2012, Burschil, T., Scheer, W., Kirsch, R., Wiederhold, H.

Within the scope of climatic change and associated sea level rise, coastal aquifers are endangered and are becoming more a focus of research to ensure the future water supply in coastal areas. For groundwater modelling a good understanding of the geological/hydrogeological situation and the aquifer behavior is necessary. In preparation of groundwater modelling and assessment of climate change impacts on coastal water resources, we setup a geological/hydrogeological model for the North Sea Island of Föhr. Data from different geophysical methods applied from the air, the surface and in boreholes contribute to the 3-D model, e.g. airborne electromagnetics (SkyTEM) for spatial mapping the resistivity of the entire island, seismic reflections for detailed cross-sections in the groundwater catchment area, and geophysical borehole logging for calibration of these measurements. An iterative and integrated evaluation of the results from the different geophysical methods contributes to reliable data as input for the 3-D model covering the whole island and not just the well fields. The complex subsurface structure of the island is revealed. The local waterworks use a freshwater body embedded in saline groundwater. Several glaciations reordered the youngest Tertiary and Quaternary sediments by glaciotectonic thrust faulting, as well as incision and refill of glacial valleys. Both subsurface structures have a strong impact on the distribution of freshwater-bearing aquifers. A digital geological 3-D model reproduces the hydrogeological structure of the island as a base for a groundwater model. In the course of the data interpretation, we deliver a basis for rock identification. We demonstrate that geophysical investigation provide petrophysical parameters and improve the understanding of the subsurface and the groundwater system. The main benefit of our work is that the successful combination of electromagnetic, seismic and borehole data reveals the complex geology of a glacially-affected island. A sound understanding of the subsurface structure and the compilation of a 3-D model is imperative and the basis for a groundwater flow model to predict climate change effects on future water resources.

2018, Zhang, Zeyu, Kruschwitz, Sabine, Weller, Andreas, Halisch, Matthias

We investigate the pore space of rock samples with respect to different petrophysical parameters using various methods, which provide data on pore size distributions, including micro computed tomography (μ-CT), mercury intrusion porosimetry (MIP), nuclear magnetic resonance (NMR), and spectral-induced polarization (SIP). The resulting cumulative distributions of pore volume as a function of pore size are compared. Considering that the methods differ with regard to their limits of resolution, a multiple-length-scale characterization of the pore space is proposed, that is based on a combination of the results from all of these methods. The approach is demonstrated using samples of Bentheimer and Röttbacher sandstone. Additionally, we compare the potential of SIP to provide a pore size distribution with other commonly used methods (MIP, NMR). The limits of resolution of SIP depend on the usable frequency range (between 0.002 and 100 Hz). The methods with similar resolution show a similar behavior of the cumulative pore volume distribution in the overlapping pore size range. We assume that μ-CT and NMR provide the pore body size while MIP and SIP characterize the pore throat size. Our study shows that a good agreement between the pore radius distributions can only be achieved if the curves are adjusted considering the resolution and pore volume in the relevant range of pore radii. The MIP curve with the widest range in resolution should be used as reference.

2015, Burschil, T., Beilecke, T., Krawczyk, C.M.

High-resolution reflection seismic methods are an established non-destructive tool for engineering tasks. In the near surface, shear-wave reflection seismic measurements usually offer a higher spatial resolution in the same effective signal frequency spectrum than P-wave data, but data quality varies more strongly. To discuss the causes of these differences, we investigated a P-wave and a SH-wave seismic reflection profile measured at the same location on the island of Föhr, Germany and applied seismic reflection processing to the field data as well as finite-difference modelling of the seismic wave field. The simulations calculated were adapted to the acquisition field geometry, comprising 2 m receiver distance (1 m for SH wave) and 4 m shot distance along the 1.5 km long P-wave and 800 m long SH-wave profiles. A Ricker wavelet and the use of absorbing frames were first-order model parameters. The petrophysical parameters to populate the structural models down to 400 m depth were taken from borehole data, VSP (vertical seismic profile) measurements and cross-plot relations. The simulation of the P-wave wave-field was based on interpretation of the P-wave depth section that included a priori information from boreholes and airborne electromagnetics. Velocities for 14 layers in the model were derived from the analysis of five nearby VSPs (vP =1600–2300 m s-1). Synthetic shot data were compared with the field data and seismic sections were created. Major features like direct wave and reflections are imaged. We reproduce the mayor reflectors in the depth section of the field data, e.g. a prominent till layer and several deep reflectors. The SH-wave model was adapted accordingly but only led to minor correlation with the field data and produced a higher signal-to-noise ratio. Therefore, we suggest to consider for future simulations additional features like intrinsic damping, thin layering, or a near-surface weathering layer. These may lead to a better understanding of key parameters determining the data quality of near-surface shear-wave seismic measurements.

2019, Bauer, Johanna F., Krumbholz, Michael, Luijendijk, Elco, Tanner, David C.

Geothermal energy is an important and sustainable resource that has more potential than is currently utilized. Whether or not a deep geothermal resource can be exploited, mostly depends on, besides temperature, the utilizable reservoir volume over time, which in turn largely depends on petrophysical parameters. We show, using over 1000 (n=1027) 4-D finite-element models of a simple geothermal doublet, that the lifetime of a reservoir is a complex function of its geological parameters, their heterogeneity, and the background hydraulic gradient (BHG). In our models, we test the effects of porosity, permeability, and BHG in an isotropic medium. Furthermore, we simulate the effect of permeability contrast and anisotropy induced by layering, fractures, and a fault. We quantify the lifetime of the reservoir by measuring the time to thermal breakthrough, i.e. how many years pass before the temperature of the produced fluid falls below the 100 ∘C threshold. The results of our sensitivity study attest to the positive effect of high porosity; however, high permeability and BHG can combine to outperform the former. Particular configurations of all the parameters can cause either early thermal breakthrough or extreme longevity of the reservoir. For example, the presence of high-permeability fractures, e.g. in a fault damage zone, can provide initially high yields, but it channels fluid flow and therefore dramatically restricts the exploitable reservoir volume. We demonstrate that the magnitude and orientation of the BHG, provided permeability is sufficiently high, are the prime parameters that affect the lifetime of a reservoir. Our numerical experiments show also that BHGs (low and high) can be outperformed by comparatively small variations in permeability contrast (103) and fracture-induced permeability anisotropy (101) that thus strongly affect the performance of geothermal reservoirs.