On the estimation of specific surface per unit pore volume from induced polarization: A robust empirical relation fits multiple data sets

The induced polarization (IP) method is an established technique in mining geophysics. One of the major challenges facing mining geophysicists is the ability to differentiate between different lithologies based on geophysical data such as IP data. In this paper, we present a study that focuses on extracting useful information about lithology from the full waveform IP data acquired by Newmont Distributed IP Data Acquisition System (NEWDAS). We first invert the 4D IP data by performing a sequence of 3D inversions. We then propose a new 4D inversion algorithm with time regularization and time weighting function. Once the inversions are done, statistical analysis of these recovered chargeabilities on a regional basis reveals some important differences on how the chargeabilities of different lithologies decay with time, and consequently, helps differentiate between various rock types. We show in this paper that lithology differentiation based on IP data is possible by combining two components: advanced data acquisition system and powerful inversion algorithms.

We have demonstrated that the induced polarization effect in the airborne electromagnetic (AEM) data can be represented well using the generalized effective-medium theory of induced polarization (GEMTIP) model. We have also developed a novel method for 3D inversion of the airborne spectral induced polarization (SIP) data into the volume distribution of the parameters of the GEMTIP model. The results show that the SIP effect can be observed by a typical AEM system. The AEM data collected by the standard systems can be used not only for producing resistivity models of the subsurface formations, but for recovering the chargeability and other GEMTIP model parameters which can be subsequently used for mineral exploration and possibly mineral discrimination.

Evaluation of a novel correction procedure to remove electrode impedance effects from broadband SIP measurements

In this study, we conducted an extensive geophysical survey to explore the potential of electrical resistivity methods in delineating deep ore deposits within between Koillismaa Intrusion and Näränkävaara intrusion, northeastern Finland. Preliminary investigations in 2022, including magnetic, gravity and audio-magnetotelluric (AMT) methods, along with drilling, uncovered significant anomalous structures in the survey area. Subsequent drilling of an exploration well provided positive lithological indications of a ultramafic igneous rock at more than 1.5 km depth, which are very likely of the same age as the layered intrusions in the area. Borehole data indeed revealed that the Archaean basement gneiss extends down to approximately 510 m, underlain by a granite dyke with interspersed thin layers of pyroxenite and peridotite. Notably, peridotite layers around 1500 m depth exhibited distinct magnetic and IP responses in core data.We employed electrical methods at the site, including electrical resistivity tomography (ERT) and induced polarization (IP). To cover a large-scale area, 25 transmitter dipoles, each 1 km long and using three different transmitter systems, were deployed and data were recorded at 119 receiver stations. This work presents the acquisition and preliminary results from the ERT-IP surveys. During the processing of ERT and IP data, we utilized full time-series data recorded across the four lowest main frequencies (from 0.0625 Hz to 8Hz) to capture voltage data in a steady state. Apparent resistivity data were derived from the stacked voltage data, while IP data were initially extracted from these decay curves of these stacked voltage data and subsequently processed in the frequency domain (outphasing). 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. This study demonstrates the efficacy of ERT and IP methods in delineating deep-seated mineral deposits, with the deep-depths IP responses being particularly noteworthy.

Hydraulic conductivity estimation from induced polarisation data at the field scale — the Krauthausen case history

Recent studies have shown that induced polarization (IP) coupled with electrical resistivity surveys can be used for in situ lithological and hydrologic discrimination of the subsurface; yet the driving factors behind the effects of water content dynamics on IP are relatively understudied. We sought to improve the understanding of the relationship of IP on variations in water saturation degree for an undisturbed agricultural soil. Our experiment consisted of collecting IP measurements concurrently with hydraulic data during multistep outflow experiments. We determined the hydraulic properties of the undisturbed soil samples and correlated saturation degree with IP data. Due to an increase in pore fluid conductivity in the column pore water with decreasing saturation degree, we found that imaginary conductivity (σ″) may offer distinct advantages for determining water content over real conductivity (σ′) measurements. Although σ″ exhibits a weaker dependence on saturation degree compared to σ′, the relative insensitivity of σ″ to pore fluid conductivity results in a simpler dependence on saturation change in the presence of varying salinities. As changes in pore fluid conductivity are likely to occur in the field simultaneously with water content variations, we suggest that although IP has mostly been used to discriminate lithology, time lapse IP measurements may additionally provide a robust indicator of changes in saturation degree.

