1,721,043 research outputs found
A combination of the Hashin-Shtrikman bounds aimed at modelling electrical conductivity and permittivity of variably saturated porous media
Full text linkIn this paper, we propose a novel theoretical model for the dielectric response of variably saturated porous media. The model is first constructed for fully saturated systems as a combination of the well-established Hashin and Shtrikman bounds and Archie's first law. One of the key advantages of the new constitutive model is that it explains both electrical conductivity-when surface conductivity is small and negligible-and permittivity using the same parametrization. The model for partially saturated media is derived as an extension of the fully saturated model, where the permittivity of the pore space is obtained as a combination of the permittivity of the aqueous and non-aqueous phases. Model parameters have a well-defined physical meaning, can be independently measured, and can be used to characterize the pore-scale geometrical features of the medium. Both the fully and the partially saturated models are successfully tested against measured values of relative permittivity for a wide range of porous media and saturating fluids. The model is also compared against existing models using the same parametrization, showing better agreement with the data when all the parameters are independently estimated. An example is also presented to demonstrate how the model can be used to predict the relative permittivity when only electrical conductivity is measured, or vice versa
Linking soil properties to permittivity data: beyond the refractive index model
No full textThe availability of reliable constitutive models linking the bulk electric properties
of porous media to their inner structure is a key requirement for useful quantitative
applications of non invasive methods. In this paper we focus on the use of dielectric
measurements for (time-lapse) monitoring of fluid saturation changes in porous
materials, e.g. via Time Domain Reflectometry (TDR) or ground penetrating radar
(GPR). A number of empirical, semi-empirical and theoretical relationships have been
proposed, linking the bulk dielectric constant with volumetric water content. Among
the most popular are mixing models that involve some form of weighted average of the
dielectric constants of the components. One such model, named CRIM (complex
refractive index model) or LR (Lichteneker-Rother) has found extensive application in
recent years. In this paper we first analyze the characteristics of the CRIM by means
of theoretical considerations, thus drawing a link between the CRIM and the well
known Archie's law used for the analysis of DC resistivity in porous media. Next, we
use pore-scale modelling and experimental results to show other characteristics of
the CRIM, and in particular the dependence of its parameters on dielectric properties
of the components, as well as on porosity. We then proceed to assess the robustness
of the identification of CRIM parameters in presence of synthetic data error, thus
concluding that CRIM parameters cannot, in general, be independently identified on
the basis of bulk dielectric constant versus moisture content data. A novel
theoretical model for the dielectric response of saturated porous media is proposed
in the second section. The new constitutive relationship incorporates the theoretical
link with Archie's law as well as a combination of the well established Hashin and
Shtrikman bounds. The proposed model is shown to be able to match both experimental
and pore-scale modelling data with no use of ad-hoc fitting parameters.Presenters:
name: Brovelli, Alessandro
affiliation: Dipartimento di Scienze dell'Ambiente e del Territorio, University of Milano-Bicocc
Constitutive models for the joint estimation of electrical conductivity and permittivity of variably-saturated soils
No full textIn the context of sustainable development, there is a growing interest in mapping soil characteristics and land use, in particular to evaluate threats to soil quality. Fast non-invasive geophysical surveys have been used to estimate the areal distribution of soil properties, such as moisture content, texture and salinity. All geophysical methodologies rely on constitutive relationships to convert the measured variables in the corresponding properties of interest. Among the most used methodologies are those that measure soil electrical properties, in particular DC electrical conductivity and permittivity in the static limit. Traditionally, these two variables have been treated separately, that is, different constitutive models have been used to parameterize and link the measured response of the soil to its hydrological state and its texture. A number of works however have shown that these two quantities depend to a large extent on the same soil characteristics, and can therefore be jointly studied. The aim of this study was to identify and compare possible petrophysical constitutive models to be used to parameterize electrical conductivity and permittivity with the same conceptual approach, and to evaluate whether soil parameters estimated with one model can be used with a different equation based on the same parameterization. In addition, one such constitutive law was developed. The new petrophysical equation is derived combining the Hashin and Shtrickman upper and lower bounds with Archie’s law. A large suite of laboratory measurements was used to test the model in different conditions (soil types, mineralogy, texture, etc), and good comparison was found in most cases. Five existing models that use a parameterization of the soil response compatible with that of the new equation were compared. In particular, in all models the soil texture and the geometrical and topological properties of the pore-space were expressed through Archie’s cementation factor and saturation exponent. It was found that only three models were fully interchangeable, while the remainders predicted a different response for the same soil texture. Following this analysis, a methodology to estimate soil salinity based on simultaneous measurements of bulk electrical conductivity and permittivity was developed and validated with laboratory experiments. The successful estimation of pore-water salinity without any knowledge of other soil properties and without tuning of the model confirmed that the combined use of simultaneous measurements of DC conductivity and permittivity is an extremely powerful tool to map soil properties over large areas, because it reduces the calibration needs.ECOLAbstract EGU2011-478
Contribution of Stern layer and membrane polarization to spectral induced polarization of variably saturated sandy soils
