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13th SAGA Biennial Conference & Exhibition
- Conference date: 06 Oct 2013 - 09 Oct 2013
- Location: Kruger National Park, South Africa
- Published: 06 October 2013
1 - 50 of 108 results
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Interpretation Of Borehole Gravity Data For Vms Deposits In The Context Of Complex Geology
Authors C. Nackers, M. Chouteau, P. Shamsipour, M. Cheman and P. BérubéA forward modeling and inversion code has been developed to study the use of the borehole gravity method for exploration of VMS deposits. Different problems are associated with the gravity method and to analyse them, synthetic data has been modeled and then inverted. The first problem encountered is the data acquisition settings including the number of boreholes, their location and data collection sampling in the boreholes. They highly influence the results of the inversion. A minimum of three boreholes within appropriate distance from the target is required to locate any structure. In the case where the geology is complex, the boreholes have to be well positioned around the deposit to be able to properly locate it. In the scenario where the borehole gravity method is used to calculate the excess mass of a deposit and to define its structure, at least one borehole should intercept the deposit. Also, fixed density constraints are highly recommended to be able to calculate precisely the excess mass of a deposit. The regional-residual anomaly separation is a very important aspect of the data interpretation. Up to now no technique is effective in performing an optimal separation. In this paper we apply four different methods and compare the inverted density model to the initial model. These methods are the graphical method, the vertical gradient, a non-linear filter and a wavelet filter. Then these methods were applied to GraviLog data that was acquired over and in the vicinity of the Virginia Gold’s Coulon deposit (Quebec, Canada). All methods were able to help locate the deposit, but densities are all underestimated.
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Multiphysics Model Of Borehole Environment And Joint Inversion Of Well Logs
Authors I. Eltsov, V. Kuvshinov, L. Nazarova, G. Nesterova, Yu. Stefanov and V. ShelukhinMultiphysics model of borehole environment was proposed. It describes main factors of generation and evolution of near well zone: mechanical impact on reservoir by well, flow of multiphasic fluids, salt transfer, lithologic characteristics of reservoir, mud cake growth and destruction. Software system for joint computer simulation of electromagnetic, hydrodynamic and geomechanical processes in the borehole environment has been constructed. Theoretical logs of VIKIZ (high frequency induction isoparametric wireline logging tool) and BKZ (Russian lateral log) are simulated for calculated electrical resistivity distribution. In consequence of deformation processes during and after drilling porosity and permeability near borehole are decreased, therefore mud filtrate invasion and salt transfer acquire another character. VIKIZ and BKZ logs are strongly depended on geomechanical parameters, such as break point stress or internal friction point and properties of pore fluid. Authors have suggested interpretation technique taking into account deformation processes and there consequences, this make it possible to improve formation evaluation. Proposed integrated interpretation methodology was verified for West Siberia well logs. Input parameters of joint model were determined from data of geophysical study of borehole and core analysis. For this purpose technique of evaluation of parameters, which are responsible for filtration-volumetric and strain characteristics of rocks were devised. Joint multidisciplinary mathematical and computer model of borehole environment make the opportunity to perfect interpretation of well data and reservoir evaluation.
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Data Driven, Objective Mapping By Clustering Satellite Imagery And Airborne Geophysical Data Suites
Authors D. Eberle, D. Hutchins and I. KahimiseConventional geological mapping using aerial photo interpretation coupled with field visits is often handicapped in areas with tropical weathering or sand cover in arid regions. High resolution airborne geophysical data acquisition over large areas covered by overburden can augment geological mapping of these areas provided information extraction is done in an adequate, automated and objective way. We suggest that the integration of airborne geophysical data with space-borne remote sensing data suites using fuzzy partitioning clustering meets these criteria. The outcome of unsupervised clustering is a classified zonal map which, in combination with some field inspections, produces a high resolution lithology map, in this case 100 m by 100 m pixel size, which is by power of spatial resolution far superior to any conventional mapping and can therefore provide new incentive for geological understanding of the area, modelling or mineral exploration. Our new approach to map hard rock geology hidden beneath overburden has been applied to a portion of largely sand and sediment covered southern Namibia by integrating airborne magnetic, radiometric and Landsat 7 data suites. The classified zonal map obtained from these data was converted to a lithology map by attributing what is known from previous geological investigations of this area. Apart from the structural control of known mineral showings, the resultant map clearly displays an area requiring further grass-root exploration.
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Regional Cartography Of Regolith Parameters Using Airborne Gamma Ray Spectrometry
Authors G. Martelet, S. Drufin, B. Tourliere, N.P.A. Saby, J. Perrin, J. Deparis, F. Prognon, C. Jolivet, C. Ratié and D. ArrouaysAmong the remote sensing techniques, the airborne gamma-ray spectrometry has been widely developed for several tens of years (e.g. IAEA, 1991), being nowadays almost systematically surveyed in regional mapping programs, for soil/regolith (e.g. Lacoste et al., 2011), solid geology reconnaissance (e.g. Metelka et al., 2011) or mineral potential assessment (e.g. Shives et al., 2000). Gamma-ray surveys provide maps of concentration in K, U and Th chemical elements at the ground level. These data are primarily sensitive to the mineralogy and geochemistry of the parent material, as well as its weathering products, including residual and transported clays, sand and gravel (e.g.Minty, 1997). In this study, instead of addressing complex concepts such as lithology, we chose to investigate these gamma spectrometric data by mapping parameters of the regolith, which are simple objective quantities. It is known that these parameters of the regolith: density, moisture, chemistry, all influence the natural gamma ray emissions (e.g. IAEA, 2003). Several papers assess correlations between gamma-spectrometric variables and one or few soil parameters including texture (e.g. Taylor et al., 2002), regolith chemical or mineral contents (e.g. ViscarraRossel et al., 2007), and other soil parameters such as soil depth, total phosphorus or total carbon (e.g. McKenzie and Ryan, 1999).
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The Bateke Plateau Structure- A New Possible 7 Km Diameter Quaternary Meteorite Impact Structure In Gabon: A Remote Sensing Study
Authors S. Master, G.R.J. Cooper and K. KlajnikA 7 km diameter circular structure, called the Bateke Plateau Structure, has been identified on Landsat imagery of eastern Gabon, in a region dominated by Palaeogene to Neogene continental sedimentary rocks of the Bateke Plateau, which unconformably overlie cratonic Archaean basement. Processing of DEMs derived from SRTM radar imagery of the structure, using various radial shading algorithms, has demonstrated that the structure consists of two nested toroidal rings, with a ring-shaped depression between them. The outer toroid has a peak-to-peak diameter of 5.8 km, and a thickness of 1.3 km, yielding a maximum diameter of 7.1 km. It is separated by a deep moat from the inner toroid, which has a diameter of 1.4 km, and a thickness of 700 m. Such a structure is consistent with a complex impact structure containing a central peak-ring uplift, formed by meteorite or cometary impact. Other possibilities for forming such a ring structure are considered highly unlikely, given the setting of the structure in a thin continental sandstone sequence overlying an Archaean craton, and the absence of young igneous intrusions, diapiric structures, karst, or deformation features. The Bateke Plateau Structure is postulated as a post-Neogene (Quaternary) complex impact structure with a peak-ring central uplift, which, because of its degraded, eroded state, is likely to be Pleistocene rather than Holocene in age.
