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Field-scale estimation of soil properties from spectral induced polarization tomography
Abstract
Estimates of soil properties such as Cation Exchange Capacity (CEC), water content, grain size characteristics, and permeability are important in geotechnical engineering, water resources, and agriculture. We develop a non-intrusive approach to estimate these properties in the field using spectral induced polarization (SIP) tomography. This geophysical method provides information about the frequency dependence of the complex electrical conductivity of porous media. Using 18 soil samples collected from a Bordeaux vineyard, we first conducted a laboratory study using SIP over the frequency range 10 mHz-45 kHz. The laboratory data were used to confirm the accuracy of a recently developed dynamic Stern layer petrophysical model. The results are consistent with published values from previous works using soils. A comparison was made by comparing the field complex conductivity spectra and the experimental data at two locations where core samples were obtained. The model was then used in concert with field data to image the spatial distribution of CEC, water content, permeability, and mean grain size along a vineyard transect. For clay and sandy textures found in the field, measured and estimated CEC agree rather well (from 6 to 40% discrepancy). Our approach provides an efficient way to estimate important soil properties in a non-invasive manner, in high resolution, and over field-relevant scales of the critical zone of the Earth.
... In the case of a broad distribution of grain sizes, the spectrum of the conductivity phase angle is rather flat and can be roughly described by a constant phase angle model, the so-called Drake's model (Van Voorhis et al., 1973). If such a model is applicable, M n is linearly related to σ (at the geometric mean frequency between f 1 and f 2 ) by the factor α, as demonstrated by laboratory and field investigations (see Revil et al., 2017cRevil et al., , 2021: ...
... We used the difference between the conductivity real part at 25 and 0.1 Hz of the MG inverse model for the calculation of M n as proposed by Revil et al. (2021). In contrast to Revil et al. (2021), the linear relation between M n and σ is weak (see Fig. 2c) with a large contrast between the predicted and the estimated slope α (10.11 and 3.37). ...
... We used the difference between the conductivity real part at 25 and 0.1 Hz of the MG inverse model for the calculation of M n as proposed by Revil et al. (2021). In contrast to Revil et al. (2021), the linear relation between M n and σ is weak (see Fig. 2c) with a large contrast between the predicted and the estimated slope α (10.11 and 3.37). This discrepancy is due to the strong increase in ϕ and the nonlinear increase in |σ * | at frequencies above 2.5 Hz as presented in Fig. 2a (here marked as "high-frequency range"), and it therefore does not fulfill the conditions of the DSLM. ...
Degrading permafrost in rock glaciers has been reported from several sites in the European Alps. Changes in ground temperature and ice content are expected to affect the hydrogeological properties of rock glaciers and in turn modify the runoff regime and groundwater recharge in high-mountain environments. In this study, we investigate the use of an emerging geophysical method in permafrost studies to understand the hydrogeological properties of the active Gran Sometta rock glacier, which consists of a two-lobe tongue (a white and a black) whose lobes differ in their geologies. We present the application of spectral induced polarization (SIP) imaging, a method that provides quasi-continuous spatial information about the electrical conductivity and polarization of the subsurface, which are linked to hydrogeological properties. To quantify the water content and the hydraulic conductivity from SIP imaging results, we used the petrophysical dynamic stern layer model. The SIP results show a continuously frozen layer at 4–6 m depth along both lobes which hinders the infiltration of water, leading to a quick flow through the active layer. To evaluate our results, we conducted tracer experiments monitored with time-lapse electrical conductivity imaging, which confirms the hydraulic barrier associated with the frozen layer and allows the pore water velocity to be quantified (∼ 10⁻² m s⁻¹). Below the frozen layer, both lobes have distinct water content and hydraulic conductivity. We observed a higher water content in the black lobe, which moves faster than the white lobe, supporting the hypothesis that the water content at the shear horizon affects the rock glacier velocity. Our study demonstrates that the SIP method is able to provide valuable information for the hydrogeological characterization of rock glaciers.
... Another lower frequency mechanism can be associated with the large grains present in the COx formation (see Figure 1). For the large grains, the associated relaxation time is given by (Revil et al., 2021) ...
... (Table 7) and the data from Revil et al. (2005), Descostes et al. (2008), and Jougnot et al. (2009Jougnot et al. ( , 2010aJougnot et al. ( , 2010b including data from the Oxfordian formation located above the COx formation. In addition, we use the data from Revil et al. (2021) for clayey (smectite-rich) soils from the Netherlands. Figure 13. ...
... where ε w denotes the low-frequency (ω ≪ 2π=τ w ) permittivity of water (78.4ε 0 at 298.15 K) also called the static permittivity of Revil et al. (2021). Note: 1 meq/100 g = 963.2 ...
A petrophysical model describing spectral induced polarization has been developed for clay-rocks accounting for the Maxwell Wagner polarization. It is also used to connect the complex conductivity to the relative permeability of the material. This model is applied to the Callovo-Oxfordian (COx) clay-rock of the Paris Basin (France) where the Meuse/Haute-Marne Underground Research Laboratory (URL) is located. Laboratory experiments are performed using 8 clay-rock cores to study the effect of desiccation on their spectral induced polarization response. The measurements are performed along the foliation plane. Complex conductivity spectra are performed over the frequency range 1 mHz to 45 kHz. These spectra are fitted with a double Cole Cole model to extract the evolution of the Cole Cole parameters with the saturation during the desiccation process. The low-frequency Cole Cole model corresponds to induced polarization phenomena while the high-frequency Cole Cole model corresponds to the Maxwell-Wagner contribution. We obtain the value of the first and second Archies exponents and we check the relationship between the surface conductivity and the cation exchange capacity of the clay-rocks. We are also able to connect the relative permeability curve to the second (saturation) Archies exponent. The monitoring of the complex conductivity can be used to predict how the permeability of the clay-rock formation changes with the water content.
... This ability has been recently used to derive textural and hydrogeological information about the subsurface, using petrophysical relations developed on a fundamental basis. [39][40][41] We propose here to extend the scope of SIP tomography (SIPT) to permafrost terrains for delineating frozen areas and detecting fine-grained, frost-susceptible sediments. Indeed, the presence of ice within a porous medium induces several specific polarization mechanisms. ...
... The method for the inversion of the SIPT data was adapted from the one of. 40 See this reference for a more detailed description. The real and imaginary parts of the complex conductivity were inverted separately as a function of frequency, using the software ResIPy. ...
... However, due to the frozen conditions and the severe filtering strategy used for the SIPT data, no quantitative estimation of the subsurface hydrological properties could be done with the petrophysical relations proposed in the literature for unfrozen conditions. [39][40][41]64 In the future, the use of the mechanistic relations developed by 22 for frozen condition would be an interesting step forward. , the presence of the water table close to the surface in the pond area also helps prevent frost penetration ( Figure 9). ...
In this study, high resolution ground‐penetrating radar (GPR), electrical resistivity tomography (ERT), and spectral‐induced polarization tomography (SIPT) were used to (i) delineate characteristic solifluction features, (ii) map the ice distribution, and (iii) assess subsurface water content and permeability in the surrounding rampart of a thermokarst pond in the discontinuous permafrost zone. The study site is located in the Tasiapik Valley near Umiujaq in Nunavik (Québec), Canada, which benefits from decades of geological mapping, geophysical investigation, and monitoring of ground temperature and thaw subsidence, providing an extensive understanding of the cryohydrogeological context of the area. The results of geophysical investigation undertaken in this study were cross validated using core sampling, laboratory core analysis, and in situ ground temperature and water content monitoring. Based on this investigation, a conceptual model was derived and compared to the stratigraphy of cross‐section described in literature in finer‐grained context. Very good consistency was found from one in situ geophysical survey to another, as well as between the derived stratigraphic models and the ground truth. The combination of all the available data allowed the development of a detailed cryohydrogeological model across the studied thermokarst pond, which highlights the effect of lithology, topography, and land cover on the distribution and mobility of water in the ground.
... Usually, the measurements are performed in the time domain in the field, or in the frequency domain in the laboratory (Figure 3, see [51]). With that being said, it is worth noticing that more and more field investigations are based on frequency-domain induced polarization measurements [28,[52][53][54][55]. In our viewpoint, it does not really matter if the measurements are performed in the time domain or the frequency domain. ...
... Usually, the measurements are performed in the time domain in the field, or in the frequency domain in the laboratory ( Figure 3, see [51]). With that being said, it is worth noticing that more and more field investigations are based on frequency-domain induced polarization measurements [28,[52][53][54][55]. In our viewpoint, it does not really matter if the measurements are performed in the time domain or the frequency domain. ...
... Although the traditional induced polarization method in the time and frequenc domains [28,[52][53][54][55] has achieved great successes, it cannot necessarily meet the needs o ...
Disseminated ores in porous or fractured media can be polarized under the application of an external low-frequency electrical field. This polarization is characterized by a dimensionless property that is called the “chargeability”. Induced polarization is a nonintrusive geophysical sensing technique that be used in the field to image both the electrical conductivity and the chargeability of porous rocks together with a characteristic relaxation time. A petrophysical model of the induced polarization of metallic ores immersed in a porous conductive and polarizable material is reviewed, and its predictions are compared to a large dataset of experimental data. The model shows that the chargeability of the material is linearly dependent on the volume fraction of the ore and the chargeability of the background material, which can, in turn, be related to the conductivity of the pore water and the cation exchange capacity of the clay fraction. The relaxation time depends on the grain sizes of the ores and on the conductivity of the background material, which is close to the conductivity of the porous rock itself. Five applications of the induced-polarization method to ore and metallic bodies are discussed in order to show the usefulness of this technique. These applications include: (i) A sandbox experiment, in which cubes of pyrite are located in a specific area of the tank; (ii) The tomography of an iron slag at an archeological site in France; (iii) A study of partially frozen graphitic schists in the French Alps; (iv) The detection of a metallic tank through the tomography of the relaxation times; and (v) The detection and localization of a deep ore body that is associated with a tectonic fault. We also discuss the possibility of combining self-potential and induced-polarization tomography to better characterize ore bodies below the seafloor.
... Even under high-salinity conditions, bacterial activities can be effectively monitored using SIP (Joo et al., 2021). However, soils are complex media, and the presence of clays can also enhance their induced polarization response Revil, Ghorbani, Jougnot, Yven, Grgic, et al., 2023;Revil et al., 2021). Recently, Song et al. (2022) conducted laboratory experiments to monitor the growth and decay of the bacteria in natural soil columns, in which the presence of clays in the overall polarization response of the mixture was considered. ...
