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Applications of geophysical inversions in mineral exploration

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Abstract

Increases in computing power and advances in mathematical optimization theory have combined to produce a new generation of algorithms that can invert geophysical data to recover 1-D, 2-D, or 3-D images of the earth’s physical properties. These images may indicate mineralization directly or delineate the associated structures. In both cases they are valuable aids to mineral exploration, and they provide information that previously had to be distilled from data maps alone. In this short article, we illustrate both the practicability of inverting geophysical data and the important, even decisive, information that it provides. Applications are in time‐domain electromagnetics, DC resistivity and induced polarization, gravity, and magnetics.

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... Analyses of such data have ranged from qualitative inferences to complex modeling. Recently, increases in computational power and advances in mathematical optimization theory have encouraged the development of a new generation of algorithms that can invert geophysical data to recover 2-D or 3-D images of the earth's physical properties (e.g., Oldenburg et al., 1997Oldenburg et al., , 1998. These images may indicate the zones of mineralization directly or the structures associated with them. ...
... These images may indicate the zones of mineralization directly or the structures associated with them. The successful inversion of 3-D magnetic data over the Raglan deposit in northern Quebec and 3-D gravity data at the Voisey's Bay nickel deposit (Oldenburg et al., 1998) provides a motivation to apply these and related procedures in conjunction with the seismic reflection technique in mining regions. Limitations in the depth of penetration of electrical signals (∼500 m) and their resolution makes the seismic reflection survey a more viable tool for studying deeper structures associated with mineral deposits-particularly in mining camps, where infrastructure is already in place. ...
... The cells must be sufficiently small for such an assumption to be valid. The model m is a vector, and the inverse problem is solved by minimizing the model objective function subject to the constraint that the observed data be reproduced (Oldenburg et al., 1998). The data misfit is a sum-of square measure, assuming the observations are contaminated with Gaussian noise and an estimate of the standard deviation is available. ...
Article
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The Guichon Creek batholith (GCB), located in south-central British Columbia, contains several large, low-grade copper deposits of considerable economic importance. The surface geology of the Guichon batholith and its surrounding region have been well mapped; however, little information about subsurface features is available. The batholith consists of four major phases, emplaced radially outward, which can be separated on the basis of their texture and composition. Previous interpretation of gravity data suggests a mushroom-shaped structure for the batholith. Data from Lithoprobe seismic reflection fine 88-11, acquired across the batholith in 1988, reveal weakly coherent east-dipping reflections on the west side and west-dipping reflections on the east in the upper 10 km. To determine if these are related to structures associated with the batholith, we reprocessed the upper 6 s with particular emphasis on applications of signal enhancement techniques (e.g., pattern recognition methods, refraction statics, dip moveout corrections) and correlation of the improved subsurface images with the geological environment associated with porphyry copper deposits. Low near-surface velocities correlate well with the phases of the batholith hosting the major copper deposits, which structurally lie in faulted and brecciated regions. Although the top 1.5 km cannot be imaged by the regional-scale seismic reflection data, the reprocessed seismic section helps define the edges of the batholith, its various concentric phases, and the stem in the depth range of 1.5 to 10 km. The seismic results are complemented by 2.5-D (profile sense) modeling and 3-D inversion of regional-scale gravity and high-resolution aeromagnetic data. These show a low-density and low-magnetic-susceptibility region associated with the batholith that extends to more than 10 km depth. The region of active mining interest lies above a circular low-susceptibility area at 2 km depth and a low-velocity region. Integrated interpretation of geophysical results and geological observations indicates the GCB is a funnel-shaped feature in which mineralization is located above the stem of the batholith.
... With Tikhonov 15,16 proposing a regularization theory and method to solve these problems, it has been applied and popularized in many fields, such as mineral exploration, magnetization imaging, and satellite electromagnetic anomalies. [17][18][19] In this paper, we first introduce the ship's magnetic signature extrapolation model based on a magnetic monopole model. Second, we analyze the applicability and stability of the Tikhonov Regularization Method (TRM) and Truncated Singular Value Decomposition (TSVD) methods under the influence of different types of noise. ...
