Calculating thickness variations of soft sediments above bedrock is important for site effect characterisation, earthquake ground motion amplification and for hydrogeological and geothermal purposes. In case seismic array instrumentation is not available and hence shear-wave velocity profiles cannot be obtained, other correlation techniques need to be applied to accurately deduce bedrock depth. Nakamura’s H/V Spectral Ratio (HVSR) analysis of ambient noise is a powerful seismological method to reveal a site’s resonance frequency. The conversion from a resonance map to a bedrock depth map in areas with a different sedimentary cover in terms of layer thickness and lithologies is, however, not straightforward. Converting resonance frequencies to depth by applying a mean shear-wave velocity (Vs) will under- and overestimate bedrock depth at higher and lower topographies, respectively. Applying an empirical log-log (powerlaw) relationship between resonance frequency (obtained from HVSR analyses) and bedrock depth (obtained from boreholes) provides a much better depth estimation that considers the non-linear increase of Vs with depth. Accurate empirical equations can however only be constructed from HVSR measurements performed in areas with similar lithological sediments. In this study we present a high-resolution microzonation study performed in Brussels (Belgium) where both the sedimentary cover and the fractured top of the bedrock (i.e. the Brabant Massif) are of interest for their geothermal potential. Using 88 ambient noise measurements above boreholes we constructed four different powerlaw equations that are applicable to convert resonance frequency to depth in areas with a clayey, sandy-clayey, alluvial-clayey and alluvial sedimentary cover. Subsequently, 405 ambient noise measurements were conducted and converted to virtual boreholes using these four empirical equations. Measurements were used to map out bedrock depth in a 15km2 and a 25 km2 area applying a 200 m and 500 m station density spacing, respectively. The results demonstrate the presence of NW-SE oriented, 20 m-high ridges at 100 m depth that stand out because of differential erosion between less-resistant slaty (Tubize Formation) and hard quartzitic (Blanmont Formation) rock formations of the Brabant Massif. Separating seismic data according to their subsurface geology results in more accurate empirical frequency-depth conversion equations than if only one equation would be used for an entire area.
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RBINS Staff Publications 2018
Geological databases resulting from the merging of various data sources and time periods jeopardize harmonization of data products. Data standardization is already common practice and a first step in avoiding semantic overlap. European marine data management infrastructures provide such standards, e.g., Geo-Seas (http://www.geo-seas.eu/) for geological data and SeaDataNet (https://www.seadatanet.org/) for marine metadata in general. In addition, metadata quality control is important, though data uncertainty is seldom quantified and to be used in modelling. Preliminary uncertainty analyses were worked out to provide an extra dimension to the cross-border 3D voxel models of the geological subsurface of the Belgian and southern Netherlands part of the North Sea (http://odnature.naturalsciences.be/tiles/). Starting from simple quality flagging in geological databases and model uncertainty calculations (probability and entropy) in the 3D modelling, data uncertainty (e.g., related to qualities in positioning, sampling and vintage) is now quantified. Combining all uncertainties remains a challenge, as well as communicating their importance in decision making. A demonstration will be given on the status of the uncertainty analyses and how these are incorporated in a newly developed decision support tool allowing interactive querying of the 3D voxel model, now comprising geological, as well as entropy, probability and data uncertainty attributes (figure 1).
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RBINS Staff Publications 2018
Geoscience, and understanding Earth’s systems, is essential to provide the resources we need while maintaining a habitable environment, contributing towards a more sustainable society. Resources from the subsurface: groundwater, geo-energy and raw materials, represent essential elements for society. Strong and integrated geological knowledge and expertise is essential to acquire data and transform it into reliable and functional information to underpin the continued development and growth of humankind. At the national and regional level, public authorities across Europe and globally have recognized these needs, leading to the establishment of Geological Survey Organizations (GSOs). In response to growing requests for pan-European data, intensified collaboration amongst GSOs, under the umbrella organization EuroGeoSurveys, recently led to the launch of the ERA-NET Cofund Action GeoERA: “Establishing the European Geological Surveys Research Area to deliver a Geological Service for Europe”. GeoERA – a demonstrator project for a Geological Service for Europe (2017-2021) – is a 30M EUR programme supported by 45 national and regional GSOs from 33 countries in Europe. It contributes to the sustainable use of the subsurface by delivering expertise, data and information to policy and decision makers through a single access point, based on the European Geological Data Infrastructure (EGDI). GeoERA and EGDI are both initiatives of EuroGeoSurveys (EGS), an international non-profit organization representing the national GSOs from 36 European countries. Equipped with additional support from the European Commission, the GSOs intend to establish a Geological Service for Europe (GS4E) that builds on the ongoing GeoERA projects and is tailored to suit the dynamic needs of society, policy and decision makers. The mission of a Geological Service for Europe represents a robust and sustainable single access point to pan-European, harmonized and interoperable expertise, geoscientific data and information, through increased collaboration of the Geological Survey Organizations within Europe. This GS4E will provide the European Commission and other stakeholders with open access to relevant and fair subsurface knowledge, represented in pan-European maps and RDI projects/publications, to support decision making and sustainable use of the subsurface. It will address the Sustainable Development Goals related to the Earth system through delivering expertise, data and information to assess our water resources, assess and develop affordable and clean energy, support sustainable economic growth and employment, support innovation in subsurface management, assess risks of subsurface use that can jeopardize safe and resilient cities, minimize and mitigate climate change impacts and support research on sustainable alternatives. The GS4E may also contribute to the so-called adaptation needs, that is, anticipating the adverse effects of climate change and taking appropriate action to prevent or minimize the damage they can cause, or taking advantage of opportunities that may arise. A well planned, early adaptation action will contribute to economic development through reducing imports of energy and mineral resources, increasing resilience and reducing the impact of extreme natural events, securing and enhancing safety in a long-term strategy on use of scarce water resources and improved land-use planning.
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RBINS Staff Publications 2019
