Search

Spectral induced electric polarization provides useful information on geoelectric parameters, which may allow mineral discrimination in exploration geophysics. Electromagnetic coupling between current and potential electrodes equally contributes to understanding the geology but represents the main source of noise in spectral induced electric polarization surveys. In this article, we propose an integrated analysis of these two phenomena in the context of complex resistivity data in order to evaluate their role in exploration geophysics taking into account their specificities. Deepening previous studies of forward modeling and inversion of mutual impedance in the frequency domain, in this present approach, we have employed an expanded apparent complex resistivity function, which includes the inductive effect in its parameters. We developed an integrated inversion of this function using the very fast simulated annealing and least-squares methods to estimate the geoelectric parameters. We also performed tests and applications on synthetic and real data, discussing the benefits of using the proposed complex resistivity model. The approach was applied to data from three profiles of the Copper District of Vale do Curaca, Bahia, Brazil. The results show that it is possible to distinguish between spectral induced polarization and induction in terms of geoelectric parameters distributed in pseudosections and to identify patterns and behaviors for each of them. They constitute an important step toward mineral discrimination via analysis of analogous circuit parameters, a fundamental tool in searching metallic ores.

Electrical geophysical prospecting methods are widely used at different stages of geological exploration. In the last two decades, new computer technologies and satellite navigation systems were successfully introduced in the geophysical industry. As a result, exploration technologies have improved, and new geophysical methods have been developed, such as electrical resistivity tomography (ERT) and spectral induced polarization (SIP) methods. An important role in ore geophysics is played by magnetotelluric (MT) methods. In this article, we focus on the issues of methodology and interpretation of electrical prospecting data for solving ore exploration problems. Special attention is paid to the induced polarization (IP) method that is most widely used in mineral exploration and mining industry as one of the most important and most dynamically developing techniques of ore geophysics. In addition, the issues of correct choices of survey scales and the use of automatic 2D and 3D inversion programs are considered.

In spread spectrum induced polarization (SSIP) data processing, attenuation of background noise from the observed data is the essential step that improves the signal-to-noise ratio (SNR) of SSIP data. The traditional correlation identification (TCI) algorithm has been proposed to improve the SNR of these data. However, signal processing in background noise is still a challenging problem. We propose an enhanced correlation identification (ECI) algorithm to attenuate the background noise. In this algorithm, the cross-correlation matching method is helpful for the extraction of useful components of the raw SSIP data and suppression of background noise. Then the formula of the TCI algorithm is used for identifying the frequency response of the observation system. Even when the signal to noise ratio (SNR) is −37.5 dB, this ECI algorithm can still be able to keep 3.0 % relative error. Experiments on both synthetic and real SSIP data show that the ECI algorithm can not only suppress the background noise but also better preserves the valid information of the raw SSIP data to display the actual location and shape of adjacent high resistivity anomalies, which can improve subsequent steps in SSIP data processing and imaging.

Spectral induced polarization (SIP) and bender element (BE) techniques show a high sensitivity to particle size, particle distribution and content of generated hydration products, which essentially govern the efficiency of ground improvement. In this context, both SIP and BE were integrated on a column setup to monitor the processes of lime and cement stabilization. A 5 mmol/L Na2CO3 solution was injected into the sand-lime mixture to produce CaCO3 precipitation, while deionized water was injected into the sand-cement mixture to induce the hydration of cement. The average diameters of the precipitated particles or clusters were calculated from the relaxation time, which was a significant parameter of SIP signals, via the Schwarz equation. Two pairs of BE were used to demonstrate the heterogeneity of the product precipitation, which was probably caused by the location of the inflow and outflow on the SIP-BE column. SIP and BE showed the capability of nondestructively monitoring the spatiotemporal chemical evolution processes, which could be helpful for engineering applications.

The spectral induced polarization (SIP) method has emerged as a well-suited laboratory technique to characterize hydrogeologic and biogeochemical parameters in soil samples. However, field applications of the SIP imaging method are still rare, which can be attributed to the particular care required to minimize the contamination of the data by electromagnetic coupling. To date, field procedures rely on the use of two different cables separating the current and potential dipoles to improve the quality of the SIP readings, although this increases the efforts in the field and might reduce the depth of investigation or the spatial resolution of the data. To overcome these limitations, we have investigated the use of a single coaxial cable, as an alternative to improve data quality and simplify field procedures. We evaluate SIP imaging data collected with the same measuring device using a coaxial cable and a combination of multicore cables of different lengths and manufacturers. Data sets collected with a single coaxial cable reveal a significantly lower number of outliers and a high spatial consistency between the phase-lag readings, even for measurements collected with a coaxial cable five times longer than the length of the profile. Furthermore, the data collected with coaxial cables reveal an improved quality for deeper measurements (with a lower signal-to-noise ratio) in comparison to data sets collected with separate cables. Our results demonstrate that the use of coaxial cables might permit the collection of SIP readings with high quality and similar field procedures to those used in resistivity surveys.

SUMMARY Electrically conductive minerals (e.g. graphite, pyrite, chalcopyrite and magnetite) occur in the various geological contexts. They might represent economic resources or serve as indicators of such resources. In addition, they can be sources of contamination of soil and groundwater. Therefore, characterization of rocks containing electrically conductive inclusions is an important task in many sectors of science and economy. We conducted laboratory measurements to study the impact of the shape, composition, size and passivation character of electrically conductive inclusions on the induced polarization (IP) parameters. This paper presents results of time-domain IP measurements performed on 22 synthetic models, which were made of sieved sand mixed with electrically conductive particles. We carried out the IP measurements while varying orientation of the electrical field relative to the long axis of the inclusions. We found that the total chargeability of the models (M) strongly depended on the volumetric content (ξ), shape and characteristic size (l) of the inclusions. It also depended on the angle between the electrical field direction and the orientation of the long axis of the inclusions (α), which made the models anisotropic. Experimental relationships between M, ξ, l and α were found consistent with predictions of the generalized Maxwell–Garnett mixing equation in the tensor form. In contrast to M, the relaxation time (τ) of the studied models was almost independent of $\alpha $. Exceptions were the models with cylindrical electrically conductive particles, which showed a strong relationship between τ and $\ \alpha $. Despite the previous assumptions, no unique relationship between τ and the characteristic length of electrically conductive inclusions was observed. However, for all particle shapes, $\tau $ was proportional to the surface area of the particles. We also studied how passivated areas on the surface of the inclusions modified the spectral IP parameters. We found that passivation of electrically conductive inclusions led to anisotropy of M, while τ remained almost unaffected by the orientation of the polarizing field. Based on the experimental data, we show that the polarization magnitude of electrically conductive inclusions is proportional to the normal component of the electrical current density on their surface. We also show that the relaxation time is proportional to the area of the active surface of the inclusions. The obtained relationships highlight the importance of the interfacial polarization mechanism of the electrically conductive inclusions.

Heterogeneity in either chemically or microbiologically induced carbonate-based ground improvement methods is a major obstacle in engineering application. Spectral induced polarization (SIP), an innovative and nondestructive method, which has demonstrated promise in monitoring microbial activity, was used in this study to monitor enzyme induced carbonate precipitation (EICP). The complex conductivities, together with the shear wave velocities (Vs), of an EICP modified sand were monitored using a self-developed spectral induced polarization–bender element column. The mean precipitate size was calculated by relaxation time (τ) and the Schwarz equation. The precipitate contents were calculated by cumulative gamma distribution function on the global polarization magnitude (mn) with R2 = 0.989. The stiffness of the enhanced geomaterial, in terms of Vs, correlates to mn with a cumulative lognormal distribution function with R2 = 0.967. Contact cementation was postulated as the dominant association pattern. The possible mechanism for this may be the formation of eddies and the nucleation of CaCO3 crystals during precipitation. The results suggest that SIP can be used as an effective nondestructive monitoring tool to assess the stiffness of geomaterials.

