Time-domain Induced Polarization Research Articles

Manganese prospecting using time domain induced polarisation (TDIP), case study: Karangnunggal district, Tasikmalaya regency

Indonesia has good manganese potential, but its utilization has not been optimal. It has been unable to meet domestic needs because of the low production level. Thus, it needs to conduct more exploration activities. Karangnunggal district, Tasikmalaya regency, is one of the manganese minerals located. The method to prospect the manganese distribution in the area was applying a geophysical approach. The measurement employed the time domain induced polarisation (TDIP) method in 17 lines. In this study, the process deployed the Wenner array. Each line has a length of about 117.5 m with an electrode spacing of 2.5 m. The outcome shows that a manganese mineral has a chargeability of more than 5 ms. Based on the 3D visualization, it can be identified that most deposits spread to the south and the east while partially located in the north and the west of the study area. The 3D chargeability modeling generates the estimating manganese volume of about 2.4 million cubic meters.

Conversion of Induced Polarization Data and Their Uncertainty from Time Domain to Frequency Domain Using Debye Decomposition

The time-domain (TD) induced polarization (IP) method is used as an extension of direct current (DC) resistivity measurements to capture information on the ability of the subsurface to develop electrical polarization. In the TD, the transient voltage decay is measured after the termination of the current injection. To invert tomographic TD IP data sets into frequency-domain (FD) models of complex electrical resistivity, a suitable approach for converting TD IP transients and their corresponding uncertainties into the FD is essential. To apply existing FD inversion algorithms to TD IP measurements, a conversion scheme must transform the measured decay curves into FD impedances and also propagate the corresponding measurement uncertainty from the TD to the FD. Here, we present such an approach based on a Debye decomposition (DD) of the decay curve into a relaxation-time distribution and the calculation of the equivalent spectrum. The corresponding FD data error can be obtained by applying error propagation through all of these steps. To accomplish the DD we implement a non-linear Gauss–Newton inversion scheme. We test the conversion scheme in a synthetic study and demonstrate its application to field data on a tomographic TD IP data set measured on the Maletoyvaemskoie ore field (Kamchatka, Russia). The proposed conversion scheme yields accurate impedance data for relaxation processes, which are resolved by the TD measurements. The error propagation scheme provides a reasonable FD uncertainty estimate, as confirmed by a Monte Carlo analysis of the underlying parameter distributions.

Three-Dimensional Joint Inversion of the Resistivity Method and Time-Domain-Induced Polarization Based on the Cross-Gradient Constraints

The resistivity method and time-domain-induced polarization (TDIP) are two branches of electric exploration that are used to solve problems in mineral exploration, hydrogeology and engineering geology. In recent years, integrating different physical parameters for joint inversion to improve the accuracy of inversion results has been extensively examined; however, three-dimensional joint inversion of the two methods above has not been realized. To further address this issue, in this research, we used the limited-memory BFGS (L-BFGS) method to develop a three-dimensional joint inversion algorithm of the resistivity method and TDIP based on the cross-gradient constraints. In the new algorithm, the resistivity method and TDIP inversion were iteratively updated alternately to ensure that the inversion results can simultaneously meet the two conditions of obtaining minimum data misfits and finding structural similarity. The three-dimensional synthetic dataset inversion results showed that the models obtained by joint inversion are more accurate in the recovery of both the boundaries and the values of the anomalies. Especially in the background of high noise, joint inversion has higher resolution for the target body. The joint inversion algorithm was also successfully applied to a groundwater detection practice in Beijing, China, in which the practicability of the algorithm was confirmed.

Integrated subsurface investigation for magmatic sulfide mineralization in Betul Fold Belt, central India