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3d Inversion Of Airborne Vertical Gradient Gravity Data
Authors E.B. Tchikaya, M. Chouteau, P. Keating and P. ShamsipourWe present an inversion method based on a geostatistical approach, i.e. cokriging and conditional simulation for three dimensional inversion of airborne gradient gravity data including geological constraints. Cokriging is a method of estimation that minimizes the error variance by applying cross-correlation between several variables. In this study the estimates are derived using gradient gravity data as secondary variable and the density as the primary variable. In the proposed method, the linearity between gradient gravity and density allows us to obtain a covariance matrix of densities using observed data, i.e, we adjust the density covariance matrix by fitting experimental and theoretical gradient gravity covariance matrices. To obtain various reasonable solutions in order to see the variability that can be expected from the density covariance model adopted, a geostatistical simulation algorithm is applied. The proposed method was first tested on synthetic data. The result shows the ability of the method to integrate complex a priori information. The technique was then applied to actual gravity gradient data collected by the Geological Survey of Canada in the area of Strange-Lake (Quebec) using the Falcon gravity system. Results of inversion (cokriging and co-simulation) are in good agreement with the geology of the studied regions.
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Getting The Best Value From Gravity Gradiometry
Authors D. Fitzgerald and R. PatersonThe critically important steps to get best value from your gravity gradiometry data, assuming your contractor has done his job well in designing and acquiring the data, is the preparation of the representation of the potential field gradients. The ~200m resolving power of existing gradiometer systems approaches what is necessary for minerals applications. In particular, beyond the aircraft, the topographic surface represents the largest and most proximal density contrast encountered in an airborne survey. Hence terrain effects can have significant impact on AGG data. The critical steps are: Terrain correction and determining ‘best’ terrain density Gridding, using all the measured gradients to constrain the interpolation Smoothing/de-noising by using the 3rd order tensor constraints Anti-alias filtering of the gradient signals so that wave lengths are properly represented in all directions Transformation of the gradients by integration to estimate the gravity or magnetic field Terrain corrections are a necessary step in the processing of observed AGG data in rugged terrain, in order to highlight subsurface density variations with a minimal overprint from the terrain. We propose a simple and rapid AGG tensor-based method to estimate an optimum bulk terrain density for subsequent terrain-correction. Each of the currently deployed systems for acquiring gradiometry is evolving driven by competition and the users’ needs. Mining applications of the technology to directly detect ore-bodies that show up as anomalies can now be successful provided the dimensions are of the order of 200m or more. High resolution 3D geology models of operating mines can be used to calibrate gradiometry surveys.
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Vs Structure Of The Crust Containing The Bushveld Complex
Authors E.M. Kgaswane, A.A. Nyblade, R.J. Durrheim, J. Julià, P.H.G.M. Dirks and S.J. WebbThe crustal structure of the Bushveld Complex is investigated by jointly inverting high-frequency teleseismic receiver functions and 2–60 sec Rayleigh wave group velocities for 16 broadband seismic stations spanning the Bushveld Complex. Rayleigh wave group velocities for 2–15 sec periods were obtained from a surface wave tomography using local and regional events, while group velocities for 20–60 sec periods were taken from a published model. The 1-D Vs models obtained for each station show the presence of a thickened crust in the centre of the Bushveld Complex, and that Vs4.0 km/s over a significant portion of the lower crust (30 km depth). The 1-D Vs models also reveal that the upper crustal structure (10 km depth) across the Bushveld Complex is characterized by Vs as high as ~3.7–3.8 km/s, consistent with the presence of mafic lithologies. These results support a “continuous-sheet” as opposed to a “dipping-sheet” model for the Bushveld Complex. However, detailed modelling of receiver functions at one station within the centre of the complex suggests that the mafic layering is inhomogeneous and could have been locally disrupted by diapirism and metamorphism.
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Tectonic Evolution Of The Limpopo Belt: Constraints From Mangetotelluric Data
More LessDespite many years of work, a convincing evolutionary model for the Limpopo belt and its geometrical relation to the surrounding cratons is still elusive. This is partly due to the complex nature of the crust and upper mantle structure, the significance of anatectic events and multiple high-grade metamorphic overprints. We use deep probing magnetotelluric data acquired along three profiles crossing the Kaapvaal craton and the Limpopo belt to investigate the crust and upper mantle lithospheric structure between these two tectonic blocks. The 20-30 km wide composite Sunnyside-Palala-Tshipise-Shear zone is imaged in depth for the first time as a sub-vertical conductive structure that marks a fundamental tectonic divide interpreted here to represent a collisional suture between the Kaapvaal and Zimbabwe Cratons. The upper crust in the Kaapvaal Craton and the South Marginal Zone comprises resistive granitoids and granite-greenstone lithologies. Integrating the magnetotelluric, seismic and metamorphic data, we propose a new tectonic model that involves the collision of the Kaapvaal and Zimbabwe cratons circa 2.6 Ga, resulting in high-grade granulite Limpopo lithologies. This evolutionary path does not require a separate terrane status for each of the Limpopo zones, as has been previously suggested.
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Lebombo: Are We On The Edge Of Africa?
By C. ReevesThe history of continental dispersal is recorded in the fracture zones and magnetic anomalies of the oceans, though for various reasons it is the earliest phases of dispersal that are, at the same time, the most interesting and the least clearly defined in this record. The relative movements of Africa and Antarctica are particularly important if a reliable model for the whole of Gondwana dispersal is to be built since it affects plate circuits through both the South Atlantic and the Indian Oceans. The absence of marine magnetic anomalies through the Cretaceous Quiet Zone (125-84 Ma) leads to a lack of time control for an important part of the dispersal process, though the fracture zones created then are still clear.
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Magnetotelluric Image Of Conductivity Distribution At The Passive Continental Margin In The Kaoko Belt In Northern Namibia And The Walvis Ridge
Authors G. Kapinos, U. Weckmann, O. Ritter and M. Jegen-kulcsarUnderstanding of processes related to the deformation and evolution of the South Atlantic margins and factors controlling the continental rifting and breakup of Gondwana 200 my ago are few objectives of the interdisciplinary SAMPLE project (South Atlantic Margin Processes and Links with onshore Evolution). In frame of this project we conducted an on- offshore magnetotelluric (MT) survey in the Kaoko Belt in Northern Namibia and the Walvis Ridge to image the subsurface electrical conductivity structure of the Southern African passive continental margin. With a conductivity model we aim to study the magmatic and tectonic activity in this area since the early Cretaceous and the influence of pre-existing neoproterozoic structures on the placement of magma and subsequent continental breakup.
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Exploration Of The Xade Complex, Botswana
Authors B. Corner, J. Blaine, R.E. Fripp, M. Jones, P.R. Hildebrand and D.R. VerranThe Xade Complex occurs in the Central Kalahari semi-desert of Botswana. It was initially identified in 1976 during the first regional aeromagnetic survey of the country (Reeves, 1978). Cover sequences comprise 200-900 m of Karoo Supergroup sediments and basalts, and overlying Kalahari Group sediments. Two boreholes were drilled as part of the follow-up Kalahari Drilling Project (Meixner and Peart, 1984), the one intersecting gabbronorite at 815 m and the other weathered basalt at 419 m, passing into dolerite. The gabbronorite has yielded an U-Pb zircon age of 1109.0±1.3 Ma (Hanson et al., 2004), which is coeval with the Umkondo Igneous Province. The Anglo American Corporation (Ambot) subsequently held exploration licenses over the Complex as part of their Kalahari Gold Project (Ambot 1998). A single Ambot borehole, drilled on a seismic line traversing the southern portion of the Complex, intersected amygdaloidal lava at 621 m, passing into dolerite and shales assigned to the Waterberg Group. More recently, Manica Minerals Ltd has held licenses over the Complex, having initially drilled a further three boreholes in partnership with Mvelaphanda Resources Ltd. Exploration of the Complex is currently being advanced in a Joint Venture with Australian company Impact Minerals Limited which has earned a majority share in the project. The current phase of exploration, still in progress, has included aeromagnetic reflying of a selected target area at a high resolution, extensive geochemical surveying using MMITM technology, trial high temperature SQUID time-domain electromagnetic profiling (HTS TDEM), and the drilling to date of a further two deep boreholes. The current target area encompasses a possible feeder (or exit) dyke system and its entry zone into the complex.