Article
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In this paper, we first study the applicability and stability of the Tikhonov regularization method (TRM) and the truncated singular value decomposition method in ship’s magnetic signature inversion modeling. To further improve the noise immunity and robustness of the model, we apply an iterative regularization algorithm based on the conjugate gradient least squares (CGLS) method. Numerical simulation shows that the proposed algorithm is more applicable and can obtain better results of ship’s magnetic field extrapolation under different noise backgrounds. In the ship model experiment, the errors of the TRM and CGLS method for magnetic signature extrapolation are, respectively, 4.77% and 3.67%, showing that the proposed algorithm can improve the inversion effect of the ship model magnetic field.
... Compared with expensive drilling, deep targets such as concealed coal-bearing strata can be more efficiently and cost-effectively discovered through geophysical methods [6,7]. Every geophysical method is directly or indirectly related to a physical property contrast of the subsurface rock and the exploration target [8][9][10]. ...
Article
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With the majority of coal mines in uncovered and semi-covered coal strata now explored and developed, most of the undiscovered coal-bearing strata are concealed. Compared with expensive drilling, deep targets such as concealed coal-bearing strata can be more efficiently and cost-effectively discovered through geophysical methods. We designed an integrated geophysical exploration approach to detect coal-bearing strata in the Yangjiazhangzi (YJZZ) area. Large-scale magnetotellurics (MT) is used to describe the geological structure along with its profile through the YJZZ area. An aeromagnetic survey was used to delineate the spatial distribution characteristics of the YJZZ syncline, a coal-bearing prospect tectonic unit. Localized exploration with controlled-source audio magnetotellurics (CSAMT) and MT reveals coal-bearing targets for drilling. Drilling results verified the targets identified through the integrated geophysical approach. Coal-bearing strata in the Benxi formation, the Taiyuan formation, and the Shanxi formation of the Permo-Carboniferous age are found between 630 and 770 m. This case study demonstrates that the multidisciplinary geophysical strategy can provide reliable results and credible data interpretation for deep coal seam resources exploration. The findings of this study can provide reference for explorers to carry out their specific exploration cases.
... Electrical resistivity and IP imaging allow the spatial distribution of the low-frequency resistive and capacitive characteristics of subsurface earth materials such as mineral ores, weathered or fresh rock, unconsolidated sediments, and, respectively, uncontaminated and contaminated soils [7][8][9][10][11][12][13]. These methods are extensively utilized in mineral exploration campaigns, engineering and environmental investigations, hydrogeological studies, and archeological prospecting [7,[14][15][16][17][18][19][20][21][22]. Both methods have proven successful in the mineral exploration arena worldwide because of their cost-effective and rapid ability to image natural shallow subsurface geologic structures with mineralization potential [7,8], with a certain degree of resolution dependent on the electrode spacing [23]. ...
Article
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This paper presents an underground silver mining operation outside Gomez Palacio, Du-rango, Mexico, which terminated around the 1930s, of which previous knowledge of its operations was poor. Durango's current silver exploration campaigns are likely to overlook historic silver mining sites due to interest in specific prospect regions. A two-dimensional (2D) Electrical Resistivity Imaging (ERI) survey coupled with reconnaissance of the area was performed at this historic silver mining site. The exploration campaign aimed to find the abandoned mineshaft, map its subsurface extent, and explore the occurrence of mineralization zones (silver ore). The ERI survey comprised five profiles measured with the extended dipole-dipole array with a consistent electrode spacing of 5 m. The smooth, robust, and damped least-squares inversion methods were used to invert the 2D data. Our field observations and ERI survey results collectively reveal the following findings: (a) reconnaissance reveals mining infrastructure consistent with historical mining activity; the infrastructure includes a complex of habitational rooms, an ore-processing pit near a concrete slab next to a dirt road, and two limestone-wall structures interpreted as the entrance of abandoned backfilled mineshafts named Mesquite and Lechuguilla; (b) high-resistivity anomalies suggest vestiges of shallow, underground mine workings including backfilled mineshafts that connect a mine gallery complex, and (c) various low-resistivity anomalies, juxtaposed against mine galleries, suggestive of unmined shallow vein-type and manto-type mineralization of hydrothermal origin. The imaging depth is estimated at ~65 m. Underground silver mining moved southwards and was limited to ~40 m depth.
