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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
101 - 150 of 157 results
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Jwaneng South Structure, Botswana: a New 1.3 km Diameter Buried Cenozoic Impact Crater Discovered by Airship-mounted Gravity Gradiometer
Authors Sharad Master, Brad Pitts and Marek WendorffDuring regional exploration for diamondiferous kimberlites by De Beers Botswana in 2006, a pronounced circular geophysical anomaly, ~1 km in diameter (now called the Jwaneng South Structure), was discovered at ~24°46’E, 24°42’S, about 15 km S of Jwaneng. The target was generated on bouguer gravity data obtained with an airship-mounted full tensor gravity gradiometer. The circular anomaly also appears on airborne EM, ground gravity and CSAMT, images. The structure was investigated by 9 diamond drill holes, which revealed a sedimentary rock sequence, consisting of evaporitic lacustrine carbonates with plant fossils, overlain by sedimentary breccias and sandstones of the Kalahari Group (maximum thickness ~300m), filling an apparent circular bowl-shaped depression about 1.3 km in diameter. Below the sedimentary rocks is a <60m thick allogenic breccia complex consisting of a series of breccias with granitic fragments, which pass downwards into brecciated mafic rocks (which resemble Karoo dolerites geochemically) and then unbrecciated granites (of the 2785 Ma Gaborone Granite Complex). No kimberlites or any other similar intrusive rocks were encountered in the boreholes. Petrographic studies show that the rocks have suffered intense shock eformation (although impact-diagnostic PDFs have not yet been found). They have been completely shattered into angular fragments in the authigenically brecciated basement, and into subangular to subrounded shapes in the allogenic breccias, which have a sandy matrix. Individual mineral grains show features found in known impact structures- including mosaicism, deformation bands and lamellae in plagioclase and alkali feldspars, and cleavage in quartz. Macroscopic indicators of shock include “gries”-textured breccias, and multiply-striated joint surfaces, formed by intersecting shock waves, analogous to shatter cones. The disposition of fractured basement, allogenic breccias, breccia dykes, and sedimentary fill, as well as the geophysical characteristics, closely resemble those found in other buried impact structures developed on granitic targets (Tswaing, Kgagodi, Brent), and modelling points to the origin of the structure due to hypervelocity impact by a meteorite (or asteroidal fragment), with a diameter, if chondritic, of ~100m. Jwaneng South is the first impact structure on earth to have been discovered using airborne gravity surveys.
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Seismic Hazard in the DRC and Western Rift Valley of Africa
Authors T. Mavonga and R. J. DurrheimA probabilistic approach was used to assess the seismic hazard in Democratic Republic of Congo and surrounding areas. Seismic hazard maps were prepared using a 90-year catalogue compiled for homogeneous Mw magnitudes; the attenuation relations of Mavonga (for the Western Rift Valley of Africa), Atkinson and Boore (for eastern North America) and Jonathan (for eastern and southern Africa); and the EZ-Frisk software package. The highest levels of seismic hazard were found in the Lake Tanganyika Rift seismic zone, where peak ground accelerations (PGA) in excess of 0.32 g, 0.22 g and 0.16 g are expected to occur with 2%, 5% and 10% chance of exceedence in 50 years, respectively. The seismic hazard in the Congo basin diminishes with distance away from the Western Rift Valley until, at a distance of about 450 km, the chance of exceeding 0.05 g (the threshold value of engineering interest) is less than 10% in 50 years.
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Detection of Buried Magnetic Objects by a SQUID Gradiometer System
We present a magnetic detection system based on superconducting gradiometric sensors (SQUID gradiometers). The system provides a unique fast mapping of large areas with a high resolution of the magnetic field gradient as well as the local position. A main part of this work is the localization and classification of magnetic objects in the ground by automatic interpretation of geomagnetic field gradients, measured by the SQUID system. In accordance with specific features the field is decomposed into segments, which allow inferences to possible objects in the ground. The global consideration of object describing properties and their optimization using error minimization methods allows the reconstruction of superimposed features and detection of buried objects. The analysis system of measured geomagnetic fields works fully automatically. By a given surface of area-measured gradients the algorithm determines within numerical limits the absolute position of objects including depth with sub-pixel accuracy and allows an arbitrary position and attitude of sources. Several SQUID gradiometer data sets were used to show the applicability of the analysis algorithm.
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1-D Velocity Model for use by the SANSN in Earthquake Location
Authors V. Midzi, I. Saunders, M. B. C. Brandt and T. MoleaKnowledge of the velocity model of an area is essential both for earthquake location and tectonic implication. Locating earthquakes using an unreliable model contributes in part to the uncertainties of active fault mapping and unexplained scatter of seismic locations. Given that we strive to continually improve our location abilities, it is necessary to always improve on the model used in the location process. The travel time inversion method was used to estimate a 1 - D velocity model that can be utilised by the South African National Seismic Network (SANSN) in seismic data analysis. It should be noted that the velocities obtained are approximately equal to the average velocity of the 3D structure within the same depth range that has been sampled by the data. In order to test the new model, it was used to relocate a sample of well recorded data from the SANSN database and results compared to previous data analyses. The new model VM1 was found to provide improved locations compared to the previously published CGS locations especially when considering the clustering of events in the mining areas as well as the observed reduction in location errors. Station corrections were calculated and showed strong lateral variations across the region. The velocity model will continue to be improved with time as more seismic stations are installed throughout the country especially in the southern part and thus more data are collected.
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Large-Scale Structural Map of the Cape Fold Belt Derived by Remote Sensing Analysis
Authors Ch. Mielke and M.J. de WitThe presented map shows the geology and structure of the Cape Fold Belt at a scale of 1:600,000. This map was produced to analyze fold structures and fault lineament trends to propose distinct structural domains within the Cape Fold Belt. This map is a first attempt to provide a coherent regional framework into which regional studies could be integrated to link the structures of the Cape Fold Belt to the structures of Gondwanide Fold Belts in South America and Antarctica. Data from Landsat-7, ASTER, SRTM and Air Photos provided the image base for an estimation of fold plunge data (azimuth and dip) and fault lineament data that were correlated with field measurements for accuracy assessment. The map shows that the Cape Fold Belt can be subdivided into six major structural domains. The two structural blocks in the Western Cape Province, separated by the Worcester Fault, show great similarities in fold plunge data. However, more detailed structural data from the field are necessary to characterize the proposed structural domains and to quantify the structural blocks north and south of the Worcester Fault in greater detail.
