Pore Water Conductivity Research Articles

Induced polarization of clay-rich materials. 4. Water content and temperature effects in bentonites

Deep geological disposals (DGDs) are widely seen to be the best solution to contain high-level radioactive wastes safely. Compacted bentonite and bentonite-sand mixtures are considered the most appropriate buffers or sealing materials for access drifts, ramps, and shafts due to their favorable physicochemical and hydro-mechanical properties. Bentonite-sand mixtures are expected to swell and seal all voids when in contact with water, forming an impermeable barrier to radioactive elements. The parameters that will most affect the hydraulic performance of these seals are their water content, dry density, water salinity, and temperature. Monitoring and assessing these parameters are, therefore, crucial to confirm that the seals’ safety functions are fulfilled during the life of a DGD. Induced polarization (IP) is a nonintrusive geophysical method able to perform this task. However, the underlying physics of bentonite sand mixtures has not been checked. The complex conductivity spectra of 42 compacted bentonite-sand mixtures were measured in the frequency range of 1 Hz–45 kHz in order to develop workable relationships between in-phase and quadrature conductivities versus water content and saturation, pore water conductivity, bentonite-sand ratio (10% to 100%), temperature (10°C–60°C), and dry density (0.97 to 1.64 g cm−3). We observe that conductivity is mostly dominated by surface conductivity associated with the Stern layer (SL) coating the surface of smectite, the main component of bentonite. At a given salinity and temperature, the in-phase and quadrature conductivities obey the power law relationships with water content and saturation. The in-phase and quadrature conductivities depend on the temperature according to a classical linear relationship with the same temperature coefficient. An SL-based model is used to explain the dependence of the complex conductivity with water content, dry density, water salinity, and temperature. It could be used to interpret the IP field data to monitor the efficiency of the seal of DGD facilities.

Influence of dissolved and non-aqueous phase toluene on spectral induced polarization signatures of soils

Fifty-two laboratory experiments are undertaken to analyze the sensitivity of spectral induced polarization (SIP) to the presence of toluene in soils. Among these experiments, four experiments are conducted to collect SIP responses of soils containing dissolved phase toluene within the pore water using columns. The results demonstrate that SIP is not sensitive to the presence of dissolved phase toluene in soils. The remaining forty-eight experiments are undertaken with four types of soils mixed with non-aqueous phase toluene. The experimental results prove that SIP is sensitive to toluene saturation under varying salinity conditions. These observations are well-explained by a published petrophysical model accounting for the effects of water saturation on complex conductivity. The water saturation exponent n and quadrature conductivity exponent p in this model are obtained by fitting complex conductivity data versus saturation at different saturation levels. The petrophysical model is tested where in-phase and quadrature conductivity responses are predicted from water saturation, soil cation exchange capacity (CEC), and pore water conductivity. The petrophysical model provides satisfactory predictions for non-aqueous phase toluene saturation. Overall, this study contributes to our understanding of SIP as a non-intrusive tool for characterizing toluene contamination in soils with applications to the field.

Electrical conductivity profiling for rapid contamination assessment in unsaturated zones: A case study of an MSW landfill

This study investigates the potential of using bulk soil electrical conductivity (ECbulk) to predict pore water conductivity (ECpw) for assessing the contamination in the unsaturated zone of an old municipal solid waste (MSW) landfill. ECbulk, ECpw, and water content were evaluated with depth at an old MSW landfill in Bhalswa, Delhi, using the Hydraulic Profiling Tool (HPT) and a dual tube soil sampling system. This data was also supplemented by a cone penetration test (CPTu) for high-resolution soil type identification. The correlation of ECbulk with ECpw was primarily influenced by volumetric water content and mineral conductivity with the latter being negligible at this site due to the high conductivity of the leachate. A reasonable linear correlation between normalized EC (ECbulk/ECpw) was observed with volumetric water content, except at low water contents. ECbulk and ECpw profiles with depth indicated attenuation of contaminants in clay layers, while sand layers exhibited constant profile with depth. ECpw was contributed by macro ions generally found in the leachate, including Na+, Mg2+, K+, Ca2+, NH4+, Cl−, SO42−, and HCO3−, as demonstrated by a strong correlation with their cumulative ionic strength. The results indicate that ECbulk profile can be used as a rapid semi-quantitative method for assessing contaminant migration in the unsaturated soil zone, supporting the remediation or control strategies at old landfills.

