We miniaturize the low-frequency (<1kHz) geoelectrical acquisition using advanced micro-fabrication technologies to investigate coupled processes in the critical zone (CZ). With this innovation in the experimental acquisition, we focus on the development of the complex electrical conductivity monitoring with the spectral induced polarization (SIP) method. The interpretation of the SIP signal is based on the development of petrophysical models that relate the complex electrical conductivity to structural, hydrodynamical, and geochemical properties or distributions. State-of-the-art petrophysical models, however, suffer from a limited range of validity and presume too many microscopic mechanisms to define macroscale parameters. Thus, direct observations of the underlying processes coupled with geoelectrical monitoring are keys to deconvolute the signature of the biochemical-physical mechanisms and, then, developing more reliable models. Microfluidic experiments enable direct visualization of flows, reactions, and transport at the pore-scale thanks to transparent micromodels coupled with optical microscopy and high-resolution imaging techniques. Micromodels are a two-dimensional representation of the porous medium, ranging in complexity from single channels to replicas of natural rocks. Cutting-edge micromodels use reactive minerals to investigate the water-mineral interactions involved in the CZ. In this work, we propose a new kind of micromodels equipped with four aligned electrodes within the channel for SIP monitoring of calcite dissolution, a key multiphase process of the CZ involved in karstification. We highlight the strong correlation between SIP response and dissolution through electrical signal examination and image analysis. In particular, degassed CO2 bubbles generated by the dissolution play a major role.  Our technological advancement brings a deeper understanding of the physical interpretation of the complex electrical conductivity and will provide a further understanding of the CZ dynamic processes through SIP observation.

How to cite: Rembert, F., Stolz, A., Roman, S., and Soulaine, C.: Development of geoelectrical monitoring of the critical zone processes on microfluidic chips, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9639, https://doi.org/10.5194/egusphere-egu23-9639, 2023.

Zeta potential is an important interfacial property that controls electrostatic interactions between mineral, water, and non-aqueous phase fluids. These interactions play an important role in defining the wetting state of reservoir rocks and transport of ionic species through porous media. The zeta potential is shown to be an efficient means for a broad range of applications including monitoring of single- and multi-phase flows in subsurface settings, characterization of fracture networks, efficiency of CO₂ sequestration, hydrogen underground storage and enhanced oil recovery. It is widely agreed that the zeta potential in carbonate rocks is controlled by the concentration of potential determining ions (PDI), but the understanding of the underlying mechanisms is still limited as there are little experimental data on quantitative characterization of the dependence of the zeta potential on concentration of negative potential determining ions (PDI) such as SO₄^2-, CO₃^2-, HCO₃^-, especially when their concentration is high and exceeds that of the positive PDIs.

In this study, the streaming potential method is used to investigate the zeta potential of natural carbonate rock samples in contact with natural aqueous solutions of low-to-high ionic strength and with varying concentration of sulphate (SO₄^2-) and carbon (C₄) related (HCO₃^-, CO₃^2-) ions. In each set of experiments the total ionic strength was kept constant to eliminate the impact of concentration on the zeta potential so that the increasing/decreasing concentration of negative PDI was adjusted by decreasing/increasing concentration of indifferent Cl^- ions. The study probed the concentration of negative PDIs that has never been reported before, with their respective lowest concentration consistent with previously reported equilibrium values, and the highest concentration being equal to the maximum achievable through stripping the tested solutions of Cl^-.

Our results demonstrate for the first time that magnitude of the negative zeta potential increases linearly with log₁₀ of C₄ concentration, however its dependence on the log₁₀ of SO₄^2- concentration is non-linear suggesting varying mechanisms of this PDI’s specific adsorption. Moreover, the results demonstrate that the zeta potential strongly depends on the total ionic strength, interpreted from slopes of the linear regressions for each negative PDI in different background solution. This observation suggests that equilibrium constants of negative PDI specific adsorption may be affected by the total ionic strength. Our findings improve the current understanding of the complex physicochemical processes that take place at calcite-water interface and provide important experimental data for surface complexation modelling of carbonate-brine systems.

