Spectral Induced Polarization Research Articles

Simultaneous inversion of spectral IP data with frequency constraints

Spectral-induced polarization (SIP) measures the apparent complex resistivities (CRs) of electrical current sources at frequencies of less than 10 kHz. The measured apparent CRs are usually inverted to recover the intrinsic subsurface CRs, which depend on the source frequencies. The Cole-Cole (CC) model is commonly used to describe the dispersion characteristics of the intrinsic CRs; electrical resistivity, chargeability, and the time and relaxation constants can be derived using this model. For quantitative estimation of these parameters, a reliable inversion method for multifrequency SIP datasets is required. Conventionally, SIP data at each frequency are independently and sequentially inverted. This does not consider the relationships between SIP datasets of different frequencies. To consider such relationships, here, we simultaneously invert multi-frequency SIP datasets by introducing frequency-smoothness constraints between adjacent frequencies. We numerically verified the utility of the approach in terms of both amplitude and phase recovery; the method yielded more reliable relaxation models (e.g., CC models) than conventional inversion. For field verification, the method was applied to SIP data of several frequencies obtained in South Korea.

Real-time monitoring of organic contaminant adsorption in activated carbon filters using spectral induced polarization

Real-time, in-situ monitoring of adsorption processes in activated carbon (AC) filters may advance the effectiveness, reliability and economical value of such systems. In this study, the applicability of spectral induced polarization (SIP) as a real-time monitoring tool was examined. The adsorption of anionic and cationic organic dyes to commercial-AC filter was examined using a set of breakthrough experiments combined with continuous SIP monitoring. The imaginary part of the complex electrical conductivity decreased in the range of 0.25−2.5Hz for both dyes. During the adsorption of the cationic dye, a new peak developed in the range of 7−40Hz, suggesting the dominance of surface processes that are not explained by the classic stern-layer polarization theory. The recorded imaginary conductivity values were used as a proxy for adsorbed dye concentration in the calibration process of a reactive transport model. The model confirmed that SIP can successfully be used for real-time monitoring of the dye progression through the filter. The applicability of SIP as an effective monitoring tool was also shown for cyclic operation (adsorption-desorption cycles).

Paraquat sorption- and organic matter-induced modifications of soil spectral induced polarization (SIP) signals

SUMMARY Quantifying the capacity of soils to immobilize sorbing contaminants of concern relies on batch sorption experiments, typically performed at skewed solid-to-liquid ratios. The geophysical method spectral induced polarization (SIP) provides a powerful non-invasive monitoring alternative that can capture changes in soil electrical properties driven by contaminant sorption, yielding an approach whereby immobilization can be monitored in situ. Here, we present SIP signals obtained from a series of columns packed with a water saturated natural sandy-loam soil, with and without solid organic matter (SOM) amendment, contaminated with increasing concentrations of the herbicide paraquat. Our results highlight that soil polarization drops proportional to increasing amounts of sorbed paraquat in the SOM-free soil, exhibiting a Langmuir-type leveling-off behaviour. The addition of 8 percent-SOM yielded an increase in both the real ($\sigma ^{\prime}$) and imaginary ($\sigma ^{\prime\prime}$) conductivity of the uncontaminated treatment, driven by the contribution of charged sites in the SOM. Further, SOM modified the dependence between $\sigma ^{\prime\prime}$ and sorbed paraquat, likely driven by continued polarization within the polydisperse SOM with continued paraquat addition. However, the time constant ($\tau $), derived using the Cole–Cole model, shed light on a saturation-type dependence governed by a drop in ion mobility with increasing sorption. Thus, aiding the interpretation of sorption-driven SIP signals.

An Integrated Approach of Spectral Induced Polarization and Electromagnetic Coupling in Exploration Geophysics

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 Curaçá, 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.

Simulation of Rock Complex Resistivity Using an Inversion Method

The complex resistivity of coal and related rocks contains abundant physical property information, which can be indirectly used to study the lithology and microstructure of these materials. These aspects are closely related to the fluids inside the considered coal rocks, such as gas, water and coalbed methane. In the present analysis, considering different lithological structures, and using the Cole-Cole model, a forward simulation method is used to study different physical parameters such as the zero-frequency resistivity, the polarizability, the relaxation time, and the frequency correlation coefficient. Moreover, using a least square technique, a complex resistivity “inversion” algorithm is written. The comparison of the initial model parameters and those obtained after inversion is used to verify the stability and accuracy of such approach. The method is finally applied to primary-structure coal considered as the experimental sample for complex resistivity measurements.

SOME PROBLEMS OF ELECTRICAL GEOPHYSICAL PROSPECTING METHODS USED FOR EXPLORATION OF ORE DEPOSITS

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.

Modeling and Experimental Study of the Effect of Pore Water Velocity on the Spectral Induced Polarization Signature in Porous Media

AbstractInduced 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 rely 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 electrical double layer (EDL) structure, accounting for the observed reduction in polarization. We found a qualitative agreement between the model and the measurements. Still, the model overestimates the impact of flow rate on SIP signature, which we explain in terms of the flow boundary conditions. Overall, our results demonstrate the sensitivity of the SIP signature to fluid flow, highlighting the need to consider 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.

