Browsing by Author "Smith, Richard S."

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Airborne electromagnetic methods: applications to minerals, water and hydrocarbon exploration
(2010-03) Smith, Richard S.
An airborne electromagnetic system with a three-component transmitter and three-component receiver capable of detecting extremely conductive bodies
(Society of Exploration Geophysicists, 2018-08-28) Smith, Richard S.
Extremely conductive bodies, such as those containing valuable nickel sulfides, have a secondary response that is dominated by an in-phase component, so this secondary response is very difficult to distinguish from the primary field emanating from the transmitter (because by definition they are identical in temporal shape and phase). Hence, an airborne electromagnetic (AEM) system able to identify the response from the extremely conductive bodies in the ground must be able to predict the primary field to identify and measure the secondary response of the extremely conductive body. This is normally done by having a rigid system and bucking out the predicted primary (which will not change significantly due to the rigidity). Unfortunately, these rigid systems must be small and are not capable of detecting extremely conductive bodies buried deeper than approximately 100 m. Another approach is to measure the transmitter current and geometry and subtract the primary mathematically, but these measurements must be extremely accurate and this is difficult or expensive, so it has not been done successfully for an AEM system. I exploit the geometric relationship of the primary fields from a three-component (3C) dipole transmitter. If the transmitter is mathematically rotated so that one axis points to the receiver, then linear combinations of the fields measured by a 3C receiver can be combined in such a way that the primary fields from the transmitter sum to zero and cancel. Alternatively, the measured transmitter current and response could be used to estimate the transmitter-receiver geometry and then to predict and remove the primary field. Any residual must be the secondary coming from a conductive body in the ground. Hence, extremely conductive bodies containing valuable minerals can be found. An AEM system with a 3C transmitter and a 3C receiver should not be too difficult to build.
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.
Approximate semianalytical solutions for the electromagnetic response of a dipping-sphere interacting with conductive overburden
(Society of Exploration Geophysicists, 2016-06-06) Desmarais, J.K.; Smith, Richard S.
Electromagnetic exploration methods have important applications for geologic mapping and mineral exploration in igneous and metamorphic terranes. In such cases, the earth is often largely resistive and the most important interaction is between a conductor of interest and a shallow, thin, horizontal sheet representing glacial tills and clays or the conductive weathering products of the basement rocks (both of which are here termed the “conductive overburden”). To this end, we have developed a theory from which the step and impulse responses of a sphere interacting with conductive overburden can be quickly and efficiently approximated. The sphere model can also be extended to restrict the currents to flow in a specific orientation (termed the dipping-sphere model). The resulting expressions are called semianalytical because all relevant relations are developed analytically, with the exception of the time-convolution integrals. The overburden is assumed to not be touching the sphere, so there is no galvanic interactions between the bodies. We make use of the dipole sphere in a uniform field and thin sheet approximations; however, expressions could be obtained for a sphere in a dipolar (or nondipolar) field using a similar methodology. We have found that there is no term related to the first zero of the relevant Bessel function in the response of the sphere alone. However, there are terms for all other zeros. A test on a synthetic model shows that the combined sphere-overburden response can be reasonably approximated using the first-order perturbation of the overburden field. Minor discrepancies between the approximate and more elaborate numerical responses are believed to be the result of numerical errors. This means that in practice, the proposed approach consists of evaluating one convolution integral over a sum of exponentials multiplied by a polynomial function. This results in an extremely simple algorithmic implementation that is simple to program and easy to run. The proposed approach also provides a simple method that can be used to validate more complex algorithms. A test on field data obtained at the Reid Mahaffy site in Northern Ontario shows that our approximate method is useful for interpreting electromagnetic data even when the background is thick. We use our approach to obtain a better estimate of the geometry and physical properties of the conductor and evaluate the conductance of the overburden.
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.
Combining spatial components and Hilbert transforms to interpret ground-time-domain-electromagnetic data
(Society of Exploration Geophysicists, 2015-06-19) Desmarais, J.K.; Smith, Richard S.