How to cite: Vinogradov, J., Atiwurcha, N., Vega-Maza, D., and Derksen, J.: New Insights into the Effect of HCO3-, CO32- and SO42- Ions on the Zeta Potential of Intact Carbonate Rock Sample, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11013, https://doi.org/10.5194/egusphere-egu23-11013, 2023.

Understanding the physical process of soil imbibition and water flow in the porous media in depth is significant in assessing the risk of forming landslides. Volumetric soil moisture sensors can be used to measure water content variations in situ. However, it has a spatial gap due to the limited number of installed sensors. On the other hand, geophysics can provide integrated measurements that can be spatially resolved. Among existing geophysical methods, Self-Potential (SP) is a method of choice to monitor water flow. Indeed, pore-water flows can generate the electrical streaming current based on the electrokinetic mechanism. This electrokinetic cross-coupling process is not only sensitive to the water flow but also depends on water content variations. This study relies on a soil-column experiment by artificially imposing rainfall to examine if the electrical SP could indicate the water infiltrating process. Combined with the observed data, our results indicate the water infiltrating stages can be characterized by the extracted SP signatures under a comprehensive numerical model. As a passive hydrogeophysical method, the capacity of SP to capture the characteristics of the spatio-temporal variations of water fluxes and soil-water conditions can offer early warning information of rainfall-induced landslides.

How to cite: Hu, K., Huang, Q., Han, P., Zhang, Y., Mo, C., and Jougnot, D.: Rainwater infiltrating process revealed from the self-potential signatures, insight for landslide monitoring, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10845, https://doi.org/10.5194/egusphere-egu23-10845, 2023.

Accurate modeling of water flow and solute transport in unsaturated soils are of significant importance for precision agriculture and environmental protection. Traditional modeling approaches are considerably challenging since they require well-defined boundaries and initial conditions. Harnessing machine-learning techniques, specifically deep neural networks (DNNs), to detect water flow and solute transport in porous media have recently gained considerable attention. In traditional DNNs, an artificial neural network with several hidden layers is trained solely using data to approximate parameter and state estimation, e.g., the spatiotemporal distribution of water content and pore-water salinity. However, data is extremely limited and sparsely available in subsurface applications. Physics-informed neural networks (PINNs) have recently been developed to learn and solve forward and inverse problems constrained to a set of partial differential equations (PDEs). Unlike traditional DNNs, PINNs are confined to physics and do not require" big" data for training. However, hydrological applications of PINNs only considered an in-silico environment with spatial measurements of hydraulic head, water content and/or solute concentrations well distributed in the subsurface. Such measurements are hard to obtain in real-world applications since they require drilling to extract soil samples or installing in-situ measurement devices at depth which also violets the soil's natural structure. As opposed to conventional subsurface characterization and monitoring techniques, non-invasive geoelectrical methods can provide continuous, extensive, and non-invasive information of the subsurface. Nevertheless, the sensitivity of the measured electrical signal to various soil parameters, mainly water content and pore-water salinity, as well as inversion errors, could result in biased hydrological interpretations. This work adopted the PINNs framework to simulate two-dimensional water flow and solute transport during a drip irrigation event and the following redistribution stage, using time-lapse geoelectrical measurements with unknown initial conditions. For that manner, a PINNs system containing two coupled feed-forward DNNs was constructed, describing the spatiotemporal distribution of both water content and pore-water salinity. The system was trained by minimizing the loss function, which incorporates physics-informed penalties, i.e., mismatch with the governing PDEs and boundary conditions, and measurement penalties, i.e., mismatch with the geoelectrical data. Two-dimensional flow and transport numerical simulations were used as benchmarks to examine the suitability of the described approach.  Results have shown that the trained PINNs system was able to reproduce the spatiotemporal distribution of both water content and pore-water salinity during both stages, i.e., irrigation and redistribution, with high accuracy, using five time-lapse geoelectrical measurements conducted with 59 electrodes placed at the surface. The trained PINNs system also reconstructed the initial conditions of both state parameters for both stages. It was also able to separate the "measured" electrical signal into its two components, i.e., water content and pore-water salinity. In addition, the subsurface geoelectrical tomograms were significantly improved compared to those obtained from a classical inversion of the raw geoelectrical data.    

