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2nd Conference on Geophysics for Mineral Exploration and Mining
- Conference date: September 9-12, 2018
- Location: Porto, Portugal
- Published: 09 September 2018
21 - 40 of 68 results
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Large-Scale Multiphysics Airborne Geophysical Survey in Saudi Arabia: Results of the Glass Earth (Pilot) Project
Authors M. Zhdanov, F. Alfouzan, L. Cox, A. Alotaibi, M. Alyousif, D. Sunwall and M. EndoSummaryRecent developments in large-scale geophysical inversions made it possible to invert the results of entire airborne geophysical surveys over the large areas into 3D models of the subsurface. This paper presents the results of interpretation of the data acquired by a multiphysics airborne geophysical survey in Saudi Arabia. The project involved the acquisition, processing, and interpretation of airborne electromagnetic, gravity, and magnetic geophysical data over an 8,000 square kilometer area. All the collected data were carefully analyzed and inverted in 3D models of the corresponding physical properties of the subsurface, including 3D density, magnetization vector, and conductivity models. This paper summarizes the interpretation of all geophysical data sets collected during the field airborne survey.
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Geologically-Driven Inversion of Magnetotelluric Data
Authors F. Golfré Andreasi, S. Re, F. Ceci, L. Masnaghetti and A. BattagliniSummaryMineral exploration in mountainous areas is challenged by several factors. The accessibility is not straightforward due to steep terrains and, in some cases, also due to forest coverage; thus, making both exploration and development plans far more expensive. A comprehensive understanding of the subsurface is of paramount importance to target the most promising prospects and to reduce the footprint – and the cost – of exploration and production activities.
Satellite imagery and airborne acquisition techniques can be used to cover large areas in an economic manner and to gain a preliminary knowledge of the subsurface. Unfortunately, these techniques lack the resolution required to infer the geologic setting at depth and to build a structural model with adequate confidence; this is the time when it becomes necessary to step into ground exploration.
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On the Optimal Strategy of Three-Dimensional Inversion of Low-Frequency Electromagnetic Data
Authors M. Malovichko, N. Yavich, N. Khokhlov and M. ZhdanovSummaryThe inverse problem of three-dimensional (3D) low-frequency electromagnetic (EM) data is usually formulated as unconditional minimization of the Tikhonov parametric functional. The Gauss-Newton method ensures fast convergence, but has high computational and/or memory complexity due to the need to factorize the Hessian matrix. This difficulty can be overcome only partially by the use of modern massively parallel distributed memory clusters (for example, Grayver et al., 2013 ; Wang et al, 2018 ). The nonlinear conjugate-gradient (NLCG) or L-BFGS methods are less demanding in terms of computational load and memory consumption, but may suffer from slow convergence at complicated models. There are two approaches, in which a special kind of transformation of model parameters is proposed: a diagonal preconditioner of Newman and Boggs (2004) and the integral-sensitivity approach of Zhdanov (2002) . These approaches are essential to achieve a tolerable convergence rate and, in fact, are very similar. Recent examples with these methods include ( Commer and Newman, 2008 ) and (Čuma et al., 2017).
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Three-Dimensional Inversion of ATEM Responses for Grounded Source
Authors M. Yi, Y. Sasaki, A. Johmori and S. JungSummaryRecently, an airborne time-domain electromagnetic (ATEM) surveying with a grounded source, called GREATEM, has gained popularity because it provides a greater depth of investigation than moving-source ATEM surveys. However, unlike moving-source ATEM responses, GREATEM responses exhibit complicated behaviors that result from galvanic currents rather than vortex currents. Our model study showed that 1D inversion of GREATEM data is most likely to result in an erroneous resistivity model even for layered structures if there are some lateral changes in layer thickness and that 3D inversion is required to recover the subsurface resistivity. Our 3D modeling is based on a finite-difference technique in the frequency domain and inverse Fourier transformation to the time-domain response. The convolution with the system response is performed in the frequency domain. For 3D inversion, we introduced a scaling factor as an unknown parameter to take into account inaccuracies in the source moment and receiver gain. The inversion algorithm has been tested with a simple 3D subsurface model and 3D inversion result for a field data set taken over a known uranium deposit in Korea showed that the recovered conductive zones are consistent with the known structural direction and distribution of mineralization zones.
