Subsidence revealed by PSInSAR technique in the Ottignies- area () related to water pumping in urban area

Declercq P-Y.(1), Devleeschouwer X.(2) & Pouriel F.(3)

(1) Royal Belgian Institute of Natural Sciences, Dpt VII: Geological Survey of Belgium, Rue Jenner, 3 - B-1000 , Belgium (2) Royal Belgian Institute of Natural Sciences, Dpt VII: Geological Survey of Belgium, Rue Jenner, 3 - B-1000 Brussels, Belgium. (e-mail: [email protected]) (2) Royal Belgian Institute of Natural Sciences, Dpt VII: Geological Survey of Belgium, Rue Jenner, 3 - B-1000 Brussels, Belgium

ABSTRACT

Radar interferometry (PS technique) has been applied in and around Brussels. The total area investigated is equivalent to 900 km². The ERS1/2 data sets covering the time span 1992-2003 have been exploited. Seventy-four scenes were used and around 173000 permanent scatterers (PS) were identified and could be used for time-series analysis. Several ground motion processes have been pointed out for the first time in Belgian urban environment. In particular, a subsidence process has been detected along the river between Ottignies and Wavre (SO-NE direction). 7186 permanent scatterers (PS) were used in the Dyle basin to interpolate (kriging) the average annual velocity data. The urban and suburban areas of Wavre affected by the strongest negative ground deformations (average annual velocity displacement from -1,3 to -4,7 mm/yr) correspond to the zone of water-catchments and pumpings in the Cretaceous aquifer. A link seems to exist between the subsidence zone and the Quaternary sediments of the alluvial plain of the Dyle. In addition to the pumpings effects, the presence of Quaternary peat layers (50 cm to 5,6 m) and the deepness of the Cambrian basement are also important parameters that influence the subsidence process. A progressive decrease of the subsidence values towards the southwest near the village of Ottignies is observed. This is related to the deepness of the Cambrian basement that ranges from 18-22 m in the centre of Wavre to 5 m deep near Ottignies. Field campaigns in the subsidence area reveal millimetre to centimetre cracks in the walls of the buildings attesting active ground motions.

1. INTRODUCTION

The Interferometric Synthetic Aperture Radar (InSAR) is a microwave imaging system of the Earth surface. It has cloud-penetrating capabilities because it uses microwaves. It has day/night operational capabilities because it is an active system. SAR interferometry is a highly effective spatial technique allowing very small surface deformations to be detected and measured from data acquired by the European C-band ERS1/2 and ENVISAT-ASAR satellites. Because of regular acquisitions since 1991, the fifteen years archives of the SAR images have been largely used in order to detect and measure with millimetre accuracy ground movements, using radar images. In urban areas, it is possible to identify numerous back scatterers that do not change their signature with time and called Permanent Scatterers or PS [1] [2]. Therefore, they can be used as natural objects to estimate the progressive ground motion. The high repeat rate of new acquisitions leads to a timely identification of changing deformation characteristics. This is important, for example, in monitoring the stability of individual buildings. It is necessary to use long temporal series of SAR data in order to identify coherent radar targets (PS) where motion and atmospheric contributions can be separated and thus very accurate measurements can be performed. The PSInSAR technique applies in various contexts for the detection and assessment of extended ground deformation having mainly a vertical component (subsidence or uplift). The PS technique has been recently applied in Brussels and revealed an uplift process in the heart of the city along the valley, which is related to the recharge of several aquifers (Hannut Formation, Cretaceous and Cretaceous-Paleozoic basement) excessively withdrawn during the industrialisation period. The uplift consists in an elastic rebound of the previously subsiding and compacting basin [3]. The goal of this paper is to illustrate a subsidence phenomenon observed from Wavre to Ottignies in the Dyle valley (20 km to the southeast of Brussels) and to present the preliminary causes that could explain this ground motion.

2. GEOGRAPHY, HYDROGEOLOGY AND GEOLOGICAL SETTINGS

2.1. Geography of the studied area

The cities of Ottignies and Wavre are situated in the southeastern part of the map (Fig.1, left picture) and included in the hydrological basin of the Dyle river where Wavre is the most important city. More exactly the studied area is located along the Dyle valley. The basin has a mean altitude of 131 meters and covers a surface of around 310 km². The source of the Dyle is located in the village of Houtain-le-Val, 30 km to the southwest of Wavre. This river has 90 tributaries and flows along a southwest - northeast direction through the Belgian provinces of the and . The length of the river is about 100 km.

