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11th SAGA Biennial Technical Meeting and Exhibition
- Conference date: 16 Sep 2009 - 18 Sep 2009
- Location: Swaziland, South Africa
- Published: 16 September 2009
141 - 157 of 157 results
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Interpretation of Airborne Magnetic and ASTER Images over Kimberley and Boshof Areas, Northern Cape Province, South Africa: Implication for the Occurrence of Diamond-bearing Kimberlite Pipes
By A. TessemaInterpretation of airborne magnetic and ASTER images acquired from Kimberley and Boshof areas was undertaken. The study area lies between Kimberley and Boshof, which is located in the Free State and Northern Cape Provinces. Geologically the area underlies three major groups of rocks, viz. Archaean granites of the oldest age, which are overlain by sedimentary rocks and andesite lavas of the Ventersdorp Supergroup. The youngest rocks are the Karoo sediments and volcanic rocks which are widespread throughout the study area. The aim of the integrated interpretation of the magnetic and ASTER images is to identify diamond-bearing kimberlite pipes and related targets. In order to achieve the objective, the interpretation of airborne magnetic and ASTER images was supplemented by enhanced Landsat TM (ETM+) images. The magnetic data was processed for mapping structural features and lithological units. In order to improve the spectral and radiometric signatures of the ASTER and ETM+ images, image enhancement techniques such as edge detection filter, band ratioing, histogram equalization and principal component analysis were applied. In addition, colour composite images derived from both ASTER and ETM+ bands were prepared based on the calculation of Optimum Index Factor. The analytic signal and the first vertical derivative of the total magnetic intensity revealed several circular features which resemble the morphology of kimberlite pipes. The ASTER composite image derived from bands 6, 2 and 1 in red, green and blue channels, respectively assisted identification of linear features such as kimberlite dykes and fissures, while colour composite image derived from bands 8, 3 and 1 displayed circular-shaped features, which can be interpreted as potential kimberlite pipes. The results from both sets of images were merged with similar features obtained from the magnetic maps. The resulting merged layers revealed areas which are potentially prospective for the occurrence of diamond-bearing kimberlite pipes. The most interesting zone lies in the central and northwestern parts of the study area, where large number of cross-cutting lineaments and circular-shaped bodies were identified. In general, the targets are controlled by cross-cutting structures and largely confined to areas that are intruded by Karoo dyke swarms.
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Platinum Value Chain
Authors M. Tredoux, T. Oberthuer, D Reid and R. TrumbullSouth Africa hosts 70% of the world’s platinum-group element (PGE) resources. Its sustainable management is of utmost economic importance to South Africa and indeed to the world, since the PGE are raw materials for a growing sector of high-tech applications. This holistic project will investigate the chain of natural and industrial processes involved in the formation and utilisation of PGE resources. Scientists from a wide range of disciplines are involved, realizing a first truly multi-disciplinary and multi-facetted scrutiny on the behaviour PGE from ore formation to end-user, and beyond. We intend to use state-of-the-art analytical tools, and theoretical advances, to generate new models for ore formation, which in turn could inform the development of enhanced beneficiation protocols. Furthermore, investigation of the natural leaching and redistribution of the PGE during weathering will be done, and the results used to assess anthropogenic PGE dispersal along highways and relevant industrial sites, with emphasis on their biologic availability. There are three sub-projects, which will focus on investigations into: 1. Primary controls on the natural distribution of the PGE (i) in the mantle, (ii) during magma generation and fractional crystallization, (iii) during magmatic ore-forming processes, and (iv) during post-depositional hydrothermal processes. 2. Sustainable mining, recovery and beneficiation of PGE ores, by quantifying preferred PGE mineralogy and mineral association modes in primary ores, and during the primary flotation circuit. 3. The environmental footprint of PGE recovery and industrial deployment, especially during the weathering cycle. The project extend far beyond the authors and their institutions. Currently, there are six researchers from four institutions in South Africa involved, and four from three German counterparts. Three mining companies have provides samples and/or data and/or support, and three students are already well advanced in their studies.