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Preparation Study Based on Borehole Data for Delphi-Distomon Mining Area to Better Design Geophysical Works
Authors C. Orfanos, K. Leontarakis, G. Apostolopoulos, K. Athanassas and P. KofakisSummaryEurope is in need of fresh aluminum for its vast variety of developments and Greece has the potential to deliver. Delphi Distomon S.A is one of the largest bauxite producers in Greece and is interested to explore new deposits in new unexploited areas. Logistics, accessibility, environmental issues and high cost are key obstacles in the application of a high-definition 3D active seismic survey. Hence, an alternative integrated method of exploration will be carried out based on gravity, magnetotelluric and passive seismic methods. As a preliminary step for an optimized acquisition scheme, a dynamical approach is followed that utilizes a lithology model created by available drilling data, its transformation to a density one, the forward modelling and the comparison of the synthetic data with a previous gravity study in the area. The preliminary results of the analysis gave the chance to identify the vulnerabilities of the lithology and the equivalent geophysical model. As it was observed, they should be enriched during the survey with additional geological information and in situ observations. The emerged models can contribute remarkable to the stage of the processing of the different geophysical methods as well as the final stage of the integrated interpretation.
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Advances in Deep Electrical Earth Imaging - Implications for Improved Targeting and Planning for Exploration
More LessSummaryAdvanced imaging in the medical world developed in the 1950’s through the 1970’s thanks to new applications of physics to measure key parameters related to internal aspects of the human body. This involved imaging methodologies, and sophisticated software and data manipulation. Damadian was the first to perform a full body scan of a human being in 1977 to diagnose cancer with Magnetic Resonance Imaging (MRI) technology. Today, MRI’s are used routinely by doctors to perform investigations and diagnoses prior to any invasive further steps, including surgery.
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3-D Magnetotelluric Study across the Capricorn Orogen (Western Australia) and its Implications for Mineral Exploration
Authors P. Piña-Varas and M. DentithSummaryWhat have conventionally been seen as ‘academic’ geophysical methods used to study the deep crust are now increasingly finding applications in mineral exploration ( Dentith et al., 2018 ). This is largely due to the adoption of the mineral systems concept, which is seen as a basis for future mineral exploration which will increasingly seek deeper targets and under cover. A mineral system comprises all the geological processes that lead to the creation of a mineral deposit. Conventionally, geophysical exploration has sought to map the near-surface geology and/or detect the mineralisation itself and the immediately surrounding geological environment.
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Seismic Exploration in The Kylylahti Cu-Au-Zn Mining Area: Comparison of Time and Depth Imaging Approaches
Authors S. Heinonen, M. Malinowski, F. Hlousek, G. Gislason, E. Koivisto, S. Buske and The COGITO-MIN Working GroupSummaryHigh-resolution seismic reflection profiles were acquired in the Kylylahti polymetallic mine area in the Outokumpu region, Finland, in the framework of the COGITO-MIN project. Acquisition was performed along two segmentally straight lines using both vibroseis and dynamite sources. We present three different versions of the data imaging, one utilizing post-stack time migration, second pre-stack time migration and third Fresnel Volume pre-stack depth migration algorithms. Seismic images reveal steep reflectors at less than 500 m depth that correlate with the black schist intercalations in the mica schist as well as strong reflectors at 5–8 km depth attributed to mafic dykes on top of Archean basement rocks. The rock assemblage hosting the mineralisation is imaged as prominent but discontinuous reflectivity due to its complex internal structure caused by folding and faulting.