... Inversion has been widely used as an effective tool for interpreting measurements arising from a wide range of geoscientific applications (e.g., Oldenburg et al., 1998;Virieux and Operto, 2009;Melo et al., 2017). Uncertainty analysis of geophysical inversions have gained considerable attention in both academia and industry. ...
... The exploitation of geophysical methods to image subsurface structures has been widely recognized as a key to the success of explorations (e.g., Eckhardt, 1940;Oldenburg et al., 1998;Virieux and Operto, 2009;Moorkamp, 2017). Inversion as one of the most important methods to analyze and interpret geophysical measurements has been commonly used in geophysical community with various applications where the primary efforts of researchers have been focusing on recovering physical property models. ...
... Some application examples in which the precise knowledge of the Earth's gravity field is fundamental are (1) to establish a global vertical datum of global reference systems (Sideris and Fotopoulos, 2012), (2) to monitor mass distributions that are indicators of climate-related changes (Tapley et al., 2004;Schmidt et al., 2006), (3) to simulate the perturbing forces on space vehicles and predict orbits in aeronautics and astronautics (Chao, 2005), (4) to explore the interior structure and geological evolution of our Earth (Wieczorek, 2015), and (5) to explore minerals or fossil fuels and to examine geophysical models developed using gravity inversion (Oldenburg et al., 1998). For most of the above-mentioned examples, representation of the Earth's global gravity field in terms of mathematical models is an indispensable need. ...
Article
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The International Centre for Global Earth Models (ICGEM, http://icgem.gfz-potsdam.de/, last access: 6 May 2019) hosted at the GFZ German Research Centre for Geosciences (GFZ) is one of the five services coordinated by the International Gravity Field Service (IGFS) of the International Association of Geodesy (IAG). The goal of the ICGEM service is to provide the scientific community with a state-of-the-art archive of static and temporal global gravity field models of the Earth, and develop and operate interactive calculation and visualization services of gravity field functionals on user-defined grids or at a list of particular points via its website. ICGEM offers the largest collection of global gravity field models, including those from the 1960s to the 1990s, as well as the most recent ones, which have been developed using data from dedicated satellite gravity missions, CHAMP, GRACE, GOCE, advanced processing methodologies, and additional data sources such as satellite altimetry and terrestrial gravity. The global gravity field models have been collected from different institutions at international level and after a validation process made publicly available in a standardized format with DOI numbers assigned through GFZ Data Services. The development and maintenance of such a unique platform is crucial for the scientific community in geodesy, geophysics, oceanography, and climate research. In this article, we present the development history and future plans of ICGEM and its current products and essential services. We present the ICGEM's data by means of Earth's static, temporal, and topographic gravity field models as well as the gravity field models of other celestial bodies together with examples produced by the ICGEM's calculation and 3-D visualization services and give an insight into how the ICGEM service can additionally contribute to the needs of research and society.
... Some application examples in which the precise knowledge of the Earth's gravity field is fundamental are: (1) To establish a global vertical datum of global reference systems (Sideris and Fotopoulos, 2012), (2) to monitor mass distributions that are 10 indicators of climate related changes (Tapley et al., 2004, Schmidt et al., 2006, (3) to simulate the perturbing forces on space vehicles and predict orbits in aeronautics and astronautics (Chao, 2005), (4) to explore the interior structure and geological evolution of our Earth (Wieczorek, 2015), and (5) to explore minerals or fossil fuels and to examine geophysical models developed using gravity inversion (Oldenburg et al., 1998;Bosch and McGaugney, 2001). For most of the above mentioned examples, representation of the Earth's global gravity field in terms of mathematical models is an indispensable need. ...
