Smith, Richard S.

Permanent URI for this collectionhttps://laurentian.scholaris.ca/handle/10219/3306

Richard Smith received a BSc and MSc from the University of Adelaide, Australia and an MSc and PhD from the University of Toronto. In 1989, he worked at Lamontagne Geophysics in Toronto (1989) where he helped develop methods for generating conductivity depth sections from airborne electromagnetic data. In 1990, Richard held an ARC post-doctoral fellowship at Macquarie University in Sydney (1990). From 1990 to 1992, Richard worked as an explorationist at Pasminco Exploration in Melbourne.

In 1993, Richard joined Geoterrex Ltd, an airborne geophysical survey company, later purchased by Fugro. Here he developed methods for processing and interpreting airborne magnetic and electromagnetic data. In May 2009, Richard took up an Industrial Research Chair in Exploration Geophysics at Laurentian University in Sudbury. Dr Smith is a member of the SEG (Chair, Mining and Geothermal committee, 2008-2010), the EAGE, KEGS, the ASEG (Federal Executive, 1992-1993), and the PDAC (conference organizing committee, 2008-2011). He is the recipient of a number of awards for the "best presentation" at conferences and is a co-recipient of the award for the best paper in the journal Geophysics for 1997 and the best paper in Exploration and Mining Geology (2012). In 2009/2010 he was the CSEG Distinguished Lecturer.

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Estimating the parameters of simple models from two-component on-time airborne electromagnetic data
(2022-01-01) Bagley, Thomas; Smith, Richard S.
The horizontal and vertical components of an on-time electromagnetic (EM) response can be used to estimate the parameters of simple models such as thin sheets, half-spaces, thin sheets over a lower half-space, and a two-layer model. The formulas used in these methods are valid in areas where the on-time response is essentially proportional to the conductivity or conductance, the so-called “resistive limit.”The half-space and thin sheet over lower half-space models can be combined to give an estimate of the conductivity for a lower half-space below a thick sheet that might be reasonable for the entire survey area. With this estimation, an equation solver can be used to estimate the thickness and conductivity of the overlying thick sheet over the whole sur- vey area. This latter approach seemed most appropriate for the Russell South area in the Athabasca Basin, Canada, where GEOTEM data have been collected. The output of the algorithm was generally stable. Although it did not always reliably reproduce the overburden thicknesses as measured in a set of reference drillholes, it did give an estimate that was reasonable in the relatively conductive areas.
Potential-field modelling of the prospective Chibougamau area (northeastern Abitibi subprovince, Quebec, Canada) using geological, geophysical, and petrophysical constraints
(2021-01-01) Maleki, Amir; Smith, Richard; Eshaghi, Esmaeil; Mathieu, Lucie; Snyder, David; Naghizadeh, Mostafa
This paper focusses on obtaining a better understanding of the subsurface geology of the Chibougamau area, in the northeast of the Abitibi greenstone belt (Superior craton), using geophysical data collected along a 128 km long traverse with a rough southwest–northeast orientation. We have constructed two-dimensional (2D) models of the study area that are consistent with newly collected gravity data and high-resolution magnetic data sets. The initial models were constrained at depth by an interpretation of a new seismic section and at surface by the bedrock geology and known geometry of lithological units. The attributes of the model were constrained using petrophysical measurements so that the final model is compatible with all available geological and geophysical data. The potential-field data modelling resolved the geometry of plutons and magnetic bodies that are transparent on seismic sections. The new model is consistent with the known structural geology, such as open folding, and provides an improvement in estimating the size, shape, and depth of the Barlow and Chibougamau plutons. The Chibougamau pluton is known to be associated with Cu–Au magmatic-hydrothermal mineralisation and, as the volume and geometry of intrusive bodies is paramount to the exploration of such mineralisation, the modelling presented here provides a scientific foundation to exploration models focused on such mineralisation.
