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HS8.1.5

This session deals with the use of geophysical methods for the characterisation of subsurface properties, states, and processes in contexts such as hydrology, agriculture, contaminant transport, etc. Geophysical methods potentially provide subsurface data with an unprecedented spatial and a high temporal resolution in a non-invasive manner. However, the interpretation of these measurements is far from straightforward in many contexts and various challenges still remain. Amongst these, the need for improved quantitative use of geophysical measurements in model conceptualisation and parameterisation, and the need to move quantitative hydrogeophysical investigations beyond the column and field scale towards the catchment scale. Therefore, we especially encourage submissions addressing advances in i) the acquisition, inversion and interpretation of geophysical data and other minimally invasive methods in a (contaminant) hydrological context, ii) model-data fusion including new concepts for joint and coupled inversion, and iii) petrophysical understanding linking hydrological and geophysical properties.

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Co-organized by SSS6
Convener: Sarah Garré | Co-conveners: Remi Clement, Damien Jougnot, Philippe Leroy, Ulrike Werban
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| Attendance Thu, 07 May, 16:15–18:00 (CEST)

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Chat time: Thursday, 7 May 2020, 16:15–18:00

Chairperson: Sarah Garré
D363 |
EGU2020-18751
Florian M. Wagner, Carsten Rücker, Thomas Günther, Friedrich Dinsel, Nico Skibbe, Maximilian Weigand, and Joost Hase

Hydrogeophysics is interdisciplinary by definition. As researchers strive to gain quantitative information on process-relevant subsurface parameters while integrating non-geophysical measurements, multi-physical geoscientific models are often developed that simulate the dynamic process and its geophysical response. Such endeavors are associated with considerable technical challenges due to coupling of different numerical models, which represents an initial hurdle for students and many practitioners. Even technically versatile users often end up with individually tailored solutions at the cost of scientific reproducibility.

We argue that the reproducibility of studies in computational hydrogeophysics, and therefore the advancement of the field itself, needs versatile open-source software. One example is pyGIMLi - a flexible and computationally efficient framework for modeling and inversion in geophysics. The library provides management for structured and unstructured 2D and 3D meshes, finite-element and finite-volume solvers, various geophysical forward operators, as well as a generalized Gauss-Newton based inversion framework.

In this contribution, we highlight some of the recent advances and use cases in research and education since its 1.0 release in 2017 (Rücker et al., 2017) including:

  • generalized modeling and inversion frameworks for conventional, joint, time-lapse and process-based inversion
  • geostatistical regularization operators for unstructured meshes (Jordi et al., 2018)
  • improved constraints in the presence of petrophysical parameter transformations demonstrated by an estimation of water, ice, and air in partially frozen systems (Wagner et al., 2019)
  • 3D visualization leveraging upon PyVista (Sullivan and Kaszynski, 2019)
  • simulation of electrical streaming potentials
  • complex-valued forward modeling and inversion of induced polarization
  • forward modeling with anisotropic parameters
  • availability for Mac OS
  • improved API and documentation

Since the library is freely available and platform-compatible, it is also well suited for teaching. We demonstrate examples from Master level university courses and public outreach, where learners can interactively change model and acquisition parameters to study their influence on a hydrogeophysical process simulation. Finally, we would like to use this opportunity to discuss future developments with the community.

References

Jordi, C., Doetsch, J., Günther, T., Schmelzbach, C., & Robertsson, J. O. (2018). Geostatistical regularization operators for geophysical inverse problems on irregular meshes. Geophysical Journal International, 213(2), 1374–1386.

Rücker, C., Günther, T., Wagner, F.M., 2017. pyGIMLi: An open-source library for modelling and inversion in geophysics, Computers and Geosciences, 109, 106-123.

Sullivan, C., & Kaszynski, A. (2019). PyVista: 3D plotting and mesh analysis through a streamlined interface for the Visualization Toolkit (VTK). Journal of Open Source Software, 4(37), 1450.

