Xavier Devleeschouwer

Laser-induced breakdown spectroscopy (LIBS) imaging is a versatile technique that can produce megapixel elemental maps of solid samples, including geological samples. We describe a mostly open-source, laboratory-based LIBS elemental imager assembled from low-cost CNC-derived components: ball-bearing linear stages controlled by the open-source Grbl_ESP32 firmware. The setup uses a fully hardware-triggered acquisition chain: the stepper motor step pulses are counted and divided by an Arduino-based pulse controller that triggers the laser which in turns triggers the spectrometers so shot spacing is locked to stage position and does not depend on software timing. The laser and the spectrometers are proprietary, but equivalent models can be used. An air pump keeps the focusing head under positive pressure to protect the focusing optics, and a HEPA-filtered aspiration at the ablation zone captures the ejected particles. A Python/PyQt5 application streams dark-corrected, wavelength-concatenated spectra into a compressed NetCDF4 hyperspectral cube, generates progress previews and recovers automatically from laser faults; a companion application provides masking, normalization, elemental maps, RGB composites and photo co-registration. We document the mechanics, electronics, firmware and software, and validate the design with megapixel-scale maps of geological samples. All design files are released under open licenses.

Spatio-temporal ground-movement measurements and mappings have been carried out in the Campine coalfield in Belgian Limburg since the closure of the mines to document post-mining effects. MT-InSAR measurements are compared to groundwater head changes in the overburden and to height data from the closest GNSS stations. Radar interferometry is used to estimate the extension and the velocity of ground movements. In particular, the MT-InSAR technique has been applied to SAR acquisitions of the satellites ERS-1/2 (1991–2005), ENVISAT (2003–2010), COSMO-SkyMed (2011–2014), and Sentinel-1A (2014–2022). The images were processed and used to highlight a switch from subsidence to uplift conditions in the western part of the coal basin, while the eastern part had already been affected by a rebound since the beginning of the ERS-1/2 acquisitions. Following the closure of the last active colliery of Zolder in 1992 and the subsequent cease of mine-water pumping, a recharge of mine-water aquifers occurred in the western part of the basin. This process provoked the change from subsidence to uplift conditions that was recorded during the ENVISAT period. In the center of the coal-mining area, measured uplift velocities reached a maximum of 18 mm/year during the ENVISAT period, while they subsided at −12 mm/year during the ERS-1/2 period. Mean velocities in the western and eastern parts of the coalfield area have decreased since the last MT-InSAR measurements were performed using Sentinel-1A, while the Zolder coal mine continues to rise at a faster-than-average rate of a maximum of 16 mm/year. The eastern part of the coalfield is still uplifting, while its rate has been reduced from 18 mm/year (ERS-1/2) to 9 mm/year (Sentinel-1A) since the beginning of the radar–satellite observations. Time-series data from the two GNSS stations present in the study area were used for a local comparison with the evolution of ground movements observed by MT-InSAR. Two leveling campaigns (2000, 2013) were also used to make comparisons with the MT-InSAR data. The station’s measurements and the leveling data were in line with the MT-InSAR data. Overall, major ground movements are obviously limited to an extension of the actual underground-mining works and rapidly diminish outside of them.

Interferometric Synthetic Aperture Radar (InSAR) provides essential information dealing with different natural hazards caused by hydrogeological processes turned into subsidence (Lu et al., 2010). Among all geodetic techniques for monitoring land surface deformation, InSAR has demonstrated the most precise observations in both high spatial and temporal resolution (Dehghani et al., 2009). Different cities of Belgium are subject to land subsidence mostly induced by groundwater over-exploitation. In this study, we present Persistent Scatterer (PS-InSAR) over Belgium to highlight land surface deformation between 2003 to 2010. To this purpose, large-scale interferometry is accomplished by applying the PS-InSAR algorithm on three tracks of ENVISAT ASAR radar images covering the entire country. To provide an integrated land surface velocity map from several single acquisitions, eight stacks of ENVISAT ASAR C-band frames of Single Look Complex (SLC) data acquired from track 380, 423, and 466 spanning from 2003 to 2010 are used. Time series of deformation and mean velocity map are two products of applying PS-InSAR on the dataset. Joining single interferograms generated from successive frames along their acquisition tracks makes a randomly directed phase ramp in the mean velocity map due to atmospheric effects. The atmospheric effect composed of vertical and turbulent components is one of the most important error sources in using space-based deformation monitoring approaches. Monitoring land surface deformations, the atmospheric artifacts could mask real values of deformation phase calculated by different InSAR methodologies (Yu et al., 2018). To address this problem, different methods having different degrees of accuracy are widely suggested in the literature (Xiao et al., 2021). The patchy active deforming areas over Belgium along with a variety of atmospheric conditions through time and very slow ground deformation velocities make it a perfect case study for applying different proposed approaches for reducing the atmospheric effects. These approaches could be categorized into two main groups: i) those which are method-dependent and reduce all stochastic residual noise through processing, and ii) those which are based on external data such as meteorological features. The main advantage of the first group is that using them will not require any external data. However, the drawback is that the high spatial variability of the atmospheric artifact is disregarded by applying them. Overcoming this drawback, using professional GPS data is the most accurate method of the second group. Zenith Total Delay (ZTD) measurements are provided with the highest temporal resolution by GPS; therefore, additional uncertainties are inhibited (Yu et al., 2018). Using this method is an ongoing part of this study, but before that the Generic Atmospheric Correction Online Service for InSAR (GACOS) has been applied. The GACOS procedure is a developed package in the TRAIN software. Applying this procedure to the interferograms showed no tangible changes in them. In this study which is being carried out as part of the BRAIN BESLSPO project: "monitoring LAnd SUbsidence caused by Groundwater exploitation through gEOdetic measurements (LASUGEO)", we benefit from ZTD files which are by-products of GNSS processing to mitigate atmospheric effects.

