Spectral Induced Polarization Research Articles

Application of the SIP method to metal resource exploration

Application of the SIP method to metal resource exploration

Prediction of permeability of cement-admixed soft clay using resistivity and time-domain IP measurements

Permeability is one of the most important petrophysical parameters, which unfortunately is quite difficult to be tested and estimated, particularly for the fine-grained soils and mixed soils. Prediction of permeability based on geophysical measurements is currently one of the most challenging issues in petrophysics. There have been recently reported some empirical relationships between permeability, resistivity and spectral induced polarization (SIP) parameters for a porous medium. However, the disadvantage of this approach is the very scarcity of SIP data as most of practical measurements are time-domain IP. In this study, a detailed overview of permeability prediction models using resistivity and spectral IP data was made. More than that, an innovative approach using resistivity and time-domain IP measurements to predict permeability of cement-admixed Bangkok clay was proposed and successfully applied for tested samples based on measurements of resistivity and time-domain IP data. A good amount of geotechnical and geophysical tests was conducted to investigate the time-dependent development of strength, porosity, and permeability of cement-mixed Bangkok soft clay samples during a 28-day curing process. The permeability predicted by resistivity and chargeability model matched well with permeability measured by consolidation testing. In addition, a series of correlations between unconfined compressive strength, porosity and permeability as measured by geotechnical testing and resistivity and chargeability as measured by geophysical testing were found.

스카른 금속광상 탐사를 위한 광대역 유도분극법 적용성

광물자원개발에 있어서 탐사 대상이 천부 내지 고품위 광체에서 심부 또는 저품위 광체로 전환됨에 따라 보다 진보된 탐사 기술 개발이 요구되고 있다. 본 연구에서는 스카른 금속광상의 하나인 가곡광산을 대상으로 정밀 탐사가 가능한 광대역 유도분극(spectral induced polarization; SIP)을 이용한 암석시료의 실내 물성 측정과 현장탐사를 수행했다. 또한 실내 측정에서 획득한 암석의 광대역 유도분극 특성과 현장 탐사자료의 역해석 결과를 종합적으로 해석함으로써 스카른 금속광상에 대한 광대역 유도분극 탐사 적용성을 검토하였다. 암상별 광대역 유도분극 특성을 파악하기 위해 기존 연구에서 사용된 89개 시료들의 자료를 재평가했다. 현장탐사는 암상의 경계와 스카른 광화대를 평가할 수 있도록 측선을 설정하고, 0.25 Hz 주파수 대역의 자료를 획득하고 역산해석을 수행했다. 광석광물을 수반하고 있는 암석과 스카른 광화대는 낮은 전기비저항 구간을 보이며, 충전성이 높고 음의 위상이 나타났다. 따라서 광대역 유도분극 탐사는 스카른 광상의 광화작용 특성을 해석하는데 유용하다는 것을 알 수 있었다. The development of more advanced geophysical exploration techniques is necessary because the orebodies as yet discovered are increasingly changing in characteristics from shallow/high-grade to deep/low-grade. In this work, laboratory measurement of physical properties of rock samples and a field survey and interpretation of spectral induced polarization (SIP) have been conducted in a skarn metallic deposit, Gagok mine. The purpose of this study is that the applicability of SIP in the exploration of skarn metallic deposits is verified by the comprehensive interpretation between SIP characteristics of rocks obtained from the laboratory measurements and inverted survey results from the field data. In order to understand the SIP characteristics of each lithology, the data of eighty nine rock samples utilized in the previous studies were revaluated. The field survey was carried out using frequency of 0.25 Hz along a survey line designed for intersecting lithological boundaries and evaluating mineralized zones. The mineralized rocks were more conductive (low-resistivity) and capacitive (high-chargeability or strong-phase) than other rocks. Thus, SIP can be one of the very useful tools for the mineral exploration of the skarn deposits.

