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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
1 - 100 of 157 results
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Dispersion of Surface Waves in Southern Africa from Inter-station Measurements
Authors Joanne Adam and Sergei LebedevWe measured Rayleigh and Love wave dispersion across southern Africa using a combination of cross-correlation and waveform inversion approaches. Phase-velocity averages between pairs of stations of the SASE array were obtained in period ranges of at least 5-200 s for Rayleigh and 5-50 s for Love waves. The new data confirm the thinning of the crust from the Limpopo Belt and northern Kaapvaal Craton towards the southern part of the craton, as observed perviously. A reduction in S-wave velocity at the bottom of the lithosphere at 150-200 km is required by the data both beneath the Kaapvaal Craton and the Limpopo Belt, although this reduction is less pronounced beneath the latter. The new data should also enable direct measurements of the depth of the lithosphere-asthenosphere boundary.
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Sequence Stratigraphic and Diagenetic Alterations within the Silisiclatic Reservoir Deposits of Orange Basin, Southwestern Africa Margin.
Authors Solomon A. Adekola, Akinsehinwa Akinlua and Oluwaseun A. FadipeThe Orange Basin in South Africa of South Atlantic Ocean consistently has shown poor reservoir quality. The reservoir been a silisiclastic deposit normally should have shown good to very good quality. The poor porosity and permeability characteristics of the reservoir across the five blocks in the basin necessitated this work. In finding the cause of the problems in the basin, some wells were subjected to sequence stratigraphic and diagenetic analyses. This was done using the digital wireline logs and 2D seismic sections, sidewall core and core samples acquired form Petroleum Agency of South Africa (PASA). The wireline logs were interpreted and broken to parasequence sets. The seismic section also interpreted based on unconformities, reflection truncations and terminations to obtain sequence stratigraphic framework. Core and sidewall core samples were collected within parasequence settings: Highstand HST, Lowstand LST and Transgressive system tracts to look at the effect of diagenetic alterations within the framework. The cores samples were subjected to: Thin sectioning, XRD, SEM, and EDS analyses to reveal the deposition and post deposition history of the basin. It was shown that authigenic minerals and quartz over growth is responsible for the poor reservoir quality of the reservoirs.
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GyroLAG: Versatile Multi-sensor Airborne Geophysics Platform
Authors Dr. L. Ameglio, G. Jacobs, Dr. B. de Cabissole and J. von LudwigEXIGE (www.exigesa.com) and its partner AirWatch, a joint venture between Wagtail Aviation (www.wagtail.co.za) and Alto Air (www.Alto-Air.com), developed the next generation airborne geophysical platform – GyroLAG (Gyrocopter Light Airborne Geophysics). GyroLAG is a single pilot operated, smart tailored, ultra-high resolution airborne geophysical platform, which combines the merits of dense data sampling, accurate positioning, low clearance above ground or sea level, slow speed, multiple parameter measurements, automatic data quality control and real-time quality assurance, with the benefit of being highly efficient, cost effective, commercially attractive, safe, easy to operate and reliable. The aviation platform, a Kriek type gyrocopter, is a South African designed and built machine with a license for commercial operations. It originated from 6 years of development, which are presented in more detail. The platform will undergo production in the fourth quarter of 2009. The final setup of the remote sensing and geophysical instrumentation on-board is at an early stage of implementation. GyroLAG will accommodate up to 7 distinct remote sensing technologies including a range of digital cameras covering the entire IR to UV spectrum, lightweight magnetic, radiometric and gravity sensors. Instrumentation’s agility and ruggedness have been proven during a sequence of separate survey and test conditions and their preliminary performances are illustrated. The platform’s positioning information is acquired utilising a DGPS and/or real time GPS. A notebook computer is used as digital acquisition system and proprietary software for flight preparation, data acquisition, and automatic flight data quality control against user-set technical specifications are being developed. Commercial launch of GyroLAG in its full capacity is planned for the first quarter of 2010 but it is worth pointing out that the platform in a magnetic, radiometric and/or still camera configuration is already operational.
