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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
51 - 100 of 157 results
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Inversion of Magnetic and Gravity Fields Applied to the Sub-Basalt Imaging Problem
Authors J. Ebbing, M. Aarset, R.F. Reynisson and T. VattekarThis study investigates the feasibility to use gravity and magnetic inversion to image basalts and sub-basalt structures in sedimentary basins affected by volcanism. A regional 3D model was constructed for the Møre margin, mid-Norway, based on a wealth of seismic and petrophysical information. Resolution of the regional 3D model prevents however detailed imaging of the basalts and sub-basaltic structures. While it is difficult to identify the lateral extend of the volcanic features (at depth of 6 km) in the gravity and magnetic data, as well as in Full Tensor Gravity (FTG) data, the sub-basaltic basement architecture can be identified. The gravity gradients provide valuable information on the vertical and lateral extent below the basalts, despite the small density contrast to the surrounding. Inversion of the gravity and magnetic residuals, after correction for the regional anomalies provided by the 3D model, gives a better insight into the extent and thickness of the basaltic and sub-basaltic layers. Especially, coupled inversion increases the depth resolution, but magnetic and gravity inversion must be weighted differently. Such inversion requires detailed pre-knowledge on the regional structures and shallow sedimentary layers, and can assist other sub-basalt imaging techniques.
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Integration of Geophysical and Satellite Imagery Data From the Alto de Ligonha Pegmatite Fields, Northern Moçambique: Implications for the Control of Mineralisation
Authors Detlef Eberle, Abera Tessema, Elônio A Muiuane, Elias X Daudi and Alfredo M. PontavidaMajor parts of Mocambique were flown a few years ago to acquire high resolution magnetic and radiometric data. It has been since then the intent of the National Geology Directorate of Moçambique to interpret these data generating value-added maps that are easier to use by the exploration and mining industries than mere airborne geophysical grid data and maps. The National Geology Directorate of Moçambique and the Council for Geoscience have joined with the financial support of the National Research Foundation of South Africa to conduct an example study case in the Alto de Ligonha pegmatite fields, northern Moçambique, with a special view to support the small scale-mining sector of the region. Analysis of the airborne geophysical, satellite imagery and geology data, in combination with ground geophysical data acquired over specific mineral showings, reveals that the occurrence of pegmatites is mostly confined to amphibolitic gneiss, which is part of the meso-Proterozoic Namama Thrust Belt. Generation of the respective value-added map was achieved using crisp exploratory K-means clustering of the airborne geophysical data. The map is the result of clustering 850,000 four-element samples (Th- and K-surface concentration, apparent magnetic susceptibility and the vertical magnetic gradient) into a number of classes. It clearly enhances the outcrop area of the amphibolitic gneiss where the occurrence of mineralised pegmatites is the most probable. The automated integration of airborne geophysical data using the well known K-means algorithm proved to be a fast, objective and effective tool to generate a value-added integrated map. The experience made in the Alto de Ligonha pegmatite fields encourages the adoption of this methodology over other parts of the Moçambique Fold Belt. This makes it an integral part of geological mapping ongoing in the country.
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An Evaluation of the Educational Impact of the EarthWise Teacher Training Project
Authors Mariette Erwee, Marian Tredoux and Huibrie JoubertThe EarthWISE geoscience project has dual objectives: To host workshops for science educators, and to offer concurrent road shows for science learners and the community. The workshops are specifically meant to provide training in the section of the National Curriculum Statement, called “The Planet Earth and Beyond”, which includes knowledge in the fields of astronomy, geography and geology. Workshops are presented by knowledgeable individuals in these fields to ensure quality training. The road shows include interactive exhibitions of various scientific fields to increase science awareness among the learners and the community as a whole, and to provide information of possible career opportunities as motivation for their performance in science at school. The main focus of the project is on rural areas, although workshops are also offered in the major cities. In 2006, the EarthWise office was moved to the University of the Free State. In 2008, funds were made available from the Inkaba phase 2 budget to partially finance a part-time manager for the project, and at least one training workshop, with road show, per year. The 2008 activity was presented in Prieska, in the Northern Cape Province, and was attended by 18 educators. The road show attracted 225 visitors. General consensus of the education department involved, as well as the Prieska community, is that it was a very successful event. The study that will be reported on was instigated to investigate in a more rigorous way the educational impact of these workshops held in rural areas. To determine whether the project has been beneficial to the community, some aspects needs to be evaluated, including the increase of the teachers’ subject knowledge and the change of attitude of the learners and the teachers towards science. This is done by means of pre- and post-tests, discussion groups, questionnaires and personal interviews. Both the response of the teachers concerning their experience of the workshops and the results of the evaluations were positive. This is evidence of the necessity of training programs like the EarthWise project. Some very enlightening comments and ideas were also obtained and will be implemented to improve the quality of future workshops.
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Source Rock Potential of the Lower Ecca Group, Tanqua Karoo Basin
Authors J. C. Ferreira and A. AkinluaThe increase in demand for hydrocarbons is motivating a new period in exploration for hydrocarbon resources in previously overlooked areas. This is supported by renewed exploration including feasibility studies, which are currently being conducted within the Karoo Basins of South Africa. The Lower Ecca Group of the Tanqua Karoo Basin is one of the many areas that are currently being re-evaluated and it consists of the Prince Albert Formation which was deposited under deep to shallow marine conditions, which is overlaid by the Whitehill Formation carbonaceous shale that was deposited under anoxic bottom conditions. The purpose of the research is to assess source characteristics of organic matter of the Prince Albert and Whitehill Formations of the Lower Ecca Group within the Tanqua Karoo Basin. Organic matter will be assessed on the basis of quantity, quality and maturity of the organic matter by means of Rock-Eval pyrolysis and organic petrography. Rock-Eval pyrolytic analyses will be undertaken, in addition vitrinite reflectance and maceral composition will be determined. Two very important techniques which can be applied for the rapid assessment of maturation and source characteristics of organic matter, which in turn is crucial in determining source-rock potential.
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Delineate 3D Iron Ore Geology and Resource Models using the Potential Field Method
Authors Des FitzGerald, Jean-Paul Chiles and Antonio GuillenMost 3D geological modelling tools were designed for the needs of the oil industry or detailed mine planning and are not suited to the variety of situations encountered in other application domains. Moreover, the usual modelling tools are not able to quantify the uncertainty of the geometric models generated. The potential field method was designed to build 3D geological models from data available in geology and mineral exploration, namely the geological map and a digital terrain model (DTM), structural data, borehole data, and interpretations of the geologist. This method considers a geological interface as a particular isosurface of a scalar field defined in the 3D space, called a potential field. The interpolation of that field, based on cokriging, provides surfaces that honour all the data. The 3D model and its parts are always consistent with the observations. New developments allow the covariance of the potential field to be identified from the structural data. This makes it possible to associate sensible cokriging standard deviations to the potential field estimates and to express the uncertainty of the geometric model. It also, for the first time, gives a statistically optimal, geologically sound way of interpolating geology, other than directly joining the dots as you do with CAD. Practical implementation issues for producing 3D geological models are presented: how to handle faults, how to honour borehole ends, how to take relationships between several interfaces into account, how to model thin beds over many kilometres, how to optimise lithological properties and how to integrate gravimetric and magnetic data. We describe all geology surfaces and volumes using implicit functions. These are then rendered onto the required sections, plans etc. The estimation of ore-body grades and tonnes, using an unbiased and optimal geostatistical technique, makes use of the stratigraphically bound 3D geology model. An application to the geological modelling of the Hamersley Iron Ore district, Australia, is briefly presented.
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Full Tensor Magnetic Gradiometry Processing and Interpretation Developments
Authors Des FitzGerald, Dominik Argast, Rod Paterson and Horst HolsteinIn recent years, Anglo/De Beers have championed the development of a Full Tensor Magnetic gradient (FTGM) signal instrument from IPHT. Multiple surveys of this quantity have been made in Southern Africa. With the advent of this new potential field full tensor gradient instrumentation, new methods have been developed to de-noise and process these curvature gradients. Traditional Fourier domain and minimum least squares residual of the linear differential tensor relationships have been adapted. This leads to levelling, gridding and grid filtering innovations. The result is a full tensor grid representation of the curvature gradients that is coherent and compliant with the physics at all points in the grid. All of the observed data is thus honoured in the Tensor grid. Isolating the signal and then refining it to be sure there are no distortions have dominated efforts to date. Superior anomaly interpretation regarding the full magnetic history and inferences can then be made. A survey from the Groblersdal Platinum mine is shown in the context of the structural geology interpretation. In particular, the dolerite dykes and faults are seen. The Hornfels footwall contact is very strong. The phase map traces the Platreef contact. The Upper Zone magnetites are more pervasive and fine layered structure is revealed there. None of the granites can be seen. A 3D geology model is in preparation. The observed FTG signal will be compared to the predicted thin-body responses from the model. There is more directly inferable structural geology in this tensor signal than can be found in a conventional TMI signal.