The relaxation phenomena observed in the electrical low-frequency range (approximately 1 MHz–10 kHz) of natural porous media such as sandstones are often assumed to be directly related to the dominant (modal) pore throat sizes measured, for instance, with mercury intrusion porosimetry. Attempts to establish a universally valid relationship between pore size and peak spectral induced polarization (SIP) relaxation time have failed, considering sandstones from very different origins and featuring great variations in textural and chemical compositions as well as in geometric pore space properties. In addition working with characteristic relaxation times determined in Cole-Cole or Debye decomposition fits to build the relationship have not been successful. In particular, samples with narrow pore throats are often characterized by long SIP relaxation times corresponding to long “characteristic length scales” in these media, assuming that the diffusion coefficients along the electrical double layer were constant. Based on these observations, three different types of SIP relaxation can be distinguished. We have developed a new way of assessing complex pore spaces of very different sandstones in a multimethodical approach to combine the benefits of mercury intrusion porosimetry, micro-computed tomography, and nuclear magnetic resonance. In this way, we achieve much deeper insight into the pore space due to the different resolutions and sensitivities of the applied methods to pore constrictions (throats) and wide pores (pore bodies). We experimentally quantify pore aspect ratios and volume distributions within the two pore regions. We clearly observe systematic differences between three SIP relaxation types identified previously, and we can attribute the SIP peak relaxation times to measured characteristic length scales within our materials. We highlight selected results for a total of nine sandstones. It seems that SIP relaxation behavior depends on the size difference of the narrow pore throats to the wide pore bodies, which increases from SIP type 1 to type 3.

In this work, we explore the use of biochar as a remediation agent, and the sensitivity of the spectral‐induced polarization method as a remediation monitoring aid. Biochar amended columns were fully saturated with industrial wastewater (olive oil mill waste) with very high concentration of phenols (∼2485 mg/L) and other substances. The biochar‐amended columns achieved very high removal rates of phenols compared to the control (sand only). Geophysical monitoring over the duration of the experiment (10 days) showed changes in the spectral‐induced polarization signal (imaginary conductivity) consistent with phenol removal as confirmed by geochemical monitoring. This experiment confirmed the utility of biochar as a remediation agent. Furthermore, spectral‐induced polarization can serve as long‐term, high resolution, monitoring aid in organic contaminant degradation processes.

Spectral induced polarization (SIP) measurements were collected at the Lapires talus slope, a long-term permafrost monitoring site located in the Western Swiss Alps, to assess the potential of the frequency dependence (within the frequency range of 0.1–225 Hz) of the electrical polarization response of frozen rocks for an improved permafrost characterization. The aim of our investigation was to (a) find a field protocol that provides SIP imaging data sets less affected by electromagnetic coupling and easy to deploy in rough terrains, (b) cover the spatial extent of the local permafrost distribution, and (c) evaluate the potential of the spectral data to discriminate between different substrates and spatial variations in the volumetric ice content within the talus slope. To qualitatively assess data uncertainty, we analyze the misfit between normal and reciprocal (N&R) measurements collected for all profiles and frequencies. A comparison between different cable setups reveals the lowest N&R misfits for coaxial cables and the possibility to collect high-quality SIP data in the range between 0.1–75 Hz. We observe an overall smaller spatial extent of the ice-rich permafrost body compared to its assumed distribution from previous studies. Our results further suggest that SIP data help to improve the discrimination between ice-rich permafrost and unfrozen bedrock in ambiguous cases based on their characteristic spectral behavior, with ice-rich areas showing a stronger polarization towards higher frequencies in agreement with the well-known spectral response of ice.

International audience; Spectral induced polarization (SIP) has shown potential to detect subsurface reactive transport processes. However, previous laboratory experiments could not associate the SIP response with the temporal development and spatial distribution of reactive transport processes in detail due to the opaque nature of porous media. We developed a novel experimental setup that consists of a millimeter scale 2D transparent porous medium and electrodes for SIP measurements to visually observe reactive transport processes down to the pore scale while performing SIP measurements (i.e. a 2D millifluidic setup). We used this setup to investigate the SIP response of calcite precipitation (CaCO₃). The image analysis and SIP measurements highlight that the SIP response does not necessarily represent the total mass of calcite precipitation in the measurement region. In addition, we numerically simulated the electric field distribution based on the observed calcite precipitation. The results suggested that the heterogeneous distribution of calcite precipitation within the investigated volume had an important influence on the SIP response. Given the heterogeneous and localized nature of many reactive processes, these findings clearly demonstrate the importance of investigating sub-resolution reactive processes for SIP measurements. The proposed novel experimental setup was shown to be suitable for this purpose.

The reliable detection of subsurface ice using non-destructive geophysical methods is an important objective in permafrost research. Furthermore, the ice content of the frozen ground is an essential parameter for further interpretation, for example in terms of risk analysis, e.g. for the description of permafrost carbon feedback by thawing processes. The High-Frequency Induced Polarization method (HFIP) enables the measurement of the frequency dependent electrical signal of the subsurface. In contrast to the well-established Electrical Resistivity Tomography (ERT), the usage of the full spectral information provides additional physical parameters of the ground. As the electrical properties of ice exhibit a strong characteristic behaviour in the frequency range between 100 Hz and 100 kHz, HFIP is in principle suitable to estimate ice content. Here, we present methodological advancements of the HFIP method and suggest an explicit procedure for ice content estimation. A new measuring device, the Chameleon-II (Radic Research), was used for the first time. It was designed for the application of Spectral Induced Polarization over a wide frequency range and is usable under challenging conditions, for example in field sites under periglacial influence and the presence of permafrost. Amongst other improvements, compared to a previous generation, the new system is equipped with longer cables and larger power, such that we can now achieve larger penetration depths up to 10 m. Moreover, it is equipped with technology to reduce electromagnetic coupling effects which can distort the desired subsurface signal. The second development is a method to estimate ice content quantitatively from five Cole-Cole parameters obtained from spectral two-dimensional inversion results. The method is based on a description of the subsurface as a mixture of two components (matrix and ice) and uses a previously suggested relationship between frequency-dependent electrical permittivity and ice content. Measurements on a permafrost site near Yakutsk, Russia, were carried out to test the entire procedure under real conditions at the field scale. We demonstrate that the spectral signal of ice can clearly be identified even in the raw data, and show that the spectral 2-D inversion algorithm is suitable to obtain the multidimensional distribution of electrical parameters. The parameter distribution and the estimated ice content agree reasonably well with previous knowledge of the field site from borehole and geophysical investigations. We conclude that the method is able to provide quantitative ice content estimates, and that relationships that have been tested in the laboratory may be applied at the field scale.

Breakthrough curves (BTCs) are a valuable tool for qualitative and quantitative examination of transport patterns in porous media. Although breakthrough (BT) experiments are simple, they often require extensive sampling and multi-component chemical analysis. In this work, we examine spectral induced polarization (SIP) signals measured along a soil column during BT experiments in homogeneous and heterogeneous soil profiles. Soil profiles were equilibrated with an NaCl background solution, and then a constant flow of either CaCl2 or ZnCl2 solution was applied. The SIP signature was recorded, and complementary ion analysis was performed on the collected outflow samples. Our results confirm that changes to the pore-water composition, ion exchange processes and profile heterogeneity are detectable by SIP: the real part of the SIP-based BTCs clearly indicated the BT of the non-reactive ions as well as the retarded BT of cations. The imaginary part of the SIP-based curves changed in response to the alteration of ion mobility around the electrical double layer (EDL) and indicated the initiation and the termination of the cation exchange reaction. Finally, both the real and imaginary components of the complex conductivity changed in response to the presence of a coarser textured layer in the heterogeneous profile.