Sensitivity of 2-D complex resistivity measurements to subsurface anisotropy

Sensitivity of 2-D complex resistivity measurements to subsurface anisotropy

Reconstruction quality of SIP parameters in multi‐frequency complex resistivity imaging

ABSTRACTComplex resistivity imaging provides information on the subsurface distribution of the electrical conduction and polarisation properties. Spectral induced polarisation (SIP) refers to the frequency dependence of these complex resistivity values. Measured SIP signatures are commonly analysed by performing a Cole–Cole model fit or a Debye decomposition, yielding in particular chargeability and relaxation time values. Given the close relation of these parameters with petrophysical properties of relevance in various hydrogeological and environmental applications, it is crucial to understand how well they can be reconstructed from multi‐frequency complex resistivity imaging with subsequent Cole–Cole or Debye decomposition analysis. In this work, we investigate, in a series of numerical simulations, the reconstruction behaviour of the main spectral induced polarisation parameters across a two‐dimensional complex resistivity imaging plane by considering a local anomalous polarisable body at different depths. The different anomaly positions correspond to different cumulated sensitivity (coverage) values, which we find to be a simple and computationally inexpensive proxy for resolution. Our results show that, for single‐frequency measurements, the reconstruction quality of resistivity and phase decreases strongly with decreasing cumulated sensitivity. A similar behaviour is found for the recovery of Cole–Cole and Debye decomposition chargeabilities from multi‐frequency imaging results, while the reconstruction of the Cole–Cole exponent shows non‐uniform dependence over the examined sensitivity range. In contrast, the Cole–Cole and Debye decomposition relaxation times are relatively well recovered over a broad sensitivity range. Our results suggest that a quantitative interpretation of petrophysical properties derived from Cole– Cole or Debye decomposition relaxation times is possible in an imaging framework, while any parameter estimate derived from Cole–Cole or Debye decomposition chargeabilities must be used with caution. These findings are of great importance for a successful quantitative application of spectral induced polarisation imaging for improved subsurface characterisation, which is of interest particularly in the fields of hydrogeophysics and biogeophysics.

Spectral‐induced polarization measurements on sieved sands and the relationship to permeability

AbstractLaboratory measurements of the permeability and spectral‐induced polarization (SIP) response of samples consisting of unconsolidated sands typical of those found in New Zealand aquifers have been made. After correction of measured formation factors to allow for the fact that some were measured at only one fluid conductivity, predictions of permeability from the grain size (d) of the samples are found to agree well with measured values of permeability. The Cole‐Cole time constant (derived from the SIP measurements) is found, as expected, to depend upon d2, but can be affected by the inclusion of smaller grains in the sample. Measurements made on samples comprising of mixtures of grain sizes show that inclusion in a sample of even 10% of smaller grains can significantly reduce both the Cole‐Cole time constant (τCC) and the permeability, and support theoretical derivation of how the permeability of a mixture of grain sizes varies with the content of the mixture. Proposed relationships for using τCC as a predictor for permeability are tested and found to be crucially dependent on the assumed relationship between the dynamic pore radius and grain size. The inclusion of a multiplicative constant to take account of numerical approximations results in good predictions for the permeability of the samples in this study. It seems unlikely, however, that there is a single global expression for predicting permeability from SIP data for all samples.

Spectral-induced polarization characterization of rocks from the Handuk iron mine, South Korea

Spectral-induced polarization (SIP) using the electrochemical reaction at the interface between mineral particle and groundwater is a powerful tool for the exploration of minerals. However, SIP responses of rocks have not been fully characterized because it is influenced by various petrological properties. In this study, the relationship between SIP responses and the petrological features of representative rocks from skarn deposits was analyzed. For the evaluation of the SIP responses, chargeability and relaxation time of time domain induced polarization (IP) were calculated by an equivalent circuit analysis. Among various circuit models used for the analysis, an existing circuit model was utilized for this study because it sufficiently describes the electrochemical reaction of water-saturated rocks and is more suitable for the analysis of heterogeneous rocks than the other models. The circuit model was found to be appropriate because the normalized root mean square error between the observed values and ones obtained through the model fittings reasonably low, ranging from 4.18 to 0.05 %. The chargeability values of the mineralized rocks were higher than those of the non-mineralized rocks (igneous and carbonates), and the relaxation time was related to the grain size of the magnetite. It was found that the SIP characteristics of the mineralized rocks are dependent on features of the ore minerals. In the non-mineralized rocks, there was little difference in the chargeability, whereas the variation in their relaxation time values was influenced by the feldspar content (sericitization) and the grain size of the calcite (recrystallization). The SIP characteristics of the non-mineralized rocks are considered to be dependent on the grade of the hydrothermal alteration.

