Surface waves such as Rayleigh, Love and Scholte waves can exhibit dispersion, that is, variations in phase velocity with wavelength as a function of frequency. This property enables the inversion of 1-D models of seismic velocity and density in the subsurface. Conventional deterministic and stochastic inversion schemes are widely applied to surface wave data but face two main challenges. The first is the identification of dispersion curves for fundamental and higher modes on wavefield-transformed images, which is often done manually. The second is the quantification of uncertainty, which can be computationally expensive in stochastic approaches or limited to data-propagated uncertainty in deterministic inversions. Our objectives are to (1) eliminate the need for manual or automatic dispersion curve picking, and (2) directly infer ensembles of 1-D velocity models-and their associated uncertainties-from the full velocity spectrum, that is, the complete dispersion image containing all modes. To this end, we employ Bayesian Evidential Learning, a predictive framework that reproduces experimental data from prior information while allowing prior falsification. In our application, ensembles of prior Earth models are sampled to predict 1-D subsurface structures in terms of seismic velocity and, where applicable, attenuation from near-surface seismic wave data. This approach bypasses traditional inversion schemes and provides a computationally efficient tool for uncertainty quantification.

A key parameter influencing crop growth is the availability of water and its spatial distribution within agricultural soils. The strong electrical response associated with variations in water content means that electrical geophysical methods such as Electrical Resistivity Tomography ( ERT ) are ideally suited for studying soil moisture dynamics. While relative changes in electrical resistivity allow conclusions to be drawn on the temporal variability of soil moisture in the subsurface, they do not provide quantitative estimates of soil moisture. Laboratory measurements of soil moisture and electrical resistivity - together with the fitting of a model to these data (e.g. Waxman-Smits) - enable the estimation of soil moisture from ERT models. While this method is established, examples are limited for Alfisols in general, and from African sites in particular. We present laboratory data on agricultural Alfisols from three sites in southern Africa, to define relationships between electrical resistivity and volumetric water content. We fit the data to several widely used models, and utilise the novel application of Akaike’s Information Criterion to identify the best model for each dataset. In addition to identifying which model is most suitable for each site, our findings also allow us to make conclusions on whether a single or multiple models are required per site, or even whether heterogeneities make the utilisation of a model inadvisable. We then compare our results against complementary laboratory datasets to explain which soil physical parameters are the main causes of variability in electrical response at each site.

SUMMARY Understanding the interplay between pore-scale fluid distribution and bulk electrical properties is critical to improving petrophysical models of partially saturated porous media. This study introduces and evaluates a novel experimental setup that enables joint spectral induced polarization (SIP) and X-ray microcomputed tomography (µCT) under dynamic saturation conditions. A custom-designed flow cell was developed to enable simultaneous high-resolution µCT imaging and accurate SIP acquisition. It includes retracted, non-polarizable potential electrodes placed in agar-filled channels, which minimize electrode polarization and preserve signal integrity during measurement. Using this setup, we conducted a drainage–imbibition experiment on an unconsolidated sand sample. High-resolution µCT images captured the water-phase distribution at successive equilibrated saturation states, while co-acquired SIP data provided complementary information on bulk resistivity and aqueous phase connectivity. We extracted the fluid distribution in the pore network from segmented images and computed both geometric and electrical tortuosity to investigate how pore-scale and transport parameters are related. A pore network model, extracted from the dry µCT scan, was used to simulate resistivity index trends, allowing for direct comparison between experimental and modelled values. Results show that saturation history significantly impacts both resistivity and tortuosity, with notable differences between drainage and imbibition. The µCT data confirm that trapped gas phases and connectivity losses are key drivers of marked resistivity increases. While the tested sample exhibited limited polarization, the experimental platform proved effective in linking microstructure to geoelectrical response. The combined SIP–µCT method offers a promising route for refining petrophysical models and holds potential for future studies involving more complex, polarization-prone materials and biogeochemical processes.