Wagner, F. M., Mollaret, C., Günther, T., Kemna, A., & Hauck, C. (2019). Quantitative imaging of water, ice and air in permafrost systems through petrophysical joint inversion of seismic refraction and electrical resistivity data. Geophysical Journal International, 219(3), 1866–1875.

How to cite: Wagner, F. M., Rücker, C., Günther, T., Dinsel, F., Skibbe, N., Weigand, M., and Hase, J.: Open-source hydrogeophysical modeling and inversion with pyGIMLi 1.1: Recent advances and examples in research and education, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18751, https://doi.org/10.5194/egusphere-egu2020-18751, 2020.

D364 |
EGU2020-17952
Timea Katona, Jakob Gallistl, Sven Nordsiek, Matthias Bücker, Sven Frei, Stefan Durejka, Benjamin Gilfedder, and Adrian Flores-Orozco

Biogeochemical hot spots are spatially confined areas where biogeochemical processes take place with anomalously high reaction rates. On the landscape scale, biogeochemical hot spots are of major interest due to the possible emission of greenhouse gases (carbon dioxide) and high nutrient turnover. Such hot spots are sensitive environments and given their environmental impact, there is a growing demand for noninvasive methods to investigate such hot spots without disturbing the biogeochemical settings. Classical geochemical sampling methods (e.g., piezometers or suction cups) often disturb the redox-sensitive settings by bringing oxygen into anoxic areas. Induced polarization (IP) is a noninvasive geophysical method that was initially developed to explore metal-ore deposits but more recently developed into a versatile tool for environmental studies.  Here, we present imaging results from a geophysical survey using the IP method to characterize hot spots in a wetland located in the Lehstenbach catchment in Southeastern Germany. We collected IP imaging data sets along 64 profiles using 64 electrodes deployed with a spacing of 20 cm. Our highly resolved measurements aimed at delineating hot spots within a thin layer (approximately 20 cm) heterogeneous peat material on top of the local granite bedrock. To validate the field-scale IP signatures, geochemical analyses (e.g., dissolved and solid iron concentration) were performed on freeze-core samples obtained in areas characterized by anomalous high and low IP responses. Furthermore, the thickness of the peat was measured with a dipstick along every fifth profile to evaluate the IP imaging results. Our imaging results reveal an increase in the IP response within the upper 20 cm of the subsurface, which correlates with the variations in the iron concentrations and variations in the geochemical composition of groundwater accompanying microbial activity within the biogeochemical hot spots observed in the soil samples. The IP response decreases in the deeper regions, which can be associated with the granite bedrock.

How to cite: Katona, T., Gallistl, J., Nordsiek, S., Bücker, M., Frei, S., Durejka, S., Gilfedder, B., and Flores-Orozco, A.: Induced polarization for the spatial characterization of biogeochemical hot spots, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17952, https://doi.org/10.5194/egusphere-egu2020-17952, 2020.

D365 |
EGU2020-10603
Philippe Leroy and Shuai Li

Exploring the electrical properties of the mineral-water interface for interpreting geophysical electrical measurements is a very challenging work because of the low specific surface area of minerals such as quartz or calcite. Only few methods exist to probe the properties of the electrical double layer (EDL) compensating the surface charge of minerals. Among them, there is the streaming potential (SP) method where the applied water pressure difference generates a pore water flow displacing the mobile counter-ions in excess in the EDL, hence creating a measurable electrical potential difference, the streaming potential. During SP measurements, the exact position of the shear plane from the mineral surface is not known and it is widely accepted that the shear plane is located between the compact Stern layer and the diffuse layer. In our study, we show that the assumption that there is no water flow in the Stern layer has no physical basis for sandstones in contact with a NaCl electrolyte because water molecules around counter-ions in the Stern layer may have bulk-like properties. Using a basic Stern model to simulate surface complexation reactions and considering water flow in the Stern layer, we reproduced the zeta potential measurements on sandstones over a large salinity range from about 10-2 to 5.5 M NaCl. The “anomalous” high salinity zeta potential data can not be reproduced by a surface complexation model considering water flow only in the diffuse layer. Our ability to explain these measurements suggests that the shear plane may be located between the mineral surface and the Stern layer, i.e. closer to the surface than previously thought, which may have strong implications for the modelling of the surface electrical properties of the minerals.