Estimating properties of unconsolidated sand-clay from spectral-induced polarization

Spectral-induced polarization (SIP) of glass beads, gravel, sand and sand-clay mixtures (5, 10 and 20% clay by weight) was measured over 10 mHz to 100 Hz by employing an SR810 DSP lock-in amplifier. Phase shift (j), and real part (s¢) and imaginary part (s¢¢) of complex electrical conductivity of the materials were calculated from the measured voltage and its x- and y-components to link them to physico-chemical properties of the materials. The phase shift decreased with the increase in salt concentration in pore water and diminished above a material-specific limiting salt concentration. The imaginary part of electrical conductivity was strongly correlated with clay content of the samples. For gravel and sand, j was strongly sensitive to pore-water content at 0.1 Hz, and there was a material-specific correlation between j and pore-water content. The ratio of the real part of electrical conductivity of the unsaturated samples to that of the saturated samples increased linearly with the increasing degree of pore-water saturation (ratio of water content of a sample to its water content at saturation) except for sand+20% clay mixture, in which the real part of electrical conductivity ratio first increased linearly and then leveled off at high (» 0.6) water saturation. The real part of electrical conductivity ratio versus the degree of pore-water saturation relationship was different for sand+10% clay and sand+20% clay samples but unique for sand and sand+5% clay samples. Such relationships of real part of electrical conductivity ratio of soils to their pore-water saturation provide prospects of predicting important soil properties such as soil salinity.

Relationship between Cole–Cole model parameters and spectral decomposition parameters derived from SIP data

SUMMARY Spectral induced polarization (SIP) data are commonly analysed using phenomenological models. Among these models the Cole–Cole (CC) model is the most popular choice to describe the strength and frequency dependence of distinct polarization peaks in the data. More flexibility regarding the shape of the spectrum is provided by decomposition schemes. Here the spectral response is decomposed into individual responses of a chosen elementary relaxation model, mathematically acting as kernel in the involved integral, based on a broad range of relaxation times. A frequently used kernel function is the Debye model, but also the CC model with some other a priorly specified frequency dispersion (e.g. Warburg model) has been proposed as kernel in the decomposition. The different decomposition approaches in use, also including conductivity and resistivity formulations, pose the question to which degree the integral spectral parameters typically derived from the obtained relaxation time distribution are biased by the approach itself. Based on synthetic SIP data sampled from an ideal CC response, we here investigate how the two most important integral output parameters deviate from the corresponding CC input parameters. We find that the total chargeability may be underestimated by up to 80 per cent and the mean relaxation time may be off by up to three ordersofmagnituderelativetotheoriginalvalues,dependingonthefrequencydispersionofthe analysed spectrum and the proximity of its peak to the frequency range limits considered in the decomposition. We conclude that a quantitative comparison of SIP parameters across different studies, or the adoption of parameter relationships from other studies, for example when transferring laboratory results to the field, is only possible on the basis of a consistent spectral analysis procedure. This is particularly important when comparing effective CC parameters with spectral parameters derived from decomposition results.

Two-dimensional inversion of spectral induced polarization data using MPI parallel algorithm in data space

Traditional two-dimensional (2D) complex resistivity forward modeling is based on Poisson’s equation but spectral induced polarization (SIP) data are the coproducts of the induced polarization (IP) and the electromagnetic induction (EMI) effects. This is especially true under high frequencies, where the EMI effect can exceed the IP effect. 2D inversion that only considers the IP effect reduces the reliability of the inversion data. In this paper, we derive differential equations using Maxwell’s equations. With the introduction of the Cole–Cole model, we use the finite-element method to conduct 2D SIP forward modeling that considers the EMI and IP effects simultaneously. The data-space Occam method, in which different constraints to the model smoothness and parametric boundaries are introduced, is then used to simultaneously obtain the four parameters of the Cole—Cole model using multi-array electric field data. This approach not only improves the stability of the inversion but also significantly reduces the solution ambiguity. To improve the computational efficiency, message passing interface programming was used to accelerate the 2D SIP forward modeling and inversion. Synthetic datasets were tested using both serial and parallel algorithms, and the tests suggest that the proposed parallel algorithm is robust and efficient.

