Three-Dimensional Inversion of Induced Polarization Effects in Airborne Time Domain Electromagnetic Data Using the GEMTIP Model

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.

The acquisition of airborne electromagnetic (AEM) data colocated with water well data provides an essential data set for constructing an accurate rock-physics relationship between resistivity from AEM and lithology from wells. Results from an AEM survey in Butte and Glenn Counties, California, USA, where 41 water wells are located within 100 m of the AEM flight lines, provide a unique opportunity to investigate the effect of water wells on AEM data. Based on systematic numerical analyses and examination of the field data, we find that the only noise source related to the wells that affects the AEM signals in our study area is the power lines; the effect of the steel casings in the study area is negligible. Our numerical simulations yield two important findings: (1) the effect of the power line is mainly dependent upon the resistivity of the ground pathway between the base of the power-line poles, and (2) for the resistivity structure in the study area, when the ground contact resistance is sufficiently low, the effect of the power line will be observed in the AEM data until there is a separation distance of 174 m from the power line; the total power-line percentage effect less than 3% has been considered to be negligible. However, we find that, in the field data, an average separation distance between the power line and high-quality AEM data (i.e., no impact from the power lines) is only 60% of this predicted distance. We attribute this to the high ground contact resistance caused by degradation of the grounded electrodes located at the bottom of the power-line poles. For our 41 wells, after removing any soundings contaminated by power-line noise, we are left with high-quality soundings located 0–160 m from the water wells, with an average distance of 77 m.

A major impediment in the path toward airborne induced polarization (IP) is an effective method to quantify data from inductive sources, such as those used in airborne electromagnetic systems. We modeled inductive IP using a combination of Warburg and exponential decay models as a basis for fitting electromagnetic data from ground time-domain electromagnetic (TEM) and airborne versatile TEM (VTEM) surveys. Observed decays were deconvolved into electromagnetic and IP constituents by constrained least-squares fitting of basis functions modified to account for transmitter waveforms. The method was confirmed through synthetic modeling of 2D and 3D structures, and when applied to ground TEM or airborne TEM data, obtained an estimate of apparent chargeability at each station or fiducial. In the case of a VTEM survey in Africa, the apparent chargeabilities mapped graphitic sediments and provided spatially consistent indications of clay concentrations. A limitation on this airborne IP for airborne applications is motion noise, which places a lower limit on usable base frequency and begins to significantly affect the signal at the later delay times, when IP effects are most visible.

Summary Induced polarization (IP) effects in airborne electromagnetic (AEM) surveys have commonly been investigated in helicopter-borne systems, leaving both a bibliographic and application gap for fixed-wing configurations. This gap partly reflects the large relative number of helicopter compared to fixed wing AEM systems, but also the geometric complexity of fixed wing platforms. In these platforms, nine geometric parameters come into play: the pitch, roll, and yaw of both transmitter and receiver, plus the three-axis offsets between the coils. Shifts in these factors can distort the measured data in ways that aren’t uniquely attributable, making it hard to pinpoint whether negative recordings truly arise from IP or from geometry-related effects. The non-fixed geometry also complicates removal of the primary field, often requiring iterative processing steps that may suppress or alter spectral content linked to IP. With advances in airborne IP understanding from helicopter-borne systems, revisiting fixed-wing platforms is both timely and necessary. Part A of this two-part study addresses this issue using the TEMPEST™ fixed-wing system connecting numerical modelling with field evidence. A suite of synthetic two-layer models with variable resistivity and chargeability parameters was developed to evaluate the system’s sensitivity to polarizable structures. The experiments demonstrate that IP effects, including negative secondary field responses, can be reliably detected in fixed-wing AEM data, both in X and Z magnetic field components. The capacity of these systems to detect IP phenomena is, however, strongly dependent on the electrical conductance of the environment. For instance, both fixed-wing and helicopter-borne systems, elevated near-surface conductance enhances the amplitude of purely electromagnetic induction currents, which in turn can dominate the recorded response and obscure the comparatively weaker polarization currents. More in general, IP detectability depends on the strength of the EM response generated by induction currents flowing elsewhere, which can dominate the small reverse current flow from a polarizable target. This highlights the critical role of near-surface conductivity in controlling the expression of IP responses and underscores the need to carefully account for these factors when interpreting survey data. The synthetic results are then connected with field-scale observations from a subset of the AusAEM dataset, over 470 000 line-km of TEMPESTTM data, where negative responses align with areas of low shallow conductance, confirming the simulation results. These finding open the way to the Part B of this study, where TEMPESTTM data are inverted taking into account IP and compared with helicopter-borne results and geological information.

