Conductive Ore Bodies Research Articles

SQUIDs for magnetic and electromagnetic methods in mineral exploration

Research on quantum sensors for the detection of magnetic fields (quantum magnetometers) is one of the fast-moving areas of Quantum Technologies. Since there exist expectations about their use in geophysics, this work will provide a brief overview on the various developing quantum technologies and their individual state of the art for implementing quantum magnetometers. As one example, the developments on superconducting quantum interference devices, so-called SQUIDs as a specific implementation of a quantum magnetometer, are presented. In the course of this, SQUID instrument implementations and associated demonstrations and case studies will be presented. An airborne vector magnetometer with ultra-low noise (<10 mathrm{fT}/sqrt{mathrm{Hz}}) and high dynamic range of >32 mathrm{bit} will be introduced which has the prospect to be applied for the magnetic method in parallel with electromagnetic methods such as passive audio-frequency magnetics or semi-airborne methods using active transmitters such as elongated grounded dipole sources. The according signals are separated in the frequency domain. A second implementation is an airborne full tensor gradiometer instrument will be discussed which has shown already a number of successful case studies and which turned into commercial operation in the past years. Besides the airborne instrument, a very successful implementation of quantum magnetometers is the SQUID-based receiver for the ground-based transient electromagnetic method. Today it is a mature technology which has been in commercial use for more than a decade and has led to a number of discoveries of conductive ore bodies. One case study will be presented which demonstrates the performance of this instrument. Finally, future prospects of using quantum magnetometers, including SQUIDs and new optically pumped magnetometers, in geophysical exploration will be discussed. Particular applications for both sensor types will be introduced.

A finite‐element time‐domain forward‐modelling algorithm for transient electromagnetics excited by grounded‐wire sources

ABSTRACTA three‐dimensional finite‐element time‐domain forward‐modelling algorithm is developed to simulate transient electromagnetics excited by grounded‐wire sources. The main advantage of this finite‐element time‐domain algorithm is that full transmitting‐current waveforms and complex‐shaped sources resulting from topography can be directly dealt with in this algorithm. The models used to test this algorithm include a homogeneous half‐space model, a stratified‐medium model, the model of a complex conductor at a vertical contact and the Ovoid Zone massive sulfide deposit at Voisey's Bay, Canada. The homogeneous half‐space model is used to determine the truncation boundary for a computational domain, and to compare with the electromagnetic responses excited by step‐off, step‐on and direct current waveforms. For the stratified‐medium model, results demonstrate that full transmitting waveforms have strong effects on the observed electromagnetic responses. The model of a complex conductor at a vertical contact is designed for the grounded electrical source airborne transient electromagnetic method and is also used to examine the effectiveness of the broadside and inline configurations for such a vertical, thin plate embedded in the subsurface. The area of the Ovoid Zone massive sulfide deposit possesses non‐negligible topography, the effects of which on the shapes of the grounded‐wire sources must be taken into account when implementing the finite‐element time‐domain solver. The results show that both the broadside and inline electromagnetic responses are strongly affected by the massive conductive ore body.

Feasibility of In-Phase Airborne TEM Utilising Gradient Measurements

Detection of highly conductive ore bodies with airborne TEM is difficult or impossible as a result of the response of the target being largely in-phase with the transmitter signal. Measurement of magnetic field spatial gradients, in addition to the usual field measurements is proposed as a means to allow in-phase measurements in a non-rigid TEM system. With in-phase measurements the most conductive targets are as detectable as any geometrically equivalent target with lower time constant, greatly extending the utility of TEM. A simplified, proof-of-concept, analytical system model, in which only coaxial transmitter, receiver and target are considered, is used to demonstrate the compensation concept and to quantify the expected performance of such a system. The model demonstrates that a compensated signal preserving (or even enhancing) the target response can be generated. An assessment of noise in this model shows that noise from the gradient measurements significantly increases the system noise, over that of an equivalent receiver, by ~ 100-fold. However this does not extinguish the benefit of in-phase measurement and only results in halving of the depth of detection for the most conductive target, in comparison with an equivalent target with optimum time constant, in a system measuring the off-time response. A numerical model of a system with 7 degrees of freedom is then used to confirm the results can be extended to physically realistic TEM systems.