SUMMARY We provide a two-stage approach to extract spectral induced polarization (SIP) information from time-domain IP data. In the first stage we invert dc data to recover the background conductivity. In the second, we solve a linear inverse problem and invert all time channels simultaneously to recover the IP parameters. The IP decay curves are represented by a stretched exponential (SE) rather than the traditional Cole–Cole model, and we find that defining the parameters in terms of their logarithmic values is advantageous. To demonstrate the capability of our simultaneous SIP inversion we use synthetic data simulating a porphyry mineral deposit. The challenge is to image a mineral body that is hosted within an alteration halo having the same chargeability but a different time constant. For a 2-D problem, we were able to distinguish the body using our simultaneous inversion but we were not successful in using a sequential (or conventional) SIP inversion approach. For the 3-D problem we recovered 3-D distributions of the SIP parameters and used those to construct a 3-D rock model having four rock units. Three chargeable units were distinguished. The compact mineralization zone, having a large time constant, was distinguished from the circular alteration halo that had a small time constant. Finally, to promote the use of the SIP technique, and to have further development of SIP inversion, all examples presented in this paper are available in our open source resources (https://github.com/simpeg-research/kang-2018-spectral-inducedpolarization).

Electrical impedance spectroscopy (EIS), also known as Spectral Induced Polarization (SIP) has proven to be a successful non-destructive technique able to characterize the chemical and physical properties of a complex structure (concrete, rocks, and soil) under various environmental conditions. Because this technique aims to measure small amplitude signals, it can easily be influenced by parasitic effects which are not representative of the intrinsic properties of the investigated material. The purpose of this paper is to validate the resolution of our experimental methodology as a preliminary step to ensure an accurate measurement of the bulk complex resistivity response of concrete samples within a wide frequency range (1.43 mHz–20 kHz). A cement mortar sample is first used as a homogeneous (isotropic) material to improve the sample holder design for reducing the errors and controlling the external parameters (such as coupling effect, geometric factor, contact impedance, electrode polarization) during the laboratory measurements. Also, the performance and installation of the measuring components such as the electrodes (current and potential) and the sample holder design are assessed. The specimen dimensions and designs are selected based on a sample standard size that will be used to determine the physico-mechanical properties of concrete (compressive strength, modulus of elasticity, ultrasonic pulse velocity, length variation). The examinations are performed in parallel with the SIP measurement on the same sample. The accuracy of our measurement setup is then validated using aluminum-bronze plates bonded to the sample using a conductive gel to transmit electric current with a density less than 10 - 3 A m−2. The results showed that by controlling the sponge moisture in the Ag/AgCl non-polarizable voltage electrodes, our SIP measurement system is able to measure the phase lag with an error smaller than 1 mrad over a frequency range from 1.43 mHz to 20 kHz.

SUMMARY Spectral induced polarization (SIP), describing the measurement of the frequency domain electrical impedance magnitude and phase of porous materials, has been widely used to characterize subsurface hydrological/biogeochemical properties and processes. SIP data collected at frequencies higher than 100 Hz are expected to describe the polarization of small particles providing insights into the physicochemical properties of clays, nanoparticles and microorganisms. However, the phase measurements at these high frequencies are often contaminated by errors due to the parasitic capacitive coupling of the SIP instrument, especially for lower conductivity samples. We developed a model showing the measured phase is the sum of the true sample phase and an error term ωCinZx, where ω is the angular frequency; Cin is the instrument input capacitance and Zx is a measurable impedance function related to the sample holder properties and the reference resistor. Based on this model, a new phase correction method is proposed that results in highly accurate SIP data up to 20 kHz as well as the determination of Cin. We tested the correction method using electric circuits, NaCl fluids and three unconsolidated samples (sand, sand-clay and sand-pyrite mixtures). The corrected phase for the circuit and NaCl fluid experiments showed excellent agreement with the theoretical phase response across the studied frequency range (errors

Applications of the spectral induced polarization (SIP) method to mineral exploration are limited by our knowledge of the relationships among rock texture, mineral composition, and electrical properties. Laboratory SIP responses were measured on rock samples from the Canadian Malartic gold deposit. Field SIP responses were also measured at the outcrop scale, along a profile that intersects a well-studied mineralized zone. The mineralogy and the texture of sedimentary rocks from this deposit were quantitatively determined with mineral liberation analysis. A systematic decrease (Pearson [Formula: see text]) in total chargeability with increasing fraction of the sulfide mineral interfaces associated with feldspar minerals (namely, K-feldspar and albite) was observed. On the other hand, total chargeability increased with the fraction of sulfide mineral interfaces associated with carbonates and micas (Pearson [Formula: see text]). At Canadian Malartic, proximal alteration in the mineralized zones is marked by rocks that lack a foliation plane and that were subjected to pervasive K-feldspar, albite, and pyrite alteration. In contrast, distal alteration in sedimentary rocks is marked by biotite, albite, carbonate, and pyrite that are oriented along the regional [Formula: see text] foliation. In the least-altered (LA) sedimentary rocks, quartz and biotite are associated with pyrrhotite and ilmenite as the main sulfide and oxide mineral phases, respectively. SIP measurements conducted at district and outcrop scales and along a drill core indicated that proximally altered sedimentary rocks were characterized by low total chargeability values ([Formula: see text] to [Formula: see text] in the laboratory and [Formula: see text] in the field). In contrast, the LA sedimentary rocks were characterized by total chargeability values up to [Formula: see text] in the laboratory and [Formula: see text] in the field. We conclude that mineralized zones associated with this type of ore deposit are characterized by low chargeability anomalies.

Degradation of organic chemicals in natural soils depends on oxidation-reduction conditions. To protect our groundwater resources we need to understand the degradation processes under anaerobic conditions . Available iron and manganese oxides are used as electron acceptors for anaerobic degradation and are reduced to the dissolved form of metallic cations in pore water . To monitor this process is a challenge, because anaerobic conditions are difficult to sample directly without introducing oxygen. A few studies have shown an impact of iron reduction on spectral induced polarisation (SIP) signature, often associated with bacterial growth. Our objective is to study the impact of iron and manganese oxide dissolution, caused by degradation of an organic compound, with spectral induced polarisation signatures. Twenty-six vertical columns (30 cm high, inner diameter 4.6 cm) were filled with a sand rich in oxides (manganese and iron) with a static water table in the middle. In half of the columns, a 2 cm high contaminated layer was installed just above the water table. As the contaminant degrades, the initial oxygen is consumed and anaerobic conditions form Every three days over a period of one month, spectral induced polarisation (twenty frequencies between 5mHz and 10 kHz) data were collected on six columns: three contaminated replicates and three control replicates. Chemical analysis was done on twenty columns assigned for destructive water sampling, ten contaminated columns and ten control. The results show an increase of the real conductivity associated with the degradation processes, independent of frequency. Compared with the pore water electrical conductivity in the saturated zone , the real conductivity measurement revealed the formation of surface conductivity before iron was released in the pore water. In parallel, we also observed an evolution of the imaginary conductivity in both saturated and unsaturated zones at frequencies below 1 Hz. Overall, the anaerobic reduction of iron and manganese oxide during the organic degradation increased both the conductive and polarisation component of the complex conductivity.