The Frome airborne electromagnetic (AEM) survey was designed to provide reliable pre-competitive AEM data to aid the search for energy and mineral resources around the Lake Frome region of South Australia. Flown in 2010, a total of 32,317 line kilometres of high-quality airborne geophysical data were collected over an area of 95,450 km2 at a flight line spacing mostly of 2.5 km, opening to 5 km spaced lines in the Marree-Strzelecki Desert area to the north.Interpretations of the data show the utility of regional AEM surveying for mapping crucial elements of sandstone-hosted uranium mineral systems as well as for mapping geological surfaces, structures and depth of cover over a wide area. Data from the Frome AEM survey allow mineral explorers to put their own high-resolution AEM surveys into a regional context. Survey data were used to map and interpret a range of geological features that are associated with, or control the location of, sandstone-hosted uranium mineral systems, and have been used to assess the uranium prospectivity of new areas to the north of the Flinders Ranges.

Traditionally, airborne time-domain electromagnetic (ATEM) data are inverted to derive the earth model by iteration. However, the data are often highly correlated among channels and consequently cause ill-posed and over-determined problems in the inversion. The correlation complicates the mapping relation between the ATEM data and the earth parameters and thus increases the inversion complexity. To obviate this, we adopt principal component analysis to transform ATEM data into orthogonal principal components (PCs) to reduce the correlations and the data dimensionality and simultaneously suppress the unrelated noise. In this paper, we use an artificial neural network (ANN) to approach the PCs mapping relation with the earth model parameters, avoiding the calculation of Jacobian derivatives. The PC-based ANN algorithm is applied to synthetic data for layered models compared with data-based ANN for airborne time-domain electromagnetic inversion. The results demonstrate the PC-based ANN advantages of simpler network structure, less training steps, and better inversion results over data-based ANN, especially for contaminated data. Furthermore, the PC-based ANN algorithm effectiveness is examined by the inversion of the pseudo 2D model and comparison with data-based ANN and Zhody’s methods. The results indicate that PC-based ANN inversion can achieve a better agreement with the true model and also proved that PC-based ANN is feasible to invert large ATEM datasets.

Induced polarization (IP) effects are becoming more evident in time-domain helicopter airborne electromagnetic (AEM) data thanks to advances in instrumentation, mainly due to improvements in the signal-to-noise ratio and hence better data quality. Although the IP effects are often manifested as negative receiver voltage values, which are easy to detect, in some cases, IP effects can distort recovered transients in other ways so they may be less obvious and require careful data analysis and processing. These effects represent a challenge for modeling and inversion of the AEM data. For proper modeling of electromagnetic transients, the chargeability of the subsurface and other parameters describing the dispersion also need to be taken into consideration. We use the Cole-Cole model to characterize the dispersion and for modeling of the IP effects in field AEM data, collected by different airborne systems over different geologies and exploration targets, including examples from diamond, gold, and base metal exploration. We determined how multiparametric inversion techniques can simultaneously recover all four Cole-Cole parameters, including resistivity [Formula: see text], chargeability [Formula: see text], relaxation time [Formula: see text], and frequency parameter [Formula: see text]. The results obtained are in good agreement with the ancillary information available. Interpretation of the IP effects in AEM data is therefore seen by the authors as providing corrected electrical resistivity distributions, as well as additional information that could assist in mineral exploration.