3D vector finite-element electromagnetic forward modeling for large loop sources using a total-field algorithm and unstructured tetrahedral grids

Unstructured tetrahedral grids with local refinement facilitate the use of total-field solution approaches to geophysical electromagnetic (EM) forward problems. These approaches, when combined with the vector finite-element (FE) method and with refinement near transmitters and receivers, can give accurate solutions and can easily handle realistic models with complex geometry and topography. We have applied this approach to 3D forward modeling for fixed- and moving-loop configurations. MUMPS, a direct solver, was used to solve the linear system of equations generated by FE analysis. A direct solver is particularly suited to the moving-loop configuration for which the right side is different for every transmitter loop, but for which the coefficient matrix is unchanged. Therefore, the coefficient matrix need only be factorized once, and then the system can be solved efficiently for all different right sides. We compared our results with several typical scenarios from the literature: a conductive brick in a homogeneous half-space and a complex conductor at a vertical contact both for fixed-loop configurations, and a homogeneous half-space for a moving-loop configuration. We also evaluated results for the massive sulfide ore deposit of the Ovoid Zone at Voisey’s Bay, Labrador, Canada, for which we considered fixed- and moving-loop configurations. This model also provides an illustration of the complex vortex current systems that are generated by time-domain EM methods within highly conductive ore bodies in a resistive host.

Down-hole magnetometric resistivity inversion for zinc and lead lenses localization at tobermalug, county Limerick, Ireland

The magnetometric resistivity (MMR) method consists of measuring the variation of magnetostatic field associated to current density variations. It is based on the principle that direct current flow between two electrodes takes the path of least resistance through the earth. Consequently, conductive or relatively conductive ore bodies such as massive, semi-massive to disseminated sulphides are preferentially energized, which increases the current density in their vicinity. MMR is more efficient than electromagnetic methods in the case of weak conductivity contrasts.To improve the signal to noise ratio the magnetic field can be measured in boreholes rather than on the surface. Down-hole magnetometric resistivity measurements have been conducted in Tobermalug prospect in County Limerick, Ireland. The survey was used as an alternative to down-hole electromagnetic to delineate subhorizontal zinc/lead mineralization lenses that can be poorly conductive. Two survey areas were investigated, DHMMR1 and DHMMR2.Interpretation is based on the regularized least-squares inversion of MMR data, in which the MMR forward problem is solved with a finite volume discretization of the electrostatic and magnetostatic equations. Inversion of synthetic data generated over two conductive horizontal discs model shows that conductive structures are well positioned but their extensions is biased along current electrodes orientation.Inversion of the field data allowed localizing a few conductive elongated targets. At DHMMR1, the conductivity is weaker and seems to be associated to disseminated mineralization. DHMMR2 contains a higher conductivity and more elongated target. It appears to be associated to semi-massive sulphides.

Review on airborne electromagnetic inverse theory and applications

Airborne electromagnetic (AEM) data processing and inversion has progressed from the early conductivity-depth imaging, to 1D inversion for a layered earth model, and most recently to 2D/3D inversions. The new processes have used methods for searching for solutions, applying constraints, and focusing images, such as Occam’s, laterally constrained, and holistic inversions. For airborne data imaging and 1D inversion, some algorithms are fast, whereas others have control over data misfit and the vertical or lateral smoothness. Beyond 1D algorithms, 2D/3D inversions are based on currently used methods such as isolated conductor models (which are good for highly conductive orebodies in resistive background) and techniques that discretized regions that are used for more complex structures and backgrounds. In the latter case, the computations are slow, so research is focusing on time-efficient computer algorithms for solving the equations such as Gauss-Newton, quasi-Newton, and nonlinear conjugate gradient algorithms. For the electromagnetic (EM) problem, the solution can be obtained in a more practical time frame if material that has minimal impact is ignored. Two approaches can be used to speedup the EM inversion process — the moving footprint and direct solver methods. We hope our work will to some extent help stimulate and focus the research in AEM inversion.