Mapping and monitoring peatlands in the Belgian Hautes Fagnes: Insights from Ground-penetrating radar and Electromagnetic induction characterization

Peatlands are vital ecosystems providing crucial ecological services such as significant carbon storage in the context of climate change. These sensitive ecosystems are subjected to degradation due to land use change. In this context, it is important to understand the actual state of degraded peatlands and their recovery potential for regaining important functions. This study focuses on a disturbed peatland in the Belgian Hautes Fagnes, previously drained and planted with spruce, and characterized by a topographic gradient. We aimed to elucidate the use of Ground-penetrating radar (GPR) and Electromagnetic induction (EMI) techniques in characterizing such peatlands, with a specific focus on their implications for peat depth and electrical conductivity assessment, related to peatland degradation. The GPR revealed a high spatial heterogeneity in peat depth, ranging from 0.2 to more than 2 m on the site. In contrast, an EMI instrument used with single coil spacing proved to be unsuccessful to deduce peat depth but demonstrated potential for the delineation of zones of mineral soil. It is also shown that the links between soil physical and chemical properties and its bulk soil electrical conductivity (ECa) measured by EMI are complex in zones of shallower peat. Additionally, this study highlights the role of pore water conductivity in influencing temporal variations in ECa. Our findings explain the intricate interplay between peat depth, topography and ECa in disturbed peatlands. The results of this study may be of substantial societal interest, as it provides insights into techniques for the rapid non destructive characterization of the conditions of previously drained peatlands.

Predicting the Electrical Conductivity of Partially Saturated Frozen Porous Media, a Fractal Model for Wide Ranges of Temperature and Salinity

AbstractThe quantitative determination of liquid water content and salinity in soils is crucial for the preservation of hydrological environments and engineering infrastructures, especially in frozen regions. Electrical conductivity, as a fundamental physical parameter in electrical and electromagnetic non‐destructive techniques, varies significantly with the physical and chemical properties, such as pore water conductivity, salinity, water saturation, and temperature. In this study, accounting for pore size and tortuous length following fractal distributions, we develop a new capillary bundle model for variation of electrical conductivity as a function of temperature in broad water saturation and salinity ranges. In this new model, we consider the contributions of bulk and surface conductivities to the total electrical conductivity. To test this model, a series of laboratory experiments were carried out for different initial water saturations and salinities using an electrical resistance apparatus and a nuclear magnetic resonance method. The experimental results show that unfrozen water saturation and ionic concentration affect the electrical conductivity of unsaturated frozen soils. Furthermore, the proposed model is capable of fitting the main trends of the experimental data from the literature and acquired in this study in unfrozen‐frozen conditions for different water contents. Relying on the proposed model, we also determine the expression of the apparent formation factor, which is significantly sensitive to porosity, water saturation, and temperature. The predicted values of the apparent formation factor also agree very well with the experimental data. This new capillary bundle model provides a new perspective in interpreting electrical monitoring to easily deduce changes in key variables in the cryosphere such as liquid water content and moisture gradients.