Magnetotelluric (MT), Electrical Resistivity Tomography (ERT), Time-Domain Induced Polarization (TDIP), Geochemical and Geological studies followed by drilling and down-hole logging were undertaken with in the Betul Fold Belt (BFB) in Central India, to demarcate zones of magmatic Ni-Cu-PGE sulfide mineralization. The BFB is predominantly composed of circular to elongate gabbro bodies of the Padhar Mafic-Ultramafic Complex, intruded into a sequence of bimodal volcanic rocks and quartzites. Near-surface samples of ultramafic rocks were subjected to precise geochemical analysis and scanned by an electron microscopy with an energy dispersive spectrometer (SEM-EDS). This work indicated the presence of pyrite, pyrrhotite, chalcopyrite, pentlandite, and minor amounts of W-Cd bearing boweiite and palladenite assemblage. These minerals are regarded as favorable to the occurrence of Ni-Cu-PGE sulfide mineralization. MT data derived from two profiles were analyzed and modeled using 2D and 3D inversion algorithms. The robust conductivity anomalies obtained from the MT model have been interpreted coupled with electrical tomography, geology, and geochemistry data. The near-surface shallow depth conductors observed in the ERT sections are interpreted as the sulfide mineralized zones. They corroborate the MT results. These conductive zones reflect the occurrence of the magmatic Ni-Cu-PGE bearing sulfide mineralization associated with rocks of the mantle-derived Padhar Mafic Ultramafic Complex. This geophysical data, in conjunction with petrological and geochemical analysis of drill core samples have allowed the identification of the origin and paragenesis of sulphide mineralization in the study area. Geochemical studies suggest that the parental magma was generated from a subduction modified, metasomatized and an enriched mantle source that was subsequently emplaced in a magmatic continental arc setting. The interpreted conductors, observed at shallow depths (∼200-300 m), have been generated by secondary hydrothermal fluid circulation leading to vein formation in the host Padhar Mafic-Ultramafic Complex. The MT and electrical tomography models delineate the geological boundaries of the sulfide-bearing mineralized deposits in the BFB.

3D time-domain induced polarization modelling considering anisotropy and topography

Induced polarization (IP) method is one of the most widely used techniques in mineral exploration, hydrogeology, and environmental monitoring, becoming indispensable especially for metallic ore exploration. Topography of the surface and anisotropy of the subsurface plays an important role in the IP data interpretation and modelling leading to wrong resistivity and chargeability models if they are not considered. We developed a 3D IP forward modelling considering arbitrary anisotropy and topography using the finite element method (FEM). The newly developed algorithm is compared with the numerical solutions and analytical results for classical two-layer anisotropic models successfully. By using unstructured mesh to consider complex topography, we implement 3D IP forward modelling for arbitrary anisotropic medium under the condition of uneven terrain and investigate the effect of anisotropy and topography on IP data interpretation. Topography has a decreased or reinforcing effect on time-domain induced polarization (TDIP) response. Vertical transverse isotropic (VTI) medium has a tiny effect and can accurately locate the anomaly in the subsurface while tilted transverse isotropic (TTI) medium has a very large effect and cannot be explored. Under the comprehensive influence of complex topography and anisotropy with an angle larger than 30 degrees, it is very difficult to interpret TDIP results. The 3D inversion of the synthetic data derived by the 3D anisotropic chargeability model indicates that topography weakens the TDIP response. However, VTI anisotropic anomaly can be detected accurately in the horizontal direction, the TTI anisotropic anomaly is difficult to be explored.

Organic Contamination Distribution Constrained with Induced Polarization at a Waste Disposal Site

Waste disposal sites are a serious concern due to their potential contamination threat. Site characterizations are the basis for contamination remediation, among which borehole-based methods are time-consuming. This study aims to utilize time-domain induced polarization (TDIP) to constrain groundwater and soil organic contamination. Fourteen TDIP profiles were arranged using three acquisition methods at a disposal site contaminated with benzene. A data quality analysis was first conducted to overcome poor signals in high-salinity groundwater conditions. The TDIP results show that solid waste can be classified into two distinct types based on chargeability. The results of induced polarization measurements and X-ray fluorescence analysis proved the presence of metal substances causing high chargeability. The waste with metals was the benzene contamination source. Finally, we propose a procedure for the integration of geophysical and geochemical methods to delineate contaminated areas. A new roll-along survey protocol was developed to meet the needs of long-profile surveys of contaminated shallow groundwater and soil sites.

