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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
41 - 60 of 68 results
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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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Reflection Seismic with DAS, Why and Where?
Authors M. Urosevic, A. Bona, S. Ziramov, R. Martin, J. Dwyer and A. FoleySummaryDuring the last decade the Distributed Acoustic Sensing (DAS) data acquisition has been tested in boreholes in a soft rock environment with a variable success. The technology is now approaching its maturity and it is showing a high potential. In hard rock environment however its use is much more challenging. In this study we show the first attempt to use this technology for surface reflection seismic rather than it is commonly attempted in boreholes. The principal reason is that in some specific environments such as a hyper-saline lakes, under which significant mineral reserves are found in Australia, the use of conventional equipment is limited and prone to significant hardware issues and equipment damage. In contrast such an environment seems appropriate for the application of the DAS technology. This study reports on the results from a comparative DAS-geophone surface reflection experimental survey conducted along a salt lake in Western Australia and shows that DAS has the potential to replace surface geophones in such environment.
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A Newly Developed 3ch System of SQUITEM III and the Result of its Field Test
Authors M. Motoori, S. Ueda, K. Masuda, E. Arai, T. Hato, A. Tsukamoto, H. Watanabe, H. Ishikawa, C. Okada and T. YamazakiSummaryExploration is much deeper these days. SQUID sensor is a highly sensitive magnetometer, and is currently applied to mineral exploration geophysics, Ground base Time Domain Electromagnetic, and Airborne Gradient magnetic. High quality data with low noise level would improve depth penetration. Time domain EM SQUID system Supracon Jessy-Deep and CSIRO LandTEM have been used worldwide these days. JOGMEC developed the SQUID system for TDEM, named SQUITEM. This paper describes our newly developed 3ch system, MLEM trial results at Mallee Bull mineralization in NSW Australia.
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Spectral Characterization of SMS Deposits from FDEM In-Loop Sounding and Rock Physical Property Analysis
Authors H. Müller, K. Schwalenberg and K. ReeckSummaryA new deep-sea frequency domain electromagnetic (FDEM) sounding system named GOLDEN EYE has been developed and applied to map active and relict hydrothermal vent fields in the German license areas at the Central and Southeastern Indian Ridges. The spectral electromagnetic data reveal electric conductivity distributions in the shallow subsurface (approx. 0–20 m below seafloor) and allow estimation of induced polarization and magnetic susceptibility of outcropping and buried seafloor massive sulfide (SMS) structures. In-situ mapping results are cross-analyzed with petrophysical data measured on rock samples using electrical impedance spectroscopy and rock magnetics. We will discuss persistent questions like: How electromagnetic methods serve to estimate the resource potential of SMS deposits? Which petrophysical properties are most meaningful and do they work as proxies for rock mineralogy? What are the uncertainties and how to unravel conductivity-depth-distribution, induced polarization and magnetic susceptibility in such geological and morphological complex deep-sea, mid-ocean ridge environments?
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Keynote: State of the Art Technologies of SMS Exploration in Japan
By E. AsakawaSummaryIn 2014, the Strategic Innovation Promotion Program (SIP) was launched by the Council for Science, Technology, and Innovation (CSTI). One of SIP issues is “Next-Generation Technology for Ocean Resources Exploration”. The main objective is to establish the advanced and effective ocean resources survey technology. The main target is Seafloor Massive Sulphide (SMS). There are other topics; scientific research to find the SMS potential area, development of the survey platforms and environmental assessment. In this paper, I show the overview of the current status of ocean resources exploration technologies in Japan.
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Microtremor Survey for Exploration Targetting Epithermal Vein Systems at the Hishikari Gold Mine, Kagoshima, Japan
More LessSummaryThis paper discusses applicability of the microtremor survey to the complicated structure in mineral exploration, taking epithermal vein systems as an example through a case study at the Hishikari mine, the largest and richest gold deposit in Japan.
In general, epithermal gold deposit itself is relatively difficult target for geophysical exploration. Almost all geophysical techniques are deployed aiming at detection of some physical phenomena associated to mineralization, e.g. basement highs and hydrothermal alteration.
The experimental microtremor survey in this study showed wide-range applicability to the exploration of epithermal gold deposits, especially the possibility to detect vein systems more directly through the higher modes in the dispersion curves.
Although microtremor survey is still in the research and development phase, and even in the particular case of this study, quantitative analysis of the elastic characteristics of the ore body and its circumference needs further validation in practice as well as in theory, it will play an expected role to detect epithermal vein systems directly in the future mineral exploration.