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Remote Sensing Heat Anomalies
More LessRemote sensing heat anomalies directly deserves more attention. Many methods and instruments exist but results from these are largely ignored in the geothermal community. A calibration range is advocated in one or more of the more prospective regions to fasten better use of geophysical methods. Clever software is also required to bridge the gap.
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Remote Sensing Reconnaissance of the Congo-Zambezi Watershed: Exploration of Palaeo-hydrological Patterns and Structural Controls on Biogeography
Authors T.J. Flϋgel, F.D. Eckardt and F.P.D. CotterillEarth’s land surface is ultimately related to (and highly affected by) the underlying geology. Landscapes are the interface between terrestrial, subaerial and climatic processes, and this is where hydrological processes hold a dominant role in determining the nature of landsurfaces. The understanding of large rivers with their associated tributaries and evolution is of fundamental importance when looking at landscape evolution. Yet, the origin of these drainage systems, especially in Africa, rarely receives deserved attention in contemporary geology and biology. Our current understanding of the Congo-Zambezi watershed often invokes data that is over 50 years old. A dearth of recent research into the Congo-Zambezi system largely reflects challenges of fieldwork across a vast area that remains difficult of access; fortunately, analyses of remotely sensed data reveals invaluable reconstructions of landforms. Although high resolution remotely-sensed data has been available since the late 1970’s, it was computationally challenging to derive sufficiently accurate river profiles from these data. The precise resolution of elevation data was significantly rectified by the 2003 Shuttle Radar Topographic Mission (SRTM). Surface elevation models (SEM) obtained from the SRTM along with Landsat 7 ETM + imagery now allows precise mapping of rivers and their profiles across the encompassing landscape. The presence of water falls and rapids (knickpoints) along river profiles reveal subtle geological structure. Integrated maps point to present and historical links (and barriers) across neighbouring drainage systems in relation to structural controls. Additional to the resolution they provide into the dynamic histories of fluvial systems, spatial contexts of knickpoints reveal valuable insights into potential barriers to dispersal of aquatic biota through fluvial systems. This poster presents the preliminary findings of selected river profiles in the study region.
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The Structure of the Karoo-age Ellisras Basin in Limpopo Province, South Africa in the Light of New Airborne Geophysical Data: a Preliminary Report
Authors C.J.S. Fourie, G. Henry and L.P. MarèThe Waterberg Coalfield is destined to become the major source of energy for South Africa in the future. In 2008, Coaltech Research Organisation funded an airborne magnetic and radiometric survey over the Karoo-age Ellisras Basin in which the coalfield is developed. Interpretation of the magnetic data has provided a novel half-graben model for the structure of this basin. The northern boundary is the block-faulted Melinda Fault Zone, with the southern, less-faulted part of the basin sloping gently to the north. The thickness of the Karoo Supergroup reaches 1 500m in the eastern part of the basin, and decreases to the west. The new geophysical data has contributed much to the understanding of the geological evolution of this important coalfield.
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Development of a Proposed Geo-science Train
Authors C.J.S. Fourie, M. De Coure and L. CroukampThe proposed Geo-Science Train (GST) facility will promote scientific research, science advancement, accumulation of scientific knowledge, service delivery of a geoscientific nature and training as its main objectives. It will attempt to reach South Africans from all walks of life strengthening the Public Understanding of Science Engineering and Technology (PUSET) program of the Department of Science and Technology (DST), but special attention will be given to rural areas and communities. The facility will be a mobile geological survey and science centre, strengthen the science awareness program of DST through the network of science centres, by hosting science exhibitions on board that will move with the train from town to town. More children and adults will be reached who would otherwise not have had the means or opportunity to view these science exhibitions. Training of students (to PhD level) through Research and Mapping of areas visited by the Geo-Science Train will contribute directly to education and economic development of the rural areas in South Africa. The Geo-Science Train will create an avenue for international research consortiums (e.g. Inkaba yeAfrica and SAMTEX) to make direct contributions towards the training of South Africans by promoting the understanding of the resources of South Africa. Services by the Geo-Science Train will be geared as training and service delivery. Day to day problems of rural communities and municipalities, such as clean drinking water, sources of suitable local building materials and the placement of waste sites and cemeteries will also be addressed from this facility. The train can provide access to a post box of the Department of Home Affairs, making it possible for persons in the rural areas to do applications for documents (I.D., birth certificates etc.). The contents of the post box will be delivered to the Department upon the return of the Geo-Science Train to Pretoria, approximately once every 3 months. Funding should be from government and industry.
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The Influence of Curved and Angled Survey Lines on 2D Resistivity Surveys Employing the Wenner (α) Geometr
More LessMulti-electrode resistivity systems, such as the Lund Imaging System, employ protocols that control switching between the various electrode pairs. The standard protocols used for 2D surveying assume that the survey lines are straight so that the various electrode pairs are collinear. However, due to the presence of infrastructure and other surface constraints it is not always possible to conduct 2D resistivity surveys along straight lines. When survey lines are curved/angled, the apparent resistivity values and assumed pseudo-depths are affected by the non-collinearity of the electrode pairs. In addition, the recorded apparent resistivity data may be representative of the subsurface conditions at positions laterally displaced from the survey line. For a curved survey line spanning an angle of less than π/2 radians, the errors in both the calculated apparent resistivities and assumed pseudo-depths are less than 3%. Similarly, for angled survey lines with angles smaller than π/4 radians, these errors remain below 6.5% and 4%. Although these errors are relatively small, it is possible that they may be propagated during the inversion process and result in distorted resistivity models. If the true positions of the electrodes along a curved/angled survey line can be measured, the error in the calculated apparent resistivity values may be compensated for by calculating the true geometric factors. Corrections to the pseudo-depths may be made by calculating the true median depth for each measurement position. The lateral displacement of the measurement position is a consequence of the survey geometry and cannot be compensated for.
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Model Studies of the Propagation of Errors During the Inversion of 2D Resistivity Data Recorded with the Wenner (α) Geometry Along Curved and Angled Survey Lines
More LessWhen recording 2D resistivity data along curved or angled survey lines, the recorded apparent resistivity data will be affected by the fact that the true geometric factors will differ from the assumed geometric factors of a straight survey line. Numerical model studies show that, although the errors in the apparent resistivities may be small even for large angles and curvatures, these errors may rapidly increase in magnitude during inversion. This is particularly true during the inversion of data recorded along angled survey lines where input errors may increase by an order of magnitude within a couple of iterations. Although it seems that angles should be avoided in favour of curves when it is not possible to perform a resistivity survey along a straight line, the apparent resistivity data recorded along an angled survey line may very readily be corrected prior to inversion if the true electrode positions can be measured.
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Inversion of Gravity and Magnetic Gradient Data
Authors Peter K. Fullagar and Glenn A. PearsGradiometry has come of age in the past decade, with the development of airborne systems capable of measuring full gravity and magnetic gradient tensors. Airborne gravity gradiometry in particular represents a stunning technical achievement, delivering a completely new capability to exploration. Gravity gradients can now be acquired rapidly in remote and inaccessible areas. Airborne magnetic gradiometry has been available for many years, albeit for a limited number of components. Full tensor magnetic gradiometry is a very recent development. There are several technical advantages of gradiometry, including interpolation between lines and, for magnetics,suppression of diurnal effects, potential for characterisation of remanence, and (for the theoretically minded) the satisfaction of dealing with true potential fields. In terms of interpretation, gradiometry offers greater sensitivity to the near-surface than conventional gravity or magnetics. This is a blessing for resolution of shallow structure and stratigraphy, and for definition of outcropping or sub-cropping targets, e.g. kimberlites. However, enhancement of shallow features can be a disadvantage in exploration for buried targets. The problem is compounded by the inherent variability of the shallow sub-surface, owing to weathering and to transported cover. Finally, sensitivity to the near-surface also implies sensitivity to topography and flying height. This paper explores these issues and others arising during geologically-constrained inversion of gradiometer data. Analysis of gradiometer data alone is not sufficient to fully prescribe the sub-surface distribution of density or susceptibility: acquisition of full tensor data does not deliver us from non-uniqueness. Geological and petrophysical information, primarily derived from drill holes, is required to reduce uncertainty in interpretation. In order to capture both geology and physical properties, a model must be both categorical and quantitative. Such ‘geo-physical’ models are a force for integration in their own right, and also offer a number of practical advantages over pure property models. In particular, geological boundaries can be recognised, and therefore modified (subject to drill hole pierce point constraints); thus, when applied to a geological model, inversion can adjust its geometry as well as its density or susceptibility distribution.