Improved imaging efficiency of large mine waste rock piles with integrated electrical and electromagnetic geophysical techniques

Acid mine drainage (AMD) can emanate from mine waste rock piles (WRPs) for extended periods, resulting in severe environmental contamination. Since the generation and release of AMD is dependent on the volume and composition of acid-generating waste rock, effective characterization of WRPs is necessary for successful long-term AMD management. Conventional methods such as coring and test pits are invasive and only provide sparsely distributed point information in WRPs which are typically large and highly heterogeneous. Geoelectrical methods such as direct current (DC) resistivity, time-domain induced polarization (TDIP) and electromagnetics (EM) can provide non-invasive and continuous information of WRPs. However, due to the enormous scale of typical WRPs, improved strategies are needed for more efficient imaging. The objective of this study was to demonstrate the improved efficiency of integrated imaging techniques with rapid but low-resolution EM, and laborious but high-resolution DC and TDIP, to characterize the acid-generating regions of a coal mine WRP in Nova Scotia, Canada. EM was first employed to rapidly image the entire pile footprint and locate zones with high conductivity. Focused 3D DC and TDIP surveys were then performed over these zones, which were <10% of entire WRP, to better characterize the associated waste rock composition. DC and TDIP images indicate coincident and continuous areas of low resistivity (<15 Ohm·m) and high chargeability (>15 mV/V), which are indicative of generated AMD (leachate) and stored AMD (sulfides), respectively. This interpretation is validated by corresponding geological and geochemical measurements within the WRP. This study demonstrates the value of integrating complementary electrical and electromagnetic methods for more efficient characterization of large mine WRPs, with EM rapidly locating zones of interest, and simultaneous DC and TDIP imaging then discriminating between areas that generate AMD and areas that are affected by AMD flow paths.

A study of time spectrum identification of induced polarization field based on Weibull distribution function

Identification of time spectrum is one of the core issues of the time-domain induced polarization (IP) method, which can be considered as one of the bases for distinguishing various polarized rocks. Fitting the IP data based on the spectrum forward model and obtaining optimal solution of the model parameters is a key step in spectrum identification. However, the suitable forward model should consider the observation conditions, such as the charging time and the time window. The time spectrum identification may be difficult to implement stably due to the lack of objective reference for the optimal solution. Therefore, our purpose is to improve the forward model and implement the spectrum identification for IP data. First, using the Weibull (WB) distribution function as the basis, a time spectrum forward model considering the charging time and observation time window is provided according to the typical measurement mode. Then, based on the WB spectrum model and Barzilai-Borwein gradient optimization, a method for solution of apparent spectral model parameters for spectrum identification is developed. Finally, this method is used to process the IP data from a mine in which the anomalies related to ore-bearing beds are identified based on the processing results. Results obtained demonstrate that the spectrum forward model based on the WB function is feasible in describing the IP data. The limited charging time and a wide observed time window should be considered to realize accurate simulations and description of the time-domain IP data. The essence of the time spectrum identification is to comprehensively reflect the time-varying state of the whole time channel through the spectral model parameters, whereas the decay field ratio of the adjacent time channel can be used as an objective reference. The parameters of the spectrum model characterizing the time-varying state are independent of polarization and resistivity and thus can be directly used for identification of IP anomalies.

Application of induced polarization imaging across different scales to understand surface and groundwater flow at the Hofermuehle landslide

Understanding changes in hydraulic properties of the subsurface is critical to delineate areas susceptible for groundwater accumulation and the triggering of landslides. Laboratory studies have demonstrated the possibility to estimate the hydraulic conductivity from induced polarization (IP) measurements. However, to-date only rare studies have been applied at the field scale and none has evaluated the frequency-dependence in field IP imaging data. We show that the application of petrophysical models linking hydraulic conductivity (K) and IP at 1 Hz, resolves for the same estimations from frequency-domain measurements as well as from time-domain IP. Moreover, our IP images reveal an evident frequency-dependence between 0.1 and 240 Hz in the electrical properties, which is also observed in lab measurements conducted in soil samples. To account for this, we fit a Cole-Cole model to our multi-frequency IP results and evaluate models linking K and the normalized chargeability, gaining a subsurface model of the hydraulic conductivity with enhanced resolution. Our results reveal clear variations at depth in the electrical conductivity and polarization associated with the presence of a plane of instability and the sliding plane. We also resolve lateral changes delineating areas with K below 10-6 m/s, which may act as barriers for groundwater flow. To evaluate our results we also estimate the hydraulic conductivity using a joint inversion algorithm that directly solves for porosity and saturation through the simultaneous inversion of seismic refraction and electrical resistivity tomography data. We observed consistent hydraulic conductivity images obtained through the inversion of the IP data and the joint inversion, with such model being in agreement with existing information of the site. We resolve a 3D model of the hydraulic conductivity at the Hofermuehle landslide from IP data that reveals the geometry of areas with poor drainage that may lead to land sliding.

