Leibniz Institute for Applied Geophysics

Abstract A comprehensive multi‐method geophysical strategy is used to investigate two faults in the Northern Upper Rhine Graben. Combining industrial 3D seismic data with our own 2D P‐ and S‐wave seismic, electrical resistivity tomography (ERT) and ground‐penetrating radar (GPR), we created comprehensive subsurface images of the faults. 3D seismic data feature structure from 450 m to 4 km deep, whereas high‐resolution 2D P‐wave data complemented this for depths of 50–600 m. S‐wave seismic imaging revealed disrupted layering and syn‐sedimentary features in the range of 5–250 m depth, whereas ERT measurements further supported the identification of offset horizons and showed syn‐sedimentary activity, particularly down to 10 m depth. GPR results remained ambiguous, though they suggested potential geological features related to fault tectonics. Both faults have varying dip angles and offsets in different stratigraphic units, proving that tectonic movement occurred during sedimentation of the Neogene and Quaternary layers. In general, both faults steepen upwards, forming typically listric shapes. They both offset the base Quaternary by about 35 m. Finally, the very‐near‐surface methods show that the faults extend almost up to the Earth's surface, proving their very recent activity. This project highlights the efficiency of multi‐method geophysical surveys in neotectonic research and seismic risk assessment and emphasizes their role in detailed subsurface analysis. It also contributes to our understanding of potential gas migration along fault zones to the Earth's surface.

Summary High-energy beaches, where salinity changes drive dynamic biochemical reactions at the land-sea boundary, were the focus of our 18-month study on Spiekeroog’s North beach (Germany). Employing continuous groundwater logging, direct-push sampling, a quasi-continuous electrode chain, and repeated ERT, we introduced a novel approach: directly inverting all data for salinity. This method outperformed traditional workflows, providing improved results by incorporating the crucial role of subsurface temperature and changes in the formation factor directly into the inversion process. Consequently, we successfully imaged the seasonal evolution of the upper saline plume and the infiltration of winter storm flood remnants into the subsurface. These findings offer valuable calibration data for groundwater models, advancing our comprehension of these highly dynamic systems.

The Tauern Window (TW) in the European Alps is one of the largest tectonic windows in the world. Its formation started in the Cretaceous with subduction of the Penninic realm beneath the northern margin of Adria leading to the collision between Europe (Subpenninic) and the Adria margin (Austroalpine). The resulting Penninic and Subpenninic nappe stack was exhumed by ca. 20 km by the approach of the Dolomites Indenter (Eastern Southern Alps) in the Miocene. This last deformation stage resulted in synkinematic N-S shortening of the western TW (ca. 70 km), W-E extension and lateral extrusion towards the east. However, how the Subpenninic core (Venediger Duplex; VD) and the Penninic and Austroalpine nappes (PN and AN, respectively) in the hanging-wall were tectonically stacked, upright folded and emplaced is poorly understood. This study investigates the deformation accommodated by each major tectonic basement unit of the western TW, and contributes to a better understanding of orogenic processes in general.We kinematically restore the cross-section of [1] along the Brenner Base Tunnel (W of the TRANSALP seismic profile) using the software MOVEtm (Petroleum Experts), focusing firstly on the VD. We choose area balancing as minimum criteria, because we do not know how much material was transported out of the plane of cross-section by extension and lateral extrusion. We integrate zircon fission-track data (ZFT) as a temporal constraint and test different geothermal gradients. Petrological data are used to define the maximum depth the VD reached at the time of indentation and as marker for the transition from brittle to viscous conditions of the felsic rocks of the VD (lowest temperature for folding). Finally, we reconstruct the hanging-wall nappes above the restored VD, thereby precisely constraining the position of the AN at that time. The surface samples taken from the AN must have reached thermal conditions between the annealing zones of apatite fission-tracks and ZFT (115°C and 180°C, respectively) as only the former system was reset in the Miocene.We first displace the entire VD down along the Sub-Tauern Ramp below the 300°C isotherm (brittle to viscous transition of felsic rocks). For this, the geothermal gradient of 50°C/km fits well to the petrological data. ZFT ages reveal upright folding of the VD terminated at ca. 17 +/- 2 Ma. Subsequent unfolding of the gneiss cores, while conserving surface area, reveals the model to be extended ca. 70 km to the south (i.e. thus equaling indenter shortening), which means that no material left the plane of cross-section by W-E extension or lateral extrusion. However, the situation for the hanging-wall nappes is different: The total thickness of the northern limbs of the AN and the PN together is twice as much after restoration compared to today. We postulate that the extension on the Brenner Normal Fault mainly caused this tectonic thinning, which is approximately 10 km.   References[1] Reiter, F., Freudenthaler, C., Hausmann, H., Ortner, H., Lenhardt, W., & Brandner, R. (2018). Tectonics, 37(12), 4625-4654.