How to cite: Leroy, P. and Li, S.: A dynamic Stern layer model to explain high salinity zeta potential measurements on sandstones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10603, https://doi.org/10.5194/egusphere-egu2020-10603, 2020.

D366 |
EGU2020-703
| Highlight
Cosimo Brogi, Johan Alexander Huisman, Michael Herbst, Anne Klosterhalfen, Heye Bogena, Lutz Weihermüller, Carsten Montzka, and Harry Vereecken

Water shortage in soil can result in a considerable reduction in crop yield, thus representing a severe threat to agricultural sustainability and profitability. It is therefore crucial to improve our understanding and prediction of the spatial variability of water stress and crop yield. Within this context, detailed soil maps obtained from the combination of hydrogeophysical methods, such as electromagnetic induction (EMI), and direct soil sampling can prove vital. However, it is still challenging to derive and exploit such data beyond the field-scale and their added value has not been fully investigated yet. In this study, we present results from two case studies where the added value of hydrogeophysical measurements in agriculture have been evaluated. In the first caone, high-resolution multi-configuration EMI data was measured on 51 adjacent agricultural fields (102 ha) near Selhausen (Germany). Each field was separately measured and six apparent electrical conductivity (ECa) maps with increasing depth of investigation were obtained. A supervised image classification method was applied to the ECa maps to obtain a 1 m resolution map of the study area that identifies 18 soil units with similar ECa signature. Afterwards, 100 ground truth locations were randomly selected and information on horizon type, depth and texture were collected until a maximum depth of 2 m. Statistical tests proved that each soil unit had unique soil characteristics in comparison to other units, thus confirming the effectiveness of the methodology in producing a highly detailed soil map in a complex environment that extends well beyond the field scale. To test its added value in agricultural applications, this geophysics-based soil map was used as input in agro-ecosystem simulations of crop growth and productivity for the 2016 growing season. For this, the one-dimensional AgroC model was used, which couples SoilCO2, RothC, and SUCROS subroutines to simulate crop growth. The necessary hydraulic parameters were estimated using pedotransfer functions. The leaf area index (LAI) of six crops simulated with AgroC showed clear correlation with LAI observed in six RapidEye satellite images. At the same time, further AgroC simulations based on commonly available soil maps performed significantly worse in terms of RMSE, model efficiency, and R2. Following these encouraging results, further simulations were performed to quantify the costs and benefits of irrigation within the study area in 2016 in terms of economical profit and CO2 sequestration. Despite the apparent added value of geophysics-based soil information, it was found that additional data recorded during the growing season would allow improving modelling and predictions, for example, through data assimilation. For this reason, the second case study considers a set-up with additional soil moisture sensors installed in two orchards near Agia (Greece). In each field, EMI and soil sampling were combined to inform the placement of SoilNet soil moisture sensor surrounding a cosmic-ray neutron probe. The purpose of this second case study is to integrate soil data, hydrological modelling, and weather forecasts to provide farmers with an efficient decision support system that would enhance financial gains and sustainability of their irrigation practices in the long term.

How to cite: Brogi, C., Huisman, J. A., Herbst, M., Klosterhalfen, A., Bogena, H., Weihermüller, L., Montzka, C., and Vereecken, H.: Improved spatial modelling of crop productivity using geophysics-based soil mapping: Two case studies beyond the field scale, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-703, https://doi.org/10.5194/egusphere-egu2020-703, 2019.