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Time Domain Electromagnetic Sounding In Southern Africa: Recent Case Studies
Authors V. Hallbauer-zadorozhnaya and P.K. NyabezeThe Council for Geoscience carried out ground based time domain electromagnetic sounding (TDEM) surveys on prospects in five countries in Southern Africa namely Democratic Republic of Congo (DRC), Mozambique, Rwanda, South Africa and Zimbabwe. The surveys were undertaken on mining, mineral exploration and engineering sites. The TEMFAST-48HPC and TSIKL-5 instruments were used for data acquisition. In addition, the Geonics Protem 47 and Zonge Nano-TEM systems are being used in South Africa for geology and groundwater investigations. The Interpretation of time domain electromagnetic data was performed using different programs namely ERA, TEM-RESEARCHER (TEM-RES-WIN), IRAF and Sτ Inversion. The parameters that were obtained from modelled data for three case studies comprised of resistivity, chargeability and magnetic susceptibility. Modelling of TDEM data collected in most parts of Southern Africa should take into consideration, the super-paramagnetic effect. In South Africa TDEM surveys were used to delineate mined-out and non-mined areas in the eMalahleni coal mining areas. Building materials and aggregates were mapped using the TDEM technique in the Democratic Republic of Congo (DRC). In Rwanda aeromagnetic interpretation targets were investigated using the TDEM method for potential mineralisation. The coal bearing and mined out areas were demarcated on the basis of resistivity contrasts for the South Africa case study. The thickness of building aggregates was estimated from resistivity data for the DRC site. Areas with high chargeability and low resistivity were identified as potential targets for mineral exploration for the Rwanda prospect. The TDEM method has proved to be an important non-invasive tool for carrying out groundwater, mineral and engineering investigations.
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Ztem-vtem-aerotem Helicopter Em Comparisons Over The 501 Project Cu-zn Vms At Mcfauld’s Lake, Northern Ontario, Canada
Authors M. Orta, J.M. Legault, A. Prikhodko, G. Plastow, S. Zhao, M. Moreton and C. UlanskyHelicopter AeroTEM, VTEM and ZTEM surveys were flown over the 501 zone in the McFauld’s Lake area, northern Ontario. The 501 zone is a relatively small VMS deposit that appears to respond well to all three active and passive airborne EM systems that have surveyed the property. Comparisons between these data sets and the geology are showcased using 1D-2D-3D EM inversion modeling.
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Evaluation Of Contamination At A Feedlot In San Pedro, Argentina, By Means Of Electromagnetic Methods
Contamination at a feedlot, enclosure of animals, in San Pedro, Argentina, was investigated using geophysical methods. Results of inversion of time domain electromagnetic (TEM) data resolved three layers at each point of investigation, namely, a surface layer of clayish soil, a second layer comprising partially water saturated clastic sediments and hydrogeological basement that was very conductive, possible clay bearing. Resistivity of sediments is very low, below 10.5 Ohm m. The three layers are characterized by different resistivity range of values. The first layer is very thin, and it is not clearly seen in the profiles. In the corral it is fully contaminated by manure. The resistivity of this layer decreases in corral up to less than 3 Ohm m (the thickness of soil is less than 3 m within the corral increasing to the test site). Resistivity of the second layer is about 9.5-10.5 Ohm m. The resistivity of clay considerably decreases beneath the corral. The identified geo-electrical layers may not always be identified with geological layers. The TEM results showed full agreement with previous results of frequency domain electromagnetic (FDEM) method and electrical resistivity tomography. It was demonstrated that electrical and electromagnetic methods are a very important tools for the evaluation of contamination.
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Natural Source Audio Magneto-tellurics (nsamt): Case Studies Over Various Structural And Mineralized Targets
More LessCase histories using a Natural Source Audio Magneto-Telluric (NSAMT) method of profiling are presented in this paper. These case histories indicate how conductive, resistive and intermediate resistive/conductive targets can be detected at depth using NSAMT. The method is also shown to work in highlighting structural elements such folding and horst and graben tectonics. Success also has been achieved in detecting and providing thickness information about conductors underneath surficial conductive overburden or overlying conductors. NSAMT can also be used to locate deep steeply dipping conductors such as nickel sulphides, negating the use of the more costly Squid TEM. Additionally, NSAMT cans be used as a deeper resistivity profiling technique to add to IP/Resistivity surveys, as is done by TITAN and MIMDAS. The NSAMT method is based around the use of hardware and software from Zonge International. Data collection, processing and modelling costs are similar to Induced Polarization (IP) and Transient Electro-magnetics (TEM) making the NSAMT profiling method a very attractive option in mineral exploration. Costs are kept to a minimum by forgoing the use of a remote reference and using novel filtering and processing schemes. NSAMT is intrinsically safer than IP and TEM as there is no transmitter or high voltages involved, and also because fewer people and vehicles are needed. Smaller crews are needed to those mobilized when doing IP and TEM as there are no heavy cables necessary or big electrodes that need to be prepared. Fly in surveys are possible on short notice.
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Magnetic Induced Polarization - Using New Technology For Greater Detection Capability Of Deep And Elusive Mineralization
More LessThe Magnetic Induced Polarization (MIP) method uses the measurement of magnetic fields to directly detect internal and external current flow from IP-generating targets, rather than the resultant surface currents as with conventional Electric Induced Polarization (EIP). Magnetometric Resistivity (MMR) measures the magnetic field produced by galvanic current flow to detect horizontal variations in resistivity. MIP/MMR is insensitive to horizontal layering, and is especially suitable for regions with highly conductive cover where EIP and resistivity responses are sharply attenuated. Magnetic fields easily propagate through such conditions; therefore MIP/MMR is minimally impacted by conductive cover. The other major advantage of MIP/MMR, over traditional electrical IP and resistivity, is that it completely eliminates the need for measurement electrodes. Hence, it is effective in difficult ground contact conditions such as dry sandy soils, frozen ground, and rocky scree slopes. For inversion purposes, MIP has an additional benefit that magnetic fields can be measured in all three axes simultaneously, which provides significantly more information about target position and attitude. By using SQUID technology and remote referencing, we are able to improve the data quality and extract useful three component MIP and MMR data. We present a number of field trials using both frequency and time domain methods to analyze the MIP and MMR responses from porphyry copper, and unconformity uranium ore bodies.
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A Multipurpose Squid Receiver For Electromagnetic Measurements
Authors A. Chwala, J. Kingman, R. Stolz, M. Meyer and H.-g. MeyerGeophysical exploration is getting more and more difficult – many of the easy explorable ore-bodies are exploited or under production. Finding new mines requires new technologies and tools. Transient Electromagnetics (TEM) is widely used in mineral exploration, but conventional sensors (especially induction coils) cannot fulfil the needs anymore: deep targets, very conductive targets or targets under conductive overburden are much better (or sometimes only) detectable using SQUIDs. IPHT and Supracon AG are developing low temperature SQUID magnetometer systems for ground based TEM for Anglo American (AOL). As AOL is applying these systems worldwide it is necessary to harden the systems for all conceivable application scenarios. The new multipurpose SQUID system features two orthogonal sets of SQUIDs of different sensitivities: the “low” sensitivity SQUIDs (about 20fT/√Hz) are meant for active methods like TEM that require high slew rates, while the sensitivity channels (below 2fT/√Hz) shall be used for passive methods (MT) or methods with low signals like TEM in slingram configuration. Here, we report on the latest development of these systems which are now routinely used in South Africa, Australia, Finland and Canada. This paper highlights the main features of the system, gives an estimate of the intrinsic sensor noise in shielded environment as well as in the typical exploration setup and will show some results from a first MIP test.
