Temporal and geometrical decorrelation often prevents SAR interferometry from being an operational tool for surface deformation monitoring and topographic profile reconstruction. Moreover, atmospheric disturbances can strongly compromise the accuracy of the results. The authors present a complete procedure for the identification and exploitation of stable natural reflectors or permanent scatterers (PSs) starting from long temporal series of interferometric SAR images. When, as it often happens, the dimension of the PS is smaller than the resolution cell, the coherence is good even for interferograms with baselines larger than the decorrelation one, and all the available images of the ESA ERS data set can be successfully exploited. On these pixels, submeter DEM accuracy and millimetric terrain motion detection can be achieved, since atmospheric phase screen (APS) contributions can be estimated and removed. Examples are then shown of small motion measurements, DEM refinement, and APS estimation and removal in the case of a sliding area in Ancona, Italy. ERS data have been used.

Permanent scatterers in SAR interferometry

We present a new differential synthetic aperture radar (SAR) interferometry algorithm for monitoring the temporal evolution of surface deformations. The presented technique is based on an appropriate combination of differential interferograms produced by data pairs characterized by a small orbital separation (baseline) in order to limit the spatial decorrelation phenomena. The application of the singular value decomposition method allows us to easily "link" independent SAR acquisition datasets, separated by large baselines, thus increasing the observation temporal sampling rate. The availability of both spatial and temporal information in the processed data is used to identify and filter out atmospheric phase artifacts. We present results obtained on the data acquired from 1992 to 2000 by the European Remote Sensing satellites and relative to the Campi Flegrei caldera and to the city of Naples, Italy, that demonstrate the capability of the proposed approach to follow the dynamics of the detected deformations.

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A new algorithm for surface deformation monitoring based on small baseline differential SAR interferograms

Synthetic aperture radar interferometry is an imaging technique for measuring the topography of a surface, its changes over time, and other changes in the detailed characteristic of the surface. By exploiting the phase of the coherent radar signal, interferometry has transformed radar remote sensing from a largely interpretive science to a quantitative tool, with applications in cartography, geodesy, land cover characterization, and natural hazards. This paper reviews the techniques of interferometry, systems and limitations, and applications in a rapidly growing area of science and engineering.

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Synthetic aperture radar interferometry

Geophysical applications of radar interferometry to measure changes in the Earth's surface have exploded in the early 1990s. This new geodetic technique calculates the interference pattern caused by the difference in phase between two images acquired by a spaceborne synthetic aperture radar at two distinct times. The resulting interferogram is a contour map of the change in distance between the ground and the radar instrument. These maps provide an unsurpassed spatial sampling density (∼100 pixels km−2), a competitive precision (∼1 cm), and a useful observation cadence (1 pass month−1). They record movements in the crust, perturbations in the atmosphere, dielectric modifications in the soil, and relief in the topography. They are also sensitive to technical effects, such as relative variations in the radar's trajectory or variations in its frequency standard. We describe how all these phenomena contribute to an interferogram. Then a practical summary explains the techniques for calculating and manipulating interferograms from various radar instruments, including the four satellites currently in orbit: ERS-1, ERS-2, JERS-1, and RADARSAT. The next chapter suggests some guidelines for interpreting an interferogram as a geophysical measurement: respecting the limits of the technique, assessing its uncertainty, recognizing artifacts, and discriminating different types of signal. We then review the geophysical applications published to date, most of which study deformation related to earthquakes, volcanoes, and glaciers using ERS-1 data. We also show examples of monitoring natural hazards and environmental alterations related to landslides, subsidence, and agriculture. In addition, we consider subtler geophysical signals such as postseismic relaxation, tidal loading of coastal areas, and interseismic strain accumulation. We conclude with our perspectives on the future of radar interferometry. The objective of the review is for the reader to develop the physical understanding necessary to calculate an interferogram and the geophysical intuition necessary to interpret it.

/pdf/radar-interferometry-and-its-application-to-changes-in-the-2xi2itqotn.pdf

Radar interferometry and its application to changes in the Earth's surface

To understand whether the 1992 M = 7.4 Landers earthquake changed the proximity to failure on the San Andreas fault system, we examine the general problem of how one earthquake might trigger another. The tendency of rocks to fail in a brittle manner is thought to be a function of both shear and confining stresses, commonly formulated as the Coulomb failure criterion. Here we explore how changes in Coulomb conditions associated with one or more earthquakes may trigger subsequent events. We first consider a Coulomb criterion appropriate for the production of aftershocks, where faults most likely to slip are those optimally orientated for failure as a result of the prevailing regional stress field and the stress change caused by the mainshock. We find that the distribution of aftershocks for the Landers earthquake, as well as for several other moderate events in its vicinity, can be explained by the Coulomb criterion as follows: aftershocks are abundant where the Coulomb stress on optimally orientated faults rose by more than one-half bar, and aftershocks are sparse where the Coulomb stress dropped by a similar amount. Further, we find that several moderate shocks raised the stress at the future Landers epicenter and along much of the Landers rupture zone by about a bar, advancing the Landers shock by 1 to 3 centuries. The Landers rupture, in turn, raised the stress at site of the future M = 6.5 Big Bear aftershock site by 3 bars. The Coulomb stress change on a specified fault is independent of regional stress but depends on the fault geometry, sense of slip, and the coefficient of friction. We use this method to resolve stress changes on the San Andreas and San Jacinto faults imposed by the Landers sequence. Together the Landers and Big Bear earthquakes raised the stress along the San Bernardino segment of the southern San Andreas fault by 2 to 6 bars, hastening the next great earthquake there by about a decade.

