SSS6.1

Sub-resolution pore space is increasingly being accounted for in Lattice-Boltzmann models of water flow and solute transport. However, robust methods to estimate the permeability and porosity of sub-resolution voxels in 3D X-ray images, which are needed for model parameterization are lacking. The grey-value of a voxel in a 3D X-ray tomography image is approximately proportional to the density. Since the density in a voxel depends on the volume fractions of solids, air and water, and the densities of these phases, a grey-value cannot be directly translated to a porosity value. The objective of this study was to develop a reliable method for 3D estimation of sub-resolution porosity in undisturbed soil samples using data obtained from standard industrial X-ray tomography images. To achieve this we used the differences in X-ray attenuation between samples saturated with water and saturated with a potassium iodide (KI) solution.

We collected ten intact soil cores (5.5 cm high, 6.5 cm diameter) in aluminium cylinders from the topsoil of an arable field in south-west Sweden. The samples had a large variations both in soil texture and organic carbon content. The samples were scanned using X-ray tomography after being slowly saturated with water from the bottom. The water was then replaced by a KI solution (30 g I L^-1) with a larger X-ray attenuation than water, and the samples were scanned a second time. The grey-values of the resulting 3D images were scaled by the known densities of air, water and aluminium and the images were registered (i.e. spatially aligned). Macropores, sand grains and gravel were then removed from the images. The difference in attenuation between the two final images was then used to calculate the sub-resolution porosity (i.e. the degree of saturation) in all voxels in the remaining image of the soil matrix. Average porosities for individual samples, which were in the range 0,34­–0.45, were significantly correlated to matrix porosities estimated from soil water retention measurements.

How to cite: Larsbo, M., Fukumasu, J., and Koestel, J.: A method for 3D mapping of sub-resolution porosity from X-ray tomography images, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11106, https://doi.org/10.5194/egusphere-egu22-11106, 2022.

Soil aggregates fracture through the coalescence of internal macropores (cracks), forming smaller fragments that change pore structure characteristics. Many studies have measured soil aggregate fracture with laboratory tests, but the impact of internal pore structure has remained elusive in the black box of soil.  This study, which is the first of its kind, uses Xray micro-CT imaging, mechanical measurement experiments and finite element simulations to investigate the relationship between soil pore-scale topology and aggregate mechanical properties including fracture energy. The soil aggregates came from a red soil (Acrisols) experimental field in Jiangxi, China that had been amended with different amounts of manure and lime. From Xray micro-CT, quantitative topology analysis extracted the pore network extraction method. Then the strain-stress relationship and fracture energy of the scanned aggregate were measured using a loading frame. The micro-CT images are used as geometry inputs to perform finite element methods to calculate effective Young’s modulus and detailed strain-stress distribution at micrometers. The experimental results showed that adding manure increased the elastic stiffness and fracture energy of the aggregate. The pore scale strain-stress distribution analysis from finite element simulations found these properties at aggregate scale were weakly correlated to bulk porosity but driven by the stress intensity distribution of the aggregates, agreeing with previous research on model soil structures.

How to cite: Qu, R., Zhou, H., and Hallett, P.: Micron-scale mechanical properties of soil aggregates amended with manure: experimental evidence and image-based finite element simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12291, https://doi.org/10.5194/egusphere-egu22-12291, 2022.

Dielectric properties of soil are often utilized for the purpose of soil moisture measurement. However, the relations between soil complex dielectric permittivity and volumetric water content depend also on other factors, such as the operating frequency of the sensor, soil texture, and temperature. The goal of the presented work is to examine the impact of temperature in the 0.5 – 40°C range on the complex dielectric permittivity spectrum of two soils of silt loam and loamy sand textures. The permittivity spectra were measured in a coaxial transmission-line cell system with the use of a Copper Mountain R60 one-port vector-network-analyzer in the frequency range from 20 MHz to 3 GHz. The relations between the real part of dielectric permittivity and soil volumetric water content were modeled at each examined frequency and the temperature dependence of the applied model parameters was determined. In the future research steps, the obtained relations will be applied and tested with the use of a prototype field soil moisture probe operating in a broadband frequency range.