... where D (+) (in m 2 s 1 ) denotes the diffusion coefficient of the counterions in the Stern layer, which can be related to the mobility B (see Revil et al., 2021). In presence of microcolonies inside a soil, it is possible that the polarization length scale is associated with the size of the microcolonies instead of the size of the single bacteria. ...
Spectral induced polarization (SIP) exhibits potential to be a nonintrusive approach to monitor bacterial activity in biological hotspots associated with the critical zone of the earth. The polarization of bacteria in a low‐frequency electrical field is related to the polarization of their electrical double layer coating their surface. However, few studies have quantified the induced polarization responses on both gram‐negative (GN) and gram‐positive (GP) bacteria in soil column experiments. To address this gap, 17 experiments using two strains, Pseudomonas aeruginosa O1 (PAO1, GN) and Brevibacillus centrosporus (L3, GP) are conducted. Complex conductivity spectra are collected in the frequency range 10 mHz–10 kHz during bacterial growth and decay phases in soils. The complex conductivity spectra are fitted using a double Cole‐Cole model to remove the effect of Maxwell‐Wagner polarization. The change in the magnitude of the polarization (quadrature conductivity or normalized chargeability of the low‐frequency contribution) is linearly related to the bacterial density, regardless of the type of bacteria. The changes in the normalized chargeability and Cole‐Cole relaxation time are directly proportional to the density of bacteria. Furthermore, it is inferred that the thickness of microcolonies plays a critical role in the relaxation time rather than the diameter of individual bacteria. This study expands the potential of SIP for in situ monitoring of microbial activity in soils.
... We can compute the distribution of the θ and CEC from M n and σ according to following equations (Revil, 2013b;Revil et al., 2017aRevil et al., , 2020Revil et al., , 2021: ...
... where R is a dimensionless number introduced by Revil et al. (2017aRevil et al. ( , 2020Revil et al. ( , 2021, with R = λ/B ≈ 0.10 (independent of temperature and saturation). In this work, the pore water conductivity is assumed to be known and is uniform in the domain. ...
Toxic organic contaminants in groundwater are pervasive at many industrial sites worldwide. These contaminants, such as chlorinated solvents, often appear as dense non-aqueous phase liquids (DNAPLs). To design efficient remediation strategies, detailed characterization of DNAPL Source Zone Architecture (SZA) is required. Since invasive borehole-based investigations suffer from limited spatial coverage, a non-intrusive geophysical method, direct current (DC) resistivity, has been applied to image the DNAPL distribution; however, in clay-sand environments, the ability of DC resistivity for DNAPLs imaging is limited since it cannot separate between DNAPLs and surrounding clay-sand soils. Moreover, the simplified parameterization of conventional inversion approaches cannot preserve physically realistic patterns of SZAs, and tends to smooth out any sharp spatial variations. In this paper, the induced polarization (IP) technique is combined with DC resistivity (DCIP) to provide plausible DNAPL characterization in clay-sand environments. Using petrophysical models, the DCIP data is utilized to provide tomograms of the DNAPL saturation (SN) and hydraulic conductivity (K). The DCIP-estimated K/SN tomograms are then integrated with borehole measurements in a deep learning-based joint inversion framework to accurately parameterize the highly irregular SZA and provide a refined DNAPL image. To evaluate the performance of the proposed approach, we conducted numerical experiments in a heterogeneous clay-sand aquifer with a complex SZA. Results demonstrate the standalone DC resistivity method fails to infer the DNAPL in complex clay-sand environments. In contrast, the combined DCIP technique provides the necessary information to reconstruct the large-scale features of K/SN fields, while integrating DCIP data with sparse but accurate borehole data results in a high resolution characterization of the SZA. Share Link:
https://authors.elsevier.com/c/1hiqkccWj2t8L
... However, monitoring instruments are usually arranged across a designated section, which easily leads to missed inspections. For decades, researchers have investigated the internal leakage of earth/rock-fill dams by various methods, including geotechnical methods such as traditional hydraulic tests (Wang and Jiang 2013) and hydraulic tomography (Dong et al. 2020), geoelectric methods such as the direct current (DC) resistivity method , induced polarization method (Revil et al. 2021), and self-potential method (Soueid Ahmed et al. 2019b), and tracing methods such as environmental isotope tracing (Mozafari and Raeisi 2017), artificial tracing (Abedian et al. 2019), and temperature tracing (Radzicki et al. 2021). In addition, investigation methods for structural surface leakage, such as unmanned aerial vehicle (UAV) infrared thermal imaging ) and flow field fitting method (Zhao et al. 2021), have recently been developed. ...
... The other, conduction on the surface of particles, is proportional to the surface area of particles and is called surface conductivity. Recent studies have shown that the application of the conductivity equation neglecting surface conduction is incorrect (Revil et al. 2021). The DC resistivity method cannot separate the volume conductivity from the surface conductivity, but the induced polarization method can be used to separate the two contributions of conductivity (Abdulsamad et al. 2019). ...
Earth/rock-fill dams and embankments are the main water retaining structures in hydraulic projects, and they can effectively resist floods and are of great significance for protecting people's lives and property. Leakage is a common problem in these structures. Investigation activities, including geotechnical, geoelectric, and tracing methods, are required to locate the leakage path and provide a basis for risk mitigation and reinforcement. These three methods provide information on different leakage characteristics, uncertainties, and spatiotemporal distributions. This work first introduces the micro-mechanism of internal erosion and then, provides a site case base for leakage investigation of earth/rock-fill dams and embankments from all over the world. For each investigation method, the basic principle, investigation process, data interpretation, and future potential are summarized. It should be emphasized that geotechnical, geoelectric, and tracing methods are placed on an equal level to assist dam managers and researchers in selecting the most appropriate method to assess dam leakage against specific geological backgrounds and structural types. Finally, the advantages, disadvantages, and applicable conditions of each investigation method are compared. The role of surface investigation methods and internal investigation methods in different stages of leakage is explained. The application of combined methods is discussed at four levels, and a new combined method is proposed.
... [17][18][19][20] SIP measures both electrical conduction and interfacial polarization in a porous medium. 21,22 SIP instruments dedicated to geoscientific applications were developed decades ago (see the reviews of Collett 23 and Seigel et al. 24 and references therein). For studies on sedimentary rocks, new high-precision instruments have been recently produced. ...
... This type of interfacial polarization is called the Maxwell-Wagner polarization. 64,65 The presence of local maxima of σ″ at various frequencies comes from specific mechanisms related to, e.g., the grain size 22,32 or surface roughness 30 (both due to EDL polarization). ...
We miniaturize geoelectrical acquisition using advanced microfabrication technologies to investigate coupled processes in the critical zone. We focus on the development of the complex electrical conductivity acquisition with the spectral induced polarization (SIP) method on a microfluidic chip equipped with electrodes. SIP is an innovative detection method that has the potential to monitor biogeochemical processes. However, due to the lack of microscale visualization of the processes, the interpretation of the SIP response remains under debate. This approach at the micrometer scale allows working in well-controlled conditions, with real-time monitoring by high-speed and high-resolution microscopy. It enables direct observation of microscopic reactive transport processes in the critical zone. We monitor the dissolution of pure calcite, a common geochemical reaction studied as an analog of the water-mineral interactions. We highlight the strong correlation between SIP response and dissolution through image processing. These results demonstrate that the proposed technological advancement will provide a further understanding of the critical zone processes through SIP observation.
... We aim to investigate the fate of phenolic pollutants, specifically in the context of oxidation processes in MnO 2enriched soil. To achieve this, we will apply both classical methodologies and an advanced geoelectrical method recently introduced to soil science: spectral induced polarization (SIP) (Gao et al., 2019;Johansson et al., 2019;Kessouri et al., 2019;Mellage et al., 2022;Revil, 2012;Revil et al., 2021;Shefer et al., 2013;Vaudelet et al., 2011;Vinegar and Waxman, 1984;Zhang et al., 2012). This approach allows us not only to track the transformation of phenolic pollutants through oxidation by MnO 2 but also to monitor the broader impacts of this oxidation process on other elements within the soil environment. ...
Understanding phenolic-pollutant interactions with soil colloids has been a focus of extensive research, primarily under controlled conditions. This study addresses the need to explore these processes in a more natural, complex soil environment. We aim to shed light on the underlying mechanisms of hydroquinone (a representative phenolic pollutant) oxidation in ambient, MnO2-rich sandy soil within soil columns designed for breakthrough experiments. Our innovative approach combines noninvasive electrical measurements, crystallographic and microscopic analyses, and chemical profiling to comprehensively understand soil–pollutant interactions. Our study reveals that hydroquinone oxidation by MnO2 initiates a cascade of reactions, altering local pH, dissolving calcite, and precipitating amorphous Mn oxides, thereby showcasing a complex interplay of chemical processes. Our analysis, combining insights from chemistry and electrical measurements, reveals that the oxidation process led to a constant decrease in polarizing surfaces, as indicated by quadrature conductivity monitoring. Furthermore, dynamic shifts in the soil solution chemistry (changes in the calcium and manganese concentrations, pH, and electrical conductivity (EC)) correlated with the non-monotonous behavior of the in-phase conductivity. Our findings conclusively demonstrate that the noninvasive electrical method allows real-time monitoring of calcite dissolution, serving as a direct cursor to the oxidation process of hydroquinone and enabling the observation of chemical interactions in soil solution and on soil particle surfaces.
... Note that the high normalized chargeability anomaly in S27 may be related to the silt layer in the vadose zone. Similar research can be referred to Johansson et al. 53 and Revil et al. 54 . ...
Light non-aqueous phase liquids (LNAPLs) that infiltrate into the subsurface are commonly described in two distinct zones: the source zone and the plume zone. A precise differentiation between these zones is essential for constraining further migration and selecting an effective remediation method. In this study, we employ the induced polarization (IP) method to characterize the contaminants. Six time domain IP survey lines were conducted at a former chemical plant contaminated with LNAPLs. Even though the contaminated areas corresponding to BTEX concentration above 180 mg/kg are less than 5 mS/m, the source and plume zones cannot be distinguished by conductivity alone. However, a noticeable difference in phase (φ) between the two zones is observed, and the threshold phase value corresponding to a critical concentration of 450 mg/kg is 20 mrad. Moreover, the normalized chargeability (Mn) threshold for the source zone is 80 mS/m, and the corresponding Mn differences between the source and plume zones are more significant than those in φ. These results illustrate that changes in polarization characteristics associated with BTEX concentrations can aid in further distinguishing between the source and plume zones. Ultimately, it is concluded that IP imaging is a well-suited method for LNAPL investigations that permits an improved characterization of different contaminated zones, which can facilitate the optimization of drillings for further site assessment and remediation.