Low-frequency on-site identification of a highly conductive body buried in Earth from a model ellipsoid

Identification of a highly conductive orebody buried in Earth using an equivalent, perfectly conducting, triaxial model ellipsoid is investigated. The real data available (three-component magnetic fields collected along a borehole due to a single-frequency current loop at the Earth surface) are simulated via a low frequency, closed-form power series expansion of the electromagnetic fields scattered off an equivalent ellipsoid within a homogeneous, conductive medium, the source itself being idealized as a vertical magnetic dipole nearby. The approach provides formulations amenable to fast yet accurate computations, most of the work being in the construction of the formulations themselves, not in the numerical computations. The inversion scheme is described, which sees the iterative minimization of the least-square discrepancy between the fields due to a given ellipsoid and the data available. Unknowns are semi-axis lengths, angular orientations and coordinates of its centre. Numerical simulations illustrate the approach, before considering experimental single-well log data in a surface-to-borehole configuration at a mining site.

Qualitative geophysical interpretation of the Sudbury Structure

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.

On mapping seafloor mineral deposits with central loop transient electromagnetics

Electromagnetic methods are commonly employed in exploration for land-based mineral deposits. A suite of airborne, land, and borehole electromagnetic techniques consisting of different coil and dipole configurations have been developed over the last few decades for this purpose. In contrast, although the commercial value of marine mineral deposits has been recognized for decades, the development of suitable marine electromagnetic methods for mineral exploration at sea is still in its infancy. One particularly interesting electromagnetic method, which could be used to image a mineral deposit on the ocean floor, is the central loop configuration. Central loop systems consist of concentric transmitting and receiving loops of wire. While these types of systems are frequently used in land-based or airborne surveys, to our knowledge neither system has been used for marine mineral exploration. The advantages of using central loop systems at sea are twofold: (1) simplified navigation, because the transmitter and receiver are concentric, and (2) simplified operation because only one compact unit must be deployed. We produced layered seafloor type curves for two particular types of central loop methods: the in-loop and coincident loop configurations. In particular, we consider models inspired by real marine mineral exploration scenarios consisting of overburdens 0 to 5 m thick overlying a conductive ore body 5 to 30 m thick. Modeling and resolution analyses showed that, using a [Formula: see text] transmitting loop with 20 A of current, these two configurations are useful tools to determine the overburden depth to a conductive ore deposit and its thickness. In the most extreme case, absolute voltage errors on the order of 10 nV are required to resolve the base of a 30 m thick ore deposit. Whether such noise floors can be achieved in real marine environments remains to be seen.

Forward and inverse self-potential modeling in mineral exploration

Natural self-potential (SP) fields observed in the vicinity of conductive orebodies can be explained in terms of electrochemical reactions in which the conductors participate. Battery-like models assume that a buried conductor creates an anode-cathode pair by conveying a flow of electrons to oxidizing areas in the shallow subsurface from reducing areas at depth. For conductors with invariant composition (behaving as an inert electrode), a quantitative formulation is already available. Numerical Green’s functions are used to allow one-step evaluation of SP fields from an inert electrode model. The model is used to simulate geoelectric targets in mineral exploration and to test a procedure to obtain current source terms by inverting an SP data set. Data inversion is constrained by charge conservation and prescribes source terms at the target surface. A background resistivity model is assumed to be known and is used to recognize interfaces and evaluate numerical Green’s functions in forward and inverse modeling. The inversion procedure is applied to interpret 2D data from two gold deposits of the Yanacocha district, Peru.

