VALIDATION OF THE PERMANENT SCATTERERS TECHNIQUE IN URBAN AREAS Alessandro Ferretti, Claudio Prati, Fabio Rocca, Carlo Colesanti Dipartimento di Elettonica e Informazione Politecnico di Milano Piazza L. da Vinci 32 20133 Milano - Italy Email: aferre@elet.polimi.it INTRODUCTION Most of the earth surface has been monitored systematically first by ERS-1, then by ERS-2 and it will be with the satellite ENVISAT, thus creating long and consistent series of data. In urban areas and where exposed rocks are visible, it is possible to identify numerous back scatterers that do not change their signature with time (the Permanent Scatterers) and therefore they can be used as natural monuments to estimate the progressive motion of the terrain. The precision of the measurement is a small fraction of a wavelength (5.6cm) and millimetric motions are appreciable with good reliability. The atmospheric contribution is rather smooth spatially and independent from take to take, so that it can be identified and removed from the data using a proper processing, provided that the density of the PS is high enough as it happens in urban areas. Then, it is possible to obtain maps of subsidence with very high spatial sampling rate (more than 50 PS/km2, in urban areas) and high quality. The sub-millimetric accuracy of this technique has been checked by means of a comparison with GPS measurements where available and by cross-correlating dilation of metallic structures in Paris and Milano with temperatures over a six years period. The PS technique has been patented at POLIMI on May 1999 [1]. It has been validated by means of ERS SAR data and the main results have been published in the last two years by the same authors [2-8]. Another independent group has exploited the PS technique for scientific purposes recognizing its advantages with respect to the conventional DINSAR technique [9]. In this paper only the last main results of the PS technique achieved by means of ERS-1 and ERS-2 repeated observations are reported. THE PS DENSITY IN URBAN AREAS The number of PSs per km 2 observed over a period of several years have been computed for two European and one US cities. In figure 1 the PS density is shown as a function of the phase dispersion for Los Angeles, Milano and Paris. It should be noted that for all these cities the mean density of PS is higher than 100 PS per km 2 when the phase dispersion is smaller than 0.6 radians. THE CASE OF LOS ANGELES In this case 56 ERS images of the area of Los Angeles have been exploited to generate a subsidence map of the area mainly due to oil/gas extraction and water pumping. The map that shows the average subsidence rate in the seven year period from 1992 to 1999 is shown in figure 2 with a color scale that saturates at 5mm/yr. Besides these antropogenic effects, the map shows the ground deformation across many faults with the accuracy of less than 1mm per year. In this area there are also several GPS stations whose data have been processed at JPL. They are available on the Web. The location of the GPS stations is shown in figure 3. These data have been compared with the PS measurements. A comparison is shown in figure 4. Here it is evident that GPS and PS measurements show almost the same trend and that the PS measurements (almost vertical) dispersion is smaller. From these images it can be clearly appreciated the much higher PS density with respect to GPS permanent stations.
Finally the PS mesurements of the Great Western Forum in L.A. downtown have been correlated with the temperatures recorded at the same days of the SAR acquisitions. Temperatures and vertical displacements (thermal dilation) have been superimposed in order to show their impressive degree of correlation. 100 PS/km 2 0.6 rad fi 2.7 mm PS AREA Figure 1. PS density as a function of the phase dispersion for Los Angeles, Milano and Paris. Figure 2. Average subsidence map of the area of Los Angeles in the seven year period from 1992 to 1999.
Figure 3. Location of the GPS permanent stations in the area of Los Angeles. Figure 4. Right: superposition of the PS (green) and GPS (red) vertical measurements for the sake of comparison. Right: Location of the PS and GPS permanent stations. Their density looks quite different especially in the area shown in upper image.
Seasonal Deformations: Great Western Forum Figure 5. PS measurements of the Great Western Forum in L.A. downtown. Upper left: road map of the area. Upper right: photograph of the building. Lower left: SAR image where the Forum is clearly visible. Lower right: superposition of scaled temperatures and measured vertical displacements.. THE CASE OF MILANO In this case 64 ERS images of the area of Milano have been exploited to generate a displacement map of most of the buildings. One interesting example is shown in figure 6. Here a road/building map and the PS displacement measurements have been superimposed in a GIS framework. All buildings showing an average subsidence rate of more than 4 mm/year have been indicated with a red mark on the background map. The detailed time series that shows the vertical displacement of one of these buildings is also reported. Here a displacement step of about 1cm is clearly evident in the period of time between June and July 1998. The effect of this displacement is documented in the picture also reported in figure 6. The exercise of cross-correlating temperatures and thermal dilation of metallic structures has been carried out in order to validate the PS measurements. The results are shown in figure 7. The degree of correlation is very high as in the case already shown in Los Angeles and many other as, for example, in Paris (see figure 8).
Figure 6. Road/building map of Milano superimposed to the PS displacement measurements in a commercial GIS framework. All buildings showing an average subsidence rate of more than 4 mm/year are indicated with a red mark on the background map. The detailed time series that shows the vertical displacement of one of these buildings is shown in the lower right plot. Here a displacement step of about 1cm is clearly evident in the period of time between June and July 1998. The effects of the June-July 1998 displacement are documented in the picture showing the fissures on the facade of the building..
