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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
1 - 50 of 68 results
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Developed Correlations Between Sound Wave Velocity and Porosity, Permeability and Mechanical Properties of Sandstone Core Samples
More LessSummarypetrophysical properties such as porosity and permeability of the reservoir rocks and rock mechanical properties are affected by the relevant physical properties and such physical properties and mechanical properties affect the drilling programs and the development plans for a reservoir. It is more convenient to use homogenous rock samples with nearly constant initial permeability, obtaining such cores is very difficult. In this paper a simulated natural and homogeneous compacted sandstone rock with known physical and petrophysical properties were used. The physical properties include grain size, cementing material concentration, and compaction (confining) pressure. The effect of these properties on the petrophysical properties of Rock such as permeability and porosity were also known. For the same simulated natural sandstone rocks, Sound wave velocity was measured using an ultra sound tool. Good relationships have been developed between sound wave velocity and other rock properties; porosity, permeability, mechanical parameters namely Young, shear modulus and poison’s ratio. This study has been carried out on dry core samples and core samples with different saturations. The results of this study were presented as graphs of simulated lithification factors, porosity, permeability mechanical parameters versus sound wave (compressional and shear) velocity under different degree of water saturation.
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Geophysics, Petrophysics and Field Data Integration for Porphyry Delineation in the Canadian Cordillera – Some Caveats
Authors I. Lenauer, H. Ugalde, B. Milkereit and W. MorrisSummaryThis contribution presents an integrated interpretation of airborne geophysical and remote sensing data over the Central Cordillera in British Columbia, Canada. Data comes from the Search II survey, commissioned by the British Columbia Geological Survey in 2016. The study area is part of the Mesozoic volcanic arcs preserved as the Quesnel and Stikane terranes in Western Canada. Sedimentary and volcanic cover as well rugged terrain and difficult access make field mapping in this area quite challenging. The first part of the work consisted on interpreting the geophysical data to determine structures that could have a role on porphyry emplacement, and the extension of these under sedimentary cover, as well as zones of potassic alteration. Subsequent field mapping was focused on obtaining structural information as well as petrophysical and alteration data over exposed faults and intrusives. Field observations did not locate faults with similar strike to the ones interpreted from the airborne data but did find numerous secondary structures with different strikes that are however consistent with the main stress regime for the area. Magnetic susceptibility traverses indicate that not all alteration features create a decrease in magnetic susceptibility. These are important findings for field structural follow-up of airborne data interpretation.
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Keynote: Helicopter Airborne Gravity Gradiometry
More LessSummaryRotary-wing airborne gravity gradiometry provides both better signal-to-noise and better spatial resolution than is possible with a fixed-wing survey in the same terrain by virtue of the low altitude, low speed and close terrain following capabilities of a helicopter. In rugged terrain, these flight characteristics provide a significant advantage for the use of helicopters for gravity data acquisition. Comparing helicopter surveys over gentle terrain at Margaret Lake, Canada, and over rugged terrain at Mount Aso, Japan demonstrates that although there is some loss of spatial resolution in the more rugged terrain, the line spacing remains the limitation to resolution in these examples. The slightly higher altitudes forced by rugged terrain make the requirements for terrain correction easier than for gentle terrain. Transforming the curvature gradients measured by the Falcon gravity gradiometer into gravity and the complete set of tensor components without loss of resolution is done by a Fourier method over gentle terrain and an equivalent source method over rugged terrain. The Fourier method is perfectly stable and uses iterative padding to improve the accuracy of the longer wavelengths. The equivalent source method relies on a smooth inversion and the source distribution must be customized to suit the survey design.