... In the case of a broad distribution of grain sizes, the spectrum of the conductivity phase angle is rather flat and can be roughly described by a constant phase angle model, the so-called Drake's model (Van Voorhis et al., 1973). If such model is 215 applicable, is linearly related to ′′ (at the geometric mean frequency between 1 and 2 ) by the factor , as demonstrated by laboratory and field investigations (see Revil et al., 2017;Revil et al., 2021) ...
Degrading permafrost in rock glaciers has been reported from several sites in the European Alps. Changes in ground temperature and ice content are expected to affect the hydrogeological properties of the rock glacier and in turn modify the runoff regime and groundwater recharge in high-mountain environments. In this study, we investigate the use of an emerging geophysical method to understand the hydrogeological properties of the active Gran Sometta rock glacier, which consists of a two lobe-tongue (a white and a black) differing in their geologies. We present the application of the spectral induced polarization (SIP) imaging, a method that provides continuous spatial information about the electrical conductivity and polarization of the subsurface, which are linked to hydrogeological properties. To quantify the water content and the hydraulic conductivity from SIP imaging results, we used the petrophysical dynamic stern layer model. The SIP results show a continuously frozen layer at 4−6 m depth along both lobes which hinders the infiltration of water leading to a quick flow through the active layer. To evaluate our results, we conducted tracer experiments monitored with a time-lapse electrical conductivity imaging which confirms the hydraulic barrier associated with the frozen layer and allows to quantify the pore water velocity (~10-2 m/s). Below the frozen layer, both lobes have distinct water content and hydraulic conductivity. We observed a higher water content in the black lobe, which moves faster than the white lobe supporting the hypothesis that the water content at the shear horizon dominates rock glacier velocity. Our study demonstrates that the SIP method is able to provide valuable information for the hydrogeological characterization of rock glaciers.
... Spectral induced polarization (SIP) tomography as a noninvasive and near-surface geophysical method is increasingly being used to address problems in earth sciences such as mineral exploration (Günther and Martin, 2016;Alfouzan et al., 2020), environmental and groundwater investigations Attwa and Günther, 2013;Mudler et al., 2022), contaminant plume identification Flores Orozco et al., 2021), landfill characterization , and landslide and soil property imaging (Flores Orozco et al., 2018;Revil et al., 2021Revil et al., , 2023. It is well known that frequency-dependent induced polarization measurements, despite the incomplete understanding of the universal mechanism, provide extra information (i.e., spectral information) beyond an individual measure of the induced polarization amplitude based on the shape of the frequency response. ...
Induced polarization tomography offers the potential to better characterize the subsurface structures by considering spectral content from the data acquisition over a broad frequency range. Spectral induced polarization tomography is generally defined as a non-linear inverse problem commonly solved through deterministic gradient-based methods. To this end, the spectral parameters, i.e., DC resistivity, chargeability, relaxation time, and frequency exponent, are resolved by individually or simultaneously inverting all frequency data followed by fitting a generalized Cole-Cole model to the inverted complex resistivities. Due to the high correlation between Cole-Cole model parameters and a lack of knowledge about the initial approximation of the spectral parameters, using the classical least-square methods may lead to inaccurate solutions and impede reliable uncertainty analysis. To cope with these limitations, we introduce a new approach based on a hybrid application of a globally convergent homotopic continuation method and Bayesian inference to reconstruct the distribution of the subsurface spectral parameters. The homotopic optimization, owing to its fast and global convergence, is first implemented to invert multi-frequency spectral induced polarization datasets aimed at retrieving the complex-valued resistivity. Then, Bayesian inversion based on a Markov-chain Monte Carlo (McMC) sampling method along with a priori information including lower and upper bounds of the prior distributions is utilized to invert the complex resistivity for Cole-Cole model parameters. By applying the McMC inversion algorithm a full nonlinear uncertainty appraisal can be provided. We numerically evaluate the performance of the proposed method using synthetic and real data examples in the presence of topographical effects. Numerical results prove that the homotopic continuation method outperforms the classic smooth inversion algorithm in the sense of approximation accuracy and computational efficiency. we demonstrate that the proposed hybrid inversion strategy provides reliable representations of the main features and structure of the Earths subsurface in terms of the spectral parameters.
... An essential point in the evaluation of the complex electrical conductivity is the assessment of the geometrical factor of the sample holder (Revil et al., 2017;Revil et al., 2021). To translate a measured complex impedance to a complex electrical conductivity, this factor depends on the position of electrodes which affects the current distribution between the current electrodes and the geometry of the sample holder. ...
The lithological and stratigraphical heterogeneity of coastal aquifers has a great influence on saltwater intrusion (SI). This makes it difficult to predict SI pathways and their persistence in time. In this context, electrical resistivity tomography (ERT) and induced polarization (IP) methods are receiving increasing attention regarding the discrimination between saltwater-bearing and clayey sediments. To simplify the interpretation of ERT data, it
is commonly assumed that the bulk conductivity mostly depends on the conductivity of pore-filling fluids, while surface conductivity is generally disregarded in the spatial and temporal variability of the aquifers, particularly, once the aquifer is affected by the presence of saltwater. Quantifying salinities based on a simplified petrophysical relationship can lead to misinterpretation in aquifers constituted by clay-rich sediments. In this study,
we rely on co-located data from drilled boreholes to formulate petrophysical relationships between bulk and fluid conductivity for clay-bearing and clay-free sediments. First, the sedimentary samples from the drilled wells were classified according to their particle size distribution and analyzed in the lab using spectral IP in controlled salinity conditions to derive their formation factors, surface conductivity, and normalized chargeability. Second,
the deduced thresholds are applied on the field to distinguish clay-bearing sediments from brackish sandy sediments. The results are validated with logging data and direct salinity measurements on water samples. We applied the approach along the Luy River catchment and found that the formation factors and surface conductivity of the different unconsolidated sedimentary classifications vary from 4.0 to 8.9 for coarse-grained sand and
clay-bearing mixtures, while normalized chargeability above 1.0 mS.m− 1 indicates the presence of clay. The clay bearing sediments are mostly distributed in discontinuous small lenses. The assumption of homogenous geological media is therefore leading to overestimating SI in the heterogeneous clay-bearing aquifers.
... (11) and (12). Instead of fitting a Cole-Cole model, Revil et al. (2021) estimate the normalized chargeability from the imaginary conductivity, using a linear approximation (i.e., σ ′′ ∝M n ). In Fig. 9, we present plots of the M n as a function of the σ′′, with the latter obtained from the inversion of the data collected at 6 Hz, which is the data set closest to the geometric mean of the frequency range used to fit the Cole-Cole parameters (between 1 and 60 Hz). ...
Understanding changes in hydraulic properties of the subsurface is critical to delineate areas susceptible for groundwater accumulation and the triggering of landslides. Laboratory studies have demonstrated the possibility to estimate the hydraulic conductivity from induced polarization (IP) measurements. However, to-date only rare studies have been applied at the field scale and none has evaluated the frequency-dependence in field IP imaging data. We show that the application of petrophysical models linking hydraulic conductivity (K) and IP at 1 Hz, resolves for the same estimations from frequency-domain measurements as well as from time-domain IP. Moreover, our IP images reveal an evident frequency-dependence between 0.1 and 240 Hz in the electrical properties, which is also observed in lab measurements conducted in soil samples. To account for this, we fit a Cole-Cole model to our multi-frequency IP results and evaluate models linking K and the normalized chargeability, gaining a subsurface model of the hydraulic conductivity with enhanced resolution. Our results reveal clear variations at depth in the electrical conductivity and polarization associated with the presence of a plane of instability and the sliding plane. We also resolve lateral changes delineating areas with K below 10⁻⁶ m/s, which may act as barriers for groundwater flow. To evaluate our results we also estimate the hydraulic conductivity using a joint inversion algorithm that directly solves for porosity and saturation through the simultaneous inversion of seismic refraction and electrical resistivity tomography data. We observed consistent hydraulic conductivity images obtained through the inversion of the IP data and the joint inversion, with such model being in agreement with existing information of the site. We resolve a 3D model of the hydraulic conductivity at the Hofermuehle landslide from IP data that reveals the geometry of areas with poor drainage that may lead to land sliding.
To ascertain the groundwater flow, hydrological conditions, and availability of water for plant growth, the hydrogeological system of a farmed fen wetland in low-latitude tropical Sudan savannah climate, geologically within south-west Nigeria basement complex, was researched. In order to achieve this, the results of the ground-truth geotechnical analyses of the moisture contents, Atterberg limits, grain-size distribution, porosity, and hydraulic conductivity of soils recovered from 3.0-m-deep bores, as well as depths control from two monitoring wells, were used to calibrate the results of the interpretations of the surficial and non-invasive geophysical methods employed: electrical resistivity tomography (ERT) and time-domain-induced polarization (TDIP). The latter geophysical method additionally enabled delineation of sand/gravel soil from clayey soil. The wetland was covered in soils with moisture contents between 10 and 62%, which increased with depth as evidenced by decreasing resistivity on inverted ERT sections, a relatively thick (2.8–3.8 m) water saturation zone diagnosis of low resistivity, chargeability, and normalized chargeability of 0.011–16.4 Ωm, 0.212–4.87 ms, and 0.0321–0.573 ms, respectively, as revealed by their corresponding inverted models, and permeable weathered zones (2.5–22.5 m thick) that could support ground water up-flow. These show that both shallow- and deep-rooted plants have access to water. Infiltration of the northward–southward-flowing groundwater into the weathered zones was made possible by structural fractured/joint system of the weathered zone. The lateral alternation of the weathered zone with highly resistive rocks ensures compartmentalization and maximum detention of the infiltrated groundwater within the fractured/weathered rock aquifer.
