Induced Polarization Response Research Articles

Spectral induced polarization of corrosion of sulfur modified Iron in sediments

Spectral induced polarization (SIP) responses are not well understood within the context of remediation applications at contaminated sites. Systematic SIP studies are needed to gain further insights into the complex electrical response of dynamic, biogeochemical states to enable the use of SIP for subsurface site characterization and remediation monitoring. Although SIP measurements on zero valent iron have been previously published, the SIP response for sulfur modified iron (SMI), a similar potential subsurface reductive amendment, has not yet been reported. Hence, the purpose of this laboratory-scale study was to evaluate SIP for nonintrusive monitoring of SMI under relevant subsurface conditions. SMI was separately mixed with silica sand or sediments from the Hanford Site (Washington, USA) and then packed into columns for geochemical and SIP analysis for up to 77 days under fully saturated conditions. SMI exhibited distinguishable phase peaks between 0.1 and 1.0 Hz, which changed in magnitude based on content and were detected as low as 0.3 wt%. In the initial days, the complex conductivity, phase maxima, and chargeability increased while the peak locations shifted to higher frequency (decreasing relaxation times), suggesting an initial increase in polarization and concurrent decrease in the length scales (potentially due to changes in particle size and mineralogy). Then, after 77 days, the phase maxima and chargeability decreased with a concurrent increase in relaxation times, suggesting that over longer periods, less polarizable phases are forming and particle size or connectivity of polarizable phases is increasing. These results demonstrated a unique SIP response to SMI transformations that might be applied to monitoring of SMI emplaced as a subsurface barrier or injected in the field.

Towards the imaging of deep land ore deposits with ERT-IP method – first results from a demonstration survey in Finland

In this study, we conducted a comprehensive geophysical survey near Kuusamo (Finland) to assess the potential of electrical resistivity methods in delineating mineral deposits at depths greater than 1 km. Preliminary investigations, including magnetic and gravity methods as well as drilling, revealed significant anomalous structures in the survey area. We employed multiple electrical and electromagnetic methods at the site, comprising controlled-source electromagnetic (CSEM), magnetotelluric (MT), electrical resistivity tomography (ERT), and induced polarization (IP). To obtain the geophysical data in very large-scale area, we used a total of 25 transmitter dipoles with 1km long using three distinct transmitter systems and recorded data at 119 receiver stations. In this paper, we present the acquisition and preliminary results from ERT-IP. Analysis of the resistivity and IP responses revealed notable IP signals at depths exceeding 1.5 km. Meanwhile, the resistivity data indicated generally very high values, around 10,000 ohm-m, with complex variations observed near the surface.

Hierarchical Fe-Co@TiO2 with Incoherent Heterointerfaces and Gradient Magnetic Domains for Electromagnetic Wave Absorption.

Induced polarization response and integrated magnetic resonance show prosperous advantages in boosting electromagnetic wave absorption but still face huge challenges in revealing the intrinsic mechanism. In this work, we propose a self-confined strategy to construct hierarchical Fe-Co@TiO2 microrods with numerous incoherent heterointerfaces and gradient magnetic domains. The results demonstrate that the use of polyvinylpyrrolidone (PVP) coating is crucial for the subsequent deposition of Co-zeolitic imidazolate frameworks (ZIF-67), the distance of ordered arranged metal ions manipulates the size of magnetic domains, and the pyrolysis of PVP layers restricts the eutectic process of Fe-Co alloys to some extent. As a result, these introduced lattice defects, oxygen vacancies, and incoherent heterointerfaces inevitably generate a strong polarization response, and the regulated gradient magnetic domains realize integrated magnetic resonance, including macroscopic magnetic coupling, long-range magnetic diffraction, and nanoscale magnetic bridge connection, and both of the intrinsic mechanisms in dissipating electromagnetic energy are quantitatively clarified by Lorentz off-axis electron holography. Owing to the cooperative merits, the Fe-Co@TiO2 absorbents exhibit enhanced absorption intensity and strong absorption bandwidth. This study inspires us to develop a generalized strategy for manipulating the size of magnetic domains, and the integrated magnetic resonance theory provides a versatile methodology in clarifying magnetic loss mechanism.

Delineation of LNAPL plumes in a clay-rich site in Gyeongsangnam-do Province, South Korea: integration of geophysical survey data with borehole data and soil sampling information.