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Early Holocene Extensional Tectonics in the South-Eastern Cape Fold Belt, South Africa
Authors M.L. Goedhart and P.W.K. BoothThe current seismo-tectonic model for South Africa depicts the south-eastern Cape Fold Belt as a tectonically stable intraplate-type environment, where earthquakes are relatively infrequent to rare, particularly large surface-rupturing events. The closest surface rupture recorded by the SANSN, which underpins the model, is the extensional Mozambique 7.2M event on 22 February 2006, related to southward propagation of the East African Rift. A similar 84km-long extensional surface rupture has been found along the Kango fault within the southern Cape Fold Belt, 50km east of Oudtshoorn. Optically stimulated luminescence dating indicates it occurred around 10,620 ± 509, at the start of the Holocene. This fault, together with the adjoining Baviaanskloof and Coega faults, and other nearby adjacent faults, is the subject of a palaeoseismic investigation to determine the Quaternary tectonic history of the south-eastern continental margin. This region of South Africa has little seismic information, and the record is too short to include the long recurrence intervals of large, surface damaging earthquakes typical of intraplate regions. The data reported includes the location and extent of the surface rupture, the local stress direction, the date and magnitude of the most recent event, the minimum recurrence interval, and maximum slip rate. The earthquake environmental effects are also evaluated according to the new Environmental Seismic Intensity Scale (ESI 2007), to provide an independent assessment of seismic intensity. A potential local tectonic driver, the Cape Isostatic Anomaly (CIA) is also discussed, as well as a possible trigger mechanism leading to the rupture. Results should facilitate PSHA of several new critical facilities planned for the province.
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Flooding-induced Seismicity in Mines
More LessMany South African gold mines will flood when they close, as the groundwater will gradually fill the mining voids. Preliminary investigations have shown that flooding of mines can generate increased levels of seismicity. The purpose of this paper is to create awareness that fluid-induced seismicity will become increasingly important in South Africa when closed mines are allowed to flood. Such flooding-induced seismicity can have significant environmental and socio-economic consequences, and may endanger neighbouring mines and surface communities. Even though fluid-induced seismicity has often been observed in non-mining settings (e.g. filling of dams, oil-well stimulation and hydrothermal fields), no detailed study of seismicity associated with flooding of deep mines has ever been conducted anywhere. It is possible that mine flooding could lead to potentially disastrous seismicity, which may result in high continuous pumping costs by the State to prevent or to contain flooding. The potential relationships between flooding and the magnitude and frequency of triggered and induced seismicity resulting from mine flooding need to be researched. A thorough understanding of the interaction between flooding and seismicity will allow the impact of mine flooding on safety to be determined. In particular, the maximum earthquake size resulting from the flooding of deep gold mines in South Africa needs to be determined. The identified risks will in turn allow appropriate mitigating strategies to be developed. Such strategies will influence South African mine closure policies.
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Anisotropy of Southern African Lithosphere and Asthenosphere
Authors Mark P. Hamilton, Alan G. Jones and The SAMTEX TeamObservations of anisotropy, when understood in terms of deformation processes, are critical to illuminating the dynamics of past and present tectonic processes. In particular anisotropy can infer how continents formed, stabilized and interacted with underlying mantle regions in the past, and how they do so today. Seismology and electromagnetic observations of anisotropy are essential if we are to understand the tectonic history of a region. Seismic anisotropy, defined from SKS arrivals, is poorly constrained in depth, whereas electrical anisotropy has inherent depth localization but lower spatial resolution. Given the limitations of both sub-disciplines a more robust characterization of anisotropy is achieved by integrating complementary datasets. Southern Africa has now two rich geophysical databases from the SASE and SAMTEX experiments that can be explored, compared and contrasted for lithospheric anisotropy. Doing so suggests a new model to explain SKS observations which is based on plausible tectonic history. The new model combines the “Silver” lithospheric anisotropy and “Vinnik” asthenospheric anisotropy models, and incorporates their differentiation using electrical anisotropy.
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Orthogonal AMS and SPO Fabrics in the MORB-like Rooi-Rand Dyke Swarm of South Africa and Swaziland
Authors Warwick Hastie, C. Aubourg and M. K. WatkeysThe cause of the ca. 185-175 Ma Karoo volcanism in southern Africa has been ascribed to the presence of a mantle plume centred on the Nuanetsi (now Mwenezi) Igneous Province, southern Zimbabwe. In the mantle plume model, this area is considered to represent a triple junction between the WNW-trending Okavango Dyke Swarm (ODS), the ENE-trending Sabi monocline and the N-S trending Lebombo monocline. The plume model predicts that magma flow in Karoo dykes of the Lebombo monocline should be away from the plume head and should be sub-horizontal in the distal regions. A brief study of the anisotropy of magnetic susceptibility (AMS) of 23 dykes in the MORB-like Rooi-Rand dyke swarm (RRDS) is presented. The AMS in the samples results from fine-grained, Ti-poor magnetite which in 20 dykes defines fabric sub-parallel to the dyke plane, consistent with the plume model. The magnetite defines a weakly anisotropic and dominantly oblate fabric. From a total of 10 dykes studied for plagioclase mineral shape preferred orientation (SPO), 8 have a dyke-parallel foliation most consistent with vertical magma flow. The plagioclase grains define a weakly anisotropic, oblate fabric, which is magmatic in origin. In 8 dykes this fabric is coaxial with the AMS fabric. However, in 40% of the dykes, the fabric defined by the SPO of opaque grains is non-coaxial with AMS and is at a high angle to the dyke plane and dips steeply. The non-coaxial AMS and SPO fabric, coupled with the orthogonal SPO fabrics suggests that late-stage lateral flow of relatively high viscosity magma has occurred. This results in a fabric which most workers would regard as “inverse” and/or non-magmatic, and, therefore, would misinterpret.
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Geotherms, Lithosphere Thickness and Sedimentary Basins
By Chris HattonA relation between the thickness of the lithosphere and geothermal gradients in the mantle can be extracted from two independent sets of observations. One set is the relation between lithosphere thickness determined from magnetotelluric measurements and surface heat flow. The other set of observations is the temperature-depth array that can be extracted from xenoliths in kimberlites. Both sets lead to the conclusion that an increase in the temperature at the Moho is accompanied by thinning of the lithosphere when the lithosphere is thinner than 180 km, but is accompanied by thickening of the lithosphere when the lithosphere is thicker than 200 km. The corollary of this conclusion is that the same high Moho temperature can be associated with very thin or very thick lithosphere. Switching from very thin to very thick lithosphere can initiate the formation of a sedimentary basin. Continued increase in Moho temperature results in further thickening of the lithosphere and growth of the sedimentary basin.
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A Lithospheric and Temperature Study from the South Atlantic
Authors Katja K. Hirsch, M. Scheck-Wenderoth, S. Fishwick, Y. Maystrenko and J. SippelThe East African continental margin is a passive volcanic margin that experienced a long post-rifting history after break up in Early Cretaceous times. The break up resulted in the formation of a number of basins along the margin. The by far largest depocentre in the South Atlantic, the Orange Basin, is the location of the here represented study. The main goal is to combine information on the geometry and properties of the sedimentary part of the system with data on the geometry and physical properties of the deep crust. It was also aimed to integrate both the continental and the oceanic parts of the margin into a consistent 3D structural model on a lithospheric scale to study how far these features give insights into the deeper lithospheric mantle structure. We compare the depth of the Lithosphere-Asthenosphere-Boundary calculated obeying the principles of Isostasy to results based on surface wave tomography and their influence on the thermal field of the margin. A 3D temperature model was evaluated for the passive continental margin of the South Atlantic including the lithospheric structure of the margin. We evaluate a case study for different scenarios to estimate the influence of sediments and crustal structures on the thermal field. The calculated conductive field is constrained by temperature measurements and 3D gravity modelling. At the Norwegian continental margin it has been found that a differentiation of the physical properties of the lower crust and the mantle is needed between the oceanic and continental domains to explain the observations. We aim to compare the younger setting of the Norwegian continental margin with the old passive margin in the South Atlantic. In particular, the South Atlantic is interesting since the southern half of the continent is elevated and the question arises how this might be linked to a deep seated thermal anomaly.
