- Home
- A-Z Publications
- Near Surface Geophysics
- Previous Issues
- Volume 15, Issue 6, 2017
Near Surface Geophysics - Volume 15, Issue 6, 2017
Volume 15, Issue 6, 2017
-
-
An analytical membrane‐polarization model to predict the complex conductivity signature of immiscible liquid hydrocarbon contaminants
Authors Matthias Bücker, Adrián Flores Orozco, Andreas Hördt and Andreas KemnaABSTRACTAn analytical membrane‐polarization model is developed to predict the frequency‐dependent complex conductivity of hydrocarbon‐contaminated sediments. In the absence of hydrocarbon contaminants, the effect of membrane polarization can be approximated using a recently developed analytical model, which describes the pore space as a sequence of two cylindrical pores of different lengths and radii. Different cation and anion concentrations in the electrical double layer at the pore wall lead to an ion‐selective behaviour causing the membrane‐polarization effect. This model can readily be adjusted to account for a wetting liquid hydrocarbon covering the pore wall. To model the effect of non‐wetting hydrocarbon, we extend the analytical model by introducing a second cylindrical body into the cylindrical pores that represents a discrete droplet of the contaminant phase. In order to account for the high electrical resistivity of liquid hydrocarbons, the corresponding volumes are assumed to be electrically insulating. Because most liquid hydrocarbon surfaces are negatively charged when in contact with an electrolyte, they are covered by a second electrical double layer, which can easily be incorporated into the analytical model. We use our extended model to study the effect of varying saturations of wetting and non‐wetting hydrocarbon on the complex electrical conductivity of the pore system. Our results predict that conductivity magnitude and conductivity phase generally decrease with hydrocarbon saturation. However, if the surface potential at the surface of non‐wetting hydrocarbon droplets is larger than the one at the pore wall, we can observe an increase in the conductivity magnitude with the hydrocarbon saturation and a slight increase in the conductivity phase at intermediate hydrocarbon concentrations. This finding is particularly interesting as it offers a possible explanation for the relation between complex conductivity and hydrocarbon saturation observed in different field and laboratory experiments.
-
-
-
Simulation of membrane polarization of porous media with impedance networks
Authors Hermann Stebner, Matthias Halisch and Andreas HördtABSTRACTThe spectral induced polarization method is supposed to have some potential to provide useful information on hydraulic properties of the subsurface. One difficulty for the practical implementation is the simulation of underlying physical processes at the pore scale and their upscaling to hydraulically relevant scales. We extend an existing semi‐analytical membrane polarization model to two‐ and three‐dimensional impedance networks, which are numerically solved to obtain a spectral induced polarization response that can be compared to measured data.
The original model consists of two cylinders with different sizes. The model has been shown to be able to reproduce some basic features of spectral induced polarization spectra measured in the laboratory, but it is unable to meet the complexity of macroscopic porous media structures, which can be found within unconsolidated sediments or rocks. To approach realistic pore space geometry, we connect different combinations of cylinders to networks. We treat the smaller of the two cylinders as a pore throat. The pore throat distribution in the network is chosen by matching measured pore throat radii distributions obtained from the mercury injection method. The large cylinder is treated as the (large) pore body, and its radii are associated to the pore throats using existing relationships.
We compare the behaviour of the networks with real porous media by using sandstone samples for which both the electrical and petrophysical properties have been measured. We adjust the geometrical parameters of the network, i.e., pore lengths, pore radii, and their relative occurrences, such that they match the measured parameters (specific internal surface area, pore throat distribution, and porosity) of the sandstone samples. The measured spectral induced polarization parameters, like the maximum phase shift and the characteristic relaxation time, are qualitatively consistent with our simulation results.
-
-
-
Geometrical constraints for membrane polarization
Authors Andreas Hördt, Katharina Bairlein, Matthias Bücker and Hermann StebnerABSTRACTMembrane polarization is one process that might describe the causes of induced polarization in sediments at the pore scale. Here, we investigate the practical relevance of one particular model, which consists of two cylindrical pores with different radii r and lengths L, for which the impedance can be calculated analytically. We derive approximate equations that relate polarizabilities and relaxation times directly to r and L. Based on these relations and on a systematic exploration of the parameter space, we investigate under which conditions membrane polarization is relevant in the sense that it produces measurable phase shifts in the frequency range typically observed in the laboratory or at the field scale.