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Magnetic Induced Polarization Survey Using Low Temperature Squids
Authors D. Khoza, L. Polome, W. Pettit and A. RobertsonFrom 20th November – 21st December 2012, Anglo American conducted a geophysical test surveys over porphyry copper systems. The purpose of the Magnetic Induced Polarization (MIP) survey was to test the viability of the MIP method in known porphyry copper prospect. There is broad correlation between the known chargeability anomaly and the MIP results, in terms of percentage frequency effect. There is a need to further test the viability of collecting MIP data using inductive sources instead of galvanic sources. Preliminary indications are that SQUID sensors are most suitable for MIP data acquisition and can be applied and used for exploration of buried porphyry copper deposits. Success in measuring inductive MIP effects using SQUID sensors will constitute a major breakthrough in terms of finding and defining new buried porphyry copper resources.
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New Processing Methods For Data Of A Full Tensor Magnetic Gradiometer System
Authors R. Stolz, M. Schiffler, A. Chwala, M. Queitsch, M. Meyer, N. Kukowski and H.-g. MeyerAdvanced system calibration and data processing methods have to be developed for high sensitive magnetic measurements using the Full Tensor Magnetic Gradiometer (FTMG) system developed at the Institute of Photonic Technology (IPHT) Jena. The instrument consists of an array of six extremely sensitive gradiometers and a triple of orthogonal magnetometers based on low-temperature Superconducting Quantum Interference Devices (SQUIDs). First we implemented in the semi-automatic processing scheme the calculation of the magnetic field component from the IGRF and developed the extension towards the gradient tensor components. The next step is to do a fast calibration of the magnetometer signals which compensates for scaling errors, misalignment, and also for arbitrary offsets introduced by the SQUID the read-out electronics using the IGRF. In a subsequent step we have to compensate for the parasitic magnetometer areas of the gradiometers called balancing. They lead to a strong influence of motion noise in the tensor components. The calculated tensor components have to be transformed from the local system frame into an Earth-Centered, Earth-Fixed coordinate system. Additional processing steps have are introduced which allow to convert between the magnetic signals. Hilbert and Hilbert-like transforms are used to transform between gradient tensor components, magnetic field vector components and total magnetic intensity (TMI). The processes are performed either in wavenumber or Fourier domain or in space domain by integral presentation. First results are shown and a comparison of the two transformation schemes discussed. The transformations allow calculating low noise TMI and the magnetic field components from the gradient tensor components. Besides the improvement of magnetic resolution the mentioned transforms are used for calibration of the system components to and for comparison with data from other surveys. Example data are shown.
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Evaluation And Interpretation Of Full Tensor Squid Magnetic Gradiometer Data
Authors A. Vorster, L. Polomé and S. Lettsence Device) systems will most likely provide the mineral exploration industry with a new tool that provides significant benefits over current airborne magnetic systems. Full tensor measurement of the magnetic field allows the application of filtering, gridding and visualisation techniques that greatly improve the information and resolution relative to data acquired with standard sensors. Potential benefits to the mining industry include faster execution of exploration work programmes and a reduction in exploration costs. Remote sensing of magnetic remanence as a physical property through the use of SQUID magnetometers has not been used as an exploration tool before and the SQUID system’s ability to measure remanence indicators will add significant benefits to exploration programmes. In 2007 Anglo American and De Beers entered into an agreement with the Institute of Photonic Technology (IPHT) in Germany to develop and operationalise a full tensor airborne magnetic SQUID system. Final field tests with a helicopter tow-bird mounted system were successfully completed in February 2012. In December 2012 a test survey was flown for De Beers over an area near Finch in the Northern Cape. The survey produced excellent results with noise levels generally below 10 pT/m. Visualisation of the full tensor and filtered products showed significant detail and resolution in the final data and allowed for the extraction of a wealth of additional geological information from the data.
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Africaarray Seismological Studies Of The Structure And Evolution Of The African Continent
Authors R.J. Durrheim, A.A. Nyblade, M.C.B. Brandt, F. Tugume, G.D. Mulibo, E.M. Kgaswane, A. Mangongolo, M.S.D. Manzi, N. El Tahir and L. LootsThe AfricaArray programme seeks to build geoscience capacity and conduct research that supports development in Africa. This paper reports on investigations of the structure and evolution of the African continent that have been concluded since the review presented at the 2009 SAGA Conference. The AfricaArray observatory network has been expanded from 33 to 49 stations, and 25 continuous GPS and 22 meteorological stations have been installed. A temporary seismic array has been deployed in Mozambique and Madagascar. Brandt and Mulibo elucidated the relationship between the African Superplume, Superswell and the East African Rift System by studying the seismic velocity structure of the mantle. Kgaswane jointly inverted P-wave receiver functions (PRFs) and surface waves, and found that the Kalahari Craton lower crust is largely mafic, except for a few terrains such as the Kimberley. Kgaswane also produced evidence that supports a link between the eastern and western lobes of the Bushveld Complex. Mangongolo used surface wave tomography to define the south-western boundary of the Congo Craton. El Tahir used PRFs to investigate the crustal structure of the Khartoum Basin. Tugume determined the Moho depths and Poisson’s ratios of the Precambrian crust in East Africa. Manzi reprocessed 3D reflection seismic data covering part of the Witwatersrand goldfields using seismic attribute analysis methods, and has provided new constraints on the evolution of the Basin during the Neoarchean. Loots interpreted a 105 km 2D seismic reflection profile immediately to the north of the Cape Fold Belt, imaging the Karoo and Cape Supergroup rocks and the seismic fabric of the basement. A zone of strong reflectors was found beneath the Beattie Magnetic Anomaly.
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What Rock Magnetism Reveals Of The Karoo Basin
Authors L.P. Maré, M.O. De Kock, B. Cairncross and H. MouriA comparative study of the variation in magnetic properties in eight boreholes situated across the Karoo Basin was performed. Four of these boreholes incepted one or more dolerite sills. The aim of this study is to determine the thermal impact of the magma intrusions on the surrounding sediments by using several magnetic experiments. The methods that will be discussed include low field anisotropy of magnetization (LFAMS), magneto-stratigraphy using the classic baked contact test, as well as the magnetic alteration index (A40). Re-magnetization due to the heating effect of the dolerite intrusions are limited to short distances within the contact aureole. Those boreholes that intersect dolerite sills indicate that the heating effect occurred no wider than half of the sill thicknesses. These results also suggest a widespread elevation in the geomagnetic temperatures of the Karoo stratigraphic sequence to temperatures above 150 °C that gradually increases from southwest to northeast across the Karoo Basin.
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Bushveld Cooling And Palaeomagnetism
Authors S.J. Webb, R.G. Cawthorn and W.A. MorrisEarly palaeomagnetic studies on the Bushveld Complex yielded widely separated poles for the various zones suggesting that the Bushveld Complex was emplaced over an extended period of time, possibly in separate pulses over a period of ~50 million years. However, modern high resolution age data have revealed that the Bushveld Complex was emplaced in a relatively short time span, likely to be less than 1 million years. These new age data prompted new palaeomagnetic studies that resulted in a single pole for all of the zones of the Bushveld Complex from magnetite with high unblocking (HB) components. These new studies also documented 7 reversals within the 8 km thick sequence of Bushveld Complex lithologies. These reversals were initially interpreted as occurring sequentially in stratigraphic order; however careful examination of the cooling process in conjunction with the reversals reveals that they are duplicated as the Bushveld Complex cools from the top and the bottom. Thus the stratigraphic location of the reversal also depends strongly on the thickness of the Bushveld Complex. In order to examine details of these reversals and their relationship to the cooling process, we are using data from the triaxial fluxgate magnetometers used in borehole orientation systems to identify the precise stratigraphic location of reversals in borehole core.
