Minerals Exploration Methods Modified for Environmental Targets

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.

Unexploded ordnance (UXO) investigation is mainly carried out by the magnetic exploration. However, in this paper, the transient electromagnetic (TEM) method was used in order to detect UXO more accurately. Although the TEM method is usually used for deep sounding of mineral exploration, we used small loop TEM method to detect buried UXO. In order to understand the characteristics of the small loop TEM method, many experiments using small empty cans were carried out at Ito Campus in Kyushu University and a lot of features were discovered. For example, if the size of loop becomes smaller, the penetration depth becomes smaller. These results showed that the small loop TEM was applicable to UXO investigation. Based on these results, the test survey to detect the imitation UXOs using 180×180-cm-square loop was carried out. In this field experiment, there are various kinds of imitation UXOs, but all of the UXOs were detected. As a result, it was found that UXO detection by small loop TEM method is effective compared to magnetic method. Moreover, the small loop TEM method could narrow the range of UXO location to the size of 30×30cm-square.

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.

The transient electromagnetic (TEM) responses measured in areas with highly polarizable media commonly show induced polarization (IP) effects, which poses difficulties to the TEM data interpretation and inversion with conventional methods. When present, the IP effects break the monotony in TEM decay curves and can even cause sign reversals, and lead to the singularity and non-monotony of inversion method. It is therefore important to take the IP effects into account when processing those TEM data. In this paper, a new inversion method is developed to invert TEM data with IP effects. The Cole-Cole model was introduced to the forward modeling solver so that the IP effects can be accurately calculated. The regularization terms were adapted to the objective function and a global optimization method, the particle swarm optimization algorithm, was used to solve the optimization problem, which weakened the singularity and non-monotony of the inversion greatly. Field data were collected in Wulong gold mine where IP effects were detected. The data were inverted using the method developed in this paper, and the inversion results were able to identify the depth extension of NO. 163 ore-bearing quartz veins and their associated fine-grained diorite veins. The global inversion method in this paper primarily solved the inversion problem of IP-affected TEM responses, and recovered the polarization parameters, which can be used as an aid to the geological interpretation.

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

We analyze the relationship between induced polarization (IP) parameters and the specific surface area normalized to the pore volume (Spor) for an extensive sample database. We find that a single linear imaginary conductivity-Spor relation holds across a range of single-frequency IP data sets composed of sandstones and unconsolidated sediments that lack an appreciable metallic mineral content. We also apply a recent approach defined as Debye decomposition (DD) to determine normalized chargeability (mn), a global estimate of polarization magnitude from available spectral IP (SIP) data sets. A strong linear relationship between mn and Spor is also found across multiple data sets. However, SIP model parameters determined for samples containing metallic minerals are approximately two orders of magnitude greater than for the model parameters estimated for the nonmetallic sample database. We propose a concept of “polarizability of the mineral-fluid interface per unit Spor” to explain this difference, which is supported by the observed dependence of IP parameters on fluid conductivity between sample types. We suggest that this linear IP-Spor relation can be considered the IP equivalent of the classical Archie empirical relation. Whereas the Archie relation describes a power-law relation between electrical conductivity due to electrolytic conduction through the available interconnected pore volume, the IP-Spor relation is an equivalent relation between mineral-fluid interfacial polarization and available pore surface area.

<p>The prospection of electrical conductors and semi-conductors has been one of the classical applications of the induced polarization (IP) method, with recent laboratory studies permitting to gain a deeper insight into the parameters controlling the polarization response. However, the application of electrochemical models developed for laboratory measurements has been rarely taken into field-scale imaging data sets. To fill this gap, here we discuss IP imaging results collected in Zettlitz (Austria), a former quarry operated between 1855 and 1967 for the extraction of graphite, an electrical conductor. The general goal of the geophysical survey is to characterize the geometry and volume of the residual graphite at the site. To this end, frequency-domain IP imaging measurements were collected along 10 main transects using different geometries, with selected data sets collected in the frequency range between 0.25 and 1 Hz to gain information about the frequency-dependence of the electrical properties. As expected, initial measurements revealed a high IP response in the graphite-rich areas. Nevertheless, the high electrical conductivity of the materials resulted in low voltage readings and an important decrease in the signal-to-noise ratio for deep measurements; thus, significantly reducing the depth of investigation. To overcome this limitation, we conducted measurements at areas of interest using transient electromagnetic (TEM) soundings, which are favored by the high conductivity of the targeted graphite and permit a better delineation of the contact to the calcareous host-rock. Initial analysis of the TEM data revealed a poor consistency with the electrical models retrieved from the IP surveys. However, taking into account the IP effect within the inversion of the TEM data significantly improved the consistency in the subsurface models resolved by the different methods. In order to resolve for adequate parameters for the modeling of TEM signatures, IP measurements were also collected at relevant positions in the frequency-range between 0.01 and 10000 Hz, with a high accuracy electrical impedance spectrometer. Further IP measurements were also collected in rock samples in the laboratory to aid in the interpretation of the field surveys and to permit the numerical modeling of the electrical signatures using a recently proposed electrochemical model. Our results demonstrate that the combination of IP and TEM surveys provide an improved modeling of the field signatures and, thus, a better characterization of the site. Additionally, we discuss the applicability of existing empirical and numerical models for the quantitative interpretation of field surveys.</p>