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FDEM & Magnetic Data Integration for Kimberlite Exploration – Remanent magnetization and apparent susceptibility mapping
Authors H. Ugalde, W. Morris and B. MilkereitSummaryWe present a case of FDEM and Magnetic data integration for kimberlite exploration in the Northwest Territories, Canada. Data comes from six high resolution helicopter-borne surveys commissioned by the Northwest Territories Geological Survey (NTGS). Analysis starts with the calculation of an apparent magnetic susceptibility distribution from the FDEM data. This is then compared to magnetic susceptibility derived from the total field magnetic data. The differences allow to recognize areas of prominent remanent magnetization. Helbig analysis on the magnetic data over these areas gives an estimate of NRM intensity and direction. Integration of these results with the Apparent Polar Wander Path for the area of study, gives us a full understanding of the different components of the NRM (thermal and viscous).
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Borehole Magnetics - Mineral Deposit Mapping and Age Estimation of the Stratmat Deposit, Bathurst, New Brunswick
Authors W. Morris, H. Ugalde and S.-.L. UnderhaySummaryBorehole vector magnetics. Stratmat deposit, Bathurst Mining Camp, improved borehole navigation, ore deposit modeling, age estimation from magnetic remanence interpretation
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Using Magnetic Data for VMS Exploration in Vihanti, Western Finland
Authors H. Leväniemi, S. Heinonen and J. LuukasSummaryThe magnetic properties of two mineralisation in the Vihanti region, western Finland, are studied. The study area is part of the 1.92–1.93 Ga Vihanti-Pyhäsalmi Belt that contains the most important volcanogenic massive sulphide deposits in Finland. The two mineralisations presented in this study, Lampinsaari and Kuuhkamo, show on aeromagnetic maps as high-amplitude anomalies that are not fully explained by the observed drill core susceptibilities. Constrained magnetic modelling and inversion are used for determining the intensity and direction of the remanent component. The results indicate that the strong magnetic anomalies are dominantly caused by remanent magnetisation that is mainly, but not completely, aligned with the current Earth’s field direction. The information can be applied in generating more reliable magnetic models for the region to enhance data integration for mineral exploration.
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ZTEM in Deep Brownfield Exploration in Outokumpu
More LessSummaryGeological Survey of Finland (GTK) ordered a ZTEM (Z-Axis Tipper Electromagnetic) survey from Geotech Ltd. as a part of its Developing Mining Camp Exploration Concepts and Technologies – Brownfield Exploration Project. The project took place in Outokumpu Mining District in North Karelia in Eastern Finland and was conducted by the Geological Survey of Finland and the University of Helsinki together. The Outokumpu Cu-Co-Zn-Ni-Ag-Au ore belt is historical mining area where there has been exploration for over hundred years. The goal of the project was to develop new deep exploration concepts when working in previously explored and exploited mining area (brownfield) in crystalline bedrock areas. Among other geophysical surveys, ZTEM survey aimed at gathering information from the depth of several kilometers. The contractor also provided 2D and 3D inversions which combined with other data sets. In this abstract, the ZTEM data is compared to GTK’s own airborne electromagnetic data.
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Targeting Supergene Manganese in the Capricorn Orogen Using Airborne EM
Authors C. Soerensen, S. Spinks, R. Thorne and T. MundaySummaryWhile airborne electromagnetics (AEM) is routinely employed as an exploration tool for Mn mineral systems in Australian, the challenges of using the derived data to better delimit the extent of supergene Mn resources in regolith settings has received limited attention. This is particularly so for geological settings where the measured AEM response is strongly affected by IP effects. Here the question as to whether inverting for conductivity and IP can improve the definition of supergene Mn mineralisation in the subsurface, thereby helping determine the resources present is examined. The focus of this study was the Butcherbird/Yanneri Ridge deposits which occur in the Collier group in the eastern part of the Bangemall Basin in Western Australia. These deposits are highly conductive, and chargeable. A full non-linear 1D inversion of the helicopter (XTEM) time domain airborne EM data set effectively outlines the lateral and vertical extent of supergene Mn mineralisation defined in drilling, particularly when inverting for both conductivity and IP. This is not the case when just inverting for conductivity or when employing transforms on the data.