We present two approaches to solving the airborne electromagnetic (AEM) problems given the induced polarization (IP) effect: with calculating the field in a polarizable medium directly in the time domain (TD) and with calculating the EM+IP field in the frequency domain (FD) followed by a transition to the time domain. The first approach is based on calculating the field at each time step given the sources that depend on the currents excited in the medium at previous time steps. This approach allows us to use any IP decay functions. The frequency domain approach is based on the Fourier series expansion of a non-stationary source and the Cole-Cole model. In order to reduce the computational cost, we use the Hermite spline interpolation. Both approaches allow modeling EM + IP processes in complex media with topography and the curved boundaries of layers containing 3D heterogeneities. 3D modeling is performed on non-conforming hexahedral meshes generated fully automatically. The analysis of computational efficiency and verification of the developed approaches are presented in comparison with the results from other researchers. Moreover, we compare the results obtained in the time and frequency domains. The results of 3D modeling the IP effects, which are obtained for some geoelectrical models typical for AEM exploration problems, are demonstrated.

The complex composition and structure of the submarine reservoir and its secondary pyrite, which are multiphase composite media, can provoke the induced polarization (IP) effect, resulting in the change of the marine controlled-source electromagnetic (CSEM) induction-polarization response, which directly affects geological interpretation results. In this paper, the generalized effective-medium theory of induced polarization (GEMTIP) model is introduced to study the influence of composition, structure and geometric characteristics of submarine reservoirs and secondary pyrite on the 3D marine CSEM induction-polarization response. We first construct the 3D finite-difference frequency-domain (FDFD) electromagnetic field governing equation based on the GEMTIP model, apply the emission source on the non-uniform grid, and solve the linear equations by using the difference coefficient matrix. Then we perform forward calculation on a typical 1D reservoir model to verify the correctness and effectiveness of the proposed algorithm. Finally, we design the reservoir and secondary pyrite models, and analyze the influence laws of IP parameters and polarization layer geometry parameters on the 3D marine CSEM induction-polarization response. These studies are of great value for understanding the relationship between submarine multiphase composite medium and electromagnetic wavepropagation.

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

This article presents a tutorial of the bases for, and problems encountered with, the resistivity and induced polarization methods in geotechnical and environmental applications. It commences with a discussion of aqueous electrolytic conduction in rocks including consideration of the effects of temperature, rock texture, rock type, geological processes, and of the presence of clay minerals. Both electrode and membrane polarization in soils and rocks are described. The elementary theory for the resistivity and induced polarization methods is introduced via formulas for electrodes on homogeneous and layered half-spaces. The notion of apparent resistivity is presented. Vertical electric sounding is described in relation to curve types, inversion, equivalence, anisotropy, and correlation. Profiling with resistivity is briefly treated while combined sounding-profiling is treated in more detail for both the resistivity and induced polarization methods. The parameters used to describe the induced polarization phenomena are introduced. A brief discourse follows on data acquisition and processing, including design considerations for transmitters and receivers, electrodes, and wire logistics. A section on arrays and the factors involved in selecting an array provokes the disclosure that while their application in geotechnical and environmental studies usually is straightforward much remains to be learned about the resistivity and induced polarization methods in general. Specific topics requiring study are comparative studies of various arrays in (a) depth of exploration in sounding and in profiling, (b) resolution of horizontal structures, (c) sensitivity to depth, (d) sensitivity to lateral effects, (e) sensitivity to buried and surface topography, and (f) the effectiveness of focused arrays. References to recent significant publications on one-, two-, and three-dimensional forward and inverse interpretation procedures follow. Problems encountered with these methods are initially treated under the section on arrays and are expanded with specific reference to geologic noise and negative induced polarization effects. The advantages of focused arrays are illustrated. The paper concludes with nine representative examples of applications.