Potential of ground-penetrating radar to calibrate electromagnetic induction for shallow soil water content estimation

Ground-penetrating radar (GPR) and electromagnetic induction (EMI) are used to determine and map soil water content (SWC) in the agricultural landscape. While GPR provides a straightforward estimation of SWC, EMI requires site-specific calibrations mainly based on point-scale measurements such as time domain reflectometry (TDR). However, there is a significant difference in the measurement volumes between EMI and point-scale TDR measurements. This study aimed to enhance the calibration of EMI for estimating SWC in the agricultural landscape by leveraging the larger sampling volumes provided by GPR. Apparent electrical conductivity (ECa) from a multi-coil EMI sensor and dielectric constant from GPR with two center frequencies (500 MHz and 250 MHz) were collected as soil proxies along with TDR measurements. Calibration models were developed under irrigated conditions and the model evaluation was conducted under natural moisture conditions. Correlations were assessed between the proxies from GPR and EMI with TDR-derived SWCs. Simple linear regression (SLR) models were developed between EMI and GPR data to predict SWC. Strong positive correlations (r ≥ 0.80) were observed between the proxies (GPR and the shorter inter-coil spacing (0.32 m) of EMI) and TDR-measured SWC. ECa data from vertical and horizontal coil orientations of the shorter inter-coil spacings were selected to develop SLR models with GPR. The SLR models with the GPR 500 MHz frequency showed a higher coefficient of determination (R2 > 0.70) for both coil orientations of EMI. Results showed the potential of using GPR to calibrate EMI for shallow SWC estimation (below 0.5 m). Nevertheless, the EMI estimated SWCs were not effectively verified with GPR-estimated SWCs, which could be attributed to specific site conditions in the study area. Further research must focus on improving the calibration and model evaluation by incorporating the variability of other soil parameters (soil porosity, pore-water conductivity, and soil salinity) that may affect the SWC–ECa relationship.

The Fundamental Flaws of the Waxman-Smits and Dual Water Formulations, Attempted Remedies, and New Revelations from Historical Laboratory Complex Conductivity Measurements

The Waxman-Smits formula was introduced in 1968 as a parallel conductance model to improve previous models. A careful inspection of the Waxman and Smits model reveals it is not a parallel conduction model by the conventional definition. First, Waxman-Smits assumed that “the electrical current transported by the counterions associated with the clay travels along the same tortuous path as the current attributed to the ions in the pore water” (Waxman and Smits, 1968), removing an essential feature of a parallel conduction model, that there be two separate conductors. Based on this assumption, they assign the same geometrical factor to both current paths. The geometrical factor is defined as the reciprocal of the formation resistivity factor (1/F or σm). Waxman-Smits found experimentally that a shaly sand appeared to have an F that was larger than a clean sand and introduced F* to account for this. Therefore, the tortuosity of the current paths through the clay and the pore water were deemed to be equivalent, with both tortuosities increasing equally as the clay content increased. Second, a parallel model requires the bulk conductivity of a volume to be weighted by the fractional volumes of the separate clay and interstitial water current paths. Clavier et al. (1977) discovered during the field testing of the new 1.1-GHz electromagnetic propagation tool that there existed a volume of clay water of near-constant salinity in shales. These two concepts are not accounted for in the Waxman-Smits model. A re-evaluation of the Waxman-Smits database by Clavier et al. (1977, 1984) revealed the F* increase was primarily due to the Waxman-Smits model not accounting for the physical presence of the volume of the clay water. The inclusion of the clay water volume in the dual water model produces a true parallel conductivity model. However, like Waxman-Smits, it assigns the same tortuosity to both the clay and pore water current paths. This assumption seems dubious based on observations of scanning electron microscope (SEM) photos showing actual clay morphologies. Laboratory measurements of pure clay and glass beads would allow one to quantify tortuosity changes due to the introduction of clay into an otherwise pure glass bead environment. Theoretically and experimentally, the value of the clay water conductivity (Ccw) at room temperature was found to be 6.8 S/m. Therefore, for a pore water conductivity (Cw) less than 6.8, the clay adds to the rock conductivity relative to an Archie rock, as written in the Waxman-Smits model. However, when Cw is greater than 6.8, the clay water subtracts from the rock conductivity relative to an Archie rock. This cannot be accommodated by the Waxman-Smits formulation. To correct for this model deficiency, B was made a function of salinity and temperature when, theoretically, it is a function of temperature only. Thirdly, neither model accurately predicts the rock conductivity at pore water salinities below approximately 0.5 to 1 S/m. Having a proper model at these lower salinities is important for geothermal evaluations, waterflooded reservoirs, and naturally occurring freshwater reservoirs. We propose a correction method based on our knowledge gained from the study of the quadrature conductivity measurement from cores and recent laboratory measurements.