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High-Resolution Seismic Imaging of The Garpenberg Ore System (Central Sweden) Through Active- and Passive-Seismic Survey
Authors A. Gil de la Iglesia, A. Malehmir, B. Lund, C. Juhlin and R. CarbonellSummaryThe global demand for mineral resources is increasing every year, with now even a strong push for green and high-tech technologies. Considering the ever-increasing hope of exploring these resources at depth, particularly at near mines or near existing infrastructures, new and improved approaches for integrating available data and use them to optimize exploration and mining are required. In this work, we have revisited an existing legacy active-source 2D seismic data acquired over Garpenberg volcanic-hosted massive sulphide (VMS) deposit. Given the strong noise contamination observed in the active-source data, we argue an approach involving the use of active-and passive-seismic data recording would be optimal for deep exploration and characterization of the top of the orebody illustrating its potential for targeting even steep geometry mineralized zones. The reprocessing reveals a strong reflection that may be associated with a VMS mineralization or contact that hosts the mineralization.
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Passive Seismic Three-Component Interferometry Experiment at the Kylylahti Mine Site, Eastern Finland
Authors S. Väkevä, E. Koivisto, M. Chamarczuk, M. Malinowski and the COGITO-MIN Working GroupSummaryRecent theoretical advances have demonstrated that passive surveys utilizing ambient seismic noise allow new possibilities for seismic imaging and may provide an alternative for controlled-source surveying. Related to the COGITO-MIN project, a passive seismic experiment employing a set of 45 three-component seismometers recording for about 30 days was conducted in the Kylylahti polymetallic mine area in Eastern Finland. Acoustic forward modelling was carried out to get an idea on how well the reflections could be retrieved with seismic interferometry. Three-component data allows to determine all nine components of the Green’s tensor. For this work, one day of passive recordings were used to retrieve Green’s function for the vertical and transverse component of the geophone lines. There are events in the vertical component panel that can be attributed to the impedance contrast between the host rocks of the mineralisation and its surroundings. The transverse component panel exhibits similar sparse reflectivity, but is lacking coherence of the reflections.
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From Regional Seismics to High-Resolution Resource Delineation: Example from the Outokumpu Ore District, Eastern Finland
SummaryWe present an overview of the seismic exploration initiatives at the Outokumpu ore district in Eastern Finland, with new results from the COGITO-MIN active-passive seismic survey in 2016. Through this example from Outokumpu, we aim to demonstrate the possibilities of different types of seismic data, including new emerging, cost-effective methodologies, for different stages of mineral exploration. Building on two earlier seismic reflection 2D campaigns in the area, the COGITO-MIN project acquired new types of active and passive seismic data at the Kylylahti sulphide mine site within the Outokumpu ore district. Overall, the new experiments consisted of 2D seismic measurements, sparse active-source and passive 3D seismic surveys, as well a VSP survey partly utilizing novel fibre-optic DAS technology. The experiments were designed with different stages of the exploration workflow in mind; from mapping of the ore host rocks at larger scale to high-resolution resource delineation.
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VSP Using Distributed Acoustic Sensing with Scatter-Enhanced Fibre-Optic Cable at the New Afton Porphyry Deposit, Canada
Authors G. Bellefleur, E. Schetselaar, D. Wade and D. WhiteSummaryWe present vertical seismic profiling results acquired with distributed acoustic sensing (DAS) in boreholes that intersect the main mineralized zone and alteration halo at the Cu-Au New Afton porphyry deposit, Canada. We used an advanced DAS system achieving signal-to-noise ratio of conventional geophones but offering high-density sensing of the entire fibre-optic cable. Straight and helically-wound fibre-optic cables specifically engineered for that system were installed in two boreholes and compared to conventional fibre-optic cables with similar configuration connected to a standard interrogator. Comparison of raw field and processed data demonstrates significantly higher signal-to-noise ratio for the new DAS system. Upgoing waves in one of the boreholes reveals a strong reflection near the bottom of the fibre-optic cable and another slightly below it. These reflections are associated with biotite-dominant potassic alteration zone and picrite, and are excellent proxies for the top and bottom of the hypogene mineralized zone.