To effectively delineate the spatial distribution of oil contaminant plumes, geophysical methods indirectly measure the physical properties of the subsurface and can provide spatial information and images on a large scale, as opposed to traditional direct methods such as borehole drilling, sampling, and chemical analysis, which are time-consuming and costly. However, interpreting geophysical responses over non-aqueous phase liquid (NAPL)-contaminated sites is not straightforward due to inconsistent responses from biodegraded oil contaminants. In this study, we performed multi-geophysical surveys including seismic refraction, ground-penetrating radar, electrical resistivity tomography (ERT), and induced polarization (IP) surveys, to locate NAPL-contaminated zones in a clay-rich site. To reduce ambiguity in discriminating between oil contaminants and clay layers, we first figure out the geological structure of the site by interpreting geophysical data incorporating with borehole data. The ERT data highlighted the heavily contaminated regions in the unsaturated zone but were less distinctive below groundwater levels. Conversely, IP responses revealed potential hotspots within the clay layers, extending beneath the groundwater. Considering the 3D geological model, NAPL-contaminated zones are properly delineated through interpretation of ERT and IP data together with borehole data, and the contaminant source zone was properly estimated within the site.

Spectral induced polarization responses of carbonate precipitation in gel-saturated media with and without electron conductors

SUMMARY Interest in the use of carbonate precipitation for the purposes of soil stabilization, carbon capture and environmental remediation has created a need for geophysical techniques capable of characterizing the 3-D extent of precipitation in the subsurface. The use of spectral induced polarization, a geophysical technique sensitive to mineral precipitation and dissolution processes, has been investigated in previous studies; nevertheless, debate still exists regarding measured induced polarization (IP) signals relating to the formation of calcite. In this study, calcite was precipitated within gel-saturated media through highly controlled double-diffusion methods. Three experiments were conducted to study both the inherent IP response of calcite in isolation, as well as the IP response when precipitated within a low-polarizability background medium (quartz sand) and a high-polarizability medium (a sand–magnetite mixture). The results support that in isolation, the polarization of the electrical double layer surrounding individual calcite grains is not a significant contributor to IP signatures from calcite precipitation. However, under certain circumstances the precipitation of calcite, in the quartz-sand and sand–magnetite mixtures, can produce a strong IP response when the precipitation blocks a current conduction pathway. If metallic mineral grains are present then they may act as conduits for current if they become embedded in an insulating calcite wall, creating a significant electrode polarization which dominates the IP signal. Equivalent circuit models indicate that the precipitation of calcite as a thin planar front normal to the current path is analogous to a simple lossy capacitor blocking most DC current but being thin enough to measurably polarize. Circuit models also suggest that the total impedance seen from the sand–magnetite sample is simply the sum of the impedance of the disseminated grains and the grains encased in the calcite front. The encased grains also have a higher relaxation time which may indicate a different polarization mechanism than from disseminated magnetite.

Induced polarization of clay-rich materials — Part 1: The effect of desiccation

A petrophysical model describing spectral induced polarization (IP) has been developed for clay rocks accounting for the Maxwell-Wagner polarization. It is also used to connect the complex conductivity to the relative permeability of the material. This model is applied to the Callovo-Oxfordian clay rock of the Paris Basin (France) where the Meuse/Haute-Marne Underground Research Laboratory is located. Laboratory experiments are performed using eight clay-rock cores to study the effect of desiccation on their spectral-IP response. The measurements are performed along the foliation plane. Complex conductivity spectra are measured over a frequency range of 1 mHz to 45 kHz. These spectra are fitted with a double Cole-Cole model to extract the evolution of the Cole-Cole parameters with the saturation during the desiccation process. The low-frequency Cole-Cole model corresponds to IP phenomena, whereas the high-frequency Cole-Cole model corresponds to the Maxwell-Wagner contribution. We obtain the value of the first and second Archie’s exponents and we check the relationship between the surface conductivity and the cation exchange capacity of the clay rocks. We are also able to connect the relative permeability curve to the second (saturation) Archie’s exponent. The monitoring of the complex conductivity can be used to predict how the permeability of the clay-rock formation changes with the water content.