We have developed a method for displaying or imaging data from a ground-time-domain electromagnetic system and for extracting the geometric parameters of a small conductor. The parameters are determined directly from the data using combinations of the spatial components of the secondary fields and their Hilbert transforms. The position of the target coincides with the peaks of the energy envelope (EE) or the 𝑇-component of the response. Here, the EE is the square root of the sum of the squares of the three spatial components and their Hilbert transforms, whereas the 𝑇-component response is an analogous quantity that excludes the Hilbert transform terms. Studies on synthetic models indicate that the 𝑇-component response is sharper than the EE in most possible target orientations. Once the position of a body has been determined using the peak of the 𝑇-component response, the dip of the target can be quantified using the ratio of the full-width at half-magnitude (FWHM) of the 𝑇-component response and the 𝑇-component Hilbert transform response, which is analogous to the EE but excludes the untransformed quantities. Finally, once all other geometric parameters have been determined, the depth of the target can be evaluated using the FWHM of the 𝑇-component response. The proposed modeling method was tested over an anomaly acquired at the Coulon field site during an InfiniTEM survey in the Abitibi greenstone belt of Quebec. The extracted geometric parameters were consistent with the available geologic information.
A comparison of magnetic susceptibility meters using samples from the Thompson Nickel Belt, Canada
(2016-10) Deng, Deng N.; Smith, Richard S.
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.
The effect of dielectric permittivity on the fields radiated from a radio-frequency electric dipole in a homogeneous whole space
(Society of Exploration Geophysicists, 2016-02-18) Naprstek, T.; Smith, Richard S.
The radio imaging method (RIM) is an electromagnetic cross-borehole method with applications in mineral exploration, as well as in the coal industry, where it is used across mine drives. Attenuation of the signal from conductive regions may indicate areas of mineralization, and these conductive effects in general dominate the response. In an effort to better understand the effect of a material’s dielectric permittivity on the response of the RIM, we have developed a simple program to model an electric dipole in a homogeneous whole space. When increasing the dielectric permittivity, the amplitude peak broadened and increased, whereas the phase peak sharpened and shifted negatively. To showcase the effect of dielectric permittivity on RIM data, data recorded from two transmitter positions in a moderately homogeneous zone in the Sudbury Basin were curve fit, and we concluded that despite the stronger effect that conductivity has on the signal, RIM is still sensitive to dielectric permittivity, and appropriate values must be used when developing conductivity tomograms. In addition, we found that for the given situation and frequencies used, an increase in either the conductivity or dielectric permittivity could be accounted for by a decrease of approximately the same factor in the other variable. However, the low-conductivity, high-permittivity case seemed to fit the shape of the amplitude and phase curves better. For the sulfide impregnated crystalline rocks at our field site, relative dielectric constants of 26.4 and 31 at 1250 and 625 kHz, respectively, were inferred.
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.
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.
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.
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.
The impact of magnetic viscosity on time-domain electromagnetic data from iron oxide minerals embedded in rocks at Opemiska, Québec, Canada
(Society of Exploration Geophysicists, 2017-07-06) Smith, Richard S.; Gaucher, Frédéric E. S.