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Exploring to Depth in the Shadow of Headframes
More LessBrownfield (near-mine) mineral exploration activity is at a record high – driven by the desire to mitigate risk in these uncertain economic times and the old adage that "the best place to explore is in the shadow of a headframe.” Many companies have purchased “old” mines to gain access to mineralization that was “missed” with previous generations of geoscience technologies and to assess new targets. Today, new deep geophysical technologies are assisting in exploration, ore delineation and ground sterilization.
Historically, however, it has been difficult to apply geophysical techniques around mines. Cultural noise, scheduling, electrical noise, remoteness and resistance to new technologies are some of the traditional obstacles (to performing geophysical surveys in brownfield areas) that have been overcome. One of the new technologies that has proven itself is deep electrical imaging -- made available thanks to the distributed acquisition system (DAS) technology. DAS technologies are characterized by a large multi-channel, fixed receiver array and several other factors that together contribute to improved depth of penetration, data quality and detectability. In this paper, we review the components and capabilities of DAS, focusing on one system in particular, for brownfield work, including near-mine and minesite applications. Three case studies, two from porphyry copper environments in western Canada and a gold project from Bulgaria, are presented. These case studies represent the state-of-the art in geophysics for brownfield work and are a novel application for current DAS technologies.
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Gravi-magnetic Anomalies of Uniform thin Polygonal Sheets
Authors Horst Holstein, Des Fitzgerald and Costas AnastasiadesThin planar sheets are useful gravitational and magnetic models of dykes and veins treated as two-dimensional limit cannot be reached this way on account of the floating point finite precision. We derive the analytical zero thickness for the gravity potential while maintaining finite total mass. We use the concept of gravi-magnetic similarity to extend the thin-sheet potential formula to include the potential, field and field gradient in both gravity and megnetic cases, thereby generalising other studies that have obtained isolated polygonal thin-sheet anomaly solutions. We compare the anomalies computed by the new formulae to those of corresponding finite thickness targets, and to the finite differences estimates of the field and field gradient obtained from numerically differentiated thin-sheet potentials. In both cases a second order rate of approach to the limit is observed, verifying the correctness of the new formulae. Thin-sheet solutions are attractive for their reduced computational burden compared to full parallelepiped solutions, while the stacking of thin sheets may be used to stimulate variable density or magnetization targets. It is anticipated than thin-sheet solutions presented here will find application in gravi-magnetic modelling.
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Shale Gas: an Unconventional Hydrocarbon Resource in South Africa? Some Preliminary Observations
Authors Brian Horsfield, Hans-Martin Schulz and Maarten de WitGas shales are currently amongst the hottest plays in the United States as a result of high gas prices, and the remarkable technological successes exploiting the Barnett Shale of the Fort Worth Basin. Furthermore, viable gas shales most likely occur in many developed basins where an underutilized distribution infrastructure exists and markets are readily accessible. Annual natural gas production from shale gas reservoirs in the US is approximately 1.0 Tcf and comes from more than 40,000 shale gas wells (around 6-10% of total natural-gas production in the US today; numbers vary according to reference). While high energy prices and advances in fracturing technology have enabled shale gas production to become a lucrative reality, it is the prediction of gas concentration, partition behaviour and rock properties ahead of drilling that is of paramount importance for reducing risk and identifying “sweet spots” or fairways. In other words, there is still a lot to learn scientifically about how shale gas systems actually work. Significant advances have been made over the last five years, but there are great opportunities for major advances in science and technology. No commercial shale-gas enterprises are currently known from outside of North America. Estimations of more than 16,000 Tcf worldwide shale-gas resources are indications that a tremendous potential exists for future growth. Approximately 500 Tcf is forecast for Europe, and targets in Europe include the Posidonia Shale (Jurassic), Alum Shale (Cambrian), Wealden (Cretaceous), Mikulov Formation (Jurassic) and Riga Formation (Silurian). About 200 Tcf shale gas potential has been forecast for sub-Saharan Africa. As far as South Africa is concerned, and while numerous black shale-containing formations occur throughout the stratigraphic column, the Whitehill and Prince Albert Formations and its equivalents (Permian) stand out as perhaps the prime candidate for exploitation. It consists of dark, carbonaceous lacustrine shale overlain in part by siliciclastic turbidites and tuffs, and was deposited in a sea-level highstand under strongly anoxic redox conditions (Faure and Cole, 1999; Summons et al., 2008). While its organic richness is beyond doubt, thermal maturity will likely play the critical role in distinguishing gas shale versus oil shale potentials. By analogy with the Barnett Shale and Marcellus Shale plays of the United States, advanced thermal maturity levels will be prerequisite in order to achieve high gas-in-place and enable high gas flow rates to be maintained. The role played by the Karoo basalts in defining maturation history is of paramount importance. Extremely high maturity levels (Rm = 2-4%) have already been documented in cores of the Whitehill Formation, and lateral continuity demonstrated by MT surveys (Branch et al., 2007 Inkaba yeAfrica Special Volume). The current contribution will outline the attributes of gas shales in the United States, and explore the potential targets in South Africa, most notably the Whitehill and Prince Albert Formations.
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Wavelet-based Semblance Analysis Applied to Geophysical Borehole Data
Authors Celeste Johnson, Susan Webb, Gordon Cooper and Fabio FrescuraWavelet-based semblance analysis is a relatively new measure which allows for the similarity investigation of local phase relationships between two data sets as a function of frequency (or wavenumber) and time (or distance). The potential for applying this method to geophysical datasets is explored, with quantitative results for two cases. The first case is where two types of data come from the same section of a borehole, and the second case where data comes from similar stratigraphic locations but from different boreholes. Source data comprises density and magnetic susceptibility data from the Bellevue (BV-1) and Moordkopje (MO-1) boreholes located within the Northern Lobe of the Bushveld Complex.
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Fresh Geoscientific Data from the Natal Valley and Mozambique Ridge
Authors Wilfried Jokat, V. Leinweber, K. Kopsch and M.K. WatkeysAlthough the broad break-up sequence of Gondwana around southern Africa during the Jurassic and Cretaceous is understood, the refit of Africa and Antarctica remains unclear. This is due to uncertainty regarding the areas both onshore and offshore of SE Africa that are underlain by oceanic crust as well as the ages of that oceanic crust. An example is the enigmatic Mozambique Ridge that has been variously proposed to be either continental or oceanic in origin (or both). In order to help resolve these issues, during 2009 a marine geophysical survey was undertaken over the Natal Valley and the Mozambique Ridge. This cruise collected magnetic and gravity data, together with swath bathymetry and echo-sounder data. The results revealed that the magnetic anomalies of the southern Natal Valley are consistent with earlier work whereas the Northern Natal Valley is more complex than previously suspected. The Mozambique Ridge contains striped magnetic anomalies indicative of the ridge having been formed at a mid-ocean ridge.
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The SAMTEX Experiment: Overview and Preliminary Results
The Kaapvaal Craton is one of the world’s best natural laboratories for studying the lithospheric mantle given the wealth of xenolith and seismic data that exist for it. The Southern African Magnetotelluric Experiment (SAMTEX) was launched to complement these databases and provide further constraints on physical parameters and conditions by obtaining information about electrical conductivity structures within the lithosphere. Initially, magnetotelluric data acquisition was planned on profiles spatially coincident with the Kaapvaal Seismic Experiment. However with seven more partners joining the original four through the course of the experiment, SAMTEX was enlarged from two to four phases of acquisition, and extended northwards to cover much of Botswana and Namibia. The complete SAMTEX dataset now comprises MT data from over 730 distinct locations in an area of over one million square kilometres, making SAMTEX the largest regional-scale MT experiment conducted to date. Preliminary images of electrical resistivity and electrical resistivity anisotropy at 100 km and 200 km, constructed through approximate one-dimensional methods, map resistive regions spatially correlated with the Kaapvaal, Zimbabwe and Angola Cratons, and more conductive regions spatially associated with the neighbouring mobile belts and the Rehoboth Terrane. Known diamondiferous kimberlites occur primarily on the boundaries between the resistive or isotropic regions and conductive or anisotropic regions.