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From Aspirin to “the Bin” in a Mere 85 years
Authors Jeanne-Claire Trickett, Wolfgang Düweke, Lindsay Linzer, Schalk Kock and Kevin TootalThe worldwide demand for platinum over the past decade together with the high costs of borehole drilling at ever-increasing depths, have provided the impetus and experimental latitude that have led to vast improvements in the acquisition, processing and interpretation of surface seismic reflection data. This short paper provides a brief overview of the historical development of the technique as applied to the Bushveld Complex from the point it was first introduced for the acquisition of two-dimensional (2-D) seismic sections, to its current high-resolution application and integration into the mine-planning process. The early 2-D data sections are compared to data derived from the most recent three-dimensional (3-D) volumes acquired for Impala Platinum and Lonmin Platinum. The discussion covers the geophysical lessons learnt as well as the challenges and limitations that are currently faced in providing accurate structural models of the target reefs. Since the first interpretation of high-resolution data, significant advances have been made in the understanding (and hence recognition) of important geological features and their associated seismic signatures. These findings are presented with focus on the improved delineation of iron-rich ultramafic pegmatoids and slump structures. Examples of the effective integration of aeromagnetic, wireline and seismic data are also illustrated.
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Integration of Borehole Radar and Acoustic Televiewer Data in Geotechnical Boreholes – A Case Study of the Use of Downhole Geophysical Data in Mitigating Risk to a new Mining Shaft Development
Authors K.K. Trofimczyk and P. du PisaniAnglo Platinum is evaluating a new underground platinum mine in the Bushveld Complex, South Africa. Major capital investment is required to sink twin shafts to access the platinum reefs from ~700 m depth. Shaft geotechnical boreholes were drilled along the two proposed shafts’ axes and were extensively logged with a suite of geophysical tools to detect potentially hazardous structures that may influence shaft sinking operations. This paper reports the integration of borehole radar (BHR) and acoustic televiewer (ATV) data down the proposed main and ventilation shaft positions. BHR was used in reflection mode in the boreholes to delineate any near-vertical structures close to the intended shaft positions. The drawback of current slim-line BHR probes is that they are omni-directional; hence it is not possible to determine the absolute orientation of radar reflectors in relation to the borehole. The ATV is used to map structures that intersect the boreholes by providing a high-resolution, oriented acoustic scan of the borehole sidewall. Borehole radar reflectors that intersect the borehole can be depth-correlated with structures identified by the ATV. In this way, BHR reflectors can be oriented in space. BHR identified a sub-vertical structure running sub-parallel and less than 15 m from the main shaft borehole, over a depth range of ~250 m. The structure was linked with a low-strength lamprophyre dyke that could pose a risk to the stability of a vertical excavation in close proximity to the structure. Based on the geophysical interpretation, the shaft positions were moved 30 m in a direction away from the indicated structure. Data captured in new pilot geotechnical boreholes revealed more suitable ground conditions. The sub-vertical dyke was identified in BHR images, but at a safe distance away from the new shaft sites.