On the Time Decay constant of AEM systems: A Semi- Heuristic Algorithm to validate calculations
(2020-01-01) Martinez, José Manuel; Smith, Richard; Vazquez, Danilo Diaz
The time decay constant or “tau” of airborne EM systems (AEM) is commonly used to indicate the presence and the relative conductivity or conductance of conductors in the survey area. In fact, it is not a constant since it depends on: the system; the survey design and the method of calculation. The system dependence is a consequence of parameters relating to the acquisition and pre– and post– processing of the signal. Here we propose a method for calculating tau, which is simply the time at which the transient voltage decays to 37%, or V37, of some initial value. The model utilises a semi-heuristic algorithm that estimates V37 for each transient in the database and then it calculates the delay time that voltage is measures, which is the estimates tau value. No calculation is involved with the data, instead tau is given by a weighted average of the delay times associated with the windows either side of the V37 value. We illustrate how this algorithm works using data collected by MEGATEM II in the Reid-Mahaffy test site. Results shown a good agreement between tau-grids reported in previous studies and those calculated with our V37 – method. To account for all effects coming from the acquisition and processing of EM data, the algorithm allows the emphasis to be shifted from early to late time parts of the transient. It is envisage that since this method does not apply any mathematical operation to the data it may serve as a robust means of for validating other methods.
Forward modeling and 3D inversion of electromagnetic data collected over the McArthur River uranium deposit in the Athabasca Basin, Canada
(2022-03-01) Mir, Reza; Fullagar, Peter; Darijani, Mehrdad; Smith, Richard; Scott, Shawn; Ross, Martin; Shamsipour, Pejman; Chouteau, Michel; Ansdell, Kevin; Gouiza, Mohamed
Detection and assessment of the deeply buried high-grade uranium deposits in the Athabasca Basin rely on geophysical methods to map conductive rocks. Variable Quaternary surface cover can mask the anomalous signals from depth and affect the interpretation of inverted conductivity models. We present the analysis of several electromagnetic (EM) modeling studies and two field data sets to demonstrate the effects of varying Quaternary cover resistivity and thickness, on the ability to resolve the parameters of underlying sandstone, alteration, and basement conductors. Synthetic data, assuming a typical shallow EM sounding system and realistic resistivities found in the Athabasca Basin, indicate that resistivity and thickness parameters of the Quaternary cover can be separately recovered in cases in which this cover is more conductive than the underlying sandstone, but not when the cover is significantly more resistive. A 3D modeling study indicates that by using airborne EM data, it is possible to detect a basement conductor of 20 S at a depth of at least 600 m below the surface, even in the presence of Quaternary cover thickness variations of the up to 20% (40–60 m). Furthermore, although Quaternary cover variations and deeper sandstone alteration can produce comparable anomalous signal amplitudes in a time-domain EM response, their effects are most visible in distinctly separate time windows. Ground-penetrating radar and other data to characterize the Quaternary cover in the McArthur River area indicate that this cover consists mostly of sandy tills ranging in thickness from 0 to 117 m. Constrained 3D inversion of an airborne EM data set from the same area indicates basement conductors consistent with the depth and location of a known fault. Elevated conductivity in the sandstone by up to a factor of two over the background values could indicate possible alteration.
Convolutional neural networks applied to the interpretation of lineaments in aeromagnetic data
(2022-01-01) Naprstek, Tomas; Smith, Richard S.
Parameter estimation in aeromagnetics is an important tool for geologic interpretation. Due to aeromagnetic data being highly prevalent around the world, it can often be used to assist in understanding the geology of an area as a whole or for locating potential areas of further investigation for mineral exploration. Methods that automatically provide information such as the location and depth to the source of anomalies are useful to the interpretation, particularly in areas where a large number of anomalies exist. Unfortunately, many current methods rely on high-order derivatives and are therefore susceptible to noise in the data. Convolutional neural networks (CNNs) are a subset of machine-learning methods that are well-suited to image processing tasks, and they have been shown to be effective at interpreting other geophysical data, such as seismic sections. Following several similar successful approaches, we have developed a CNN methodology for estimating the location and depth of lineament-type anomalies in aeromagnetic maps. To train the CNN model, we used a synthetic aeromagnetic data modeler to vary the relevant physical parameters, and we developed a representative data set of approximately 1.4 million images. These were then used for training classification CNNs, with each class representing a small range of depth values. We first applied the model to a series of difficult synthetic data sets with varying amounts of noise, comparing the results against the tilt-depth method. We then applied the CNN model to a data set from northeastern Ontario, Canada, that contained a dike with known depth that was correctly estimated. This method is shown to be robust to noise, and it can easily be applied to new data sets using the trained model, which has been made publicly available.
Multiple-order moments of the transient electromagnetic response of a one-dimensional earth with finite conductance – the Gaussian variation applied to a field example
(2022-01-01) Smith, Richard S.; Lee, Terry J.