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Raising the Bar by Lowering AEM Base Frequency to 7.5 Hz with Helitem35C
Authors G. Konieczny, A. Smiarowski, G. Paleolog and P. MilesSummaryLowering the base frequency of AEM systems is important for increasing depth of exploration in conductive terrain, but is difficult because receiver coil motion noise dramatically increases noise levels. By re-designing the receiver coil and coil suspension, the receiver coil motion noise has been largely eliminated in the Helitem35C system. In this paper we compare survey data acquired at 30 Hz with 15 Hz and 7.5 Hz data. In order to generate appreciable ground signal at 7.5 Hz, a 50% duty cycle square pulse has been used. The survey data had signal well above noise levels to the 30 ms channel centre time possible with the 7.5 Hz, providing greater depth of exploration and better interpretation of conductors.
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Application of Airborne Magnetic Data to Gold Mines in Namuno, Mozambique
Authors C. Macuacua, T. Mogi, K. Ishitsuka and M. UtsugiSummaryThe availability of high-resolution aero-geophysical data was a monumental step toward the discovery of new gold and mineral deposits in very remote areas in Mozambique. However, many mineral occurrences and deposits were identified previously by either local prospectors or small-scale miners with limited structural and geological knowledge. Therefore, the exploitation of resources has proceeded chaotically, which is not beneficial for local communities or state revenues. To contribute to improving this circumstance, we studied the Namuno District, where there is high rate of small-scale gold mining but limited knowledge of the geological structures that host the gold mineralisation.
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Identification of Kimberlite Bodies in Brazil from a 3D Audio-Magnetotelluric Survey
Authors E. de Oliveira, P. Pastana De Lugão and B.F. KriegshäuserSummaryWe report on a successful identification of kimberlite bodies in Brazil through the use of the electromagnetic technique audio-magnetotelluric (AMT). Macnae (1979) writes that “In one large survey in South Africa, electromagnetic (EM) techniques have proven to be remarkably effective in detecting the presence of weathered clays or epiclastic kimberlite contained within the pipes.”
Full tensor AMT data were acquired at 65 stations in a 3D configuration with frequencies ranging from 10 kHz to 1Hz in the NW portion of the Mato Grosso state, Brazil, an area of thick jungle coverage. During the AMT survey, few outcrops were seen, because of the dense forest cover. Usually, the occurrences found were of sand deposits, indicating the occurrence of Fazenda Casa Branca and Utiariti Formations and gravel from Salto das Nuvens Formation, widely used in paving trails in this region. In the area of the survey, three main targets were identified and confirmed by the 3D resistivity model in depth obtained from inversion of the AMT data, helping the exploration in a region of difficult logistics due to the dense jungle cover and remote location.
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Combined Interpretation of ERT, SP and VLF-EM Data for the Identification of Manganese Deposits
Authors N. Kordatos, G. Vargemezis, P. Tsourlos, G.N. Tsokas and P. TsitsanisSummaryThe signature of a mineral deposit can vary to different geophysical methods, depending on its mineral concentration, size and depth. For this reason, a single geophysical method in several cases could be insufficient, if the expected geophysical response is weak or the interpretation is not unique.
Combined interpretation of different geophysical methods can contribute to this problem in two ways.
In the first place, the existence of a mineral deposit is verified by the fact that different geophysical methods point to a certain location. In addition, geophysical signatures can be related to the characteristics of the mineral deposit and contribute to its qualitative evaluation.
In this work we apply the Very Low Frequency electromagnetic (VLF-EM), Electrical Resistivity Tomography (ERT) with Direct Current (DC) and Induced Polarization (IP) data and Self-Potential (SP) geophysical methods over surficial manganese mineralization located in Piavitsa area (Chalkidiki, Northern Greece). For the final interpretation of geophysical data, information from geological maps and borehole data have also been used. Results over the same profile are presented, where each method has a very good response over the mineralization body. Moreover, the characteristics of the geophysical models are related to the properties of the mineralization.