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Spectrometric Nuclear Logging as a Tool for Real-time, Downhole Assay – Case Studies Using SIROLOG PGNAA
Authors K. Trofimczyk, S. Saraswatibhatla and C. SmithThe SIROLOG Prompt Gamma Neutron Activation Analysis probe is a spectrometric borehole logging tool developed by the CSIRO in Australia. The principle of the technique is the measurement of the characteristic intensity and energy of gamma (γ) rays that result from nuclear interactions when fast neutrons bombard the formation. The intensity and energy of the emitted γ rays is related to the elemental composition of the rock, thus the volumetric occurrence of the elements in a bulk rock mass intersected by a borehole, can be directly estimated. Downhole nuclear logging has certain benefits over conventional core and drill-chip assaying. A much larger volume of the material surrounding the borehole is sampled than the sample taken for laboratory analysis, thus providing better sampling statistics. Provided a suitable calibration exists, data processing can be automated for quick turnaround of results (near real-time processing). Since a continuous log is measured in situ, the data is not limited by zones of core loss or breakage or potential sample contamination. The SIROLOG PGNAA probe was demonstrated on four Anglo American mines in 2004. At Namakwa sands heavy mineral sands mine in South Africa, the technique indicated the potential to deliver quantitative estimates of whole rock geochemistry, including heavy mineral concentrations. At Sishen Iron Ore mine in South Africa, quantitative in situ determination of Fe (iron) grade was demonstrated. This is in line with published results from studies done at other iron ore mines. Encouraging results were obtained for P (phosphorous), one of the important penalty elements, although low concentrations and a limited range of grade values in the small data sample, affected the outcome. At Skorpion mine in Namibia, a good estimate of Zn (zinc) grade, the primary ore mined, was obtained in blast boreholes; however the penalty element, Ca (calcium), could not be accurately predicted due to the low abundance of this element intersected by the test boreholes. Similarly, low abundance of Cu (copper) returned poor calibrations in a study on waste dumps at Mantos Blancos mine in Chile. The results demonstrate that the PGNAA technique can quantitatively predict the abundance of certain elements in situ, although site-specific calibrations are required for best results. However, calibrations are difficult to establish where the abundance and the range of grade values of the target element are low, or the typical signature of the target element overlaps that of another element. The development of higher resolution LaBr3 γ-ray detectors can potentially alleviate these limitations. Another drawback of downhole nuclear logging is the hazard associated with the use of chemical radioactive sources. Alternative, safe sources, such as neutron activation systems, are currently being developed. These systems use non-isotopic, switchable sources such as D-T neutron generators that can be switched off when not being used.
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Airborne Gamma-ray Spectrometry - How to Settle Rn Atmospheric Correction?
Authors C. Truffert, J. Perrin, G. Martelet, J. Deparis, A. Duffy and D. EberleIn the workflow of airborne gamma-ray spectrometry processing, background correction is the most substantial component. The major part of this correction requires knowing the contribution of atmospheric 222Rn and its daughter products within airborne recorded spectra. To evaluate the atmospheric Rn component, specific calibration flights are needed. Two types of techniques are commonly used: i) upward looking detector technique, ii) spectral ratio technique. For both, the issue lies first in the ability to get the spectra of atmospheric radon and the spectra of uranium from ground. This ability is strongly dependent on local conditions. Two local conditions have to be satisfied: (i) presence in the vicinity of the survey of a wide water surface, (ii) presence of high radon content in the air during calibration flights. BRGM, the French Geological survey, has designed, quality controlled and processed a wide variety of surveys. The experience gained assisted in the monitor atmospheric Rn spectra depending on local condition. Examples from BRGM experience in Tropical forest and temperate climate zones are presented. Constraints and limitations of atmospheric background correction techniques are debated.
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Active vs. Passive Rifting in the South Atlantic from a Petrologic Perspective: an Upwelling Controversy
Authors Robert B. Trumbull, David L. Reid, Coenraad de Beer and Rolf L. RomerThe Atlantic margin of southern Africa is a classic example of a volcanic rifted margin, but the strong gradient in the intensity of magmatism from north to south challenges a conventional plume model. A petrologic study of breakup-related mafic dikes from the northern and southern margin was undertaken to provide data on magma compositions and P-T conditions of mantle melting with which to test the plume (excess heat) hypothesis. The results demonstrate not only more magma but greater diversity of magma types and stronger crustal influence in the dikes from high-flux northern margin in Nambia. Some of these dikes have picritic bulk compositions (>12 wt.% MgO), and olivines Fo90-93, indicating about 200°C excess mantle temperature. Dolerite dikes from the Cape peninsula derive from a single tholeiite series and yield petrologic temperature estimates in favor of a passive upwelling model.
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Multi-Scale Depth Estimation
By R. van BurenThis paper documents the development of an empirical depth estimation technique applicable to potential field data. The technique is demonstrated by application to synthetic magnetic data generated from bodies with varying properties. Application to a sample field dataset is also documented. The estimates produced by the technique and its various derivatives are compared with Euler results providing a benchmark. The results are encouraging and comparable to those obtained from the 3D (grid) Euler method. Although the robust nature of the technique is illustrated, it requires more development in order to formalise the methodology and to provide a theoretical understanding of the observations.
