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
Shells are powerful climate archives - they add growth increments on timescales as short as sub-daily, and often live for decades, some even more than 100 years. With the aid of isotope and trace-elemental geochemistry, the effects of climate change on temperature, seasonality and extreme weather can be read from them. Belgium is one of the few countries blessed with extensive records of exquisitely preserved fossil shells dating to the Pliocene, a geologic period dating from 5.3 to 2.6 million year ago. Critically, the Pliocene is the youngest geologic time during which CO2 levels were >400 ppm and mean annual temperatures comparable to those to be reached by the end of this century, following Shared Socioeconomic Pathways (SSP) 2-4.5 of the IPCC. It therefore presents an ideal near-future analogue. Rich collections of well-preserved Belgian Pliocene shells are in the Royal Belgian Institute of Natural Sciences (RBINS), and more material is collected from temporary outcrops like building sites in and around Antwerp with the aid of citizen-scientists. In recent years, RBINS collaborated with national (VUB, KULeuven) and international (VU Amsterdam, Naturalis, UDerby) researchers to start tapping into these exquisite climate archives, unraveling previously unknown details on Belgian past climate, predicting amplified seasonality in Europe in a warmer world, and investigating the potential of fossil shells to document heat waves and storms. The poster will highlight some of this recent collaborative work, and, why the RBINS, through its collections, fieldwork and expertise can play a pivotal role in climate research in Belgium.
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RBINS Staff Publications 2024
Based on the current spread of exotic mosquito species (EMS) in Europe, the number of interceptions in Belgium and suitability models developed for Aedes albopictus (Skuse) in Europe, EMS are likely to establish and spread in Belgium. A prerequisite for their control is their early detection. Therefore, the Belgian federal authorities and the federated entities funded a 3-year active monitoring project (MEMO) (July 2017–June 2020). The aims are early detection of EMS in Belgium, quantifying locally established EMS populations, evaluating the EMS import risk at possible points of entry (PoE), expand reference collections and make recommendations for a future, long-term, cost-effective EMS monitoring plan in Belgium. Monitoring activities are implemented at 23 PoE using adult trapping with CO2 and lure traps, egg sampling with oviposition traps and larval sampling with dipping nets. DNA barcoding is used to validate morphological identifications and to expand the DNA reference database. Specimens are also added to the morphological reference collection at Royal Belgian Institute of Natural Sciences. Since July 2017, four EMS were intercepted. The colonised area of Ae. koreicus (Edwards) in Belgium increased from 7 to 113 km2. Aedes japonicus (Theobald) was detected again in southern Belgium, from where it was thought to be eliminated. This species has now also been collected on the border with Germany. Anopheles pharoensis (Theobald) entered Belgium via cargo transport. Aedes albopictus was intercepted at four PoE. To conclude, EMS are effectively entering and spreading in Belgium and appropriate control management strategies on the national level are urgently required.
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RBINS Staff Publications 2018