This study investigates the potential of field‐based induced polarization (IP) methods to provide in‐situ estimates of soil cation exchange capacity (CEC). CEC influences the fate of nutrients and pollutants in the subsurface. However, estimates of CEC require sampling and laboratory analysis, which can be costly, especially at large scales. Induced polarization (IP) methods offer an alternative approach for CEC estimation. The sensitivity of IP measurements to the surface properties of geological materials ought to make them more appropriate than DC resistivity and electromagnetic induction methods, that are sensitive to bulk electrical properties . Such abilities of IP are well demonstrated in the laboratory; however, applications are lacking at field scales. In this work, the ability of field‐based IP to characterize the CEC of floodplain soils is assessed by implementing a methodology that allows for direct comparison between IP and soil parameters. In one field, soil polarization and CEC exhibited the expected positive correlation; but multi‐frequency measurements showed no clear advantage over single‐frequency measurements. In another field, coarser soils (with low CEC) exhibited a high polarization. These coarser soils were characterized by anomalous magnetic susceptibility values, and hence the polarization was attributed to the presence of magnetic minerals. Although better than order‐of‐magnitude estimates of CEC were possible in soils without substantial magnetic minerals, better characterization of porosity, saturation, cementation and saturation exponents, and pore fluid conductivity would improve predictions. However, the measurement of these parameters would require similar efforts as direct CEC measurements. This study contributes to bridging the gap between laboratory‐derived relationships and their applicability in field applications. Overall, this work provides valuable insight for future studies seeking to understand polarization mechanisms in soils at the field scale.
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.
There is growing interest in the use of spectral induced polarization (SIP) surveys to characterize the near-surface environment. Few attempts have been made to perform field SIP surveys in a 3D configuration; when done, they are typically conducted using a series of parallel 2D electrode lines with collinear measurements. However, such measurements are limited in the resolution between the lines which is critical in the case of heterogeneous subsurface conditions, such as in landfills. To overcome this, we investigate here the enhanced resolution in SIP measurements through true 3D measurements, i.e., the resolving capabilities of different electrode configurations distributed across measuring planes. First, we investigate, through a synthetic study, the difference between results from using 2D parallel collinear electrode arrays and true 3D configurations. Second, we collected SIP data (in the frequency range between 1 and 240 Hz) using 2D and 3D configurations in two landfills to evaluate the application of our results in real field conditions. Both the synthetic and the field experiments demonstrate that measurements of parallel 2D collinear arrays result in the creation of artifacts and the loss of resolution in the 3D structure, especially of polarizable features. In contrast, the 3D configurations are able to resolve polarizable anomalies in synthetic and field measurements, resulting in a better delineation of the geometry of waste units. Our results also demonstrate that 3D configurations are better suited to recover the frequency-dependence of the electrical properties; thus, permitting an improved interpretation of waste composition and the quantification of waste volume.
Computed tomography (CT) in combination with advanced image processing can be used to non-invasively and non-destructively visualize complex interiors of living and non-living media in 2 and 3-dimensional space. In addition to medical applications, CT has also been widely used in soil and plant science for visual and quantitative descriptions of physical, chemical, and biological properties and processes. The technique has been used successfully on numerous applications. However, with a rapidly evolving CT technologies and expanding applications, a renewed review is desirable. Only a few attempts have been made to collate and review examples of CT applications involving the integrated field of soil and plant research in recent years. Therefore, the objectives of this work were to: (1) briefly introduce the basic principles of CT and image processing; (2) identify the research status and hot spots of CT using bibliometric analysis based on Web of Science literature over the past three decades; (3) provide an overall review of CT applications in soil science for measuring soil properties (e.g., porous soil structure, soil components, soil biology, heat transfer, water flow, and solute transport); and (4) give an overview of applications of CT in plant science to detect morphological structures, plant material properties, and root-soil interaction. Moreover, the limitations of CT and image processing are discussed and future perspectives are given.
Core Ideas
A Bayesian inference approach (INLA‐SPDE) was used to map topsoil and subsoil CEC.
DEM, gamma‐ray spectrometer and EM induction data were combined to map CEC.
Posterior marginal distributions of the model parameters and responses were estimated.
Gamma‐ray data performed best in the topsoil, followed by DUALEM421S and elevation.
Elevation data performed best in the subsoil, followed by gamma‐ray and DUALEM421S.
Cation exchange capacity (CEC) affects soil fertility, acidity, and structural resilience. This is particularly the case in sugarcane growing areas of Australia because the soil there is sandy (>60%), strongly acidic (pH < 5.5), and strongly sodic (exchangeable sodium percentage [ESP] > 15%). Unfortunately, obtaining information on CEC at the field extent is time‐consuming and expensive. Here, we used a digital soil mapping approach to add value to limited (40) topsoil (0–0.3 m) and subsoil (0.6–0.9 m) CEC information. We first collected proximally sensed ancillary data from three sources, including a digital elevation model (DEM), γ‐ray (γ‐ray) spectrometer (RS700) and electromagnetic (EM) induction instruments. We then use a Bayesian inference approach (Integrated Nested Laplace Approximation with Stochastic Partial Differential Equation, INLA‐SPDE) implemented in R software to model the CEC and ancillary data. Accuracy (RMSE), bias (ME), and concordance (Lin's) of models were also generated from the different sources of ancillary data, either in combination or alone. We concluded, overall, that the INLA–SPDE approach could provide estimations of the posterior marginal distributions of the model parameters as well as the model responses as reported by other researchers. We also concluded that using the ancillary data sources in combination was most accurate (e.g., RMSE = 0.72) to predict CEC, least biased (e.g., ME = 0.07) and had the highest concordance (e.g., Lin's = 0.69) in both the topsoil and subsoil than using the ancillary data alone. The best ancillary data, when used alone for mapping CEC in the topsoil, was γ‐ray spectrometry, followed by EM data and elevation. For subsoil CEC, it was elevation, followed by γ‐ray spectrometry and then soil electrical conductivity (EC a ) data. The maps of the credibility interval (CI) indicated that better predictions were achieved in the topsoil and indicated where improvements in prediction could be achieved in the subsoil.
Whether close evolutionary relatives can coexist is expected to depend on evolutionary divergence in niches relative to divergence in competitive abilities. We investigated how plant species' responses to soil texture might affect coexistence by analysing distributions, seedling emergence and performance, and competitive abilities of the winter annuals Clarkia speciosa ssp. polyantha and C. xantiana ssp. xantiana. A landscape survey showed that the species have distinct associations with soil texture, C. speciosa presence correlating with fine soil and C. xantiana correlating with coarse soil. At the scale of population presences, the species co-occur less often than would be expected at random. On small scales within sites where they do co-occur, each species was negatively associated with the other. Clarkia xantiana presence and/or density also correlated positively with coarse soil texture and steep, poleward slopes, suggesting limitation by water availability. Lab experiments that varied substrate texture and imposed drought revealed contrasting species' fundamental niches at the seed and seedling stages. In coarse substrates, C. xantiana seedlings emerged at several-fold higher rates than C. speciosa, and, unlike C. speciosa, emerged when seeds were buried 0.5 cm. Clarkia speciosa seedlings had superior drought tolerance, independent of substrate. Competition coefficients estimated in a response surface experiment in artificial substrates predicted competitive exclusion of C. speciosa by C. xantiana in coarse substrate, with possible founder control of competitive outcome in fine substrate. Species' differences in responses to soil texture generate spatial segregation that likely facilitates coexistence, despite competitive ability differences that oppose it.
Big sagebrush (Artemisia tridentata Nutt.) plant communities are widespread in western North America and, similar to all shrub steppe ecosystems worldwide, are composed of a shrub overstory layer and a forb and graminoid understory layer. Forbs account for the majority of plant species diversity in big sagebrush plant communities and are important for ecosystem function. Few studies have explored geographic patterns of forb species richness and composition and their relationships with environmental variables in these communities. Our objectives were to examine the fine and broad-scale spatial patterns in forb species richness and composition and the influence of environmental variables. We sampled forb species richness and composition along transects at 15 field sites in Colorado, Idaho, Montana, Nevada, Oregon, Utah, and Wyoming, built species-area relationships to quantify differences in forb species richness at sites, and used Principal Components Analysis, non-metric multidimensional scaling, and redundancy analysis to identify relationships among environmental variables and forb species richness and composition. We found that species richness was most strongly correlated with soil texture, while species composition was most related to climate. The combination of climate and soil texture influences water availability, which our results indicate has important consequences for forb species richness and composition, and suggests that climate change-induced modification of soil water availability may have important implications for plant species diversity in the future. [Springer Nature SharedIt Initiative Full-Text Article Available : http://rdcu.be/tFJu].
Mapping of soil water content (SWC) by electromagnetic induction (EMI) is an established method to obtain field-scale SWC information. However, the relationship between SWC and the apparent electrical conductivity (ECa) measured with EMI is complex and affected by several confounding factors at the catchment scale such as variable porosity (ϕ) and pore water electrical conductivity (σw). In this study, we investigated these confounding factors using a time-lapse EMI data set obtained in a forest ecosystem with soils of low ECa and catchment-wide SWC data provided by a wireless soil moisture sensor network. To assess the impact of variable ϕ on the accuracy of SWC estimates, we compared three different models to relate SWC and ECa: (i) a linear regression model and two nonlinear models based on Archie’s equation with (ii) constant ϕ and (iii) variable ϕ. The linear model reached a prediction accuracy of RMSE = 5.83 vol%, while the Archie models increased the accuracy to RMSE = 4.55 vol% (constant ϕ) and RMSE = 4.20 vol% (variable ϕ). Although we found strong spatial similarities between SWC and ECa maps, the temporal trends in SWC and ECa were inconsistent. This was attributed to temporal variations in σw due to seasonal changes in ion concentrations of the soil pore water. To support this hypothesis, σw was calculated from the measured ECa and the known soil saturation from SoilNet. The resulting σw maps showed highly structured and consistent patterns. We thus conclude that in addition to variation in SWC and ϕ, spatiotemporal variations of σw affected the ECa measured with EMI. These potentially confounding factors in the interpretation of EMI measurements in terms of SWC have not been sufficiently recognized in the literature so far, and the results presented in this study indicate a range of limitations for the use of EMI to monitor spatiotemporal changes in SWC at test sites with low ECa.