This study investigates the origin of spectral induced polarization (SIP) signals associated with microbial systems, aiming to distinguish direct microbial electrical effects from indirect geochemical processes. Although SIP has been proposed for detecting subsurface microbial activity, its interpretation is complicated by overlapping polarization mechanisms. We therefore performed controlled experiments in simplified, non-geological environments to isolate the roles of microbial presence, attachment, and activity. Using a custom millifluidic chip, SIP measurements were conducted with \textit{Pseudomonas putida} KT2440 under three configurations: bacteria suspended in solution, bacteria attached to metal surfaces, and bacteria affecting SIP through redox reactions.Bacterial cells in suspension did not produce a measurable SIP response: phase shifts were indistinguishable from those of the background medium, despite small increases in bulk conductivity. Reanalysis of previous studies indicates that interpretations based solely on imaginary conductivity can be misleading, and that phase shift is the more reliable indicator. Likewise, bacterial attachment to metal surfaces did not modify the SIP response, even though microscopy confirmed surface colonization.In contrast, strong and time-dependent SIP signals were observed when iron was immersed in oxygenated media, closely tracking oxygen consumption and attributable to surface oxidation. When oxygen was removed—by microbial respiration or chemical scavenging—the change in SIP response was significantly reduced, and systems with and without bacteria behaved similarly. Numerical simulations with redox-modified boundary conditions reproduced these trends and linked SIP evolution to changes in surface capacitance and electron transfer. Overall, SIP is sensitive not to microbial cells themselves, but to microbially mediated redox processes involving conductive materials.

Understanding and mapping saltwater intrusion (SI) in coastal aquifers is crucial for safeguarding freshwater resources, yet its spatial characterization remains highly uncertain. Conventional well sampling delivers accurate salinity measurements but is spatially limited, whereas geophysical imaging captures broader subsurface patterns but provides only indirect information constrained by uncertain petrophysical models. To address this, we integrate hydrochemical and geophysical datasets within a probabilistic geostatistical framework. Specifically, total dissolved solids (TDS) concentrations from 70 shallow wells (5-10 m) are combined with electrical resistivity tomography (ERT) data collected along 28 transects in the Luy River catchment, Vietnam. Laboratory experiments established two distinct resistivity-salinity relationships for clay-free and clay-bearing sediments. Uncertainty arises from the unknown clay content at specific locations, representing a key source of ambiguity in petrophysical relationships. Accordingly, each resistivity value was expressed as a probability of exceeding predefined salinity thresholds derived from the TDS distribution. Both a traditional binary indicator transformation and an extended encoding with continuous probabilities were tested. As highlighted by a crossvalidation analysis, the combined dataset produces a more consistent and spatially detailed delineation of SI than TDS-only kriging. Moreover, the probabilistic framework explicitly accounts for alternative petrophysical scenarios and quantifies the likelihood of crossing critical salinity limits. This study illustrates the effectiveness of integrating hydrochemical observations with geophysical measurements using indicator kriging to improve the reliability of SI mapping in heterogeneous coastal aquifers.

Polders are coastal low-lying areas reclaimed on the sea mainly for agriculture. They are constantly drained by pumps, ditches and subsurface drains. In West-Flanders, Belgium they are composed of a shallow freshwater lens on top of denser saline groundwater. This freshwater lens is essential for crop growth but during dry periods, it can disappear and the saline groundwater can enter the root zone by capillary rise and endanger yield. To prevent this, farmers can use controlled drainage to raise the water table in their field during winter months to increase the freshwater lens thickness. To study the effectiveness of the technique, we monitored three fields with piezometers, resistivity sticks alongside field mapping using electromagnetic induction for three years. The resistivity sticks highlighted lithological differences, but also enabled us to monitor the fresh–saline water interface and the water table accurately. Controlled drainage retained additional rainfall during intense summer precipitation but little effect was observed on the fresh–saline water interface. Soil heterogeneity and past land use of the field seem to have a larger effect on the fresh–saline water interface. Despite the limited effect on the freshwater lens, the value of geophysical methods for monitoring its thickness for field-scale study was demonstrated.