We investigated the relative effect of different bacterial biofilm components to influence spectral induced polarization (SIP) signals and Cole-Cole model parameters (i.e. normalized chargeability (S m−1) and relaxation time (τ)) in order to reduce the ambiguity associated with interpretation of SIP signatures in biostimulated environments. SIP measurements were collected over a frequency range, (i.e. 0.1 Hz–1 kHz) from experimental columns containing Pseudomonas aeruginosa PAO1 (non-mucoid bacterial strain) cell suspensions alone and cell suspensions mixed with different molecules associated with P. aeruginosa biofilm matrix (alginate, DNA and phenazine). Results from SIP measurements of cell density investigations suggest an increase in phase (~ 200%), imaginary conductivity (σ″ S m−1), and normalized chargeability (S m−1) values as cell densities increase (1.50–5.55 × 109 CFUml−1), yet no significant change in the real conductivity (σ′ S m−1) or relaxation time was observed. The addition of alginate resulted in a suppression of phase (φ mrads), imaginary conductivity (σ″ S m−1), and normalized chargeability (S m−1) response. However, a mixture of cells and alginate increased all parameters but with no significant changes in real conductivity (σ′ S m−1) or relaxation time. Similar trend was observed in the DNA treatments; increasing concentrations of DNA reduced the imaginary conductivity (σ″ S m−1), phase (φ mrads), and normalized chargeability (S m−1) response. In contrast, the addition of phenazine, a secondary metabolite of P. aeruginosa, resulted in a significant increase in both imaginary conductivity (σ″ S m−1), phase (φ mrads), and normalized chargeability (S m−1) values especially at larger concentrations (>50 mM). Our work suggests that cell density and metabolites are significant factors increasing the polarization response in biofilms and advances the interpretation of electrical signals associated with biofilms in the subsurface.

Dehydration is a common process occurring in natural environments, rocks, and building materials. Suitable methods for monitoring the changing moisture content should be identified. We have investigated the impact of dehydration of sandstones on complex conductivity spectra. Spectral induced polarization measurements were performed on five sandstone samples during evaporative drying. The complex conductivity spectra indicate considerable changes with decreasing saturation. The relationship between conductivity and saturation can be described by a power law known as the second empirical Archie equation. Separate saturation exponents have been determined for the real and imaginary parts of conductivity. The imaginary part of conductivity indicates higher saturation exponents for the investigated sandstones compared to the real part. Obviously, the saturation exponents depend on the method used for changing water saturation. Evaporative drying, which is used in our experiments, causes an increase of pore-water salinity and results in lower saturation exponents for the real part of conductivity but higher exponents for the imaginary part in comparison with the classical imbibition/drainage technique. We evaluate a theoretical approach that considers the influence of pore water salinity on the saturation exponents of the real and imaginary parts of conductivity. The complex conductivity spectra are processed by a Debye decomposition procedure. The resulting integrating parameters such as direct current resistivity, total chargeability, normalized chargeability, and mean relaxation time indicate a power law dependence on saturation. Our experiments indicate that the imaginary part of conductivity and normalized chargeability have a high sensitivity related to the drying process of rocks and might be suitable indicators to monitor the changes of moisture content in sandstones.

Spectral induced polarization (SIP) measurement requires synchronous current acquisition and real-timetransmission devices or precise phase-measuring apparatus. To carry out SIP investigations in the absence of these auxiliary devices, a new SIP measurement has been developed in this research. The new method uses high-accuracy phase-calculation technology based on all-phase spectrum analysis (APSA) to obtain the initial phase of the emission current and uses time information recorded in the file header of the voltage data file and the ‘Frequency-Time Table’ of the emission current to calculate the current phase information at any given moment. The phase-calculation method for the voltage data was the same as that used for the current initial phase. Subsequently, the phase of the complex resistivity was obtained by subtracting the current phase from the voltage phase, and the amplitude was obtained by normalization. Simulation tests showed that the algorithm was superior for absolute phase measurement compared with other spectrum analysis methods and confirmed that the accuracy of the new method was equal to that of the measurement that collects voltage and current synchronously. Indoor and field tests showed that the new APSA-based SIP method can successfully measure the complex resistivity without a synchronous current- or phase-measuring device. Moreover, this method reduces the requirement for synchronization of the transmitter and receiver.

Carbonate formations still present a challenging target with regard to reservoir characterization as they possess a wide range of pore types and sizes as well as interconnected and isolated pore space . In this study a set of micritic and oomoldic carbonate samples was investigated with three measurement techniques (magnetic resonance (NMR), mercury intrusion (MICP), and spectral induced polarization (SIP, in a frequency range from 5mHz to 100 Hz)) which provide information about a rock's pore space indirectly based on different physical principles. The aims of the study were to combine these pieces of information to characterize the pore space of mud-dominated and grain-dominated carbonate rocks and to investigate potential correlations between pore space properties and SIP-derived parameters. Pore body and pore throat size distributions were derived from NMR and MICP data, respectively, and interpreted in terms of dominant pore throat (rt) radius, pore body (rb) radius and the rb/rt ratio. For validation of the NMR-derivedrb distributions, scanning electron microscope images were used for the computation of image-derived pore size distributions. The SIP data were interpreted in terms of characteristic features for oomoldic and micritic samples. Phase shift, real ( σ′) and imaginary part (σ′′) of complex conductivity, respectively, were fitted with Cole-Cole models and the characteristic parameters relaxation time τ, chargeability m, and frequency dependence c were derived for each sample. Subsequently, these parameters were interpreted with regard to correlations with rb,rt, and rb/rt and specific surface Spor. The results indicate that grain-dominated carbonates possess comparatively large radii rb and rt and low Spor, which seems to go along with sharp phase peaks (i.e. high c values) and a low absolute phase level (and therefore low chargeability m). The mud-dominated carbonates, in contrast, possess relatively small radii rb and rt and high Spor which is obviously associated with broad phase peaks (i.e. low c values) and a comparatively high phase level resulting in high m values. τ, in contrast, was found to decrease with increasing ratio rb/rt and may therefore have similar values in both micritic and oomoldic samples. Consequently, the results of this study contribute to a better understanding of the petrophysical properties of carbonate rocks as a function of their pore space characteristics.

Groundwater contamination with iron caused by mining and landfill activities has fueled the development of remediation strategies. Permeable reactive barriers (PRBs) are commonly applied in subsurface remediation because of their high removal effect and low costs. Spectral induced polarization (SIP) technique has been approved for its nondestructive ability to monitor the geochemical processes in porous media. In this study, SIP technique was applied for monitoring iron remediation by limestone at column scale. The chemical analysis showed the pH of the porous fluid increased - attributed to the dissolution of limestone, which promoted the precipitation of iron. The precipitate phases included both γ-FeOOH and Fe2O3 based on X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) results. The micro computed tomography (CT) technique investigated the uneven distribution of the precipitates in the column, which indicated the existence of preferential flow. SIP signals revealed the quantity of the accumulated iron precipitates, which was proved by the chemical measurement and calculation. SIP signals also derived the time evolution of both the average precipitate size and size distribution, which elucidated the processes of precipitate crystal growth and aggregation during Fe flow-through. Above results suggest that SIP holds the promise of monitoring the engineering barrier performance.

International audience; Clay material characterization is of importance for many geo-engineering and environmental applications, and geo-electrical methods are often used to detect them in the subsurface. Spectral induced polarization (SIP) is a geo-electric method that non-intrusively measures the frequency dependent complex electrical conductivity of a material, in the mHz to the kHz range. We present a new SIP data set of four different types of clay (a red montmorillonite sample, a green montmorillonite sample, a kaolinite sample, and an illite sample) at five different salinities (initially de-ionized water, 10−3, 10−2, 10−1, and 1 mol/L of NaCl). We propose a new laboratory protocol that allows the repeatable characterization of clay samples. The complex conductivity spectra are interpreted with the widely used phenomenological double-Pelton model. We observe an increase of the real part of the conductivity with salinity for all types of clay, while the imaginary part presents a non-monotonous behavior. The decrease of polarization over conduction with salinity is interpreted as evidence that conduction increases with salinity faster than polarization. We test the empirical petrophysical relationship between the imaginary and real surface conductivities and validate this approach based on our experimental data and two other datasets from the literature. With this data set we can better understand the frequency-dependent electrical response of different types of clay. This unique data set of complex conductivity spectra for different types of clay samples is a step forward toward better characterization of clay formations in situ.