Electromagnetic imaging of a complex ore body: 3D forward modeling, sensitivity tests, and down-mine measurements

This study investigated the capability of audio-magnetotellurics (AMT) not only to detect, but also to delineate complex conductive ore bodies at minable depths. A detailed 3D numerical-electrical resistivity model of the Bathurst no. 12 deposit (New Brunswick, Canada) was constructed using available geologic and geophysical information. Different geologic and data acquisition conditions were simulated: presence of overburden; different geometries, dimensions, and positions of the ore body; and different data sampling regimes. The behavior of the surface 3D electromagnetic fields was compared with that from bodies of infinite strike extent. The 3D and 2D AMT responses were similar at high frequencies, so 2D modeling was shown to be both valid and sufficient. However, at low frequencies only those responses for current flow perpendicular to the body (the transverse magnetic mode in a 2D case), were reasonably similar. The 2Dinversions showed that the position and the top of the 3D ore body were well resolved, but the bottom of the ore body's resis-tivity were poorly resolved. To increase resolution at depth below ore bodies, and to possibly extend mine life, we recommend that AMT measurements are taken from within mines. Simple mod-els of ore bodies were simulated to show responses at depth, and to undertake body stripping of the overlying structures. The tests showed that conductive structures above the measurement level can have a strong influence on imaging of conducting zones below, and can produce distortion effects in apparent resistivity and phase. Although the body-stripping approach reduces these effects and gives an indication of whether there are conductive structures below, the resulting image is considerably different from that of a model without overlying conductive structures. Full 3D inversion, holding known structures at constant, is required.

Time-Domain Electromagnetic Data Interpretation using Moving-Loop Configurations for Sheet-Like Base Metal Ore Deposits in Resistive Hosts

A simple, useful, and practical tool is proposed for the interpretation of moving-loop time-domain electromagnetic (TEM) surveys over two-dimensional (2D) sheet-like conductors embedded in a resistive host. It is based on the relationship observed between attributes of the displayed responses and the geometrical and electrical parameters of the conductive ore body. The ore body is modelled as a plate conductor for which the depth, dip, and conductance parameters are estimated. Numerical and scale modelling are used to establish the interpretative expressions. Responses computed for the various plate parameters are classed according to the following response attributes: time constant, asymmetry, and peak-to-peak distance. Three expressions relating depth, dip, and conductance to the response attributes are determined using multiple linear regression. The relationships are validated using scale-modelling data. The method allows the determination of the plate depth, conductance, and dip with an accuracy of ±10%, ±10% and ±5° respectively. The method is tested on SIROTEM survey data from Chutes-des-Passes in Quebec (Canada), where drill hole information is available. The results show that the regression relationships provide accurate estimates of the basic characteristics of the deposits.

The Application of Airborne Electromagnetics to the Search for High Conductance Targets

The airborne electromagnetics (AEM) technique has been used very successfully for over 50 years to locate conductive ore bodies. For most users of the technique, the primary purpose of AEM has been to provide a rapid and inexpensive means to locate targets, whereupon the best features were then followed up with ground geophysical techniques prior to possible drill testing. In this role as a first-pass detection tool, AEM equipment development has focused primarily on spatial resolution, depth of detection and rejection of conductive cover. Traditionally little emphasis was placed upon the need to provide quantitative information about a target?s conductance. This situation has been changing in the last 10 years, driven in large part from concerns expressed from those groups exploring for magmatic nickel deposits. Drawing upon both petrophysical studies and field observations, a case is developing that magmatic nickel deposits have conductances that place them far outside the conductance bandwidth of effectively all AEM systems. This being the case, then the use of AEM as a primary target identification tool for new nickel deposits is a seriously flawed strategy. So as to better understand this issue, AEM data sets acquired over the Voisey?s Bay deposits, Labrador, Canada have been analysed using standard commercial processing techniques such as CDIs and time constant analysis. Additionally, a series of numerical models have been generated to try to better explain the observed field results, as well as simulate the response that other AEM systems would produce over the deposit. While the Voisey?s Bay deposit undoubtedly contains mineralisation of very high conductivity, the processing and modelling shows the AEM technique to be an effective means to locate and discriminate targets of high conductance. Although such conductance discrimination may only be achievable on a relative scale, it is seen as generally adequate for most exploration situations.