The research has been supported by the Polish National Agency for Academic Exchange under Grant No. PPI/APM/2018/1/00048/U/001. The soil dielectric spectra have been obtained in the scope of the project No. 2014/15/D/ST10/04000 financed by the Polish National Science Centre (NCN).

How to cite: Szypłowska, A., Lewandowski, A., Kafarski, M., Szerement, J., Wilczek, A., Majcher, J., and Skierucha, W.: The influence of temperature on soil complex dielectric permittivity in the 0.02 – 3 GHz frequency range, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4330, https://doi.org/10.5194/egusphere-egu22-4330, 2022.

Magnetic resonance imaging (MRI) of the freezing and thawing process was performed on a series of repacked samples of sand, soil, and sand-soil mixture. The freezing/thawing is performed in the sample container placed inside the vertical bore MRI scanner within the 66 mm inner diameter of the radiofrequency coil. The sample container was vacuum insulated from the sides and bottom to allow for the minimum thickness of the insulation layer. The vacuum was constantly maintained by a vacuum pump. The sample assembly was built from PMMA and other nonmetallic - MRI compatible materials. A porous material in the sample container was cooled at the top by the flow of cold gaseous nitrogen released from the liquid nitrogen stored in the Dewar flask. The cooling took place across the glass plate positioned at the top of the sample in the headspace above the sample. The temperature of the gas that was delivered to the headspace and leaving the headspace was monitored. Additionally, the temperature was monitored in the headspace above the glass disk and directly in the glass disk by fiber optics temperature probes. A 4.7 T magnet at the FZJ was used for MRI. Multiple-Slice Spin-Echo and Zero Echo Time pulse sequences were utilized. The contrast between the frozen and unfrozen water is given by the difference in T1 and T2 relaxation times. The time-lapse 3D imaging was done during the entire course of the experiment. Once the freezing front reached the bottom of the sample, the thawing process was induced. The small changes in sand structure as a consequence of volumetric ice-water changes were studied. The spatiotemporal analysis of the freezing front advancement and frozen water volume has been performed. The data are available for the development of two-phase ice-water simulation models.

How to cite: Snehota, M., Koestel, J., Pohlmeier, A., Princ, T., Sobotkova, M., Cislerova, M., and Sklenar, J.: Dynamics of freezing and thawing of water in saturated sand and soil: Magnetic resonance imaging study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13065, https://doi.org/10.5194/egusphere-egu22-13065, 2022.

Electromagnetic (EM)-based sensors used for water content determination are now being widely used across the globe in research, environmental monitoring networks, weather stations, irrigation management, feed and grain quality control in addition to a host of other applications. The multi-million-dollar EM sensor market continues to expand and yet lacks test standards and is generally lacking in information about sensor quality and performance. Decades of past sensor assessments have presented mixed testing approaches and a commensurate measure of mixed results. Confusion regarding EM sensor-function, -failure rate and -value, stems from testing results that often use non-standard targets including inhomogeneous (variable density and water content) and complex materials (e.g., soils) that may not be widely available for subsequent testing and verification by others. Electromagnetic sensors employ time- or frequency-domain measurements to estimate real (and sometimes including imaginary) permittivity and electrical conductivity, with different sensors measuring at varied and often unknown frequencies. Sensor output is affected by environmental impacts on circuitry (temperature) combined with effects of porous medium temperature, electrical conductivity, interfacial polarization and dielectric relaxation, all of which often combine to alter the apparent permittivity and resulting water content. Although a few attempts have been made to standardize testing, more work and research is required before an international standard can be recognized and adopted. Here we point to standardizing 1) granular porous test media, 2) media packing approaches and 3) permittivity-water content calibration functions with examples and comparison of different EM sensors.  

How to cite: Jones, S. B., Chang, C.-Y., González-Teruel, J. D., Robinson, D. A., Friedman, S. P., Szyplowska, A., and Skierucha, W.: A Framework for Standardizing Electromagnetic Water Content Sensor Assessment using Granular Porous Media, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6597, https://doi.org/10.5194/egusphere-egu22-6597, 2022.