Formulae for the moments of the magnetic field response can be derived for simple models which have conductivities that vary suddenly as a function of depth (thin and thick sheets) or not at all (half space). In a companion paper we have derived expressions for the moments of a conductivity-depth profile that varies smoothly, taking the form of a Gaussian function. In this paper we apply the Gaussian model to data from Russell South, an area in the Athabasca Basin of Canada. The low signal-to-noise ratio in this area means that estimating the overburden thickness is a challenging problem, so this dataset is a good candidate for demonstrating the applicability of our approach. The estimated thicknesses can be compared with drill information, also somewhat problematic as a reliable source of information. If we constrain the Gaussian model to be similar to a thin sheet or a thick sheet at surface, we get estimates of the overburden thickness which are much greater than what is inferred from drill information. However, if the overburden is allowed to vary gradually and the depth and value of the maximum conductivity can vary, then we find that the depth of the most conductive part of the overburden is realistic as it is generally above the base of overburden as determined from drilling. Features of geological interest that are not apparent on the original data can be identified on the derived images.
A grid implementation of the SLUTH algorithm for visualizing the depth and structural index of magnetic sources
(2012-01-01) Smith, Richard S.; Thurston, Jeffrey B.; Salem, Ahmed; Reid, Alan B.
The SLUTH method requires first-order derivatives at two or more different heights above the ground and can estimate the location and depth of source bodies from magnetic data. Results of this method are independent of a specific model type and can be used to estimate the most appropriate model (structural index). This paper presents a grid implementation of the SLUTH method to visualize both depth and structural index from magnetic anomaly data. The implementation uses the Geosoft GX programming language. The method has been tested using theoretical magnetic gridded data and of two methods have been used for estimating depth; the estimate from the width of the imaged feature gives an underestimate and the estimate from the rate of fall off of the field with height gives an overestimate. The practical utility of the algorithm is demonstrated using field data from the Saskatoon area of Canada.
Transformation of magnetic data to the pole and vertical dip and a related apparent susceptibility transform: Exact and approximate approaches
(2022-03-01) Smith, Richard S.; Roots, Eric A.; Vavavur, Rajesh
The dipolar character of magnetic data means that there is a high and a low associated with each source. The relative positions and sizes of these highs and lows vary depending on the magnetic latitude or the inclination of the earth’s magnetic field. One method for dealing with this complexity is to transform the data to what would be collected if the inclination were vertical (as at the magnetic pole), a process that is unstable at low magnetic latitudes. Unfortunately, remanent magnetization adversely impacts the success of this transformation. A second approach is to calculate the analytic-signal amplitude (ASA) of the data, which creates a single positive feature for each source or edge, with the shape being only weakly dependent on the inclination and the presence of remanent magnetization. The ASA anomalies can appear to be relatively broad, so features sometimes merge together on map views of the ASA. A subsequent transformation of the ASA using an appropriate transforming tilt angle can generate a magnetic field of a body that is at the pole and has a vertical dip. The transformation is exact for contacts when calculated from the first- order ASA, but the sign of the transformed data can be incorrect depending on whether you are over one edge or the other edge of a discrete source body. Another approximate transfor- mation of the zeroth-order ASA does not have this issue and gives good results on synthetic data provided that any noise is handled appropriately. The resulting maps outline the magnetic source bodies and have amplitudes proportional to an apparent magnetic susceptibility. On field data from Black Hill, South Australia, the approximate transformation generates an image that is simple to interpret and enhances some features less obvious on other enhancements of the data.
Multiple-order moments of the transient electromagnetic response of a one-dimensional earth with finite conductance – theory
(2021-01-01) Lee, Terry J.; Smith, Richard S.
The concept of moments of the electromagnetic response is useful in electromagnetic interpretation. Analytic formulae exist for low-order moments of a few conductivity models, enabling source parameters such as time constant, depth, conductance and conductivity to be estimated from the measured moments of the electromagnetic response. However, most models for which analytic formulae exist have conductivity varying abruptly as a function of depth or position. In this paper, we have derived a procedure that allows moments of any order to be calculated for a conductivity which has finite conductance but can otherwise vary arbitrarily with depth. The horizontal loop transient electromagnetic step response is computed as a sum of residues. Integration of the step response over time yields a mathematical expression for a moment of any order. We illustrate the procedure for a Gaussian conductivity function which varies smoothly with depth. The Gaussian model produces results that agree in specific limits with the thin sheet, thick sheet and half-space cases.
Airborne electromagnetic methods: applications to minerals, water and hydrocarbon exploration
(2010-03) Smith, Richard S.
A comparison of magnetic susceptibility meters using samples from the Thompson Nickel Belt, Canada
(2016-10) Deng, Deng N.; Smith, Richard S.