Fig. 1. Left picture: ground motion velocities (mm/yr) on the 900 km² processed area from Brussels (upper part) and the Wavre - Ottignies area (lower right corner). Right picture: close up view of the PS points available in the Wavre - Ottignies area along the Dyle. Topographic background at 1/100.000 scale-map © NGI.

2.2. Hydrogeological setting

Water catchments in the region are well distributed along the Dyle (Fig.2). Water is extracted from four different aquifers observed successively from the surface to the basement: the alluvial aquifer in the Quaternary sediments of the alluvial plain, the sands of the Brussel Formation (Middle Eocene), the Cretaceous sediments and the fissured Cambrian rocks which is subdivided into two aquifers. The alluvial sediments of the valley composed of sands and gravels constitute the surface aquifer directly related to the river drainage. The sands of the Brussel Formation correspond to the largest water reserve in the area. It is an unconfined aquifer made of sands and silty sands, sometimes consolidated by a carbonaceous matrix. A large confined aquifer is present in the chalks of the Cretaceous. This aquifer is isolated from the Brussel Formation by clayish rocks of the Kortrijk Formation that constitute an aquitard. In the valley, the aquifer is semi-confined due to the removal of post-Cretaceous sediments during erosive action phase [4] [5]. Altered sandstones and quartzites constituting the top of the Cambrian give an interesting sandy aquifer. Different tectonic phases have fissured and faulted the Cambrian rocks during the successive orogens. Faulting zones in the Palaeozoic basement correspond to potentially important aquifer due to the high number of faults filled with sands, which correspond to conduit-type flow in the Cambrian aquifer [4] [6].

2.3. Geological situation

The city of Wavre is built on the valley of the Dyle. In this valley, the geological layers consist of different formations. In the southern part of the valley, the Paleozoic basement outcrops directly on each side and under quaternary alluvial sediments in the centre of the valley. This is visible only around the city of Ottignies and towards the south. Paleozoic rocks are composed of Devillian or Revinian (Cambrian) quartzites, phyllites and quartzophyllites sometimes altered or kaolinized. The irregular and mostly unknown surface shape of the top of the Paleozoic results from Triassic continental erosion. Theses series are also faulted and the general dip is oriented towards the North. Cretaceous formations lie directly below the alluvial deposits in the valley from the Ottignies area towards the north and northeastern areas. The thickness of the Cretaceous increases in the same direction. Cretaceous sediments consist of glauconitic white chalks and argillaceous chalk layers with flint gravels at the base of the Cretaceous. The Cretaceous rocks are covered outside the valley by fine sands and clays of the Hannut (Upper Paleocene) and Kortrijk (Lower Eocene) Formations. Above theses layers, the Brussel Formation is around 40 meters thick and composed by medium to coarse yellow sands. Finally, Holocene and Pleistocene deposits cover all the area with loess on the plateau and loams, sands and peat layers in the valley. The thickness of these series ranges from a few meters to more than 10 meters [4].

3. PS DATA OVERVIEW

The Geological Survey of Belgium (GSB) received the ERS 1/2 data imaging set covering the time span 5th July 1992 to 19th November 2003 and corresponding to seventy-four scenes whereas twelve were removed due to high Doppler Centroïd values. The 10th November 1999 corresponds to the master scene. Tele-Rilevamento Europa (TRE) realized the processing and the identification of the Permanent Scatterers during the Terrafirma program stage1. The total area processed covers a surface of ± 900 km2 (Fig 1) centred on the city of Brussels from Ternat--Zaventem (north side) to Halle - Louvain-la-Neuve (south side). 173767 PS were identified in this area corresponding to a density of 193 PS/km2. This density is highly variable spatially. Very high PS densities are clearly observed in urban environments like the city of Brussels and also in every village like the one of Louvain-la-Neuve where 1080 PS are observed. In contrary, forest areas in the southern part of Brussels like the “Forêt de Soignes”, agricultural landscape and some motorways show a small numbers of available PS. The ERS1/2 data have been imported and displayed into a GIS (Fig.1, left picture). The legend indicates the annual average ground motion (mm/year) for the 173767 PS points detected. The red-orange colours reveal high rates of subsidence in the Wavre-Ottignies - Louvain-la-Neuve area (right lower corner). In contrary, blue-mallow colours in the centre of Brussels (upper left corner) along the Senne valley indicate uplift movements. PS points correspond to the roof of the buildings, metallic structures and prominent natural features that are not present in the forest area (Forêt de Soignes) such as in the centre of the Fig.1. This paper is focused on the ground movements observed in the Dyle basin that includes the cities of Ottignies and Wavre (Fig.1, right picture). This area contains 7182 PS points, which are classified using the average annual ground motion velocities (mm/year): red to orange colours indicate strong subsidence (-4.73 to -1 mm/yr) and blue colours correspond to uplifting movements (1 to 2 mm/yr). The centre of Wavre (Fig.2) is characterized by a strong subsidence phenomenon with an average ground motion velocity rate of 2 mm/yr. A clear decreasing gradient is observed along the Dyle towards the south and southeast direction. Close to Ottignies, the ground motion velocities of the PS are relatively stable and no more clear movements could be detected. An ordinary kriging interpolation (Fig.2) based on the average annual ground motion of the PS has been realized in order to discriminate the regions with strong subsidence. The interpolation reveals negative ground movements in the Dyle valley located essentially in the old alluvial plain bordering the Dyle. The interpolation reveals also a clear change of the ground motions close to the Ottignies-Limal villages. Strong negative velocity values ranging between -4.73 and -1 mm/yr underline the subsidence process observed in the northern part of the Dyle. Positive and slightly negative velocity values are observed in the southern part of the Dyle (ground motion velocities ranging between -1 and +1.1 mm/yr).