Plant testing with hemp and miscanthus to assess phytomanagement options including biostimulants and mycorrhizae on a metal-contaminated soil to provide biomass for sustainable biofuel production

The need of biofuels from biomass, including sustainable aviation fuel, without using agricultural land dedicated to food crops, is in constant demand. Strategies to intensify biomass production using mycorrhizal fungi, biostimulants and their combinations could be solutions for improving the cultivation of lignocellulosic plants but still lack well-established validation on metal-contaminated soils. This study aimed to assess the yield of Miscanthus x giganteus J.M. Greef & Deuter and Cannabis sativa L. grown on a metal-contaminated agricultural soil (11 mg Cd, 536 mg Pb and 955 mg Zn kg−1) amended with biostimulants and/or arbuscular mycorrhizal fungi, and the shoot Cd, Pb and Zn uptake. A pot trial was carried out with soil collected from a field near a former Pb/Zn smelter in France and six treatments: control (C), protein hydrolysate (a mixture of peptides and amino acids, PH), humic/fulvic acids (HFA), arbuscular mycorrhizae fungi (AMF), PH combined with AMF (PHxAMF), and HFA combined with AMF (HFAxAMF). Metal concentrations in the soil pore water (SPW), pH and electrical conductivity were measured over time. Miscanthus and hemp shoots were harvested on day 90. Both PH and PHxAMF treatments increased SPW Cd, Pb, and Zn concentrations (e.g. by 26, 1.9, and 22.9 times for miscanthus and 9.7, 4.7, and 19.3 times for hemp in the PH and PHxAMF treatments as compared to the control one, respectively). This led to phytotoxicity and reduced shoot yield for miscanthus. Conversely, HFA and HFAxAMF treatments decreased SPW Cd and Zn concentrations, increasing shoot yields for hemp and miscanthus. Shoot Cd, Pb, and Zn uptakes peaked for PH and PHxAMF hemp plants (in μg plant-1, Cd: 310–334, Pb: 34–38, and Zn: 232–309 for PHxAMF and PH, respectively), while lowest values occurred for PH miscanthus plants mainly due to low shoot yield. Overall, this study suggested that humic/fulvic acids can be an effective biostimulant for increasing shoot biomass production in a metal-contaminated soil. These results warrant further investigations of the HFAxAMF in field trials.

Induced polarization of volcanic rocks. Part 7. Kimberlites

SUMMARY In the field, kimberlites are characterized by high electrical conductivities (about 0.1 S m−1) compared to most igneous rocks. The reason for these high conductivities has not been fully elucidated to date. We investigate here the spectral induced polarization of seven core samples of kimberlites in the frequency range 1.43 mHz–20 kHz. The measurements are made at pore water conductivities ranging from 0.07 to 2.4 S m−1 (NaCl, 25 °C). We also measured the cation exchange capacity (CEC), the specific surface area (SSA) and the magnetic susceptibility of the core samples. We characterized the samples by optical microscopy as well as the X-ray diffraction and thermogravimetric analyses. Based on the electrical measurements, we obtained values of the surface conductivity produced by the double electrical layer coating the solid particles, and the normalized chargeability values characterizing the polarization magnitude of these materials. Mineralogical analyses show significant amount of magnetite (from 2 to 9 wt. per cent, approximately 1 to 4 per cent in vol. content) and smectite (from 1 to 44 wt. per cent) in the core samples. The main contributor of the CEC is smectite because of its very high CEC. The quadrature conductivity, the normalized chargeability, and the surface conductivity are controlled by the CEC normalized by the tortuosity of the pore space (product of the formation factor by the porosity). Our data demonstrate that the conduction and polarization of kimberlites are both controlled by the presence of smectite rather than associated with magnetite. Comparing the new data set and data recently obtained with volcanic rocks from both shield and strato-volcanoes in the previous papers of this series, we show that the model of polarization of the dynamic Stern layer correctly describes the complex electrical conductivity of kimberlites as well. Our results also explain the cause of electrical conductivity anomalies detected at kimberlite pipes and offer new perspectives in using induced polarization method for the exploration of kimberlites around the world.