Preprint
The International Centre for Global Earth Models (ICGEM, http://icgem.gfz-potsdam.de/) hosted at the GFZ German Research Centre for Geosciences (GFZ) is one of the five Services coordinated by the International Gravity Field Service (IGFS) of the International Association of Geodesy (IAG). The goal of the ICGEM Service is to provide the scientific community with a state of the art archive of static and temporal global gravity field models of the Earth, and develop and operate interactive calculation and visualisation services of gravity field functionals on user defined grids or at a list of particular points via its website. ICGEM offers the largest collection of global gravity field models, including those from the 1960s, as well as the most recent ones that have been developed using data from dedicated gravity missions, advanced processing methodologies and additional data sources such as satellite and terrestrial gravity. The global gravity field models have been collected from different institutions at international level and after a validation process made publicly available in a standardized format with DOI numbers assigned through GFZ Data Services. The development and maintenance of such a unique platform is crucial for the scientific community in geodesy, geophysics, oceanography, and climate research. The services of ICGEM have motivated researchers worldwide to grant access to their gravity field models and also provide them an access to variety of other gravity field models and their products. In this article, we present the development history and future plans of ICGEM and its current products and essential services. We present the Earth’s static, temporal, and topographic gravity field models as well as the gravity field models of other celestial bodies together with examples produced by the ICGEM’s calculation and 3D visualisation services and give an insight how the ICGEM Service can additionally contribute to the needs of research and society.
... This compensates the depth-attenuation of the diffusive electromagnetic signal (Zach et al. 2008). Similar depth weighting schemes have been used in inversion of magnetic and gravity data (Oldenburg et al. 1998). Here, () zs is the vertical distance to the current transmitter position in the wellbore, () f  is the skin depth for a given frequency, and () kf is a parameter based on the near-wellbore resistivity that adjusts the scaling factor. ...
Article
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The interpretation of resistivity measurements acquired in high-angle and horizontal wells is a critical technical problem in formation evaluation. We develop an efficient parallel 3D inversion method to estimate the spatial distribution of electrical resistivity in the neighborhood of a well from deep directional electromagnetic induction measurements. The methodology places no restriction on the spatial distribution of the electrical resistivity around arbitrary well trajectories. The fast forward modelling of triaxial induction measurements performed with multiple transmitter-receiver configurations employs a parallel direct solver. The inversion uses a preconditioned gradient-based method whose accuracy is improved using the Wolfe conditions to estimate optimal step lengths at each iteration. The large transmitter-receiver offsets, used in the latest generation of commercial directional resistivity tools, improve the depth of investigation to over thirty meters from the wellbore. Several challenging synthetic examples confirm the feasibility of the full 3D inversion-based interpretations for these distances, hence enabling the integration of resistivity measurements with seismic amplitude data to improve the forecast of the petrophysical and fluid properties. Employing parallel direct solvers for the triaxial induction problems allows for large reductions in computational effort, thereby opening the possibility to invert multiposition 3D data in practical CPU times.
... In geodesy and geophysics these so called global gravity field models (GGM) or potential models are fundamental resources and are needed, e.g., to establish the vertical datum of global reference systems and to realize global height unification (Sideris and Fotopoulos, 2012) or to explore the interior structure and retrieve the geologic evolution of Earth (Wieczorek, 2015). Also in many other disciplines the precise knowledge of the Earth's gravity field is of great importance: in aeronautics/astronautics gravity models are used to simulate perturbing forces that act on (space) vehicles and predict orbits (Chao, 2005); inertial navigation needs precise information on deflections of the vertical that can be derived from gravity field models (Wu et al, 2016); the exploration of minerals or fossil fuels and geophysical models rely on gravity inversion (Oldenburg et al, 1998;Bosch and McGaughey, 2001); orthometric heights are measured w.r.t. to the geoid -an equipotential surface of the gravity field -and are relevant for a wide field of applications and sciences, such as construction or hydrological modelling; melting of ice-covered regions and ground-water depletion can be observed through spatio-temporal gravity variations (Tapley et al, 2004); or in oceanography where the geostrophic ocean circulation is derived from the dynamic ocean topography that is given by the difference of the heights measured by satellite altimetry and the geoid heights of a static gravity field model (Bosch and Savcenko, 2010). ...