Exploring for copper–gold deposits exhibiting a wide range of conductivities with time–domain electromagnetics at Opemiska, Canada
(2017) Gaucher, Frédéric E. S.; Smith, Richard S.
Finding and delineating new economic Cu-Au ore zones corresponding to poorly conductive disseminated mineralization and narrow massive chalcopyrite veins in the Chapais-Chibougamau mining district of Québec is a challenging exploration problem. The site of the former Opemiska underground mine was the location for conducting an experimental ground time-domain electromagnetics (EM) survey for mapping the conductivity, the anisotropy of the conductivity, and the chargeability estimated from shape reversals. Measurements at fourteen different sites confirmed the variability of the EM response. The trends, sizes, shapes and conductances of the relatively strong conductors were identified with success and modelled using thin plates in full space. The vein direction in the weakly conductive zones was quantified from the x-component data. Petrophysical measurements and microscopic observations suggest complex interrelations between the amount of ore, the fabric of the rock, texture, mineralogical associations and impurities. This explains a wide range of bulk conductivity values ~0.01 S/m to 4000 S/m measured on rock samples and also suggests that chalcopyrite might be a semiconductor at some locations at Opemiska.
Integrated Multi-Parameter Exploration Footprints of the Canadian Malartic Disseminated Au, McArthur River-Millennium Unconformity U, and Highland Valley Porphyry Cu Deposits: Preliminary Results from the NSERC-CMIC Mineral Exploration Footprints Research Network
(2017) Lesher, M.; Hannington, M.; Galley, A.; Ansdell, K.; Astic, T.; Banerjee, N.; Beauchamp, S.; Beaudoin, G.; Bertelli, M.; Bérubé, C.; Beyer, S.; Blacklock, N.; Byrne, K.; Cheng, L.-Z.; Chouinard, R.; Chouteau, M.; Clark, J.; D'Angelo, M.; Darijani, M.; Devine, M.; Dupuis, C.; El Goumi, N.; Farquharson, C.; Enkin, R.; Fayol, N.; Feltrin, L.; Feng, J.; Gaillard, N.; Gleeson, S.; Gouiza, M.; Grenon, C.; Guffey, S.; Guilmette, C.; Guo, K.; Hart, C.; Hattori, K.; Hollings, P.; Joyce, N.; Kamal, D.; King, J.; Kyser, K.; Layton-Matthews, D.; Lee, R.; Lesage, G.; Leybourne, M.; Linnen, R.; Lypaczewski, P.; McGaughey, J.; Mitchinson, D.; Milkereit, B.; Mir, R.; Morris, W.; Oldenburg, D.; Olivo, G.; Perrouty, S.; Piercey, S.; Piette-Lauzière, N.; Raskevicius, T.; Reman, A.; Rivard, B.; Ross, M.; Samson, I.; Scott, S.; Shamsipour, P.; Shi, D.; Smith, Richard S.; Sundaralingam, N.; Taves, R.; Taylor, C.; Valentino, M.; Vallée, M.; Wasyliuk, K.; Williams-Jones, A.; Winterburn, P.
Mineral exploration in Canada is increasingly focused on concealed and deeply buried targets, requiring more effective tools to detect large-scale ore-forming systems and to vector from their most distal margins to their high grade cores. A new generation of ore system models is required to achieve this. The Mineral Exploration Footprints Research Network is a consortium of 70 faculty, research associates, and students from 20 Canadian universities working with 30 mining, mineral exploration, and mining service providers to develop new approaches to ore system modelling based on more effective integration and visualization of multi-parameter geological-structural-mineralogical-lithogeochemical-petrophysical-geophysical exploration data. The Network is developing the next generation ore system models and exploration strategies at three sites based on integrated data visualization using self-consistent 3D Common Earth Models and geostatistical/machine learning technologies. Thus far over 60 footprint components and vectors have been identified at the Canadian Malartic stockwork-disseminated Au deposit, 20–30 at the McArthur-Millennium unconformity U deposits, and over 20 in the Highland Valley porphyry Cu system. For the first time, these are being assembled into comprehensive models that will serve as landmark case studies for data integration and analysis in the today’s challenging exploration environment.
Application of Occam’s inversion to airborne time-domain electromagnetics
(2009-03-01) Vallée, Marc A.; Smith, Richard S.