Warming of permafrost regions with an associated increase in subsurface temperatures has been reported worldwide. Thus, long-term monitoring of the thermal state of permafrost and the associated ground ice contents has become an essential task also for the European Alps. Geophysical methods have proven to be well-suited to support and interconnect spatially sparse borehole data and investigate the distribution and temporal evolution of permafrost. In particular, electrical resistivity tomography (ERT) is a widely applied technique for permafrost characterization, commonly associated with a significant increase in the electrical resistivity upon freezing. However, air is also characterized by high electrical resistivity values complicating the interpretation of ERT results. Recent studies have revealed that the spectral induced polarization (SIP) response of frozen rocks is affected by the temperature-dependent polarization behaviour of ice at higher frequencies. Thus, the SIP or complex resistivity method offers potential for an improved characterization of permafrost sites.We here present SIP imaging results conducted over a broad range of frequencies (0.1-225 Hz) at an operational long-term permafrost monitoring site covering a period of one and a half years. The selected study area Cervinia Cime Bianche (Italian Alps) is situated at an elevation of ~3100m and provides comprehensive geophysical, borehole temperature and water content data for validation. Shielded cables and an adequate measuring protocol were deployed to minimize the electromagnetic coupling in the SIP data. Data were collected as normal and reciprocal pairs for the quantification of data error, and we developed an analysis scheme for data quality that considers changes in time and in the frequency to remove spatial and temporal outliers and erroneous measurements. To understand the temperature dependence of the polarization response, we compare our field results with SIP laboratory measurements on water-saturated rock samples, collected in close proximity to the monitoring profile, in a frequency range of 10 mHz to 45 kHz during controlled freeze-thaw cycles (+20°C to -40°C).Our field results show clear seasonal changes in the complex resistivity images. Resistivity magnitude shows an increase in winter and decrease in summer throughout the image plane, with most prominent changes at shallow depths, where also resistivity phase shows distinctly increased (absolute) values in winter for frequencies above 10 Hz. This region coincides with the active layer as monitored by borehole temperature logging, suggesting that especially the polarization response is indicative of the seasonal freezing and thawing of the ground. This interpretation is confirmed by the laboratory measurements on the rock samples from the site, which upon freezing and thawing exhibit an absolute phase increase with decreasing temperature at higher frequencies (above 10 Hz for temperatures down to -10°C), with the general spectral behaviour being consistent with the known polarization properties of ice. We conclude that with appropriate measurement and processing procedures, the characteristic dependence of the SIP response of frozen rocks on temperature, and thus ice content, can be utilized in field surveys for an improved assessment of thermal state and ice content at permafrost sites.

Detailed information on the clay content of the subsurface and its spatial distribution plays a critical role in the interaction between surface- and groundwater. In this study, we investigate a new methodology to integrate data measured with electromagnetic and electrical geophysical methods, namely, the transient electromagnetic (TEM) and the spectral induced polarization (SIP) to quantify subsurface clay content in an imaging framework. The methodology is tested in data sets collected at a quarry close to Vienna and consists of a ca. 10 m thick clay layer below a ca. 8 m thick overburden of sandy silts. Our data set includes SIP data collected along a 315 m long profile with an electrode separation of 5 m in a frequency range from 0.1 to 225 Hz. Along this profile, we measured 26 TEM soundings using a 12.5 m loop with 24 windows recording in a time range between 4 and 140 μs. Ground truth information corresponds to grain size analysis conducted in 25 soil samples collected in a depth from 5 to 28 m. SIP inversion results at a single frequency provided structural a-priori information to improve the inversion of the TEM data. The inverted TEM conductivity model, nearest to the position of soil sample collection, was correlated to the grain size distribution and the resulting positive exponential relationship was used to obtain vertical 1D variations of clay content with depth. All sounding positions were interpolated to obtain a 2D image of subsurface clay content. This clay content variations were then compared to images of the Cole-Cole parameters, describing the frequency dependence of SIP imaging results. To evaluate the uncertainty in our clay estimations, we applied the Bayesian evidential learning 1D imaging (BEL1D). We obtained uncertainties of layer thickness, resistivity, and clay content by integrating the clay-conductivity relationship derived from TEM data into the BEL1D framework.

Exploration of plant roots and monitoring their conditions during growth is of great importance. A promising method for the non-invasive investigation of plant roots is spectral induced polarization (SIP). To enhance understanding of the mechanism controlling the plant root’s induced polarization response, we have conducted a series of experiments and constructed a physical-based numerical model. We measured the SIP signal of wheat root grown in the nutrient solution. The experiments have demonstrated a relationship between the SIP parameters (chargeability and relaxation time) and the root biomass and surface area. Monitoring the SIP response of roots poisoned by cyanide has revealed that the root polarization source is the cell membrane potential. In addition, we modeled plant root as a collection of 2-dimensional individual cells surrounded by an electrolyte. The SIP signal was calculated based on the numerical solution of the Poisson-Nernst-Planck equation. The model has supported the experimental results with the correlation between the magnitude of polarization and the root surface area. According to the model, the root polarization magnitude is related to the root’s external surface area. The polarization length scale is the root’s diameter, not the cell diameter. Based on these results and data from the literature, we suggest that at the low-frequency range associated with the SIP method, passing the current through the plant results in polarization of the individual cells, a relatively high polarization and relaxation time that is related to the cell length. On the other hand, injecting current to the growing medium results in the polarization of the external surface area of the root and polarization length scale related to the root diameter.

Induced polarization (IP) is increasingly applied for hydrological, environmental and agricultural purposes. Interpretation of IP data is based on understanding the relationship between the IP signature and the porous media property of interest. Mechanistic models on the IP phenomenon relay on the Poisson-Nernst-Plank equations, where diffusion and electromigration fluxes are the driving forces of charge transport, and are directly related to IP. However, to our knowledge, the impact of advection flux on IP was not investigated experimentally, and was not considered in any IP model. In this work, we measured the spectral IP (SIP) signature of porous media under varying flow conditions, in addition to developing and solving a model for SIP signature of porous media, which takes flow into consideration. The experimental and the model results demonstrate that as bulk velocity increases, polarization and relaxation time decrease. Using a numerical model, we established that fluid flow near the particle deforms the structure of the electrical double layer (EDL), accounting for the observed decrease in polarization. Using simple physical arguments, we developed a new model for the relaxation time, taking into account the impact of bulk fluid velocity. The model and the measured and synthetic data were found to be in good agreement. Overall, our results demonstrate the sensitivity of the SIP signature to fluid flow, highlighting the need for considering fluid velocity in the interpretation of the SIP signature of porous media, and opening an exciting new direction for noninvasive measurements of fluid flow at the EDL scale.

SUMMARY Spectral information obtained from induced polarization (IP) measurements can be used in a variety of applications and is often gathered in frequency domain (FD) at the laboratory scale. In contrast, field IP measurements are mostly done in time domain (TD). Theoretically, the spectral content from both domains should be similar. In practice, they are often different, mainly due to instrumental restrictions as well as the limited time and frequency range of measurements. Therefore, a possibility of transition between both domains, in particular for the comparison of laboratory FD IP data and field TD IP results, would be very favourable. To compare both domains, we conducted laboratory IP experiments in both TD and FD. We started with three numerical models and measurements at a test circuit, followed by several investigations for different wood and sandstone samples. Our results demonstrate that the differential polarizability (DP), which is calculated from the TD decay curves, can be compared very well with the phase of the complex electrical resistivity. Thus, DP can be used for a first visual comparison of FD and TD data, which also enables a fast discrimination between different samples. Furthermore, to compare both domains qualitatively, we calculated the relaxation time distribution (RTD) for all data. The results are mostly in agreement between both domains, however, depending on the TD data quality. It is striking that the DP and RTD results are in better agreement for higher data quality in TD. Nevertheless, we demonstrate that IP laboratory measurements can be carried out in both TD and FD with almost equivalent results. The RTD enables a good comparability of FD IP laboratory data with TD IP field data.