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History of Stress at Vaalputs, Namaqualand, South Africa: Evidence for a Mid-Cretaceous “Wegener-type Orogeny” in Western Southern Africa
The paper reviews more than 20 years of structural, stratigraphic and seismic monitoring studies focused on the Vaalputs radioactive waste disposal site, 100 km SSE of Springbok, in Namaqualand. Our finds, supported by the recordings of two 3-components seismometers, show that the frequency of seismic events in this region may be slowly increasing over time, that the predicted Mmax is ~5.8, and that deformation is governed by a NNW-SSE oriented horizontal σ1, typical of an Andersonian strike-slip regime (σ1 > σv > σ3). The history and dynamics of this large scale (≥ 2x 106 Km2) stress field, known as the Wegener stress anomaly, appears to be complex. The palaeostress record suggests that a stress field comparable to the current one became established after the opening of the Atlantic, perhaps at ~102 Ma and waned at about ~72 Ma, when it was replaced by a markedly different Andersonian thrust regime (σ1 > σ2 > σ v) oriented NNE-SSW. It is uncertain when the current Wegener stress field was re-established, but some evidence points to a pre-Quaternary event. Our finds at Vaalputs are consistent with published accounts of mid-Cretaceous, NW-SE oriented crustal shortening through reverse faulting, thrusting, and folding in Namibia and also in the offshore Bredasdorp Basin. This tectonic activity locally resulted in mountain building such as the Groot and Klein Karas Mts. of southern Namibia. As such, this tectonic style is difficult to reconcile with the extensional regime of a classic (passive) “Atlantic-type” continental margin, and calls for a new approach to the way the Kalahari epeirogeny of southern Africa is perceived.
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Source Rock Potential, Palaeoenvironment & Thermal Maturity of the Lower Ecca Group, South Africa: Implications for Hydrocarbon Exploration.
Authors Andy Arnold and A. AkinluaThe Prince Albert and Collingham formations of the lower Karoo succession contain organic rich shales deposited in the extensive anoxic to dyoxic Gondwana basin during the early to middle Permian. A total of 15 samples were subjected to Rock-eval pyrolysis and X-ray florescence spectrometry for the determination of its source rock characteristics, thermal maturity and palaeoenvironments. These were then compared to 6 analyzed samples from the Agbada formation of the Niger Delta. The total organic carbon content of the Karoo shales varied between 0.07-1.30 wt%, with an average of 0.46 wt%. Rock-Eval pyrolytic yields and calculated values of hydrogen and oxygen indexes imply that organic matter type is predominantly inert kerogen (Type IV). The measured Tmax values as determined by Rock-Eval Pyrolysis range between 215oC-424oC, this corresponds to thermally immaturity. Despite the age of the samples the thermal maturity of these kerogens are generally low, therefore suggesting uplift and exposure of these sediments. This claim is further confirmed by low trace metal concentrations relative to those of the Niger Delta samples. Preliminary studies indicate the Karoo basin holds no significant hydrocarbon generating potential, apart from gas. Further geochemical studies and petroleum systems analysis may however reveal hydrocarbon accumulations within the Karoo basin.
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Petrogenesis of the False Bay Dyke Swarm
More LessThe False Bay Dyke swarm is the southern NW-SE trending end member of the Cretaceous dolerite dyke intrusions on the western African margin associated with rifting of Gondwana and opening of the Atlantic Ocean. This southern dyke swarm has been associated with a low-flux magma volume compared to its northern high-flux counterpart: the Henties Bay-Outjo dike swarm in Namibia. The contrast in basaltic magma types and magma flux between north and south has been related to different tectonic settings (i.e. magma sources) along the current coast line during Cretaceous rifting. Further, the False Bay Dyke swarm is characterised by olivine-tholeiites with quite extreme differentiation to ferro-tholeiites within a monogenetic magma system.
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Can the Spectrem Transmitter and Processing Work on a Small Helicopter Platform?