Mechanistic induced polarization (IP) models describe the relationships between the physical properties of geomaterials and their frequency-dependent complex conductivity. However, practitioners rarely use mechanistic models to interpret the IP data because the uncertainties associated with estimating petrophysical properties from complex conductivity spectra are still poorly understood. We propose a framework for critically assessing any IP model’s sensitivity and parameter estimation limitations. The framework consists of a conditional variational autoencoder (CVAE), an unsupervised Bayesian neural network specializing in data dimension reduction and generative modeling. We apply the framework in a case study of the “perfectly polarized interfacial polarization” model by training the CVAE on the IP signatures of synthetic mixtures of metallic mineral inclusions hosted in electrolyte-filled geomaterials. First, the CVAE’s Jacobian reveals the relative importance of each petrophysical property for generating the spectral IP data. The most critical parameters are the conductivity of the host, the volume fraction of the inclusions, the characteristic length of the inclusions, and the permittivity of the host. Contrastingly, the inclusions’ diffusion coefficient, permittivity, and conductivity, as well as the host’s diffusion coefficient, have marginal importance. A parameter estimation experiment using various model constraints yields the standardized accuracy of petrophysical properties and corroborates the sensitivity analysis results. Finally, we visualize the effects of data transformations and model constraints on the petrophysical parameter space. We conclude that a common logarithm data transformation yields optimal parameter estimation results and that constraining the electrochemical properties of a geomaterial improves the estimates of the size of its metallic inclusions and vice versa.

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

The search and recovery for base and precious metals in recent years has led to surveys in more challenging areas over complex deposits and in extreme terrains. Such deposits frequently have accessory minerals that can be detected by induced polarization (I.P.) surveys. Due to their complex shapes and host terrains, 3-D surveys and inversion models are necessary to accurately resolve them. However, in some cases, the survey lines are not arranged rectilinearly. To accommodate an arbitrary arrangement of the electrodes, a model discretisation that is independent of the electrode positions is used. The rugged terrain can be accurately modelled by the use of the finite-element method where the surface of the mesh matches the topography. Innovative arrays such as the offset pole-dipole array have been used to rapidly survey large areas at a lower cost compared to traditional dipole- dipole arrays. Such arrays frequently have large geometric factors that make it difficult to accurately calculate the I.P. anomalies with the conventional linear perturbation approach that uses the difference of two resistivity calculations. The complex resistivity method, where the I.P. component becomes the imaginary component of the resistivity model, avoids this problem as it effectively decouples the resistivity and I.P. calculations. Furthermore, time-lapse 3-D surveys using surface and borehole electrodes have been conducted to monitor the flow of sodium cyanide solution directly injected in steep-sided ore rock piles for secondary recovery of gold. A 4-D resistivity inversion method is used to map the flow of the solution during the injection process.

Past metallurgical sites and deposits account for a significant proportion of potentially contaminated sites in the European Union (EU):  about 100,000 have been identified only in the North West regions of the EU. While recent wastes from sites still in operation are commonly recovered, this is not the case for old aggregated materials with a high content of ferrous (and other) metals, white and black slag, etc., which are considered to be sources of pollution and are costly to manage or dispose of. These sites could be considered as opportunities to recover large volumes of resources (metals, materials and land) using urban mining techniques if they were better characterized.The induced polarization (IP) method is a geophysical method known to be sensitive to the presence of various metallic particles disseminated in the soil layers. If qualitative interpretation of the measured IP parameters in the field (i.e. resistivity and chargeability) are widespread, quantitative interpretation in terms of concentrations of different metallic particles is yet to be developed.The example of the Pompey field site (FR), investigated as part of the NWE-REGENERATIS project (https://www.nweurope.eu/projects/project-search/nwe-regeneratis-regeneration-of-past-metallurgical-sites-and-deposits-through-innovative-circularity-for-raw-materials/), is used in this study to present the interest in using time domain IP (TDIP) field measurements to characterize metallurgical past deposits. Several paths are explored to convert resistivity and chargeability TDIP tomographies into quantitative interpretation of metallic element concentrations: (1) extraction of frequency data from TDIP field measurements; and (2) upscaling of lab results through numerical simulations.Regarding (1), TDIP measurement were made with different time windows (different frequencies), giving us access to spectral IP (SIP) processing and interpretation at 5 frequencies. These new frequency interpretations of the TDIP can be compared to lab measurements and facilitate the upscaling of the found petrophysical relationships.Regarding (2), in order to interpret the TDIP results in terms of concentration of metallic particles, known petrophysical relationships and geochemical measurements obtained at the lab scale need to be interpreted at the field scale. We propose to use a Bayesian framework for inferring field-scale metallic particles concentrations, taking into account heterogeneity and anisotropy within the inversion schemes. This work is ongoing.For both (1) and (2), it is crucial to find the best petrophysical relationships linking the IP parameters to concentration and size of metallic particles. Wong (1979) developed a physics-based electrochemical model that is still used today. We further investigate the Wong model to explore the role of the background porous medium itself in determining the IP signature of disseminated metallic particles and discuss the sensitivity of the model to estimate metallic grains concentration.All these different research paths lead to a better understanding of metallic particles IP signature at a small scale, as well as discussions on how to use these findings to better characterize and reevaluate past metallurgical sites and deposits.This study was funded by the North West Europe (NWE) Interreg project called NWE-REGENERATIS that aims at the regeneration of past metallurgic sites and deposits through innovative circularity for raw materials, and by Schlumberger-Doll Research Center (USA, MA).