International audience; Usually, in situ electrical polarization measurements (in geophysical prospection referred to as induced polarization or spectral induced polarization (SIP)) are carried out at frequencies below 1 kHz. These techniques have mainly been used for mining exploration, followed by a larger panel of environmental applications. However, in this ultra low and extremely low-frequency domain, the duration of each individual measurement is long, i.e., typically several tens of minutes for a single full SIP spectrum down to the mHz range. This restriction makes it unrealistic to implement high-density measurement mapping campaigns over large areas, which would otherwise be possible at higher frequencies. In the intermediate frequency range [3 kHz–3 MHz], laboratory studies of soil and rock samples have shown that they can be strongly polarized notably in the presence of clays, and this property has been confirmed by several in situ mapping experiments using electromagnetic induction in the time and frequency domains (FDEM and TDEM), as well as by an electrostatic method (often named capacitive-coupled resistivity or CCR). The present paper recalls these results in an effort to promote polarization measurements at intermediate frequencies and to emphasize the importance of measuring this phenomenon.

Summary In this study, we investigate the pore space of a sample of the Portland sandstone using various methods with different resolution, and based on different physical principles (Mercury injection capillary pressure, MICP, Micro-computed tomography, µ-CT, and Spectral induced polarization, SIP). According to their physical principles, each method provides either the pore body size (µ-CT) or the pore throat size (MICP and SIP). Moreover MICP characterizes the pore-throat radii, whereas SIP rather gives the pore-throat lengths. The objective of this work was to compare results provided by each technique in a common parameter framework. The comparison approach between the different techniques is based on the relationships of ‘the incremental porosity – pore size’. Our results show that the recovered porosity can increase when using a method with higher resolution. We believe each method gives a specific attribute of the pore space topology similar to reflector attributes obtained in exploration seismology. We also believe that extensive works must be done to improve our understanding of these attributes to better characterize the pores space topology, and, consequently to better predict the transport and storage properties of soils, rocks and sediments.

Summary Aiming at the development of an efficient prospection technique, we have investigated the capabilities of the spectral induced polarization (SIP) method for the prospection of graphite and the quantification of its textural parameter. Here, we present SIP imaging results from the grounds of a former graphite mine. The study area is located close to Zettlitz in Lower Austria, where a graphite mine was active between 1855 and 1966. Moreover, SIP laboratory measurements have been performed on synthetic samples using well-defined composition and content of graphite and samples retrieved from the field site. We demonstrate here the application of the SIP method for the characterization of graphite. Based on laboratory and field measurements, we demonstrate that SIP measurements are sensitive to changes in the grain size and volumetric content of graphite soil and rock samples. Moreover, both field and laboratory results reveal a significant frequency dependence on the electrical properties of graphite.

Summary The Spectral Induced Polarization (SIP) method measures the frequency dependence of the electrical resistivity of rocks and sediments by magnitude and phase. Usually, the resistivity spectrum is measured sequentially with mono-frequency signals. This method proves to be advantageous for large-scale measurement set-ups and high interference voltages, as it provides the highest measurement accuracy. For small-scale field measurements or laboratory measurements, interference voltages often play only a subordinate role. Here, it is also important to achieve the highest possible measurement progress. This can be doubled by multi-frequency excitation compared to mono-frequency excitation. However, the price of faster measurement is a somewhat higher measurement error. We have implemented and successfully tested both excitation techniques in a new 88-channel laboratory measuring instrument (SIP-LAB-FAST). The user now has the choice between the most accurate or the fastest possible measurement. He can thus optimally adapt the measurement process to the requirements of the object under examination.

Summary In this abstract, we focus on the raw signatures for Spectral Induced Polarization (SIP) imaging data sets collected at 11 representative permafrost sites in the Swiss-, Austrian-, Italian- and German Alps. The selected study areas range from ice-rich rock glaciers and talus slopes to ice-poor bedrock permafrost sites. SIP data were collected within a frequency range between 0.1–225 Hz. Our results show that the polarization is different for ice-rich and ice-poor permafrost sites and that the increase in the polarization at high frequencies (> 10 Hz) correlates with an increase in ice content. Thus, we demonstrate the potential of SIP data to discriminate between alpine permafrost areas with high, medium and low ice content, which helps to improve the interpretation of ERT data.

In a column set-up, Fe modified biochar produced from date palm leaves was used to remove As (1 mg L−1) from a laboratory-prepared wastewater. The wastewater treatment process was monitored in real-time by spectral induced polarization (SIP), over a wide range of frequencies (0.01–1000 Hz). Both 5 and 10% biochar-amended columns achieved As removal exceeding 98%. The SIP parameters appear to be sensitive on As removal processes, with the recorded trend following the conventional geochemical monitoring, while offering higher temporal resolution.

Spectral induced polarization (SIP) is a non-intrusive geophysical method that collects chargeability information (the ability of a material to retain charge) in the time domain or its phase shift in the frequency domain. Although SIP is a temporal method, it cannot measure the dynamics of flow and solute/species transport in the subsurface over long times (i.e., 10–100 s of years). Data collected with the SIP technique need to be coupled with fluid flow and reactive-transport models in order to capture long-term dynamics. To address this challenge, PFLOTRAN-SIP was built to couple SIP data to fluid flow and solute transport processes. Specifically, this framework couples the subsurface flow and transport simulator PFLOTRAN and geoelectrical simulator E4D without sacrificing computational performance. PFLOTRAN solves the coupled flow and solute-transport process models in order to estimate solute concentrations, which were used in Archie’s model to compute bulk electrical conductivities at near-zero frequency. These bulk electrical conductivities were modified while using the Cole–Cole model to account for frequency dependence. Using the estimated frequency-dependent bulk conductivities, E4D simulated the real and complex electrical potential signals for selected frequencies for SIP. These frequency-dependent bulk conductivities contain information that is relevant to geochemical changes in the system. This study demonstrated that the PFLOTRAN-SIP framework is able to detect the presence of a tracer in the subsurface. SIP offers a significant benefit over ERT in the form of greater information content. It provided multiple datasets at different frequencies that better constrained the tracer distribution in the subsurface. Consequently, this framework allows for practitioners of environmental hydrogeophysics and biogeophysics to monitor the subsurface with improved resolution.

Breakthrough curves (BTCs) are a valuable tool for qualitative and quantitative examination of transport patterns in porous media. Although breakthrough (BT) experiments are simple, they often require extensive sampling and multi-component chemical analysis. In this work, we examine spectral induced polarization (SIP) signals measured along a soil column during a BT experiment in a homogeneous and heterogeneous soil profiles. Soil profiles were equilibrated with an NaCl background solution and then a constant flow of CaCl2 solution was applied. SIP signature was recorded, and complementary ion analysis was performed on the collected outflow samples. Our results confirm that changes to the pore-water composition, ion exchange processes and profile heterogeneity are detectable by SIP: the real part of the conductivity-based BTCs clearly indicated the BT of the non-reactive ions as well as the retarded BT of Ca2+. The imaginary part of the conductivity-based curves reacted to the changes in ion mobility around the electrical double layer (EDL) and indicated the initiation and the termination of the Na+–Ca2+ exchange reaction. Finally, both the real and imaginary components of the complex conductivity reacted to the presence of a coarser textured layer in the heterogeneous profile.