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Magnetotelluric Experiments Across The Southern Barberton Greenstone Belt, South Africa
Authors S. Kuetter and U. WeckmannThe first formation of the crust is a controversially discussed topic amongst geoscientists. Tectonics on the Early Earth might have been similar to the plate movement and their driving forces that we observe today. However, regarding that some fundamental conditions like the thermal setting were considerably different at this time other processes like vertical mass transport might have played the governing role in tectonics. Unfortunately, there are only few remnants, e.g. the Barberton Greenstone Belt (BGB) in South Africa, where Early Earth’s tectonics can be studied. In the framework of the research initiative Inkaba yeAfrica two high resolution magnetotelluric (MT) field experiments were carried out in 2009 and 2010 across the southern part of the BGB. Unlike the surface structure which is well known from a number of geological studies, there is only little information on its deeper architecture. Imaging the complex subsurface is therefore the major aim of this project. The MT method allows resolving the electrical conductivity distribution at least down to lithospheric depths. Against the surrounding more conductive geological units the BGB is outlined as a highly resistive body. Mapping tectonic features within the BGB like faults and sutures which can be electrically conductive due to mineralizations along shear planes could be the first step to better understand the evolution of the BGB. However, various electromagnetic noise sources, e.g. power lines and electric fences, hinder the analysis of the MT data. Applying advanced filtering methods is therefore mandatory to improve the data quality as much as possible. Based on the good areal coverage of the ~ 200 MT sites (Figure 1), 2D interpretations along the six profiles with lengths between 80 and 110 km and a 3D analysis of the whole data set are conducted. In 2D models, the faults appear as zones of high conductivity down to a depth of 5 to 10 km; however, it seems difficult to follow these structures across neighbouring profiles. Therefore 3D inversion is more appropriate to correctly image this part of the Barberton Greenstone Belt.
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Maximising The Benefit Of Historic Airborne Em Through New Modelling; 36 Surveys Over A Decade For Building A Basin-wide Conductivity Model For Uranium Exploration
More LessSince 2001, Cameco has been acquiring airborne electromagnetic data over their tenements in the Arnhem Land region of northern Australia to assist in uranium exploration. Airborne EM has predominantly been used for mapping the depth to the sandstone-basement unconformity, and geological structures that favour uranium mineralisation. Thirty-six surveys have been acquired using the TEMPEST® system over the last 12 years in the region. These surveys have historically been limited to individual tenements that were being explored at the time, and as such have been typically treated individually and in isolation to one another. Originally, conductivity models were generated using EMFlow on Z component data only. In 2012, Cameco re-modelled these datasets using the latest inversion algorithms utilising the non-geometry corrected “raw” X and Z component data. This has increased the spatial resolution of the resultant conductivity model, providing an improved product for geological interpretation. The individual surveys were merged together with no stitching, and the resultant basin wide compilation shows the excellent stability of the TEMPEST® system throughout the 12 years of surveying. Re-modelling has markedly improved the conductivity models from these surveys and allowed a basin wide compilation to be constructed to aid in exploration, particularly by providing a regional structural framework which is not always obvious at the tenement scale.
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Vtem And Ztem Helicopter Em Case-study Over The Nuqrah Cu-pb-zn-au Sedex Massive Sulphide Deposit In Western Arabian Shield, Ksa
Authors J.M. Legault, A. Prikhodko, C. Izarra, S. Zhao and E.M. SaadawiHelicopter VTEM active source, ZTEM AFMAG passive source EM and aeromagnetic survey results are compared over the Nuqrah Sedex massive sulphide deposit in Western Arabian Shield of KSA. Field data and 1D-2D- inversions are used to show that all surveys map major controlling structures that host the Nuqrah deposits. VTEM directly detects more massive sulphide mineralized vent portions of Sedex orebodies; whereas ZTEM likely defining larger, less conductive and weakly mineralized distal portions of Sedex system. ZTEM also maps possible conductive down-dip extension of Nuqrah South Sedex below 750m depths.
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Looking For Graphite To Find Copper
More LessExploration for copper in the Zambian Copperbelt has been guided for most of the past hundred years by the search for a known favourable stratigraphic horizon known as the Ore Shale. Historical efforts have also been informed by a belief that the ores are syngenetic and stratiform. Both of these criteria are now known to be too restrictive. Economic mineralisation beyond the Ore Shale has been amply demonstrated by stratigraphically higher deposits close to the traditional Copperbelt, such as Frontier and Lonshi, as well as deposits such as Kansanshi and Sentinel that extend the Zambian Copper Belt all the way to the northwest of the country, and by the most recent discovery at Kamoa, in the DRC, by Ivanplats Ltd, far outside of, and above what was considered Copperbelt stratigraphy. The hydrothermal nature of many of the ores has also been validated, with the attendant role of structure, both deep-seated and local. These cases demonstrate that copper-bearing fluids precipitate their metals in (or near) the first reducing horizon they reach above the basement. The geophysical contribution becomes focused on mapping basement topography and graphitic (reducing) horizons, as well as crustal and local structure that can focus this fluid flow. Examples from a geographic spread of deposits along the Zambian Copperbelt highlight the role of electromagnetic imaging in the search for copper.
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The Geophysics Of The Jacomynspan Proje
Authors M. Ushendibaba, S.J. Du Plessis, R. Hornsey and N. NonoiseAfrican Nickel Ltd applied a number of geophysical methods from May 2011 to March 2013 with state of the art equipment at the Jacomynspan nickel-copper project in the Northern Cape in order to guide drilling and to detect the better mineralised parts of the deposit. The Ni-sulphide mineralisation is hosted in a 100m thick mafic to ultramafic sill that dips 650-750 south. A high resolution aeromagnetic survey was conducted covering the main target zone and the strike extent of historical drilling. The survey successfully delineated the sub-crop of the east-west trending ultramafic rocks. This information was used to plan ground electromagnetic (EM) surveys. Physical property measurements in the form of magnetic susceptibility and electrical conductivity were collected on historical drill core. The relative conductivity readings show the mineralised harzburgite to be more conductive than the gneissic host rock and the tremolite schist mineralisation. It was anticipated when planning the EM techniques, that the current would gather in massive sulphide stringer zones and the net-textured higher grade mineralised harzburgite. Therefore the EM plate model targets could potentially indicate these units rather than the lower grade tremolite schist mineralisation. A moving loop Time Domain Electromagnetic (TDEM) survey was carried out using a Low Temperature SQUID sensor over the main target zone to assess whether multiple stacked conductors exist, to map the best mineralised zones, and to explore for massive sulphide mineralisation. Downhole Electromagnetic (DHEM) surveys were also carried out in order to confirm whether off-hole mineralisation was present. To image both deep and near surface geological structure and mineralisation, vector Audio-frequency Magnetotellurics (AMT) was carried out on the main target zone. The results confirm that the intrusion is steeply dipping to the south and clearly mapped the higher grade harzburgite mineralisation down to 500m depth. The final 3D model was integrated into Micromine software with modelled geology and EM plates. This work confirms that AMT is applicable at Jacomynspan to rapidly and cost-effectively prospect for higher grade mineralisation.
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Magnetic Tensor Measurements Validated By Means Of A Simulated Magnetic Source
Authors B. Du Plooy and R. StolzAn experimental facility was established in which a helicopter borne magnetic gradient sensor was flown over a known magnetic source. A comparison of the measured gradients and the calculated gradients served as a validation of the proper functioning of the complete system.