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Multi-Parameter Visualization of Petrophysical and Geochemical Data for Base Metal Exploration
Authors C. Hebert, E. Veglio, S. Liu, L. Sun and B. MilkereitSummaryA large petrophysical and geochemical dataset has been obtained from archived drill core for a base metal exploration project located north of the Bathurst Mining camp (New Brunswick, Canada). The integration of this large, heterogeneous dataset presents a unique opportunity for statistical analysis of correlating and visualising multi-parameter behaviour in a bi-modal felsic/mafic geological setting. Large variation in parameter distribution enables questioning the linkage between petrophysical and geochemical parameters. To better characterize mineralization and alteration zones, a statistical approach has been developed for interactive visualization of the contribution of each parameter to the exploration model. The integration of geophysical and geochemical datasets produced robust results for imaging wide-spread alteration zones and associated base metal mineralization.
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Gravity and Magnetic Modeling of Polyhedron and Iterative SVD Applied to Regional-Residual Gravity Data Separation
Authors E. Amarante and E. SampaioSummaryWe have developed a method of forward modeling to determine the gravity and magnetic potential of a convex polyhedron and an iterative procedure of singular value decomposition (SVD) to filter and extract residual gravity anomalies in order to obtain useful information to improve geological mapping and the search of sulphide deposits. The forward modeling method is based on the fundamental work of Werner (1994) . It employs Green’s divergence theoremto change potential volume integrals into asumof surface integrals, with each face of the polyhedron divided into triangles. So, each surface integral consists of the contribution of a triangle calculated relative to its own coordinate system – z perpendicular to the face and x parallel to one of its sides – which is subsequently transformed into the global coordinate. The resulting potentials are derived to yield the analytical solutions of the gravity and magnetic anomalies.
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Interpretation of High Resolution Aeromagnetic (HRAM) Data over Chibok and Damboa Chad Basin, North Eastern Nigeria, Usi
Authors G.A. Bala and D.N. ObioraSummaryHere we present the interpretation of high resolution aeromagnetic (HRAM) data of Chibok and Damboa areas (sheets 112 and 134), north-eastern Nigeria, using forward and inverse modelling methods on Oasis montaj 6.4.2 software. Quantitative depths estimates obtained by employing SPI have shown minimum to maximum depth to anomalous source at 72.7m to 5026.8m.Oasis montaj 6.4.2 software. The estimated depths from the forward and inverse modelling methods for profiles 1–7 are 2749, 2201, 2088, 703, 521, 927 and 481 m respectively. The respective susceptibility values of 0.20407, 0.47779, 7.8043, 0.1349, 0.0889, 0.3083 and 0.4820 SI, which indicate the presence of sedimentary intrusions (basalt or limestone), few metamorphic rocks (schist) and minerals (magnetite).
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Locating Potential Field Sources Using Wavelet Analysis
More LessSummaryCooper (2006) introduced a particularly simple wavelet based semiautomatic interpretation method. When applied to magnetic data the wavelets were based on horizontal derivatives of the first order analytic signal amplitude of the response from different sources, such as dykes and contacts. The wavelets were not applied to the data itself, but to the horizontal derivatives of its first order analytic signal amplitude. While the method worked well, the use of second order derivatives made it sensitive to noise. This abstract introduces several improvements to the original methodology, with the aim of increasing its robustness to noise, namely
- Use wavelets based on derivatives of the zero order analytic signal amplitude
- Use wavelets based on the Hilbert transform of the analytic signal amplitude of different orders
- Application of a sharpening technique to improve the response of the transform
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Deep Targeting Iron-Oxide Mineralization Using Reflection Seismic Method: A Case Study from the Ludvika Mines Of Sweden
Authors M. Markovic, G. Maries, A. Malehmir, E. Bäckström, M. Schön, J. Jakobsson and P. MarsdenSummaryBlötberget, in the Ludvika Mines of Bergslagen mineral district of central Sweden, is well-known for its iron-oxide, sometimes apatite-rich, deposits. There is also a renewed interest in exploring and mining the deposits due to accessibility to the market and recent advancements made in the mining and metallurgical technologies. During two field campaigns (2015 and 2016), high-resolution reflection seismic data were acquired using both cabled-and wireless-recorders as well as a landstreamer system. In this study, we have merged the two datasets and process them together to provide deeper information on the extension of the mineralization and potential unknown resources at depth. We show how the merged dataset images the mineralization much better and deeper than known also potential reflections under the known ones that can be targeted through a drilling program. This study demonstrates reflection seismic method is a powerful tool for imaging iron-oxide mineralization at depth. We argue that they should be acquired more routinely at the site for mineral exploration purposes. It also paves the way for justifying a 3D seismic survey in the area.