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Magnetotelluric Study in Northeastern Botswana
The proposed boundaries of geological terranes in northeastern Botswana are mainly based on regional magnetic and gravity data, because there are not many outcrops available due to the thick Phanerozoic cover rocks. The extent of the Zimbabwe craton into Botswana as well as the location of the boundaries to its neighbouring mobile belts (Limpopo Belt, Magondi Mobile Belt and Ghanzi-Chobe Belt) are not very well known. Magnetotelluric (MT) profiles of the Southern African MagnetoTelluric EXperiments (SAMTEX) are present in this area and provide information about lithospheric strike directions and the resistivity distributions as well as possible locations of terrane boundaries, which verify some of the proposed terrane boundaries and suggest modification for others (e.g., Ghanzi-Chobe Belt to Magondi Mobile Belt terrane boundary and the western boundary of the Zimbabwe craton).
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Maximum Flooding Surface; the Ultimate Correlator.
By Daniel Mikeš‘Sequence stratigraphy’ claims to be a method that describes the 3D/4D basin-fill. However, it comes short in a number of ways: there is no concensus on its concepts and terminology, some of the assumptions are known to be incorrect, there is no general model that fits all environments and locations. The scientific community has been working hard on these issues and the latest joint effort (Catuneanu et al., 2009) gives an excellent overview of the present state of the method. The authors make a wise distinction between model-independent and model-independent features of the technique. Although impressive in its review of existing models and including a range of techniques, areas and environments, it continues to build on the same assumptions and hence leaves the same shortcomings as all previous models, even though it mentions them explicitly. The main assumptions on which sequence stratigraphy seems to be built are the following. (1) The base level curve is cyclical. This is very unlikely the case, the more so since it is known that base level is a combination of tectonic, eustatic, and climatic signals. Added to this is the fact that although the insolation curve can be calculated there is no adequate high-resolution model for the other three signals. It is highly unlikely of any of the individual signals to by cyclic or symmetric, let alone the compound signal. A look at the insolation curve itself shows us its irregular shape. As a result, it is unlikely for the succession of geometric elements to be regular. (2) Length translates directly to time. The behaviour of a sedimentary system is known to be extremely variable in both space and time, and both on larger and smaller space and time scales. It is therefore highly unlikely that a geometric element will be laterally continuous in both facies and time. The natural migration of sedimentary systems makes timelines naturally cut through lithological boundaries and the diachronicity of the same geometric element within a sedimentary system can be substantial. This is even enhanced by the fact that sedimentation and erosion most probably occur in only a fraction of the entire time. As a result a sequence at one place is unlikely to correlate with a similar one some space away. The basic concepts of ‘sequence stratigraphy’ are relatively straightforward and have been proposed as early as 1949 (Sloss et al., 1949), all concepts thereafter have intended to expand and improve the method. However, there is to date no concensus on many of those concepts. This means that either the concepts are not sufficiently general, or that there is disagreement on the assumptions. One thing is sabotaging all attempts and that is the fact that we study the output signal and try to reconstruct the input signal from it. One of the main differences between the alternative models is the choice of main correlative surface. All but one (Galloway, 1989) use the erosional unconformity. The question is: if we don’t know the input-to-output relationship, can we employ the output-to-input interpretation? To answer this question we concentrate our studies on the growth of deltaic systems with and without external forcing. To that end we simulate a deltaic system in a scaled environment for a selection of prototype scenarios and we intend to run numerical models for a large range of scenarios. Our preliminary results show that the erosional unconformity is a strongly diachronous local phenomenon, whereas the maximum flooding surface is a weekly diachronous regional phenomenon. From our findings we recommend the following: the maximum flooding surface as correlator; the shoreline migration to establish purely geometric behaviour like e.g. aggradation, progradation, retrogradation; a combination of geometric behaviour and lithofacies to establish ‘stratotypes’ or elements. We recommend to not use the following: orders of cyclicity (e.g. 4th, 5th, 6th), unless the period is known through age-control; systems tracts or socalled ‘genetic’ elements. Overall we recommend the use of well-established existing techniques like sedimentary structure analysis, facies analysis, basin analysis to establish the areal extent of 3D geometric elements like lithofacies and their bounding surfaces.
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A Structural and Petrographical Study of the Peninsula Formation at Moregrove Quarry, Port Elizabeth
Authors J. A. Moodley, P. W. K. Booth and G. BrunsdonA 100 metre section of quarry face in the Moregrove quarry in Port Elizabeth was mapped with the purpose of carrying out a detailed structural analysis of Peninsula Formation rocks. Along the quarry face primary sedimentary structures and deformation structures (faults, cleavages, joints) were measured in detail with a Brunton compass, and results plotted on an accurately constructed section, with accompanying stereogram data. Rock types are predominantly quartzites with lesser phyllite horizons. The latter occur as lense-shaped pods along faults zones and are highly sheared. Several thrust faults dipping shallowly towards the southwest have in places imbricate faults ramping up to join thrust planes, thus forming duplex structures. Normal faults are also present showing displacements of the order of only a few metres. A fracture cleavage, probably related to thrust development, is prominent in hangingwall blocks. At least two open joint sets are present in the quartzites which enhance the close-spaced fracture patterns in these rocks. In addition there are en echelon quartz filled fractures, which relate to the normal fault development probably during a post- thrusting episode. Structures formed as a result of compressional tectonism are interpreted as having developed during the Cape Orogeny, during the Late Palaeozoic, whereas normal faulting is probably related to the breakup of Gondwana, during the Mesozoic.
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Using 3-D Gravity Gradient Space Plots Instead of 2-D; a Forward Step for Basin Analysis
More LessThe historical works in gradient space had 2-D space plots of almost all possible geological structures. Evaluation of recent work by this method for interpretation of a sedimentary basin showed that error entrance caused by lots of approximations must be eliminated. As a more complete and less approximated geometrical interpretation method, 3-D space plots were offered. These kinds of plots contain more details and also they are suitable to use for not only basin but also all dominant anomaly's shape. There isn't any estimation of density in this method because density estimation doesn't appear in this method procedure at all.