Authors John Bell and Braam du PlooyThe Spectrem waveform and processing has many advantages. The possibility of implementing this on a central-loop helicopter platform was investigated. Data was acquired in early 2008 with ground and flight tests. The amplitude of the anomalies produced by the Step Response processing are modulated by the amount of primary field present at the receiver. The smaller the primary field (larger “Bucking Factor”), the larger the anomaly amplitudes, however this makes the system more sensitive to geometric changes. The ground and flight tests suggest that the Spectrem waveform and processing is not suitable for a central loop helicopter system due to it’s sensitivity to geometric changes.
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Stochastic Modelling of the Velocity Structure: Beyond Joint Inversion Methods
Authors K. Boomer and R. BrazierThe traditional approach to modelling seismic velocity structure of the Earth with multiple data sets (such as receiver functions and surface wave dispersion curves) is to perform a joint inversion. This method produced a single solution and little information on uncertainty in the models. We explore an alternative approach enhancing the multiple objective forward modelling method with a stochastic search and optimization. A genetic algorithm is well-suited to the velocity modelling setting as it supports parallel optimization, maintains diversity in the search space, and is more likely than random search procedures to converge to the global minima.
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A Review of the Structural Geology of the Cape Fold Belt and Challenges Towards Future Research
More LessThis review focuses on our understanding of the structural and tectonic setting of the Cape Fold Belt (CFB) based on contributions made by academics and professional geologists, mainly during the last two decades. Most of the research concentrated on the identification and mapping of thrust faults in the southern arm of the CFB, as well as seismic surveys which provided data for putting forward tectonic models to account for structural and stratigraphic features of the fold belt. Thrust stacking is a common characteristic in all three stratigraphic sequences of Palaeozoic Cape Supergroup rocks. A complex pattern of ductile and brittle deformation occurs in the Table Mountain Group whereas in the overlying Bokkeveld and Witteberg Groups there is a close association of folding with development of thrust faults. Thin-skinned characteristics are prevalent in the southern arm of the CFB, but give way to thick-skinned features in the southernmost central part of the fold belt where basement rocks have been thrusted northwards towards the foreland. A variety of tectonic models proposed for the CFB have generated some controversy among researchers. Currently popular are the Andean and the strike-slip (transpression) models. The former accounts for the northward vergence of structures in the southern arm of the fold belt, but thicknesses of crustal substrate obtained from recently carried out deep sounding seismic surveys across the fold belt do not entirely corroborate this model. Structural characteristics in cover rocks that favour the transpression model are en echelon folds and faults as well as flower structures which are commonly associated with strike-slip regimes.
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Investigating the Variability of Earth Gravity Field’s J2 Spherical Harmonic Coefficient Using Satellite Laser Ranging Data
Authors M. C. Botai and W. L. CombrinckSpatial-temporal variations in the Earth’s gravity field (expressed as a set of spherical or ellipsoidal harmonic coefficients: the geo-potential model) are caused by mass redistribution within the Earth system. Accurate determination of the Earth’s gravity field is essential for a variety of geophysical applications such as oceanography, hydrology, geodesy, solid Earth science as well as being essential for precise orbit determination. As a result, improved knowledge of the variations of the Earth’s gravity field could yield improved understanding of the Earth’s system dynamics and associated long term climate change. The main objective of the current analysis is to investigate the variability of coefficients computed using a new Satellite Laser Ranging (SLR) program being developed at Hartebeesthoek Radio Astronomy Observatory (HartRAO). In the analysis program, Earth tide, ocean tide, pole tide and atmospheric loading models are included in SLR station position adjustment in order to improve calculated range and therefore minimise the observed minus computed (O-C) residuals. The time series for lower degree and order harmonics has been recovered utilising a priori J2 coefficients based on the GRACE gravity model GGM03C. We present results of J2 coefficients computed from about 20 SLR stations tracking LAGEOS-1 and LAGEOS-2. In addition we estimate a known secular decrease in J2. The ability to estimate gravity coefficients will enable further investigations into seasonal annual variations and other longer term inter-decadal and decadal variations which can be linked to global mass redistributions.