How to cite: Moreno, Z. and Haruzi, P.: Simulating water flow and solute transport at unsaturated soils with unknown initial conditions using physics-informed neural networks trained with time-lapse geoelectrical measurements, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-956, https://doi.org/10.5194/egusphere-egu23-956, 2023.

In the unsaturated zone of fractured and karstified media, although the fractures and incipient karst conduits generally account for a minor volume in the bulk geologic formations, they contribute significantly to the flow and transport properties. The reason is due to the significant difference (i.e., several orders of magnitude) of permeability in comparison with the surrounding porous matrix. Thus, the simulation of groundwater flow in such heterogeneous porous media requires knowledge of geometry and hydrodynamical characteristics of the fractures. However, identifying fractures and incipient karst conduits in the unsaturated zone has been a challenge in hydrogeology. Electrical resistivity tomography (ERT) as a non-invasive geophysical method has the potential to deliver vast information rapidly in a relatively economical way related to fractures and incipient karst conduits. However, the sole use of ERT for the interpretation of geological formation and hydrodynamic properties of fractured domains is not possible as it may lead to ambiguity of interpretations. The main goal of this work is to investigate the performance of coupling water flow and electrical current for fracture characterization. Thus, we develop a new numerical model for the simulation of coupled water flow and electrical current. In this model, the fractures or incipient karst conduits are simulated with the discrete fracture matrix approach which is known to be the most accurate approach for addressing flow in fractured domains as it considers fractures without any simplification. This approach is applied for both electrical current and water flow. The hybrid dimensional approach, which assumes 1D fractures in a 2D porous matrix is used to improve the computational efficiency of the developed model. The partial differential equations describing the flow and electrical current are solved using the mixed hybrid finite element method for space discretization and the method of lines for time integration. These numerical techniques have been selected to ensure an accurate solution to the nonlinear problem in a time-efficient and effective way. The newly developed model is validated for simplified test cases against the results obtained by an equi-dimensional approach based on the conforming finite element method (i.e., using COMSOL Multiphysics®). The effect of major fracture orientation and length on the temporal response of bulk resistivity during a percolation scenario has been studied numerically. In addition, the effect of injection of electrical dipole has been simulated and discussed for the same problem. The results show that the proper placement of the electrical dipole can significantly affect the resolution of the fracture signature response and the bulk resistivity measured on the surface of the domain through time.

How to cite: Koohbor, B., Fischer, P., Fahs, M., Younes, A., and Jourde, H.: An advanced hybrid-dimensional discrete fracture-matrix model for coupled simulation of water flow and electrical current in variably saturated fractured porous media, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5444, https://doi.org/10.5194/egusphere-egu23-5444, 2023.