Airborne time-domain electromagnetics (ATDEM) methods are regularly used for mining, hydrocarbon, and groundwater exploration. A large quantity of data is collected along survey lines from an aircraft, and there is an incentive to interpret these data in a systematic way. When the geology is appropriate, the use of 1D inversion methods is justified. Among these methods are: conductivity-depth transform (CDT) (Wolfgram and Karlik, 1995), layered-earth inversion (Sattel, 1998), Zohdy's method (Sattel, 2005), and Occam's inversion (Constable et al., 1987; Sattel, 2005). These methods either require considerable tuning to get realistic results, are limited to step response data, or require considerable experimentation with the initial guess to ensure a reasonable result. The advantage of the Occam's algorithm is that it can be easily adapted to different ATDEM methods and is not strongly dependent on the initial guess. Furthermore, there are not a lot of parameters to tune in order to get a reasonable result. The weakness of the Occam's inversion is that for ATDEM data, the process requires a great deal of computer time. In this paper, we review details of the application of Occam's method to ATDEM data and we present the results of some of our experiments.
How to make better use of physical properties in mineral exploration: The exploration site measurement
(2012-03-01) Smith, Richard S.; Shore, Mark; Rainsford, Desmond
In recent years, there has been a growing awareness that a better understanding of physical property information is required in mineral exploration. As a consequence, there has been a strong push to collect more data and to use these data more intelligently. There are a multiplicity of reasons behind this impetus: geophysicists want more information about physical property data to enable better surveys to be planned and better interpretations to come from the data acquired and geologists want physical properties to provide addition information about the geology that might allow them to see variations in rocks that are not easy to see using traditional or more expensive methods (hand specimen examination, thin sections, lithogeochemistry, assays, etc.). If a hole is drilled on a geophysical target, then a physical property measurement of the core or the rocks surrounding the core can confirm if the target was intercepted and provides data that can be used to model the target response.
Case history of combined airborne time-domain electromagnetics and power-line field survey in Chibougamau, Canada
(2010-03-01) Vallée, Marc A.; Smith, Richard S.; Keating, Pierre
Exploration for volcanogenic massive sulfides requires good geologic understanding. Geologic knowledge often is limited by a lack of outcrops. This is especially true in Canada under residual glacial covers. Geologic information must therefore be complemented by information obtained using means such as geophysical and geochemical observations. Electromagnetic (EM) methods extend lithological understanding to depths beyond the overburden. Massive sulfides are highly conductive and, depending on their depth and volume, may be detected easily by airborne EM surveys. They are more often equant than graphitic sediments, which typically have longer strike length. Current EMtechniques that identify massive sulfides operate in the frequency or time domain, the latter being more common. Additional information can be provided by using power-line fields as a source of EM signals when the powerlines are appropriately located in the area of interest. We have worked in an active exploration area near Chibougamau, Canada, known for a large occurrence of massive sulfide deposits. The geology is a sequence of volcanic formations with felsic and mafic intrusions. Our magnetic technique responded well to mafic rocks. An airborne time-domain EM survey mapped localized and intrasedimentary conductors in that area. We learned in our study that power-line EM fields can be used to map large-extent conductive formations and narrow geologic faults.
Metalliferous mining geophysics — State of the art after a decade in the new millennium
(2011-06-03) Vallée, Marc A.; Smith, Richard S.; Keating, Pierre
Mining exploration was very active during the first decade of the twenty-first century because there were numerous advances in the science and technology that geophysicists were using for mineral exploration. Development came from different sources: instrumentation improvements, new numerical algorithms, and cross-fertilization with the seismic industry. In gravity, gradiometry kept its promise and is on the cusp of becoming a key technology for mining exploration. In potential-field methods in general, numerous techniques have been developed for automatic interpretation, and 3D inversion schemes came into frequent use. These inversions will have even greater use when geologic constraints can be applied easily. In airborne electromagnetic (EM) methods, the development of time-domain helicopter EM systems changed the industry. In parallel, improvements in EM modeling and interpretation occurred; in particular, the strengths and weaknesses of the various algorithms became better understood. Simpler imaging schemes came into standard use, whereas layered inversion seldom is used in the mining industry today. Improvements in ground EM methods were associated with the development of SQUID technology and distributed-acquisition systems; the latter also impacted ground induced-polarization (IP) methods. Developments in borehole geophysics for mining and exploration were numerous. Borehole logging to measure physical properties received significant interest. Perhaps one reason for that interest was the desire to develop links between geophysical and geologic results, which also is a topic of great importance to mining geologists and geophysicists.
Using combinations of spatial gradients to improve the detectability of buried conductors below or within conductive material
(2012-12-12) Smith, Richard S.