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Assessing Source Rock Maturity within the Prince Albert and the Whitehill Formations
Authors Z. Mowzer and R. DomoneyPetroleum plays a pivotal role in society, so much so that petroleum accounts for a huge percentage of the world’s energy consumption. The population boom has made the demand for this commodity increase exponentially. New ventures into the petroleum sector are increasing to keep up with the demand of hydrocarbons. The Karoo in South Africa is an area in which petroleum studies are being reviewed with particular interest, especially within the Tanqua and Laingsburg subbasins of the Karoo. Research in this area is becoming more readily available as the oil industry seeks to gain more knowledge for exploration purposes. Understanding this region can help aid in offshore development that is not as readily accessible as the onshore sediments. Using new modern technologies as well as old geochemical techniques we can expand our knowledge on hydrocarbons within this onshore environment. The particularly formations of interest comprises of the Prince Albert and the Whitehill of the Lower Ecca group, located within the southern tip of the Laingsburg subbasin. The Prince Albert consists of siliceous shale, whereas the Whitehill is made up of very fine grained black shale that has been weathered white. These sedimentary rocks are important for hydrocarbon exploration research due to the fact that it is found onshore and has not been altered by faulting, fracturing and metamorphism. The present research work deals with interpreting and distinguishing the different type of rock lithologies, to identify the source rock zones and organic matter within the Whitehill and Prince Albert formations. This will be achieved by utilizing different geochemical analysis (such as XRD, XRF, rock eval pyrolysis, vitrinite reflectance, TOC and SEM) to assess the maturity and the quality of the source rocks.
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Mineralogy of the Middle Group Chromitite Cumulates at the Tweefontein Chrome Mine Section, Central Critical Zone, Eastern Bushveld Complex
Authors C.T. Mphatse, C.D.K. Gauert, E. Denge and M. Maluma.The understanding of the silicate mineralogy in the Middle Group chromitite section of the central Critical Zone at Tweefontein in the eastern Bushveld Complex south of Steelpoort is essential for the investigation of the genesis of the cumulate rocks; in particular the crystallization history of the cumulus minerals and the development of the interstitial liquid are considered to be key parameters controlling the PGM distribution in those mafic rocks. The detailed ongoing investigation of the textural relationships and the mineral chemistry of the rock forming minerals may therefore assist in explaining the PGE distribution. Vertical geochemical and modal mineral distribution patterns are prime indicators of magmatic system conditions. The investigations will help to better understand the PGE distribution in a dynamic ultramafic to mafic system on the theoretical side, and will characterize the possibly variable PGE patterns of the different Middle Group chromitite sections within the Bushveld Complex. Practical conclusions such as PGE abundances and their occurrence in various minerals, as well as grain sizes and textures relations relevant for liberation are important for economic extraction processes.
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Rapid Mesozoic Thermal and Chemical Modification of the Rehoboth Terrane and Kaapvaal Craton from Broadband Magnetotellurics and Xenolith Geochemistry
Authors Mark R. Muller, Alan G. Jones, Rob L. Evans, Herman S. Grütter, Chris Hatton, Xavier Garcia, Mark P. Hamilton, Marion P. Miensopust, Patrick Cole, Tiyapo Ngwisanyi, Dave Hutchins, C.J.S. Fourie, Hielke A. Jelsma, Shane F. Evans, Theo Aravanis, Wayne Pettit, Susan J. Webb, Jan Wasborg and the SAMTEX TeamA 1400 km-long, 2-D magnetotelluric (MT) profile across the Archaean Kaapvaal Craton, the Proterozoic Rehoboth Terrane and the Late Proterozoic/Early Phanerozoic Ghanzi-Chobe/Damara Belt reveals significant lateral heterogeneity in the electrical resistivity structure of the southern African lithosphere. The profile indicates the following present-day average lithospheric thicknesses, to a precision of about ± 20 km, for each of the terranes traversed (inferred conductive geotherms in brackets): Eastern Kimberley Block of the Kaapvaal Craton 220 km (41 mWm-2), Western Kimberley Block 190 km (44 mWm-2), Rehoboth Terrane 180 km (45 mWm-2) and Ghanzi-Chobe/Damara Belt 160 km (48 mWm-2). Previously published mantle xenolith pressure-temperature (P-T) arrays from the Gibeon, Gordonia and Kimberley fields, however, suggest that the Rehoboth Terrane had equilibrated to a cooler conductive palaeo-geotherm (40 – 42 mWm-2 ) very similar to that of Eastern Kimberley Block of the Kaapvaal Craton, at some (unconstrained) time prior to the Mesozoic eruption of the kimberlites. A model consisting of the penetration of heat transporting magmas into the lithosphere, with associated chemical refertilisation, at an early stage of Mesozoic thermalism appears to be the most plausible model at present to account for both the present-day lithospheric structure of the Rehoboth Terrane and an earlier, cooler palaeo-geotherm. Some problems, however, remain unresolved in terms of the isostatic response of the model. Based on a compilation of xenocryst Cr/Ca-in-pyrope barometry observations, the extent of depleted mantle in the Rehoboth Terrane is found to be significantly reduced with respect to the Eastern Kimberley Block: 117 km versus 138 – 167 km. It appears most likely that the chemical depletion depth in both terranes, at least in the vicinity of kimberlite eruption, is accounted for by refertilisation of the lower lithospheric mantle.
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Exploring Exploration Play Models with FTG Gravity Data
Authors Colm A. Murphy and Jade L. DickinsonExploration play models are employed by mineral and hydrocarbon exploration companies to establish and help explain the geological setting of their targeted resource. These models vary from salt domes to structural fault blocks, from igneous intrusives such as dykes and sills to kimberlite pipes. Such geological models are based on geological observations that are enhanced through the usage of geophysical data.
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The Proposed Use of Seasonal Forecasts to Improve Maize Production in the Free State
Authors K.M. Nape, A.S. Steyn and S. WalkerClimate variability is one of the major causes of instability in crop production, especially in semi-arid regions where high variability of rainfall results in uncertainty of water availability for rainfed farming. Improvement of rainfall predictions at a 1-3months lead-time can give a benefit in South Africa due to the high contribution from the agricultural economy. If rainfall predictions are available in time, strategic management decisions can be adjusted to reduce impacts and take advantage of favourable conditions. However, this has not been applied to maize in the Free State. The aim is to use the long-term climate data in a locally tested crop simulation model (Agricultural Production Systems Simulator Model (APSIM)) to generate a range of maize yields. The study area is quaternary catchments along Modder-Riet rivers using different arable soils. APSIM will predict maize yield under various management strategies (fertilizer applications, plant density, planting dates, cultivar types, etc) to identify optimal yields. These outputs will be assessed with two 3-month rainfall forecast periods for the maize growing season (6 possible combinations). The comparative advantage of maize production using seasonal rainfall forecasts will be evaluated against standard practices. This information could be incorporated into a decision support system based on the financial benefit of maize production using the seasonal rainfall forecast each year as an input. The success of the crop yield forecasting application strongly depends on the crop simulation model’s ability to quantify the influence of weather, soil and management conditions on crop yield. Wise utilization of the seasonal rainfall-crop model information by farmers, agribusiness and policy makers could contribute substantially towards achieving sustainability in crop production.