/pdf/static-stress-changes-and-the-triggering-of-earthquakes-4r29mmr407.pdf

Static stress changes and the triggering of earthquakes

GEODETIC data, obtained by ground- or space-based techniques, can be used to infer the distribution of slip on a fault that has ruptured in an earthquake. Although most geodetic techniques require a surveyed network to be in place before the earthquake1–3, satellite images, when collected at regular intervals, can capture co-seismic displacements without advance knowledge of the earthquake's location. Synthetic aperture radar (SAR) interferometry, first introduced4 in 1974 for topographic mapping5–8 can also be used to detect changes in the ground surface, by removing the signal from the topography9,10. Here we use SAR interferometry to capture the movements produced by the 1992 earthquake in Landers, California11. We construct an interferogram by combining topographic information with SAR images obtained by the ERS-1 satellite before and after the earthquake. The observed changes in range from the ground surface to the satellite agree well with the slip measured in the field, with the displacements measured by surveying, and with the results of an elastic dislocation model. As a geodetic tool, the SAR interferogram provides a denser spatial sampling (100 m per pixel) than surveying methods1–3 and a better precision (∼3 cm) than previous space imaging techniques12,13.

The displacement field of the Landers earthquake mapped by radar interferometry

ALTHOUGH the 1992 Landers, California, earthquake sequence occurred in an area well sampled by geodetic networks1–3, the postseismic deformation in the months following the earthquake has been measured at only 15 geodetic stations4. Another shortcoming in the geodetic coverage occurs west of the primary rupture, where the existing geodetic observations suggest, but cannot resolve, sympathetic slip on secondary faults1. Such measurements, which are needed to place the Landers earthquake sequence in the context of a recurring seismic cycle in California, can be obtained with the dense spatial coverage provided by satellite radar interferometry5–9. Here we present radar maps of the surface deformation field which reveal features that would otherwise have been poorly sampled, particularly if the earthquake had occurred in a less accessible area. We see triggered slip at the level of several centimetres as far as 100 km from the primary rupture, and can resolve the geodetic signal of at least one small (magnitude ˜5) aftershock. The amount of surface slip following the main shock is less than a decimetre, and is consistent with an exponential decay time of several months for the postseismic deformation.

Radar interferometric mapping of deformation in the year after the Landers earthquake

[1] We have analyzed a set of ERS-2 images to investigate surface deformation within the city of Paris using the radar interferometry technique (InSAR). We show that a large scale uplift phenomenon occurred during the summer 1998 south of the Saint-Lazare railway station (maximum amplitude of 1.3 cm ± 0.3 cm). This area corresponds to an important underground working site, where the piezometric level has been lowered during 1995 and 1997. We show that the uplift correlates in time with the increase of the underground water level during the summer 1998, some ten months after the end of the main water pumping work. The amplitude of the uplift corresponds to the amplitude of a subsidence which was previously observed in the same area. This suggests that the deformation mechanism is elastic reversible rather than irreversible. A simple elastic deformation theoretical model matches the InSAR measured surface deformation.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002GL015630

A ground uplift in the city of Paris (France) detected by satellite radar interferometry

CNES has designed a new SAR correlator to process both airborne and spaceborne data. This correlator allows data presumming prior to their radar processing. The migration compensation is performed in the range frequency/azimuth frequency domain and a constant phase reference is kept for further interferometric applications. The correlator offers great flexibility for the output geometry and formats: standard single-look complex product, any kind of multilooking, and a quick-look. Some image quality functions are included: raw data correction through look-up tables, a contrast seeker, and a radiometric equalization map. The architecture of the correlator is based on only two computer intensive functions: the fast-Fourier transform and a polynomial law generator. As a result, the software has good portability and good efficiency. Proved performances concerning various spaceborne or airborne sensors are given. >

CNES general-purpose SAR correlator

We have analyzed a set of ERS-2 images to investigate centimetric surface deformations within the city of Paris (France) using the radar interferometry technique. We show that a large scale uplift phenomenon occurred during the summer 1998 south of the Saint-Lazare railway station (maximum amplitude of 1.3 cm +/- 0.3 cm). This area corresponds to an important underground working site, which required to lower the piezometric level during 1995 and 1997. The uplift correlates in time with the increase of the underground water level at the end of the construction work. The amplitude of the uplift corresponds to the amplitude of a subsidence which was previously observed in the same area, suggesting that the deformation mechanism is elastic and not simply due to compaction.

Frédéric Adragna

Papers

The displacement field of the Landers earthquake mapped by radar interferometry

Radar interferometric mapping of deformation in the year after the Landers earthquake

A ground uplift in the city of Paris (France) detected by satellite radar interferometry

CNES general-purpose SAR correlator

Ground uplift in the city of Paris (France) revealed by satellite radar interferometry