Effects of changes in soil properties caused by progressive infiltration of rainwater on rainfall-induced landslides

Slope failure is associated with changes in pore water pressure induced by rainfall, this paper further attempt to bridge knowledge gaps that exist in understanding the relationship between mechanism of rainfall-induced regional-scale widespread shallow landslides in cohesive soils and change in pore water pressure. We designed in situ slope experiments with pre-designed vertical profiles at the toe of the slope, and used pore water pressure, conductivity, and moisture content sensors to capture the changing characteristics of soil properties during slope failure. The experiment finds that failure occurred first where internal material was continuously taken out of the slope, even before the maximum static pore water pressure is reached. However, the slope remains stable when the pore water pressure near the failure surface reached the maximum static pore water pressure that could be provided by rainfall. Instead, slope failure during subsequent rainfall periods, even if the pore pressure is then lower. Meanwhile, the soluble minerals in the soil continuously dissolve and migrate, causing the electrical conductivity before slope failure is approximately 1.38 times that after failure, on average. Here, we estimate that the changes in soil properties caused by progressive water infiltration only may have decreased test slope stability by an average of approximately 33.0%. Therefore, we suggest that the early attenuation of soil strength caused by the gradual changes in soil properties makes a significant contribution to soil failure caused by pore water pressure. This was verified in the case of the 7.25 Tianshui group-occurring landslides. Finally, by using an automatic laser particle size analyzer, scanning electron microscope, X-ray diffraction, ion chromatography, and quadruple direct shear instrument to analogize the changes in soil properties exposed to different leaching environments on loess platform, we further show that the early attenuation soil strength is caused by deteriorate in soil properties in the processes of successive years of rainfall, where the content of cemented minerals tends to decrease, fine particles tend to be lost, and pore structure tends to take the form of single macro-pores, which are more likely distributed around mineral particles >10 μm. Our results provide a new perspective on the widespread shallow landslides caused by changes in pore water pressure in a region and their associated gradual deteriorations in soil properties.

An investigation into the impact of soil particle conductivity and percolation threshold on the Hilhorst model to estimate pore water conductivity in soils

AbstractThe aim of this work has been to assess the Hilhorst model, used for estimating in‐soil pore water conductivity, against soil properties of percolation threshold and soil particle conductivity. The Hilhorst model has the benefit of requiring a single soil‐specific parameter which makes this model easy to apply from soil permittivity and bulk conductivity measurements. However, the Hilhorst model requires that the bulk conductivity measurement is dominated by pore water conductivity, which is not always the case in many ‘‘real‐world’’ settings. This work examines a mathematical framework derived from combining the Hilhorst and Ewing and Hunt models which allows the Hilhorst soil parameter to be derived for a range of soil particle conductivities (0 to 10 mS m−1) and percolation thresholds (0 to 0.1 m3 m−3). The analysis in this work indicates that the Hilhorst parameter is highly sensitive to both soil properties with respect to the default value of 4.1 that is often employed. This assessment indicates that: (i) soil particle conductivity can result in the Hilhorst model overestimating pore water conductivity, which becomes more significant for lower bulk conductivities, and, (ii) percolation threshold can result in the Hilhorst model underestimating pore water conductivity, which becomes more significant for lower soil moisture contents. The results from this analysis suggest that the application of the Hilhorst model should be considered against the context of soil particle conductivity and percolation threshold for a given soil under test.