Relaxation time distribution obtained from a Debye decomposition of spectral induced polarization data

We have developed an alternative formulation for Debye decomposition of complex electric conductivity spectra, by recasting it into a new set of parameters with a close relationship to the continuous formulation for the complex conductivity method. The procedure determines a relaxation time distribution (RTD) and two frequency-independent parameters that modulate the complex conductivity spectra. These two parameters represent (1) the direct current contribution and (2) the conductivity range spanned by the low- and high-frequency limits. The distribution of relaxation times quantifies the contribution of each distinct relaxation process. Assuming that characteristic times with insignificant contributions can be ignored, a minimum set of characteristic relaxation times is determined. Each contribution can then be associated with specific polarization processes that can be interpreted in terms of electrochemical or interfacial parameters of mechanistic models derived from inverted parameters obtained from the proposed approach. Synthetic tests show that the procedure can fit spectral induced polarization (SIP) data and successfully retrieve the RTD. We have applied the procedure to laboratory SIP data from experiments with sand and oil mixtures undergoing microbial degradation of hydrocarbons. The RTD reveals evidence of a length scale at which a new polarization process takes place as a result of the biodegradation process.

Using electrical anisotropy for structural characterization of sediments: an experimental validation study

ABSTRACTImproved characterization of subsurface heterogeneity is important to better understand a range of key processes in the hydrologic cycle, such as overland flow, infiltration into the soil, evaporation to the atmosphere, and transpiration by plants. Recently, synthetic modelling studies have shown that information on subsurface heterogeneity can be obtained from the anisotropy in electrical resistivity. The objective of this paper is to experimentally validate the findings of this synthetic modelling study. In order to do so, we developed a new measurement procedure to determine the effective complex electrical resistivity from a set of current injections and voltage measurements on a heterogeneous sample. A synthetic modelling study showed that this new measurement procedure was able to reproduce the results of the previous study that showed how the electrical properties and the correlation length ratio of bimodal distributions of two materials can be obtained from the effective complex electrical resistivity measured in two perpendicular directions. After validation of the new measurement approach, we constructed two bimodal distributions in a 2D measurement cell and applied the newly developed measurement strategy. We were able to estimate the electrical properties, the volume fraction, and the correlation length ratio with good accuracy from the complex resistivity measurements in two directions. The remaining differences were attributed to variations in sediment thickness that occurred during sample preparation. We conclude that anisotropy should not be ignored but embraced when dealing with subsurface heterogeneity and that proper interpretation of anisotropy may actually be used to characterize subsurface heterogeneity.

Salinity dependence of the complex surface conductivity of the Portland sandstone

Induced polarization can be used to estimate surface conductivity by assuming a universal linear relationship between the surface and quadrature conductivities of porous media. However, this assumption has not yet been justified for conditions covering a broad range of fluid conductivities. We have performed complex conductivity measurements on Portland sandstone, an illite- and kaolinite-rich sandstone, at 13 different water salinities (NaCl) over the frequency range of 0.1 Hz to 45 kHz. The conductivity of the pore water [Formula: see text] affected the complex surface conductivity mainly by changing the tortuosity of the conduction paths in the pore network from high to low salinities. As the fluid conductivity decreases, the magnitude of the surface conductivity and quadrature conductivity was observed to decrease. At relatively high salinities ([Formula: see text]), the ratio between the surface conductivity and quadrature conductivity was roughly constant. At low salinities ([Formula: see text]), the ratio decreased slightly with the decrease of the salinity. A Stern layer polarization model was combined with the differential effective medium (DEM) theory to describe this behavior. The tortuosity entering the complex surface conductivity was salinity dependent following the prediction of the DEM theory. At high salinity, it reached the value of the bulk tortuosity of the pore space given by the product of the intrinsic formation factor and the connected porosity. The relaxation time distributions were also obtained at different salinities by inverting the measured spectra using a Warburg decomposition. The mode of the relaxation time probability distribution found a small but clear dependence on the salinity. This salinity dependence can be explained by considering the ions exchange between Stern and diffuse layers during polarization of the former. The pore-size distribution obtained from the distribution of the relaxation time agreed with the pore-size distribution from nuclear magnetic resonance measurements.

The dependence of induced polarization on fluid salinity and pH, studied with an extended model of membrane polarization