The magnetic viscosity (MV) effects observed at time scales between 0.01 and 10 ms at Opemiska are associated with magnetic grains of variable size in rocks. Recent observations made during a ground time-domain electromagnetic (TDEM) survey at Opemiska are consistent with four aspects of the spatial and amplitude characteristics of a MV response: (1) the ∂Bz/∂𝑡 decay rate is roughly proportional to 1/𝑡1+𝛼, where −0.4<𝛼<0.4, (2) the anomalies are mainly visible on the 𝑧-component, when the EM receiver sensor is located inside or just outside the transmitter loop, (3) there is no obvious 𝑥- or 𝑦-component response, and (4) the sites where MV effects are seen in the TDEM data are coincident with an airborne magnetic anomaly. Previous studies have demonstrated that MV could be caused by (1) fine-grained particles of maghemite or magnetite in the overburden, regolith, or soil that were formed through lateritic weathering processes, (2) volcanic glass shards from tuff containing approximately 1% by weight magnetite, which occur as grains approximately 0.002–0.01μm in size precipitated in a spatially uniform way, or (3) from the Gallionella bacterium that precipitates ferrihydrite that oxidizes to nanocrystalline maghemite aggregates. The sites investigated at Opemiska are outcropping and well-exposed with relatively little or no overburden, and they are unfavorable for the formation of maghemite; hence, it is assumed that the source of MV seen at Opemiska cannot be the maghemite, or the other aforementioned causes. Hand samples were collected from Opemiska to identify the minerals present. Polished thin sections observed under an optical reflecting microscope identified the accessory minerals magnetite, ilmenite, and pyrrhotite, all known for their relatively high magnetic susceptibility. The use of the scanning electron microscope confirmed fine-grained magnetite grains as small as 0.667μm. An electromagnetic induction spectrometer confirmed the viscous nature of the susceptibility of the Opemiska samples. This suggests that MV could originate not only from fine-grained magnetite and maghemite particles located in the weathered regolith but also from other iron oxides and magnetic minerals embedded in the rock itself.
Inductive electromagnetic data interpretation using a 3-D distribution of 3-D magnetic or electric dipoles
(Society of Exploration Geophysicists, 2017-05-25) Kolaj, M.; Smith, Richard S.
In inductive electromagnetics, the magnetic field measured in the air at any instant can be considered to be a potential field. As such, we can invert measured magnetic fields (at a fixed time or frequency) for the causative subsurface current system. These currents can be approximated with a 3D subsurface grid of 3D magnetic (closed-loop current) or electric (line current) dipoles whose location and orientation can be solved for using a potential-field-style smooth-model inversion. Because the problem is linear, both inversions can be solved quickly even for large subsurface volumes; and both can be run on a single data set for complementary information. Synthetic studies suggest that for discrete induction dominated targets, the magnetic and electric dipole inversions can be used to determine the center and top edge of the target, respectively. Furthermore, the orientation of plate targets can be estimated from visual examination of the orientations of the 3D vector dipoles and/or using the interpreted location of the center and top edge of the target. In the first field example, ground data from a deep massive sulfide body (mineral exploration target) was inverted and the results were consistent with the conclusions drawn from the synthetic examples and with the existing interpretation of the body (shallow dipping conductor at a depth of approximately 400 m). A second example over a near-surface mine tailing (a near-surface environmental/engineering study) highlighted the strength of being able to invert data using either magnetic or electric dipoles. Although both models were able to fit the data, the electric dipole model was considerably simpler and revealed a southwest−northeast-trending conductive zone. This fast approximate 3D inversion can be used as a starting point for more rigorous interpretation and/or, in some cases, as a stand-alone interpretation tool.
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.
Introduction to special section: Mining and minerals exploration interpretation
(Society of Exploration Geophysicists, American Association of Petroleum Geologists, 2015-04-22) Lo, B.; Li, Y.; Smith, Richard S.; Arce, J.; Shore, M.