4. INTERPRETATION OF THE RESULTS

The subsidence process detected by the PSInSAR technique in the area between Ottignies and Wavre must be related to some geological-human causes. Geological databases (lithology, drill holes, pedology, etc.) and digital geological maps stored in the GSB archives have been integrated in the GIS to compare the ground movements revealed by the PSInSAR interpolation map with the geology. Drill holes descriptions stored in the GSB archives indicate the presence of peat layers in the Quaternary sediments of the alluvial plain. The presence of alluvial peat layers ranging in thickness from 60 cm to 5.6 m in the close vicinity of Wavre could be regarded as a major effect controlling local ground movements. Indeed, the thickness of peat layers could be reduced to nearly 100% of the initial thickness. This happens during a long drawdown of the water table of the alluvial aquifer allowing drying up the peat layers. This phenomenon is only possible if the water table dropdown sufficiently in the Cretaceous aquifer. An inverse trend appears and leads to the development of a percolation phase of the groundwater from the alluvial aquifer towards the Cretaceous one’s. This process leads to dropdown the groundwater level of the alluvial aquifer. It could imply thereafter to dry up the peat resulting in peat layers compaction. This complex process could explain the increasing subsidence ground movement observed towards the centre of Wavre. Nevertheless, a detailed cartography of the peat layers in the Wavre area does not exist at the moment and would be necessary for further investigations. As shown on Fig.2, most of the more productive water catchments (in terms of volume of water extracted), represented by the blue circles in the Dyle valley, are currently pumping water in the Cretaceous aquifer. They are mostly located in the subsidence area close to the city of Wavre.

Fig. 2. Interpolation of the average annual motion rate of the PS points present in the area. Water catchments (blue circles) and piezometers (black triangles) in the Wavre-Ottignies area are presented. Three red squares correspond to field pictures showing the damages observed on the walls of some buildings. They are illustrated on Fig.3.

It seems thus that a relation exists between subsidence and the exploitation of the Cretaceous aquifer. Unfortunately, the measures of the evolution of the piezometric water level since the eighties are not available until now. This situation does not allow to verify if a water level dropdown of the Cretaceous aquifer could be seen in this part of the basin. On the Fig.2, red squares correspond to pictures taken during fieldworks (Fig.3) where cracks on the walls of different buildings were observed. It illustrates probably the impact of the subsidence on the buildings. A time-series of the nearest PS point related to the picture n°3 is presented on the Fig. 3. The time-series corresponds to the evolution of the ground motion velocity in mm/year (Y-axis) against the time. The main evolution of the ground movement corresponds to a progressive decreasing trend, which is more or less irregular in detail. The 11.5 years of subsidence reveals a mean deformation value ranging between -8 mm and -5.3 cm. The cracks seen on the walls and the broken lintel of the window on the picture n°2 confirm that active ground motions affect the buildings in this area.

Fig.3. Cracks observed on the walls of different buildings located in the subsidence zone in and around Wavre. The nearest time-series of the building on the picture n°3 illustrates the ground motion (in mm/yr, Y-axis) against the time.

Fig. 4. The old geological map of the area is drapped on the DEM realized on this area. The black surfaces indicate the zones where the Paleozoic outcrops in the Dyle valley.