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Assessing the Degree of Synchronization Between Geophysical Records Using the Method of Instantaneous Phase Differences
Authors O. J. Botai, W. L. Combrinck and V. SivakumarRecovering geodetic parameters such as tropospheric delay and geodetic site positions and monitoring their variations in time has important applications for studying the processes of the dynamic Earth. Geodetic position time series exhibit non-linear motions that are associated with seasonal signals caused by loading effects, and seismic deformation processes such as earthquakes etc. This implies that the fluctuations in the station coordinates and tropospheric parameters could be synchronized because they are driven by common underlying processes. In the analysis of space geodetic measurements, space geodetic solutions are often co-assessed in order to determine geophysical signals present in both parameters. The main objective presented in the current contribution is to determine the linkage between temporal structure of the zenith tropospheric delay and the geodetic station height coordinates fluctuations in the time-frequency-energy space. The temporal structures of the combined solution of the zenith tropospheric wet delay (ZWD) and the geodetic station height at the Hartebeesthoek geodetic station (HartRAO) have been studied. The oscillation patterns in these geophysical signals have been analysed by using the noise-assisted data analysis (NADA) methodology known as ensemble empirical mode decomposition (EEMD). The instantaneous phase differences among the associated modes of the intrinsic mode functions (IMFs) have been computed and used to assess the degree of synchronisation between the two series. Our results show that the ZWD and the HartRAO geodetic station height show modes that are temporally correlated and some of the IMF modes exhibit temporal structures that can be associated with both local and global forcing mechanisms.
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Update on Laser Development for Lunar Laser Ranging in South Africa
Authors R.C. Botha, C. Bollig and W.L. CombrinckA new fundamental space geodetic observatory for South Africa has been proposed. Lunar Laser Ranging (LLR) is one of the space geodetic techniques to be hosted on-site. LLR requires a pulsed laser that can operate at 100 to 400 mJ energy per ~ 20 pico-second pulses, at a pulse repetition frequency in the kHz region. It must also have excellent beam quality at M2 < 1.1. Since no commercial laser matching the exact specifications could be obtained, it was decided to develop such a laser.
The most feasible candidate for such laser parameters was found to be a crystal-based diode pumped system. Several measures must be adhered to in order to obtain the required results: end-pumping will reduce losses and therefore minimise excess heat, a very good overlap between the pump and laser beams will further reduce losses, thermal lens effects must be kept to a minimum and corrections for astigmatism must be incorporated, to name a few.
Nd:YLF was identified as a suitable candidate for the gain medium crystal. 4 such crystals were used within a single resonator to multiply the output power. The setup was also built in such a way that the astigmatism of 2 crystals overlapped at right angles with the other 2 crystals’. This minimised the effect of the overall astigmatism. The total output power of this initial continuous wave laser was 87 W at 1053 nm, less than the anticipated 100 W. A revised setup with smaller pump beam diameter is underway.
A 4 crystal Nd:YLF laser still seems like the best candidate to achieve the required output for LLR. Future work includes a redesign of the resonator, pulsing the laser and frequency doubling it to green.
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Matjiesfontein: A possible Future for Space Geodesy in South Africa
Authors R.C. Botha and W.L. CombrinckSpace Geodesy provides many important products. These products are applicable in numerous scientific, industrial and commercial applications. The current hub for Space Geodetic activities is the Space Geodesy Programme at the Hartebeesthoek Radio Astronomy Observatory. A new fundamental space geodetic observatory for South Africa has however been proposed and various workshops were held to date in this regard. The main drivers for a new station are the ageing and subsequent breakdown of current equipment as well as deteriorating atmospheric seeing conditions at HartRAO. Current equipment that still functions also does not provide the resolution and sensitivity needed by modern techniques. Initial surveys highlighted Matjiesfontein as a very good option in terms of numerous factors. These factors will be discussed. The actual site is located about 4 km south of the village in a shallow valley in the mountains and is ideal to become a new fudicial site for Africa. A GPS Reference station as well as a weather station has already been installed on this site. Current equipment at the Matjiesfontein site as well as planned equipment and infrastructure will be discussed.