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Seismic Interferometry for Mineral Exploration: Passive Seismic Experiment over Kylylahti Mine Area, Finland
Authors M. Chamarczuk, M. Malinowski, D. Draganov, E. Koivisto, S. Heinonen and S. JuurelaSummaryWe present the results of adapting seismic interferometry (SI) for reflection imaging in mineral exploration. We use a unique dataset of one-month ambient-noise recordings acquired with large-N array (~1000 receivers) deployed in a regular grid (50 m receiver interval, 200 m line interval) directly above the known mineralisation and underground mine infrastructure at the Kylylahti polymetallic mine located in Eastern Finland. Ambient-noise in the study area is dominated by the road traffic and mine activities (both surface and underground) providing quasi omni-directional distribution and broad freqeuncy spectrum of the noise sources. We start from the simple 2D forward modelling using existing geological model. Results of the SI applied to field data from selected receiver lines exhibit reflections related to the bottom of the ore body as well as the reflection from the target area confirmed by synthetics. Finally, we develop robust mineral exploration SI workflow (MESI) tailored for reflection imaging and apply it to our 3D ambient-noise dataset. Migrated sections obtained from the MESI-processed data exhibit high reflectivity, compatible with the active-source seismics and directly related to the known geological structures.
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Application of Surface-Wave Analysis for Mineral Exploration: A Case Study from Central Sweden
Authors M. Papadopoulou, F. Da Col, L.V. Socco, E. Bäckström, M. Schön, P. Marsden and A. MalehmirSummaryNear-surface velocity models are important for deep imaging of mineral deposits with seismic exploration. The near-surface can be quite complex from loose, highly heterogeneous materials to stiff, fractured rocks. Surface-wave analysis can be an effective method to image the shallow subsurface of such challenging environments. Here, we propose a workflow that includes several processing and inversion steps. Initially, for the optimization of the processing parameters, we assess the presence of sharp lateral variations with a method based on the measured energy of Rayleigh waves. Then, using a moving window of receivers, we extract Rayleigh-wave dispersion curves along the acquisition line as the maxima of the f-k spectrum. Finally, the dispersion curves are inverted using a laterally constrained inversion scheme. The proposed methodology has been tested on legacy data from a mining field.
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Improved Subsurface Imaging through Re-processing of Legacy 2D Seismic Data - A Case Study from a Deep South African Gold Mine
Authors M. Manzi, A. Malehmir and R.J. DurrheimSummaryOver the last few years, there has been a proliferation of seismic solutions, which employ specific combinations of equipment, acquisition, and processing techniques that can be applied in hard rock situations to improve the imaging resolution. The latest developments in seismic processing, in particular, make it worthwhile to re-process the legacy data to enhance the resolution of the data. This is particularly important in the mining regions where no new data are available or the acquisition of new data is expensive or not allowed due to new environmental regulations. In this work we demonstrate, through a case study from one of the world’s deepest gold mines in South Africa, how revisiting, recovering and re-processing of the seismic data acquired decades ago can significantly improve the quality of the interpretations. The information can benefit future mine planning operations by providing a better estimation of the resources and inform in the siting of the sinking of future shafts. Thus, any future mineral exploration plans could take the information obtained from the re-processed legacy seismic data into account when planning either 2D or 3D seismic surveys.