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Some Applications of VTEM for Environmental Studies
More LessThis paper presents two examples of Geotech’s VTEM (Versatile Time-Domain Electro-Magnetic) results obtained over suspected contaminant pollution plumes due to acid mine drainage from tailings storage facilities. These were defined with high resolution airborne electromagnetic (AEM) profiling which traced out subsurface contact zones and plume morphology from conductivity depth imagery.
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Paleo-Stress Orientation and Brief Sedimentology of the Lower Ecca Group, Karoo Basin, South Africa.
More LessThe Karoo basin has undergone multiple episodes of deformation and has lots of controversy surrounding its structural evolution. The sedimentary rocks which make up the lower Ecca have been deposited under deep to shallow marine conditions progressively and can be deduced from sedimentary structures or absence thereof. Tectonism took place between 278Ma and 230Ma in the Cape Fold Belt which was during the deposition of the Karoo sediments and is responsible for the structures formed in the deformed SW Karoo basin. Using stereographic projections, I was able to determine paleo-stress orientations and deduce structural trends which can be extrapolated to regional trends. Evidence of residual hydrocarbons exist in the lower Ecca sandstones and can be associated with the intense tectonic history in this particular region.
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Comparison Between Time- and Frequency-Domain Induced Polarisation Parameters
Authors M. van Schoor, L.P. Marè and C.J.S. FouriePhysical properties of rock samples from exploration or mining sites are often needed to assist in the planning of geophysical surveys or in the interpretation of geophysical results. For example, the output from a physical property study may be used in numerical model studies aimed at assessing the applicability or performance of a geophysical technique. Physical properties are also useful for constraining model parameters during processing and in the interpretation of field data. Electrical property data are often recorded in the time-domain, yielding resistivity and chargeability values; however, a frequency-domain, or even complex resistivity approach, may also be followed. The latter approach, for example, would produce resistivity magnitude and phase angle outputs. It is shown that both the commonly used standard time-domain chargeability parameters and the (single-frequency) complex resistivity phase angle parameter could produce misleading estimates of the polarisable nature of mineralised rocks and of the contrast between different rock types. It is further shown that a multi-frequency (spectral) approach can be used to avoid this pitfall; similarly, the calculation of different time-domain induced polarisation (IP) parameters, (using different definitions) may provide better insight into the polarisable nature of rock samples than a single, arbitrary chargeability value.
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Tree Root Mapping with Ground Penetrating Radar
Authors M. van Schoor and C. ColvinIn this paper, the application of ground penetrating radar (GPR) for the mapping of near surface tree roots is demonstrated. GPR enables tree roots to be mapped in a non-destructive and cost-effective manner and is therefore a useful prospecting tool in a variety of diverse study fields, including geohydrology, ecology and civil engineering. Case study examples are presented of how GPR can play a role in detecting and quantifying subsurface biomass and also how GPR can be used to discriminate between subsurface roots and man-made utilities.
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Increased Accuracy in Mineral and Hydrogeophysical Modelling of HTEM Data via Detailed Description of System Transfer Function and Constrained Inversion
Authors Andrea Viezzoli, Anders Vest Christiansen and Esben AukenThis paper aims at providing more insight into the parameters that need to be modelled during inversion of Helicopter TEM data for accurate modelling, both for hydrogeophysical and exploration applications. We use synthetic data to show in details the effect, both in data and in model space, of the low pass filters present in any system, and of waveform repetition. Low pass filters affect the shallow to intermediate part of the model, whereas the waveform repetition the deeper part. Results show how filters and waveform are parameters, like frame altitude, Tx-Rx timing and so on, that need to be taken into account and modelled correctly during inversion of HTEM data. We then present an application of this approach on real VTEM data from an exploration survey. The results from constrained inversion of the VTEM, compared with borehole information and with other modelling methodologies, show its validity.