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High-temperature Squid (hts) Tem Trials In Africa
Authors T. Le Roux, C. Naudé and B. SteenkampTrial surveys have been carried out successfully with High Temperature SQUID (HTS) sensors for Time Domain EM applications in Africa where previously SQUID technology had been available only for LTS under exclusive rights to Anglo American. The HTS sensor coupled with a 24-bit SMARTem receiver proved a practical and cost effective field solution capable of acquiring much better low-noise TEM data than conventional coils and 16-bit receivers. A trial line of TDEM and MIP data over a Xade Complex target in Botswana is shown here to illustrate that effective penetration of severely conductive (100 Siemens) Kalahari and Karoo cover can be achieved. This means that the HTS technology now available has tremendous potential for exploring for sulphide or gold targets elsewhere in Africa below previously debilitating conductive cover with these geophysical methods.
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The Terrascanner - A Ground Based Magnetic Mapping System
Authors H.-g. Meyer, S. Linzen, M. Schneider, M. Schulz, S. Dunkel, J. Bemmann, M. Meyer and R. StolzThe aim of the Terrascanner instrument developed at IPHT Jena is the fast geomagnetic prospection of large areas. This enables applications like as archaeology, building-ground-analytics, geology and detection of unexploded ordnance. The instrument is based on nine thin-film planar-type SQUID gradiometers of first order, three of which are assembled in each of the three liquid helium cryostats. A fast SQUID electronics and a low-drift and low-noise data acquisition systems enables rapid exploration using an all-terrain vehicle. Hence, extended archaeological or geologic objects are mapped with high efficiency and quality of magnetic data. In order to allow high spatial resolution (better than 10cm) and real time geo-referencing of the sensor signals a high-end differential GPS system with base station was implemented. Therefore, precise topographic models of the scanned area can be calculated which are useful for interpretation of the data. In this work we present data acquired with the Terrascanner during a campaign within the Orkhon valley, Central Mongolia, carried out in autumn 2010. The joint interpretation of the magnetic and topographic maps of surveyed areas of up to 1km x 1km proved to be very helpful for large-area archaeological investigations of e.g. steppe regions like the Mongolian ones. We will show additional examples of magnetic scans for archaeology and geology in Germany.
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New Developments On The Full Tensor Magnetic Gradiometer System
Authors R. Stolz, V. Zakosarenko, M. Schulz, M. Schmelz, A. Chwala, M. Meyer and H.-g. MeyerINFLUINS is a German research program which investigates the movement of fluids (and associated gases) in the Thuringian sedimentary basin. The phrase "integrated fluid dynamics" in the project name highlights the interest in the relationship between near surface and deeper fluid and material flows. One major part of the program is to examine shallow and medium depth structures, for instance to understand fluid-fault interactions, using magnetic field measurements. The main tool is an instrument measuring the full tensor of the Earth’s magnetic field (FTMG) in airborne or ground based operation. The sensors are planar gradiometer of first order with a base line of 3.5 cm and a sensitivity of below 100 fT/(m*sqrt(Hz)) based on low temperature Superconducting Quantum Interference Devices so called SQUIDs. The gradiometers effectively suppress the Earth’s magnetic field by about four orders of magnitude and are well suited to map the gradient of the Earth’s magnetic field. A low sensitive reference magnetometer triplet is used in order to enhance the suppression by another three orders of magnitude. The system can be operated in a nonmagnetic bird towed by a helicopter at low altitude or on a nonmagnetic cart towed by a 4x4 car. Both operations allow the investigation of huge areas in short time and record the full tensor gradient of the magnetic field. Here, we introduce the new features of an airborne FTMG system which provides superior performance in fixed wing and helicopter based applications compared to conventional Caesium type magnetometers. First of all we could half the length of the liquid helium cryostat. This enabled to build a new tow fish with a much more aerodynamic shape. Additionally, we improved the performance of the inertial measurement unit which leads to a better transformation from the body to a georeferenced coordinate system. First results of the new system in a newly developed bird flown in three survey areas in the Thuringian basin are shown. Standard deviation of noise level is lower than 60pT/m throughout a whole survey area. Examples of 2D maps of tensor components with high spatial resolution are presented.
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A 3d Potential Field Model Of The Pilanesberg Complex Shape And Structure
Authors S-a. Lee, S.J. Webb, M.Q.W. Jones, R.J. Durrheim and M. GanerødThe Mesoproterozoic Pilanesberg Complex, South Africa, is the world’s largest alkaline intrusion. While surface field relationships suggest an inward dipping structure, it is unclear how these dips extend to depth. The 3D geometry of the Pilanesberg Complex is also unknown. 2D and 3D forward and inversion modelling of gravity and magnetic data are used to set limits on the 3D shape of the Complex. Based on age and chemical affinity, it is known that the Pilanesberg Complex forms part of a larger system of alkaline intrusions that includes two dyke swarms that radiate to the north-west and south of the Complex, as well as smaller circular clinopyroxene intrusions throughout the Bushveld Complex. The Pilanesberg dyke swarms and the circular clinopyroxenite intrusions are reversely magnetised to that of the normally magnetised Pilanesberg Complex, suggesting that a magnetic reversal occurred during emplacement of the system.
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New Evidence For The Asthenospheric Origin Of The Cameroon Volcanic Line From 1d Shear Wave Velocities
Authors A.-p. Tokam, R. Durrheim, A. Nyblade, C. Tabod and S. NguiyaThe composition of the lithosphere beneath Cameroon and the origin of the Cameroon Volcanic Line (CVL) are still debated by the Science Community. Although many studies based on regional or global observations provide good arguments for the setting of the CVL, none of them are strong enough to be considered as unique. In this work we used the joint inversion of Rayleigh wave group velocities and Rayleigh wave group velocities to provide shear wave velocity profiles of the lithosphere beneath Cameroon. Preliminary results show that lithosphere is, on average, faster beneath the Congo Craton than the Pan-African lithosphere beneath Cameroon. Due to the limited resolution of the dispersion curves, the maximum investigation depth was taken to be 200 km. The calculated velocity-depth profiles do not show any sharp discontinuity that could be interpreted as the lithosphere-asthenosphere transition. In addition there is no clear evidence of the existence of a low velocity zone beneath any geologic province within Cameroon. The smooth velocity contrasts observed on the velocity models are believed to be influenced by lateral mantle heterogeneities rather than vertical ones. The shear wave velocities for the uppermost mantle are in general greater than 4.3 km/s at all stations. This is higher than the values obtained in the Main Ethiopian Rift, and suggest that the lithosphere is not globally perturbed by thermal anomalies. This suggests that the source of volcanism along the CVL is from small scale convection in the asthenosphere and occucontrolled by lithospheric fractures that are probably driven by the cold (and fast) edge of the Congo Craton.
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Uncovering Prospective Palaeo-proterozoic Basins In Western Australia In The Search For Distal Footprints Of Large Ore Systems
Authors T. Munday, S. Johnson, R. Hough and I. TylerThe Bryah Basin is part of the Capricorn Orogen, a collision zone between the Archaean Pilbara and Yilgarn Cratons in Western Australia. The Basin is host to significant mineralisation, including mesothermal orogenic gold, copper–gold volcanogenic massive sulphides. Among the challenges in the exploration for these mineral systems is the paucity of outcrop and the extent and variability of a complex regolith cover. To better understand this regolith, a regional-scale, fixed-wing time domain AEM survey was undertaken over the Bryah Basin in 2012. The resulting data were inverted using a smooth model layered earth inversion, taking account of system geometry to better resolve regolith variability in the near surface, to define regolith thickness and to understand the geometry of the regolith-basement boundary. The inverted data show the most dominant regolith features are associated with deep sediment filled palaeovalleys. The regional regolith framework determined from this study provides a basis for better understanding and interpreting an extensive regolith geochemical data set with respect to metalloid anomalies that may be linked to buried Cu-Au mineral systems. It also indicates where cover is relatively thin and therefore offers the opportunity to prioritize areas for exploration.