Mineral deposits are found in a variety of geologic settings and ore-forming minerals can have a vast range of physical properties. The search for these deposits is also relatively near-surface thus far. These factors allow for a large number of possible airborne and ground-based techniques to be used in geophysical exploration. Deciding on the proper geophysical technique and survey layout requires an understanding of the target, its associated alteration, the variations in physical properties and the geologic and structural setting. Knowing the exploration history is important, particularly in exploration programs that are more mature. Interpretation of the data requires the integration of the myriad of information ranging from physical property models constructed from inversions or forward modeling, physical property data, geochemical data, mineral deposit model, and host geology. We envisioned a special section on mining geophysics to highlight the integrative nature of mining geophysics through a collection of papers using multiple geophysical data to provide geology and exploration rationale along with the interpretations. So in collaboration with the editor of Interpretation, we issued a call for papers that discussed geophysics as applied to mining, discussed all relevant geophysical data and provided geologic information and the exploration rationale along with the interpretations. The following papers provide insight into the importance of geophysics in mineral exploration from the belt or camp scale to exploration focused on a specific orebody. Wright and Koehler combine controlled-source audio magnetotelluric and gravity data in a previously explored terrain of the Carlin trend. The authors demonstrate that successive geophysical surveys, combined with geologic understanding and target model development were key to a significant gold discovery. Martinez and Li demonstrate that lithological interpretation techniques based on inversion of airborne gravity gradiometry and aeromagnetic data can be used to characterize an iron-ore formation in Minas Gerais. The authors show that lithology differentiation using either generic physical property constraints or geologic constraints can contribute to a geologic understanding at the deposit scale. Olaniyan et al. study the 3D geologic and structural setting of the Sudbury structure using an integration of geologic data with airborne gravity and magnetic data. Using standard 2.5D modeling and 3D Geomodeller software, they generate continuous surfaces in three dimensions for each geologic interface, which leads them to suggest a possible deformation history of the Sudbury structure. Woolrych et al. present data from a range of airborne and ground-based geophysical techniques that have contributed to the discovery of the Kitumba iron oxide copper gold (IOCG) deposit in central Zambia. The interpretation of geophysical data following an exploration criteria of an IOCG-type deposit model has opened up exploration for this style of deposit in Central Zambia. Lü et al. present a case study that demonstrates the use of integrating seismic, magnetotelluric, gravity, and magnetic data to interpret the 3D structure and deformation at depth in the Lu-Zong ore district of Eastern China. Insights were obtained into the fault systems and crustal architecture that are essential for understanding the Lu-Zong ore district mineral system and for mineral exploration at depth. Legault et al. present the results of three different airborne electromagnetic (EM) surveys over the Lalor copper-zinc-gold volcanogenic massive sulfide deposit, which is more than 500 m deep and is in the Flin Flon Greenstone Belt of north-central Manitoba. The active and passive source EM surveys span a five year period, which means that the development of EM systems over this period can be assessed.
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.
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.
A multiple transmitter and receiver electromagnetic system for improved target detection
(Society of Exploration Geophysicists, 2015-06-22) Kolaj, M.; Smith, Richard S.
We have developed an alternative strategy for the inevitable deeper inductive electromagnetic (EM) exploration, which will be required as shallow deposits are exhausted. Rather than using very large magnetic moment ground loops, measurement stations are repeated using many smaller sized loops with smaller moments. The multiple transmitter data are then weighted and summed into a single high signal-to-noise ratio (S/N) composite transmitter. The composite transmitter can be thought of as a postprocessing method that uses the collected multitransmitter data to construct/simulate a transmitter, which maximizes the coupling to a particular target. The appropriate transmitter weights to use will depend on the target location and geometry, and, as such, different weighting schemes allow for the construction of different composite transmitters, each of which will maximally highlight different targets. We have assumed no prior knowledge of the location and orientation of the exploration targets, and we constructed composite transmitters for each possible location of a discretized subsurface and 324 possible target orientations (dipole embedded within a fully resistive medium). A modified difference of squares and a dipole look-up table was used to assess the fit between each composite transmitter and the suggested target location and orientation. Synthetic studies using conductive plate target(s) embedded within a fully resistive medium found that the target locations and orientations could be accurately determined and that the S/N of the composite transmitter was significantly higher than that of standard fixed-loop ground and airborne surveys. In a ground time-domain EM field test, 23 transmitter positions were used, and a shallow target (conductive dike) could be identified using the developed methodology. The composite transmitter data we produced was considerably easier to interpret and had a larger amplitude than that of any one single transmitter.