## 2 Geographical and Geological Setting  
The area of Brussels, and more particularly the historical heart of the city, is crosscut by the Senne valley along a SW-NE axis (Figure 1). Some less important, parallel valleys (such as the Maelbeek and Woluwe) have incised the eastern part of the urban landscape of Brussels. The altitude difference between the top of the hills and the base of the valleys reaches more than 80 m. The large alluvial plain of the Senne River inclines gently towards the north from an altitude of 19 m in the south to 13 m in the north. Figure 1: Localisation of the studied area around the Brussels Region (bold black line). The Quaternary alluvial plains (blue hatched surfaces) of the Senne, Maelbeek and Woluwe rivers are highlighted and the common reference point taken for all the Synthetic Aperture Radar (SAR) images is located in the NE side of the area (red star). The region of Brussels (Figure 2) is located over a major geological structure from the Paleozoic age called the Brabant Massif, which covers large areas of Belgian territory. The Brabant Massif consists of a compressed wedge, the core of which is formed by steeply deformed Cambrian formations [19] that are flanked to the NE and SW by younger Ordovician and Silurian formations [20]. The paleotopographical surface of the Brabant Massif basement is affected along the Senne valley by a chain of NW-SE trending ridges and depressions that have a width ranging between 0.5 to 1 km and a spacing of 1 to 1.5 km [21]. The steeply dipping Lower Paleozoic deposits that occur in subcrop in the mapped area belong to the Bhanmont, (quartzites), Tubize (from shales to sandstones) and Oisquercq (claystones to siltstones) formations. The Brabant Massif was a persistent positive area with only reduced sedimentation during the Late Paleozoic period. The Carboniferous was probably removed by erosion during a Jurassic uplift [22]. Late Cretaceous chalk deposits (Senonian Age) are partly preserved and were eroded by an uplift phase of the Brabant Massif during the middle Paleocene Laramide phase [23]. Sedimentary Tertiary marine series, composed of clays, silts and sands, occur in numerous eusattic cycles. The subsurface geology of the latter is detailed in the following paragraphs. Anthropogenic fillings are present almost everywhere and have a thickness up to several meters. Holocene alluvial sediments, composed of loam, sand, clay, peat and gravel layers, are essentially located in the valleys. Their thickness ranges generally from 10 to 20 m. Pleistocene deposits (mainly loess) cover the whole area. Their thickness varies strongly and can reach up to 20-30 m. Underneath the Quaternary continental sediments, Cenozoic marine formations are present, their age spanning from the Upper Miocene down to the Upper Paleocene (Figure 2). The Diest Formation (Upper Miocene) and the Bolderberg Formation (Lower Miocene) correspond to limited sand deposits at the top of the hills in the northern part of Brussels. The Sint-Huibrechts-Hern sandy Formation (Upper Eocene) has a reduced thickness in the north while in the southeast it can reach more than 10 m. The Maldegem Formation (Middle Eocene) contains four Members: Zomergem, Onderdale, Ursel and Asse and, finally Wemmel. The Maldegem Formation is relatively good for house foundations except when it is saturated with water. The Lede Formation (Middle Miocene) is made of sands and sandy calcareous layers. The Bruxelles Formation (Middle Eocene) is characterised by coarser sands than those of Lede. The Gent Formation (Lower Eocene) reaches a thickness of 8 m and is only observed in the north-western part of Brussels. The clay and fine sands of the Tielt Formation (Lower Eocene) are generally 20 m thick. The Kortrijk Formation (Lower Eocene) is subdivided into three Members and mainly composed of clays, silts and sands. The average total thickness of the Kortrijk Formation reaches 70 m. The Upper Paleocene Formation (Hannut) contains an upper sandy Member (Grandglise) and a lower greenish clay Member (Lincent).