Developments in the methods available for root investigation in recent years have enabled many studies to be carried out on roots, which represent the hidden half of the plant. Despite the increased number of studies on roots, there are still knowledge gaps in our understanding of the electromagnetic properties of plant roots, which will be useful to quantify plant properties and monitor plant physiological responses to dynamic environmental factors amidst climate change. In this study, we evaluated the suitability of spectral induced polarization (SIP) for the noninvasive assessment of root activity. We investigated the electrical properties of the primary roots of Brachypodium distachyon L. and Zea mays L. during the uptake of fresh and saline water using SIP measurements in a frequency range from 1 Hz to 45 kHz. The results show that SIP is able to detect the uptake of water and saline water in both species and that their electrical signatures were influenced by the solute concentration. The resistivity and phase response of both species increased with solute concentration until a certain threshold before it decreased. This concentration threshold was much higher in maize than in Brachypodium, which implies that tolerance to salinity varies with species and that maize is more tolerant to salinity than Brachypodium. We conclude that SIP is a useful tool for monitoring root activity and could be adapted for the early detection of salt stress in plants.

BACKGROUND:In 2019, AABB released the bulletin "Recommendations on the Use of Group O Red Blood Cells" in which the recommendations about pediatric and neonatal blood transfusions were limited. Eight U.S. pediatric hospitals sought to determine trends in pediatric group O blood use and clarify which pediatric populations receive group O blood transfusions despite a non-group O blood type. STUDY DESIGN AND METHODS:Eight U.S.-based institutions serving a pediatric population provided data from their respective Electronic Health Records. Data submitted included unit blood type, patient blood type, patient age, sex, and discharge diagnosis. If the discharge diagnosis was not available, the admitting diagnosis was substituted. GPT-4 was used to sort diagnoses into categories for analysis. Data were visualized using a series of alluvial plots, spaghetti plots, and tables. Tables were stratified on variables of interest (blood type, age, sex, diagnosis) to explore O blood type distribution among different patient populations. RESULTS:A total of 142,227 discrete transfusion events were identified, of which 52,731 recipients were non-O blood type. Overall, 35,575 transfusion events of O blood went to A, B, or AB blood type recipients (67%). Additionally, 26% of Rh(D) negative transfusion events went to recipients who were Rh(D) positive. Top diagnostic categories for receiving O blood type were cardiovascular disorders (22%) and sickle cell anemia (15%). DISCUSSION:This study highlights opportunities to address O blood supply challenges by identifying where non-O blood may be utilized safely in the vulnerable pediatric population.

The geophysical investigation of geologic structures is an essential prerequisite for the preparatory phase of large subsurface construction projects like caverns or tunnels and to study structural geology. This type of investigation is crucial to guide boreholes for relevant ground truth, knowledge of the local geology, or to adjust the position of underground constructions.Typical approaches to image large structures are seismic reflection surveys or Airborne Electromagnetic surveys (AEM). However, seismic surveys might fail due to an absorbing soft top layer or steeply inclined layers. AEM surveys generally are poorly adapted to applications in urbanized areas. To overcome these issues, another method for large-scale subsurface imaging is the application of Deep Electrical Resistivity Tomography (Deep ERT), a recent approach where employing separated injection and measurement systems allows for large injection dipoles that retrieve information from depth.We conducted a Deep ERT campaign in the framework of the E-TEST project for the investigation for a suitable location for the Einstein Telescope, a gravitational wave observatory consisting of a set of subsurface laser interferometers in a triangular shape at a depth around 300 m. Here, we present the results from a 2D Deep ERT survey in Val Dieu, Belgium with a total length and maximum injection dipoles of 7,5 km and a total of 14040 measured datapoints. We show the challenges during preparation, performance and data processing and discuss its capability in imaging large and deep geological structures.