Influence of Inner Surface Roughness on the Spectral Induced Polarization Response—A Numerical Study

AbstractSpectral induced polarization (SIP) laboratory measurements on water‐saturated rocks show a strong correlation between the electrical polarization strength and the inner surface area of rocks. We investigate the influence of inner surface roughness on the SIP response by simulating the frequency‐dependent complex conductivity of micro‐scale rock models. Starting with smooth grain models, we introduce surface roughness using two different approaches: increasing the surface roughness in a fractal‐like manner, and creating random surface structures, resulting in more natural‐looking surfaces. We find that surface roughness has two distinct effects on the SIP response: (a) a shift in the position and magnitude of the primary relaxation frequency to lower frequencies and lower magnitudes, respectively, and (b) the formation of secondary polarizations above the polarization frequency of the primary polarization. We also compare the relaxation time and normalized chargeability obtained by Debye decomposition and the imaginary conductivity at 1 Hz of our models with mechanistic models and empirical relations. We point out the congruences and offer explanations for the discrepancies between our models and the empirical observations. We conclude that the results of our study are applicable to real rocks and that the SIP method has the potential to detect inner surface roughness. However, the SIP method it not able to discriminate between signals from rough particles and a distribution of smooth particles.

Time-domain induced polarization response and cementation index estimation of synthetic sandstones

Time-domain induced polarization response and cementation index estimation of synthetic sandstones

DNAPL flow and complex electrical resistivity evolution in saturated porous media: A coupled numerical simulation

Induced Polarization (IP) is a non-intrusive geophysical method to monitor Dense Non-Aqueous Phase Liquid (DNAPL) contamination and remediation processes underground. In this study, an advanced numerical code simulating DNAPL flow and complex electrical resistivity is presented. The model was validated against existing IP results and image measurements that were carried out previously in a series of 2D tank experiment. Multiphase flow modeling in porous media is coupled with electrical current modeling to simulate the process of DNAPL migration and the associated IP response. This brings a broader view of the contamination in space and time compared to surface and borehole measurements, especially when the results are supported by field measurements or laboratory experiments. The simulations are developed in 3D and are performed in COMSOL Multiphysics®. The simulations using petrophysical relationships for in-phase and quadrature resistivity and the results of the experiments are in complete accordance with each other in the parts of the tank where the saturation of DNAPL is relatively low (i.e. especially in the cone of depression in the pumping scenario). However, the parts associated with high saturation of DNAPL show high errors between the in-phase resistivity simulations and the results from experiments. The present work can be regarded as a preliminary study toward further applications of coupled IP-multiphase flow for more accurate detection and monitoring of DNAPLs. It is suggested that the choice of tool/approach in this study be extended to larger-scale studies for further investigation.

Response of Homogeneous Conducting Sphere in Non-Integer Dimensional Space

In this paper, we have investigated electric potential and field analytically for homogeneous conducting sphere by solving the Laplacian equation in fractional dimensional space. The laplacian equation in fractional space describes complex phenomena of physics. The separation variable method is used to solve the Laplace differential equation. The mathematical formulae governing the interaction of a low-frequency source of electric current with a spherical anomaly are derived in fractional dimensional space. These formulae are used to determine the apparent resistivity and induced-polarization response. The potential due to the current point source in fractional space is derived using Gegenbauer polynomials. The electric field inrensity of the homogeneous conducting sphere is calculated using the electric potential due to a current point source outside the sphere. The results are compared analytically with classical results by setting the fractional parameter α=3.

Geoelectrical properties of saline permafrost soil in the Adventdalen valley of Svalbard (Norway), constrained with in-situ well data