Effectiveness of Laboratory Physical Modeling in Acquiring the Response of Time Domain Induced Polarization (TDIP)

Induced Polarization (IP) is one of the Geophysical methods that utilize the polarization properties of the rocks for metallic mineral exploration. The problems are how to distinguish the metallic minerals deposit based on the chargeability. The physical modeling of IP is used to study the behavior of TDIP response. The study of a simple mathematical model is carried out to obtain the theoretical curves which are presented as the subsurface parameter. These curves are used as a reference to assess the result of physical modeling. The laboratory physical modeling uses a tank model with a size of (200x100x70) cm3, with a block as a target and water as a host medium. The results show laboratory physical modeling which has been made is quietly good. The approximate position and geometry of the target can be identified. The resistivity inversion modeling is sensitive to recognizing the geometry and position, while the chargeability inversion is on the distribution of metallic minerals. The quantitative correlation between chargeability and iron-ore content is obtained by Dipole-dipole and Wenner configuration. Both are connected exponentially, with a different exponential constant for different block targets.

Exploration of a Copper Ore Deposit in Elbistan/Turkey Using 2D Inversion of the Time-Domain Induced Polarization Data by Using Unstructured Mesh

We present the results of a direct current (DC) resistivity and time-domain induced polarization (TDIP) survey exploring a copper ore deposit in Elbistan/Turkey. The ore deposit is elongated below a valley and is of disseminated form with sulfide content. DC and IP data were acquired using the pole-dipole array on eight parallel profiles crossing the valley perpendicularly. The length of each profile was 300 m with an inter-profile distance of about 50 m. The data were interpreted by a newly developed 2D DC/TDIP inversion algorithm. The finite element algorithm uses a local smoothness constrained regularization on unstructured meshes. The finite element forward solution, as well as the inverse problem, is solved by an iterative preconditioned conjugate solver. The depth of investigation (DOI) was determined from cumulative sensitivities of the 2D inversion algorithm results. Because of the dissemination of the ore, the 2D inversion of the DC data was ambiguous: However, due to the sulfide content, a strong chargeability anomaly associated with the ore body was detected. We show that chargeability anomalies can generally be detected in the absence or presence of corresponding resistivity anomalies. This highly chargeable structure was confined in lateral direction. Although the lower boundary of the structure could not be resolved by the applied field set-up, a rough estimation of it could be derived at a depth of 90 m using synthetic modeling analyses. The 2D chargeability models are consistent with existing borehole information.

Identification of Alteration-Mineralization in Skarn Deposit System based on Time-Domain Induced Polarization Method Approach in Kasihan Area, Pacitan, East Java

This research was conducted in Kasihan, Pacitan, East Java with the aim of knowing the alteration zone and ore mineralization from mineral sediment systems based on the conductivity and resistivity of subsurface rocks in the study area. A geophysical method is needed to map area area area of the prospect of mineral deposits as well as mathematical models in the area research. One of the geophysical methods used in studies of mineral sediment cases is Time Domain Induced Polarization (TDIP). Measurement of Time Domain Induced Polarization method done with configuration dipole-dipole which is arranged relatively straight northeast-southwest as much 2 tracks, with each track having the length of 260 m with an initial space of 20 m electrodes. Targets of the TDIP Method can describe subsurface conditions as far as 48 m. The position of mineralization in skarn deposits in the Time Domain Induced Polarization method at a depth of 25 m below the surface topography shown by high resistivity values &gt;2000 Ωm and &gt;500 msec chargeability in dacite and limestone

Data-driven modeling of time-domain induced polarization

Induced polarization (IP) measurements are affected by various types of noise, which should be removed prior to data interpretation. However, existing data processing methods often rely on empirical assumptions about the standard shape of IP decay curves. Our goal is to introduce a data-driven approach for modeling and processing time-domain IP measurements. To reach this goal, we train a variational autoencoder (VAE) on 1,600,319 IP decays collected in Canada, the United States, and Kazakhstan. The proposed deep learning approach is unsupervised and avoids the pitfalls of IP parameterization with empirical Cole-Cole and Debye decomposition models, simple power-law models, or mechanistic models. Four applications of VAEs are key to modeling and processing IP data: (1) synthetic data generation, (2) Bayesian denoising, (3) evaluation of signal-to-noise ratio, and (4) outlier detection. Furthermore, we interpret the IP data compilation’s latent representation and reveal a correlation between its first dimension and the average chargeability. Finally, we determine that a single real-valued scalar parameter contains sufficient information to encode IP data. This new finding suggests that modeling time-domain IP data using mathematical models governed by more than one free parameter is ambiguous, whereas modeling only the average chargeability is justified. A pretrained implementation of the VAE model is available as open-source Python code.