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Correlation of Lithospheric Velocity and Electrical Conductivity for Southern Africa
Authors Alan G. Jones, Stewart Fishwick, Rob L. Evans and The SAMTEX TeamSouthern Africa is the world’s premier location for studying the Sub-Continental Lithospheric Mantle (SCLM) given the abundance of geophysical and geochemical data that now exist for it. In particular, the Southern African Seismic Experiment (SASE) and the Southern African Magnetotelluric Experiment (SAMTEX) have added a wealth of seismological and electrical data that can be explored for physical property information and compared and contrasted both qualitatively and quantitatively. Qualitatively there is significant spatial correlation between low velocity and low resistivity regions and between high velocity and high resistivity regions. Adopting a quadratic relationship between shear-wave velocity and resistivity, based on mineral physics arguments, and predicting the velocity from the observed resistivity shows that the two are compatible except for two distinct regions. This relationship requires that resistivity be controlled by bulk property effects, particularly temperature variation, and not by minor conductive phases.
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SAMTEX – Quo Vadimus?
More LessThe SAMTEX project is now in its interpretation phase after completing four large phases of acquisition. Broadband and long period magnetotelluric data have been acquired at over 730 locations in an area in excess of one million square kilometres, making it the largest survey of its kind to date. Coverage by SAMTEX is of almost all of Botswana, most of Namibia, and much of South Africa. SAMTEX has demonstrated the utility of regional-scale magnetotellurics for lithospheric imaging, so the natural question to ask is “Where next?” This presentation is one with audience participation to define outstanding problems in Southern and Central African tectonics that can be addressed using this unique geological mapping tool.
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Source Mechanisms of Mining-related Seismic Events in the Far West Rand, South Africa
Authors B.B. Kassa, J. Julià, A.A. Nyblade and R.J. DurrheimThe Far West Rand mining district is known for its deep gold mines and the associated high levels of seismic activity. Thousands of tremors are recorded every day by geophones installed in the mine workings. These events range in local magnitude ML from -1 to 4. Some of the biggest events (ML>3) are external to the network deployed on a single mine. We combined data from three mines (Savuka, Tautona and Mponeng) to improve the location of these events. Source mechanisms were calculated by minimizing the L2 norm of the difference between the observed and predicted P, SV and SH spectral amplitudes, with visually assigned polarities. The preliminary results from the Savuka mine show that the major principal stress is compressional, oriented near to the vertical, and with a significant isotropic component in the moment tensor solution.
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Magnetotelluric Imaging Across a Neoproterozoic Collision Zone: Damara Belt and Surrounding Tectonic Blocks
The Southern African Magnetotelluric Experiment (SAMTEX) is the largest ever land-based magnetotelluric (MT) project. The main objective of the project is to define the geo-electric structure across the region in order to gain a better understanding of Archean and Proterozoic tectonic processes. Only the MT profiles crossing the Rehoboth Terrane, the Neoproterozoic Ghanzi-Chobe/Damara belts (collectively termed the DMB) and the southern Angola craton are the focus of this study. One of the ways in which geo-electrical structural information is obtained is by detailed analysis of the measured impedance tensor. The Groom and Bailey decomposition technique was applied to the MT data and indicates significant depth and along-profile variations in geo-electric strike and dimensionality on all transects crossing these three tectonic units (i.e. Rehoboth Terrane, Angola craton and the DMB). The geo-electric strikes are generally parallel to the north-east trending tectonic fabric as inferred from the magnetic data, but the significant strike variations with depth are expressions of heterogeneity in the lithospheric structure. The Rehoboth terrane, south of the DMB, exhibits a strongly one dimensional (1D) to moderate two dimensional (2D) structure, with preferred strike directions in the range 200-450 20for the crust-mantle period (i.e. depth) range, indicating little crust-mantle decoupling. The DMB appears to be moderately 2D at lower crustal and upper mantle depths (10-100 s) with no consistent/preferred strike direction and significant phase differences between the conductive and resistive directions. North of the DMB and into the Angola craton there are significant variations in geo-electric strike direction and dimensionality at most sites for lower-crustal and upper mantle lithosphere. Our results further indicate that the profiles have to be divided into smaller areas having similar strike directions to allow for 2D modelling and inversion.
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New Developments in Probabilistic Seismic Hazard Analysis: Common Problems and their Resolution
More LessThe purpose of this short paper is to highlight and discuss several frequent problems which one faces in Probabilistic Seismic Hazard Analysis (PSHA). The potential difficulties are:
- how to take into account the incompleteness of seismic event catalogues,
- how to handle the uncertainties within the seismic event catalogues,
- how to take into account the inadequacies inherent in the selected model of seismicity,
- how to delineate seismogenic source zones.
The lecture attempts to address the above problems, to provide tools for their solution and to present a coherent methodology for efficient assessment of PSHA.
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Biomarker and Stable Carbon Isotope Analyses of Sedimentary Organic Matter from Lake Tswaing: Evidence for Deglacial Wetness and Early Holocene Drought from South Africa
Authors I. Kristen, H. Wilkes, A. Vieth, K.-G. Zink, B. Plessen, H. Oberhänsli, F. Schmidt and T. C. PartridgeThe 1.13-km-wide crater Tswaing (25°24’30’’ S, 28°04’59’’ E) is located approximately 40 km northwest of Pretoria at an altitude of 1045 m above sea level. The crater was formed in granitic bedrock by a meteorite impact. Sediments from Lake Tswaing document hydrological changes in southern Africa over the last 200 Ka. In this study we investigate modern plant and lake water as well as modern and ancient sediment samples from the Tswaing Crater in South Africa using biomarker and stable carbon isotope analyses on bulk OM and specific biomarker compounds. The characteristic molecular markers for higher land plants (predominantly C3-type deciduous angiosperms) in Lake Tswaing are long-chain n-alkanes (n-C27-33), n-alkanols (n-C28+30), stigmasterol, ß-sitosterol, ß-amyrin, α-amyrin and lupeol. The C17 n-alkane, tetrahymanol, gammaceran-3-one and C29 sterols dominate autochthonously produced OM. By comparing carbon isotope signatures of bulk OM and the characteristic biomarkers, we trace the modern carbon cycle in the crater environment and find indications for methanogenic activity in the lake from isotopically depleted moretene. A comparative study of core sediments reveals changes in the terrestrial (C3 vs. C4) and aquatic bioproductivity and allows insights into the variability of the carbon cycle under the influence of changing climatic conditions for the time from the end of the last glacial (Termination I) to the late Holocene, c. 14,000–2,000 calibrated years before present (yr BP). The most pronounced changes occur in the aquatic realm at c. 10,000 yr BP and 7,500 yr BP when our results imply climate swings from more humid to more arid and to gradually more humid conditions again, which can be related to a shift in the position of the Inter-Tropical Convergence Zone or to changes in the tropical atmosphere-ocean interaction.
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Signals of Mass Redistribution Observed at the South African Geodynamic Observatory Sutherland
Authors C. Kroner, H. Pflug, S. Werth, A. Güntner, B. Creutzfeldt, P. Fourie and Ph. CharlesSince the year 2000 a superconducting gravimeter (SG) has been operating at the South African Geodynamic Ob-servatory Sutherland (SAGOS). It is one of the few instruments installed in the southern hemisphere and presently the only one of its kind on the African continent. Time-dependent gravity observations carried out with supercon-ducting gravimeters are long-term stable and of high resolution (~ 1 nm/s² in the time domain) have thus the poten-tial to provide constraints for hydrological models since they completely contain the changes in all water storage components in contrast to hydrological point observations. Esp. in combination with satellite-derived temporal gravity field variations as from the GRACE mission they provide a powerful tool for the investigation of changes in continental water storage. SAGOS is located on a hill in the Karoo, a semi-arid area with an average annual precipitation in the range of 200 to 400 mm. A seasonal effect on gravity related to local hydrological variations is clearly seen in the SG record. It is in the or-der of magnitude of several nm/s². Depending on the hydrological model the large-scale hydrological impact on gravity amounts to 10 – 20 nm/s² with or without a pronounced seasonal variation. Substantial differences exist between both, the amplitudes and the features of the gravity changes derived from global hydrological models. The same applies to the observed hydrology-related gravity changes and the model-based gravity variations. The results of the recently initiated investigations will contribute to an improved local and regional quantification of hydro-logical dynamics and thus establish additional means for the management of the valuable resource water.