Broadband complex permittivity and conductivity measurements in the millimeter-wave bands over variable temperatures using a balanced-type circular disk resonator

A broadband measurement system is developed to address the issue of temperature dependence of the complex permittivity and conductivity of low-loss substrates in the millimeter-wave bands for fifth/sixth generation wireless communication applications. The developed system can provide broadband material measurements from less than 20 GHz to over 100 GHz over variable temperatures by utilizing higher order mode resonances of a balanced-type circular disk resonator (BCDR). The broadband measurement capability of the developed system is attributed to the mode-selective behavior of the BCDR over a wideband. To demonstrate the developed system, we measure the complex permittivity of the following three substrate materials over variable temperatures from 25 °C to 100 °C: cyclo-olefin polymer (COP), ceramic-filled polytetrafluoroethylene composites, and fused silica. Furthermore, the temperature dependence of the conductivity of the surface-mounted metal is characterized for the COP substrate. The measurements indicate that the loss tangents and conductivities of the substrates increase and decrease, respectively, with the temperature and frequency in the millimeter-wave bands. The full-wave simulations using the measured complex permittivity and conductivity of the COP substrate reveal that the propagation loss of the microstrip line using the COP substrate increases with the temperature and frequency and that the increase in the loss is primarily attributed to the decrease in the conductivity.

Classification of slag material by spectral induced polarization laboratory and field measurements

Historical slag dumps are of increasing interest due to economic, environmental or archaeological reasons. Geophysical investigations can help accessing the potential reuse of slag material to recover metallic raw material or for the estimation of the hazard potential of the buried slag material due to dissolution occurrence.In our study, we have investigated various slag material in the laboratory with the spectral induced polarization (SIP) method, obtained from different historical slag dumps, located in the Harz Mountains, Germany. We also present SIP results from field measurements at a historical slag dump where most of the slag samples reveal high amounts of iron, zinc, silica, and barium.Our results reveal a discrimination between three different slag grades (low, medium, high) by using the imaginary conductivity σ″ at a medium frequency (1–10 Hz) in both laboratory and field. Furthermore, additional information is obtained by a classification based on the spectral polarization behaviour and considering the field frequency range (0.1 Hz – 100 Hz). Five different types of spectra (ascending, descending, constant, maximum and minimum type) can be discriminated and recognized in the laboratory and in distinct areas of the slag dump. Even though a direct comparison between the laboratory and field results still needs to be proven, the buried slag material can be differentiated from the surrounding material by the polarization magnitude.

Evaluating iron remediation with limestone using spectral induced polarization and microscopic techniques

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.

Field-scale estimation of soil properties from spectral induced polarization tomography

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.

State‐of‐the‐art geophysics for metal exploration

AbstractThe new discovery of the economical metal deposit is getting more difficult year by year because the unexplored areas tend to be more remote areas than ever, and the metal prospective zones are deepening in the recent decades. Therefore, the advent of the state‐of‐the‐art or advanced geophysics technology to peer the deeper parts of the ground with higher accuracy and spatial resolution has been aspired in metal exploration industry. The author reports and introduces the drone‐based magnetic survey, the three‐dimensional time‐domain IP inversion algorithm using Cole–Cole parameter, the time domain electromagnetic method using superconducting quantum interference device (SQUID), and passive seismic survey as the state‐of‐the‐art or advanced geophysics technology for metal exploration.In the early stage of metal exploration, airborne magnetic survey has been used in a wide variety of exploration programs for large areas to map basement structures and to delineate other magnetic structures. The drone‐based magnetic survey is recently getting prevailing in the metal exploration because it can be conducted at much lower cost with simpler clearance for its flight by the governmental authority, and with higher spatial resolution than the conventional fixed‐wing or heliborne magnetic survey.In the target identification stage, induced polarization (IP) method has been recognized as one of the extremely powerful tools in the discovery of many new metal deposits. The historical various studies on complex resistivity measurements for metal sulphides, graphite, magnetite and pyrrhotite have revealed that there are significant differences in the spectral responses among metal sulphides, graphite, magnetite and pyrrhotite which is capable of being parametrized using the empirical Cole–Cole parameter model. JOGMEC developed the three‐dimensional time‐domain IP inversion code using Cole–Cole parameter model so as to interpret the difference of IP sources using Cole–Cole parameters, and has applied this code to its exploration activities. Transient electromagnetic method (TEM) also has a well‐established place in metal exploration. Since recent exploration targets are generally concealed by thick conductive overburdens such as weathered layers and black shale, these geological covers preclude induction currents generated by the TEM transmitter loop on the surface from diffusing into the exploration targets plentifully beneath the conductive covers. B‐field measurement using SQUID magnetic sensor is one of the technologies which enable TEM to investigate deeper than dB/dt measurement by the conventional induction coil system, because B‐field decays slower than dB/dt and time decay of B‐field reaches the system noise floor at later time than dB/dt if dynamic ranges of the both systems are almost identical. JOGMEC had developed the TEM system using a high‐temperature superconductor (HTS) SQUID, and has been using it in its metal exploration.The other current trend of mining geophysics is application of seismic method to metal exploration. In the target identification stage, deep geochemistry using aircore drilling is much utilized over the areas covered by transported cover. In order to design the aircore drilling survey, passive seismic survey has been utilized to evaluate thickness of transported cover in order to make plans of specification (length) of the aircore drilling survey, which is also a new trend in mining geophysics.