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Sequence Stratigraphic Characterization of Petroleum Reservoirs in Block 3013, Deep Water Orange Basin, South Africa
Authors Emmanuel N. Nformi and Reginald Domoney“What’s next” for hydrocarbon exploration in South Africa’s Frontier Basin- the Orange Basin? Remaining one of the largest under explored basins in the world, (Pickford, 2001), exploration has concentrated on the shelf, in water depths of under 500m with numerous gas/condensate discoveries and one oil discovery. The potential of the deep water is so far untested, with clear indications that suitable reservoirs, source rocks and traps exist. This research in an attempt to show that the lowstand reservoir components (often developed in down dip deepwater and slope paleoenvironments) of the Orange Basin should hold unexplored/untapped massive hydrocarbon resources. It is geared at exploration Play Analysis of the deepwater Orange Basin of South Africa from a Sequence Stratigraphic Perspective. A modus operandi is developed which uses the integration of a range of tools and techniques including sequence stratigraphy and seismic reflection analysis. These are in turn linked with core and outcrop study/analyses of the exceptionally exposed Permian submarine fans (3 and 4) of the Tanqua sub basin (Karoo Basin) to constrain the range of possible fan types, reservoir architecture and pinch out which are very valuable in appraisal, development and production.
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Measurement of Electrical Properties of Bushveld Complex Rocks for Assessment of Radar Performance
Authors B. Ngwenya, M.T. Mafiri and P.G. SefaraA vector impedance meter was used to measure the complex impedance of samples from Bushveld complex (BC) Mines for frequencies between 1 MHz and 64 MHz. The samples were obtained from Rustenburg and Boschfontein Platinum Mines. The obtained impedance magnitude and phase angle data were imported into the Winpro computer program, resistivity and relative dielectric constant values were then computed for each frequency setting. The frequency range within which the measurements were done is very low for GPR, but ideal for borehole radar (BHR). From the recently done electrical property studies, there is sufficient dielectric contrast between chromitite and Merensky reef, and host rocks such as norites and anorthosite, which was predicted to lead to good radar reflection signals. The hosts have high resistivity values, implying low radar signal attenuation or a longer radar range.
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Investigation of Techniques to Determine Astronomical Aeeing Conditions at Matjiesfontein
Authors M. Nickola, R. Botha and W.L. CombrinckA new fundamental space geodetic observatory for South Africa has been proposed. Matjiesfontein in the Karoo has been identified as a suitable site for locating the proposed new fundamental space geodetic observatory. Lunar Laser Ranging (LLR) is one of the space geodetic techniques to be hosted on-site. LLR requires optical seeing conditions of ~ 1 arc-sec resolution level for a site to be considered suitable. Seeing quality for various locations on the Matjiesfontein site has to be measured to determine the most suitable location for the LLR. Investigation revealed that the following are feasible techniques to determine seeing quality at the Matjiesfontein site: double star separation, Light Detection and Ranging (LiDAR) and automated seeing monitor. Atmospheric turbulence in the boundary layer contributes significantly to the degradation of seeing quality. Atmospheric boundary-layer structure and behaviour may be simulated by a turbulence-resolving numerical model, such as the Large Eddy Simulation (LESNIC) model. The model’s predictions will be compared with the quantitative seeing measurements to determine whether LESNIC is suitable for modelling seeing conditions as well as to fine-tune the model to deliver more accurate seeing predictions.
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The Mount Cameroon Volcano, West Africa: an Active Link Between Recent Eruptives and Mantle Signatures of the Deep Past Beneath the Margins of Africa
Authors M.S. Njome, C.E. Suh and M.J. de WitMount Cameroon, a 4095 m high stratovolcano is arguably the most active volcano in Equatorial West Africa, with seven eruptions during the last century. It is the only active member of the 1600 km long volcanic belt – “Cameroon Volcanic Line (CVL)” - with a history going back ~ 80 Ma at the initial opening of the Central Atlantic Ocean during the Cretaceous. In order to decipher recent petrogenetic and geotectonic evolution of the CVL at this volcano, Lava samples of unknown ages but older than all 20th century lavas are investigated for their mineral chemistry and whole rock geochemistry and compared with the 20th century lavas that have been the only focus of petrological investigations for the past three decades. The data show all lavas as within plate and lava types (basalt, basanite/trachy-basalt, hawaiite and basaltic trachy-andesite) indicating these historical eruptions involved even more evolved magmas than those of the 20th century. The lavas are nepheline normative and show an alkaline affinity associated with rift-graben structures, fed from fissures and central vents. Most samples are olivine, clinopyroxene and plagioclase feldspar phyric with additional Fe-Ti oxide phenocrysts. Mineral chemistry data show the compositional range of olivine for older lava samples to be very wide (Fo58%-84%) and a broader plagioclase composition extending to intermediate andesine, a composition which has not been reported for any 20th century lavas. Despite these differences, primitive mantle normalized trace element spidergrams show similar pattern for these older lavas as the 20th century lavas, similar to OIB, confirming they all have a common mantle source of HIMU type. Thus, whilst the precise origin of the CVL is still disputed, it is clear that its mantle source area is not a transient plume. This has major implications for understanding mantle processes marginal to evolving passive Atlantic margins.