Reliable information about the aquifer geometry and its spatial variability from geophysical methods plays a critical role for various hydrological research questions. Such information can be obtained with the transient electromagnetic (TEM) method that reaches a larger depth of investigation with a smaller survey layout compared to electrical or seismic methods. However, the quantitative interpretation of the resistivity model obtained from TEM data in terms of groundwater level and aquifer geometry might be biased by the non-uniqueness of the deterministic inversion. To overcome such limitations, we propose here to use Bayesian evidential learning (BEL1D) to evaluate the uncertainty of the groundwater level and aquifer thickness interpreted from TEM results using a classical deterministic inversion approach. Additionally, we investigate the effect of different prior model spaces on the uncertainty obtained from BEL1D and use the distance-based global sensitivity analysis (DGSA) to determine whether model parameters with a large uncertainty are actually non-influential on the model response. To test the uncertainty quantification, TEM data were measured at three sites in Austria representative of different hydrogeological settings and groundwater levels, namely: 1) a shallow (1 m - 10 m) aquifer located in the soda lakes of the Neusiedl-Seewinkel Basin in Burgenland, 2) an aquifer in intermediate (5 m – 15 m) depth located in the hydrological open-air laboratory (HOAL) in lower Austria and 3) a deep (> 35 m) aquifer in a farm land located in Upper Austria. We obtain TEM data with the TEM-FAST 48 instrument with a 6 m, 12.5 m and a 25 m square single-loop configuration to achieve sensitivities corresponding to the three different aquifer depths. Additionally, we use electrical resistivity tomography (ERT) and multi-channel analysis of surface waves (MASW) data to assess the TEM inversion results. The interpretation of the TEM inversion results is evaluated with BEL1D to obtain the uncertainty of the groundwater level from the cumulative uncertainty of all layers above the layer representing the groundwater level as well as the thickness of the aquifer. Additionally, we achieve a quantitative evaluation of the solved TEM model uncertainties with the DGSA method as well as with the ERT and MASW inversion results. Our results show a lower uncertainty for the electrical resistivity than for the layer thickness, while the DGSA reveals a decrease of sensitivity with depth.

How to cite: Aigner, L., Roser, N., Hettegger, A., Höfelmaier, D., Cimadom, A., Michel, H., Hermans, T., and Flores Orozco, A.: Uncertainty quantification of aquifer geometry and groundwater level using electrical resistivity models obtained from transient electromagnetic data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9723, https://doi.org/10.5194/egusphere-egu23-9723, 2023.

The use of near-surface geothermal energy implying geothermal infrastructure increases significantly in Germany [1]. Therefore it becomes an essential subject for impact analysis of our groundwater resource. Rock physical properties change through the alteration of the pore fluid properties such as temperature. Those alterations happen under the influence of heat in the subsurface. Variations in geophysical proxies can provide information about the subsurface changes. Laboratory measurements by Jaya et al. [2] show that P-wave velocities decrease with increasing temperature.

Within the BMBF funded follow-on project – TestUM II a “cyclic high temperature aquifer thermal energy storage (ATES) experiment” has been conducted in the north or Germany to verify these findings. With this field experiment at a shallow aquifer environment we are able to avoid difficulties in frequency dependent upscaling procedures [3]. We investigated the coherence between geophysical proxies and the temperature distribution in the near surface with a combined hydrogeological, microbiological and geophysical monitoring system covering an area of approximately 100 m². A cyclic heat injection at a depth between 7 – 14 m was monitored over 15 months with seismic cross-hole measurements in 16 different wells to cover heat propagation and direction-dependent heterogeneities across the field. The purpose was to investigate geophysical proxy responses on the alteration of the pore fluid that is likely to change rock physical properties in the near-surface. To predict the effect of temperature on P-wave velocity we took advantage of the Jaya’s [2] modification of the Gassmann equation which accounts for the related thermophysical characteristics of the pore fluid. Unlike the findings of Jaya et al. [2], that the P-wave velocity decreases, we see the opposite, an increase in P-wave velocity in our non-closed system assuming different thermophysical characteristics of the saturating fluid. We excluded a bubble-formation since we only cover temperatures below 80°C. However, a decrease in the amplitudes due to the P-wave attenuation can be observed. According to Jaya et al. [2] this can be attributed to the decrease in water viscosity.