In electrical geophysical methods it is often assumed that the resistivity or chargeability, behaves linearly with the applied electrical current. However, non-linearity has been reported in Induced Polarization (IP) and can be caused by oxidation-reduction reactions or reactions at the surface of clay minerals. Methods to determine the presence of non-linear effects exist and involve analysis of the harmonic distortion of spectral IP data. Measurement of reciprocal data is standard practice in many ERT surveys in the context of data filtering and error quantification. When assuming a system behaves linearly, the normal and reciprocal measurements should yield the same distribution of values. Consequently, a systematic misfit between the two can indicate non-linearity. Here, we present the case of time domain IP (TDIP) data acquired in a volcanic hydrothermal system (VHS), with a strong discrepancy in both observed data and inversion of normal and reciprocal data, presented as a positive shift of the reciprocal decay curves (>100 mV/V). The inversion shows a strong discrepancy in the imaginary part, of 20 mS/m, localized on the southern part of the profile. In the complex environment of a VHS we interpret the observed non-linear IP effect as the result of oxidation-reduction reactions at the interface of iron oxides or sulfides. Our study suggests that analysis of normal-reciprocal misfit can be further used as an interpretation tool for the presence of non-linear effects.

Research into the early urban history of Ostia (4th century BC – 2nd century AD), Recent geophysical surveys and core drillings in the excavated area of the city, New data on the palaeoenvironment at the mouth of the Tiber in the 1st millennium BC

Understanding the interplay between pore-scale fluid distribution and bulk electrical properties is critical to improving petrophysical models of partially saturated porous media. This study introduces and evaluates a novel experimental setup that enables synchronized spectral induced polarization (SIP) and X-ray micro-computed tomography (µCT) measurements under dynamic saturation conditions. A custom-designed flow cell was developed to accommodate both high-resolution µCT imaging and accurate SIP acquisition at the same time. It includes retracted, non-polarizable potential electrodes placed in agar-filled channels, which minimize electrode polarization and preserve signal integrity during measurement. Using this setup, we conducted a drainage–imbibition experiment on an unconsolidated sand sample. High-resolution µCT images captured the evolving spatial distribution of the water phase, while simultaneous SIP data provided complementary information on bulk resistivity and phase connectivity. We extracted the fluid distribution in the pore network from segmented images and computed both geometric and electrical tortuosity to investigate how pore-scale and transport parameters are related. A pore network model (PNM), extracted from the dry µCT scan, was used to simulate resistivity index (RI) trends, allowing for direct comparison between experimental and modelled values. Results show that saturation history significantly impacts both resistivity and tortuosity, with notable differences between drainage and imbibition. The µCT data confirm that trapped gas phases and connectivity losses are key drivers of marked resistivity increases. While the tested sample exhibited limited polarization, the experimental platform proved effective in linking microstructure to geoelectrical response. The combined SIP–µCT method offers a promising route for refining petrophysical models and holds potential for future studies involving more complex, polarization-prone materials and biogeochemical processes.

An artificial run-off hydrogeophysical experiment was conducted in cultivated fields (Gembloux (BE) to study the infiltration patterns of water in the transition between an empty beet field and a band where miscanthus has been planted. Such set-ups are designed to mitigate the flooding risk and the erosion from runoff during intense rainfall events. The objective of this experiment was to determine with geophysical methods whether miscanthus enhance water infiltration in addition to blocking mud and slow water flow. The experiment was repeated 3 times, each time next to each other with the same experimental setup: a 1-meter-wide and 6-meter-long long band isolated with plastic boards, 3m is uncovered and the other 3m is covered with miscanthus plant base and roots. The band has a slight inclination, and saline water was poured to create a surface run-off at a rate of 1L/s at the top of the band. A primary longitudinal profile composed of 16 electrodes (0.4m spacing) was used to monitor the infiltration, with measurements taken approximately every 2.5 minutes. Two perpendicular profiles (4.5m long, 0.3m spacing) in each section were used to do background measurements and after the experiment. A first analysis has been carried out on apparent resistivity to avoid any inversion bias. Each parcel shows a greater starting mean apparent resistivity in the miscanthus parcels. During the infiltration, apparent resistivities decrease more rapidly in the miscanthus parcel during the first minutes of the experiment and reach a lower value than in the bare parcel. Subsequently, resistivities in both parcels decrease at a slower rate but do not reach a steady state, even after 3 hours of infiltration. Once water injection ceases, resistivities quickly stabilize within a few minutes at a lower value than the starting value but higher than at the end of injection. Timelapse inversion revealed a decrease in resistivity in the top 40cm soil after only a few minutes following the start of the water injection. We estimate that the layer below this horizon corresponds to the plough layer, where the higher density and lower permeability of the soil beneath this level doesn’t allow the infiltration at this time scale. However, inversions of perpendicular profiles reveal lateral extension of the resistivity decrease in the parcels without miscanthus, a pattern which is absent in the miscanthus parcel. Further data processing will focus on the inversion problem and on the influence of the surface water height during the water injection and pedophysics experiments will allow us to estimate the water content. Along with other inversion parameters, this will help provide a better understanding of the dynamics of infiltration rates in the different parcels.