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Magnetotelluric Measurements Across the Southern Barberton Greenstone Belt: Data Analysis
Authors A. Nube, U. Weckmann, O. Ritter, X. Chen, J. Deacon, S. MacLennan, L. Moodley, M. DeWit, G. Willkommen, R. Barth, R. Streich and K. TietzeThe Kaapvaal Craton of South Africa is one of the oldest well-preserved continental fragments on our 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 the 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. A 120 km long profile and two complimentary shorter 60 km long profiles with nearly 100 MT sites provide a good areal coverage of the Barberton suture and its complex geology. Strong cultural electromagnetic noise, possibly originating from nearby mines and the DC railway system, is present in the entire area of investigation. As this man-made noise is much larger than the natural electromagnetic signal, which we use to calculate the magnetotelluric transfer functions, advanced data processing schemes have to be used to obtain the cleanest possible sounding curves. This is of major importance for all later analysis steps as we have to make sure that our impedance tensor represents the conductivity structure of the subsuface and does not mirror the electromagnetic noise. We present first results of a standard data analysis in comparison with the remote reference technique and give an outlook on other approaches aiming to improve the data quality
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Oil Sands Exploration in Ijebu-Imushin using Magnetic and Electrical Resistivity Methods
Authors R.K. Odunaike, G. C.Ijeoma, R. O. Edigbe and A. H. BabatopeThere has been tremendous interest in the exploration and production activities of readily available alternatives to conventional oil. Oil seeps observed in Ijebu-Imushin, Ogun State, south-western Nigeria, served as an indication of the occurrence of oil sands – a non-conventional hydrocarbon resource – in the area. Electrical and ground magnetic geophysical survey techniques were thus employed to map the occurrence of the oil sand deposits in the study area as well as to determine the depth to the basement in order to obtain information on the subsurface for exploitation activities. Three vertical electrical soundings and 2-D electrical imaging along a single profile were carried out using the Schlumberger and Werner configurations, respectively. Modelling the likely subsurface structures from the measured magnetic profiles indicates a likely maximum depth to basement of 300m across the study area. The constructed strata from the electrical methods revealed that oil sands deposit are likely to be present and substantial in the south-western part of the study area. The deposit appears to commence from a depth of about 2 m, extending to a depth of approximately 40 m in this part of the study area.
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A Lithological and Structural Study of the Witteberg Group (Cape Supergroup) with Focus on the Witpoort Formation Near Kirkwood, Eastern Cape
Authors W. Olivier, P. W. K. Booth and G. BrunsdonThis study of Late Palaeozoic Witteberg Group rocks (Cape Supergroup) near Kirkwood, Eastern Cape, focused mainly on quartzitic rocks because these rock types crop out in mountainous terrain where lithological and structural data collection was possible. A detailed structural analysis has been carried out and the results compared to the regional structural pattern of the Cape Fold Belt in the Eastern Cape. Field methods include air photo interpretation of lithological and structural features, and the measurement of structural elements with the aid of a Brunton compass. Thin sections of the main rock types analysed by microscopic and SEM techniques show that arenaceous rocks are composed predominantly of quartz, whereas argillaceous rocks contain mainly micaceous minerals with lesser quartz, feldspar and iron oxides. Strata are folded into open anticlines and synclines that plunge at shallow angles to the east-southeast as well as in the opposite direction and show northward vergence. The general east–west orientation of thrust faults, and their southward dips indicate that they formed during a northward-directed stress field that was present during the Late Palaeozoic. Predominantly south-dipping normal faults formed during the Mesozoic, as part of the process of the break-up of Gondwana.
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Reversed Multi-offset VSP Traveltime Tomography Employing Direct-push Technology for Near-surface Seismic Velocity Imaging
Authors H. Paasche, U. Werban and P. DietrichFor a wide variety of environmental and engineering geophysical applications reliable knowledge of the seismic velocity distribution in near-surface sediments is essential. We evaluate the suitability of the minimally invasive direct-push technology for rapid 2D traveltime tomography imaging P-wave velocity variations in near-surface unconsolidated sediments. We acquire reversed multi-offset vertical seismic profiles (VSP) employing geophones placed on the earth’s surface and a seismic source temporarily installed in the subsurface using direct-push technology. The 2D seismic velocity distribution is tomographically reconstructed from first arrival traveltimes obtained from a number of closely spaced reversed multi-offset VSP measurements. First, we test this approach in a synthetic study that utilises realistic geometries. This is followed by a field data example measured over alluvial deposits in a former river flood plain. The P-wave velocity model finally obtained contains information about high and low velocity anomalies and structurally matches the results of cone penetration tests (CPT) and gamma radiation data acquired along the profile. Additionally, the method enabled a significantly larger penetration depth compared to a conventional refraction tomography survey.
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Zonal Cooperative Inversion of Partially Co-located Crosshole Tomographic P- and S-wave Traveltime Data Sets under Automated Incorporation of a Priori Information
Authors H. Paasche, J. Tronicke and P. DietrichIn many near-surface geophysical studies it is now common practice to collect co-located disparate geophysical data sets to explore subsurface structures. Reconstruction of physical parameter distributions underlying the available geophysical data sets usually requires the use of tomographic reconstruction techniques. To improve the reliability of the obtained model parameters, the information content of all data sets should be considered during the tomographic model generation process, e.g., by employing joint or cooperative inversion approaches. Here, we extend the recently developed zonal cooperative inversion methodology based on fuzzy c-means cluster analysis and conventional single-input data set inversion algorithms for the cooperative inversion of two crosshole tomographic traveltime data sets with partly co-located model areas. This is done by considering recently published modifications made to the fuzzy c-means cluster analysis. Additionally, we show how supplementary a priori information can be incorporated in an automated fashion into the zonal cooperative inversion approach. The only requirement is that the additional information considered can be expressed numerically, e.g., by physical parameters or indicator variables. The approach results in a single zoned multi-parameter model, which is consistent with all available geoscientific information and outlines the major subsurface units. Additionally, physical parameter models underlying the zoned multi-parameter model are obtained for each of the input data sets.
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Automated Integration of Large Geophysical Data Sets using Three Partitioning Cluster Algorithms: a Comparison
Authors H. Paasche and D. EberleSince the advent of modern desktop computers, attempts have been made in various geoscientific fields towards rapid, automated and objective information extraction from suites of co-located data sets. Multivariate unsupervised classification techniques, such as cluster algorithms, have been proven valuable tools for largely automated information extraction and are for example routinely used for structural exploration and integration of multi-spectral remote sensing data sets. However, so far very few attempts have been made towards using unsupervised classification techniques for rapid, automated and objective information extraction from large geophysical data sets. In this study, we employ the crisp k-means, fuzzy c-means (FCM) and Gustafson-Kessel (GK) cluster algorithms and compare their suitability for rapid and largely automated integration of complementary geophysical data sets comprising airborne radiometric and magnetic as well as ground-based gravity data. All three data sets cover a survey area of 5000 km2 located south-east of Johannesburg, South Africa. Integrated geophysical maps outlining dominant subsurface structures are obtained from each of the used cluster algorithm. Fuzzy cluster algorithms, such as the FCM and GK algorithm provide additional quantitative information about the trustworthiness of the detected subsurface units, which is considered very valuable when interpreting the finally obtained zonal maps. We will also show that the GK algorithm is most robust when it comes to the integration of data sets containing a few extreme anomalous values, e.g. as typically present in magnetic data sets, resulting in strongly skewed histograms of the data.