Direct Current (DC) Resistivity and Induced Polarization (IP) response of six profiles were measured using the Gradient electrode configuration in Adventdalen, Svalbard, to characterise the near-surface stratigraphy of the soil and to account for geotechnical and environmental aspects of global warming in the arctic region. In addition, Wenner array data was collected for the selected profiles to examine its effectiveness as compared to the Gradient array, given the characteristics of the study site. Two commercial inversion software programs, Res2DINV and AarhusINV, were used for the inversion of the DC resistivity and IP data, to compare the software. Physical soil properties, including porosity, water saturation, water salinity, freezing temperature and grain size distribution, previously measured from samples retrieved from wells along the studied profiles, were integrated in this study to investigate the correlation with geoelectrical properties of the sediments inferred from the DC resistivity and IP data.Results from processing of the Wenner array DC resistivity data provided higher resolution as compared to the Gradient array data, especially from deeper parts of the models, due to its higher signal-to-noise ratio. The Wenner array data also indicated better inversion result for the IP data as distinctive anomalies were better indicated in data from Wenner array survey. The Wenner array data also provided a realistic trend for the anomalies, thanks to the symmetrical geometry of the electrodes during the survey, although at the cost of time and higher expenses. Inversion results proved that AarhusINV resolved the geometry of the subsurface layers with higher resolution compared with the Res2DINV. However, the two inversion algorithms use slightly different parameters for the processing and for presenting the results, thus only allowing qualitative comparison. Based on the interpretations of the DC resistivity and IP data, four distinctive zones were identified from the surface to the maximum depth of 26 m, consisting of (i) unfrozen active-layer-(silts and sands), with intermediate resistivity values 200–300 Ω·m; (ii) frozen soil with 3–10 m thickness and resistivity values between 2500 and 5000 Ω·m; (iii) unfrozen soil (cryopeg) with high salinity and low resistivity of 40 Ω·m; and finally (iv) clayey-unfrozen soil sediments with low resistivity ranging 10–20 Ω·m, at depths between 13 and 26 m. The IP data allowed for the delineation of a low chargeability zone near the surface and a high chargeability zone at greater depth which denote the active layer, lower parts of unfrozen soil sediments and cryopeg respectively, within the top 10 m of the subsurface.The 3D subsurface model of the study area was created based on interpretations of the DC resistivity and IP data and was constrained by the description of the subsurface stratigraphy from nearby wells, which provided detailed information about the vertical stratigraphy of the study area. In addition, a good correlation was observed between the studied physical properties of the sediments and the DC resistivity data for the intersecting profile SVAER04, as the interface between high and low resistivity data at ca. 10 m depth coincided the sedimentary formation with intermediate-fine grain size, high porosity, high water saturation and high salt content. Our findings show that joint application of the geoelectrical surveys and laboratory analysis of soil samples are an efficient complement to each other. These methods can be used as an alternative to each other to investigate larger areas where achieving high resolution data is not necessary.Our study results underline the importance of choosing the right survey design for collecting the DC resistivity and IP data. Contribution of the IP data in this study mostly concerns the shallow parts of the subsurface due to the low signal-to-noise ratio at greater depths, and frozen ground which limited the ion mobility and hence the IP response. An appropriate inversion program coupled with integration of the physical properties of the sediments, can be successfully applied to characterise the near-surface morphology of the sedimentary formations to address the geotechnical and environmental challenges related to the permafrost in the Arctic.

Induced Polarization as a Tool to Assess Alteration in Geothermal Systems: A Review

The mineral alteration patterns in high- to low-temperature geothermal fields affect the induced polarization (electrical conductivity and chargeability) properties of volcanic rocks. Indeed, these properties are sensitive to the cation exchange capacity and the porosity of the rock, which are both dependent on the alteration path, temperature, and depth of burial. Therefore induced polarization tomography appears as a powerful non-intrusive geophysical method to investigate alteration patterns in geothermal fields. Among clay minerals, the production of smectite through prograde reactions occurs progressively in volcanic rocks up to 220 °C. The presence of smectite dominates the induced polarization response of the volcanic rocks because of its very large cation exchange capacity. It follows that induced polarization can be used as a non-intrusive temperature proxy up to 220 °C for both active and inactive geothermal fields, recording the highest temperatures reached in the past. The influence of magnetite and pyrite, two semi-conductors, also has a strong influence regarding the induced polarization properties of volcanic rocks. Various field examples are discussed to show how induced polarization can be used to image volcanic conduits and smectite-rich clay caps in volcanic areas for both stratovolcanoes and shield volcanoes.

High-resolution induced polarization imaging of biogeochemical carbon turnover hotspots in a peatland

Abstract. Biogeochemical hotspots are defined as areas where biogeochemical processes occur with anomalously high reaction rates relative to their surroundings. Due to their importance in carbon and nutrient cycling, the characterization of hotspots is critical for predicting carbon budgets accurately in the context of climate change. However, biogeochemical hotspots are difficult to identify in the environment, as methods for in situ measurements often directly affect the sensitive redox-chemical conditions. Here, we present imaging results of a geophysical survey using the non-invasive induced polarization (IP) method to identify biogeochemical hotspots of carbon turnover in a minerotrophic wetland. To interpret the field-scale IP signatures, geochemical analyses were performed on freeze-core samples obtained in areas characterized by anomalously high and low IP responses. Our results reveal large variations in the electrical response, with the highest IP phase values (> 18 mrad) corresponding to high concentrations of phosphates (> 4000 µM), an indicator of carbon turnover. Furthermore, we found a strong relationship between the electrical properties resolved in IP images and the dissolved organic carbon. Moreover, analysis of the freeze core reveals negligible concentrations of iron sulfides. The extensive geochemical and geophysical data presented in our study demonstrate that IP images can track small-scale changes in the biogeochemical activity in peat and can be used to identify hotspots.