A model is proposed regarding the polarization of dispersed metallic conductors (e.g., pyrite and magnetite) in porous media free of redox-active ionic species in the pore water. We studied two cases corresponding to having a background material with or without chargeability. The model was based on the polarization mechanism of a single particle using well-established bounds for the reflection coefficient entering the definition of the dipole moment of the metallic grains. We used the Maxwell-Clausius-Mos-sotti mixing equation to obtain the complex conductivity of the mixture of dispersed metallic particles in the background porous material composed of the pore water and the insulating grains coated by an electric double-layer. This equation can be generalized to a mixture of various types of metallic particles (with their own properties) dispersed in the background porous material. Our model led to a very simple linear relationship between the chargeability and the volume content of metallic particles in the material. In addition, the chargeability depended weakly only on the shape of the spheroidal metallic particles as long as their orientation was random. The relaxation time defined from the phase peak frequency related to the diffusion coefficient of the n-and p-charge carriers in the metallic particles. This diffusion coefficient was consistent with the mobility of the charge carriers derived from theoretical considerations or electric conductivity measurements. In the presence of a polarizable background (e.g., a clayey matrix), we found that the total chargeabil-ity of the material can be determined from the chargeability of the metallic particles and the chargeability of the background material.
Over the last 15 years significant advancements in induced polarization (IP) research have taken place, particularly with respect to spectral IP (SIP), concerning the understanding of the mechanisms of the IP phenomenon, the conduction of accurate and broadband laboratory measurements, the modelling and inversion of IF data for imaging purposes and the increasing application of the method in near-surface investigations. We summarize here the current state of the science of the SIP method for near-surface applications and describe which aspects still represent open issues and should be the focus of future research efforts. Significant progress has been made over the last decade in the understanding of the microscopic mechanisms of IP; however, integrated mechanistic models involving different possible polarization processes at the grain/pore scale are still lacking. A prerequisite for the advances in the mechanistic understanding of IP was the development of improved laboratory instrumentation, which has led to a continuously growing data base of SIP measurements on various soil and rock samples. We summarize the experience of numerous experimental studies by formulating key recommendations for reliable SIP laboratory measurements. To make use of the established theoretical and empirical relationships between SIP characteristics and target petrophysical properties at the field scale, sophisticated forward modelling and inversion algorithms are needed. Considerable progress has also been made in this field, in particular with the development of complex resistivity algorithms allowing the modelling and inversion of IF data in the frequency domain. The ultimate goal for the future are algorithms and codes for the integral inversion of 3D, time-lapse and multi-frequency IF data, which defines a 5D inversion problem involving the dimensions space (for imaging), time (for monitoring) and frequency (for spectroscopy). We also offer guidelines for reliable and accurate measurements of IP spectra, which are essential for improved understanding of IP mechanisms and their links to physical, chemical and biological properties of interest. We believe that the SIP method offers potential for subsurface structure and process characterization, in particular in hydrogeophysical and biogeophysical studies.
Approaches to soil conservation are in constant evolution and improvement. This paper summarizes some of the modern approaches, ranging from no till to conservation agriculture to sustainable land management. These approaches are not separate, but components of a continuum of conservation approaches applicable at different levels and different scales. No tillage is important at the detailed, farm level, while CA and SLM are important at the farming systems and corporate levels. The successes achieved with no till in Argentina (also Brazil, Paraguay, Uruguay, Mexico, Canada, Australia, and others) illustrate how these concepts relate to each other.
Soil quality, in a viticultural context, may be defined as the soil's capacity to support grapevine growth without resulting in soil degradation or otherwise harming the environment. In other agricultural systems, various approaches for evaluating soil quality have been adopted, and numerous soil physical and chemical properties have been used to characterise it. Here, we consider the relevance and suitability of these approaches and the choice of soil properties for Australian viticulture. As a consequence, the soil physical and chemical properties suggested to comprise a minimum data set for ongoing monitoring of soil quality in Australian viticulture are aggregate stability, air-dry soil consistence, pH, electrical conductivity, cation exchange capacity (if pHCa<5.5), exchangeable cations and total organic carbon. Biological parameters are considered in a companion paper. The rationale for not including other soil physical and chemical properties that may be part of the minimum data set in other agricultural systems or be considered important in Australian vineyard soils is discussed. An area still to be considered is the selection of an indicator(s) of vine or grape quality since yield, which is used in many other agricultural industries, may not necessarily be an appropriate management goal in viticulture.
Resistivity and self-potential tomography can be used to investigate anomalous seepage inside heterogeneous earthen dams. The self-potential (SP) signals provide a unique signature to groundwater flow because the source current density responsible for the SP signals is proportional to the Darcy velocity. The distribution of the SP signals is also influenced by the distribution of the resistivity; therefore, resistivity and SP need to be used in concert to elucidate groundwater flow pathways. In this study, a survey is conducted at a small earthen dam in Colorado where anomalous seepage is observed on the downstream face at the dam toe. The data reveal SP and direct current resistivity anomalies that are used to delineate three anomalous seepage zones within the dam and to estimate the source of the localized seepage discharge. The SP data are inverted in two dimensions using the resistivity distribution to determine the distribution of the Darcy velocity responsible for the observed seepage. The inverted Darcy velocity agrees with an estimation of the Darcy velocity from the hydraulic conductivity obtained from a slug test and the observed head gradient.
Electrical impedance tomography (EIT) is gaining importance in the field of geophysics and there is increasing interest for accurate borehole EIT measurements in a broad frequency range (mHz to kHz) in order to study subsurface properties. To characterize weakly polarizable soils and sediments with EIT, high phase accuracy is required. Typically, long electrode cables are used for borehole measurements. However, this may lead to undesired electromagnetic coupling effects associated with the inductive coupling between the double wire pairs for current injection and potential measurement and the capacitive coupling between the electrically conductive shield of the cable and the electrically conductive environment surrounding the electrode cables. Depending on the electrical properties of the subsurface and the measured transfer impedances, both coupling effects can cause large phase errors that have typically limited the frequency bandwidth of field EIT measurements to the mHz to Hz range. The aim of this paper is to develop numerical corrections for these phase errors. To this end, the inductive coupling effect was modeled using electronic circuit models, and the capacitive coupling effect was modeled by integrating discrete capacitances in the electrical forward model describing the EIT measurement process. The correction methods were successfully verified with measurements under controlled conditions in a water-filled rain barrel, where a high phase accuracy of 0.8 mrad in the frequency range up to 10 kHz was achieved. The corrections were also applied to field EIT measurements made using a 25 m long EIT borehole chain with eight electrodes and an electrode separation of 1 m. The results of a 1D inversion of these measurements showed that the correction methods increased the measurement accuracy considerably. It was concluded that the proposed correction methods enlarge the bandwidth of the field EIT measurement system, and that accurate EIT measurements can now be made in the mHz to kHz frequency range. This increased accuracy in the kHz range will allow a more accurate field characterization of the complex electrical conductivity of soils and sediments, which may lead to the improved estimation of saturated hydraulic conductivity from electrical properties. Although the correction methods have been developed for a custom-made EIT system, they also have potential to improve the phase accuracy of EIT measurements made with commercial systems relying on multicore cables.
The cation exchange capacity (CEC) at pH 7 was measured for samples of 347 A horizons and 696 B horizons of New Zealand soils. The mean CEC was 22.1 cmolc/kg for the A horizons and 15.2 cmolc/kg for the B horizons. Multiple regressions were carried out for CEC against organic carbon (C), clay content, and the content of seven groups of clay minerals. The results, significant at p <0.001, were consistent with most of the CEC arising from soil organic matter. For the samples of A horizon, the calculated CEC was 221 cmolc/kg per unit C and for the B horizons was 330 cmolc/kg C. There was also a contribution from sites on clay minerals. Multiple regression indicated that smectite had a higher CEC (70 cmolc/kg) than other minerals but it was not as high as that of type smectites; kaolin minerals had the lowest CEC. There was a significant effect of interaction between organic matter and some clay minerals on the CEC. Samples from B horizons containing allophane had lower CEC than those not containing allophane which is consistent with allophane reacting with carboxyl groups on organic matter. For the samples from the A horizons, however the CEC was higher when allophane was present.
At the hillslope scale, surface runoff depends to a great extent on the water deficit of the soils, i.e. the difference between the effective porosity and the water content, all through the soil profile. The aim of this study was to assess water deficits using Electrical Resistivity Tomography (ERT). ERT offers several advantages: (i) it is not intrusive, (ii) it exhibits spatial patterns at a decametric or hectometric scale, and (iii) it provides information on both water content and soil depth. The study was conducted on a Mediterranean shaley sandy soil, in the Cevennes (South of France). First, the factors (nature of the pores and organization of the material, water content, soil solution and temperature) on which electrical resistivities depend were analyzed in the laboratory. Soil samples were used to derive the ‘m’ and ‘n’ parameters of Archie’s relation, changes in the soil solution over time, and the effect of temperature on resistivities. These results were then used to interpret ERT in situ at different times of the year. The water content and water deficit obtained from ERT were then compared with local measurements made using Time Domain Reflectometry (TDR) and the comparison was satisfactory. The results of this study show the potential of measuring soil water content and water deficit using electrical resistivities, and describe how uncertainties in temperature, porosity, and fluid conductivity impact ERT-obtained estimates of soil water content and deficit.
Probabilistic formulation of inverse problems leads to the definition of a probability distribution in the model space. This probability distribution combines a priori information with new information obtained by measuring some observable parameters (data). As, in the general case, the theory linking data with model parameters is nonlinear, the a posteriori probability in the model space may not be east to describe (it may be multimodal, some moments may not be defined, etc.). When analyzing an inverse problem, obtaining a maximum likelihood model is usually not sufficient, as we normally also wish to have information on the resolution power of the data. In the general case we may have a large number of model parameters, and an inspection of the marginal probability densities of interest may be impractical, or even useless. But it is possible to pseudorandomly generate a large collection of models according to the posterior probability distribution and to analyze and display the models in such a way that information on the relative likelihoods of model properties is conveyed to the spectator. This can be accomplished by means of an efficient Monte Carlo method, even in cases where no explicit formula for the a priori distribution is available. The most well known importance sampling method, the Metropolis algorithm, can be generalized, and this gives a method that allows analysis of (possible highly nonlinear) inverse problems with complex a priori information and data with an arbitrary noise distribution.