The detection of conductive bodies is an important capability when exploring for massive sulfide deposits or looking for unexploded ordnance. When these bodies are buried below conductive overburden or embedded in conductive material, the use of an electromagnetic system to identify the bodies becomes problematic because the response of the overlying conductive material can be much greater that the response of the buried conductor. I calculated the response of five models representing different conductivity distributions (a buried conductor, a uniform overburden with changes in the system altitude, a paleochannel, a thicker overburden, and a thinner overburden). The subtle response of the buried conductor was difficult to identify because it looked very similar to the responses of other structures that are not necessarily of interest. The spatial gradients for the same five models showed that the greatest improvement in the relative size of the anomalous gradient response compared with the background gradient came for the cases in which the material closest to the surface changes, in particular the paleochannel and thickening overburden models. However, identification of the deeper buried conductor was still problematic because of the large background gradients. In theory, the cylindrical symmetry of a dipole transmitter over a layered earth ensured that there were exact relations between the spatial derivatives. Hence it was possible to define two specific combinations that should be zero over a layered earth. Calculating these combinations for the five models showed that the anomalous zones stood out with significantly greater anomaly-to-background ratios. The measurement of the gradients and the calculation of these combinations therefore provided a means of identifying anomalous zones in and below a conductive earth. Different relative sizes and shapes of the two combinations for different models provided a way of discriminating between the vertical conductor model and the four other models.
Qualitative geophysical interpretation of the Sudbury Structure
(2013-07-26) Smith, Richard S.; Olaniyan, Oladele; Morris, Bill
The Sudbury Structure is one of the most studied geologic structures in the world due to its enigmatic nature and mineral wealth. The available geologic work from the literature and mining industry operations accumulated for more than a century was recently assessed and compiled into a bedrock geologic map. Most regional geophysical investigations of the Sudbury Structure have been quantitative — modeling and depth estimation without a clear definition of surface control. Airborne total magnetic intensity data over the Sudbury Structure were compiled, processed, and interpreted, to define magnetic stratigraphy boundaries and near-surface lineaments. Traditional directional and normalized derivatives were computed to enhance the high-frequency information in the magnetic field. Available airborne frequency-domain electromagnetic (EM) data were also interactively interpreted along profiles and in a gridded format to isolate conductive structures. On-screen geographic information system-based information extraction from multiple derivatives was used to interpret the magnetic contacts, dykes, and lineaments. The magnetic interpretation was compared with published bedrock maps of the Sudbury Structure. Magnetic contacts based on the qualitative classification of the magnetic texture did not always correspond to the geologic boundaries on the existing maps. Some magnetic lineaments corresponded with well-defined geologic structures, some were further extensions of partially mapped structures, and others are newly identified linear structures. Conductive locations identified from the EM profiles were probably due to responses from conductive ore bodies, faults, dykes, lithological contacts, and cultural objects.
Mapping lateral changes in conductance of a thin sheet using time-domain inductive electromagnetic data
(Society of Exploration Geophysicists, 2013-11-05) Kolaj, M.; Smith, Richard S.
With the inductive electromagnetic geophysical method, the laterally varying conductance of thin sheet models can be estimated either through a direct transform of the measured data or through inversion. The direct transform (called the simplified solution) does not require grid or line data and is simple enough to be performed in the field because the conductance at a location is calculated directly from the ratio of two measured magnetic fields (the vertical spatial and temporal derivative of the vertical magnetic field) at that location. However, the simplified solution assumes that the secondary horizontal magnetic fields are zero and/or that the sheet has a uniform conductance. Our nonapproximate solution (called the full inversion) does not make these assumptions, but requires gridded data, measurements of the secondary horizontal magnetic fields, and more complicated inversion algorithms. Through forward modeling, we found that the full inversion provides better results than the simplified solution when the spatial gradient of the resistance is strong and/or when the horizontal magnetic fields are large. Because the simplified solution may be preferable due to its simplicity, we introduce two unreliability parameters, which assess the unreliability of the conductance calculated using the simplified solution. A comparison of the simplified solution and full inversion in a fixed in-loop survey collected overtop a dry tailings pond in Sudbury, Ontario, Canada, revealed that there were small differences around large conductance contrasts, which coincided with elevated unreliability parameters. The simplified solution is recommended if fast in-field interpretations are required, or additionally, as a first-pass survey that can be performed with sparse station spacing to identify areas of interest. Denser grid data can then be collected, for the more reliable full inversion, over areas of interest and/or zones where the simplified solution is expected to be unreliable as predicted by the unreliability parameters.

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