Influence of the water content on the complex conductivity of bentonite

Compacted bentonite is considered as a potential buffer material for deep geological disposals of high-level nuclear wastes. Methodologies to non-intrusively monitor the water content of such sealing materials are important in the context of the safety of these storage facilities and for various engineering applications as well. Induced polarization is a non-intrusive geophysical method sensitive to the water content of porous media. We investigated the complex conductivity spectra of 69 samples made of 2 distinct MX80 bentonites, one in the form of powder from crushed pellets (Type I) and the other in the form of a granulated bentonite mixture (GBM, type II). The samples are prepared at different compaction states and saturations. The pore water conductivity of the porous samples is estimated to be ∼2.5 S m−1 (25 °C) by two different methods. The complex conductivity spectra were obtained at room temperature (25 ± 2 °C) in the frequency range 1 Hz-45 kHz. In-phase and quadrature conductivities reflect conduction (electromigration) and polarization processes, respectively. At a given frequency, both the in-phase and quadrature conductivities increase with the water content along a trend that is independent of the compaction state. An induced polarization model based on the dynamic Stern layer model is used to explain these results. The first Archie's exponent m is inferred from the formation factor F using the in-phase conductivity data versus the pore water conductivity data at different salinities (NaCl, 25 °C). The dynamic Stern layer model applied to polarization correctly predicts the dependence of the in-phase conductivity, quadrature conductivity, and normalized chargeability with the water content and the Cation Exchange Capacity (CEC). This petrophysical work can easily be applied to time-domain induced polarization data collected in field conditions to monitor hydraulic barriers.

Effect of temperature on electrical conductivity of soils – Role of surface conduction

Temperature has a significant effect on the bulk electrical conductivity (σb) of soils because fluid properties, such as viscosity, which affect σb, are functions of temperature. The governing mechanisms between pore water conduction (Kw) and surface conduction (Ks) in terms of temperature dependency are completely different. Thus, soils with a higher relevance of Ks can exhibit a different temperature dependency of σb when compared to Kw-dominant soils. Therefore, in this study, we aim to determine the temperature compensation parameter (α) of σb of soils as a function of the ratio of Ks to σb. Silica sand and kaolin clay were selected as the Kw- and Ks-dominant materials, respectively. The electrical conductivity of tested materials with varying initial porosities and pore fluid concentrations was measured as a function of temperature in the range of 10 °C to 50 °C. The results of this study showed that Kw-dominant soils have an α value of approximately 2%/°C, whereas the α value of Ks-dominant soils ranges from 1%/°C to 2%/°C as a function of porosity and pore water conductivity. The experimentally determined α value was compared with the α value calculated from the theoretical series–parallel model. Additionally, an empirical model to capture the α value of soils with varying magnitudes of Ks and Kw was suggested. Finally, the suggested model was verified using the results of Korean residual soil (i.e., clayey sand) and two other clayey soils (i.e., kaolin and ca-bentonite clays).

Ecological Responses of Meiofauna to a Saltier World—A Case Study in the Van Uc River Continuum (Vietnam) in the Dry Season

Increasing intensity of storms, typhoons, and sea level rise in conjunction with high water demand, especially for agriculture, in dry seasons in the Red River Delta may have led to seawater intruding deeper into the rivers’ estuaries. Given that losses of agricultural productivity and shortages of freshwater resources are projected, a reliable early warning of salinity invasion is, therefore, crucially needed. To evaluate the impact of salinity variations on riverine ecosystems, distribution patterns of meiofauna were examined at 20 stations along the Van Uc River continuum in the dry season. Meiofaunal richness indices were higher in the estuary and slightly decreased upriver. Nematoda was the most dominant taxon in salty stations, while Rotifera was more abundant in the less salty ones. A multiple variate analysis showed a strong interplay among salinity, nutrients, and pore water conductivity, which shaped the meiofaunal distribution. The inclusion of pore water salinity, nutrients, and meiofaunal community structure indicated a greater extent of the saline ecosystem in the estuary, posing a greater risk of freshwater salinization. Our results highlight the potential role of meiofauna as bioindicators but also call for a reformation of salinity assessment for better freshwater conservation and management.