The estimation of hydraulic parameters from spectral induced polarization (SIP) measurements is difficult partly because the electrical impedance of sediments depends on several parameters that are not related to the texture. Important parameters that influence the spectral response are fluid salinity and pH. In order to understand the behaviour of SIP spectra from a mechanistic point of view, we carry out simulations with a membrane polarization model. The geometry consists of a sequence of wide and narrow pores with finite radii. The charge distribution at the mineral surface is described by a triple layer model, characterized by the zeta potential and the partition coefficient. We extended an existing model by incorporating known dependencies of the zeta potential and the partition coefficient on fluid salinity and pH.Our simulation results predict a decrease of the maximum phase shift of the complex electrical conductivity with increasing salinity, consistent with experimental observations. For very small pore radii, the phase shift may also show the opposite behaviour and increase with salinity. The imaginary conductivity at 1Hz increases with increasing salinity, followed by a peak and a decrease at high salinities. The fact that our model predicts a decrease of the imaginary conductivity at high salinities is particularly important, because strong experimental evidence was recently found for such a decrease, which was theoretically unexplained so far.Both the maximum phase shift and the imaginary conductivity at 1Hz decrease when pH decreases. The reason is that at low pH, the zeta potential and the partition coefficient both decrease, corresponding to a smaller charge density at the mineral surface, resulting in a weaker impact of the electrical double layer. The few existing experimental studies on pH dependence are qualitatively consistent with our simulation results.

Anomalous frequency and temperature-dependent scattering and Hund's coupling in the almost quantum critical heavy-fermion systemCeFe2Ge2

We present THz range optical conductivity data of a thin film of the near quantum critical heavy fermion compound CeFe$_2$Ge$_2$. Our complex conductivity measurements find a deviation from conventional Drude-like transport in a temperature range previously reported to exhibit unconventional behavior. We calculate the frequency dependent effective mass and scattering rate using an extended Drude model analysis. We find the inelastic scattering rate can be described by a temperature dependent power-law $\omega^{n(T)}$ where $n(T)$ approaches $\sim1.0 \pm 0.2$ at 1.5 K. This is compared to the $\rho \sim T^{1.5}$ behavior claimed in dc resistivity data and the $\rho \sim T^{2}$ expected from Fermi-liquid theory. In addition to a low temperature mass renormalization, we find an anomalous mass renormalization that persists to high temperature. We attribute this to a Hund's coupling in the Fe states in a manner similar to that recently proposed in the ferro-pnictides. CeFe$_2$Ge$_2$ appears to be a very interesting system where one may study the interplay between the usual $4f$ lattice Kondo effect and this Hund's enhanced Kondo effect in the $3d$ states.

Comparison of the surface ion density of silica gel evaluated via spectral induced polarization versus acid–base titration

Surface complexation models are widely used with batch adsorption experiments to characterize and predict surface geochemical processes in porous media. In contrast, the spectral induced polarization (SIP) method has recently been used to non-invasively monitor in situ subsurface chemical reactions in porous media, such as ion adsorption processes on mineral surfaces. Here we compare these tools for investigating surface site density changes during pH-dependent sodium adsorption on a silica gel. Continuous SIP measurements were conducted using a lab scale column packed with silica gel. A constant inflow of 0.05M NaCl solution was introduced to the column while the influent pH was changed from 7.0 to 10.0 over the course of the experiment. The SIP measurements indicate that the pH change caused a 38.49±0.30μScm−1 increase in the imaginary conductivity of the silica gel. This increase is thought to result from deprotonation of silanol groups on the silica gel surface caused by the rise in pH, followed by sorption of Na+ cations. Fitting the SIP data using the mechanistic model of Leroy et al. (Leroyet al., 2008), which is based on the triple layer model of a mineral surface, we estimated an increase in the silica gel surface site density of 26.9×1016sitesm−2. We independently used a potentiometric acid–base titration data for the silica gel to calibrate the triple layer model using the software FITEQL and observed a total increase in the surface site density for sodium sorption of 11.2×1016sitesm−2, which is approximately 2.4 times smaller than the value estimated using the SIP model. By simulating the SIP response based on the calibrated surface complexation model, we found a moderate association between the measured and estimated imaginary conductivity (R2=0.65). These results suggest that the surface complexation model used here does not capture all mechanisms contributing to polarization of the silica gel captured by the SIP data.

Assessing the high frequency behavior of non-polarizable electrodes for spectral induced polarization measurements