Thesis
Global gravity field models are fundamental resources in geodesy and geophysics and required for a range of applications in different fields, such as aeronautics/astronautics and oceanography. The dissertation investigates to which extent the topographic potential (TP) – based on spectral spherical harmonic forward modelling techniques – together with up-to-date topographic elevation data is a means to improve global gravity field models at short scales, i.e. up to and beyond the resolution of present day models (~10 km). The analysis of high-resolution continental gravity maps of near-global coverage – condensed into a new empirical degree variance rule up to degree 90,000 – shows that significant gravity signal amplitudes are present at those scales, justifying the attempt to model the high-resolution constituents of the gravity field implied by the topographic masses in this work on global scale. For this purpose a variety of spectral forward modelling techniques that can be used to compute the short-scale gravitational attraction in terms of solid spherical harmonic coefficients of the uncompensated TP are studied in a comparative manner. Existing rigorous and efficient single- and multi-layer methods are reviewed and adapted, e.g. by embedding arithmetic extensions and parallel structures into the algorithms, to accommodate ultra-high degree computations. In this context a new spectral approach has been derived that allows to compute the TP from a multitude of volumetric-layers in ellipsoidal approximation, i.e. by an integration with respect to an oblate reference ellipsoid. This method leads to the ellipsoidal topographic potential (ETP) that in contrast to the widely used spherical topographic potential (STP) avoids spherical approximations. All methods are studied up to degree 5,400 and 21,600 using a new 1 arcmin source-mass model (Earth2014) that provides the geometric boundaries for the definition of layers of the solid crust, the oceans, lake water masses and polar ice sheets. Amongst a number of new insights, the experiments revealed that the efficient methods in their presented form (double precision environment) are useful to model the complete TP to degrees not much further than 10,800. Main reasons are large computational costs associated with a full convergence of the involved binominal series expansions that in this case require k=46 and j=94 binominal terms of the k- and j-series, respectively. Additionally, raising the boundary functions to the k-th power also requires oversampling of the boundary functions in order to avoid aliasing which may result in extremely large grids, demanding massive parallelisation in the spherical harmonic analysis. The latter is found to be the "bottleneck" of spectral forward modelling in terms of computation time and is facilitated efficiently with the help of exact numerical quadrature algorithms in this work. At short scales (2160 < n <= 5480) the spherical approximation locally leads to errors of up to several mGal amplitude over mountainous terrain (~0.4 mGal global RMS), therefore ETP modelling should become common practice and replace spherical approaches. The error introduced by the rock-equivalent-topography (RET) concept of single-layer modelling techniques is in the range of ~0.5 mGal (global RMS) at short scales (< 10 km). However, at these scales the RET effect is negligible over continental and coastal marine areas. Only for large bathymetric features, such as deep-ocean trenches, multi-layer modelling seems justified at short scales. This makes single-layer ETP modelling based on RET an attractive method when aiming at high-resolution global modelling of land and coastal areas, underpinned further by its additive convergence behaviour that allows an early truncation of the k-series. Simple combinations with state-of-the-art observation-based satellite-only and combined gravity models (GOCO05s and EGM2008) show that the short-scale gravitational signal from ETP models up to degree 5480 improves the agreement with ground-truth gravity observations (at more than 1 million points) over various regions of Earth between ~6 and ~46 %. Further improvements can be expected using more sophisticated combination techniques that allow for regional weighting. However, the latter is complicated by enormous computational costs and lacking stochastic modelling within the here used and proposed spectral techniques. (Download original publication: http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:91-diss-20170531-1349781-1-7)
... The use of 3D modelling of potential fields (e.g., magnetics and gravity) has become an important tool in mineral exploration Oldenburg et al. 1998), as it can provide relevant information for target generation (Fitzpatrick and Large 2015). Three-dimensional modelling has been mainly implemented within academic environments (Oldenburg and Pratt 2007;Lelièvre et al. 2009); and its use in mining exploration has been increasing over the last two decades, as the algorithms and the computer calculations have been improved (Vallée et al. 2011). ...
Article
In unconformity-related uranium deposits mineralization is associated with hydrothermal clay-rich alteration haloes that decrease the density of the host rock. In the Kiggavik Uranium project, located in the eastern Thelon Basin, Nunavut (Canada), basement-hosted shallow deposits were discovered by drilling geophysical anomalies in the 1970s. In 2014, gravity data was inverted for the first time using the Geosoft VOXI Earth ModellingTM system to generate 3D models to assist exploration in Contact, the most recent discovery at Kiggavik. A 3D unconstrained inversion model was calculated before drilling and a model constrained with petrophysical data model was computed after drilling. The unconstrained inversion provided a first approximation of the geometry and depth of a low density body and helped to collar the discovery holes of the Contact mineralization. The constrained inversion was computed using density values measured on 315 core samples collected from the 21 drill-holes completed between 2014 and ...
