically applied, in addition to a radiometric one. This paper focuses on the new detection algorithm for high resolution images, which includes six main steps: (1) generation of 20 m resolution preview images in both multispectral and panchromatic bands, (2) estimation of the misregistration between panchromatic and multispectral previews using a geometric model, a global DTM and an image matching algorithm, (3) computation of the residual parallax consisting in the difference between prediction model and image matching, (4) cloud detection through high parallax value thresholding (5) radiometric analysis for snow and low altitude clouds detection, (6) masks fusion and confidence status computation. This method, still under assessment over various SPOT5 and Quickbird images, seems to be very promising. The results, presented in the paper, show that the method is efficient even for Quickbird images, in spite of the very low B/H ratio value (0.002) close to Ple/spl acute/iades one.

A method for the automatic characterization of InSAR atmospheric artifacts by correlation of multiple interferograms over the same site

Theoretically SAR interferometry allows for the extraction of very precise digital elevation models (DEM) with a potential accuracy of a few meters (3 m rms or better for ERS), as well as for the determination of ground displacements with a centimeter level accuracy in the case of differential interferometry. In practice, the presence of atmospheric fringes limits the confidence in the results. Varying atmospheric conditions between the two acquisitions induce a path difference generating InSAR artifacts that cannot be corrected if only one interferogram is available. However, if an interferometric triplet (three interferometric pairs based on three different radar acquisitions over the same site) can be formed, the atmospheric artifacts related to each date can be characterized by correlation, the objective being to subsequently filter them out.

Fine registration of SPOT5 and Envisat/ASAR images and ortho-image production: a fully automatic approach

Multi-sensor image registration is needed in a large number of applications of remote sensing imagery. The registration accuracy obtained with methods usually used (manual control points extraction, estimation of an analytical deformation model) is not good enough for many applications where an accuracy better than the pixel for every pixel of the image is needed. In this paper we propose a fully automatic procedure for the fine co-registration of SPOT5 images with space-borne SAR images. This procedure has successfully been applied for the case of ENVISAT/ASAR IMS data but is generic and could also be applied to other SAR systems.

CNES Research and Development and Available Software in the Framework of Space-Images Based Risk and Disaster Management

CNES has been involved for four years in the so called International Charter “space and major disasters”. In this framework, both software development and research activities have been carried out, which aim at testing the usefulness of space based images for risk and disaster management, and at improving the space image based products deliverable to the end users.