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Distribution and Origin of Natural Gas Leakage Features on Block 2 of the Orange Basin as Interpreted from 2D Seismic Reflection Data Off-shore the South African Margin.
Authors Donna Boyd, Z. Anka, R.di Primio, Maarten de Wit and Jeff AldrichWe have interpreted more than 300 2D seismic-reflection profiles located across the Orange Basin, offshore the western South Africa margin. The dataset covers an area of aprox. 18750 km2 which comprises the exploration Block 2. The main goal of this work was to analyse the occurrence of natural gases within the sedimentary column and the distribution of gas leakage features in relation to tectonic and sedimentary structures, in order to assess quantitatively the hydrocarbon generation, migration and seepage dynamics through geologic time in the basin. A long term aim is to integrate the results into climate change models as thermogenic methane is a strong greenhouse gas and such natural emissions may play a role in controlling climate evolution.
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Faulting of the Witteberg Group Rocks, Steytlerville, Eastern Cape.
Authors G. Brunsdon and P.W.K. BoothA structural study of Witteberg Group Rocks was conducted along the Soutkloof River, approximately 14 km east of Steytlerville, Eastern Cape Province of South Africa. Here a north to south geotraverse was studied in an attempt at unravelling the structural geology of the rocks belonging to the Upper Devonian to Lower Carboniferous Witteberg Group (Upper Cape Supergroup). These rocks are mostly arenaceous and include quartzite, sandstone, siltstone and shale which have been folded, faulted and metamorphosed. Thrust, normal and strike-slip faulting occur in the area. Shallow south dipping low-angle thrust fault planes are displaced by steep south-dipping thrust planes and subordinate north-dipping backthrusts. Displacement along thrust planes is predominantly northwards. Steeply dipping thrust fault planes are often reactivated by east-west striking normal faults. Strike-slip faulting post dates all structural features and displaces normal and thrust fault planes. Open to tight folds are present and are mostly north-vergent and often steepened or truncated by steep south-dipping thrust fault planes. South-vergent folds are related to backthrusting and post-fold faulting. The study has revealed that the current geologic map and the local stratigraphy were compiled without recognising major structural features such as thrust, normal and strike-slip faulting, and its validity is therefore questioned. Extensive faulting suggests that the conventional stratigraphic interpretation of the Witteberg Group strata should be revised.
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Fixed Wing Time Domain Electromagnetic Surveys to Promote Mineral Development in the Republic of Uganda.
Authors D. Burrows and J. TuhumwireFugro Airborne Surveys (Pty) Ltd conducted fixed wind time domain electromagnetic (TDEM) surveys over two blocks in The Republic of Uganda (Uganda) during 2007 and 2008 using the Genesis system. This Mineral Resource Management and Capacity Building Project (MRMCBP) was funded by the African Development Bank in order to promote mineral development in Uganda. The TDEM data acquisition was accompanied by a much larger airborne magnetic and radiometric data acquisition program with the same aims. Block EM7 is situated in the North West of Uganda. This survey area straddled the boundary between Tertiary-Quaternary sedimentary packages and the Precambrian basement. The complex nature of this interface is depicted in Figure 1 which shows the conductivity surface at 100m depth. The more competent basement also creates a topographic high in the survey area. Block EM8 is situated in the South East of Uganda where the survey area is comprised principally of mafic volcanic rocks and sediments. The area is known for the Gold-bearing greenstone belt that is part of the Lake Victoria Gold Field of Northern Tanzania and Western Kenya. The magnetic data collected prior to the TDEM survey highlighted the structural complexity of the area. This can now be better understood due to the additional perspective gained by acquiring the electromagnetic data.
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2D Seismic Reflection Surveys and Structural Mapping: Faults, Dips and Domes
More LessIn the local mineral exploration environment the high resolution imaging capabilities of 2D seismic reflection surveys continue to provide rapid, cost-effective mapping solutions over areas of structural mapping concern not otherwise speedily available from borehole data. This is particularly so where time-lines are short. 2D seismic sections sourced from the Wits Gold, Karoo coal, Canadian diamond and Bushveld platinum exploration sectors demonstrate in particular; • technique versatility in mapping orebodies or marker horizons having a tabular sheet geometry, and satisfying minimum thickness plus physical property requirements • a large spectrum of structural environments ranging from the benign to the complex. This spectrum ranges from flat dips with minor rolls and grabens through large scale wrench and listric faulting, to moderate dips with structural terraces or large grabens above Floor doming.