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MODIKWA – a Successful Example of a Shallow Ore-body Imaging by Surface 3D Seismic
Authors C. Pretorius, M. Gibson, Eric Gillot and M. DenisSurface 3D is now a widely imaging tool used in the Western Bushveld to derive continuous structural image for ore bodies with a depth ranging from 600m to 1700m below surface. The cost of such surveys is directly linked to the source and receiver surface sampling. Such structural model was requested for the Modikwa Platinum mine, in the Eastern Bushveld, with an ore body whose depth is ranging from 200m till 450m. In order to balance the costs versus the expectations, a trial was conducted in 2004 whose main focus was to determine the most optimum surface sampling scenario and also benchmark the results of surface seismic versus initial model derived from boreholes. This test has delivered as expected an acquisition model for shallow UG2 surveys in Eastern Bushveld and also a structural model with reliable small features such as fault throws down to 12m due to the achieved vertical resolution. In 2006 the main survey took place and the expected structural image was achieved after processing. The fault network when compared to the initial model had its accuracy and reliability push down till fault throws around 12m for a cost balancing current density of boreholes.
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Exploration and Mining Geophysics and Remote Sensing in 2009 – Where Have We Come From and Where Are We Going To?
By C. PretoriusGeophysical and remote sensing (RS) techniques, integrated with the other geosciences and engineering disciplines, have made a significant contribution across the full mining lifecycle (MLC), extending from exploration to mine closure. Although much of the published data deals with the high profile contributions that geophysics and RS have made to exploration and mining feasibility studies, many of the exciting new contributions are expected in the mining production part of the MLC. In the future there will probably be an increasing focus on geophysical and RS contributions to mine safety, mineral resource management, geotechnical systems, hyperspectral core logging and ore sorting, as well as improved ground and airborne exploration multi-systems. A balanced portfolio across the MLC in the multi-disciplinary Geoscience / Engineering / Metallurgical environment should ensure sustainable success.
Our stock of in-house corporate geophysicists and remote sensors with an owner-manager / shareholder culture needs to be replenished. These individuals are key drivers of the macroeconomic demand for specialist geosciences. The in-house specialists should capitalise on past successes and continue to capture the methodologies in quality assurance systems, best practice manuals, procedures and guidelines in cooperation with our geological and engineering colleagues. These formalised best practices should be appropriately communicated and rolled out and will help our companies to maintain a high standard of compliance with important industry codes, for example in safety risk management and mineral resource management. Organisations such as SAGA and our tertiary education institutions can make a significant contribution to the formal compilation of best practice guidelines for the geophysical and RS geosciences.
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Recent Seismicity in the Mozambique Region and its Impact / Effects on South Africa
Authors T. Pule and I. SaundersThe seismic pattern of the southern extension of the East African Rift in Mozambique is presented in this study with special reference to the M=7.0 earthquake that occurred on 23 February 2006. Seismicity in Mozambique is associated with tectonic activity along the East African Rift, which forms the boundary between the African (Nubian) plate in the west and the Somalian plate in the east. The seismic event that occurred in the Manica province of Mozambique on 23 February 2006 was the largest event prior to 2006 was recorded in 1951 as an M=6.6 earthquake. However, it should be noted that a maximum credible earthquake of M=7.3 was assigned during the Global Seismic Hazard Assessment Programme study for a broad zone encompassing the area. Damage caused by the earthquake was limited though it was felt across a large part of southern Africa. Reports of two fatalities were received from Beira indirectly linked to the earthquake while most damage was centered on the town of Machaze where an unconfirmed number of people were either killed or injured (unconfirmed reports indicate 5 fatalities and 30 injuries). Reports also indicated that numerous houses and businesses were damaged during the earthquake. The estimated intensity levels in Durban and Johannesburg from reports indicate II-III on the Modified Mercalli scale. The shaking levels of the event reveal a surprising trend that is counter-intuitive to normally observed damage from similar events, given that Durban is more than 1000km from the epicenter. This work looks at the Gutenberg-Richter frequency magnitude distribution of earthquakes in this region.
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Electrical Conductivity of the Cape Fold Belt, South Africa.
Authors O. Ritter, U. Weckmann and Kristina TietzeWithin the framework of the German - South African geo-scientific research initiative Inkaba yeAfrica several geophysical field experiments were conducted along the Agulhas-Karoo Transect in South Africa. This 600km long transect crosses the Cape Fold Belt (CFB) with the Kango and Oudtshoorn Basins, the Namaqua-Natal Mobile Belt, the Karoo Basin and the transition into the Kaapvaal Craton. Magnetotelluric imaging reveals structural details of the CFB and its collision zone with the Namaqua Natal Mobile Belt (NNMB) at a scale of the entire crust. The CFB is generally characterized by similarly high electrical conductivities as observed in the NNMB. Exceptionally high electrical conductivities are found in the Mesozoic/Cenozoic inliers, the Kango and Outshoorn Basins, whereas the Swartberg and the Outeniekwa Mountain ranges seem to have mid crustal resistive roots.
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Geological Applications for FTMG
More LessThe geological applications of the Full Tensor Magnetic Gradiometer, or short “Mag SQUID”, are manifold. At a similar resolution to the commercially available high-resolution aeromagnetic surveys, the Mag SQUID provides vector data and information on remanence. The vector data is of high importance as it may allow a better 3D modelling of magnetic bodies such as kimberlites, dykes and iron-rich ultramafic replacement pegmatitoids (IRUPs). In the data one may detect mineralisation directly, but the information can also be used to sense negative structural interferences with the mining process such as dykes and IRUPs (hazard zones). Further we expect to detect weaker magnetic features, e.g. the so-called non-magnetic dykes in the Mpumalanga coalfields. The anticipated information on remanence is required for enhanced anomaly discrimination and determination of age of intrusive or alteration events.