Using the Loki 3D edge-finite-element program to model EM dipole-dipole drill-hole data

Electromagnetic drill-hole measurements are an important tool for locating conductive orebodies in detailed exploration scale. Dipole-dipole systems have good spatial resolution, especially if all three magnetic components are measured over a wide frequency range. Interpretation of field data requires the use of sound 3D modelling tools capable of dealing with high conductivity contrasts and the close proximity of boundaries to the source or receiver. We modelled the response of the SlimBoris dipole-dipole drill hole system going through a block target using Loki, a 3D, full-domain, edge finite-element approach based on vector shape functions and scalar unknowns. This method achieves considerable speed advantage over conventional finite-element methods since only the one tangential component rather than three orthogonal components need be solved at each ``node'. Moreover, since no boundary conditions are violated in this approach, contrasts in excess of one million to one can be modelled accurately as verified through semi-analytical layered earth solutions. Computation time can be reduced by a further factor of five by solving initially for Schelkunoff potentials rather than electric or magnetic fields. This is due to the superior condition number of the resulting matrices. Accuracy is maintained using Green?s function projectors to obtain the fields at the receivers rather than differentiating the potentials. A 30,000 cell model takes 20 seconds on a 1.9GHz Intel chip per frequency per transmitter position. Control files for complex models can be set up rapidly using either the Encom EMGUI or Maxwell from EMIT. The results were in close agreement with scale model results at all contrast ranges. Another check was made using a 3D integral equation program. As expected, close agreement was obtained at contrasts of less than 300 but the integral equation results deteriorated at high contrast.

Extended Born domain integral models of diffusive fields

Diffusive fields in three-dimensional cases, e.g., voluminous inclusions, voids, and cracks in planar metal slabs and conductive orebodies and other obstacles in a half-space earth subsoil, can be modeled via rigorous vector integral formulations that require Green dyads. The latter contain most of the complexity of the embedding environment whilst many situations can be attacked from precalculated discrete sets of such dyads. However, exact models remain computationally costly, and inversion problems (nondestructive evaluation of made structures and geo-electromagnetic probing of the earth) might require faster yet accurate models. Those provided by application of several so-called extended Born approximations are considered herein and their efficiency is illustrated from a number of numerical simulations whose results are compared to existing datasets.

On removing the primary field from fixed‐wing time‐domain airborne electromagnetic data: some consequences for quantitative modelling, estimating bird position and detecting perfect conductors

In the process of removing the primary field from fixed‐wing time‐domain airborne EM data, the response is decomposed into two parts, which are referred to here as the time‐domain ‘in‐phase’ and ‘quadrature’ components. The time‐domain in‐phase component is dominated by the primary field, which varies significantly as the transmitter–receiver separation changes. The time‐domain quadrature component comes solely from the secondary response associated with currents induced in the ground and this is the component that has traditionally been used in the interpretation of data from fixed‐wing towed‐bird time‐domain EM systems. In the off‐time, the quadrature response is very similar to the total secondary response. However, there are large differences in the on‐time and even some small differences in the off‐time.One consequence of these differences is that when airborne EM data are to be interpreted using a synthetic mathematical model, the synthetic data calculated should also be the quadrature component. A second consequence relates to the time‐domain in‐phase component which is sometimes used to estimate the receiver‐sensor (bird) position. The bird‐position estimation process assumes there is no secondary field in the in‐phase component. If the ground is resistive, the secondary contained in the in‐phase component is small, so the bird‐position estimate is accurate to about 30 cm, but in highly conductive areas the secondary contribution can be large and the position estimate can be out by as much as 5 m. A third consequence arises for highly conductive bodies, the response of which is predominantly in‐phase. This means that any response from these types of body is lost in the component that has been removed in the primary‐field extraction procedure. However, if the bird position is measured very accurately, the actual free‐space primary field can be estimated. If this is then subtracted from the estimated primary (actually free‐space primary plus secondary in‐phase response), then the residual is the secondary in‐phase response of the ground. Using this methodology, very conductive ore bodies could be detected. However, a sensitivity analysis shows that detection of a large vertically dipping very conductive body at 150 m depth would require that the bird position be measured to an accuracy of about 1.4 cm and the aircraft attitude to within about 0.01°. Such tolerances are very stringent and not easily attainable with current technology.

