SUMMARY: Induced polarization (IP) is an acknowledged method in ore exploration and can be applied to evaluate the metal content in dumps containing the residues of ore processing facilities. Existing models explain the relationships between ore content and grain size of the ore particles with IP parameters. However, the models assume full water saturation of the ore containing samples, which is often not the case in field conditions at dump sites. Hence, our study investigates the effect of desaturation on the resulting IP signal. We used six different sand–pyrite mixtures with varying amount and grain sizes of the pyrite particles. Evaporative drying desaturated the samples. Complex conductivity spectra were recorded in the frequency range between 0.02 and 1000 Hz at certain saturation levels. The resulting spectra indicate an decrease of the conductivity amplitude with progressing desaturation. This effect agrees with the second empirical Archie equation. The saturation exponent of the conductivity amplitude shows values slightly larger than one. The measured spectra were processed by a Debye decomposition. We observe a nearly constant total chargeability during desaturation. This finding is in agreement with existing models that relate the total chargeability to the metal content in the sample. However, the mean relaxation time decreases remarkably during the drying process, whereas the normalized relaxation time, which considers the ratio between the mean relaxation time and the resistivity of the embedding material, does not indicate any dependence on water saturation. This behaviour contradicts existing models that predict a decreasing relaxation time with increasing water salinity, which results from evaporative drying. In order to explain the observed effects, we propose a conceptional model that compares a mixture of pyrite particles in an embedding material (sand, water and air) with an electrical RC circuit. The pyrite grains behave as small condensers that are charged and discharged via the conductive background material. According to this simple physical model, the relaxation time is proportional to the resistivity of the embedding material. A resistivity increase while desaturation causes an increase of relaxation time as observed in our experiments. This conceptional model is in good agreement with other experiments that change the resistivity in the background material by varying water salinity or clay content. The capacitive behaviour of the dispersed particles is characterized by the normalized relaxation time that depends on the grain size and the volume content of the pyrite particles.

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.

SUMMARY: Induced polarization (IP) is a common method in ore exploration. IP spectra measured over a wide frequency range can be used to characterize material properties of ores, slags and other residual material from mines and processing facilities. Previous studies have shown that IP parameters are sensitive to type, content or grain size of electronically conductive or semi-conductive minerals. Up to now, a variety of experiments has been performed on sand mixtures with fractions of ore minerals. Most experiments consider only a single fraction with a fixed grain size. We continue a series of experiments that have been done with sand–pyrite mixtures. The presented study compares IP spectra recorded for samples either with a single grain radius fraction (E-samples) or with two different grain radii fractions (Z-samples). The spectra are fitted to Pelton models. A Debye decomposition that provides a relaxation time distribution (RTD) is applied to the complex conductivity spectra. The RTD indicates separated maxima only if the ratio of mean grain radii is larger than a factor five. The resolution of the phase spectra and the spectra of imaginary part of conductivity is lower. Even though the volumetric pyrite content is equal in each fraction, the phase spectra and RTD of the Z-samples indicate much higher signals for the pyrite fraction with smaller grain radius. The same observation is made for the chargeability that shows larger values for decreasing grain radii. This finding contradicts existing theories that consider the chargeability as a suitable proxy of the volumetric content of ore minerals. We explain the observed effect by an interaction between neighbouring pyrite particles. The conductivity of the mixtures of the E-samples increases with decreasing grain radius of the pyrite fraction. This effect is attributed to dissolution effects on the surface of the pyrite particles during the sample preparation. We find that the additive superposition of the phase spectra of two E-samples is in good agreement with the measured phase spectra of the Z-samples (measured superposition) containing the two corresponding pyrite fractions. The agreement is slightly worse for the spectra of imaginary part of conductivity, where the measured superposition overestimates the mathematical superposition. The experimental results of our study motivate a further improvement of existing mechanistic models.

To help meet emission standards, hydrogen sulfide (H₂S) from geothermal production may be injected back into the subsurface, where basalt offers, in theory, the capacity to mineralize H₂S into pyrite. Ensuring the viability of this pollution mitigation technology requires information on how much H₂S is mineralized, at what rate and where. To date, monitoring efforts of field‐scale H₂S reinjection have mostly occurred via mass balance calculations, typically capturing less than 5% of the injected fluid. While these studies, along with laboratory experiments and geochemical models, conclude effective H₂S mineralization, their extrapolation to quantify mineralization and its persistence over time leads to considerable uncertainty. Here, a geophysical methodology, using time‐domain induced polarization (TDIP) logging in two of the injection wells (NN3 and NN4), is developed as a complementary tool to follow the fate of H₂S re‐injected at Nesjavellir geothermal site (Iceland). Results show a strong chargeability increase at +40 days, interpreted as precipitation of up to 2 vol.% based on laboratory relationships. A uniform increase is observed along NN4, whereas it is localized below 450 m in NN3. Changes are more pronounced with larger electrode spacing, indicating that pyrite precipitation takes place away from the wells. Furthermore, a chargeability decrease is observed at later monitoring rounds in both wells, suggesting that pyrite is either passivated or re‐dissolved after precipitating. These results highlight that a sequence of overlapping reactive processes (pyrite precipitation, passivation, pore clogging and possibly pyrite re‐dissolution) results from H₂S injection and that TDIP monitoring is sensitive to this sequence.

Summary: Characterizing ore deposits or mining dumps in terms of mineral content and grain size remains a challenge. Since the 1950s the Induced Polarization (IP) method has been successfully applied in ore prospecting. However, reliably interpreting field survey data requires comprehensive laboratory studies to establish a link between the IP parameters from empirical or phenomenological models and the type and quantity of ore minerals. In this study, we use numerical electrical networks to replicate the complex electrical resistivity spectra observed in experiments on sand-pyrite-water mixtures. A network consists of a 3D assembly of resistors, representing the saturated pore space, and leaky capacitors simulating the electrical behaviour of ore minerals. A sophisticated fitting procedure enables the precise determination of resistor and capacitor parameters, ultimately leading to strong agreement between measured and synthetic IP spectra. The results obtained from the 3D network align well with the classical Pelton model, which is based on a simple equivalent circuit. Our findings indicate that the network's chargeability depends on the fraction of capacitors in the system (i.e. the number of capacitors divided by the number of capacitors and resistors), and that the Pelton time constant of the measured spectra is closely related to the resistor and capacitor parameters. We argue that a 3D approach offers a more realistic framework, paving the way for future studies on the effects of ore grain size distribution, and the spatial arrangement of ore grains.