... This compensates the depth-attenuation of the diffusive EM signal ( Zach et al. 2008). Similar depth weighting schemes have been used in inversion of magnetic and gravity data (Oldenburg et al. 1998). Here, z(s) is the vertical distance to the current transmitter position in the wellbore, δ( f ) is the skin depth for a given frequency, and k( f ) is a parameter based on the near-wellbore resistivity that adjusts the scaling factor. ...
... porous or fractured zones). This technique is widely used to detect voids [37][38][39][40][41], underground water [42][43][44], fractural zone recognition [45][46][47][48][49][50][51][52][53][54][55][56][57], and mineral exploration [58,59]. The difference in resistivity between the target and its surrounding materials is a key factor to ensure this technique is viable. ...
Article
The tailing management is a long term concern for environment and social security. Following the discovery of a new gold mineralization, a mining company aims at reusing one of tailings storage facility at the mining site. However, there is a small treated water reservoir (polishing pond) downstream of the Northwest dam. It is therefore important to know the present state of the Northwest dam’s internal structure. Geophysical methods of electrical resistivity and georadar were used for the auscultation of the Northwest dam. Numerical models were subsequently used to assess the geotechnical behavior of the dam in different deposition situations by simulating the restoration. The image processing results of georadar data show a layered structure near the surface. Changes in electrical resistivity offer deeper information than georadar. The lateral variability of the electrical resistivity corresponds to heterogeneity within each layer. As we cannot collect samples for the characterization of materials, the geophysical interpretation results help to estimate the compositional structure of the dam; ultimately it helps in numerical modeling on the safety factor estimation.
... The 3D inverse problem is solved by finding the model that minimizes the data misfit and adjusting the observed anomalies for a predetermined amount, spatially dependent of point position in space, and factors that affect the relative importance of the different components of the objective function ( Oldenburg et al., 1998). When minimized, the algorithm produces a relatively smooth model in three spatial directions, with data reproduced within an error tolerance. ...
Article
A joint magnetic and gravity survey was carried out in a Cenozoic magmatic plug in the northernmost of the Brazilian equatorial margin. The São João plug (0.5 km²) is located in the southern border of the Potiguar Basin, belonging to N–S oriented Macau-Queimadas Alignment formed by Macau magmatism, the youngest magmatism of the Borborema Province. This plug is hosted in Açu and Jandaíra formations, composed of sandstones and limestones that suffered significant thermal changes. The study area is located in a region characterized by a sequence of magnetic highs and lows. Based on the MaxMin technique, no significant remnant magnetization could be detected in the magmatic body. We measured magnetic susceptibility and density of the igneous rocks and their metamorphic and sedimentary bedrocks. The thermal metamorphism promoted an overall increase of bulk density. Joint magnetic and gravity modeling revealed the complex internal geometry of the São João plug, formed by a 400-m deep, probably a lopolith like body with a magmatic conduit as deep as 970 m. In addition, 3D modeling allowed mapping other igneous bodies, with no superficial expression, near to the studied plug.
... Although the co-occurrences of graphitic schist with sulphide mineralizations, both indicating a high-chargeability signature, often mask the related massive sulphide deposit (Salmirinne and Turunen 2007), the IP method is the best proven technique to differentiate between these two (Padget et al. 1969). Therefore, even if the geoelectrical/IP techniques do not entirely succeed to directly detect the VMS ore, they are still capable of delineating the ore-bearing horizons and high-strain zones, hence aiding mineral exploration (Oldenburg et al. 1998). ...
Article
Geoelectrical and induced polarization data from measurements along three profiles and from one 3D survey are acquired and processed in the central Skellefte District, northern Sweden. The data were collected during two field campaigns in 2009 and 2010 in order to delineate the structures related to volcanogenic massive sulphide deposits and to model lithological contacts down to a maximum depth of 1.5 km. The 2009 data were inverted previously, and their joint interpretation with potential field data indicated several anomalous zones. The 2010 data not only provide additional information from greater depths compared with the 2009 data but also cover a larger surface area. Several high-chargeability low-resistivity zones, interpreted as possible massive sulphide mineralization and associated hydrothermal alteration, are revealed. The 3D survey data provide a detailed high-resolution image of the top ∼450 m of the upper crust around the Maurliden East, North, and Central deposits. Several anomalies are interpreted as new potential prospects in the Maurliden area, which are mainly concentrated in the central conductive zone. In addition, the contact relationship between the major geological units, e.g., the contact between the Skellefte Group and the Jörn Intrusive Complex, is better understood with the help of 2010 deep-resistivity/chargeability data. The bottommost part of the Vargfors basin is imaged using the 2010 geoelectrical and induced polarization data down to ∼1-km depth.