The permeability of a soil is one of its most fundamental and important properties. It enters into nearly all seepage, settlement, and stability problems confronting the soil engineer. The amount of leakage through and under dams, the rate at which a building settles, and the ate at which the strength of a deposit increases after it has been subjected to a consolidating pressure are typical of the many problems in which the permeability of a soil can be a critical factor. The importance of evaluating the permeability of a pervious soil has been long recognized and test techniques for measuring it have been well developed and are widely used. The permeability of fine-grained soils, however, has not received extensive study. Soils with permeabilities of less than 1 /t per sec are often considered “impervious” and are not subjected to permeability testing. Soil permeabilities less than 1 y. per sec are becoming increasingly important to soil technologists from both practical and theoretical considerations. More use is being made of “impervious” soil to line canals and reservoirs and to construct cores for earth dams. Research workers are studying the permeability of finegrained soils to learn more of the nature of particle surfaces and the thickness of adsorbed water (7). A look at some of the factors influencing the permeability of fine-grained soils, therefore, is timely.
Kilauea is an active shield volcano located in Hawaiʻi. An induced polarization survey was performed in 2015 at the scale of the caldera. The data were acquired with a 2.5 km cable with 64 electrodes and a spacing of 40 m between the electrodes. A total of 6210 measurements were performed. The apparent chargeability data were inverted using a least square technique to obtain a chargeability tomogram. The normalized chargeability tomogram is obtained by multiplying cell-by-cell the chargeability by the conductivity. Once the conductivity and normalized chargeability tomograms are obtained, they are jointly interpreted using a dynamic Stern layer conduction/polarization model, which explains the low-frequency polarization spectra of volcanic rocks. This conductivity/polarization model is tested here on new laboratory experiments performed on 24 samples from a drill-hole located on the Kilauea East Rift Zone (Hole SOH-2). We show that for Kilauea, the ratio between the normalized chargeability and the conductivity is equal to a dimensionless number R = 0.10 ± 0.02 proving that the conductivity and the normalized chargeability are both controlled by the alteration products of the volcanic rocks with a minor role of magnetite except close to the ground surface. In turn, the degree of alteration is controlled by temperature and therefore normalized chargeability and electrical conductivity can both be used as a non-intrusive temperature sensor. This approach is then applied to the field data and temperature tomograms can be produced from the electrical conductivity and normalized chargeability tomograms.
Understanding the spatiotemporal distribution of soil volumetric water content (θ, m³ m⁻³) at field level is required to maximise water-use efficiency in irrigated agriculture. Several commercial sensors are available; however, they only provide point-information. To value-add to this soil data, mathematical models can be used in conjunction with proximal sensed data, such as soil apparent electrical conductivity (ECa, mS m⁻¹) or inverted ECa (σ, mS m⁻¹). In this research, we determine if ECa from an electromagnetic (EM) instrument (EM38) at various heights (0, 0.2, 0.4, 0.6, 0.8 and 1.0 m) or σ estimated from ECa can be used to value add to limited θ at four depths (i.e., 0.15, 0.45, 0.75 and 1.35 m). Moreover, we compare which mathematical (i.e. multiple linear regression (MLR), random forest (RF), Cubist, support vector machine (SVM) and Artificial Neural Networks (ANN)) model can best be used to predict θ from σ. We also determine the number of calibration sites required along a uniform heavy-clay transect used for furrow irrigated cotton. In terms of a leave-one-out cross validation, the best Lin’s concordance between measured and predicted θ was achieved using SVM (0.91) when estimates of σ and depth used to model θ . We showed that satisfactory results could be achieved using a single calibration site. Considering the results at day 10 when permanent wilting point was evident, irrigation scheduling could be recommended based on the use of the EM38h0 (80 mS m⁻¹) and EM38v0 (100 mS m⁻¹) reaching critical measurements.
Characterizing key petrophysical parameters of dams and embankments (including water content, specific surface area or cation exchange exchange capacity, and permeability) is an important task in estimating their degree of safety. So far, induced polarization tomography has not been investigated to check if it can play such a role. We have conducted a time domain induced polarization profile along the embankment of a canal in the South East of France. The profile is 560 m long. It comprises 1696 apparent resistivity and chargeability data and was accomplished by separating the current and voltage electrode cables to improve the signal-to-noise ratio. In order to complement the study, we performed induced polarization measurements on six core samples (including a clayey material and five carbonate rocks) collected from outcrops. A petrophysical induced polarization model called the dynamic Stern layer model is tested to see how the electrical conductivity and the normalized chargeability can be both connected to the porosity, the cation exchange capacity (CEC), and permeability of these materials. Then, these results are applied to interpret the electrical conductivity and normalized chargeability tomograms into water content and CEC tomograms. In turn, these two parameters are used to compute a permeability index.
Cation exchange capacity (CEC, cmol(+) kg⁻¹) is a measure of the capacity of soil to retain and exchange cations. However, it is expensive to sample and directly measure across a heterogenous field and at different depths. To add value to limited data, proximally sensed apparent soil electrical conductivity (ECa, mS m⁻¹) from electromagnetic (EM) instruments has been coupled to CEC at each depth through a linear regression (LR) model. In this study, LR between ECa and depth specific CEC was compared with a LR developed between true electrical conductivity (σ, mS m⁻¹), inverted from ECa, and CEC from various depths, including topsoil (0–0.3 m), subsurface (0.3–0.6 m), shallow subsoil (0.6–0.9 m) and deeper subsoil (0.9–2.1 m). We estimate σ using quasi-3d (q-3d) inversion software (EM4Soil) considering inversion of EM38 and EM31 ECa either alone or in combination (joint inversion), in horizontal (ECah) and vertical (ECav) modes, and EM38 at two different heights (i.e. 0.2 or 0.4 m). The calibration results showed LR between ECa and depth specific CEC in the topsoil (R² = 0.31), subsurface (0.37) and shallow subsoil (0.52) was unsatisfactory. Stronger LR could be established for deeper subsoil CEC (> 0.60). However, a single LR could be developed between CEC at all depths with σ (R² = 0.72) estimated by jointly inverting EM38 (0.2 m) and EM31 ECa in both modes using a forward model (CF), inversion algorithm (S2) and small damping factor (λ = 0.03). A leave-one-out-cross-validation showed CEC prediction was precise (RMSE, 2.39 cmol(+) kg⁻¹), unbiased (ME, -0.01 cmol(+) kg⁻¹) with good concordance (Lin’s = 0.82). To improve areal prediction closer spaced transects are required, while to improve vertical resolution of prediction we recommend the use of a single-frequency multi-coil array DUALEM-421.
Together, the three particle size fractions (PSFs) of clay, silt, and sand are the most fundamental soil properties because the relative abundance influences the physical, chemical, and biological activities in soil. Unfortunately, determining PSFs requires a laboratory method which is time-consuming. One way to add value is to use digital soil mapping, which relies on empirical models, such as multiple linear regression (MLR), to couple ancillary data to PSFs. This approach does not account for the special requirements of compositional data. Here, ancillary data were coupled, via MLR modelling, to additive log-ratio (ALR) or isometric log-ratio (ILR) transformations of the PSFs to meet these requirements. These three approaches (MLR vs. ALR-MLR and ILR-MLR) were evaluated along with the use of different ancillary data that included proximally sensed gamma-ray spectrometry, electromagnetic induction, and elevation data. In addition, how the prediction might be improved was examined using ancillary data that was measured on transects and was compared to data interpolated from transects spaced far apart. Although the ALR-MLR approach did not produce significantly better results, it predicted soil PSFs that summed to 100 and had the advantage of interpreting the ancillary data relative to the original coordinates (i.e. clay, silt, and sand). For the prediction of PSFs at various depths, all ancillary data were useful. Elevation and gamma-ray data were slightly better for topsoil and elevation and electromagnetic (EM) data were better for subsoil prediction. In addition, a smaller transect spacing (26 m) and number of samples (9À16) might be adopted for mapping soil PSFs and soil texture across the study field. The ALR-MLR approach can be applied elsewhere to map the spatial distribution of clay minerals.
Geophysical methods are widely used for landslide investigation to delineate depth and geometry of the sliding plane. In particular, electrical resistivity tomography (ERT) is often used because both porosity and water saturation control the electrical resistivity of the subsurface materials and are critical for slope stability. Moreover, ERT can be employed to monitor changes in pore-fluid pressure which is an important factor triggering landslides. However, the interpretation of ERT results in clay-rich landslides can be challenging considering that high electrical conductivity values may not only be related to an increase in saturation but also to the surface conduction mechanism, which becomes dominant in the presence of clays. Recently, environmental investigations have demonstrated an improved subsurface characterization through induced polarization (IP) imaging, an extension of the ERT method, which permits to gain information about electrical conductive and capacitive (i.e., polarization effect) properties of the subsurface. As the polarization effect is mainly controlled by surface charge, which is large in clays, IP images are expected to improve the lithological interpretation and overcome the limitations of the ERT method. Additionally, measurements collected over a broad frequency bandwidth, the so-called spectral IP (SIP), have been successfully used in laboratory experiments to quantify textural and hydrogeological parameters. However, the application of SIP field measurements for the delineation of hydrogeological structures in landslides has not been addressed to date. To fill this gap, in this study we present SIP imaging results for data collected at the La Valette landslide (South East French Alps), where an existing geotechnical model of the landslide is available for evaluation. Moreover, our study provides a detailed revision on the collection and processing of SIP datasets, as well as a description of the diverse sources of error in IP surveys, to stress the importance of data-error quantification for a quantitative application of the SIP method. Our results demonstrate that adequate data processing allows obtaining consistent results at different frequencies and independently of the measuring protocol. Furthermore, the frequency dependence of the complex conductivity obtained in the field-scale SIP survey is consistent with earlier laboratory experiments. In conclusion, our study shows the potential of the SIP method to improve our understanding of subsurface properties, and an improved delineation of the contact between the mobilized material and the bedrock as well as variations in the clay content within the landslide and the bedrock.
Efficient monitoring of soil moisture is becoming increasingly important. To understand soil–plant–water dynamics, we evaluate the potential of using a multiple‐coil‐array electromagnetic induction instrument and inversion software to map soil moisture beneath an olive tree. On twelve different days, we collected apparent electrical conductivity (ECa) data using a DUALEM‐21S and the volumetric soil moisture (θ) using a bank of soil moisture sensors on opposite sides of the tree. Using EM4Soil, we inverted the ECa data on five of the days and established a site‐specific calibration between estimates of true electrical conductivity (σ) and θ. The strongest calibration relationship between σ and θ (R² = 0.65) was obtained for a full‐solution, S2 algorithm and damping factor of 1.2. A leave one out cross‐validation (LOOCV) showed the calibration was robust, with a root mean square error (RMSE) of 0.046 m³/m³, a mean error (ME) of 0.001 m³/m³ and a Lin's concordance of 0.72. We subsequently evaluated the calibration relationship on the seven remaining days and over a drying period of 120 days. This approach provides information about the temporal evolution of θ by a LOOCV of validation with a RMSE of 0.037, ME of −0.003 and a Lin's concordance of 0.54. Improvement could be achieved by aligning the DUALEM‐21S in the same orientation as the sensors, with time‐lapse inversion also being advantageous.
