ALBERT

Description: Freshwater lenses, as important resource for drinking water, are sensitive to climate changes and sea level rise. To simulate this impact on the groundwater systems, hydraulic subsurface models have to be designed. Geophysical techniques can provide information for generating realistic models. The aim of our work is to show how ground-penetrating radar (GPR) investigations can contribute to such hydrological simulations. In the pilot area, Borkum island, GPR was used to map the shape of the groundwater table (GWT) and to characterise the aquifer. In total, 20 km of constant offset (CO) profiles were measured with centre frequencies of 80 and 200 MHz. Wave velocities were determined by common midpoint (CMP) measurements and vertical radar profiling (VRP) in a monitoring well. The 80 MHz CO data show a clear reflection at the groundwater table, whereas the reflection is weaker for the 200 MHz data. After correcting the GPR water tables for the capillary rise, they are in good accordance with the pressure heads of the observation wells in the area. In the centre of the island, the groundwater table is found up to 3.5 m above sea level, however it is lower towards the coastline and marshland. Some local depressions are observed in the region of dune valleys and around pumping stations of the local water supplier. GPR also reveals details within the sediments and highly-permeable aeolian sands can be distinguished from less-permeable marine sediments. Further, a silt loam layer below the water table could be mapped on a large area. The reflection characteristics indicates scattered erosion channels in this layer that cause it to be an aquitard with some leakage. GPR provides a high resolution map of the groundwater table and insight into the stratigraphy of the sediments and their hydraulic properties. This is valuable complementary information to the observation of sparsely distributed monitoring wells as input to hydraulic simulation.

Description: A numerical, density dependent groundwater model is set up for the North Sea Island of Borkum to estimate climate change impacts on coastal aquifers and especially the situation of barrier islands in the Wadden Sea. The database includes information from boreholes, a seismic survey, a helicopter-borne electromagnetic (HEM) survey, monitoring of the freshwater-saltwater boundary by vertical electrode chains in two boreholes, measurements of groundwater table, pumping and slug tests, as well as water samples. Based on a statistical analysis of borehole columns, seismic sections and HEM, a hydrogeological model is set up. The groundwater model is developed using the finite-element programme FEFLOW. The density dependent groundwater model is calibrated on the basis of hydraulic, hydrological and geophysical data, in particular spatial HEM and local monitoring data. Verification runs with the calibrated model show good agreement between measured and computed hydraulic heads. A good agreement is also obtained between measured and computed density or total dissolved solids data for both the entire freshwater lens on a large scale and in the area of the well fields on a small scale. For simulating future changes in this coastal groundwater system until the end of the current century, we use the climate scenario A2, specified by the Intergovernmental Panel on Climate Change and, in particular, the data for the German North Sea coast. Simulation runs show proceeding salinisation with time beneath the well fields of the two waterworks Waterdelle and Ostland. The modelling study shows that the spreading of well fields is an appropriate protection measure against excessive salinisation of the water supply until the end of the current century.

Description: Freshwater lenses within islands are an important resource for drinking water. The aim of the GPR investigation was to map the shape of the groundwater table and sedimentary structures on Borkum island as input parameters for hydrogeological simulation. In total, 20 km of constant offset (CO) radar profiles were measured with centre frequencies of 80 and 200 MHz. Wave velocities were determined by common midpoint (CMP) measurements and vertical radar profiling (VRP) in a monitoring well. The 80 MHz CO data show a clear reflection at the groundwater table, whereas the reflection is blurry and shifted to lower frequencies for the 200 MHz data. This is caused by the gradual increase of water content above the capillary fringe. The GPR-derived water tables are in good accordance with the observation of the monitoring wells in the area. In the centre of the island, the groundwater table is found up to 3.5 m above sea level, however it is lower towards the coast line. Some local depressions are observed in the region of dune valleys and around pumping stations of the local water supplier. GPR also reveals details within the sediments and highly-permeable aeolian sands can be distinguished from less-permeable marine sediments. A sharp horizontal reflection below the water table can be seen on many profiles and is identified as a hydraulically-tight silt loam layer by hand-drilled boreholes. Moreover, GPR data indicate scattered erosion channels in this layer that cause it to be an aquitard with some leakage. GPR provides a high resolution map of the groundwater table and insight into the stratigraphy of the sediments that are a valuable complementary information to the observation of monitoring wells.