Rising industrial interest in the application of nanomaterials for the remediation of contaminated sites has led to concern over the environmental fate of the nanoremediation agents used. A critical requirement in evaluating and understanding nanoparticle (NP) behaviour in porous media is the development of analytical methods capable of in situ monitoring of complex NP transport dynamics. Spectral induced polarization (SIP), a non-invasive geo-electrical technique, offers a promising tool for detecting and quantifying NPs in soil and aquifer media. However, its application for monitoring the spatial migration and attachment behaviour of NPs remains uninvestigated. Here, we present results from flow-through experiments where we monitored the transport of cobalt ferrite nanoparticles (CoFe-NPs) coated with Pluronic, an amphiphilic polymer, in natural aquifer sand columns. We coupled concentration breakthrough curve analysis with SIP monitoring and reactive transport modeling to relate spatiotemporal NP concentration distributions to geo-electrical signals. Changes in the real (σ′) conductivity at three different locations along the columns closely correlated with model-computed total (solid plus aqueous phase) NP concentrations during the propagation of a NP slug. The imaginary conductivity (σ″) correlated closely with the arrival of the NP-slug. However, during the receding front, bimodal σ″-signal peak behaviour was observed propagating through the columns, indicating the existence of complex in situ NP transport dynamics, potentially revealing the rupture of nanoclusters upon straining and their effect on bulk charge storage that may not be obvious from breakthrough curve data alone. Fitting of a double Cole-Cole relaxation model yielded distinct shifts in relaxation time (τ) associated with the polarization of smaller length-scale particles. Post-NP pulse τ and σ″ did not return to pre-injection values; these lingering signals were caused by retained NP concentrations as low as 8.8 mg kg−1. Our results support the applicability of SIP for spatial and temporal monitoring of NP distributions, with implications for the investigation of NP transport and nanoremediation strategies.

Soil and groundwater contamination with lead (Pb) poses serious challenges for the environment. Activated carbon (AC) and biochar have huge potential application in the in-situ remediation processes through permeable reactive barriers (PRB). Spectral induced polarization (SIP) technique recently showed promises in nondestructively monitoring the spatio-temporal characteristics of physical, chemical and biological processes in porous media. In this study SIP technique was used for monitoring Pb remediation by AC and biochar in column scale. The calculated characteristic grain/pore size evolutions from SIP signals on AC, agreed well with the size of precipitates measured by SEM and mercury intrusion porosimetry (MIP) methods. The content increment process of the retained Pb on AC was also recorded via the magnitude increment of the imaginary conductivity. The mechanisms of Pb-AC and Pb-biochar interactions were investigated using SEM-EDS, TEM, FTIR, XRD, and XPS measurements. It showed that AC immobilizes through physical adsorption and precipitation, whereas complexation with functional groups is the remediation mechanism for biochar. Furthermore, the observed SIP responses of both AC and biochar are two orders of magnitude higher than those of typical natural soils or silica materials. This distinct difference is an additional advantage for the field application of SIP technique in PRB scenarios.

We present an approach for the estimation of ore processing residue volumes in historical mine waste dumps by the use of different geophysical methods in combination with mineralogical investigations. The stamp mill dump in the Harz mountains, Germany was examined with the methods electrical resistivity tomography (ERT), ground penetrating radar (GPR) and spectral induced polarization (SIP) flanked by mineralogical studies at many drilling points. The mineralogical results were used to calibrate the geophysical results and to distinguish between valuable and non-valuable waste material. With SIP we investigated individual profiles and took lab samples. These lab results emphasize the differences between the fine-grained tailings of clayey silt to silty sand in the top layer and the sandy tailings underneath in both resistivity and phase. From the GPR results we can distinguish between different layers and various backfillings in the first two meters due to the much higher resolution than the other methods. From ERT we achieved an overview about the dimension and inner structure of the dump and the boundary between the sandy residual material and the host rock. To estimate the volume of the residual body we carried out 2D inversion of all ERT profiles followed interpolation between the inverted profiles. From the drilling interpretation, the SIP lab results and the ERT field measurements we defined a resistivity threshold of 350 ohm-m for the ore processing residues to achieve a 3-dimensional body of the dump. The volume of this body was then corrected by a factor due to consideration of uncertainties, e.g., forest areas, inaccessible dump sections, small-scale anomalies (geological or different anthropogenic nature) and inversion coverage. As a result, we were able to calculate the volume of the ore processing residues which can be used further for the determination of the economic potential (remaining metal content).

Permafrost regions are highly sensitive to climate changes, which has significant implications for the hydrological regimes and the mechanical state of the subsurface leading to natural hazards such as rock slope failures. Therefore, a better understanding of the future evolution and dynamics of mountain permafrost is highly relevant and monitoring of the thermal state of permafrost has become an essential task in the European Alps. Geophysical methods have emerged as well-suited to support borehole data and investigate the spatial distribution and temporal changes of temperature and the degradation of permafrost. In particular, electrical resistivity tomography (ERT) has developed into a routine imaging tool for the quantification of ice-rich permafrost, commonly associated with a significant increase in the electrical resistivity. However, in many cases, the interpretation of the subsurface electrical resistivity is ambiguous and additional information would improve the quantification of the ice content within the subsurface. Theoretical and laboratory studies have suggested that ice exhibits a characteristic induced electrical polarization response. Our results from an extensive field programme including many morphologically different mountain permafrost sites now indicate that this IP response may indeed be detected in the field suggesting the potential of the Induced Polarization (IP) method to overcome such ambiguities. We present here Spectral IP (SIP) mapping results conducted over a broad range of frequencies (0.1-225 Hz) at four representative permafrost sites of the Swiss-, Italian- and Austrian Alps. The mapping results have been used to install long-term permafrost monitoring arrays for a better understanding of subsurface variations associated to climate change. All SIP study sites are located at elevations around 2600 - 3000 m and include comprehensive geophysical and temperature data for validation. We focus on the spatial characterization of each site to address different research questions: to (i) reproduce and improve the mapping of the spatial permafrost extent inferred from previous investigations in the Lapires talus slope,Western Swiss Alps, to (ii) improve the geophysical characterization of the Sonnblick monitoring site located in the Austrian Central Alps, to (iii) determine the transition between permafrost and non-permafrost at the Schilthorn site, Bernese Alps, Switzerland, and to (iv) find the best-suited location for a SIP monitoring profile and conduct year-round measurements at the Cime Bianche site, Western Italian Alps. Our various field applications demonstrate the potential of the IP method for characterizing and monitoring permafrost systems in high-mountain environments.

The properties of clays and oxides govern many environmental processes, consequently, ongoing effort is invested in developing non-destructive, in-situ analytical tools that reflect these properties. Herein, the physicochemical properties of montmorillonite (MMT) and iron-oxide coated montmorillonite (FeOx-MMT) were characterized using common analytical techniques, and the results were compared to spectral induced polarization (SIP) measurements. FeOx-MMT particles showed a lower CEC, higher pH dependency of the surface charge, and lower suspension stability. Also, the size of the primary particles increased following iron-oxide deposition. SIP measurements over a range of salinities showed that the effective polarization length of the clays was in the order of several microns, suggesting the measurements of aggregates (not primary particles). Moreover, FeOx-MMT particles were more compact than MMT, and their size decreased with increasing salinity due to compaction of the EDL and arrangement of primary particles in the aggregate. The SIP-response to pH changes agreed with zeta potential measurements; at low pH values, MMT exhibited higher polarization due to the higher CEC. However, at a high pH, the differences diminish due to deprotonation of the Fe-OH surface groups. These findings suggest that SIP is a sensitive method that can detect changes in the surface chemistry of soil particles.