In principle, a wide range of spectra can be obtained. Maximum phase shifts up to hundreds of milliradian can be simulated, and the characteristic time scales cover the entire range typically measured in the laboratory. We discuss some specific constraints in the context of results from mercury injection porosimetry and recently published laboratory data and show that the required geometries are not unrealistic, even if a moderate ratio between pore length and width is included as an additional condition. We conclude that membrane polarization as a possible mechanism is not limited to a particular frequency range. We also provide evidence that the pore length of the wide pore is likely to control the measured relaxation times in practical situations. The results encourage further attempts to combine impedances of two‐pore systems to approach the simulation of real sediments, with the aim to extract pore space parameters from measured data.
-
-
-
Pore‐scale modelling of complex conductivity of saturated granular materials
More LessABSTRACTThe complex conductivity method has been frequently used in solving hydrogeological, engineering, and environmental problems in practice. Macroscopic geophysical responses are governed by pore‐scale rock properties; therefore, a clear petrophysical understanding of how the pore‐scale structure controls electric conduction and polarization in geomaterials is necessary to fully interpret laboratory‐/field‐scale geophysical observations. In this study, we have developed a pore‐scale numerical approach to calculate the complex conductivity of water‐saturated granular materials. This physics‐based model has advantage over phenomenological or empirical models such that the intrinsic relations between pore structure and geoelectrical signals could be revealed. In the modelling, the influence of electrical double layer, which is quantified by complex surface conductance, can be converted to an apparent volumetric complex conductivity of either solid particles or pore fluid. The effective complex conductivity of the sample is determined by directly solving the finite‐difference representation of the Laplace equation in the domain of a representative elementary volume. The numerical approach is tested on one synthetic sample and one glass beads sample. For the synthetic sample, the consistency between the numerical and the analytical solution confirms the accuracy of the approach. Comparison with experimental measurements of the glass beads sample reported previously indicates that the developed numerical approach can well reproduce the key characteristics of the complex conductivity of water‐saturated granular materials in the frequency range between 10‐1 and 104 Hz. Furthermore, the numerical examples also show that the proposed approach can capture the salinity‐dependent electric conduction and polarization in saturated granular materials.
-
-
-
Electrical properties of two‐component mixtures and their application to high‐frequency IP exploration of permafrost
Authors Nikita Zorin and Dmitriy AgeevABSTRACTWe investigate the problem of determination of the volumetric ratio between the two components of a heterogeneous mixture with unknown internal structure. If both resistivity and permittivity of one component are known to be much higher than those of the other within a sufficiently wide frequency range, the volumetric ratio may be roughly estimated from measured electromagnetic response of the mixture by making use of the variational approach. Otherwise, such estimation requires the exact knowledge of the inherent electrical properties of the mixed materials and application of some universal mixing model, such as the weighted power mean formula.
The high‐frequency induced polarization measurements are strongly influenced by the presence of ice inclusions in an investigated rock formation, which is commonly used for mapping of frozen ground within the permafrost regions. We show that for sedimentary rocks with low clay content, it is also possible to roughly estimate the ice concentration from broadband induced polarization data by using the two‐component, weighted power mean model, which is confirmed by a lab experiment on a frozen core sample with known ice content.
-
-
-
Specific polarizability of sand–clay mixtures with varying ethanol concentration
Authors Sundeep Sharma, Lee Slater, Dimitrios Ntarlagiannis, Dale Werkema and Zoltan SzaboABSTRACTWe utilise a concept of specific polarizability , represented as the ratio of mineral‐fluid interface polarization per pore‐normalised surface area , to demonstrate the influence of clay‐organic interaction on complex conductivity measurements. Complex conductivity measurements were performed on kaolinite‐ and illite‐sand mixtures as a function of varying ethanol (EtOH) concentration (10% and 20% v/v). The specific surface area of each clay type and Ottawa sand was determined by nitrogen‐gas‐adsorption Brunauer‐Emmett‐Teller method. We also calculated the porosity and saturation of each mixture based on weight loss of dried samples. Debye decomposition, a phenom‐enological model, was applied to the complex conductivity data to determine normalised chargea‐bility . Specific polarizability estimates from previous complex conductivity measurements for bentonite‐sand mixtures were compared with our dataset. The for all sand–clay mixtures decreased as the EtOH concentration increased from 0% to 10% to 20% v/v. We observe similar responses to EtOH concentration for all sand–clay mixtures. Analysis of variance with a level of significance suggests that the suppression in responses with increasing EtOH concentration was statistically significant for all sand–clay mixtures. On the other hand, real conductivity showed only 10% to 20% v/v changes with increasing EtOH concentration. The estimates reflect the sensitivity of complex conductivity measurements to alteration in surface chemistry at available surface adsorption sites for different clay types, likely resulting from ion exchange at the clay surface and associated with kinetic reactions in the electrical double layer of the clay‐water‐EtOH media. Our results indicate a much larger influence of specific surface area and ethanol concentration on clay‐driven polarization relative to changes in clay mineralogy.