D367 |
EGU2020-5923
Nesrine Chaali, Daniel Bravo, Sofiane Ouazaa, Jose Isidro Beltrán Medina, and Javier Benavides

Increasing consideration is being placed on the environmental impact of soil contamination with heavy metals (HM), especially in productive agricultural areas. So, a key task is to characterize this contamination qualitatively and quantitatively in order to understand the spatial distribution of HM and decide about the adequate management. Traditional sampling to monitor HM distribution is time, cost-consuming and often unrepresentative. Additionally, sparse and punctual data measurements may not allow understanding the real dynamic of HM in the soil profile, and in many cases the collected data fails in providing the needed information. Recently, in-situ geophysical techniques based on electrical resistivity tomography measurements (ERT) were implemented in agriculture as a “proxy” to determine spatial and temporal distribution of HM. The objective of this study was to provide an accurate information for future efficient measures of soil remediation, by understanding the HM distribution, specifically cadmium (Cd) and arsenic (As), using electrical resistivity measurements combined with soil chemical analyses. A UNI-T UT523A devise was used in a “Wenner Alpha” configuration to perform ERT survey at 2 m depth in nine locations of Tolima department-Colombia. 2D-ERT cross sections “Tomograms” were obtained by the Res2Dinv software which allowed characterizing qualitatively the spatial distribution of Cd and As. Chemical concentration values for both Cd (0.36±0.06 mg.kg-1) and As (3.00±0.28 mg.kg-1) were introduced in the inverse modelling procedure as a solution to provide an easier and reliable alternative to determine the shape, size, and path of the likely electrical resistivity distribution of the studied HM. Tomograms revealed that Cd distribution was mainly observed in deeper soil profile (0.80 m), while As was observed basically in shallower soil layers (0.45 m). Higher electrical resistivity values (330–48000 Ω m) correspond to Cd distribution and lower electrical resistivity values (138-291 Ω m) are related to As distribution. A high positive Pearson correlation (ρ) between electrical resistivity measurements and soil chemical properties (for Cd and As content) was obtained for the nine locations; ρ values of 0.97 and 0.98 were obtained for Cd and As; respectively. A linear regression was performed between ERT measurements and Cd and As contents; (R2=0.94, RMSE=0.33) and (R2=0.97 RMSE=0.18) for Cd and As; respectively. The results underlie the utility of the combined geophysical techniques, based on electrical resistivity measurements, and soil chemical properties to improve the understanding of HM dynamic.

Key words: Geophysical techniques, tomograms, heavy metals, soil chemical properties, spatial distribution, Pearson correlation.

How to cite: Chaali, N., Bravo, D., Ouazaa, S., Beltrán Medina, J. I., and Benavides, J.: Predicting spatial distribution of heavy metals in agricultural soils using electrical resistivity tomography technique 2D-ERT, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5923, https://doi.org/10.5194/egusphere-egu2020-5923, 2020.

D368 |
EGU2020-21978
Edmundo Placencia-Gomez, Victor Burgeon, Ramin Heidarian-Dehkordi, Jeroen Meersmans, Mihai Cimpoiasu, Julien Fouché, Frederic Nguyen, Jean-Thomas Cornelis, and Sarah Garré

Electrical resistivity and induced polarization tomography and electromagnetic induction are widely used in hydrogeophysical applications. In this work we perform a multi-scale analysis of DC-resistivity, spectral induced polarization (SIP) and electromagnetic induction (EMI) measurements to evaluate soil water dynamics of a century-old biochar enriched agroecosystem. Our study aims at comparing the spatio-temporal variations of the electrical signature (resistivity or conductivity) between the natural (reference) soil and soil enriched with biochar visible as black patches ( 0.30 m thick x 20 m of diameter) in the study area and relate this signature to a soil moisture status. In this first overall and qualitative approach we combine 1) field large-scale time-lapse electrical resistivity tomography (ERT) transects (12.6 m) and EMI conductivity maps covering the whole study area (13 ha), 2) intermediate-scale ERT/SIP profiles from on-site pits (2 m L x 1 m W x 1 m D), and 3) laboratory columns-scale (0.10 m L x 0.044 m ID) SIP signatures of undisturbed soil samples.