On the other hand, the urban development of the city on alluvial sediments, which are the worst places to build, could be also responsible for a part of the local compaction processes and the related ground movements observed. Drill holes illustrate the presence of the Cambrian underneath the Quaternary sediments in the centre of the Dyle valley and directly outcropping on each side of the valley in the southern part. The Cretaceous aquifer is reduced towards the south and disappears at the limit between the subsidence area and the stable one close to Limal and Ottignies. Only few water catchments pump directly in the Cambrian aquifer (Paleozoic rocks) (see Fig.2). The volumes of water extracted are probably not sufficient to modify the groundwater level of this aquifer. Geology is superimposed on a Digital Elevation Model (DEM) (Fig.4) along the Dyle valley. The 3D-model shows that the top of the Paleozoic is outcropping in the south and disappears underneath the Cretaceous and Tertiary units towards the north. This is clearly revealed by the isohypses of the top of the Paleozoic. These data explain the stability of the southern part of the processed area in the vicinity of Ottignies.

5. CONCLUSIONS AND PERSPECTIVES

The Ottignies-Wavre area (Belgium) becomes an area of interest for research since several ground motions have been pointed out by the Permanent Scatterers technique. This is the first time that a subsidence effect has been discovered in the area as there is no special geological risk present in the vicinity of Wavre such as karts, faults, landslides, etc. The average annual velocity displacement of the ground ranges between -1,3 and -4,7 mm/yr for a time-span between 1992 and 2003. The subsidence process occurs essentially along the Dyle (almost SW-NE direction) and covers a surface of 33 km². Field campaigns in Wavre and surroundings showed that several houses and buildings have cracks on the walls probably due to recent ground motions. The most productive water catchments in the Cretaceous aquifer, which are present in the city and the Dyle valley, combined with the presence of peat layers in the Quaternary sediments of the alluvial plain of the Dyle could be responsible for the ground motions observed in that area. Unfortunately, water table evolution given by the piezometric level data in the Cretaceous aquifer is almost unknown since the eighties. A detailed cartography of the peat layers is also essential to understand where ground movements could appear. Excessive withdrawn of the water level in the Cretaceous aquifer by the increasing volume of water extracted create an inversion process between the alluvial and the Cretaceous aquifers. A balance between the recharge of the water in the Cretaceous aquifer and the volume of water extracted seems to be understood at the scale of the basin. In addition, new PSInSAR analyses must be also undertaken along the northern branch of the river to detect how the subsidence (if any) affects this area. In the north of Wavre, other water catchments are present along the Dyle. An update of the PSInSAR since end- 2003 will help to follow up the evolution of the subsidence phenomenon. The authorities must also envisage a survey, in the same time, of the evolution of the subsidence with the one of the water table of the Cretaceous aquifer. Subsidence caused by compaction of overdrafted aquifer systems is a problem in urban areas heavily dependent on groundwater supplies. Subsidence processes are known and described in the literature in many areas around the world such as those in the US basins: Las Vegas (Nevada, [7]), Santa Clara Valley (San Francisco Bay of California, [8]). Pumping of groundwater results in surface deformation and raises critical issues from the standpoint both of public protection and economic impact.

6. REFERENCES

[1] Ferreti A., Prati C. & Rocca F., Nonlinear subsidence rate estimation using permanent scatterers in differential SAR Interferometry. IEEE Transactions on Geoscience and Remote Sensing, 38(5), 2202-2212, 2000. [2] Ferreti A., Prati C. & Rocca F., Permanent scatterers in SAR Interferometry. IEEE Transactions on Geoscience and Remote Sensing, 39(1), 8-20, 2001. [3] Devleeschouwer X., Pouriel F. & Declercq P.-Y., Vertical displacements (uplift) revealed by PSInSAR technique in the centre of Brussels, Belgium. Proceedings of the 10th IAEG Congress, Nottingham, United Kingdom, September 2006, submitted. [4] Duchateau J., Herbigniaux J., Lhoir L. & Raphael M., Etude des ressources en eaux du Brabant Wallon, Rapport interne, Contrat Région Wallonne - I.B.W., 102 pages, 1987. [5] Gulinck M. & Loy W., Hydrogéologie du Crétacé du Bassin de la Dyle (Brabant), Bulletin de la Société belge de Géologie, de Paléontologie et d’Hydrogéologie, 80(1-2), 77-83, 1971. [6] Laurent E. (Ed.) Monographie du Bassin de la Dyle, Ministère de la Santé Publique et de l’Environnement, Noyau de l’eau - Commission Dyle, 302 pages, 1978. [7] Amelung F., Galloway D.L., Bell J.W., Zebker H.A. & Laczniak R.J., Sensing the ups and downs of Las Vegas: InSAR reveals structural control of land subsidence and aquifer-system deformation. Geology, 27(6), 483-486, 1999. [8] Ferreti A., Novali F., Bürgmann R., Hilley G. & Prati C., InSAR Permanent Scatterer Analysis reveals ups and downs in San Francisco Bay area. Eos, 85(34), 317-324, 2004.