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Delineating The Kitumba Iocg Deposit With The Orion 3d Dcip System
Authors D.L. Mcgill, K.J. Killin, T. Woolrych and J. RuddAn ORION 3D DC/IP survey was conducted over Blackthorn Resources’ Kitumba IOCG deposit in Zambia. The survey results provided 3D models that successfully delineated the known deposit, and provided an enhanced understanding of the three-dimensional geometry of the mineralization. With this improved understanding of their deposit, Blackthorn was able to refocus their ongoing drilling program to best target possible extensions of the existing mineralization.
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Evaluation Of Various Electrical And Electromagnetic Methods In The Exploration For Weathered Kimberlites In Wajrakarur Kimberlite Field, Ap, India
By S. VermaIn Wajrakarur Kimberlite Field in Dharwar Craton, south India, there are more than 100 Proterozoic kimberlite pipes and some of the world famous diamonds like Kohinoor, Nizam, Orloff, etc. were won from this field. The top regions of many of these kimberlites are weathered and altered and have the presence of clayey minerals, thus providing good targets for electrical and EM exploration. As kimberlites outcrop in a limited area, the weathered top material acts as a localized 3D conductor with confined spatial distribution. Highly resistive Archaean granitic gneiss host the kimberlites and the geological setting is ideal for the application of various electrical and EM methods. Depending on the availability of equipment, different electrical and EM surveys were carried out over a long period stretching more than a decade to study some of these kimberlites. Here a comparison of the performance of the following methods is presented: electrical resistivity tomography (ERT), frequency or time-domain electromagnetic (FEM or TEM) surveys, VLF or Radio MT surveys, etc. Suitability of a particular method in delineating such 3-D conductors critically depends on factors like scale and cost of the survey, available time-frame, required details on the subsurface, availability of survey space and logistic constraints, topography, habitation, vegetation, ambient electromagnetic noise, interpretational rigor, etc. Choice of the most appropriate method thus becomes important in efficiently procuring the desired information optimally. The results are expected to be useful in optimizing survey strategy not only for kimberlites but for a variety of mining and urban problems requiring mapping of 3-D conductors.
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Application Of Aem To Shallow Geothermal Potential Mapping
Authors A. Viezzoli, A. Menghini, A. Manzella, D. Montanari, S. Maggi and A. SantilanoThe “VIGOR” project aims at assessing geothermal resources in 4 regions of Southern Italy. The SkyTEM survey is a part of the planned geophysical activities and has been chosen since it is able to collect resistivity data on wide areas, with great resolution and in short times. To date, the application of AEM methods to geothermal targets has been limited, due to the depth at which the target is usually found. Kirsh and Siemon (2008) suggested its application in low enthalpy study. In this project AEM is tested in an area of known geothermal activities, and where a wealth of ancillary data is also present, in western Sicily. The results are composed in a 3D model of the electrical resistivity of the subsurface, and then interpreted for geothermal potential, following 2 main paths. The first is to produce a model of heat exchange capacity. The second aims at identifying directly conductive anomalies that might be associated to locally shallow hydrothermal fluids. “Termini” area is covered by extensive geological surveys. Since geological conditions of Sicily, even at shallow depth, are very complex, this area provided a good place for defining the resistivity values of the main geological units outcropping in the region. The “Western Sicily” area covers the main thermal manifestations of Western Sicily. Based on near real time preliminary results from the more regional mapping at 1 km line spacing, three infill areas were selected as being the most promising to test the concept. The obtained resistivity volume has then been the base for a detailed lithological and geothermal interpretation. Lithological and geological maps were used to constrain surface condition and to understand the resistivity ranges of the different lithological units. On the base of AEM derived resistivity values, and of laboratory measurements of thermal and electrical conductivity on samples, it was possible to establish the main links between lithology, electrical resistivity and thermal conductivity. The work in progress aims at extending the correlation also at depth, producing a 3D model of thermal exchange capacity for the areas surveyed. This detailed interpretative modeling provides also the basis for detecting resistivity anomalies within carbonate units, which may possibly represent hydrogeological or hydrothermal bodies.
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The Importance Of The Tdem Waveform And Characterization Of Discrete Conductors
Authors F. Effersø, N.S. Nyboe and P. GisseløIn airborne time-domain EM (ATEM) the signal-to-noise ratio (SNR) is paramount for the detection of small responses from discrete conductors. In this paper it is examined how shortening the linear current turn-off can enhance the target responses significantly for discrete 3D conductors. For the objective of the paper two methods were applied (1) synthetic 2D thin sheet modelling as approximations to 3D discrete conductors and (2) comparison of real datasets collected over discrete conductors using two different ATEM systems having respectively a 200 s and a 1,200 s turn-off ramp. The findings of both the synthetic modelling and the real datasets show that the target response can be amplified by a factor 2 or even more if the length of the current turn-off ramp is shortened from 1,200 s to 200 s. The enhancement of the target response, and thereby the SNR, occurs for a large group of discrete conductors for which the time constant is comparable to or smaller than the duration of the current turn-off ramp. Shortening the current turn-off ramp will improve the capability to detect such conductors.
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Lithospheric Structure Of An Archean Craton And Adjacent Mobile Belt Revealed From 2d And 3d Inversion Of Magnetotelluric Data: Example From Southern Congo Craton In Northern Namibia
More LessArchean cratons, and the stitching Proterozoic orogenic belts on their flanks, form an integral part of the Southern Africa tectonic landscape. Of these, virtually nothing is known of the position and thickness of the southern boundary of the composite Congo craton and the Neoproterozoic Pan African orogenic belt due to thick sedimentary cover. We present the first lithospheric-scale geophysical study of that cryptic boundary and define its geometry at depth. Our results are derived from two-dimensional (2D) and three-dimensional (3D) inversion of magnetotelluric data acquired along four semi-parallel profiles crossing the Kalahari craton across the Damara-Ghanzi-Chobe belts (DGC) and extending into the Congo craton. Two dimensional and three-dimensional electrical resistivity models show signifi cant lateral variation in the crust and upper mantle across strike from the younger DGC orogen to the older adjacent cratons. We find the Damara belt lithosphere to be more conductive and significantly thinner than that of the ad jacent Congo craton. The Congo craton is characterized by very thick (to depths of 250 km) and resistive (i.e. cold) lithosphere. Resistive upper crustal features are interpreted as caused by igneous intrusions emplaced during Pan-African magmatism. Graphite-bearing calcite marbles and sulphides are widespread in the Damara belt and account for the high crustal conductivity in the Cen tral Zone. The resistivity models provide new constraints on the southern extent of the greater Congo craton, and suggest that the current boundarydrawn on geological maps needs revision and that the craton should be extended further south.
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Geophysical Delineation Of A Quartz-hematite-fluorite Prospect In The Bushveld Granites, South Africa: Implications For Exploration
Authors E. Chirenje, M. Sethobya and A. MbongonyaThe success of geophysical exploration depends on the characteristics and ore genetic processes of the deposit being sought after. These properties include mineralogy, structural setting, hydrothermal alteration, geochemistry and weathering processes. The Ysterkop North Prospect is located in the Western limb of the Bushveld Igneous Complex, and its mineralization is endogranitic and hosted within anorogenic and altered Nebo Granite of the Lebowa Granite Suite. The ores at Ysterkop North consist of quartz, hematite, and goethite which are locally accompanied by actinolite and fluorite with hematite commonly replacing the precursor actinolite rock or granite (Crocker et al., 2001). Results showed that the Ysterkop North Prospect is very responsive to geophysical techniques owing to the contrasting physical properties existing between the Nebo granite and hydrothermally altered mineralized part.