Salinity distribution in the subterranean estuary of a meso-tidal high-energy beach characterized by Electrical Resistivity Tomography and direct push technology

Understanding the interaction of terrestrial freshwater and seawater in the subterranean estuary (STE) is an important factor when considering nutrient fluxes from land to sea. State-of-the-art research describes the STE by a tide-induced upper saline recirculation cell, a freshwater discharge tube and a deep saltwater wedge. However, recent numerical modelling and shallow hydrogeochemical investigations for high-energy beaches indicate that multiple saline recirculation cells may exist and affect the land-sea interaction. Electrical Resistivity Tomography (ERT) and Direct Push (DP) technologies are common tools to explore the subsurface. Due to their sensitivity to the electrical conductivity of pore water, they permit investigating the STE. This study combines ERT and DP to image the salinity distribution within the STE of a meso-tidal, high-energy beach. We actively incorporate the DP data into the ERT inversion and use geostatistical regularization for closing the resolution gap. For the first time, our experimental results confirm the existence of several 10–20 m deep reaching upper saline recirculation cells and corresponding brackish discharge locations generated by a pronounced runnel-ridge beach system in 2019, whereas in 2021 only a single cell was displayed for a flat topography at the time.

A FRACTAL ELECTRICAL CONDUCTIVITY MODEL FOR WATER-SATURATED TREE-LIKE BRANCHING NETWORK

Electrical conductivity is an important physical property of porous media, and has great significance to rock physics and reservoir engineering. In this work, a conductivity model including pore water conductivity and surface conductivity is derived for water-saturated tree-like branching network. In addition, combined with Archie’s law, a general analytical formula for the formation factor is presented. Through the numerical simulation of the analytical formula above, we discuss the impact of some structural parameters ([Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] in tree-like branching network on the resistance, conductivity and formation factor. The results show that the total resistance [Formula: see text] is proportional to [Formula: see text], [Formula: see text], and inversely proportional to [Formula: see text], [Formula: see text]. The relation between conductivity and porosity in this model is contrasted with previous models and experimental data, and the results show considerable consistency at lower porosity. It is worth noting that when [Formula: see text], the conductivity and porosity curve of this model overlap exactly with those plotted by the parallel model. The fractal conductance model proposed in this work reveals the operation of the current in the tree-like branching network more comprehensively.

Determining osmotic suction through electrical conductivity for unsaturated low-plasticity soils

Determining osmotic suction from the electrical conductivity (EC) of soil pore water was widely reported in the literature. However, while dealing with unsaturated soils, they do not have enough soil pore water to be extracted for a reliable measurement of EC. In this paper, the chilled-mirror dew-point hygrometer and contact filter paper method were used to determine the total and matric suctions for low-plasticity soils with different salinities (0.05‰, 2.1‰, and 6.76‰). A new piecewise function was proposed to calculate the osmotic suction, with the piecewise point corresponding to the first occurrence of precipitated salt in mixed salt solutions (synthetic seawater). EC, ion and salt concentrations used for osmotic suction calculation were transformed from the established relationships of mixed salt solution instead of experimental measurement. The calculated osmotic suction by the proposed equation and the equations in the literature was compared with the indirectly measured one (the difference between the measured total and matric suctions). Results showed that the calculated osmotic suction, especially the one calculated using the proposed function, was in fair agreement with the indirectly measured data (especially for specimens with higher salinity of 6.76‰), suggesting that the transformation of EC and concentrations from the established relationship is a good alternative to direct measurement for low-plasticity soil. In particular, the proposed method could be applied to unsaturated low-plasticity soils which do not have enough soil pore water for a proper EC measurement.