-
-
-
Pore radius distribution and fractal dimension derived from spectral induced polarization
Authors Zeyu Zhang, Andreas Weller and Sabine KruschwitzABSTRACTThe pore size distribution provides a suitable description of the pore space geometry that can be used to investigate the fractal nature of a pore space or to determine the fractal dimension. The fractal dimension describes the size of the geometric objects as a function of resolution. It can be integrated into the models that are used for permeability prediction. We investigated the fractal dimension of the pore volume of 11 Eocene sandstone samples from China. This study describes an approach to use spectral induced polarization spectra to estimate the pore size distribution and to determine the fractal dimension of the pore volume. Additionally, the fractal dimension was derived from data of the capillary pressure curves from mercury intrusion and the transversal relaxation time distribution of nuclear magnetic resonance. For samples with an effective pore radius larger than 1 μm, a good agreement exists between the values of the fractal dimension derived from the three different methods, which implies an identification of similar pore structures. Spectral induced polarization can be a non‐invasive laboratory technique for the estimation of the pore size distribution, but the application of the methodology to field measurements remains a challenging problem considering the limited frequency range.
-
-
-
The effect of compaction on complex electrical resistivity of shaly sands
Authors Frank Börner, Edith MüllerHuber, Daniel Branka and Carsten RückerABSTRACTPore space properties of sedimentary rocks are of fundamental importance with regard to reservoir evaluation and fluid flow modelling or special geotechnical applications such as stability considerations for reservoirs, dams, or embankments. Processes such as compaction due to pressure drawdown in weakly consolidated reservoirs, effects caused by stimulation of reservoirs with decreasing productivity, or the compaction of building ground can be effectively studied by verifying and monitoring changes in porosity. The close connections between electrical resistivity or conductivity and pore space geometry as well as a high sensitivity to even slight changes in pore space characterising parameters are well‐established.
Therefore, the results of complex electrical conductivity measurements in the frequency range from 0.05 Hz to 1 kHz on a set of heterogeneous shaly sand samples during increasing compaction are presented in this study. The major objective of these investigations was to quantify the porosity reduction in shaly sands during the step‐wise compaction of the samples in a specially designed measurement cell. Overall, ten unconsolidated shaly sand samples with varying grain size distribution were analysed. In addition, selected shaly sandstone data are presented to enhance the observations made for the sand samples with regard to the potential influence of cementation as expressed by the cementation exponent.
With increasing compaction, the measured complex conductivity data of the fully water‐saturated samples show two co‐occurring effects. On the one hand, the real part decreases due to the dominating effect of Archie’s law. On the other hand, the imaginary part increases due to the increasing contribution of interface conductivity. This effect is due to an increase in the internal surface‐area‐to‐porosity ratio. Cementation exponent and the considered porosity range seem to be controlling the magnitude of this effect. These observations may be explained by using a simple complex conductivity model that relates conductivity components to porosity, specific surface area, and cementation exponent. An interpretation algorithm is proposed that allows determining relative porosity variations based on a baseline and a single repeat measurement without prior knowledge of further rock characteristics.
To demonstrate the applicability of the algorithm on field measurements, data from spectral induced polarization soundings obtained at a test site for compaction techniques were interpreted. It could be shown that these observations bear further potential for enhancing the prediction of porosity, changes of compaction, and, hence, changes in hydraulic permeability.
-
-
-
Spectral induced polarization for the characterisation of biochar in sand
ABSTRACTThe use of biochar as a soil amendment attracts increasing research interest. However, the lack of methods to detect and monitor biochar in situ limits the validation of the field‐scale application of biochar. Spectral induced polarization is a potential tool to characterise biochar in soil. The aim of this study is to investigate the sensitivity of spectral induced polarization to biochar in sand and to understand how the physicochemical properties of both the biochar and the surrounding matrix influence the spectral induced polarization response. To this end, spectral induced polarization measurements were conducted on four types of biochar with different mass fractions disseminated in saturated sand as a host media with changing electrical conductivity. In addition, it was investigated how the spectral induced polarization response depends on the particle size of biochar. The measured SIP data were interpreted by Debye decomposition to obtain values for the peak relaxation time, ; total chargeability, ; and normalised total chargeability, . Spectral induced polarization showed a clear and specifically differentiated response to the presence of all four types of biochars. was found to be proportional to the mass fraction of biochars, although relationships varied for each type of biochars. of biochars increased with increasing particle size. Increased electrolyte concentration enhanced for all biochars, although again, the specific response was different for each biochar. In addition, higher electrolyte concentrations decreased for biochars derived from wood through pyrolysis but did not affect of biochar derived from miscanthus through hydrothermal carbonisation. It was concluded that the spectral induced polarization response of pyrolytic biochars resembled that of conductors or semiconductors, whereas the spectral induced polarization response of hydrothermal carbonisation biochar more closely resembled that of clay. Overall, the findings in this study suggest that spectral induced polarization is a promising method for the detection and characterisation of biochar in soil.