The intricate architecture of plant root systems is crucial for nutrient and water uptake, significantly influencing plant growth and productivity. Induced polarization is a promising non-destructive technique for analysing plant roots in their natural conditions. This study introduces a novel theoretical and numerical model to explain the significant low-frequency polarization of plant root cells observed in previous experiments. Our approach addresses the limitations of existing models by incorporating geometric constraints and internal mechanisms of cell polarization, particularly focusing on interfacial polarization across the cell membrane. Through comprehensive simulations, we investigate various geometries and boundary conditions, demonstrating that densely packed root cells exhibit significant polarization signals within a measurable frequency range due to coupling effects. Our findings align with experimental observations, indicating that the peak frequency is highly sensitive to cell arrangement and membrane properties, while the maximum phase shift remains consistent. This model provides a robust framework for interpreting polarization signals in root systems, offering potential applications for in-situ characterization of plant roots and enhancing the understanding of root dynamics under different environmental conditions.

Many volcanoes host a hydrothermal system, responsible for a large fraction of volcanic eruption. These eruptions do not expel magma but involve the forceful ejection of pre-existing rocks, volcanic gases, and steam, posing a significant threat to human safety. Recent catastrophic incidents underscore the difficulty in foreseeing sudden hydrothermal explosions, exposing our limitations in prediction. The challenge lies in the absence of distinct precursory signals, making it difficult to anticipate these events. These eruptions may be triggered by the introduction of mass and energy originating from magma, or alternatively, by the development of mineralogical seals above vents, devoid of any direct magmatic influence. Understanding and predicting these hydrothermal phenomena remain critical for mitigating their potential human and environmental impacts.In the ERUPT research project, we study the geoelectrical response of volcano hydrothermal systems (VHS). Here, we focus on the laboratory scale, where we amalgamate electrical properties, namely SIP (Spectral Induced Polarization) measurements, with X-ray pore-scale (4D µCT) imaging to unravel the intricate electrical signatures of volcanic systems on rock samples collected from Gunnuhver region (Iceland). SIP is a geophysical method that measures the complex electrical impedance of a material as a function of a wide range of frequencies (Zimmermann et al., 2008). It is particularly useful for characterizing the electrical properties of porous media, and have been widely used to study rock samples from VHS (e.g., Lévy et al., 2019). SIP responses are sensitive to factors like surface area, pore size distribution, fluid content, as well as movement of fluids within the rock. On the other hand, X-ray µCT is an imaging technique that uses X-rays to create detailed, 3D images of the internal structure of a sample, such as internal morphology, porosity, other structural features of rocks at a micrometer scale, and quantify fluid pathways and flow dynamics within the rock. The synergy of combining these two methods can provide a more comprehensive understanding of the geoelectrical properties and internal structure of a rock sample as follow: by analyzing SIP responses at different frequencies and correlating them with the µCT images, we gain insights into how variations in geoelectrical properties relate to the movement of fluids within the rock matrix, as well as the influence of alteration or precipitation of minerals. As a first step, we developed a unique experimental set-up that enables to combine both methods (SIP and µCT) simultaneously. The noval prototype was thoroughly designed following specific technical features (e.g., dimensioning, materials) to ensure an optimal SIP signal acquisition under well controlled conditions of temperature and pressure, together with a high resolution 4D µCT imaging. This integrated approach is valuable for studies in geophysics, hydrogeology, and reservoir characterization, among other various relevant domains. ReferencesLévy, L. et al. (2019) ‘Electrical resistivity tomography and time-domain induced polarization field investigations of geothermal areas at Krafla, Iceland: Comparison to borehole and laboratory frequency-domain electrical observations’, Geophysical Journal International, 218(3), pp. 1469–1489. https://doi.org/10.1093/gji/ggz240.Zimmermann, E. et al. (2008) ‘A high-accuracy impedance spectrometer for measuring sediments with low polarizability’, Measurement Science and Technology, 19(10). https://doi.org/10.1088/0957-0233/19/10/105603.