The forward modeling for surface-borehole observation responses of polarized sphere in uniform electric field

Surface-borehole induced polarization method is a common geophysical method in metal exploration. The forward calculation results of the borehole observation responses can provide reference and support for practical work. In particular, the forward results of spherical polarized target can be equivalent to massive ore body or karst caves and other low-resistivity anomalies. In this paper, the forward simulation of borehole induced polarization responses is carried out by using the electric dipole model equivalent spherical polarized body. The results show that the polarization degree of the excitation source at different positions is different, and the closer the proximity is, the stronger the secondary field anomaly is. Borehole observation has advantages for the identification of deep ore bodies. Under the same offset condition, the closer the distance from the borehole is, the stronger the responses will be. The results of this paper can provide reference and basis for relevant research work.

Analysis of IP response characteristics in wells on undulating topography

This paper studies the effect of undulating topography on resistivity method and induced polarization (IP) method between the wells. The 3D forward modeling of borehole resistivity/induced polarization method is realized by abnormal potential algorithm based on unstructured finite-element method (FEM). The algorithm is verified in layered medium and spherical abnormal bodies. We studied the response characteristics of apparent resistivity and apparent polarizability in well-well mode and the situation when the source is at different positions in the well, and analyzed the effects of undulating topography on resistivity and induced polarization. The results show that the apparent polarizability and apparent resistivity response have a great relationship with the depth of the emission source in the well, the relative position of the emission source and the measurement electrode. The undulating topography only affects the measuring electrodes at a short distance, and it has a greater impact on the low-resistance anomalous body.

Induced polarization effect on grounded-wire transient electromagnetic data from transverse electric and magnetic fields

Transient electromagnetic (TEM) data can be seriously distorted by induced polarization (IP) phenomena when a polarizable body is present. The TEM field generated by a grounded-wire source contains transverse electric (TE) and transverse magnetic (TM) modes. The IP effect is most commonly studied with the TEM total field, rather than considering the difference between TE and TM fields. To investigate the effect of IP phenomena on the TE and TM fields, we have performed a detailed analysis on IP-distorted TEM data based on numerical and field examples. We first compare the IP effect on the TE and TM fields when polarizable bodies with different polarizable parameters are present. The TM field is more severely affected by the IP effect than the TE field. Compared to a single grounded-wire source, a double-line grounded-wire source can generate a larger TM field in the horizontal electric field. We compare the IP effect on TEM data from single- and double-line grounded-wire TEM configurations, and find that the data from the double-line configuration have a higher TM/TE ratio and are more severely affected by IP phenomena than in the single-line case. Thus, it would be easier to identify and extract the IP response from field data acquired with a double-line grounded-wire source configuration. These results have been verified by a field survey of the Kalatongke copper-nickel ore district, which has predominantly layered geology, in Xinjiang, China.

Excitation process under the ramp-step waveform of inductive source-induced polarization method

Most previous studies explain the induced polarization (IP) effects in transient electromagnetic (TEM) data using an idealized but unrealizable step-waveform transmitter current. However, the ramp-step waveform, which is commonly applied in TEM measurement, has been given less attention. To explore the effects of the switch-off time, we have compared the IP responses induced by two waveforms: the step and the ramp step. We apply a wire-filament circuit composed of a Debye model and an inductor to identify the differences in the aspect of the energy transfer process. Furthermore, we extend the analysis to illustrate the IP effects in a frozen-soil zone, metallic sulfide ore, and graphite ore and to analyze the relationship between the switch-off time, IP effects, and the polarization parameters. The results indicate that the primary and secondary fields act as excitation sources of the polarization field. In the step waveform case, the excitation source of the polarization field is the secondary field. As the switch-off time increases, the contribution rate of the primary field gradually increases, especially in the high-resistivity media. The finding provides a new understanding of the excitation process of the IP effects and indicates that source contributions are variable in different situations. Moreover, a longer switch-off time weakens the IP effects severely, and in the high-resistivity, high-polarizable media, the IP effects are more sensitive to the switch-off time. Therefore, a suitable switch-off time should be chosen based on the properties of the polarizable media, such as resistivity and time constant. To detect a relatively high-resistivity, high-polarizable body, the switch-off time should be as short as possible. Nevertheless, to detect a relatively low-resistivity polarizable body, the IP effects are fairly insensitive to the switch-off time, so the transmitter waveform can easily meet the requirements.