Spectral induced polarization imaging is a valuable method in near-surface hydrogeological and environmental investigations, both in frequency and time domain. Despite the electromagnetic inductive effects always occurring in induced polarization measurements, they are usually disregarded in the spectral inversion of the data. Consequently, in order to invert only data with negligible inductive effects, the frequency and time ranges of both frequency-domain and timedomain data have often to be restricted, limiting the higher invertible frequency and the smaller invertible time. This study investigates the induction-free acquisition range of both frequency-domain and time-domain acquisitions, computing the electromagnetic inductive effects of homogeneous and layered media for different cable layouts, with theoretical and field examples. It is found that it is much easier to get induction-free early times than induction-free high frequencies. For nested arrays it is difficult to measure a significant spectral range in the frequency domain, but also with the dipole-dipole array the inductive effects are not negligible with the typical settings used in field applications. Furthermore, it is shown that a wider acquisition range means an increased spectral content and parameter resolution, for both frequency-domain and timedomain data, and that the time-domain approach has an advantage in terms of resolution of the spectral parameters when the induction-free acquisition range is used. Moreover, the differences in spectral content between electromagnetic induction and dielectric-like polarization are evidenced, showing that an induction of the same magnitude of the dielectriclike polarization in frequency domain is negligible for time-domain gradient array. Finally, it is advocated that a full electromagnetic modeling of the inductive effects is necessary for using data affected by electromagnetic induction in the spectral inversion of induced polarization data.

Summary With advances in data acquisition and processing methods, spectral inversion of time domain induced polarization (IP) data is becoming more common. Geological interpretation of inverted spectral parameters, for instance Cole-Cole parameters, often relies on results from systematic laboratory measurements. These are most often carried out with frequency domain systems on sandstone samples. However, the two different methods of measuring the spectral IP response differ in both measurement technique and scale. One of the main objectives of the current study is, thus, to perform a direct comparison of inverted spectral parameters from time domain IP field data with frequency domain IP spectra from laboratory measurements. To achieve this, field measurements were carried out before a ∼50 m long rock core was drilled down along one of the measurement lines. Solid parts of the core were vacuum-sealed in plastic bags to preserve the natural groundwater in the samples, after which the core samples were measured with frequency domain spectral IP in laboratory. The results showed that the inverted Cole-Cole parameters closest to the borehole were comparable to the IP spectra measured at the core samples, despite differences in measurement techniques and scale. The field site chosen for the investigation was a limestone succession spanning over a well-known lithological boundary (the Cretaceous—Paleogene boundary). Little is known in previous research about varying spectral IP responses in limestones, and an additional objective of this study was, therefore, to investigate possible sources of these variations in the laboratory. The IP spectra were interpreted in light of measured lithological and physicochemical properties. The carbonate texture differed strongly across the Cretaceous—Paleogene boundary from fine-grained calcareous mudstone (Cretaceous) to more well-lithified and coarse-grained wackestone and packstone (Paleogene). Both laboratory and field spectral IP results showed that these differences cause a large shift in measured bulk conductivity across the boundary. Furthermore, carbonate mound structures with limestone grains consisting mainly of ∼cylindrical bryozoan fragments could be identified in the inverted Cole-Cole parameters as anomalies with high relaxation times. A general conclusion of this work is that limestones can give rise to a wide range of spectral responses. The carbonate texture and the dominant shape of the fossil grains seem to have important control over the electrical properties of the material. A main conclusion is that the inverted Cole-Cole parameters from the field scale time domain IP tomography were comparable to the magnitude and shape of frequency domain IP spectra at low frequencies. This opens up large interpretational possibilities, as the comprehensive knowledge about relationships between lithological properties and IP spectra from laboratory research can be used for field data interpretation.

Measuring root properties, the “hidden half” of the plant, is challenging due to their heterogeneous and dynamic nature. A promising method for noninvasive mapping of roots and their activity, spectral induced polarization (SIP), has been introduced. However, measurements of root properties together with their SIP responses are missing, limiting the interpretation of a root's SIP signature. In this study, we coupled SIP measurements of roots in hydroponic solution with measurements of root biomass, surface area, and diameter. Furthermore, we monitored the SIP response of roots poisoned by cyanide, which results in depolarization of the root's cell membrane potential. We found a linear correlation between root biomass and surface area, and the low‐frequency electrical polarization. In addition, we demonstrate the relationship between root cell membrane potential and root polarization. Based on the results, we suggest that in comparison with the stem‐based approach used by other researchers, the polarization in the contact‐free method used in this study is related to the external surface area of the root and external architectural structures such as root diameter and root hair. Overall, a direct link between root properties and their electrical signature was established.

While some recent studies have discussed various effects of temperature on the electric and electromagnetic properties of multiphase porous media , the effect of temperature changes in multiphase porous media polluted by dense non-aqueous phase liquids (DNAPLs) has rarely been documented. We attempted to characterize how relative permittivity and electrical resistivity vary with temperature in multiphase porous media. The measurements were carried out using two different column sizes. Glass beads with a 1 mm diameter were used to simulate porous media. Spectral induced polarization (SIP) and time domain reflectometry (TDR) were used to measure complex electrical resistivity and relative permittivity, respectively. We investigated the geophysical characteristics of two DNAPLs, coal tar (CT) and chlorinated solvent (CS), from 20 to 50 °C; ultra-pure water was used as the reference fluid. Experimental data on the relative permittivity and complex resistivity of pure water obey empirical models, validating our experimental setup and protocol. Results demonstrated that the real parts of relative permittivity and electrical resistivity are functions of temperature in the medium with the presence or absence of a solid phase. While we did not study the imaginary part of relative permittivity, our observations in the DNAPL samples indicated that temperature increases decreased the imaginary parts of the complex electrical resistivity of DNAPLs tested, whether in the presence or absence of the solid phase. Temperature dependency of relative permittivity and complex resistivity were also studied in multiphase porous media, after drainage (80% DNAPL and 20% water) and after imbibition (8% DNAPL and 92% water). The effect of temperature increases on complex resistivity has a secondary effect on frequency domain and Cole-Cole parameters. It was found that the relationship between temperature and resistivity is linear; therefore, resistivity temperature coefficients were obtained for water and both DNAPLs with the presence and absence of solid phase.

Spectral induced polarization (SIP) measurements have been demonstrated to correlate with important parameters in hydrogeological and environmental investigations. Although SIP measurements were often collected in the frequency domain (FDIP), recent developments have demonstrated the capabilities to solve for the frequency-dependence of the complex conductivity through measurements collected in the time domain (TDIP). Therefore, the aim of our field investigations is a comparison of the measured frequency-dependence at a broad frequency range resolved through FDIP and TDIP. In contrast to previous studies, we conducted measurements with different instruments and measuring technologies for both FDIP and TDIP. This allows for investigating the robustness of different measurements and assessing various sources of errors, for the assessment of the advantages and drawbacks from different measuring techniques. Our results demonstrate that data collected through different instruments are consistent. Apparent resistivity measurements as well as the inversion results revealed quantitatively the same values for all instruments. The measurements of the IP effect are also comparable, particularly FDIP readings in the low frequencies (< 10 Hz) revealed to be quantitatively the same for different instruments. TDIP measurements are consistent for data collected with both devices. As expected, the spatial distribution of the values is also consistent for low frequency data (in FDIP) and late times measurements in TDIP (> 0.1 s). However, data quality for higher frequencies in FDIP (i.e., early times in TDIP) show larger variations, which reflects the differences between the instruments to deal with the electromagnetic contamination of the IP data. Concluded in general, the different instruments and measuring techniques can provide consistent responses for varying signal-to-noise ratio and measuring configurations. © 2020 The Authors