<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>

An airborne electromagnetic (AEM) survey was conducted using the Resolve™ frequency-domain system over a buried bedrock valley near Elora, Ontario, Canada. A statistical bootstrapping approach was used to establish a relationship between the electrical resistivity from spatially interpolated one-dimensional AEM resistivity models and the lithostratigraphy of Quaternary sediments logged in continuously cored holes located within and adjacent to the buried bedrock valley. Three lithology types were classified using a bootstrapping approach: (i) clay, (ii) sandy to muddy diamicton with the presence of clasts, and (iii) sand/gravel. The statistically derived ranges in electrical resistivity from the model were used to generate a lithostratigraphic model of the Quaternary deposits along the valley axis. The resulting lithology model differentiated more electrically resistive coarse-grained sand and gravel from electrically conductive finer-grained clay-rich tills; but was not able to resolve interbedded layers associated with complex fluvial deposits. Modeled Quaternary deposit architecture and bedrock morphology along two transects orthogonal to the valley axis were consistent with co-located surface electrical resistivity tomography models and borehole natural gamma logs, indicating that the AEM method, when calibrated using high-quality continuous-core logs, can support quantitative conceptualizations of complex Quaternary architecture within and around a buried bedrock valley. Key limitations in this approach were the reduced vertical resolution of the AEM method and the inability to resolve thinly bedded layers (meter scale) identified in the core logs that may have a hydrogeologic influence. This study demonstrates the utility of combining airborne electrical methods with high-resolution geological logs through statistical analysis to constrain hydrostratigraphic architecture at scales relevant to municipal groundwater flow systems.

Resistivity is fundamental in mineral exploration. Airborne electromagnetic (EM) surveys efficiently map subsurface resistivity in large or remote areas. Among these, MobileMT—a passive-source airborne EM system—stands out for its ability to detect resistivity contrasts to depths of up to 2.5 km, deeper than any active-source system. MobileMT is sensitive to a wide range of resistivity contrasts and can detect geoelectrical boundaries regardless of orientation, making it effective in complex mineral systems. Resistivity reflects lithological variations, structures, and mineralization-related alterations, allowing detection of conductive ores and subtle features linked to magmatic-hydrothermal systems. Depth models derived from MobileMT data successfully characterize resistivity signatures in various geological settings, including porphyry, epithermal, orogenic, and IOCG deposits. Field results confirm MobileMT’s ability to image mineral systems, even when beneath conductive covers.

ABSTRACTIn the past few years, the focus on Alum shale hazards and the need for efficient mapping tools have increased in Norway. Alum shale is highly toxic and poses a substantial obstacle to infrastructure development such as tunnel projects. We present an evaluation of the ground‐based electrical resistivity tomography, induced polarisation, and airborne electromagnetic methods for mapping purposes using a recent case study. This evaluation is done in combination with resistivity and chargeability laboratory measurements applied to drill cores. The aim of the geophysical survey was to improve the knowledge of Alum shale occurrence to assist a tunnel project in Gran, southeast Norway. Resistivity and chargeability models derived from an electrical resistivity tomography/induced polarisation survey enabled us to map the presence of Alum shale during the tunnel investigation. The resistivity models point to geological layers that are in agreement with the rock types observed from early drillings together with subsequent geological logging during tunnelling. The time‐domain chargeability models are imperfect but nonetheless reveal the presence of polarisable minerals. These are likely due to the high levels of sulphides contained in black shale. An airborne electromagnetic survey was done close to the area of interest, which enabled us to fly some sparse lines across the tunnel alignment as a piggyback survey. Although the airborne electromagnetic resolution is lower than electrical resistivity tomography, the successful test flight lines illustrate the potential of airborne electromagnetic surveys for Alum shale mapping in Norway and affirm the promise of airborne electromagnetic in the early stages of project exploration.