Modern gravitational-waves astronomy is moving underground. Geological units act as noise-dampening covers to isolate highly sensitive gravitational-wave detectors from ambient noise disturbances, which poses challenges for the siting, planning, and construction of detectors. The current feasibility study for the Einstein Telescope (ET), the European third-generation gravitational-waves detector, aims to find the most suitable site for construction and operation from various perspectives. One potential site is the Euregio-Meuse-Rhine (EMR) area in the border region between the Netherlands, Belgium, and Germany. The Einstein Telescope is planned as a large-scale underground infrastructure with over 30 km of tunnels and various cavern constructions, approximately 250 m below the surface. This paper presents the preliminary results of the first site characterization phase for this site from an engineering geological perspective. The results show (1) complex geological conditions of siliciclastic and carbonate, brittle and hard Paleozoic (Frasnian to Westphalian) rocks beneath a cover of soft, partly unconsolidated Cretaceous and Cenozoic sediments, (2) the strong influence of several tectonic events, especially the thrust-and-fault belt of the Variscan Front and the opening of the Lower Rhine Graben, (3) variable and in places high hydraulic conductivities (10− 5 m/s) and, (4) variable rock mechanical properties of potential host rock formations (e.g. Famennian and Namurian rocks). Finally, an engineering geological assessment of the suitability, challenges, and requirements for future work associated with the construction of the ET in the EMR region is presented.

The increasing need to find alternative stocks of critical raw materials drives to revisit the residues generated during the former production of mineral and metallic raw materials. Geophysical methods contribute to the sustainable characterization of metallurgical residues inferring on their composition, zonation and volume(s) estimation. Nevertheless, more quantitative approaches are needed to link geochemical or mineralogical analyses with the geophysical data. In this contribution, we describe a methodology that integrates geochemical and geophysical laboratory measurements to interpret geophysical field data solving a classification problem. The final aim is to estimate volume(s) of different types of materials to assess the potential resource recovery. We illustrate this methodology with a slag heap composed of residues from a former iron and steel factory. First, we carried out a 3D field acquisition using electrical resistivity tomography (ERT) and induced polarization (IP), based on which, a sampling survey was designed. We conducted laboratory measurements of ERT, IP, spectral induced polarization (SIP), and X-ray fluorescence analysis, based on which, 4 groups of different chemical composition were identified. Then we carried out a 3D probabilistic classification of the field data, based on 2D kernel density estimators (for each group) fitted to the inverted data collocated with the samples. The estimated volumes based on the classification model were: 4.17 × 103 m3 ± 12 %, 1.888 × 105 m3 ± 12 %, 59.4 × 103 m3 ± 19 %, and 2.30 × 104 m3 ± 21% for the groups ordered with an increasing metallic content. The uncertainty ranges were derived from comparing the volumes with and without considering the probabilities associated to the classification. We found that a representative sampling and the definition of the KDE bandwidths are defining elements in the classification and ultimately the estimation of volumes. This methodology is suitable to quantitatively interpret geophysical data in terms of the geochemical composition of the materials, integrating uncertainties both in the classification and the estimation of volumes. Furthermore, several crucial elements in the investigation of metallurgical residues could be applied in a real case study, e.g., geophysical field acquisition, sampling and lab measurements.