... In particular, the in-cell densities are found to fit the observed data by minimizing an objective function, and Tikhonov regularization and depth weighting are also introduced into the objective function to ensure that the solution is geologically reasonable. To further handle the complex scenarios in salt imaging, additional geologic information is usually incorporated into inversion algorithms (Oldenburg et al., 1998; Li, 2001), such as imposing lower and upper bounds on the density contrast. For example, in Li (2001), the inversion algorithm allows different density bounds for different individual cells; one can freeze the density contrast value and enforce an exact salt structure in certain regions (such as the top of the salt body) by applying a very tight pair of density bounds. ...
Article
We have developed a level-set method for the inverse gravimetry problem of imaging salt structures with density contrast reversal. Under such a circumstance, a part of the salt structure contributes two completely opposite anomalies that counteract with each other, making it unobservable to the gravity data. As a consequence, this amplifies the inherent nonuniqueness of the inverse gravimetry problem so that it is much more challenging to recover the whole salt structure from the gravity data. To alleviate the severe nonuniqueness, it is reasonable to assume that the density contrast between the salt structure and the surrounding sedimentary host depends upon the depth only and is known a priori. Consequently, the original inverse gravity problem reduces to a domain inverse problem, where the supporting domain of the salt body becomes the only unknown. We have used a level-set function to parametrize the boundary of the salt body so that we reformulated the domain inverse problem into a nonlinear optimization problem for the level-set function, which was further solved for by a gradient descent method. Both 2D and 3D experiments on the SEG/EAGE salt model were carried out to demonstrate the effectiveness and efficiency of the new method. The algorithm was able to recover dipping flanks of the salt model, and it only took 40 min in a 2.5 GHz CPU to invert for a 3D model of 97,000 unknowns.
... In order to estimate the depth to the potential mineralized zone and to produce better recovery of the conductive zone, a 3D inversion was implemented using the UBC-GIF program EH3DInv for frequency-domain inversion over 3D structures (Haber et al, 2000;Haber et al, 2004;Oldenburg et al, 1998). ...
Article
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A low-frequency resistivity survey using ERA electrical profiling (middle gradient array) has been completed over a nickel prospect in Porcupine District, Ontario, Canada. The results of this EM-Resistivity were used to calculate the apparent resistivity and were further inverted using UBC-GIF 3D Frequency Domain Inversion Software, EH3DInv. The results of the inversion have shown the anomalous distribution of electrical properties, consistent with a known geological contact between metavolcanic rocks and an ultramafic pluton. The conductive zone has been interpreted to be located in the interval between 70 and 120 meters. The interpreted conductive zone was further subjected for a drilling program. The drilling program was carried out in March-April 2011 and revealed a 21m thick zone with disseminated to massive mineralized intervals starting at 82 meters, which is consistent with the results of 3D inversion of ERA data.
... Downhole MS measurement involves sending a probe directly down a borehole and recording MS readings as a continuous function of depth. Such downhole measurements have been used in petroleum exploration (Pozzi et al. 1993), ore deposit characterization (Oldenburg et al. 1998), marine geology (Barthes et al. 1999, and archeology (Dalan 2008). Applications of downhole MS loggers in lithologic and archeological mapping are described in detail in some instances (McNeill et al. 1996;Dalan 2008). ...