Description: A numerical variable-density groundwater model is set up for the North Sea Island of Borkum to estimate climate change impacts on coastal aquifers and especially the situation of barrier islands in the Wadden Sea. The database includes information from boreholes, a seismic survey, a helicopter-borne electromagnetic survey (HEM), monitoring of the freshwater-saltwater boundary by vertical electrode chains in two boreholes, measurements of groundwater table, pumping and slug tests, as well as water samples. Based on a statistical analysis of borehole columns, seismic sections and HEM, a hydrogeological model is set up. The groundwater model is developed using the finite-element programme FEFLOW. The variable-density groundwater model is calibrated on the basis of hydraulic, hydrological and geophysical data, in particular spatial HEM and local monitoring data. Verification runs with the calibrated model show good agreement between measured and computed hydraulic heads. A good agreement is also obtained between measured and computed density or total dissolved solids data for both the entire freshwater lens on a large scale and in the area of the well fields on a small scale. For simulating future changes in this coastal groundwater system until the end of the current century we use the climate scenario A2, specified by the Intergovernmental Panel on Climate Change and in particular the data for the German North Sea coast. Simulation runs show proceeding salinization with time beneath the well fields of the two waterworks Waterdelle and Ostland. The modelling study shows that spreading of well fields is an appropriate protection measure against excessive salinization of the water supply until the end of the current century.

Description: Dissolution of rocks such as anhydrite, gypsum, limestone, dolomite or salt rock enlarges fractures or pore space and finally results in voids and cavities within karst rocks. Especially in populated areas such collapses may have catastrophic consequences, but the current knowledge about the evolution, governing processes and capabilities of prediction are insufficient. Geophysical methods can contribute to the understanding, first of all by detailed investigation of known sinkhole structures and - later on - by monitoring areas of high risk as well as localising those areas before a surface expression is visible. As voids and cavities are often fluid- or air-filled (or both) and the electrical properties of the fluid and air are in many cases significantly different to those of the surrounding karst rocks, electric and electromagnetic methods such as, e.g., electrical resistivity tomography (ERT), ground-penetrating radar (GPR) and nuclear magnetic resonance (NMR) are potentially useful tools for the investigation of sinkhole structures. In the case study presented here we focus on the application of electric and electromagnetic methods on the karst system of the Innerste Mulde in Lower Saxony which is surface-mapped by numerous sinkhole structures.

Description: Understanding Faults: Detecting, Dating, and Modeling offers a single resource for analyzing faults for a variety of applications, from hazard detection and earthquake processes, to geophysical exploration. The book presents the latest research, including fault dating using new mineral growth, fault reactivation, and fault modeling, and also helps bridge the gap between geologists and geophysicists working across fault-related disciplines. Using diagrams, formulae, and worldwide case studies to illustrate concepts, the book provides geoscientists and industry experts in oil and gas with a valuable reference for detecting, modeling, analyzing and dating faults.

Description: With amplitude and full-waveform based ambient noise tomography and monitoring methods on the horizon, knowledge of the underlying noise source distribution is vital to avoid possible misinterpretations, e.g., in terms of time-varying Earth structure. Particularly the oceanic microseisms have strong spatio-temporal variations on multiple scales which could influence full waveform or travel time measurements if not taken into account properly. In this work, we present daily Seismic Ambient Noise Source (SANS) maps of the secondary microseisms (0.1 to 0.2 Hz) on a global scale which are made available to the public here: sans.ethz.ch (Igel et al., 2022). The computation of daily global SANS maps is possible due to recent improvements of non-linear finite-frequency noise source inversion methodology including pre-computed wavefields and spatially variable grids (Igel et al., 2021). Furthermore, by introducing an initial model from a different noise source imaging method - Matched Field Processing (MFP) - we accelerate the convergence of the inversion and improve the final maps. In collaboration with the Swiss National Supercomputing Centre (CSCS), we are able to run daily global SANS inversions which can be viewed, downloaded, and implemented into other studies. This paves the way for future full waveform ambient noise source and structure inversion workflows.