Dh ~1 cm H DT ~25 C Figure 7. Comparison between temperatures and thermal dilation time series of metallic structures in Milano. 10 1 0.8 5 0.6 0.4 0.2 0 0-0.2-5 -0.4-0.6-0.8-10 -1 1996 1997 1998 1999-10 -5 0 5 10 15 20 25 30 35 40 Figure 8. Superposition and correlation between temperatures and thermal dilation time series of metallic structures in La Villette Paris.
THE CASE OF INNSBRUCK In this case 34 ERS images of the area of Innsbruck have been exploited to generate a reference DEM (see figure 9) and a displacement map of all the available PS in a mix of urban and non-urban areas (see figure 10). The density of PSs is shown in figure 11. Several clusters of PSs appear in correspondence of the urban areas. They are connected each other through a few isolated PS thus allowing the estimation and removal of the atmospheric phase screen from the whole image. An example of the estimated non-linear motion of a couple of PS is shown in figure 12. The achieved results will be now analyzed by the team directed by prof. H. Rott in the framework of the MUSCL EU project in order to get a correlation with known geological phenomena in that area. Figure 9. Digital Elevation Model of the area around Innsbruck derived from the Tandem pairs present in the ERS data set.
Rum Thaur Wattens Vils Gitzens Innsbruck Hall in Tirol Figure 10. Multi-look full resolution ERS image of the area of Innsbruck. Figure 11. PS density in the area of Innsbruck.
1. Rum 2. 1. Hall in Tirol Thaur 2. [mm/yr] Figure 12. An example of the estimated non-linear motion of a couple of PS located in the town of Rum. CONCLUSIONS More than 650 ERS images over 12 different sites have been already processed using the PS technique. The achieved results show that, whenever the PS density is higher than 3-4 PS/km 2, and target motion is strongly correlated in time, APS, DEM and motion components can be separated and a time series analysis of the displacement of every PS in the area of interest can be carried out. Due to the high number of data available the accuracy is generally better than 1 mm/yr for velocity estimation and 1 m for PS elevation. Moreover, millimetric measurements on individual acquisitions are possible (after APS removal). PS analysis may play a major role where accurate geodetic measurements are needed, especially in urban areas. This may open new possibilities for the monitoring of hazardous areas for Civil Protection applications, including monitoring of seismic faults, subsiding areas and slope instability, as well as precision stability check of buildings and infrastructures. Finally the PS technique could be made even more attractive if cheap and stable reflectors are used to fill the areas with no natural PSs. Monitoring on demand every building in a town and landslides in vegetated areas will be then new possibilities offered by the PS technique.
ACKNOWLEDGMENTS The authors wish to thank ESA-ESRIN for financing part of this work and for the continuous support, the Italian Space Agency for the support and the EU for financing the MUSCL project. REFERENCES [1] Ferretti A., Prati C., Rocca F., Sistema per misure di spostamento di aree urbane e zone franose, patent n. MI99 A 001154. (International extension in progress). [2] Ferretti A., Prati C., Rocca F., Permanent Scatterers in SAR Interferometry - Proceedings of the IGARSS 99 International Geoscience and Remote Sensing Symposium, 28 June-2 July 1999, Hamburg, Germany. [3] Ferretti A., Prati C., Rocca F., Permanent Scatterers in SAR Interferometry - Proceedings of the EOS/SPIE Symposium on Remote Sensing - 20-24 September 1999 - Florence, Italy [4] Ferretti A., Prati C., Rocca F., Monitoring Terrain Deformations Using Multi-Temporal SAR Images Proceedings CEOS99 (Committee on Earth Observation Satellites) 26-29 October 1999 - Toulouse, France - http://www.estec.esa.nl/confannoun/99b02/99b02.html [5] Ferretti A., Prati C., Rocca F., Non-Uniform Motion Monitoring Using the Permanent Scatterers Technique, Proceedings FRINGE99-10 12 November 1999, Liège, Belgium. http://www.esa.int/fringe99/ [6] Ferretti A., Prati C., Rocca F., Monitoring of Terrain Motion Using the PS Technique, Proceedings EUSAR2000-22 25 May 2000, Munchen, Germany, pp.115-118. [7] Ferretti A., Prati C., Rocca F., Permanent Scatterers in SAR Interferometry to be published IEEE Trans. on Geoscience and Remote Sensing, [8] Ferretti A., Prati C., Rocca F., Non-linear Subsidence Rate Estimation Using Permanent Scatterers in Differential SAR Interferometry - IEEE Trans. on Geoscience and Remote Sensing, Vol. 38, no. 5, September 2000. [9] Van der Kooij Marco W. A., Permanent Scatterer Approach to Surface Change Detection using SAR Interferometry over the Eagle Oilfields, B.C., Geoide 2000, Calgary 25-26 May 2000.