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Soil Water Saturation in the Cathedral Peak VI Catchment, KwaZulu-Natal
Authors B.T. Kuenene, C.W. van Huyssteen and M. HensleyDegree of water saturation and water storage capacity has been studied in the Cathedral Peak VI catchment in South Africa. Soil water content data for the Cathedral Peak VI collected since 1990 in various Water Research Commission projects were used to calculate the annual degree of water saturation (s) above 0.78 of porosity (ADs>0.78) for the soils of the catchment. The ADs>0.78 value was used to determine the soil water regime of soils of the catchment. The degree of water saturation (s) was found to be heterogeneous downslope in the catchment, controlled primarily by topography. Soils in the mid and upper slopes had a dry water regime while soils in the toe slope had a wetter water regime. The soil water storage capacity of the catchment was estimated to be approximately 248 583 m3, using rainfall and streamflow data. Results reported here increases understanding of the hydrological characteristics of soils in the Cathedral Peak VI catchment and provide key information on the role of soils in catchment hydrology.
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Seismic Structure of Southern Africa: New Constraints from Surface Waves
Authors Sergei Lebedev and Joanne AdamContinental-scale seismic tomography shows the lithospheric keels beneath the cratons of Africa as pronounced high-velocity anomalies, bottoming at depths of up to about 200 km. A pronounced sub-cratonic low-velocity zone is observed only in Tanzania, where it is neighboured by volcanic areas and may be showing hot material—possibly of mantle-plume origin—trapped or spreading beneath the thick cratonic lithosphere. Regional-scale, surface-wave array analysis in South Africa shows variations in lithospheric structure across the Kaapvaal Craton and the Limpopo Belt. A reduction in S-velocity at the bottom of the lithosphere (150-200 km depths) is required by the data both beneath the Kaapvaal Craton and the Limpopo Belt, being less pronounced beneath the latter.
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Amplitude Effects of Magnetic Signals on Depth Estimation Routines
More LessIncreased acquisition of geophysical data in conjunction with limited geological information places greater importance on accurate geophysical processing and interpretation methodologies. There are numerous semi-automated data processing routines presently being implemented which specialize in location, geometry, and depth of potential source bodies. Most routines rely on the innate relationship that magnetic signal frequency is a function of a magnetic source’s depth and geometry. An additional relationship that is often not addressed is amplitude, which is a function of a source’s magnetization. Most analytic routines incorporate some sort of mathematical entity in their fundamental equations to address magnetization effects. However, there are associated limitations of these analytic routines that must be clearly defined to accurately interpret the calculated results. The mathematical expression of tilt-angle has recently been developed into a depth-estimation routine, known as “tilt-depth”. It has been shown to work reliably on sources that generate non-complex anomalies, which include a non-dipping body under the influence of a vertical magnetic field vector. It is important to consider how depth estimations will be effected when the amplitude of the source magnetization varies. Tilt-depth solutions become more inaccurate once the source approaches the surface and increases in magnetization. This is due to the creation of more complex signal geometry and reiterates the premise that the limitations of any of depth estimation routines must be considered and mitigated.
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Investigation of the Behaviour of Platinum-group Minerals and Base Metals Sulphides During Flotation of UG2 Ore
Authors Thelma B. Lekgetho, Deshenthree Chetty and Marian TredouxPlatinum-group element (PGE) mining in South Africa originally concentrated on the Merensky reef (MR) in the western limb of the Bushveld igneous complex. However, most of the mineable MR ore in the western limb has been extracted and therefore the mines are now focussing on the Upper Group 2 chromitite (UG2) ore. The increased PGE production from the UG2 ore poses challenges to the beneficiation process because of the ore’s high chromite content and smaller (2 μm to 30 μm) platinum-group mineral (PGM) size as compared to the fairly large (10 μm to 200 μm) grains in the MR. Traditionally, PGMs have been recovered from the MR ore by sulphide flotation and this sulphide flotation is still used for the UG2 ore with continuing improvements to the circuit. Due to the uncertainty of the mode of occurrence of the PGM in the UG2 ore, this research project is using mineralogy to optimise the PGE beneficiation. We have investigated the behaviour of the PGMs and base metal sulphides (BMS) during flotation, as well as looked at whether flotation of the BMS can be used as a proxy to predict the flotation behaviour of the PGM. The work is ongoing. This investigation is being done on a primary circuit of a UG2 concentrator plant. The mineralogy of the primary feed, various concentrator cells and tailings was analysed using a mineral liberation analyser (utilizing energy dispersive X-ray fluorescence (XRF)) and X-ray diffraction for mineralogical information, and wavelength dispersive XRF for chemical assays of the bulk samples. To date, it appears that 75% of the total PGMs fed into the flotation cells is in a liberated form, while 16% is associated with the BMS and 9% with the gangue (silicates). The results show that both BMS and PGMs respond in a similar way to sulphide flotation, but behave differently in some ways, because of differences in their grain size distributions.
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The Elusive Flying Squids: Interim Results from Recent Tests with a Heli-borne EM Low Temperature SQUID System.
Authors C. L Le Roux, Ronny Stolz, Braam Du Plooy and Jaco SmitFollowing the positive outcomes and successful application of Low Temperature SQUIDs (LTS) as ground TEM B-field sensors, a research and development program was launched to try and realize similar benefits in airborne EM applications. The AeroTEM III helicopter-slung bird had been selected as the prototype platform for the integration of the IPHT / Supracon LTS EM SQUID sensor technology with a new generation experimental airborne EM system. A number of ground-based and two airborne flight tests were conducted, first in December 2007 with a 100% duty cycle Spectrem (“on-time”) waveform and then in September 2008 with an AeroTEM triangular “off-time” waveform.
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Revealing the Beattie Magnetic Anomaly and the Anatomy of the Crust of Southernmost Africa: Geophysics and Deep Sub-surface Geology Where the Cape Fold Belt and Karroo Basin Meet
Authors A. S. Lindeque and M.J. de WitThe deep crust of the southernmost margin of Africa contains unresolved tectonic features such as the Paleozoic Cape Fold Belt (CFB), the Paleozoic-Mesozoic Karroo Basin and the largest terrestrial magnetic anomaly, the Beattie Magnetic Anomaly (BMA). Without resolving these structures, our understanding of the evolution of the southern margin will be incomplete and limited. Under the auspices of the Inkaba yeAfrica framework, several geophysical datasets were acquired from 2004 to 2007, along two transects across the margin and its unique tectonic features. This research presents a tectonic model and crustal geometry, at the centre 100 km of the western transect. The model is derived from the joint interpretation of: surface geology, aeromagnetic data, nearby deep boreholes, teleseismic receiver functions, impedance spectroscopy measurements on borehole samples, near vertical reflection seismic data (NVR), shallow P- and S-wave velocity data, wide angle refraction data and magnetotelluric data. The model differentiates a four component ~ 42 to 45 km thick crust and constrains the two part BMA to a ~10 to 12 km wide northern zone, and a ~5 to 7 km wide southern zone, both at ~7 to 8 km below surface, continuing for a depth of ~5 km, and, contained in the Mesoproterozoic Namaqua-Natal mid-crust. The BMA source is interpreted to be a Namaqua-like massive to disseminated, deformed/metamorphosed strataform sulphide ore body. The model presents evidence in support of a thin-skinned tectonic thrust model for the evolution of the flat-based CFB and shows no significant fore-deep stratigraphic thickening in the Karroo Basin towards the CFB front. The tectonic model suggests a Meso-Proterozoic collision orogen setting, overprinted by a mid-Phanerozoic thin-skinned fold and thrust belt coupled to the far-field accretion/subduction margin to the south.