The kinetic interface-sensitive (KIS) tracer test is a newly developed tracer approach to measure the fluid-fluid interfacial area (IFA) during dynamic two-phase flow in porous media. This new tracer approach can be applied for multiple geological applications, where dynamic two-phase flow is involved, e.g. monitoring the plume during geological storage of carbon dioxide. The obtained concentration breakthrough curves by measuring reacted tracer concentration in water samples are interpreted with a specialized Darcy-scale numerical model to determine the IFA. The previous design of the drainage experiments has one major limitation that the volume of the usable water sample after breakthrough for the measurement is often insufficient. An alternative is to employ KIS tracers in a “push-pull” experimental set-up, i.e. primary drainage is followed by a consequent main imbibition process, with the flow direction being reversed. This study applies both the pore-scale numerical simulation and the core-scale column experiments to study the KIS tracer reactive transport during push-pull processes. The pore-scale numerical simulation is done with a phase-field method-based continuous species transport model. The reactive transport of the tracer and the characteristics of the concentration breakthrough curves are analyzed. The Darcy-scale reactive transport model is validated by comparing it to the pore-scale results. Finally, the new method is applied in the column experiment, where the determined specific interfacial area is found to be close to the literature data.

This study focuses on the role of karst aquifers in the Mediterranean Basin as a buffered storage of freshwater, especially considering the anticipated increase in drought periods due to climate change. Climate change underscores the need for innovative groundwater management approaches to maximize the storage capacity of these aquifers. This study emphasizes the importance of enhancing aquifer recharge during normal or high rainfall to mitigate the impacts of droughts. Notably, many karst aquifers in this region, which developed extensively during the lower base levels of the Messinian Salinity Crisis, exhibit a dual-domain flow pattern. This pattern consists of a slower flow through the rock matrix and a faster flow through conduits. Despite the rapid drainage of these mature karst systems, some, particularly those in the Mediterranean, are limited in their outflow to the sea by marine clay deposits, as highlighted by Bakalowicz (2015, Environmental Earth Sciences). These systems have shown a significant capacity for storage over several years.In our study, we applied dual-permeability flow modeling to evaluate the storage potential of the Western Mountain Aquifer in Israel and the West Bank. The model utilizes the volume-averaged Richards' equation and integrates a term to account for the characteristic preferential infiltration in karst aquifers, even under nearly dry conditions. The model includes phreatic and vadose zone flows to comprehensively assess the storage capacities of the aquifer comprehensively. The results indicate that despite its advanced karst development, the Western Mountain Aquifer possesses a notable long-term storage capability. This is attributed to its extensive vadose zone and the restricted outflow, which is constrained by surrounding and overlying low-permeability formations (such as the Talme-Yafe, Negba, Daliya, and Menuha Formations, composed mainly of chalk and marl). The study explores various infiltration sites for managed aquifer recharge and considers current and future climatic conditions based on the RCP4.5 climate change scenario.

The eastern sector of the Alps has been heavily influenced by post-collisional indentation tectonics since the Miocene. In current times Adria-Europe convergence, albeit slow, is accommodated and distributed across several faults in the Eastern Alps.