Full text linkSpectral induced polarization is receiving increasing attention as a tool to map subsurface properties in a non-invasive manner. Although empirical correlations have been devised to link measurements to porous medium properties, such as the time constant of the Cole-Cole model to grain size distribution and hydraulic conductivity, a comprehensive process-based model is still missing. Two fundamentally different mechanisms have been proposed so far, (i) electrical double layer polarization, in particular of the Stern layer and (ii) membrane polarization. This latter mechanism is due to the accumulation of ions at the opposite sides of narrow pore-throats, which effectively act as ion-selective channels and lead to the formation of a membrane potential. Both mechanisms have so far shown the ability to explain to some degree experimental observations, although not in a completely convincing manner. The goal of this work was to test whether the two process concur to the observed polarization of the porous medium or rather are mutually exclusive. The Hashin-Shtrickman Average (HSA) model of Brovelli and Cassiani (2010, 2011) was extended to compute the complex bulk conductivity of variably saturated porous media. Complex surface conductance was computed from EDL polarization theory, whereas membrane polarization affects pore-water conductivity. The frequency-dependent HSA model was compared with the measured spectral induced polarization of variablysaturated sandy soils. A satisfactory comparison was found for most samples, in particular with water saturation above 0.8. It was observed that the two polarization mechanisms lead to separate phase peaks, which are related to the characteristic diffusion length and tortuosity of grains and pore-throats. When saturation is decreased, Stern layer polarization becomes the dominant mechanism, as the water phase is progressively less abundant and more disconnected. In addition, the measured polarization becomes more difficult to explain with the model, perhaps because additional mechanisms – such as the polarization of the air-water interface – come into play.ECOLAbstract EGU2012-734
Effective permittivity of porous media: a critical analysis of the complex refractive index model
No full textThe availability of reliable constitutive models linking the bulk electric properties of porous media to their inner structure is a key requirement for useful quantitative applications of noninvasive methods. This study focuses on the use of dielectric measurements to monitor fluid saturation changes in porous materials. A number of empirical, semi-empirical and theoretical relationships currently exists that link the bulk dielectric constant with volumetric water content. One such relationship, named complex refractive index model or Lichteneker-Rother model has been extensively applied in recent years. Here we first analyse the characteristics of this Lichteneker-Rother model by means of theoretical considerations. This theoretical analysis indicates that the Lichteneker-Rother exponent is dependent upon the geometrical properties of the porous structure, as well as the permittivity contrast between the different phases. Pore-scale modelling and experimental data further support this result. The parameter estimation robustness in presence of synthetic data error is also assessed. This demonstrates that Lichteneker-Rother parameters cannot, in general, be independently identified on the basis of bulk dielectric constant versus moisture content data
Electrophoretic and lectin-binding properties of glycopeptides released from the membrane during "in vitro" aging of human erythrocytes.
No full textDuring in vitro aging of human erythrocytes sialopeptides are lost from the membrane in a process which appears to act on glycophorins. This glycopeptide material can be purified by affinity chromatography on Wheat germ agglutinin-Sepharose, as glycophorin does. The electrophoretic behaviour of the purified material suggests that the glycopeptide comes from the breakdown of the domain of glycophorin exposed on the surface of the membrane. The binding properties toward Phaseolus vulgaris E lectin indicate that the only N-linked sugar chain of glycophorin is present in the sialopeptide released from the membrane; therefore we can argue that the glycophorin breakdown during in vitro aging of red cell takes place beyond the 26th residue of the sequence, and probably quite near the lipid bilayer
Combined estimation of effective electrical conductivity and permittivity for soil monitoring
Full text linkThe mapping of moisture content, composition, and texture of soils is attracting a growing interest, in particular with the goal of evaluating threats to soil quality, such as soil salinization. Fast noninvasive geophysical surveys are often used in this context. The aim of this work was to study constitutive models that can be used to parameterize electrical conductivity and permittivity starting from a unifying conceptual approach, and to evaluate whether the information carried by one measurement type can be used to identify soil parameters that are then used to predict the other geophysical quantity. To this end, a recently developed constitutive model was extended and modified to consider the grain surface conductivity, a critical component in most natural situations. The extended model was successfully tested against laboratory measurements. In addition, the new model was compared against five other equations that use similar soil parameterizations. It was concluded that only three out of the five selected models yield similar predictions, while the remaining two predict a different geophysical response for the same soil texture. Following this analysis, a methodology was developed to estimate soil salinity starting from the simultaneous measurements of bulk electrical conductivity and permittivity and validating this methodology against laboratory experiments. The method is valid in situations where the conductivity of the pore water remains approximately constant during the measurement period. Key features of the approach proposed to map soil salinization are (1) simplicity, (2) absence of fitting parameters, and (3) the fact that moisture content does not need to be measured or estimated independently. The methodology was tested on a large number of soil samples and proved robust and accurate
Sensitivity of Intrinsic Permeability to Electrokinetic Coupling in Shaly and Clayey Porous Media
Full text linkClassical Darcy's law assumes that the intrinsic permeability of porous media is only dependent on the micro-geometrical and structural properties of the inner geometry of the medium. There are, however, numerous experimental evidences that intrinsic permeability of shaly and clayey porous material is a function of the fluid phase used in the experiments. Several pore-scale processes have been proposed to explain the observed behavior. In this study, we conduct a detailed investigation of one such mechanism, namely the electrokinetic coupling. We have developed a numerical model to simulate this process at the pore-scale, incorporating a refined model of the electrical double layer. The model is used to conduct a detailed sensitivity analysis to elucidate the relative importance of several chemical-physical parameters on the intensity of the electrokinetic coupling. We found that permeability reduction due to this mechanism is likely to occur only if the effective pore-radius is smaller than 10(-6) m. We also observed that electrokinetic coupling is strongly sensitive to electrophoretic mobility, which is normally reduced in clays compared to free-water conditions. Based on these findings, we set up a suite of stochastic pore-network simulations to quantify the extent of permeability reduction. We found that only if the effective pore-radius is ranging from 5 x 10(-7) m to 5 x 10(-8), electrokinetic coupling can be responsible for a 5-20% reduction of the intrinsic permeability, and, therefore, this mechanism has a minor impact on situations of practical environmental or mining interest
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