The coupled effects of climate change, sea-level rise, and land sinking in estuaries/alluvial plains prone to inundation and flooding mean that reliable estimation of land movements/subsidence is becoming more crucial. During the last few decades, land subsidence has been monitored by precise and continuous geodetic measurements either from space or using terrestrial techniques. Among them, the Persistent Scaterrer Interferometry (PSInSAR) technique is used on the entire Belgian territory to detect, map and interpret the identified ground movements observed since 1992. Here the research focuses on one of the biggest cities in Belgium that became the second European harbour with giant docks and the deepening of the Scheldt river allowing the navigation of the largest container vessels. The areas along the embankments of the Scheldt river and the harbour facilities are associated to Holocene fluviatile deposits overlain by recent landfills. These sedimentary deposits and human-made landfills are affected by important and ongoing land subsidence phenomena. The land subsidence process is highlighted by an annual average Line of Sight (LOS) velocity of about −3.4 mm/year during the years 1992–2001 (ERS1/2 datasets), followed by an annual average LOS velocity of about −2.71 mm/year and −2.11 mm/year, respectively, during the years 2003–2010 (ENVISAT) and 2016–2019 (Sentinel 1A). The Synthetic Aperture Radar (SAR) imagery data indicate a progressive decrease in the average annual velocities on a global scale independently of important local variations in different districts along the Scheldt river. On the contrary, the city centre and the old historic centre of Antwerp are not affected by negative LOS velocities, indicating stable ground conditions. A geological interpretation of this difference in settlement behaviour between the different areas is provided.

StratigrapheR is an open-source integrated stratigraphy package. It is available in the free software environment R (https://CRAN.R-project.org/package=StratigrapheR) and is designed to generate lithologs in a semi-automated way, to process stratigraphical information, and to visualize any plot along the lithologs in the R environment.The basic graphical principle behind StratigrapheR is the incremental addition of elements to a drawing: a plot is opened, and graphical elements are successively added. This allows compartmentalisation of the drawing process, as well as the superposition of different plots for comparison. For instance a litholog of a single section can be written as a single function including all the drawing sub-functions, and be integrated in a larger plot, for instance to be correlated to other sections or to show proxy data.The StratigrapheR package is designed for efficient work, and minimum coding, while still allowing versatility. The lithological information of beds (upper and lower boundary, hardness, lithology, etc.) is converted into polygons. All polygons are drawn together using a single function, and each polygon can have its personalised symbology allowing to distinguish lithologies. A similar workflow can be used for plotting proxies while distinguishing each sample by their lithology. Vector graphics can be imported as SVG files, and precisely drawn with the lithologs to serve as symbols or complex elements. Every type of symbol is plotted by calling one single function which repeats the drawing for each occurrence of the represented feature. This illustrates that the amount of work invested to make lithologs using StratigrapheR is related to their complexity rather than their length: a long but monotonous litholog (e.g. of marl-limestone alternations) only takes a few lines of code to generate.The StratigrapheR package also provides a set of functions to deal with selected stratigraphic intervals (for instance in the [0,1[ form): they allow simplification, merging, inversion and visualisation of intervals, as well as identifying the samples included in the given intervals, and characterising the relation of the intervals with each other (overlap, neighbouring, etc.). StratigrapheR includes PDF and SVG generation of plots, of any dimension. The generated PDF can even store multiple plots in a single file (each plot on a different page) to document data processing comprehensively.