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Kalahari Borehole Stratigraphy from the Tsodilo Hills Area, NW Botswana: Clues to Understanding Uplift and Climate History of the Kalahari Plateau
Authors B. Linol, M. J. de Wit, F. Guillocheau and J. BruchsWe report analysed stratigraphic records across the central Kalahari highlands, including facies analysis and logging of a number of boreholes near the Tsodilo Hills of NW Botswana, which track the geological changes associated with the formation of the Kalahari Plateau. The study of these condensed sedimentary sequences from the interior of this plateau reveal that the evolution of its palaeo-environments are linked to both regional and local tectonic and climate changes, e.g. those related to the plateau’s episodic vertical movements, the opening of new oceans and continental margins flanking the plateau, and associated magmatism. Dating these geological events derived from the preserved stratigraphy is one of the main objectives of our ongoing drilling program. Reconstructions of the sedimentary architectures on profiles between boreholes, and correlations at more regional scales, are based on bio- and litho-stratigraphy. The total succession (100 to 200m thick) comprises relics of Karoo sequences that include diamictites, turbidites and deltaic deposits overlying high grade gneisses. This lower succession records significant tectonic, climate and topography changes from the carboniferous Dwyka period (350-300 Ma) to the Jurassic Stormberg magmatism (180 Ma). These successions are variably altered and intruded by Late Cretaceous (100-70 Ma) kimberlite deposits. Overlying sediments comprise a thick Kalahari sequence of carbonates and sands, usually interpreted as calcretes and aeolian deposits, respectively. In sub-surface, the carbonates are extensive and vary in thickness from 30 to 60m. These appear to be related to lacustrine processes as recorded in various biomarkers (no fossils are visible), and pedogenesis. Carbon and oxygen isotopes of the sediments provide distinct trends that will be completed by the study of a stalagmite from caves across southern Africa to test for high resolution signatures of climate changes. The uppermost unconsolidated sand-silt successions are difficult to sample by conventional drilling. New techniques will be tested later this year. Our first tests reveal sequences include muddy and carbonated intervals with sufficient plant remains to date wet-dry episodes to reconstruct relatively recent climatic variations.
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Advances in airborne EM: Introducing ZTEM
Authors Bob Lo, Jean Legault, Petr Kuzmin and Keith FiskA series of demonstration tests were conducted using the Z-TEM, airborne AFMAG system over deep targets in the Athabasca Basin of Saskatchewan, Canada. These tests were conducted in mid-2008 and were flown to test Z-TEM’s ability to detect large conductive targets at depth; deeper than conventional airborne EM methods. Data are presented over areas where the conductors are located 450-600 metres beneath the surface. As well, a case of Z-TEM following the plunge of a conductor to over 800 metres depth is shown
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Reservoir Characterization of Wells KD1, KE1, KF1, KH1, in Block 3, Orange Basin, Offshore South Africa
Authors Donovan Lombard and A. AkinluaPetroleum reservoir characterization can be described as a process that includes integrating, analyzing and understanding all available data from the well. Understanding the reservoir characteristics plays a pivotal role for the optimization towards the well performance. The two fundamental properties that govern the quality of reservoir rocks are porosity and permeability. Clay minerals in contrast, have a major influence on reservoir quality. Clay minerals occlude pore spaces, coats grain surfaces, and swells in the presence of water. Thus petroleum drilling operations and reservoir management also becomes affected. The main aim of this study is to provide a clear understanding and description on reservoir heterogeneities of four wells (i.e. KD1, KE1, KF1, and KH1) allocated within block 3 of the Orange Basin. The objective of this study are 1) to quantify the effect of clay minerals on reservoir quality (RQ), 2) to identify and characterize potential reservoir zones, 3) evaluate the reservoir properties 4) use of graphical techniques 5) implications on production of hydrocarbons. The objectives of this study will be achieved and accomplished by using wireline logs, core data, X-ray diffraction (XRD), thin-sections and scanning electron microscope (SEM) analyses.
Expected outcomes of this study includes, tight gas reservoir sands, fair to relatively high reservoir quality rocks, high net to gross ratios, variable poroperm values, increasing clay content with burial depth that influences RQ, high quartz cementation and diagenetic processes controlling RQ.
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Neutron Tomography of Geological Samples – Preliminary Results.
Authors L. Longridge, F. C. de Beer, L. Coney, P. Ogilvie and S. WebbNeutron tomographic imaging has been conducted on a variety of geological samples, including migmatitic metapelite, suevitic breccia, as well as mafic granulite, in order to investigate the success of this technique in non-destructively distinguishing their component minerals in three-dimensions. Preliminary results indicate that coarse textures found in these samples are clearly resolved, but μm-scale resolution of mineral distribution is not yet possible. The variable composition of minerals in geological samples due to solid solution results in overlapping attenuation coefficients for some minerals, and these minerals may be indistinguishable using neutron tomography. Hydrous minerals and voids present in samples are well resolved, and volumes of component phases in samples can be easily calculated, provided these phases are resolvable.
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The Integration of Borehole Geophysical logs for Geotechnical Risk Assessment at the Paardekraal 13-Level Ventilation Shaft Project
Authors Bathandwa Mabedla and Kazek TrofimczykDownhole wireline logging and borehole radar surveys were undertaken at Paardekraal in two shaft geotechnical boreholes for the planning and design of ventilation shafts for underground platinum mining. Paardekraal is one of production areas of Anglo Platinum’s Rustenburg Section, Rustenburg Platinum Mines Limited, on the Western lobe of the Bushveld Complex. The horizons mined for platinum group metals are the Merensky Reef (MR) and the UG2. Old MR support pillars are targeted to provide stable ground conditions for a vertical shaft to access to the UG2 horizon, which lies stratigraphically below the mined out MR. Studies show that most structures (jointing) in the Bushveld Complex are steep-dipping and could pose a threat to mining operations even over short distances. Mapping of structures intersecting the borehole, their location and their orientation is used to assist in highlighting zones of potential hazard and to characterize rock formations. Key geophysical tools were deployed in the Paardekraal geotechnical boreholes to complement standard drill core geotechnical analyses. The acoustic televiewer records high resolution (2 mm pixel), oriented images of the full circumference of the borehole sidewall and maps in detail the location and orientation of structural features intersecting the borehole. Density and sonic velocity logs provide information on rock strength and competence. Flowmeter, differential temperature, neutron and fluid conductivity logs identify zones of possible ground water ingress (fluid pathways). A method for integrating the structural and geotechnical information, interpreted from the geophysical logs, into a single, visual log that highlights hazardous geotechnical zones was applied. This so-called Hazard Index (HI) is a weighted combination of the following parameters, namely: intact rock strength (IRS), shear wave slowness (SWAV), fracture frequency (FRAC), joint intersection rating (INTS) (based on fracture tilt combination), water ingress (FLOW) and fracture density projected within the shaft barrel (FDEN). Borehole radar in reflection mode penetrates the formations around the borehole to distances of up to 50 m (and sometimes further). This gives information about the condition of the rock mass surrounding the borehole and the lateral extension and continuity of major structural features and dykes. Critically, the radar data was analysed for information on the location of old MR workings with respect to the proposed position of the shaft excavations and the integrity of the MR pillar surrounding the shafts.
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Aspects of Palaeo-terrace Formation and Stream Sediment Petrography of the Orange and Vaal River Confluence in the Douglas Area
Authors N. MacDonald, J.H.A. Clark, P.G. Holmes and C. GauertThe mineralogy and geochemistry of a representative number of stream sediments in combination with geomorphologic mapping of the Orange/Vaal River terraces is used to assess the provenance of the sediments and the diamond-bearing potential of the terrace material in the confluence area around Douglas, Northern Cape. Utilizing sediment profile descriptions, grain size analyses, heavy mineral separation and identification techniques we were able to distinguish sediment provenance groups at different sites in the Orange/Vaal confluence area near Douglas. First assessment of the rock fragment/mineral spectrum reveals Ventersdorp volcanics material as well as Karoo material to be predominant provenances of these sediments, and to a lesser extent glacial material derived from Dwyka series to be the source of sediments; based on the frequency of diggings, the possible diamond-bearing potential appears to be by far higher in the terraces of the lower Vaal River and along the Orange River downstream Douglas than along the Orange River section between Hopetown and Douglas. Looking specifically at the heavy minerals further studies will attempt to correlate the terraces. Also, relative ages will be determined by looking at the post-depositional alteration or weathering. First quantification of the (heavy) mineral content displays a large contribution of quartz, epidote, magnetite, and amphibole grains from Karoo sediment and volcanics, whereas altered granite and pegmatite fragments of unknown origin form the lesser part of the sediment spectrum. At Bucklands, on both sides of the confluence point, relatively large estimated gravel thicknesses (>5m) are overlain by several cycles of up to 3 m thick mud to silty sediments with occasional larger pebble to gravel occurrence, both part of the Rietputs and Riverton Formations. Orange and Vaal River sediments are sorted selectively by hydraulic processes causing the sorting of according to size, shape and density, concentrating heavy minerals in trap sites such as potholes, along bars, and in areas of turbulent water flow. Therefore river bars and slip-off slopes in an area of moderate relief appear to be prime loci of heavy mineral deposition. Principally it can be concluded that the Orange River section between Hopetown and Douglas shows less pronounced younger gravel terrace formation due to a relatively low erosion basis and a stronger incision into the Ventersdorp bedrocks; the lower Vaal River and the Orange River downstream Douglas however have a more pronounced terrace formation due to a higher erosion basis caused by bedrock morphology.