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Analysis of Complex Apparent Resistivity Data Conjugating Spectral Induced Polarization and Electromagnetic Coupling
Authors T. Sant’Ana and E. SampaioSummaryElectromagnetic coupling (EMC) between current and potential electrodes is a phenomenon which yields a dependence of the modullus and the phase of electrical resistivity with frequency. EMC plays a dual role in geophysical surveys because it is a source of spurious anomalies in spectral induced polarization (SIP) data ( Millett, 1967 ) but it helps to improve the analysis of the variation of electrical parameters in controlled source electromagnetic (CSEM) data ( Wynn and Zonge, 1977 ).
The characterization of EMC as a noise caused the emergence of several techniques of minimization and removal of its effect in SIP measurements both in time and frequency domains ( Çaglar, 2000 ; Fullagar et al., 2000 ; Xiang et al., 2002 ). Furthermore, the removal of its effect may allow the analysis of the role of SIP in mineral discrimination ( Sampaio et al., 1998 ).
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Full-3D Electrical Resistivity Tomography and Induced Polarization for Enhancing Deep Mineral Exploration
Authors C. Truffert, J. Gance, O. Leite and B. TexierSummaryInduced Polarization (IP) and resistivity, expansively used in mineral exploration for decades, provide additional information on the spatial variation in lithology, chemistry, mineralization, moisture condition… Recent innovative development of cable less and autonomous geophysical equipment significantly enhanced deep resistivity and IP surveys in 2D and in 3D. Associated with an ergonomic processing chain, the Full-Waver system allows to record full-3D IP and resistivity block of data even in rough environment.
Simulations will be shown for comparing “pseudo 3D” and “full-3D” acquisition. Authors will also present two full-3D case studies carried out in Canada and Southern Africa dedicated for gold and diamond exploration. Despite a very weak IP contrast between the gold bearing ore body (2 to 3 mV/V) and the hosted rocks for the Canadian survey, results revealed a very clear anomaly down to 100 m depth. In Southern Africa, the survey helps to reveal two almost vertical conductive anomalies down to 300 m depth. Both are located over the magnetic analytic signal positive anomaly which leads to interpret it as two kimberlite pipes.
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Assessment of the Geoelectrical Anomalies of the Disturbances of Phosphate Series Using the Triangular Prism Surface Area Method (Sidi Chennane -Morocco)
Authors A. Ayad and S. BakkaliSummaryMorocco owns more than three-quarters of all phosphate-rock reserves in the world (50 000 billion metric tons). These appreciable reserves of phosphate known since 1908 were deposited in several basins, from northeast to southwest of Morocco and has been exploited since the 1920s by l’Office Chérifien des Phosphates (OCP) ( Northolt et al., 1989 ). In this study we focus on Sidi Chennane area that it is one of the most important phosphate deposits and considered to be of world-class significance. Although the phosphate series of this deposit is present at a grade sufficient, it is frequently affected by many sterile bodies called disturbances or derangements ( Kchikach et al., 2002 ).
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Distributed Acoustic Sensing versus Conventional VSP Imaging of the Kylylahti Polymetallic Deposit
Authors M. Riedel, C. Cosma, N. Enescu, E. Koivisto, K. Komminaho, K. Vaittinen and M. MalinowskiSummaryThis work presents a high-resolution seismic imaging study of a target sulphide mineralization located in the Kylylahti polymetallic mine, Eastern Finland. For this purpose, in-mine VSP measurements were carried out, using two different technologies: a conventional three-component VSP and a fibre-optic Distributed Acoustic Sensing (DAS) system. The two acquisition systems were placed in existing exploration boreholes adjacent to the deposit and successfully recorded reflections from the target and its host rocks. Subsequent application of a target-specific VSP processing and imaging workflow to the conventional and DAS data provided high-resolution reflectivity images from both systems.
To support geological interpretation, a 3D seismic forward modelling study was conducted using a geological model created from exploration drilling data and laboratory petrophysical measurements. Cross-validation of the acquired VSP data with the synthetic seismograms and extensive logging data enabled identification of the target reflection as well as interpretation of several new geological contrasts.
Between the two VSP methods, the conventional system exhibits slightly superior signal-to-noise ratio and bandwidth. However, the DAS system also provides images of satisfactory quality at much less acquisition time and logistical effort. Thus, DAS offers a powerful and particularly cost-effective exploration technology suitable for detailed delineation of mineral resources.
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Aquifer Variation Characteristics of Overlying Strata in Coal Mining Based on Time-Lapse Apparent Resistivity Monitoring
More LessSummaryTime-lapse apparent resistivity monitoring carries out multiple observations for the same stratum at certain time intervals to study the temporal-spatial variations of aquifer characteristics of overlying strata, which can be repeated multiple 2D observation. Time-lapse apparent resistivity monitoring involves aspects such as data acquisition, processing and interpretation and inversion.
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