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.

Environmental applications of the induced polarization (IP) and the spectral induced polarization (spectral IP or SIP) methods, i.e., the possibility to detect organic and inorganic contaminants in glacial soils, has been studied the Geological Survey of Finland (GTK) for some years. This presentation deals with the IP of natural and contaminated soils (the membrane polarization), and gives two field examples; an area with an organic (hydrocarbon) contamination (Fig. 1), and an area with an inorganic (heavy metal) contarnination.

Abstract. Field and laboratory spectral induced polarization (SIP) measurements are integrated to characterize the hydrogeological conditions at the Schillerslage test site in Germany. The phase images are capable of monitoring thin peat layers within the sandy aquifers. However, the field results show limitations of decreasing resolution with depth. In comparison with the field inversion results, the SIP laboratory measurements show a certain shift in SIP response due to different compaction and sorting of the samples. The SIP data are analyzed to derive an empirical relationship for predicting the hydraulic conductivity (K). In particular, two significant but weak correlations between individual real resistivities (ρ') and relaxation times (τ), based on a Debye decomposition (DD) model, with measured K are found for the upper groundwater aquifer. The maximum relaxation time (τmax) and logarithmically weighted average relaxation time (τlw) show a better relation with K values than the median value τ50. A combined power law relation between individual ρ' and τ with K is developed with an expression of A · (ρ')B · (τlw)C, where A, B and C are determined using a least-squares fit between the measured and predicted K. The suggested approach with the calculated coefficients of the first aquifer is applied for the second. Results show good correlation with the measured K indicating that the derived relationship is superior to single phase angle models as Börner or Slater models.

There are a variety of noise sources that affect the accuracy of Induced Polarization (IP) data, e.g., cultural noise, varying self- potential, lightning strikes, telluric currents, and electromagnetic (EM) coupling. In this paper, we describe a new IP data collection system that collects full time-series data rather than just a single averaged windowed decay. The recording of the full time series allows for greater flexibility in analysing the data so that noise can be identified and removed.This paper outlines the process used to collect the time-series data, illustrates some of the many types of noise observed in the field, and demonstrates the superiority of the resulting IP data.

In recent years, several traditional mining exploration methods have been successfully modified for use in the environmental geophysics field. In these cases, "successfully modified" refers primarily to acquiring data fast enough, and therefore economically efficient enough, to accommodate the relatively small budgets that are available in most environmental studies.For example, transient electromagnetics (TEM) methods have been increasingly applied to environmental problems, particularly in unexploded ordnance (UXO), underground storage tank (UST) and utilities detection. Small, in-loop, cart-mounted TEM measurements are excellent for these deep metal detection applications. A major research effort is now underway to use, among other methods, multi-component, multi-time-gate mobile TEM systems (measuring Hx and Hy, as well as the standard Hz) in order to discriminate targets of interest (UXO, for example) from non-UXO objects such as metallic debris.A second good example is the induced polarization (IP) method. Although resistivity has been used extensively in shallow environmental applications, IP data acquisition has always been too slow, and therefore too expensive, for most environmental targets. Multichannel receivers, multiplexers, and laptop computers now allow us to acquire IP data at rates of 2500 to 3000 data points per day (in the dipole-dipole configuration, for example), providing low cost, high density data. IP data have been shown to be particularly useful in delineating buried waste, such as at old landfills. In numerous surveys, resistivity data have provided useful information on buried, back-filled pits and trenches, while IP data have been used to discriminate which of the pits and trenches located with resistivity actually contain waste material.