Effect of electrode types and soil moisture on the application of electrical resistivity tomography and time-domain induced polarization for monitoring soil stabilization

There is a growing demand for an efficient noninvasive method for monitoring stabilization sites. Geophysical monitoring method was applied in a pilot site. Electrical resistivity tomography (ERT) and time-domain induced polarization (TDIP) surveys showed that the topsoil and stabilized soil exhibited an electrical resistivity of 335.7 ± 251.8 and 74.8 ± 16.9 O-m, respectively, and a chargeability of 11.7 ± 7.6 and 6.0 ± 2.3 mV/V, respectively. Both methods distinguished the stabilized layer and topsoil, and 3D ERT and TDIP provided additional information related to the long-term stability of the stabilized site. The influence of the electrode type on electrical resistivity and chargeability noise was negligible. As the water content in the soil increased, the electrical resistivity and chargeability decreased in the stabilized soil, while chargeability increased at a water content of 20–30% in the topsoil. This study highlights the potential of geophysical methods for monitoring stabilized sites.

Integrated land and water-borne geophysical surveys shed light on the sudden drying of large karst lakes in southern Mexico

Abstract. Karst water resources play an important role in drinking water supply but are highly vulnerable to even slight changes in climate. Thus, solid and spatially dense geological information is needed to model the response of karst hydrological systems to such changes. Additionally, environmental information archived in lake sediments can be used to understand past climate effects on karst water systems. In the present study, we carry out a multi-methodological geophysical survey to investigate the geological situation and sedimentary infill of two karst lakes (Metzabok and Tzibaná) of the Lacandon Forest in Chiapas, southern Mexico. Both lakes present large seasonal lake-level fluctuations and experienced an unusually sudden and strong lake-level decline in the first half of 2019, leaving Lake Metzabok (maximum depth ∼25 m) completely dry and Lake Tzibaná (depth ∼70 m) with a water level decreased by approx. 15 m. Before this event, during a lake-level high stand in March 2018, we collected water-borne seismic data with a sub-bottom profiler (SBP) and transient electromagnetic (TEM) data with a newly developed floating single-loop configuration. In October 2019, after the sudden drainage event, we took advantage of this unique situation and carried out complementary measurements directly on the exposed lake floor of Lakes Metzabok and Tzibaná. During this second campaign, we collected time-domain induced polarization (TDIP) and seismic refraction tomography (SRT) data. By integrating the multi-methodological data set, we (1) identify 5–6 m thick, likely undisturbed sediment sequences on the bottom of both lakes, which are suitable for future paleoenvironmental drilling campaigns, (2) develop a comprehensive geological model implying a strong interconnectivity between surface water and karst aquifer, and (3) evaluate the potential of the applied geophysical approach for the reconnaissance of the geological situation of karst lakes. This methodological evaluation reveals that under the given circumstances, (i) SBP and TDIP phase images consistently resolve the thickness of the fine-grained lacustrine sediments covering the lake floor, (ii) TEM and TDIP resistivity images consistently detect the upper limit of the limestone bedrock and the geometry of fluvial deposits of a river delta, and (iii) TDIP and SRT images suggest the existence of a layer that separates the lacustrine sediments from the limestone bedrock and consists of collapse debris mixed with lacustrine sediments. Our results show that the combination of seismic methods, which are most widely used for lake-bottom reconnaissance, with resistivity-based methods such as TEM and TDIP can significantly improve the interpretation by resolving geological units or bedrock heterogeneities, which are not visible from seismic data. Only the use of complementary methods provides sufficient information to develop comprehensive geological models of such complex karst environments

Spectral induced polarization: frequency domain versus time domain laboratory data

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.