During the last decades, the usage of spectral induced polarization (SIP) measurements in hydrogeology and detecting environmental problems has been extensively increased. However, the physical mechanisms which are responsible for the induced polarization response over the usual frequency range (typically 1mHz to 10–20kHz) require better understanding. The phase shift observed at high frequencies is sometimes attributed to the so-called Maxwell–Wagner polarization which takes place when charges cross an interface. However, SIP measurements of tap water show a phase shift at frequencies higher than 1kHz, where no Maxwell–Wagner polarization may occur. In this paper, we enlighten the possible origin of this phase shift and deduce its likely relationship with the types of the measuring electrodes. SIP Laboratory measurements of tap water using different types of measuring electrodes (polarizable and non-polarizable electrodes) are carried out to detect the origin of the phase shift at high frequencies and the influence of the measuring electrodes types on the observed complex resistivity. Sodium chloride is used to change the conductivity of the medium in order to quantify the solution conductivity role. The results of these measurements are clearly showing the impact of the measuring electrodes type on the measured phase spectrum while the influence on the amplitude spectrum is negligible. The phenomenon appearing on the phase spectrum at high frequency (>1kHz) whatever the electrode type is, the phase shows an increase compared to the theoretical response, and the discrepancy (at least in absolute value) increases with frequency, but it is less severe when medium conductivity is larger. Additionally, the frequency corner is shifted upward in frequency. The dependence of this phenomenon on the conductivity and the measuring electrodes type (electrode–electrolyte interface) seems to be due to some dielectric effects (as an electrical double layer of small relaxation time formed at the electrodes interface). Therefore, this dielectric response should be taken into account at high frequency to better analytically separate the medium own response from that linked to the measuring electrodes used. We modeled this effect by adding a capacitance connected in parallel with the traditional equivalent electric circuit used to describe the dielectric response of medium.

Development of a three-dimensional icing simulation code in the NSMB flow solver

An icing model is developed in the Navier-Stokes multi-block (NSMB) compressible solver. The code implemented in this study is based on an Eulerian formulation for droplets tracking solved implicitly by means of CGSTAB or SIP methods, a modified iterative Messinger model using an improved water runback scheme for ice thickness calculation and three-dimensional mesh deformation to track the ice/air interface through time. The whole process is parallelised with MPI for efficient calculations. Validation is performed on several test cases including NACA23012 and NACA00012 airfoils and ONERA-M6 swept wing. Pressure coefficients around iced 23012 airfoil are compared with experimental data. Rime icing is computed on a DLR-F6 wing/body configuration. In all studied cases, the results are in good agreement with the literature.

Connecting complex conductivity spectra to mercury porosimetry of sedimentary rocks

The measured spectral-induced polarization (SIP) response of highly porous granular materials can be modeled by upscaling the Nernst-Planck equation for a single grain and performing a convolution product with the particle size distribution. The distribution of relaxation times associated with the polarization of consolidated porous media seems, intuitively, to be controlled by the pore size distribution because of the radius of curvature of the interface between the solid and the pore water. We have explored a mechanistic model connecting the SIP response of consolidated media to mercury injection capillary porosimetry (MICP) data. First, we developed a mechanistic electric double layer (EDL) polarization model for porous media, which considered the polarization of the Stern layer, which was possibly enhanced by the polarization of the diffuse layer and the effect of the pore size distribution. The new model revealed that the relaxation time of porous media such as sandstones could be overestimated if the enhancement of the overall polarization by the diffuse layer was not considered. Second, we have developed an empirical method to obtain the pore size distribution from MICP data by assuming a linear relation between the pore size and the pore throat size using a pore aspect ratio. Then, we compared the measured quadrature conductivities of nine samples with the theoretical quadrature conductivity obtained by convoluting the pore size distribution with the Warburg model. The Warburg model is considered to represent the quadrature conductivity response of a consolidated porous material characterized by an infinitively narrow unimodal pore size distribution coated by an EDL. The position of the peak of the phase was well-reproduced from the MICP data. In addition, the calculated broadness of the phase spectra was usually well-reproduced by convolving the Warburg function with the pore size distribution.

Indirect determination of soil water content

This manuscript reviews the recent developments in indirect methods used to measure soil watent content (SWC). It focuses on three types of methodological approaches: (i) high-frequency electromagnetic methodologies (dielectric methods and Ground Penetrating Radar), (ii) low-frequency electromagnetic methods (resistivity method and spectral induced polarization-SIP) and (iii) emerging methods (laboratory and ground-based nuclear magnetic resonance-NMR techniques and methods based on fiber optic temperature sensing systems). For each method the physical principles and the corresponding methodology are briefly described first, followed by the recent methodological advances. These recent advances address the following issues: (a) integration of different types of sensors to design new multi-functional devices, (b) miniaturization of low-cost and easily-installed electromagnetic sensors, (c) fusion and assimilation of data acquired from multiple modalities with different sampling rates and spatial resolutions, (d) physical understanding of spectra and distributed parameters measured by SIP and NMR methods, (e) improvement of geophysical inversion techniques combined with fast data acquisitions (4D monitoring of SWC).