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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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The Gramian Constraint for Incorporating A Priori Geoelectrical Model into Seismic Full-Waveform Inversion
Authors M. Malovichko, N. Yavich, N. Khokhlov and M. ZhdanovSummaryThe methods of integrated interpretation of multiphysics geophysical data have been advanced considerably in recent years. One widely used approach is based on imposing so-called structural constraints. The concept itself and its numerical implementation, known as the cross-gradient inversion, have been introduced in seminal paper by Gallardo and Meju (2003a) . For a recent review we refer to Moorkamp et al. (2016). The method and its numerous variants possess some limitations, such as non-uniqueness, non-linearity of the resulting model functional and difficulties to extend this approach to more than two physical properties. For these reasons, we develop another approach to the joint inversion based on the Gramian constraints (Zhdanov et al, 2012). This approach uses a very general mathematical formulation, which makes it possible to impose different types of constraints on the jointly inverted model parameters, including the structural constraint as a special case. One important advantage of this approach over the cross-gradient method is that, it results in a quadratic model functional that can be readily generalized on any number of physical domains.
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Data Density and Resolution Power in 3D DC Resistivity Surveys
Authors M. Gharibi and R. SharpeSummaryResolution power of a field 3D DC dataset is examined by gradually reducing the data density by removing selected transmitters from the dataset. A mineralized target zone with known geometry and resistivity is used to compare the inversion results for each subset. The results indicate that a high density measurements are essential in retrieving accurate geometry of the target. True 3D grid distribution provides an omni-directional dataset in which the subsurface is sampled by measurements having many different orientations.
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A High-Resolution Ground Magnetic Survey at Rizo’s Chromite Mine, Mt. Vourinos, N.Greece
Authors C. Christoforou, A. Stampolidis, G.N. Tsokas, P. Tsourlos, V. Lambos and A. MpatsiSummaryThe exploitation of chromite ore deposits in Greece is evident by the presence of many abandoned mines. The fresh interest by AKTOR S.A. in chromite ore deposits in Mt. Vourinos, N.Greece, is directly related to the boost in Cr prices controlled by international market trends.
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Preconditioning the Time-Harmonic Acoustic Wave Equation Based on a Special Contraction Operator Transformation
Authors N. Yavich, M. Malovichko, N. Khokhlov and M. ZhdanovSummaryContemporary interest in full waveform inversion (FWI) drives development of 3D forward modeling algorithms. Performing FWI in the frequency domain effectively reduces an inverse problem to the series of monochromatic forward simulations. Still, forward modeling consumes bulk of the FWI run time. In this paper, we focus on efficient solution of the acoustic-wave equation. The most time-consuming step in numerical solution of the acoustic-wave equation is iterative solution of the arising system of linear equations. A slow convergence of iterative solvers is related to the spectral properties of the system matrix [ Ernst and Gander, 2012 ]. Consequently, various preconditioning techniques were applied to this kind of problems.
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Modelling of Fractures Using the Chimera Grid Approach
Authors A. Ruzhanskaya and N. KhokhlovSummaryCharacterization of fractures is important in seismic exploration. In the geophysical context, cracks are usually modelled by the finite-difference method on regular grids. One of the main limitations of this approach is that only fractures parallel to coordinate axes can be modelled. At the same time the use of overset grids (arbitrary rotated rectangular grids that fit each individual model feature) is a very established approach in computational fluid dynamics. This technique is also known as the Chimera grid approach. We applied a similar technique for simulation of seismic cracks. Rectangular overset grids are used in combination with the grids-characteristic method. Our numerical experiments demonstrate that this approach accurately models a set of arbitrary oriented cracks in a complicated geological environment. Comparing to the curvilinear or unstructured grids, this approach has lower computational complexity, because processing of rectangular grids is fast.
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Co-Operative Inversion of Magnetotelluric and Seismic Reflection Data from the Nevada Gold District
More LessSummaryVarious geophysical methods are sensitive to different physical properties and have different resolution. Consequently, each geophysical dataset usually provides different subsurface images. Moreover, as inversion is non-unique; the inverted models from different methods can cause further ambiguity or even contradict in geological interpretation. Thus, it is more efficient to narrow the domain of solution by using complementary information by explicitly incorporating multiple data into the inversion algorithms/process. We use a challenging data set from a mineral exploration environment to demonstrate a novel method of integrating prior and complementary information from two geophysical methods into a co-operative inversion scheme. The proposed method exploits the advantages of the fuzzy c-means clustering technique to provide a common geostatistical model for inversion. Inclusion of borehole information provides confidence in choosing the number of clusters and defining centre values, which then improve both the MT and seismic inversion processes. Interpretation of the acoustic inversion resulting from our process allowed identification of three prospective target zones in the region of a deep borehole. These targets were verified as prospective by geochemical analysis with elevated levels of pathfinder elements and gold.