Quick, reliable and accurate estimates of soil water content (SWC) at intermediate (slope) to larger scale (catchment) are important for understanding hydrological processes and may be provided by electromagnetic induction (EMI). EMI measures the apparent electrical conductivity of the subsurface (ECapp) which represents a depth weighted average value of the bulk soil electrical conductivity (ECb). The relation between ECb and SWC has generally been investigated in soil cores or using local measurements of SWC and ECb. Studies that investigated the relation between ECapp measured with EMI and SWC in considerably larger and internally more heterogeneous support volumes are far scarcer and cover a limited range of environments with a limited range of factors contributing to ECapp. This study developed a new calibration method to obtain quantitative estimates of SWC using EMI measured ECapp data in a sub-tropical region in Southern Brazil at sites with different soil properties. SWC and ECb were measured in soil pits with Time Domain Reflectometry (TDR) probes. Collocated ECapp was simultaneously measured with EMI using different coil separations and orientations to measure over increasing sensing volume. EMI measured ECapp data were first calibrated against calculated ECapp, which were derived from ECb profiles inserted in an exact EMI forward model. A depth averaged SWC (SWCavg) was calculated and different calibrations that relate ECapp to SWCavg were evaluated. ECapp measurements of the deeper sensing coil configurations could predict best the variability of SWCavg using a non-linear relation. Spatio-temporal variations of pore water electrical conductivity (ECw) were found to be an important cofounding factor. Temporal variations of ECw and the small temporal variability of SWCavg prevented the prediction of temporal variability of SWCavg using ECapp measurements. Overall, the combination of both calibration steps resulted in the description of 83% of the spatial variability of SWCavg from ECapp measurements.
Together, the three particle size fractions (PSFs) of clay, silt, and sand are the most fundamental soil properties because the relative abundance influences the physical, chemical, and biological activities in soil. Unfortunately, determining PSFs requires a laboratory method which is time-consuming. One way to add value is to use digital soil mapping, which relies on empirical models, such as multiple linear regression (MLR), to couple ancillary data to PSFs. This approach does not account for the special requirements of compositional data. Here, ancillary data were coupled, via MLR modelling, to additive log-ratio (ALR) or isometric log-ratio (ILR) transformations of the PSFs to meet these requirements. These three approaches (MLR vs . ALR-MLR and ILR-MLR) were evaluated along with the use of different ancillary data that included proximally sensed gamma-ray spectrometry, electromagnetic induction, and elevation data. In addition, how the prediction might be improved was examined using ancillary data that was measured on transects and was compared to data interpolated from transects spaced far apart. Although the ALR-MLR approach did not produce significantly better results, it predicted soil PSFs that summed to 100 and had the advantage of interpreting the ancillary data relative to the original coordinates ( i.e. clay, silt, and sand). For the prediction of PSFs at various depths, all ancillary data were useful. Elevation and gamma-ray data were slightly better for topsoil and elevation and electromagnetic (EM) data were better for subsoil prediction. In addition, a smaller transect spacing (26 m) and number of samples (9–16) might be adopted for mapping soil PSFs and soil texture across the study field. The ALR-MLR approach can be applied elsewhere to map the spatial distribution of clay minerals.
Induced polarization well logging can be used to characterize sedimentary formations and their petrophysical properties of interest. That said, nothing is really known regarding the complex conductivity of low-porosity sedimentary rocks. To fill this gap of knowledge, we investigate the complex conductivity of 19 tight sandstones, one bioclastic turbidite, and four sand/smectite mixes. The sandstones and the bioclastic turbidite are characterized by low to very low porosities (in the range of 0.8%-12.3%) and a relatively narrow range of cation exchange capacity (CEC - 5-15 meq/100 g). The sand-clay mixtures are prepared with pure smectite (Na-Montmorillonite, porosity approximately 90%, CEC 75 meq/100 g) and a coarse sand (grain size approximately 500 μm). Data quality is assessed by checking that the percentage frequency effect between two frequencies separated by a decade is proportional to the value of the phase lag measured at the geometric frequency. We also checked that the normalized chargeability determined between two frequencies is proportional to the quadrature conductivity at the geometric mean frequency. Our experimental results indicate that the surface conductivity, the normalized chargeability, and the quadrature conductivity are highly correlated to the ratio between the CEC and the bulk tortuosity of the pore space. This tortuosity is obtained as the product of the (intrinsic) formation factor with the (connected) porosity. The quadrature conductivity is proportional to the surface conductivity. All these observations are consistent with the predictions of the dynamic Stern layer model, which can be used to assess the magnitude of the polarization associated with these porous media over the full range of porosity. The next step will be to extend and assess this model to partially saturated sandstones.
The complex conductivity of soil remains poorly known despite the growing importance of this method in hyrogeophysics. In order to fill this gap of knowledge, we investigate the complex conductivity of 71 soils samples (including 4 peat samples) and one clean sand in the frequency range 0.1 Hertz to 45 kHz. The soil samples are saturated with 6 different NaCl brines with conductivities (0.031, 0.53, 1.15, 5.7, 14.7, and 22 S m−1, NaCl, 25°C) in order to determine their intrinsic formation factor and surface conductivity. This dataset is used to test the predictions of the dynamic Stern polarization model of porous media in terms of relationship between the quadrature conductivity and the surface conductivity. We also investigate the relationship between the normalized chargeability (the difference of in-phase conductivity between two frequencies) and the quadrature conductivity at the geometric mean frequency. This dataset confirms the relationships between the surface conductivity, the quadrature conductivity, and the normalized chargeability. The normalized chargeability depends linearly on the cation exchange capacity and specific surface area while the chargeability shows no dependence on these parameters. These new data and the dynamic Stern layer polarization model are observed to be mutually consistent. Traditionally, in hydrogeophysics, surface conductivity is neglected in the analysis of resistivity data. The relationships we have developed can be used in field conditions to avoid neglecting surface conductivity in the interpretation of DC resistivity tomograms. We also investigate the effects of temperature and saturation and, here again, the dynamic Stern layer predictions and the experimental observations are mutually consistent.
We investigate the relationship between complex conductivity spectra and both permeability and pore mean size and distribution of 22 core samples (21 volcanic rocks and 1 clayey sandstone). The volcanic core samples were extracted from awellbore drilled for theHumu'ula Groundwater Research Project in the Humu'ula saddle region between Mauna Kea and Mauna Loa volcanoes (Hawaii). The quadrature conductivity spectra of volcanic rocks exhibit a subtle, but generally detectable, relaxation frequency in the range 0.3 Hz to 45 kHz similar to the relaxation frequency observed for clayey sandstones. We find a fair relationship between this relaxation frequency and the pore size determined by mercury porosimetry. Combined with the intrinsic formation factor of the core samples, the relaxation frequency can be used as an indicator of the permeability of the material. The predicted values of the permeability are grossly consistent with the permeability values to air (in the range 0.001-100 mD) within two orders of magnitude. The measured permeability values are highly correlated to the peak of the pore size distribution determined from mercury porosimetry divided by the intrinsic formation factor. By fitting the complex conductivity spectra with the pore size distribution, we obtain the normalized chargeability of the core samples, which is, in turn, highly correlated to the measured cation exchange capacity. © The Authors 2016. Published by Oxford University Press on behalf of The Royal Astronomical Society.
We performed complex conductivity measurements on 28 core samples from the hole drilled for the Humu'ula Groundwater Research Project (Hawai'i Island, HI, USA). The complex conductivity measurements were performed at 4 different pore water conductivities (0.07, 0.5, 1.0 or 2.0, and 10 S m⁻¹ prepared with NaCl) over the frequency range 1 mHz to 45 kHz at 22 ± 1 °C. The in-phase conductivity data are plotted against the pore water conductivity to determine, sample by sample, the intrinsic formation factor and the surface conductivity. The intrinsic formation factor is related to porosity by Archie's law with an average value of the cementation exponent m of 2.45, indicating that only a small fraction of the connected pore space controls the transport properties. Both the surface and quadrature conductivities are found to be linearly related to the cation exchange capacity of the material, which was measured with the cobalt hexamine chloride method. Surface and quadrature conductivities are found to be proportional to each other like for sedimentary siliclastic rocks. A Stern layer polarization model is used to explain these experimental results. Despite the fact that the samples contain some magnetite (up to 5 per cent wt.), we were not able to identify the effect of this mineral on the complex conductivity spectra. These results are very encouraging in showing that galvanometric induced polarization measurements can be used in volcanic areas to separate the bulk from the surface conductivity and therefore to define some alteration attributes. Such a goal cannot be achieved with resistivity alone. © The Authors 2016. Published by Oxford University Press on behalf of The Royal Astronomical Society.
Upscaling and/or estimating saturated hydraulic conductivity Ksat at the core scale from microscopic/macroscopic soil characteristics has been actively under investigation in the hydrology and soil physics communities for several decades. Numerous models have been developed based on different approaches, such as the bundle of capillary tubes model, pedotransfer functions, etc. In this study, we apply concepts from critical path analysis, an upscaling technique first developed in the physics literature, to estimate saturated hydraulic conductivity at the core scale from microscopic pore throat characteristics reflected in capillary pressure data. With this new model, we find Ksat estimations to be within a factor of 3 of the average measured saturated hydraulic conductivities reported by Rawls et al. (1982) for the eleven USDA soil texture classes.