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Potential of Legacy 2D Seismic Data for Deep-Targeting and Structural Imaging at the Neves-Corvo Mining Site, Portugal
Authors J.A. Donoso, A. Malehmir, N. Pacheco, V. Araujo, M. Penney, J. Carvalho, S. Beach and B. SpicerSummaryVolcanogenic-hosted massive sulphide (VMS) deposits show a strong seismic contrast (mainly due to their density) against almost all lithological host rocks therefore justifying their direct targeting using seismic methods ( Salisbury et al., 2000 ; Malehmir et al., 2012 and references therein; Malehmir et al., 2013 ) when there is adequate signal-to-noise (S/N) ratio and suitable geometry. While there are earlier published accounts illustrating the use of seismic methods for direct targeting of deep-seated VMS deposits elsewhere ( Matthew, 2002 ; Malehmir and Bellefleur, 2009 ), a number of attempts were done in Europe during the early 90s for this purpose and these have been overlooked for unknown reasons.
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High-Resolution Near-Surface Velocity Model for Depth Imaging of Mineral Deposits in the Ludvika Mining Area, Sweden
Authors L. Schreiter, S. Buske, A. Malehmir, E. Bäckström, M. Schön and P. MarsdenSummaryWithin the H2020-funded Smart Exploration project existing legacy seismic data acquired in the Ludvika Mines are analysed in order to delineate the deposits in depth. Here we present a velocity model derived using first-break traveltime tomography, which represent the near-surface materials at high resolution and can be directly used for refraction static calculations or incorporation and for depth imaging algorithm. Our results are consistent with derived velocities from downhole logging data and show a strong vertical velocity gradient in the upper first hundred meters. In mineral exploration clear images of the subsurface and an improved characterization of mineral deposits are required to reduce the risk before drilling. Especially in prestack depth imaging workflows, which are successfully applied to hardrock seismic data, a reliable velocity model is required that represents the lateral and vertical variations in lithology and assures the robustness of the velocity model within the application of depth migration routines at the same time. A special focus of this work lies on the derivation of a detailed near-surface velocity model, which accounts for strong scattering effects due to lateral inhomogeneities as well as for topographic effects on the reflections.
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Depth to Precambrian Basement Terranes Underneath the Caledonian Nappes - A Case Study from Northern Norway
Authors Y. Nasuti, D. Roberts and A. NasutiSummaryThe bedrock geology of northern Norway is dominated by rock complexes of Precambrian to Early Palaeozoic age, large parts of which have been involved to varying extent in the Caledonian orogeny ( Figure 1 ). In general, there is a basic two-fold division into the Caledonides sensu stricto and a mid-crustal continental lithospheric basement comprising autochthonous crystalline complexes that range in age from Neoarchaean to Late Palaeoproterozoic and form the northern margin of the Fennoscandian Shield (Gaál et al., 1989; Daly et al., 2006 ). These older Precambrian rocks occur mainly in eastern Finnmark and western Troms, and some are either affected locally by Caledonian deformation or incorporated as thrust slices in the Caledonian nappes.
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A Comparison of SED, Special Functions and Euler Deconvolution to Identify Magnetic Lineaments in Khuzestan Plain
Authors Z. Mahmoodpour, F. Molaei and H. SiavoshiSummaryGeological structures are not often cylindrical and can result from complex deformation histories. Lineament is one of the complex structures on the land surface. It is a linear feature in a landscape that is an expression of an underlying geological structure such as a fault. Typically, a lineament is comprised of a fault-aligned valley, a series of fault or fold-aligned hills, a straight coastline or a combination of these features. Fracture zones, shear zones and igneous intrusions such as dykes can also give rise to lineaments. Lineaments are often apparent in geological or topographic maps and can appear on aerial or satellite photographs or aeromagnetic maps.
