Ground movement analysis in the north of Belgium by radar interferometry

Document available in extended abstract form only. In the frame of the long-term management of high-level and/or long-lived radioactive waste in Belgium, the Belgian Agency for Radioactive Waste and Enriched Fissile Materials (ONDRAF/NIRAS) presently investigates the plastic clays (i.e., Boom Clay and the Ypresian clays) occurring in the northern part of Belgium as a potential host-rock for geological disposal. Boom Clay is a Rupelian (i.e., 34-28 Ma) silici-clastic sediment deposited in an open marine environment at the southern border of the epi-continental North Sea Basin. The Ypresian clays refer to the nearly continuous sequence of the clay-rich sediments deposited early in the Ypresian stage (i.e., 56-48 Ma) in a similar basin configuration. The investigated area (6400 km2 extents E-W from the Roer valley graben in The Netherlands to Antwerpen and N-S from Tilburg to Diest) is characterized by a succession of sub-horizontal (1-2 deg. dip towards N to NE) layers of Cenozoic clays and sands; this thick sedimentary cover overlies an old Palaeozoic basement and is covered by some Quaternary sediments. Moreover, the northern end of the region of interest (ROI) is characterized by the famous Roer Valley Graben. At Mol, a reference site for the research, development and demonstration of the safety and the feasibility of geological disposal, the Boom Clay is present at a depth of approximately 190 to 290 meters below a thick Neogene sandy sequence. At Doel, another reference site for the RD and D, Boom Clay is present at shallow depth (from 56 m to 113 m) whereas Ypresian clays occur at depth relevant for geological disposal (i.e., 340-440 m). Both clay units are separated by a mid to latest Eocene intercalation of sands, clayey sands and clay layers. Boom clay at the Doel site is overlain by Neogene deposits and by the alluvium of the Quaternary Flemish Valley. The aim of the study is to determine, quantify and cartography ground movements, if any, in the region of interest by the Persistent Scatterer Radar Interferometry technique. In parallel, Mol and Doel were specifically processed to extract as much PS as possible from the SAR images to increase the resolution and the quality of the interpretation around the two RD and D reference sites. This technique is indeed able to measure millimetric ground deformation over large area (>100 km2) on a raw from satellite images. In the case such ground motion would be identified, it is of major importance to find the origin and the magnitude of the on-going processes. Considering the geological setting of the area and related human activities, numerous processes could be envisaged (e.g., tectonic activity related to the Roer valley graben and its faults system, groundwater (over)exploitation, recent sediments compaction, landfills, former coal mines areas, etc...). The first processing was carried out over 68 ERS1/2 images covering the years 1992-2001 and 67 ENVISAT images spanning the years 2003-2010. The usual time period between each satellite image acquisition is 35 days. The total number of PS for ERS and ENVISAT is respectively 320,734 and 145,052 over the entire studied area, the PS density reached usually more than 200 PS/km2 in urbanised area and dropped to less than 20 PS/km2 in rural zone. Several localized and specific ground motions are detected in the ROI. The western border of the Kempen coal basin faced a negative vertical movement (-16 mm/year max) during the time span of ERS 1/2 images. The vertical ground movement monitored with the ENVISAT images shows an inversion of the velocities since 2003 with positive velocities (maximal annual mean velocity values of +10 mm/year ). The movements are related to the ground water extraction during the exploitation of the coal mines generating subsidence and to the recharge of the mine aquifer after the closure exploitations in 1992 facilitating the uplift of the region. In the vicinity of Mol, the PS data shows that this region is slightly uplifting at an average rate of about 1 mm/year with a range between -1.67 and 2.45 mm/year. However negative values exist representing only 1 % of the data available. These PS are isolated and surrounded by positive velocities. Local movements around Mol are part of a broader context of uplifting in the Kempen. The whole city of Antwerpen and more specifically the harbour is clearly subsiding both in the ERS and ENVISAT data with mean annual velocities ranging between -4 to -2 mm/year. The origin of this strong subsidence would be the compaction of the recent sediments of the Schelde valley and several landfills areas. The Doel area is also concerned by this highlighted subsidence, nevertheless the present interpretation concerns very shallow (<13 m thick) Quaternary (deposits and should therefore only affect constructions at the surface. (authors)