Electrical Resistivity And Induced Polarization Imaging For Refuse Dump Site

Contamination from landfills as a result of leaching from organic and inorganic waste poses a threat to the environment because, subsoil and groundwater are affected. The contamination is more serious in developing countries where waste management is inefficient. The aim of this study was to determine the extent of pollution at this site. Three profiles were surveyed: two in North-South direction and the third in Northeast-Southwest. 2D electrical resistivity and Time Domain Induced Polarization (TDIP) data sets have been acquired along the three profiles in Boadi Community. Electrical resistivity and induced polarization together with excavations were successful in mapping the extent of the leachate plume. The study demarcated clearly three main zones: the first, second and third zones indicated chargeabilities > 12.8 msec near the surface, > 6.4 msec and > 12.8 msec at the base respectively. Similarly, resistivities 192 Ωm for the first, second and third zones respectively were delineated. The relatively lower electrical resistivity areas were thought to be due to the presence of the leachate plume and the comparatively higher chargeability zones were interpreted as duricrust. The relatively lower chargeability represents absence of polarizable materials. The study also delineated the weathered basement granodiorites showing relatively higher resistivities. These TDIP and ERT results show that the methods have been successful in mapping the landfill leachate plume. The results from the resistivity and chargeability compare very well with the excavations.

Spectral induced polarization in anisotropic rocks with electrically conductive inclusions: synthetic model study

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.

Time-domain induced polarization forward modeling and an apparent spectral parameter solution based on the Weibull growth model

The application of the Cole-Cole model within time-domain induced polarization (TDIP) forward field modeling shows that the model parameters can characterize time-varying states of the TDIP field and support observed data analysis. The Cole-Cole model contains real and imaginary parts, and it requires a frequency-to-time conversion for TDIP forward modeling. However, the TDIP field is usually expressed by a real number, and its intuitive time-varying states field intensity increases with charging time. Therefore, the forward model should be constructed in a simpler form. We have aimed to develop a forward model using mathematical functions not based on physical principles. The Weibull (WB) growth model, which is primarily used to describe the time-varying curve features in regression analysis, is introduced into the basic algorithm of the TDIP forward model. Subsequently, a forward expression of the TDIP effect is established. Based on the time-varying shape and scale parameters, this expression describes the time-varying rate and relaxation states of the TDIP fields. Furthermore, based on the extensively used conjugate gradient optimization, an apparent WB parameter scheme is initiated to calculate the spectral parameters that represent the relaxation and time-varying rate obtained from the multi-time-channel TDIP data. Finally, this scheme is applied to interpret the different simulated and actual TDIP data. The results demonstrate that the WB growth model can be used for the TDIP forward model without involving physical principles, the model parameters without specific physical significance can be used to represent the time-varying states of TDIP fields, and apparent WB parameters can be used to discern different TDIP observed data. The setting of the TDIP forward model and model parameters can actually be more flexible and diverse, so as to obtain simpler forward expressions and ensure a highly efficient inverse solution.

Induced Polarization as a Proxy for CO2-Rich Groundwater Detection—Evidences from the Ardennes, South-East of Belgium

CO 2 -rich mineral groundwaters are of great economic and touristic interest but their origin and circulation paths in the underground are often poorly understood. A deeper understanding of the system plumbery and the development of non—to minimally—invasive near-surface geophysical methods for the prospection of potential productive areas is therefore of great interest to manage future supply. The objective of this contribution is to assess the ability of the time-domain induced polarization (TDIP) method, combined with the electrical resistivity tomography (ERT) method, to make the distinction between CO 2 -rich groundwater from non-gaseous groundwater. Three combined ERT/TDIP tomographies were performed above known uplift zones in the south-east of Belgium where thousands of CO 2 -rich groundwater springs exist. On all profiles, important contrasts in both electrical resistivity and chargeability distributions were observed in the vicinity of the upflow zone, also reflected in the normalized chargeability sections computed from the measured data. Low resistivity vertical anomalies extending in depth were interpreted as a saturated fracture network enabling the upflow of deep groundwater to the surface. High chargeability anomalies appearing directly close to the CO 2 -rich groundwater springs were inferred to metallic oxides and hydroxides precipitation in the upper part of the aquifer, linked to pressure decrease and changing redox conditions in the up-flowing groundwater approaching the land surface. The combined interpretation of electrical resistivity and induced polarization datasets provides a very promising method for a robust prospection of CO 2 -rich groundwater.