Low-frequency quadrature conductivity spectra of siliclastic materials exhibit typically a characteristic relaxation time, which either corresponds to the peak frequency of the phase or the quadrature conductivity or a typical corner frequency, at which the quadrature conductivity starts to decrease rapidly towards lower frequencies. This characteristic relaxation time can be combined with the (intrinsic) formation factor and a diffusion coefficient to predict the permeability to flow of porous materials at saturation. The intrinsic formation factor can either be determined at several salinities using an electrical conductivity model or at a single salinity using a relationship between the surface and quadrature conductivities. The diffusion coefficient entering into the relationship between the permeability, the characteristic relaxation time and the formation factor, takes only two distinct values for isothermal conditions. For pure silica, the diffusion coefficient of cations, like sodium or potassium, in the Stern layer is equal to the diffusion coefficient of these ions in the bulk pore water, indicating weak sorption of these couterions. For clayey materials and clean sands and sandstones whose surface have been exposed to alumina (possibly iron), the diffusion coefficient of the cations in the Stern layer appears to be 350 times smaller than the diffusion coefficient of the same cations in the pore water. These values are consistent with the values of the ionic mobilities used to determine the amplitude of the low and high-frequency quadrature conductivities and surface conductivity. The database used to test the model comprises a total of 202 samples. Our analysis reveals that permeability prediction with the proposed model is usually within an order of magnitude from the measured value above 0.1 mD. We also discuss the relationship between the different time constants that have been considered in previous works as characteristic relaxation time, including the mean relaxation time obtained from a Debye decomposition of the spectra and the Cole-Cole time constant. This article is protected by copyright. All rights reserved.
Complex resistivity spectra, in the frequency range 0.001 to 10 hz, have been obtained through computer analysis of waveforms tape recorded at porphyry copper deposits. When care is taken to avoid distortions caused by the geometric effects of electrical inhomogeneities, the spectra of typical porphyry copper mineralization are remarkably uniform in character. The geometric effects of veins, which generally complicate the response of hand samples, can be reduced through in‐situ measurements using electrode separations of a few meters. In the frequency range of interest for inducedpolarization (IP) exploration, the observed spectra are accurately described by
ρ ( ω ) = K ( j ω ) - b ,
where K is a constant and b is a positive fraction. The fraction b has a value less than 0.1 and is a complete measure of IP. In the frequency range of interest, Laplace transformation gives the step response to be approximated by ρ ( t ) = Kt b .
Typical current waveforms used in IP prospecting can be synthesized by superposition of steps. Using these frequency and time response functions, percent frequency effect, phase, and pulse‐transient parameters are compared as measures of IP. By using a volume distribution function, it is shown that a distribution of lossy capacitors will explain the observed response. Physically, this might correspond to the double‐layer capacitance of metallic particles in mineralized rock.
Spectral-induced polarization (SIP) is widely used for environmental and engineering geophysical prospecting and hydrogeophysics, but one major limitation concerns the electromagnetic (EM) coupling effect. The phase angles related to EM coupling may increase even at frequencies as low as 1 Hz, depending on the ground resistivity, the array type, and the geometry. Most efforts to understand and quantify the EM coupling problem (e.g., theory and computer codes) have been developed for dipole-dipole arrays. However, we used a Schlumberger array to acquire SIP data. We found that with this array, the use of an appropriate cable arrangement during data acquisition can reduce EM coupling effects in the same proportion as for the use of a dipole- dipole array, which is the pure response of the studied earth. To measure the influence of the cable layout, four cable configurations with the same electrode spacing were compared for modeling and experimental data. We discovered that the classical DC inline array was the worst one. As soon as the cables were arranged in another shape (triangle or rectangle), the coupling effect decreased significantly. The best configuration we checked was the rectangular one with an acquisition unit located at a lateral offset of 100 m from the electrode line, even if there was still some difference between the modeled and measured data.
Petrophysical interpretation of resistivity measurements is often hindered by the dependence of resistivity on the interconnected pore fluids and the interconnected pore surfaces. Induced polarization (IP) measurements yield parameters that are only controlled by the interconnected pore surfaces, thereby offering the opportunity to constrain interpretation of resistivity measurements. Using a database composed of 63 sandstone and unconsolidated sediment samples covering nine independent investigations, we identified a strong linear relationship between the real part of surface conductivity (σ'surf ) determined from multisalinity (σw) resistivity measurements and the imaginary conductivity (σ") measured with IP at a frequency of about 1 Hz. We found σ"/σ'surf = l = 0.042 with a coefficient of determination (R2) of 0.911 and a standard deviation of l of 0.022. We found a similar relation when the normalized chargeability (from Debye decomposition) of the frequency dependence of the IP response is used instead of σ'. By estimating the true formation factor (F) recorded at high salinity, we solved for σ'surf(σw) and found that it parallels the salinity dependency of the imaginary conductivity, σ"(σw), as reported in recent studies. We also found that the value of the l determined from this experimental study was generally consistent with predictions of the POLARIS model when the mobility of the ions in the Stern layer was assumed to be 1/350 of the mobility of the ions in the diffuse layer (considered equal to the mobility of the ions in the bulk solution).We discovered how the identified relationship can be used to significantly improve (1) the estimation of the true formation factor and (2) the groundwater conductivity, from a single salinity resistivity measurement when an IP measurement is also made. The approach offers an opportunity to improve estimation of porosity, formation factor, and salinity in well logging and hydrogeophysical investigations.
Electric polarization is described as the sum of charge ac-cumulations (free charge density) and orientation of polar molecules such as those of bound and free water molecules (bound charge polarization). Charge accumulation in porous materials cannot be described with Ohm's law alone. Non-equilibrium thermodynamics or the upscaling of the local Nernst-Planck equation imply that the drift of ions in porous media is controlled by the gradient of their electrochemical potentials and not solely by the electric field. In porous me-dia, electrochemical capacitance is at least six to eight orders of magnitude larger than electrostatic capacitance associated with bound charge polarization. In other words, the low-frequency (<1 kHz) effective permittivity entering Am-père's law is six to eight orders of magnitude larger than high-frequency dielectric permittivity (measured for in-stance at 1 GHz). Low-frequency polarization of porous me-dia, with no metallic particles (no electronic conductors and semiconductors) is controlled by polarization of the inner component of the electrical double layer coating the grains. This layer, called the "Stern layer," plays a strong role in defining the cation exchange capacity of a material. A polarization model based on the polarization of the Stern layer explains a large number of experimental observations and could be used in the interpretation of hydro-and petro-leum geophysical measurements.
This book is the first to cover the fundamentals of hydrogeophysics from
both the hydrogeological and geophysical perspectives. Authored by
leading experts and expert groups, the book starts out by explaining the
fundamentals of hydrological characterization, with a focus on
hydrological data acquisition, hydrological measurement analysis, and
geostatistics. The fundamentals of geophysical characterization are then
presented, with a focus on electrical resistivity, induced polarization,
electromagnetic, GPR and seismic methods.
High-resolution X-ray Computed Tomography (HRXCT) or micro-CT (μCT) is a frequently used non-destructive 3D imaging and analysis technique for the investigation of internal structures of a large variety of objects, including geomaterials. Although the possibilities of X-ray micro-CT are becoming better appreciated in earth science research, the demands on this technique are also approaching certain physical limitations. As such, there remains a lot of research to be done in order to solve all the technical problems that occur when higher demands are put on the technique. In this paper, a review of the principle, the advantages and limitations of X-ray CT itself are presented, together with an overview of some current applications of micro-CT in geosciences. One of the main advantages of this technique is the fact that it is a non-destructive characterization technique which allows 4D monitoring of internal structural changes at resolutions down to a few hundred nanometres. Limitations of this technique are the operator dependency for the 3D image analysis from the reconstructed data, the discretization effects and possible imaging artefacts. Driven by the technological and computational progress, the technique is continuously growing as an analysis tool in geosciences and is becoming one of the standard techniques, as is shown by the large and still increasing number of publications in this research area. It is foreseen that this number will continue to rise, and micro-CT will become an indispensable technique in the field of geosciences.
We present a comprehensive review of methods to measure soil water content with ground penetrating radar (GPR). We distinguish four methodologies: soil water content determined from reflected wave velocity, soil water content determined from ground wave velocity, soil water content determined from transmitted wave velocity between boreholes, and soil water content determined from the surface reflection coefficient. For each of these four methodologies, we discuss the basic principles, illustrate the quality of the data with field examples, discuss the possibilities and limitations, and identify areas where future research is required. We hope that this review will further stimulate the community to consider ground penetrating radar as one of the possible tools to measure soil water content.
Equations have been developed that relate induced polarization (IP) in shaly sands to measurable petrophysical parameters. The induced-polarization process has been modeled in terms of two mechanisms: clay counterion displacement and membrane blockage. The resulting equations can be used to determine shaliness, brine conductivity, and oil saturation from in-phase and out-of-phase conductivities. Laboratory measurements have confirmed the IP dependence on these variables, as well as on temperature. Refs.
Among numerous methods for cation exchange capacity (CEC) determination for soils and sediments, the cobaltihexamine chloride method is frequently used due to its ability to measure CEC at soil pH. After exchange with Co(NH3)63+ ions, CEC is estimated via the measurement of the Co remaining in solution. The modified method proposed allows a more rapid determination of CEC based on the measurement of the absorbance at 472nm of the cobaltihexamine chloride solution before and after exchange. This method has been applied to various soil's horizons from four sites, selected to cover a wide range of CEC and pH values. The model obtained allows one to calculate CEC from absorbance at 472nm with 95% confidence intervals. As CEC is of relevant meaning in agronomical and environmental purposes, and more recently in ecotoxicological studies, this modified method can be proposed as a rapid test for CEC evaluation.
A model to predict the moisture characteristic of a soil from its particle‐size distribution, bulk density, and particle density parameters is presented. The model first translates a particle‐size distribution into a pore‐size distribution. Then, the cumulative pore volumes corresponding to progressively increasing pore radii are divided by the sample bulk volume to give the volumetric water contents, and the pore radii are converted to equivalent soil water pressures using the equation of capillarity.
To compute the pore volumes and the pore radii, the particle‐size distribution curve is divided into a number of segments. The solid mass in each segment is assumed to form a matrix with a bulk density equal to that of a natural‐structure sample. For a unit of sample mass, an equivalent pore volume for each segment is computed from V vi = ( W i /ϱ p )e and the corresponding pore radius from: r i = R i [4 en i (1‐α) /6] 1/2 , where V vi is the pore volume, W i is the solid mass, ϱ p is the particle density, e is the void ratio, r i is the mean pore radius, R i is the mean particle radius, n i is the number of particles, and α is an empirical constant ranging in value from 1.35 to 1.40. The formulation for the pore radius is based on spherical particles and cylindrical pores.
Model predictions for several soil materials show close agreement with the experimental data.