Large-scale results show a heterogeneous-resistive soil top horizon in both soil types, but with similar hydrodynamic behaviour following precipitation events. The column scale SIP signatures reveal that texture and pore structure are the main driver of soil moisture dynamics with insignificant role of the biochar content. Large and intermediate scale monitoring campaigns during the entire growing season of two different crops are planned for the current and next year. The ultimate objective is to quantify the effect of century-old biochar on soil water dynamics and root water uptake.

How to cite: Placencia-Gomez, E., Burgeon, V., Heidarian-Dehkordi, R., Meersmans, J., Cimpoiasu, M., Fouché, J., Nguyen, F., Cornelis, J.-T., and Garré, S.: Integrated analysis of multi-scale electrical signatures for characterizing soil water dynamics in century-old biochar enriched agroecosystems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21978, https://doi.org/10.5194/egusphere-egu2020-21978, 2020.

D369 |
EGU2020-16391
Salma Ammor, Jean Michel Baltassat, Clara Jodry, Anatoly Legtchenko, Mohammed Azaroual, Nadia Amraoui, and Arnaud Isch

 

In the Region Centre - Val de Loire, groundwater quality of the largest reservoir in France is menaced by strong pressures related to high agricultural activities. Scientific efforts are being made to well understand both hydrological systems and water transfers in the Vadose Zone (VZ). The O-ZNS observatory, currently under development at an agricultural site in Villamblain (Centre Val de Loire, France), will provide a detailed insight of hydrological transfer processes in the VZ. It is based on an exceptional well (20m-depth and 4m-diameter) associated with several surrounding boreholes. These will allow, an accurate characterization of the heterogeneous structure of the unsaturated limestone, by geophysical imaging and a monitoring of the hydrogeological parameters by various techniques. The aim is to understand the hydrogeological processes governing water transfer in heterogeneous limestone VZ, and to elaborate hydrogeological models integrating these processes.

Our approach for this first characterization of the O-ZNS site is mainly based on a qualitative comparison between surface and borehole geophysical prospections and laboratory tests. Geophysical prospection consisted especially in 2D Magnetic Resonance Sounding (MRS), which allows non-invasive determination of water content in the VZ. Several sizes of MRS loops were tested to explore scale effects and different measurement configuration were applied for filtering. MRS results are compared with laboratory tests (petrophysical, mineralogical and hydraulic properties) and the other geophysical prospection methods, such as neutron logging and cross-borehole radar tomography results.

These preliminary results are used in order to define MRS monitoring measurement configuration including loop size and position, as well as identify optimized filtering strategy and measurement time frequency. Finally, these results will provide the baseline for the research projects, which aims at determining MRS contribution to the improvement of taking into consideration scale variability and heterogeneity in water transfers models within the vadose zone.

How to cite: Ammor, S., Baltassat, J. M., Jodry, C., Legtchenko, A., Azaroual, M., Amraoui, N., and Isch, A.: Geophysical characterization of a Limestone Heterogeneous Vadose Zone – Beauce Aquifer (France), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16391, https://doi.org/10.5194/egusphere-egu2020-16391, 2020.

D370 |
EGU2020-19011
Marc Dumont, Julien Guillemoteau, Luis Cavalcante-Fraga, Roger Guérin, Cyril Schamper, Ningxin Chen, and Danièle Valdés

The underground quarry of Chalk at Saint-Martin le Nœud (80km north of Paris, France) is an instrumented site of particular interest to study infiltration processes in the Chalk critical zone. The outcropping of the water table creates permanent underground lakes (20 and 30m below the surface) showing spatial and temporal variations of groundwater hydrodynamics and geochemistry within the 1.2km long quarry. Previous studies showed a correlation between infiltration variations and the geometry of the clay covering the Chalk. Here, we present a methodology coupling electrical and electromagnetic surveys to analyze the control of the critical zone structures on infiltration processes.