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Chemostratigraphy of Wells A-U1, A-F1, K-M1 and A-G1 in Block 3A Used to Facilitate Reservoir-scale Interwell Correlations in the Orange Basin Offshore the Southwest Coast of South Africa
By X. SchalkwykChemostratigraphy is the study of the inorganic geochemistry of sedimentary sequences where it is used to study variations in the geochemistry of sediments as a correlation tool. This is done by analyzing cutting samples, conventional core samples, sidewall cores or outcrop samples which allows regional to reservoir-scale interwell correlations to be made (F. Schwab, 2003). This project will entail the correlation of four wells for oil and gas provinces, with petrophysical logs and in the end determine potential reservoir compartmentalization. The four wells will serve as a good basis for correlation of sedimentary successions based on subtle changes in concentrations of key major, trace, and rare earth elements. With the aid of logs (Gamma ray, Resistivity log, Neutron – Density log, Spontaneous Potential [SP] and Sonic log the relevant reservoir zones can be determined. The main aim of this project is to use chemostratigraphy analysis as the bases of reservoir-scale interwell correlation. And to also compare geochemistry of the core or side wall samples (if available) or ditch cuttings with the stratigraphic column to establish possible correlations. From these aims the end result will be to estimate the recoverable hydrocarbon reserves and the possible prediction of future production for the reservoirs. Chemostratigraphy then enables the main reservoir units to be differentiated and correlated on the reservoir-scale and subregionally. Core data will provide a direct measurement of well properties but for this project ditch cuttings will enable a more detail description of major, minor and trace elements. The proposed area for this study is located in the Orange basin which is a quintessential passive margin that formed during the break up of Gondwana and the subsequent rifting of the South Atlantic Ocean. The syn-rift portion of the basin is characterized by a series of grabens and half-grabens that trend approximately parallel to the present day margin. The syn-rift fill, which is Upper Jurassic and Lower Cretaceous in age, predominantly comprises siliciclastic and lacustrine strata with a widespread occurrence of volcanic sequences (Gerard and Smith et al, 1983).
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Folding and Faulting in the Weltevrede Formation on Varsfontein, Steytlerville Area
Authors T. E. Scharf, P. W. K. Booth and G. BrunsdonA field study of the lowermost stratigraphic unit of the Witteberg Group (Cape Supergroup, Steytlerville area, Eastern Cape), has shown that rocks of the Weltevrede Formation are composed predominantly of folded argillaceous rocks intercalated with lesser thinly-bedded arenaceous beds. The purpose of this study is to document and provide detailed lithological and structural data, and interpret these in relation to regional geological and structural patterns in the area. Rocks of the Weltevrede Formation show colour variations in both shales and quartzites as a result of variable quantities of iron oxide minerals in the rock. Sedimentary structures are plentiful, especially in quartzites and are useful in the interpretation of facing direction of strata. Trace fossils characteristic of the Witteberg Group, especially Zoophycos and Monocraterion are frequently present in all rock types. Strata are folded into open anticlines and synclines that plunge at shallow angles towards the east-southeast. Folds all verge northwards, indicating deformation forces originated from the south. One set of faults have an east-west strike orientation, whereas a second set strike approximately north-south. The former are interpreted as products mainly of flexure-slip folding, whereas the latter are probably related to a prominent strike-slip fault in close proximity to the study area. From the orientation of structures in the study area we conclude that all structures in the study area conform with the regional structural pattern, and therefore have an integral relationship with deforming forces that were present during the Cape Orogeny, during the Late Palaeozoic.
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Results from the Initial Field Trials of a Borehole Gravity Meter for Mining and Geotechnical Applications
Authors H.O. Seigel, C.J.M. Nind, A. Milanovic and J. MacQueenScintrex is in the final stages of the development of a borehole gravity meter, for mining and geotechnical applications. It is designed to log inside NQ (57 mm I.D.) drill rods to a depth of 2,000 m, using standard four (4) conductor wireline cable. The achieved sensitivity is better than 5 μgal, and is operable in boreholes inclined from 300 to vertical. École Polytechnique of Montreal has developed forward modelling software, as part of this project. Partial financial support was provided by the Ontario government (IRAP) and through a CAMIRO project sponsored by BHP Billiton, Vale Inco, AREVA Resources Canada and Schlumberger. The first field test of the prototype probe was successfully conducted in December 2008 for Vale Inco in a borehole located in Norman Township near Sudbury, Ontario. The results of this test show a large amplitude bipolar residual gravity anomaly, with the crossover at 1,400 m down the hole where the borehole intersected sulphides. A repeat log of the hole indicates that the Gravilog system achieved operational specifications close to its targets. The second field test was conducted in March 2009 for AREVA in a newly drilled, vertical borehole at Shea Creek in northern Saskatchewan. The results clearly show the unconformity, and the bulk density calculations show an indication of a low density zone at the base of the Athabasca sandstone. The data is undergoing further analysis. The third and fourth field tests are planned in July and August 2009 for Schlumberger and BHPB respectively. Gravity measurements inside boreholes provide evidence of density variations both in the immediate vicinity and at a distance from the hole. Scintrex’s development of a new borehole gravimeter will, for the first time, allow the application of gravity logging in typical mining and geotechnical boreholes. Primary applications of the Gravilog system in mining include the sensing and mass-estimates of massive sulphide bodies, either intersected by or in close proximity to the borehole; or accurate bulk density measurements of formations intersected by the hole. In some cases (e.g. iron deposits) there is a semi-quantitative relationship between bulk density and grade of the deposit.
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Interpretation and Modelling of VTEM Data in Difficult Circumstances, Bon Accord Nickel-Sulphide Deposit
By G. R. SelfeThe Bon Accord nickel-sulphide deposit, near Barberton, is surrounded by cultural noise in the form of an operating talc mine, a railway line, a metal pipeline and high voltage power lines which combine to render ground TDEM follow-up and modelling ineffective. A heliborne VTEM survey was therefore flown over the deposit in order to try and delineate the orebody and generate additional targets for follow-up. Although compromised by noise, the VTEM data could be modelled directly using EMIT’s Maxwell software. The modelling matches drilling results closely and has engendered confidence in the new VTEM targets awaiting drilling.