Geothermal energy, as one of the renewable energy types, has great potential to balance the variable energy supply from photovoltaic and wind energy types. This, however, requires the development of an Enhanced Geothermal System (EGS). Injecting cold water into deep geo-reservoirs induces thermoelastic deformation and changes the fracture apertures, which may significantly affect the EGS lifetime and energy performance. In this case, this study develops a fully coupled thermal–hydraulic-mechanical model to investigate the impacts of temperature, pressure, and aperture heterogeneity on the alterations of fracture aperture. Then, considering the reservoir lifespan, the research takes a further step into the influence of heterogeneous aperture variation on EGS production temperature and long-term energy performance. Results show that aperture significantly varies once a temperature breakthrough occurs in the production well for both homogeneous and heterogeneous aperture fields. The thermoelasticity-induced aperture variations enhance water flow rate, leading to a considerable increase in reservoir heat production rate, but a shorter EGS lifespan. Furthermore, temperature variation shows a more substantial influence on aperture alteration compared to pressure change. Additionally, a sensitivity analysis indicates that the 30-year total energy output is proportional to the injection/production pressures and injection temperatures. However, the fracture with either excessively large or small initial apertures leads to worse EGS energy performance.

Core-drilling and groundwater well operation in topographic recharge areas have been rarely applied for groundwater quality monitoring or ecosystem exploration, simply due to lacking of productive water bodies in up to >100 m thick aeration zones. The scarcity of observers and data, and neglection in modelling opposes our efforts to unravel the role of thick hillslope aeration zones for services like water provision, water purification and biogeochemical cycling (Lehmann & Totsche, 2020). To fundamentally understand how groundwater quality comes about as a function of inputs and how climate and land use change will affect this quality, we concentrate our investigations on the soil-aeration zone-phreatic zone continuum in topographic highs. From our Hainich Critical Zone Exploratory in central Germany, we present methods and workflows for low-impact scientific drilling and construction of monitoring wells optimized for representative sampling of the total mobile inventory (Lehmann et al. 2021), and results from the analysis of drill cores, borehole geophysical data and multi-year environmental monitoring data. We found that transient (fluid) flow patterns contribute to groundwater quality dynamics, whereby overall aeration zone-phreatic zone-interactions cause quality fluctuations even in deep and isolated habitats. Results from recent drilling campaigns (2023) comprise the detection of narrow oxic zones (fractures, flow paths) also within mudstone-dominated strata (anoxic aquifer-storeys) and weathering-induced hydrofacies differences that indicate further complexity of habitat structures and ecosystem functioning across recharge(-discharge) zones.    

This study investigates the capillary-associated interfacial area during primary drainage in unconsolidated porous media for different mean grain diameters, employing pore-scale numerical simulation with the phase-field method. Five, two-dimensional porous media are generated with the same grain/pore size uniformity, but different mean grain diameters, ranging between 0.24 mm and 0.94 mm. Three cases are considered, representing stable displacement, crossover zone and viscous-fingering regimes. The findings reveal that the capillary-associated interfacial production rate increases with decreasing grain size. Specifically, the capillary-associated interfacial production rate is higher for the viscous-fingering regime compared to the stable displacement. The maximum capillary-associated interfacial area obtained for different grain sizes agrees with the correlation established in our previous study using the kinetic interface-sensitive tracer. Furthermore, the capillary-associated interfacial area is divided into two parts: one related to the disconnected water clusters and the other related to the connected water. The disconnected capillary-associated interfacial area correlates with the average volume of the clusters, following an exponential function. Furthermore, the average volume of the clusters depends on both, the grain size and displacement regime. Moreover, it is found that the displacement regime has a great impact on the connected capillary-associated interfacial area. For stable displacement, the connected capillary-associated interfacial area is larger for larger mean grain diameter. In contrast, for fingering, the connected capillary-associated interfacial area is smaller for increasing mean grain diameter.