Numerical Simulation Study on the Relationships between Mineralized Structures and Induced Polarization Properties of Seafloor Polymetallic Sulfide Rocks

The induced polarization (IP) method plays an important role in the detection of seafloor polymetallic sulfide deposits. Numerical simulations based on the Poisson–Nernst–Planck equation and the Maxwell equation were performed. The effects of mineralized structures on the IP and electrical conductivity properties of seafloor sulfide-bearing rocks were investigated. The results show that total chargeability increases linearly as the volume content of disseminated metal sulfides increases when the volume content is below 20%. However, total chargeability increases nonlinearly with increasing volume content in vein and massive metal sulfides when the volume content is below 30%. The electrical resistivity of disseminated metal sulfides mainly depends on the conductivity of pore water. The electrical resistivity of vein and massive sulfides mainly depends on the volume content and the length of sulfides. Increase in the aspect ratio (0.36 to 0.93) of seafloor massive sulfides causes relaxation time constants and total chargeability to decrease. Relaxation time constants and total chargeability also decrease with increase in the tortuosity of seafloor vein sulfides from 1.0 to 1.38. This study is of great value for the electrical survey of seafloor polymetallic sulfide deposits.

Induced polarization images alteration in stratovolcanoes

We investigate the complex conductivity of 32 volcanic rock samples from two stratovolcanoes, La Soufrière Volcano (Guadeloupe Island, Caribbean) and Papandayan volcano (Java Island, Indonesia). These stratovolcanoes are characterized by high degrees of kaolinite-related alteration associated with the upwelling of acidic ground waters as well as the formation of smectite-rich clay caps. Our goal is to assess the dependence of two geoelectrical properties, the electrical conductivity and normalized chargeability, on the conductivity of the pore water and the Cation Exchange Capacity (CEC) for volcanic rocks at near-neutral pH conditions. An alteration index based on CEC is built for both smectite- and kaolinite-rich zones. The data are discussed in the context of a previously acquired experimental dataset based on samples collected from shield volcanoes in Hawaii. We show that all the core samples display the same trends in their petrophysical geoelectrical properties whatever the type of volcanoes. Surface conductivity and normalized chargeability are strongly controlled by the CEC of the material and the bulk tortuosity of the pore space (product of the formation factor by the porosity). This implies in turn that the conductivity and the normalized chargeability are controlled by rock alteration in the same way as long as surface conductivity dominates the bulk conductivity contribution, which is salinity and pH dependent (both being interrelated in acidic pore waters). These petrophysical results are then applied to the interpretation of a 3D induced polarization survey performed at Papandayan stratovolcano in Indonesia. The tomograms are used to image the alteration of the volcanic edifice and we conclude that surface conduction is non-negligible in these volcanoes. Only above a pore water conductivity higher than 10 S m−1 at 25 °C) the bulk conductivity dominates the surface conductivity associated with alteration.

Estimating the electrical conductivity of clayey soils with varying mineralogy using the index properties of soils

Electrical conductivity assessment provides the valuable information required for comprehensive subsurface characterizations. This study aims to extend the existing modified Archie's equation for capturing the electrical conductivity of soils to various clayey soils in a broad range of plasticity. Five clayey soils that involve two kaolinite-, two illite-, and one montmorillonite-dominant fines were selected, and the bulk electrical conductivity σmix for the five clay samples in slurry state prepared in a modified oedometer cell was measured in a broad range of porosity n = 0.6–1.0 and pore water conductivity σw ≈ 0.013–4.5 S/m. The results showed that surface conduction plays a critical role in the determination of the electrical conductivity of clayey soils when subjected to a relatively low pore water conductivity environment. In this context, detailed analyses revealed that there exist robust correlations between the clay index properties and the unmeasurable input parameters for the modified Archie's equation such as the matrix conductivity λ and m exponent relevant to the tortuosity for pore water conduction. Comparison between the measured and predicted electrical conductivity for the raw clayey sediment used for model validation indicated that the unique global equation extended and verified in this study can be used for the first-order estimates of electrical conductivity of the in-situ clayey sediments.