In 2019, the whole quarry has been covered by an electromagnetic induction (EMI) mapping, providing an estimation of the shallow bulk electrical conductivity at several depths of investigation with a good lateral resolution (0.5m spacing along lines spaced 5 to 25m apart). The EMI instrument used for this study requires a calibration with conductivity/resistivity values as provided by rocks samples or electrical resistivity tomography (ERT). For this purpose, and additionally to characterize deeper structures, five ERT profiles have been acquired above underground lakes with contrasted hydrodynamical behaviors. Such setting poses several challenges for the calibration of EMI data as (i) the EMI and ERT methods have, by definition, different lateral sensitivities and (ii) the numerical modellings used for both methods in the calibration procedure are based on different geometric assumptions (2D for ERT, and 1D for EMI).

Our study explores an approach to refine and improve EMI calibration using contrasted ERT profiles. The first step tackles, for each ERT profile, the difference between EMI and ERT resolution in order to improve the consistency between the imagery of both methods. The second step analyzes the consistency of the five calibrations for each ERT profiles and the calibration validity in regards to the lateral heterogeneity at the quarry scale. As a result, we are able to provide reliable EMI calibration while taking advantage of deeper ERT imagery in areas of interest. These results are improving our understanding of the geometry of the clay covering the Chalk formations. This geometry is confronted with previous geochemical and hydrodynamical results to understand how the critical zone structures control temporal variations of groundwater infiltration within the critical zone.

How to cite: Dumont, M., Guillemoteau, J., Cavalcante-Fraga, L., Guérin, R., Schamper, C., Chen, N., and Valdés, D.: Exploration of electromagnetic induction potential to understand groundwater infiltration within the Chalk critical zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19011, https://doi.org/10.5194/egusphere-egu2020-19011, 2020.

D371 |
EGU2020-9014
Agathe Defourny, Thomas Kremer, Arnaud Collignon, Patrick Jobé, Alain Dassargues, and Frédéric Nguyen

CO2-rich mineral groundwaters have been exploited for centuries for both bottling and thermal activities. The detection and understanding of productive areas is therefore of great interest to manage future supply in a sustainable way. CO2-rich mineral water systems are complex since they usually involve an intricated network of water bearing fractures enabling the uplift of CO2-rich groundwater to the surface, a process that is still poorly understood. Geophysical prospection is crucial to detect potential uplift zones and to address corresponding uncertainties before drilling operations.

In this context, non - to minimally - invasive near-surface geophysical methods can prove to be efficient. The objective of this contribution is to assess the ability of the induced polarization method, combined with the electrical resistivity technique, to make the distinction between CO2-rich groundwater from non-gaseous groundwater.

Several combined electrical resistivity and induced polarization tomography profiles were performed in the Ardennes (Belgium) where thousands of CO2-rich groundwater springs are observed. The profiles were all set immediately above known uplift zones. Inversion results were consistent between all profiles and important contrasts in both electrical resistivity and chargeability distributions in the vicinity of the uplift zone were observed, which were also reflected in the normalized chargeability sections computed on the basis of the measured data.

Low resistivity vertical contrasts extending in depth were observed and interpreted as saturated fractures enabling the uplift of deep groundwater to the surface. In addition, high chargeability anomalies appeared directly close to the CO2-rich groundwater resurgence. Those anomalies are thought to be associated to the presence of metallic oxides and hydroxides, as a result of dissolved metallic species precipitation in the upper part of the fractured aquifer due to the pressure decrease and change in redox conditions in up-flowing groundwater towards the land surface.

We conclude that the combined interpretation of electrical resistivity and induced polarization datasets is a very promising method for a more robust prospection of naturally sparkling groundwater.

How to cite: Defourny, A., Kremer, T., Collignon, A., Jobé, P., Dassargues, A., and Nguyen, F.: Electrical signature of CO2-rich mineral groundwater systems - Application in the Ardennes, South-East of Belgium, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9014, https://doi.org/10.5194/egusphere-egu2020-9014, 2020.