-
-
-
Mapping geological structures in bedrock via large‐scale direct current resistivity and time‐domain induced polarization tomography
ABSTRACTAn investigation of geological conditions is always a key point for planning infrastructure constructions. Bedrock surface and rock quality must be estimated carefully in the designing process of infrastructures. A large direct‐current resistivity and time‐domain induced‐polarization survey has been performed in Dalby, Lund Municipality, southern Sweden, with the aim of mapping lithological variations in bedrock. The geology at the site is characterised by Precambrian granitic gneisses and amphibolites, which are intensely deformed, fractured, and partly weathered. In addition, there are northwest‐trending Permian dolerite dykes that are less deformed.
Four 2D direct‐current resistivity and time‐domain induced‐polarization profiles of about 1‐km length have been carefully pre‐processed to retrieve time‐domain induced polarization responses and inverted to obtain the direct‐current resistivity distribution of the subsoil and the phase of the complex conductivity using a constant‐phase angle model. The joint interpretation of electrical resistivity and induced‐polarization models leads to a better understanding of complex three‐dimensional subsoil geometries. The results have been validated by lithological descriptions from several drillings. In addition, direct‐current resistivity and time‐domain induced‐polarization logging has been carried out in two different boreholes, showing a good match with the results of the surface direct‐current resistivity and time‐domain induced‐polarization profiles.
The direct‐current resistivity and time‐domain induced‐polarization methodology proved to be a suitable technique for extensively mapping weathered zones with poor geotechnical characteristics and tectonic structures, which can lead to severe problems for infrastructure construction and/or constitute risk zones for aquifer contamination.
-
-
-
Spectral induced polarization in a sandy medium containing semiconductor materials: experimental results and numerical modelling of the polarization mechanism
Authors Feras Abdulsamad, Nicolas Florsch and Christian CamerlynckABSTRACTInduced polarization is widely used for mineral exploration. In the presence of sulfides (more generally speaking, semiconductors), the charge carriers inside the particles are electrons and electron gaps. The mechanism of induced polarization is not fully understood in those cases. In order to improve our knowledge about the mechanisms controlling induced polarization in such media, we carried out spectral induced polarization measurements on unconsolidated mineralised medium and performed a numerical modelling based on the solution of the Poisson–Nernst–Planck equations set. Different types of semiconductors (graphite, pyrite, chalcopyrite, and galena) have been included in the experiments. The polarization effects of grain radius, semiconductor content, and electrolyte salinity and type have been investigated at the lab scale. The experimental results showed that the chargeability or frequency effect of the medium is a function of the mineral volume and is independent of electrolyte salinity and type. Furthermore, the time constant is highly dependent on grain radius and electrolyte salinity, whereas it is slightly dependent on mineral type. The observed dependence of the chargeability and time constant on salinity could be explained by considering the semiconductor grain as an electric dipole impacting the potential and consequently the charge distribution in its vicinity. This dipole is generated inside the particle to compensate the primary electrical field. Since the Poisson–Nernst–Planck equations are coupled, the potential depends in return on the resulting ions distribution. Therefore, it could be used to explain the induced polarization phenomena. Using the finite element method, we computed the solution of the Poisson–Nernst–Planck equations for a grain of pyrite surrounded by an ionic solution, where the charge diffusions in both grain and electrolyte were considered. Regarding the electrolyte salinity, the dependence of the relaxation time observed from the experimental study is qualitatively consistent with that extracted from the numerical simulation.
-
Volumes & issues
-
Volume 22 (2024)
-
Volume 21 (2023)
-
Volume 20 (2022)
-
Volume 19 (2021)
-
Volume 18 (2020)
-
Volume 17 (2019)
-
Volume 16 (2018)
-
Volume 15 (2017)
-
Volume 14 (2015 - 2016)
-
Volume 13 (2015)
-
Volume 12 (2013 - 2014)
-
Volume 11 (2013)
-
Volume 10 (2012)
-
Volume 9 (2011)
-
Volume 8 (2010)
-
Volume 7 (2009)
-
Volume 6 (2008)
-
Volume 5 (2007)
-
Volume 4 (2006)
-
Volume 3 (2005)
-
Volume 2 (2004)
-
Volume 1 (2003)