Article
Successful in situ groundwater remediation relies on the adequate distribution of treatment materials within the subsurface. Zero-valent iron (ZVI) is widely used for the remediation of soils contaminated with chlorinated organic compounds. Because ZVI is a solid, various techniques are used to distribute ZVI in the subsurface; however, a major uncertainty in this process involves determining the distribution of the iron during emplacement. A method of mapping the distribution of ferromagnetic material such as ZVI is by magnetic susceptibility (MS), a novel approach that is highly sensitive, quantitative, objective, and easily applied in field. The method has been tested in the laboratory on synthetic cores containing EHC®, an organic amendment containing 40 to 50% ZVI, using an MS meter with two types of sensors (loop and handheld). Both sensors have high sensitivity (e.g., 1% disseminated EHC is easily detected), whereas the hand-held sensor has greater spatial resolution (e.g., differences are notable on a scale of 1 cm). Following the laboratory studies, the handheld instrument was used to perform field measurements for multiple pilot studies and a full-scale application of EHC using various delivery methods (pneumatic fracturing, hydraulic fracturing, and direct injection) to construct a biobarrier at a field remedial site. The MS method has proven invaluable in quantifying amendment distribution and ensuring appropriate application of this remedial technology.
... Recent and emerging technological advances with the IP method include 3D pole-dipole acquisition (Collins and White, 2003), distributed data acquisition (Ritchie andSheard, 1999, Goldie, 2007), and 3D forward modelling and inversion (Oldenburg et al., 1998). A combination of these advances used in conjunction with astute geological interpretation holds the promise of significant improvement in the usefulness of IP for porphyry exploration. ...
... The reliability of these results is enhanced by the fact that, despite small differences, inversion results for the two different components, agree quite well with each other. We should finally mention that our results agree remarkably well with the image obtained by Oldenburg et al. (1998) as a result of their 3-D gravity inversion over the same area. ...
Article
Three-dimensional (3-D) electromagnetic (EM) inversion is increasingly important for the correct interpretation of EM data sets in complex environments. To this end, several approximate solutions have been developed that allow the construction of relatively fast inversion schemes. We have developed a localized quasi-linear (LQL) approximation that is source-independent and, therefore, appropriate for multisource array-type surveys, typical in many geophysical applications, such as airborne EM, cross-well tomography, and well logging. This method is based on the assumption that the anomalous electric field within an inhomogeneous domain is linearly proportional to the background electric field through an electrical reflectivity tensor λ^. This tensor is determined by solving a source-independent minimization problem based on the integral equation for the scattering currents. We have also developed a new, fast 3-D EM inversion method, based on this new approximation, and applied it to synthetic and real helicopter-borne EM data. The results demonstrate the stability and efficiency of the method and show that the LQL approximation can be a practical solution to the problem of 3-D inversion of multitransmitter frequency-domain EM data.
... [4] The term ''data fusion'' has gained portent in the Earth sciences community [Oldenburg et al., 1998]. This process also goes by the names ''joint data assimilation,'' ''objective analysis, '' ''coinversion,'' and ''joint inversion.'' ...
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1] A novel data-driven artificial neural network (ANN) that quantitatively combines large numbers of multiple types of soft data is presented for performing stochastic simulation and/or spatial estimation. A counterpropagation ANN is extended with a radial basis function to estimate parameter fields that reproduce the spatial structure exhibited in autocorrelated parameters. Applications involve using three geophysical properties measured on a slab of Berea sandstone and the delineation of landfill leachate at a site in the Netherlands using electrical formation conductivity as our primary variable and six types of secondary data (e.g., hydrochemistry, archaea, and bacteria). The ANN estimation fields are statistically similar to geostatistical methods (indicator simulation and cokriging) and reference fields (when available). The method is a nonparametric clustering/classification algorithm that can assimilate significant amounts of disparate data types with both continuous and categorical responses without the computational burden associated with the construction of positive definite covariance and cross-covariance matrices. The combination of simplicity and computational speed makes the method ideally suited for environmental subsurface characterization and other Earth science applications with spatially autocorrelated variables. Citation: Besaw, L. E., and D. M. Rizzo (2007), Stochastic simulation and spatial estimation with multiple data types using artificial neural networks, Water Resour. Res., 43, W11409, doi:10.1029/2006WR005509.
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... These methods construct a density contrast distribution as a function of spatial position and image the base of salt by the transition in density contrast (Li 2001). Similar approaches have also been used widely in mineral exploration problems (Green 1975;Last & Kubik 1983;Guillen & Menichetti 1984;Oldenburg et al. 1998). ...
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Geophysics for the Mineral Exploration Geoscientist - by Michael Dentith April 2014
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Geophysics for the Mineral Exploration Geoscientist - by Michael Dentith April 2014
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