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Geophysical Exploration of Submarine Massive Sulfide Deposits Based on Integration of Multimodal Geophysical Data
Authors M. Endo, L. Cox, D. Sunwall, M. Zhdanov and E. AsakawaSummaryIn this paper, we introduce an innovative integrated approach to geophysical exploration of submarine massive sulfide deposits. The developed approach is based on three-dimensional inversions of multimodal geophysical data guided by other geophysical data and known geological information. The developed approach was applied to the quantitative interpretation of multimodal geophysical data acquired over a known hydrothermal deposit in Izena area, offshore Japan. There are good correlations between multimodal geophysical anomalies and known hydrothermal deposits. This paper demonstrates that integrated interpretation of the multimodal geophysical data can be effectively used for exploration of the seafloor hydrothermal (massive sulfide) deposits.
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Surface and Borehole Magnetotelluric Measurements to Delineate an Ore Deposit in Northern Sweden
Authors T. Kalscheuer, L. Dossow, N. Juhojuntti and L. DynesiusSummaryIn recent synthetic studies, combinations of magnetotelluric (MT) surface and borehole measurements have been demonstrated to reduce the ambiguity in models of electrical resistivity as compared to models computed from surface data. We took advantage of this and recorded combinations of MT surface impedances and axial voltages (i.e. integrals of electric fields along boreholes) at a mineral deposit in northern Sweden. In this contribution, we introduce an axial voltage transfer function in frequency domain, present a preliminary 2D inversion model of the surface MT data and discuss the compatibility of the axial voltage transfer function derived from our borehole field data with the preliminary 2D inversion model. In the next step, we will include the axial voltage transfer functions in the inversion of the field data.
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2D Joint Inversion of Gravity and Magnetic Data of a Potash Ore with the Cross-Gradient Constraint
Authors M. Tavakoli, A. Nejati Kalateh, M. Rezaie and A. RazaviSummaryThe joint inversion of different geophysical data sets can reduce considerably the inherent ambiguity in the inversion of individual data sets. The Cross-gradient method, first developed by Gallardo and Meju (2003) , tries to obtain subsurface images with parallel parameter changes without restricting the parameter values or the magnitude of their variations ( Demirci et al., 2018 ).
In order to do a joint inversion of gravity and magnetic data, we followed the algorithm of Demirci et al. (2018) who added a new regularization parameter into the second term of the parameter correction vector of Gallardo and Meju (2003) . The new regularization parameter controls the contribution of the cross gradient term in the solution.
Available gravity and magnetic data (on a potash source) have been already inverted separately in an integrated approach ( Abedi, 2018 ). In order to acquire two more compatible models a joint inversion method is necessary.
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Comparison of Stainless Steel and Non-Polarizable Electrodes in IP Method - A Metallic Mine Deposit Example
Authors Y. Gundogdu, I. Demirci, H. Aktarakci and E. CandansayarSummaryIn this study, the use of stainless steel and non-polarizable electrodes was compared in the data acquisition of induced polarization (IP) data with multi electrode-channel measurement systems. The measurements were conducted in the metallic mine site by using stainless steel, thin and fat non-polarized electrodes with Cu-CuSO4 solution. The IP values obtained from all measurements were compared with the decay curve and pseudo-sections. Moreover, comparison was made using two-dimensional subsurface models which are obtained from two-dimensional inversion of each data set. The results show that stainless steel electrodes can be used instead of non-polarized electrodes in measuring systems with high measurement accuracy and powerful transmitter.
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