D372 |
EGU2020-16956
Andrés González Quirós and Jean-Christophe Comte

Seawater intrusion is a natural phenomenon occurring in coastal aquifers, which is exacerbated by borehole abstraction, which may lead to contamination of water supplies. Borehole data are usually too scarce spatially to enable accurate identification and delineation of seawater intrusion. For that purpose, electrical resistivity tomography (ERT) and electromagnetic (EM) methods are proven and popular geophysical techniques because of their high spatial resolution and sensitivity to pore water salinity, which is particularly beneficial in heterogeneous aquifers characterised by complex saltwater intrusion patterns. Inverted resistivity models can be converted into pore water salt concentration through the application of so-called petrophysical relationships, and further interpreted to map seawater intrusion patterns or used as constraining dataset for groundwater models. However, the conversion procedure is prone to conceptual and computational errors including the application of the appropriate petrophysical relationship —considering heterogeneity or clay content—, and the numerical limitations in the inversion of the geophysical data. These errors are cumulated and transferred throughout all steps from geophysical acquisition to the hydrogeological model. In this work, using ERT geophysics as an example, we evaluate the magnitude and spatial distribution of relative errors and their imprint in the final recovered salinity section to be used for interpretation or model calibration. Results highlight the importance of applying the appropriate petrophysical relationship when delineating spatial salinity patterns from the resistivity model, along with the errors associated with the conceptual simplifications in heterogeneous systems on one hand, and with the geophysical inversion procedure on the other hand. These three sources of errors add-up and may lead to large inaccuracies in the estimated position and spreading of the seawater-freshwater mixing zone. They need to be accounted for when using geophysical model for mapping and modelling of saltwater intrusion. The analysis also provides practical insights for the integration of inverted geophysical data in early-warning seawater intrusion monitoring strategies, for improved estimations of freshwater resources and for the management of coastal aquifers.

How to cite: González Quirós, A. and Comte, J.-C.: Cumulative errors in the use of geophysically-derived salinity for the characterization of seawater intrusion in heterogeneous coastal aquifers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16956, https://doi.org/10.5194/egusphere-egu2020-16956, 2020.

D373 |
EGU2020-21589
Stefan Klingler, Ulrike Werban, Carsten Leven, Peter Dietrich, and Olaf A. Cirpka

A biogeochemical site characterization aims at delineating zones of reducing and oxidizing conditions in the subsurface to infer their influence on solute and contaminant turnover processes along groundwater flow paths. Hereby, large values of the total organic carbon (TOC) content mark reducing conditions in soils and sediments. Dark sediment colors are good predictors for high-TOC zones and thus indicate hotspots of biogeochemical turnover and microbial activity. Traditionally, obtaining the sediment color requires costly sampling, resulting in poor horizontal resolution and related uncertainty caused by interpolation. We suggest using a direct-push soil color optical screening tool to acquire multiple high-resolution vertical color profiles and demonstrate its applicability in floodplain sediments down to 12 m depth. We use Gaussian mixture models for a cluster analysis of the color logs in the CIE L*a*b* color space to identify color-facies, determine facies-specific relationships between the L*a*b* color-values and the TOC content of the sediments, and to construct the 3-D distributions of three distinct facies and organic matter. Direct-push color-logging may also be used for in-situ mapping of redox-zonation, iron content, or sedimentary structures.

How to cite: Klingler, S., Werban, U., Leven, C., Dietrich, P., and Cirpka, O. A.: Detection of High-Organic-Carbon Features in Sediments by Direct-Push Color Logging, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21589, https://doi.org/10.5194/egusphere-egu2020-21589, 2020.

D374 |
EGU2020-19400
Yi-Chen Chu and Chien-Chih Chen

In Taiwan, most of the rivers in the Pingtung Plain are ephemeral stream. In the dry season, the water source is unstable and cannot be used all the year round. Due to the uneven distribution of time and space, the supply and demand of water resources are often imbalanced. In order to provide a stable groundwater source strategy, it is necessary to understand the geologic characteristics of groundwater.