Combining geophysics and material science for environmental remediation: Real-time monitoring of Fe-biochar arsenic wastewater treatment

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 Characterization of Non‐Consolidated Clays for Varying Salinities—An Experimental Study

AbstractClay 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 nonintrusively 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 nonmonotonous 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 and 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.

Modeling Plant Roots Spectral Induced Polarization Signature

AbstractMonitoring the growth, architecture, and function of plant roots is of great interest. One promising noninvasive geoelectrical monitoring approach is spectral induced polarization (SIP). However, the roots' SIP signature is underexplored, not well understood, and a mechanistic model has not been proposed. Here, we developed a mechanistic model for SIP's response of roots, which is based on the Poisson‐Nernst‐Planck equation. The modeling results suggest that the magnitude of root polarization is linearly related to the root's external surface area and that the polarization length scale is the root's diameter. We suggest that injecting a current to the plant's stem results in higher polarization associated with the root‐cells' total surface area. In this case, the polarization length scale is the cell diameter. Overall, we quantified the link between the root's dimensions and their electrical signature, which may inspire SIP application for root phenotyping.

Cole-Cole Model Parameter Estimation from Multi-frequency Complex Resistivity Spectrum Based on the Artificial Neural Network

In near surface electrical exploration, it is often necessary to estimate the Cole-Cole model parameters according to the measured multi-frequency complex resistivity spectrum of ore and rock samples in advance. Parameter estimation is a nonlinear optimization problem, and the common method is least square fitting. The disadvantage of this method is that it relies on initial value and the result is unstable when data is confronted with noise interference. To further improve the accuracy of parameter estimation, this paper applied artificial neural network (ANN) method to the Cole-Cole model estimation. Firstly, a large number of forward models are generated as samples to train the neural network and when the data fitting error is lower than the error threshold, the training ends. The trained neural network is directly used to efficiently estimate the parameters of vast amounts of new data. The efficiency of the artificial neural network is analyzed by using simulated and measured spectral induced polarization data. The results show that artificial neural network method has a faster computing speed and higher accuracy in Cole-Cole model parameter estimation.

Characterizing Physical Properties of Streambed Interface Sediments Using In Situ Complex Electrical Conductivity Measurements

AbstractStreambed sediment physical properties such as surface area, are difficult to quantify in situ but exert a high‐level control on a wide range of biogeochemical processes and sorption of contaminants. We introduce the use of complex electrical conductivity (CC) methods (also known as spectral‐induced polarization (SIP)) that measure both real and imaginary conductivity to noninvasively and efficiently characterize shallow streambed sediments. We explore the method through synthetic modeling, laboratory, and field measurements to demonstrate the sensitivity of imaginary conductivity to sediment surface area, controlled in part by fine‐grained iron oxides produced by anoxic groundwater discharge. Laboratory measurements verify expected relationships between CC parameters and sediment properties. Synthetic modeling using a 1D analytical model illustrates the influence of water layer depth and conductivity on the field CC measurements made at the streambed‐stream water interface. Specifically, the inverted sediment imaginary conductivity is less impacted by uncertainty in the water layer depth and conductivity relative to the real conductivity and phase shift. Field CC measurements along a landfill‐impacted river reveal discrete streambed zones with enhanced bulk surface area generally corresponding to anoxic groundwater discharge zones with high concentrations of fine‐grained iron oxide precipitates.

Spatiotemporal geo-electrical sensing of a Pluronic-coated cobalt ferrite nanoparticle slug in natural sand flow-through columns

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.

Delineation of hydrocarbon contaminants with multi-frequency complex conductivity imaging

The characterization of contaminated sites is a serious issue that requires a number of techniques to be deployed in the field to reconstruct the geometry, hydraulic properties and state of contamination of the shallow subsurface, often at the hundreds of meter scale with metric resolution. Among the techniques that have been proposed to complement direct investigations (composed of drilling, sampling, and laboratory characterization) are geophysical methods, which can provide extensive spatial coverage both laterally and at depth with the required resolution. However, geophysical methods only measure physical properties that are indirectly related to contamination, and their correlation may be difficult to ascertain without direct ground truth. In this study, we present a successful example where the results of complex conductivity measurements conducted in an imaging framework are compared with direct evidence of subsoil contamination at a jet fuel impacted site. Thus, proving that a combination of direct and indirect investigations can be successfully used to image a site in its complex (potentially 3D) structure in order to build a reliable conceptual model of the site.

The Case for Considering Polarization in the Interpretation of Electrical and Electromagnetic Measurements in the 3 kHz to 3 MHz Frequency Range

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.