Transient electromagnetic exploration techniques: can they be applied to the landmine discrimination problem?

The use of metal detectors in the humanitarian demining industry closely resembles the use of electromagnetic techniques in the mineral exploration industry to find conductive orebodies. An overview is given for the landmine problem and some of the current technologies used in the demining process. The similarities of metal detection to geophysical transient electromagnetic data acquisition, processing and interpretation are used to suggest ways in which more information than just the “presence of metal” may be obtained when using pulse induction (PI) metal detectors.

Conductive masses in a half-space Earth in the diffusive regime: fast hybrid modeling of a low-contrast ellipsoid

Electromagnetic three-component magnetic probes at diffusion frequencies are now available for use in slim mineral-exploration boreholes. When a source is operated at or below the surface of the Earth in the vicinity of a conductive orebody, these probes provide, after appropriate processing, the secondary vector magnetic field attributed to this body. Proper inversion of the resulting datasets requires as a first step a clear understanding of the electromagnetic interaction of model signals with model bodies. In this paper, the response of a conductive ellipsoid buried at shallow depth in a half-space Earth is investigate by a novel hybrid approach combining the localized nonlinear approximation and the low frequency scattering theory. The ellipsoidal shape indeed fits a large class of scatterers and yet is amenable to analytical calculations in the intricate world of ellipsoidal harmonics, while the localized nonlinear approximation is known to provide fairly accurate results at least for low contrasts of conductivity between a scattering body and its host medium. In addition, weak coupling of the body to the interface is assumed. The primary field accounts for the presence of the interface, but multiple reflection of the secondary field on this interface is neglected. After analyzing the theoretical bases of the approach, numerical simulations in several geometrical and electrical configurations illustrate how estimators of the secondary magnetic field along a nearby borehole behave with respect to a general-purpose method-of-moments (MoM) code. Perspectives of the investigation and extensions, in particular, to two-body systems, strong coupling to the interface, and high contrast cases, are discussed.

Robust estimation of the band‐limited inductive‐limit response from impulse‐response TEM measurements taken during the transmitter switch‐off and the transmitter off‐time: Theory and an example from Voisey’s Bay, Labrador, Canada

Modern transient electromagnetic systems are able to take measurements in the transmitter on‐time. Integrating measurements taken during the transmitter switch‐off and those collected in the transmitter off‐time yield an estimate of the primary field plus the secondary inductive‐limit response. If the transmitter loop position is known and the position and orientation of the receiver dipoles are known, it is possible to calculate the primary field. When the theoretical primary field is subtracted from the measured inductive‐limit‐plus‐primary response, the inductive‐limit response can be isolated. An anomalous inductive‐limit response is a diagnostic feature of highly conductive ore bodies. On‐ and off‐time PROTEM data collected in a drill hole proximal to the Reid Brook Zone (one of the Voisey’s Bay deposits in Labrador, Canada) shows a strong inductive‐limit anomaly corresponding to an off‐hole conductor. A drill hole targeted to test this conductor intersected 20.4 m of mineralization, including 8.25 m of massive sulfide.

Reflection models and ramp response for downhole TEM data

Very conductive ore bodies are difficult to detect using standard TEM techniques. Eddy currents are established at the surface of these bodies to oppose the flux associated with the primary field in accord with Faraday’s law of induction. These currents persist with negligible losses through resistivity when the primary field is removed. Consequently during the off-time dB/dt remains close to zero and signal levels at the surface are negligible. However an examination of the primary field, or ramp response, rather than the secondary field, will often show sudden changes in amplitude consistent with interference effects from source multiples. Ramp response data obtained for highly conductive targets have now been interpreted using models based on multiple current filaments. The results are consistent with conventional interpretations based on filament inversion theory and also with models based on a reflection of the primary field at the conductor surface.