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GPR Characteristics of Ore-Bearing Layered Igneous Bodies
More LessFor many geological materials, the magnetic properties of the constituent parts are often considered unimportant when compared to their ‘dielectric’ characteristics (i.e., the permittivity and conductivity). However, if significant amounts of iron sulphide and/or magnetic minerals exist (such as magnetite, pyrrhotite, etc|) then the EM relaxation phenomena of these minerals can have a noticeable effect on the GPR wave’s attenuation and propagation velocity. Both surface and borehole-based GPRs have been successfully used to delineate and map layered igneous ore bodies but subtle, natural, variations in magnetite/ore mineral composition, grain size and mineral fabric can all have an effect on the nature of the GPR responses. In this paper, we present the latest findings of a measurement and analysis study on the GPR-related apparent permittivity, attenuation and propagation velocity characteristics of a range of natural occurring, magnetite/ore-rich igneous samples collected from the Bushveld complex, South Africa.
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Downhole Wireline Density Versus Drill Core Density Measurements in Porous and Vuggy Rocks
Authors Marcus Chatfield, Kazek Trofimczyk, Dirk Harney and Taurai KachigundaThe determination of in situ rock density is an important process in most opencast mining operations where an accurate estimate of total resource tonnage allows forecasting of metal production and life-of-mine. At the Skorpion zinc mine in Namibia, various techniques have been tried to improve density measurements, which are used to estimate mined tonnage and predict ore recovery based on laboratory measurements of ore yield per rock mass. Hitherto, the Archimedean submersion technique has been the basis of measurement. Downhole wireline density logging was put forward as a technology that could assist in getting accurate density determinations, particularly in the porous and vuggy arkose host rock. A borehole logging trial was conducted where a dual spaced gamma-gamma density sonde and other complementary downhole measurements, such as the photo electric density and optical televiewer, were surveyed for comparison with core density measurements. The wireline logs were run successfully in both wet and dry borehole conditions and a high degree of precision was achieved. The accuracy of the logs was based on industry standard calibration and borehole compensation with some quality assurance using core data in non-porous sections of the borehole sample. Results showed good agreement between core and wireline density in the non-porous rocks, but the wireline logs measured somewhat higher density in porous and vuggy zones. Analysis of the Archimedean technique used at Skorpion showed that the precision achieved was good, but accuracy was compromised by unsaturated and unmeasurable drill core porosity. The extreme geological environment and predominantly dry boreholes highlights the limitations of the gamma-gamma density measurement and demonstrates the critical requirement for sound data quality control and adherence to rigorous calibration standards. The availability of complementary measurements is found to be important to developing the right understanding of the wireline density response in an unfamiliar geological environment. Skorpion mine has recognised the differences in density measured by the Archimedean and the wireline logging techniques, particularly in porous and vuggy formations and supports further test work for quantitative calibration of the response. Following the successful outcome of that work, the wireline method could be deployed on an ongoing basis to supplement and enhance the accuracy of the core-based measurements.
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Joint Inversion of the Magnetic Anomaly Due to a Kimberlite Pipe and its Analytic Signal
Authors Jinfu Chen, Michel Chouteau and Pierre KeatingA three-dimensional (3D) joint inversion of the magnetic anomaly due to a vertical right circular cylinder with arbitrary polarization and its analytic signal is presented. This geometry is commonly used for the modelling of kimberlite pipes. The analytical derivatives of the body parameters are derived and used in the inversion to avoid numerical problems that may result from the use of numerical derivatives based on finite differences. The inversion uses the Levenberg-Marquardt algorithm. It is found that the inversion of the analytic signal gives better results for the geometry of the source body than the inversion of the magnetic anomaly alone. Finally, the proposed joint inversion gives more stable results and is more robust than the separate inversions of these two data sets. The joint inversion also allows better estimation of the parameters of the cylinder than the inversion of the magnetic anomaly or its analytic signal alone. This is especially true for the orientation and strength of remanent magnetization when present.