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Geomagnetic Field Strength Recorded in Iron Age Ceramics from Southern Africa
The Earth’s dipole magnetic field has decreased in intensity by 5 to 10 percent over the last 150 years. To understand more about the nature of this decline, it is desirable to have a longer record, extending back in time beyond the data available from modern magnetic observatories. Firing of archeologic objects (e.g. ceramics and hut floors) raises temperatures above the Curie temperatures of their constituent magnetic minerals. Paleointensity analyses of fired artefacts-- commonly known as archeomagnetism—can provide longer term geomagnetic records. Such studies are well developed at North Hemisphere sites, but there are no extensive records from the Southern Hemisphere for the millennium prior to the era of observatory recording. There is an extensive record of Iron Age ceramics from southern Africa, with associated burnt structures that has the potential to provide a Southern Hemisphere geomagnetic field record. We discuss our efforts to test the potential of this record using rock magnetic and paleointensity analyses of Iron age test samples. Rock magnetic results to date suggest that Iron age Iron age ceramics contain magnetic minerals of appropriate composition and size to retain a high resolution record of past field intensity. Data from preliminary paleointensity experiments further support this conclusion.
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Overview and Preliminary Results of a Magnetotelluric Experiment Across the Southern Barberton Greenstone Belt
Authors U. Weckmann, A. Nube, X. Chen, O. Ritter and M. deWitThe Kaapvaal Craton of South Africa is the oldest well-preserved continental fragment on the planet and thus is a key area for studies of geodynamic processes of the early Earth. One major controversy concerns the importance of plate tectonic processes in Archean times and the time when this may have been first initiated. Several potential ancient suture zones have been identified at surface within the Barberton greenstone Belt, and these provide an ideal natural laboratory to test for possible subsurface remnants of mid-Archean plate tectonic processes on lithospheric scale. Within the framework of the German-South African geo-scientific research initiative Inkaba yeAfrica a high resolution magnetotelluric (MT) field experiment, ELIBABA, was carried out in April/May 2009 in the Barberton/Badplaas area, eastern Mpumalanga, RSA. An 110 km long profile and two complementary shorter 70 km long profiles with almost 100 MT sites provide a good areal coverage of the Barberton suture and its complex geology. Preliminary two-dimensional inversion models show images of the crustal electrical conductivity distribution which will be compared to existing shallow geophysical results.
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The Good, the Bad, and the Ugly: Airborne EM and the Regolith
By C. WijnsEM signals may be highly influenced by the regolith in weathered terrain, to the point of obscuring geologic targets at deeper levels. In time-domain data, this influence can extend from the earliest measurement times into the mid-range time gates. The degree of interference by the regolith varies according to the terrain, and is mostly influenced by the longevity and variability of weathering. Examples from different parts of Africa illustrate the extent to which long-lived regolith dominates the signal of airborne EM surveys.
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Interpretation of Laboratory Measured Data: New Information on Pore Structure and Anisotropy using Relating to the IP Effect
Authors V. Zadorozhnaya and L.P. MaréMembrane polarization is a fundamental phenomenon of geophysical methods such as induced polarization (IP) method and frequency domain induced polarization method (FD IP). Determination of the characteristic parameters of induced polarization is required for studying physical properties of rocks. Mathematical modeling of a little known model of IP referred to as “induced polarization caused by constrictivity of pores” was conducted. Diffusion equations with specified boundary conditions that are different for current on- and off-times were used in the model. It was shown that membrane polarization occurs in all types of rocks if surface areas and transfer numbers are different for connected pores. During the polarization process all contacts between pores of different transfer numbers will be blocked and the electrical current will flow through the remaining canals. The new algorithm was tested on laboratory measurements. Several samples were selected: shale, mudstone, tillite, hematite, lava and manganese ore. Each theoretical model includes pores of more than 40 sizes sporadically distributed in the sample. The output of this stage of work is the pore size distribution in the sample, anisotropy and the relative amount of pores able to transport ions. It was shown that the size (pore radii) of pores can be different even when the porosity of samples is the same. The prevalent radii of investigated samples varied from 10 μm up to 1 μm. Laboratory data showed good agreement with theory and provided new information on the pore structure of rocks. Mathematical modeling provides reliable information of pores space of rocks, their dynamic porosity and permeability and transportation, especially of contaminant compounds
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