Quantifying Induced Polarization of Conductive Inclusions in Porous Media and Implications for Geophysical Measurements

Induced polarization (IP) mapping has gained increasing attention in the past decades, as electrical induced polarization has been shown to provide interesting signatures for detecting the presence of geological materials such as clay, ore, pyrite, and potentially, hydrocarbons. However, efforts to relate complex conductivities associated with IP to intrinsic physical properties of the corresponding materials have been largely empirical. Here we present a quantitative interpretation of induced polarization signatures from brine-filled rock formations with conductive inclusions and show that new opportunities in geophysical exploration and characterization could arise. Initially tested with model systems with solid conductive inclusions, this theory is then extended and experimentally tested with nanoporous conductors that are shown to have a distinctive spectral IP response. Several of the tests were conducted with nano-porous sulfides (pyrite) produced by sulfate-reducing bacteria grown in the lab in the presence of a hydrocarbon source, as well as with field samples from sapropel formations. Our discoveries and fundamental understanding of the electrode polarization mechanism with solid and porous conductive inclusions suggest a rigorous new approach in geophysical exploration for mineral deposits. Moreover, we show how induced polarization of biologically generated mineral deposits can yield a new paradigm for basin scale hydrocarbon exploration.

Feature Extraction and Intelligent Identification of Induced Polarization Effects in 1D Time-Domain Electromagnetic Data Based on PMI-FSVM Algorithm

The transient electromagnetic method can obtain resistivity and chargeability simultaneously in polarizable medium detection. Typically, we assume that the earth may contain a chargeable medium if the electromagnetic (EM) data appear negative values or sign reversals. Unfortunately, with barely perceptible characteristics, some EM responses with the induced polarization (IP) effects are considered to be non-polarizable responses. Insufficient understanding of features and inaccurate identification of the IP responses limits the use of the IP effects for broader purposes. For these reasons, we perform 1D forward modeling to discuss the degree of EM response affected by the IP effects and to extract polarization characteristics. To identify the IP effects, we combine partial mutual information (PMI) and the fuzzy support vector machine (FSVM) methods to complete the intelligent identification algorithm. We verify the efficiency and practicality of the algorithm by building Debye loops in field experiments. From the analysis, we distinguish the strong and weak IP effects by introducing the impact ratio. The strong IP responses manifest fast decays and sign reversals, and the weak IP responses primarily show fast decays or outward concavity. The identification algorithm validation results show that the recognition accuracy reaches 90.7%. In the field experiment verification, the Debye loop successfully simulates the IP effects of different intensity, and the identification results indicate that the algorithm has potential in the measured data. With this intelligent identification algorithm, the measurements can provide access to the weak polarizable medium when the impact ratio exceeds 30%.

Effect of solute concentration on the spectral induced polarization response of calcite precipitation

SUMMARY Induced calcite precipitation is used in geotechnics to modify the mechanical and hydrological properties of the underground. Laboratory experiments have shown that spectral induced polarization (SIP) measurements can detect calcite precipitation. However, the results of previous studies investigating the SIP response of calcite precipitation were not fully consistent. This study aims to investigate how the SIP response of calcite depends on solute concentration to explain the differences in SIP response observed in previous studies. A four-phase experiment with SIP measurements on a column filled with sand was performed. In phase I, calcite precipitation was generated for a period of 12 d by co-injecting Na2CO3 and CaCl2 solutions through two different ports. This resulted in a well-defined calcite precipitation front, which was associated with an increase in the imaginary part of the conductivity ($\sigma ^{\prime\prime}$). In phase II, diluted solutions were injected into the column. This resulted in a clear decrease in $\sigma ^{\prime\prime}$. In phase III, the injection of the two solutions was stopped while calcite precipitation continued and solute concentrations in the mixing zone decreased. Again, this decreased $\sigma ^{\prime\prime}$. Finally, the injection rate of the Na2CO3 solution was reduced relative to that of the CaCl2 solution in phase IV. This resulted in a shift of the mixing zone away from the calcite precipitation front established in phase I and an associated decrease of $\sigma ^{\prime\prime}$. These results imply that the SIP response of calcite is highly sensitive to the solute concentration near the precipitates, which may explain previously reported conflicting results.