The lithological and stratigraphical heterogeneity of coastal aquifers has a great influence on saltwater intrusion (SI). This makes it difficult to predict SI pathways and their persistence in time. In this context, electrical resistivity tomography (ERT) and induced polarization (IP) methods are receiving increasing attention regarding the discrimination between saltwater-bearing and clayey sediments. To simplify the interpretation of ERT data, it is commonly assumed that the bulk conductivity mostly depends on the conductivity of pore-filling fluids, while surface conductivity is generally disregarded in the spatial and temporal variability of the aquifers, particularly, once the aquifer is affected by the presence of saltwater. Quantifying salinities based on a simplified petrophysical relationship can lead to misinterpretation in aquifers constituted by clay-rich sediments. In this study, we rely on co-located data from drilled boreholes to formulate petrophysical relationships between bulk and fluid conductivity for clay-bearing and clay-free sediments. First, the sedimentary samples from the drilled wells were classified according to their particle size distribution and analyzed in the lab using a SIP in controlled salinity conditions to derive their formation factors, surface conductivity and chargeability. Second, the deduced thresholds are applied on the field to distinguish clay-bearing sediments from brackish sandy sediments. The results are validated with logging data and direct salinity measurements on water samples. We applied the approach along the Luy River Catchment and find that the formation factors and surface conductivity of the different unconsolidated sedimentary classifications are varying from 4.0 to 8.9 for coarse-grained sand and clay-bearing mixtures, while normalized chargeability above 1.5 mS/m indicates the presence of clay. The clay-bearing sediments are mostly distributed in discontinuous small lenses. The assumption of homogenous geological media is therefore leading to overestimating SI in the heterogeneous clay-bearing aquifers.

Polders are areas reclaimed on the sea thanks to hydraulic structures like dikes. To prevent flooding, these low-lying areas are constantly drained by a network of ditches that release excess water in the sea (e.g. at low tide). The use of subsurface drainage pipes connected to existing drainage ditches further enabled the drainage of the lands and made them suitable for agriculture. While the groundwater remains saline water from its seaborn nature, with years and precipitation, a fresh water lens, lighter than the deeper saline water, developed near the soil surface, on top of the saline water. This fresh water lens is essential for most conventional crops that would suffer from saline conditions. The thickness of freshwater lenses varies throughout the year as a function of the recharge from rainfall and evapotranspiration.However, intensive rainfall events and prolonged summer droughts are becoming more frequent with Climate Change and lead to decreasing freshwater lens thickness, endangering crop yield. Controlled drainage systems that enable to regulate the water level in the subsurface drains has the potential to mitigate this issue by imposing a temporary higher water level, hence increasing recharge of the freshwater lens. To better understand the dynamics of the fresh/saline water interface throughout the year, we equipped two fields with multilevel piezometers with both head and salinity sensors replicated three times in each field. Along each multilevel piezometer we also installed 1D resistivity sticks with 16 electrodes to obtain a vertical electrical resistivity profile. In addition, electromagnetic induction surveys enabled us to expand the local observations to the entire area (4 ha in total).The datasets collected in the two fields in the conventional scenario (i.e. without controlled drainage) during the first year, showcase the usual dynamics of the interface, its lateral as well as vertical variability. The use of geoelectrical techniques enable us to distinguish fresh and saline water boundaries and its variability per soil layers. The electromagnetic induction surveys reveal old paleochannels that influence the dynamics of the freshwater lens at the field-scale. Moreover, the dataset also demonstrates how different crops (grass and flax) lead to different ground water and salinization dynamics. In this work, we present our first year of collected field data and related interpretation before the installation of the controlled drainage system.