INTRODUCTION In the 19th and in the beginning of the 20th century the city centre of Brussels benefited of the industrialisation process. Many industries and breweries settled along the axis of the Senne river using the water supply of the area. The population extent of Brussels and its economic development into services lead to the installation of the large industries at the borders of the city. These changes locally affect the underground exploitation of the aquifer under the city resulting in measurable ground motions. The Persistent Scatterer Interferometry (PSI) technique (Ferretti, Prati & Rocca 2000 & 2001) has been applied on an area covering the city and its suburbs. The PSI technique makes it possible to measure specific displacements of radar reflecting objects present on the ground called persistent scatterer (PS) from a stack of Synthetic Aperture Radar (SAR) image. PROCESSING AND OBSERVATION

The first processing was carried out over ERS1/2 images covering the years 1992-2003. An update covering the time span 1992-2010, including ERS1/2 and Envisat SAR images, is being processed at the time of writing. The discussion will be based on the first processing results where 173,000 PS were identified on a large area around Brussels. Working on these data in the GIS made it possible to highlight and relate movements of the ground with geological data. The density in the centre of Brussels varies from 800 PS/Km² to a maximum of 1632 PS/Km² for a coherence (quality) higher than 0.65. Such high densities of PS data give the opportunity to precisely look at a high resolution scale, from districts to single house. The city of Brussels contain 92,777 PS for a surface of 161 km². The figure 1 gives the location and a colour classification based on the annual average velocity of all the PS. The red colour represents negative velocities while on the opposite blue colour corresponds to positive values. It appears that Brussels is facing a global vertical positive movement (uplift) of the area centred along the Senne river with velocities ranging from 1 to 6.61 mm/year.

Figure 1 – Colour classification based on the annual average velocity (mm/year) of the PS around Brussels

Several red-yellowish lineaments with negative velocities value (< -0.5 mm/year) pop out of the blue colour in the southeastern part of the region.

GEOLOGY-HYDROGEOLOGY To understand the movements, a brief description of the aquifers present in the region are described here. The hydrogeological structure of the area of Brussels is composed by a set of superposed aquifers separated by layers mainly formed by clay deposits (Gulinck 1966). An alluvial aquifer lies within the Quaternary deposits of the Senne river flowing through the city in NE-SW direction. The underground of the eastern part of the region hold a significant aquifer in the sands of the Lede and Bruxelles Formations. The glauconitic sands of the Hannut Formation (Late Paleocene) contain also an aquifer. The Cretaceous is absent in the southern and south western parts of Brussels. The thickness of the Cretaceous thickens from a few metres to around 20 m to the north and to more than 40 m in the most eastern parts of Brussels. Finally, the artesian aquifer of the Cambro-Silurian basement is the major aquifer of the southern part of Brussels.

INTERPRETATION The area affected by the strongest positive ground deformations corresponds to the zone of water catchments and groundwater pumpings from the Cretaceous aquifer since the industrialisation of Brussels (1880-1950). During the initial years, the data in the archives indicate that artesian wells were used for industrial purposes (breweries, dyeing, distilleries, refineries, etc). The old artesian wells has been superimposed on the PS annual average velocities values. It indicates that the uplift and the location of these wells match perfectly. In details, the artesian wells are specifically located along the Senne river axis. More than 50 wells are present in the uplifting zones. In northern part of the city (Vilvoorde), the Cretaceous aquifer level raises by 30 m since 1992 (50 m since the 1970s when records began). It appears that a combined effect of the recharge of the Cretaceous aquifer and the phreatic aquifer of the Senne river explain the observed ground motion.

With the help of old geological and topographical map of Brussels, the interpretation of the lineaments with negative annual velocities value has been realised. It appears that these area corresponds to former alluvial plain deposits where the waterways are now canalized or into pipes. The strong compaction capability and the competence of the Quaternary deposits are responsible for the subsidence recorded by the radar interferometry.

CONCLUSION The PSInSAR technique combined with geological, hydrogeological and topographical data has permitted to highlight and interpret the ground movements observed in the centre of Brussels. The former industrial activities and its groundwater needs were correlated with the uplifting zones and the recharge of the aquifer. The recent alluvial sediments are subject to compaction as shown by slight subsidence evidences in the interferometry data.

REFERENCES

FERRETI, A., PRATI, C. & ROCCA, F. (2000) - Nonlinear subsidence rate estimation using permanent scatterers in differential SAR Interferometry. IEEE Transactions on Geoscience and Remote Sensing, 38(5), 2202-2212.

FERRETI, A., PRATI, C. & ROCCA, F. (2001) - Permanent scatterers in SAR Interferometry. IEEE Transactions on Geoscience and Remote Sensing, 39(1), 8-20.

GULINCK, M. (1966) - Hydrogeology. Atlas of Belgium. Geographic National Committee, Committee of the National Atlas, Brussels.