[1] Blöcher, G., Reinsch, T., Regenspurg, S., Henninges, J., Brehme, M., Saadat, A., Kranz, S., Frick, M., Spalek, A., Huenges, E. (2019): Geothermie in urbanen Räumen: thermische Untergrundspeicherung und Tiefe Geothermie in Deutschland. - System Erde, 9, 1, 6-13.
https://doi.org/10.2312/GFZ.syserde.09.01.1

[2] Jaya, Makky S. / Shapiro, Serge A. / Kristinsdóttir, L\iney H. / Bruhn, David / Milsch, Harald / Spangenberg, Erik
Temperature dependence of seismic properties in geothermal rocks at reservoir conditions, 2010-03, Geothermics , Vol. 39, No. 1, Elsevier BV

[3] Müller, T. M. / Gurevich, Boris / Lebedev, Maxim 
Seismic wave attenuation and dispersion resulting from wave-induced flow in porous rocks - A review, 2010-09, Geophysics , Vol. 75, No. 5 ,Society of Exploration Geophysicists 

How to cite: Birnstengel, S., Koedel, U., Pohle, M., Hornbruch, G., Nordbeck, J., Werban, U., and Dietrich, P.: Monitoring of an aquifer thermal storage system on the field scale using cross-hole seismics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5777, https://doi.org/10.5194/egusphere-egu23-5777, 2023.

How to cite: Mary, B., Kaffas, K., Censini, M., Manca di Villahermosa, F. S., Dani, A., Verdone, M., Preti, F., Trucchi, P., Penna, D., and Cassiani, G.: Supporting subsurface preferential flow in a small forested catchment from geophysical data and hydrological modelling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5954, https://doi.org/10.5194/egusphere-egu23-5954, 2023.

In the current climate change context, the quality and availability of water resource are important society issues. Indeed, the consumption of water and fertilizers by farms and their discharge into the soil and aquifers leads to a critical environmental situation. To manage the effect and fate of these contaminations, it is necessary to have a relevant knowledge of the vadose zone dynamics and exchanges especially flow paths from the soil to the deep aquifers. Parts of the vadose zone are well studied such as the first meters with an adapted instrumentation, and the water table properties with piezometric measurements and pumping tests. The geophysics methods allow studying the deep vadose zone properties. The surface nuclear magnetic resonance (SNMR) is a direct geophysical method based on the protons magnetic resonance to measure water content in the subsurface. This method can be used to detect the water table level coupled with hydrogeological measurements (piezometric measurements and pumping tests) or to characterize aquifers properties and boundaries coupled with other geophysical methods (electrical, EM or gravimetric methods).

The current study is carried out in Villamblain (France) at the heart of the Beauce region; one of the most cultivated and highly nitrate-contaminated area in France. The vadose zone is a highly heterogeneous limestone with geochemical alteration, complex network of fractures and karstification. This field site was chosen to develop an observatory of transfers in the Vadose Zone named O-ZNS (https://plateformes-pivots.eu/o-zns/). This observatory consists of an exceptional well (20 m-deep and 4 m-diameter) equipped with multiple sensors and accessible for direct characterization of the heterogeneous vadose zone, surrounded by 8 boreholes used for water sampling, geophysical well-logging, piezometric level monitoring and vadose zone geochemical properties monitoring.   

In this study, the SNMR method is used (1) to characterize the spatial heterogeneities of the 3D aquifer and of the vadose zone limestone, in a 3D-model jointly interpreted with other geophysical measurements (3D-ERT, 3D-IP, gravimetric, GPR profiles) and soundings; (2) to characterize the vadose zone water content, in combination with GPR and NMR logging water content profiles. Both these works are ongoing.

This research aims to know more accurately the vadose zone water content measured by SNMR in the context of a heterogeneous limestone with the goal to monitor the vadose zone flow dynamics with time-lapse measurements coupled with hydrogeological measurements.   

How to cite: Ryckebusch, C., Baltassat, J.-M., Legchenko, A., Kessouri, P., Amraoui, N., Abbas, M., and Azaroual, M.: Vadose zone water content characterization of a heterogeneous limestone by 3D-SNMR, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9156, https://doi.org/10.5194/egusphere-egu23-9156, 2023.