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Magnetotelluric Experiment Across Southern Barberton Greenstone Belt: a Students’ Perspective of the Field Work
Authors S. A. MacLennan, L. Moodley, U. Weckmann, O. Ritter, A. Nube, X. Chen, J. Deacon, G. Willkommen, R. Barth, R. Streich, K. Tietze and M. J. de WitThe 3.2-3.5 Ga Baberton Greenstone Belt contains some of the planets oldest structural lineaments, shear zones and tectonic melanges that may represent surface features of sutures zones. A magnetotelluric survey was conducted in April 2009 across the NE structural grain of the greenstone belt to image at depth one of the most prominent sutures in this area, the Saddleback- Inyoka Fault Zone(SIFZ). This will provide a key set of observations with which to unravel Archean tectonics. The MT field campaign took place over 6 weeks with a stringent timetable in effect throughout the campaign in order to complete the planned survey. Each team would typically check one MT station, pack one up and set a new station up in the next location every day. In the evenings the data was processed and its quality was scrutinised. The field campaign required a great deal of planning, negotiation and physical labour in order to make it a success! To complement this geophysical data, ongoing field mapping by one of us, Scott MacLennan, is focussed on a transect across the SIFZ in the area around the Stolzberg syncline. In addition, and in order to increase the accuracy of the modelling of the magnetotelluric data, a representative rock sample set of the Baberton Greenstone Belt and beyond will be collected. Conductivity measurements will be carried out on these samples in Germany in 2010. This will complement the MT work to provide a more robust conductivity model for this area of the Kaapvaal craton. The integration of these different geological and geophysical techniques will result in a data set that could help elucidate what geodynamic processes were important in shaping this remnant of Archean crust.
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The Nickel Deposit at Bon Accord, Barberton, South Africa
Authors Fulufhelo Madala and Marian TredouxThe Barberton greenstone belt in the Mpumalanga, RSA, covers an area of some 6 000 km to the southeast of the town of Nelspruit. The Bon Accord (BA) Ni-oxide deposit is located in the southwest of the area. The deposit was initially discovered in the 1920s, but technology of that time could not smelt the extremely Ni-rich material and the ore was abandoned. It was rediscovered during regional Ni exploration by Eland resources Limited (in the 1970s). It has been estimated that about 22.5 t of material was removed in the 1920s. The host rock consists of ultramafic rocks of the 3.2 Ga Onverwacht Group of the Barberton greenstone belt. The Ni-oxide deposit forms a elliptical body about 0.5 m thick and 20 m wide along its longest axis. It is situated close to a contact between the ultramafic rocks of the Jamestown igneous suite and a siliceous mylonite zone. The BA deposit has a unique mineralogy of rare Ni-rich minerals, such as liebenbergite (olivine), trevorite (spinel), willemsite (talc), nimite (chlorite), and bonaccordite (borate). The levels of Cu and S are both low (<100 ppm), and that of Ni unusually high (>30%), which means that BA differs from other all known Ni deposits chemically. The aim of this research has been to do a thorough optical microscopic and XRD investigation of unclassified samples of the BA body and surrounding rocks, identifying all minerals in the samples and comparing them with the results obtained from the previous reports. XRF analysis was also done to determine the full suite of major elements (something that was not available before) and some selected trace elements. Owing to the unusual chemistry of the BA material, special adjustments had to be made to the ‘normal’ sample preparation techniques routinely used for making fusion beads for major element XRF analysis, to prevent the sample from reacting with the Pt crucible.
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Three-dimensional Modeling of Petroleum Generation in the Southeast Flank of the Bredasdorp Basin, off the South Coast of South Africa
Authors Lihle Madyibi, Rolando di Primio and George SmithThe aim of this study was to investigate the evolution of petroleum system in the southeast flank of the Bredasdorp Basin. In this study, we have applied a two-dimensional, later up-scaled to a three-dimensional, modeling approach in order to advance our understanding of the petroleum systems evolution in the southern flank of the Bredasdorp Basin. Prior to the modelling, mapped seismic horizons were depth converted using the VoK method. The model is based on the interpretation of 64490m two-dimensional seismic-reflection profiles, as well as the analyses of well logs and cores that constrain the sedimentary facies distribution and the units’ age. Three main source rock intervals (Barremian, Aptian and Turonian) were modeled for maturity and hydrocarbon generation. The simulation was performed using Petromod® 3D package (version 10, IES Germany). Source rock intervals were assigned properties such as Total Organic Content (TOC) data from the respective wells that intersected these intervals. Calibration data to the model included vitrinite reflectance and bottom-hole temperature (BHT). Results obtained from petroleum generation indicate that the Barremian and Aptian source rocks are mature whereas the Turonian source is immature. Modelling output suggests that the said source rocks started petroleum generation as from 80 Ma but halted about 69 to 35 Ma and generating very slow from period between 12 to 0 Ma both periods of generation owing to minimal burial depth of sediments.
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Techno-advantages of Acoustic Televiewer Logging on Shaft Boreholes
Authors Mary N. Mahlatji and Warren KrynieGeophysical borehole logging of two shaft boreholes separated by 300m was used to complement geological logging of un-oriented borehole core in a geotechnical investigation aimed at mapping rock strengths and stability in advance of shaft-sinking operators. High-angle (>60º) dip magnitude fracture systems interpreted from acoustic televiewer imagery were ubiquitous throughout both holes and showed large variations around a mean strike orientation of NW-SE, in line with borehole “breakout” events also mapped from the televiewer. Fracture depths correlates well with those mapped from geological core logging. A unique, 150m wide zone of anomalously high fracture frequency density in borehole A has been confirmed from ongoing shaft-sinking as reflecting “disturbed” ground, and points to the presence of a sub-vertical structural feature intersecting the borehole. Fracture orientations over the “disturbed” ground differ considerably from those above and below this zone. Ancillary geophysical data (density) does not uniquely define the extent of the highly fractured zone. Elsewhere, shallow temperature anomalies in both boreholes possibly reflect intersections of the same aquifer
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Palaeomagnetic Results from a Neoarchaean Dyke Swarm in the Badplaas-Barberton Area, South Africa
By L.P. MaréA palaeomagnetic study is in progress on dyke swarms in the Badplaas-Barberton area and the preliminary results are presented here. The main aims of the study are to contribute towards the debate on Neoarchaean crustal evolution, and attempt to constrain the age, tectonic setting and genesis of these mainly tholeiitic dyke swarms.
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Distinguishing Between Impact Craters and Volcanic Pipes using Ground Magnetics: Field Examples from Zimbabwe
Authors Sharad Master and David J. RobertsonImpact structures produced by meteorite or cometary impacts are generally circular features, which are accompanied by a range of diagnostic shock phenomena. Many other processes can also produce circular structures that resemble impact structures. We report on two case studies from Zimbabwe, where we investigated two circular possible impact structures, in similar host rocks, using N-S traverses with a hand-held proton precession magnetometer. The West Save structure, in eastern Zimbabwe is a ~600m diameter structure, developed in Triassic sandstones of the Karoo Supergroup. The ~220m diameter Sinamwenda structure, in western Zimbabwe, near Lake Kariba, is also developed in similar Triassic Karoo sandstones. The West Save structure contains basalt and gabbroic intrusions, and is formed by differential erosion of a more indurated sandstone rim around a volcanic pipe. At West Save a strong asymmetric jagged dipole magnetic anomaly, ranging from 30140 to 31080 nT, is interpreted as a signal produced by a vertical cylinder of intrusive rock in a volcanic vent. The Sinamwenda structure, by contrast, has no trace of volcanic or intrusive rocks, and it shows not the slightest magnetic anomaly, with a constant total magnetic field value of 31000 nT, with a variation of the magnetic intensity of only 2 nT. An overturned rim, multiply striated joint surfaces, microbrecciation, and planar deformation lamellae in quartz indicate an impact origin for the Sinamwenda structure. The lack of a magnetic anomaly rules out alternative interpretations such as kimberlitic intrusions. We conclude that ground magnetic study of circular structures can easily distinguish between structures produced by impact processes (absence of magnetic anomaly), and structures produced by volcanic intrusions (dipole magnetic anomaly), if the magnetic susceptibility of the surrounding rocks is minimal.
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