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.

We have developed a new high-resolution sea floor mapping system, based on the induced polarization (IP) principle. This system uses a weighted streamer, designed to be towed directly on the sea floor at speeds up to 3 knots. Signal injection is achieved by means of a pair of titanium transmitter electrodes that force 2+ amps in the form of a 4 Hz square-wave into the sediments. Two staggered receiver dipole arrays are used to detect polarizing materials in the top 5+ meters below the sediment-water interface. Using this system, we can continuously characterize the sea floor at two different depths by means of a geophysical acquisition and processing system (providing resistivity and phase for two different depths) and a GPS receiver positioned on a ship. Data acquisition and control are via a laptop computer, and the entire system is highly portable and battery-powered. We have demonstrated that the array can detect ilmenite (a titanium-bearing heavy placer mineral) to concentrations at or below 1% by volume, and can also characterize lithologic changes beneath an overlying veneer of sand. We believe that the system has additional potential applications, including rapid characterization of anthropogenic materials (including urban waste), identification and mapping of buried cables, unexploded ordnance (UXO), and possibly even to map buried archeological sites. Theoretically we can acquire more IP measurements in a week at sea than have been acquired historically on land over the past 50 years.

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.

This research was conducted to assess the possibilities of siting sustainable aquifers for borehole drilling and to assess the quality of water in hand-dug wells and existing boreholes in the Oti Pendjari and Obosum Groups of the Atebubu municipality. Groundwater is the main source of water to the people of Atebubu, but access and sustainability has been a major challenge over the years, owing to its complex underlying rocks. Groundwater occurrence and exploitation in the Voltaian has been reported by many researchers to be poor. In this research, the induced polarization (IP) method has been employed to delineate locations of high groundwater potential in the Atebubu municipality and to assess the quality of groundwater used by inhabitants of the area. The IP data was processed using the RES2DINV. version 4.9.18 Inversion and Modelling software, while the groundwater samples were analysed with the SPSS for Windows, version 22.0. The results revealed moderately to extremely fractured rocks with high possibility of water contents at greater depths, where rocks such as sandstone, pebbly beds and limestone are expected to exist. The low groundwater potential zones are likely to be covered by rocks such as mudstone or shale. A minimum drilling depth of 120 m is generally recommended for the municipality. The water quality analysis results also revealed that the water the people of the municipality drink is of high quality as all physicochemical parameters and heavy metals tested for were well within the World Health Organization’s (WHO’s) standards for drinking water.

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.

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.

Summary This paper explores the integration of 3D Electrical Resistivity Tomography (ERT) and 3D Induced Polarization (IP) for detecting unexploded ordnance (UXO) in subsurface environments. The methodology leverages the conductive and chargeable properties of metallic UXO materials, providing a powerful alternative to conventional geophysical techniques such as Ground Penetrating Radar (GPR), Electromagnetics (EM), and magnetometry, especially in urban or noise-contaminated settings. Using high-resolution, multi-channel acquisition systems and advanced inversion software, detailed resistivity and chargeability models are constructed to highlight subsurface anomalies. A critical indicator is the dipolar signature-spatially correlated resistivity and chargeability anomalies-that characterizes metallic targets. Field case studies in Trento and Naples validate the method’s effectiveness, confirming its ability to identify small-scale UXO targets in challenging site conditions. Compared to 2D methods, the 3D approach enhances detection accuracy, eliminates interpolation errors, and captures complex geometrical features. The results support ERT3D-IP as a non-invasive, cost-effective, and adaptable solution for UXO detection, with broader implications for geophysical applications in civil and defense projects.

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