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Results from the Main DESMEX Experiment Southeast of Greiz
Authors U. Meyer, H. Petersen, S. Costabel, A. Steuer, M. Cherevatova, M. Becken, C. Nittinger, T. Günther, P. Yogeshwar, W. Mörbe and B. TezkanSummaryThe DESMEX Project aims to develop semi-airborne systems for mineral exploration down to depths of about 1 km. The main system components have been tested and improved over last two years. After several preliminary test surveys, the main experiment was conducted in October 2017. Different layouts for the ground based survey set up were tested and evaluated. Large parts of the acquired data have been processed and first results are ready for display.
Parallel to these works, petrographic studies have been performed in order to be able to complement the geophysical and geological data needed for a combined 3D-model of the main DESMEX survey region.
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New Approach to Magnetovariational (MVP) and Magnetotelluric (MT) Methods Application for Mining Exploration
Authors I. Ingerov, E. Ermolin, A. Lozoviy and I. MendriiSummaryAustralian geoscientists have made a significant breakthrough in mining exploration by establishing that anomalies of high electrical conductivity in the lower part of the Earth’s crust and upper mantle are commonly observed in the regions with large polymetalic and gold deposits. From these deep zones conductive pathways (fingers) rich with thermal fluids extend all the way to the Earth’s surface (ore deposits). To further develop this idea, Australia is currently conducting broadband MT surveys on a regional scale (1: 5 000 000) of its entire continent (AusLamp project) with the objective to discover new mining provinces. The analysis of MT and MVP data obtained in the territory of other countries which have been actively conducting deep investigations fully support findings of Australian geoscientists. Based on the above, as well as accumulated extensive MT/AMT and MVP surveys experience, it will be expedient to further subdivide exploration in wide depth intervals for new large mineral deposits into 4 stages.
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1D AEMIP Inversion Using MPA Reparameterization for HTEM Survey at Lamego Gold Mine, Quadrilátero Ferrífero, MG, Brazil
Authors M.A. Couto, G. Fiandaca, E. Auken, A.V. Christiansen and J.L. PorsaniSummaryThis paper presents the result of the Airborne Electromagnetic Induced Polarization (AEMIP) 1D inversion using the Maximum Phase Angle (MPA) Cole-Cole model reparameterization for a Helicopter Time Domain (HTEM) survey in Quadrilátero Ferrífero area, MG, Brazil. The inversion was conducted for a set of four soundings points over the mineralized structure of Lamego Gold Mine. The results demonstrate good agreement with the drill-hole lithological description data and indicates two polarizable layers: one thin layer associated with the Aumineralized banded iron formation (BIF) and the other one associated with a thick graphite schist bellow the Aumineralization. Additionally, the AEMIP inversion results might suggest improvements in the underground resistivity model, which could not be achieved if the non-complex resistivity model is considered. Keywords: AEMIP, MPA Reparameterization, HTEM, Quadrilátero Ferrífero, Brazil
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Reduction to Pole of Non-Equidistantly Measured Magnetic Data Using an Inversion-Based Fourier Transformation Algorithm
Authors D.O.B. Nuamah, A. Kiss and M. DobrokaSummaryA new, robust and resistant, inversion based 2D Fourier transformation is presented where the spectrum is discretized by series expansion (S-IRLS-FT) using Hermite-functions as basis functions. The series expansion coefficients are given by the solution of a linear inverse problem. Taking advantage of the beneficial properties of Hermite-functions, that they are the eigenfunctions of the inverse Fourier transformation, the elements of the Jacobian matrix can be calculated fast and easily, without integration. The procedure can be robustified using Iteratively Reweighted Least Squares (IRLS) method with Steiner weights. This results in a very efficient robust and resistant inversion procedure. Its applicability is demonstrated in the reduction to the pole of magnetic data set measured in regular (equidistant sampling) and “random walk” measurement arrays.
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