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The Role Of Falcon® Agg In The Mumbwa, Zambia, Iron Oxide Copper-gold Discovery
Authors A. Christensen and T. WhitingThe Mumbwa iron oxide copper-gold (IOCG) exploration project is located in west-central Zambia. A FALCON® airborne gravity gradiometer (AGG) survey, flown by Fugro Airborne Surveys in 2004, was the catalyst to advance IOCG exploration efforts in the Mumbwa area, leading to the Kitumba discovery. The FALCON AGG survey mapped several regional elements considered prerequisite for prospective IOCG terrains: post-tectonic igneous intrusions and intersection of district-scale fault structures, thus highlighting two prospects, Kitumba and Mutoya, as particularly amenable to IOCG mineralization. The FALCON AGG survey accurately delineated the spatial extents of the Kitumba and Mutoya prospects and identified significant uranium anomalism, upon which the Kitumba discovery hole S36-001 was sited. In 2007 AIM Resources (now Blackthorn Resources), in a joint venture with BHP Billiton, drilled the Kitumba discovery hole, S36-001, intersecting significant Cu mineralisation over a 655 m interval grading 0.46% Cu. In-fill drilling in 2011 intersected high-grade mineralisation in several drill holes, including drill hole S36-038 with the best drilled thickness intersections containing 223m at 4.67% Cu. In 2012, Blackthorn Resources reported the total JORC Inferred and Indicated Mineral Resource of the Kitumba prospect as 187Mt of 1.14% Cu and 0.04 g/t Au, using a 0.5% Cu cut off. The FALCON AGG survey also identified an 19km N-S striking, hitherto unknown, massive hematite-siderite IOCG system, Mushingashi, under 250m-350m sedimentary cover. Widely spaced reconnaissance drilling has intersected low tenor Cu anomalism in hematite-siderite breccia systems with large areas remaining untested. The understanding of the mineralization in the Mumbwa area remains patchy. This is primarily due to the sheer size of the IOCG footprints of the Kitumba, Mutoya and Mushingashi systems, but also due to extensive leaching and possible supergene processes at Kitumba. Ground based geophysics (3D IP/MT) and additional drilling has been undertaken at Kitumba and at the new FALCON AGG prospect, Kakozhi, to the west of Mutoya, to address this issue.
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Shallow Alluvial Diamond Exploration With Genesis Airborne Tem (225hz) System
Authors M. Combrinck and R. Van BurenThe Vaalbos project area is situated in the Northern Cape Province, South Africa, and contains diamondiferous gravel deposits. Eluvial Kalahari sand covers most of the area and limits the surficial exposure of these gravel deposits. Shallow gravel channels are challenging targets to directly resolve with airborne geophysics, however bedrock geology and structure, which are controlling factors in gravel concentration, can be mapped successfully. A GENESIS electromagnetic, magnetic and radiometric survey was interpreted, resulting in a number of likely alluvial gravel deposit targets in the project area to be followed up.
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Structural Mapping Of A Coal Deposit With Genesis And Dighem - A South African Case Study
By R. Van BurenThe benchmark geotechnical and structural mapping airborne electromagnetic technology, DIGHEM, is used to corroborate the newly developed GENESIS time-domain fixed wing electromagnetic system. Examples of these data are presented from a South African Permian age coal deposit. Both 2D and 3D products deliverable from the GENESIS system are shown; their value addition to the structural understanding of the coal deposit illustrated.
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Automatically Determining The Location And Depth Of Magnetic Sources Without The Need For Pole Reduction
More LessThis paper demonstrates how to automatically obtain the location and depth of magnetic field sources of known structural index, such as contacts and dykes, for both profile and map datasets. The method does not require that the magnetic data be pole-reduced or have a known magnetisation vector. The method is applied both to synthetic data and aeromagnetic data from South Africa.
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Assessing The Performance Of Compensation In Aeromagnetic Gradiometry
By G. NoriegaAeromagnetic compensation plays an important role in airborne magnetometry, and in particular, in gradiometer systems. This paper provides a perspective on the range of performance levels achieved in the compensation of typical commercial gradiometer systems, in terms both of residual errors, and the robustness and stability of model solutions. For brevity, the analyses presented are limited to lateral gradients on fixed-wing platforms.
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The Nuqrah Massive Sulphide Sedex Deposit In Saudi Arabia - A Fugro Tempest Perspective
Authors D. Burrows, R. Van Buren and E. Al SaadawiA TEMPEST airborne fixed wing Time Domain Electromagnetic (TDEM) and magnetic survey was flown over the Nuqrah – Mardah project area of central Saudi Arabia for Ma’aden Gold and Base Metals. The Nuqrah massive-sulphide sedex deposit is located within graphite schist and carbonates near the top of the pyroclastic Halaban formation. The survey has identified known mineral occurrences, and located new mineral targets. The north and south zones of the Nuqrah Deposit have been imaged, and further drilling may be indicated at Nuqrah North based on a strike length interpreted from CDI profiles of 1.6 km.
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Botswana’s Nationwide Tie Line Survey And New Magnetic Compilation
Authors S. Reford, M. Nyepetsi, Dr. G. Tshoso, M. Koketso, B. Steenkamp and A. CrousIn 2010-2011, the Department of Geological Survey (DGS) acquired a new airborne magnetic Tie Line Survey over Botswana. It was flown on a 10 km by 50 km grid and totalled 70,176 line-km of data. The control lines were flown in single segments over a short time period to provide the most accurate levelled data set. Strategic deployment of magnetic base stations facilitated diurnal correction using a distance-weighting scheme. After compilation of the Tie Line Survey, some 70 higher resolution magnetic surveys were recompiled, most acquired by DGS supplemented by some from industry. They were then levelled to the magnetic datum provided by the Tie Line Survey, from which a new nationwide grid of total magnetic intensity at 50 m resolution was prepared. These data and enhanced nationwide magnetic grids were released by DGS in 2012.
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Geophysics Of Kimberlites In A Magnetically Noisy Environment: Syferfontein And Goedgevondon Case Study
Authors S.J. Webb and R. Van BurenThe Goedgevonden and Syferfontein kimberlites are located ~1 km apart and are ~150 km from Johannesburg, and ~25 km NNE of Klerksdorp in South Africa. Both pipes are small (~0.2 ha) and Goedgevonden is diamondiferous, although uneconomic due to low grade and quality. While the location of the Goedgevonden kimberlite has been known since the 1930s, the Syferfontein kimberlite was discovered by a speculative airborne EM survey in 1994. Exploration in the area is complicated due to the presence of thick layers of ferricrete and calcrete which partially resorb and trap heavy minerals making traditional soil sampling methods ineffective. The area around the Goedgevonden kimberlite hosts the highly magnetic shales of the West Rand Group, magnetically variable Ventersdorp lavas and several meters of ferricrete, rendering traditional airborne magnetic data virtually useless due to large numbers of anomalies not associated with kimberlite. However the frequency domain EM method was instrumental in discovering the Syferfontein pipe and also shows a pronounced response over the Goedgevonden pipe. Data for various ground geophysical methods (gravity, magnetic, TDEM, HLEM) were also collected over both pipes and show prominent anomalies. Further follow up with fixed wing and helicopter TDEM shows a significant response over both kimberlites. The EM responses are most likely due to the weathered kimberlite, which is well exposed in the Goedegevonden pipe. In spite of the presence of alluvial diamonds and diamondiferous kimberlites, this region remains under explored; we suggest that EM surveys could prove effective for exploring in this region.
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