Electrical resistivity surveying methods have been widely used to determine the thickness and resistivity of layered media for the purpose of assessing groundwater potential and siting boreholes in fractured unconfined aquifers. In this study, we used CP configuration on the Electrical Resistivity Tomography (ERT) monitoring system at two study sites. One is the surface-borehole survey line at the Dashu, Kaohsiung City, Taiwan, and the other is the surface survey line at the Daliao, Kaohsiung City, Taiwan. Both of sites located on Quaternary alluvium of Pingtung Plain which composed of coarse sand and gravel.

The resistivity difference might be caused by the dynamic of the groundwater. We analyzed the change in the electrical properties of the gravel layer during the rainfall season at the Dashu site and analyzed the groundwater level change by ERT method during the pumping event at the Daliao site which is the pumping station to understand the groundwater replenishment situation. The ERT result can be calculated Relative Water Saturation (RWS) of the shallow formation fluid, and it reveal the permeability of the gravel layer and the hydrogeological characteristics of the sites. Furthermore, we assumed the different particle size and porosity to estimate the resistivity and the hydraulic conductivity coefficient theoretical trend line, compare the observation well data is used to estimate the actual porosity and the actual hydraulic conductivity range. Finally, for the groundwater conditions in the large area of the Pingtung Plain, we use the theoretical trend line to analyze the data of WRG’s 34 wells in western Pingtung Plain. The results show (i) the well logging resistivity is positively correlated with the hydraulic conductivity. (ii) the sandstone and mudstones with small variables have smaller well-measured resistivity and a lower hydraulic conductivity. (iii) affected by compaction, the porosity tends to decrease with increasing depth. (iv) on the west side of the Pingtung Plain, the particle sizes are relatively consistent, and the hydraulic conductivity is 10 -3~10 -4 m/s.

This study is using ERT to provide hydrological data analysis on small areas and large areas in the Pingtung Plain, and also contributes to groundwater operations and management

How to cite: Chu, Y.-C. and Chen, C.-C.: Estimation of hydraulic parameter from geoelectrical measurements : A case study of the western Pingtung Plain in Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19400, https://doi.org/10.5194/egusphere-egu2020-19400, 2020.

D375 |
EGU2020-2895
Thomas Kremer

The surface Nuclear Magnetic Resonance method is gaining momentum as an efficient geophysical method for the detection and characterization of groundwater. However, the method still suffers from a low signal to noise ratio mostly due to electromagnetic noise of anthropic origin.

To solve this problem, signal processing in surface nuclear magnetic resonance surveys often relies on the reference-based noise cancellation technique. This method consists of capturing the main characteristics of the noise through a secondary loop ideally located and obtaining an estimate of the noise affecting the primary loop which can be subtracted from the noisy sNMR signal.

The main problem associated with the method occurs when the spatial distribution of the noise is heterogeneous, which can result in a low correlation between the reference loop and the primary loop, and hence in a poor noise reduction. Difficulties may also arise when the field survey location prevents the display of a reference loop for logistics or physical reasons.

To remediate these situations we have investigated the possibility of recording noise-only signals through the primary loop, prior to the sNMR measurement, and use those signals as references for subsequent calculation of the local transfer function. The correlation between a series of noise-only signal recorded on a primary loop was analyzed through the computation of the magnitude squared coherence function, and comparison was made with noise records from a secondary loop. The analysis demonstrates that temporal reference noise cancellation (TRNC) can provide more efficient noise reduction results than the classical spatial reference noise cancellation if the temporal noise-only database is large enough.

Such a technique would be particularly suited for the development of long-term sNMR monitoring systems, where noise records could be acquired for long periods without any difficulty. In addition to two field survey examples, we present a synthetic statistical analysis to estimate the minimal volume of the signal database required for optimal noise reduction.

How to cite: Kremer, T.: Using the temporal distribution of noise for reference noise cancelation in sNMR surveys, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2895, https://doi.org/10.5194/egusphere-egu2020-2895, 2020.