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New Discovery of Ni-mineralisation in Northern Namibia Geophysical Case Study
By G. R. SelfeIndications of potential Ni-mineralisation at Ombuku, on the western edge of the Kunene Anorthosite Complex in northern Namibia, were obtained from a regional stream sampling program undertaken by Anglo American plc from 1998 to 2003. Ground IP, AMT and subsequent TDEM geophysical surveys and a VTEM survey flown by ANL, have delineated several drill-targets around the new ultramafic satellite intrusions discovered at Ombuku. Drilling to date has intersected 0.4% Ni and 0.15% Cu over 24.66m (0.3% Ni cut-off) and 0.53% Ni and 0.24% Cu over 4.3m (0.5% Ni cut-off), in disseminated sulphides. This paper discusses the geophysical surveys undertaken and the potential for identification of a large Ni-sulphide resource through further exploration.
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Prediction of DC Current Flow Between the Otjiwarongo and Katima Mulilo Regions, Using 3D DC Resistivity Forward Modelling and Magnetotelluric and Audio-magnetotelluric Data Recorded During SAMTEX
SAMTEX (Southern African Magnetotelluric Experiment) is a multinational project that was initiated in 2003 to study the regional-scale electrical conductivity substructure of southern Africa and to infer from it the tectonic processes involved in the formation of the southern African subcontinent. Audio-magnetotelluric (AMT) data recorded during the most recent phase of the experiment are evaluated to investigate the local-scale conductivity substructure in the Otjiwarongo and Katima Mulilo regions, where in future the installation of high-voltage direct current (HVDC) earth electrodes will commence. Both of the AMT surveys are situated close to the edge of the orogenic Damara Mobile Belt in northern and north-eastern Namibia. Previous studies using magnetotellurics (MT), magnetometer arrays and geomagnetic observatory data all provide evidence of the relatively conductive nature of the Damara Mobile Belt. The Damara Mobile Belt represents in part the collision between the Congo and Kalahari cratons during the amalgamation of South Gondwana and its high conductivity is explained by the presence of interconnected fluids and conductive materials (graphites, sulphides). In contrast, the lithospheric structure of the ancient Archaean cratons, the Congo and Kalahari, are generally found to be electrically resistive and therefore it is hypothesized that the return path of DC current, flowing along the path of least resistance between the two electrodes, is most likely to lie somewhere within or in the vicinity of the Damara Mobile Belt. To obtain a better understanding of the current flow we propose using geological information, previous results of studies of the conductivity of the Damara Mobile Belt and surrounding regions and 2D inversion results from the AMT and MT data recorded during SAMTEX in northern Botswana and Namibia, as input to a 3D DC resistivity forward modelling code, and try to predict the return path that the DC current will follow.
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Advances in Drill Rig Deployed Radars
Authors Tim Sindle and Carina KempThe deployment of geophysical instruments in underground mining is a rarity. The primary reason for this is that the implementation is often seen as cumbersome and time consuming by geologists and mine managers alike.
This paper explores the recent improvement in deploying borehole radar in hard rock mines. Firstly the paper examines the minimum gear needed to complete the task to simplify logistics. It then looks at the processing of data collected by borehole radar deployed on a core drill rig. The non-ideal stop-start motion of the deployment presents issues in the data quality. With the aid of an onboard accelerometer and a time logging based procedure, these quality issues are addressed by examining three different algorithms to process the data. The strengths and weaknesses of each are discussed, before concluding that an intelligent combination of velocity and statistical methods will reliably produce smooth “winch like’ data.
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TDEM Survey at Shea Creek Uranium Deposit Utilising a Low-Temperature Superconductor SQUID
Authors J.P. Smit and T. Le RouxThe Shea Creek uranium deposit is situated in the Athabasca basin, northern Saskatchewan, Canada. A number of TDEM (time-domain electromagnetic) profiles were surveyed in a variety of commonly used survey configurations at mainly two locations over the conductor associated with the Shea Creek uranium deposit using the IPHT - Supracon - Anglo American low-temperature superconductor (LTS) SQUID (superconducting quantum interference device). The LTS SQUID is a B-field sensor that effectively measures small secondary transient EM decays in a conductive earth due to a sudden change in the primary field of a transmitter loop located beneath, on or above the surface of the earth.
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An Overview of Geophysical Experiments Across the Continental Margins of Southern Africa
More LessIn the framework of the multi-disciplinary Inkaba yeAfrica initiative a number of geophysical experiments have been carried out across the continental margins of Southern Africa. Across the southern margin the so-called Agulhas-Karoo Geoscience Transect extends from the offshore Agulhas Plateau, across Southern Africa’s sheared margin, across the Cape Fold Belt and into the Karoo Basin. Across the western margin, a classic volcanic rifted margin, the Springbok profile was conducted across the offshore Orange Basin, extending to the onshore exposure of the Gariep Belt. Goals of the experiments included broadening our understanding of the features crossed by the transects, especially in the framework of formation of Southern Africa’s margins and the break-up of Gondwana.
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Airborne TDEM by He-filled Balloon
More LessPerforming Time Domain Electromagnetic surveys in rugged terrain is a challenging task and as an alternative to mundanely laying out loops or using a helicopter system, a central loop configuration Time Domain EM system has been fitted to a 5m diameter He–filled balloon with a capacity to lift a ~48kg payload. The transmitter and receiver loops have a diameter of 10 and 5m, respectively. The balloon is handled by an operator and 3 assistants and measures while drifting above the topography. The TDEM electronics was custom built by Elta-Geo in Novosibirsk, Russia and records the soundings to a HP IPAQ handheld PC via BlueTooth communication. A maximum transmitter current input of up to 20 Amp is possible using a conventional half sine waveform.
One of the areas where the balloon was employed was the N’teisha gold occurrence in Yemen where the gold is situated in narrow shear zones, between 0.5 and 2m wide. Although the shear zones are possibly too narrow to be detected by the TDEM system an interesting conductivity anomaly was detected in the vicinity of the shear zones that still has to be further investigated.
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Magnetic Full Tensor Gradiometry
Authors R. Stolz, M. Schulz, V. Zakosarenko, N. Bondarenko, A. Chwala and L. Fritzsch and H.-G. MeyerAirborne geophysics is a commonly used tool for the geophysical prospecting of large areas with high spatial resolution. Especially the mapping of anomalies of the Earth’s magnetic field is well suited for airborne operation. Up to now the measurement of the total field and it’s gradients with caesium vapour magnetometers is the most widely used method in airborne magnetic surveys. But there are advantages of using the tensor gradient instead of the total field gradients, which will be shortly discussed.
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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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