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The Use of Multi-electrode Resistivity Tomography to Map Shallow Old Gold Mine Workings in the Primrose Area, Boksburg
Authors E. Chirenje and S. DiopThe shallow old mine workings along the Main Reef road, in Johannesburg pose a serious hazard to the growing squatter settlements in the area. Recently, there has been documented loss of life and property through subsidence of the ground, in particular, in the Primrose area. In the interest of ensuring public safety, the Department of Mineral Resources and the Ekurhuleni Municipality commissioned the Council for Geoscience to investigate the spatial and depth distribution of the old mine workings in the squatter settlement areas. The idea being to fill the shallow workings (less than 60m), if possible, and certify for settlement (town development) where they are deeper and meet South African civil engineering standards. A survey was carried using multi – electrode resistivity method at Maraisburg along Main Reef road where there are known south dipping shafts, as well as in the Primrose area. The dipole – dipole and Wenner – Schlumberger arrays were used with 10m dipoles. The Res2DINV program was used to model the collected resistivity data. Interpretation of the resulting models show what appears to be locations of old mine shafts, a long tunnel along the strike of the reef and other mine workings. The anomalies are identified at typical first mining level one (approximately 30 m) and some slightly above or below. One can conclude from these results that the method used showed the existence of systematic ‘voids’ in the area, whose geophysical signature can not be associated with naturally occurring geological phenomena.
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Where to Next with SQUIDs in Exploration?
Authors Andreas Chwala, Ronny Stolz, Viatcheslav Zakosarenko and Hans-Georg MeyerFirst experiments on using SQUIDs for geophysical purposes are dated back to the early 1970s. But it took more than 20 years until SQUIDs found some commercial application in exploration. Nowadays, SQUIDs have reached a quite mature status in some systems, e.g. rock magnetometers and Transient Electromagnetics (TEM). But many more explorational methods could also benefit from the advantages of SQUIDs, such as high sensitivity and a flat frequency response from DC to up to several MHz.
In this paper I will highlight fields where I see that SQUID could have a major impact in the near future, inter alia Airborne TEM, AFMAG, remote referencing for TEM and Magnetotellurics.
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Empirical Correction for Compton Effects in Airborne Radiometric Data
By Henk CoetzeeDuring the development of an ultralight-borne airborne geophysical system, conventional radiometric calibration was found to be impracticable owing to the lack of suitable calibration facilities within range of the base of operations and, in one case, the need to perform Compton stripping on spectra recorded over areas contaminated with anthropogenic radionuclides.
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Ground and Airborne Geophysical Surveys Identify Potential Subsurface Acid Mine Drainage Pathways in the Krugersdorp Game Reserve, Gauteng Province, South Africa
Authors Henk Coetzee, Emmanuel Chirenje, Philip Hobbs and Janine ColeFollowing the cessation of underground mining activities in the West Rand Goldfield in the late 1990s all pumping of water from the mine void was halted and water levels were allowed to recover due to ground and surface water inflows. In 2002 the water in the mine void reached the level of the lowest lying shaft and began to decant on surface. This water was of poor quality with low pH and high dissolved solids. Significant effort has been expended since on addressing the impacts of this water on the receiving surface water system. Far less effort has been expended on the groundwater systems in the receiving environment. Ground and airborne geophysical surveys have identified potential subsurface pollution pathways in the Krugersdorp Game Reserve, immediately adjacent to and down-gradient from the West Rand Goldfield.
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Application of the Airborne Radiometric Method in Radiation Protection Applications
Authors Henk Coetzee and James LarkinThe airborne radiometric method has been extensively applied to uranium exploration and geological mapping problems since the 1940s, taking advantage of the natural variations in radioelement concentrations in the upper part of the earth’s crust and the information which can be deduced from these regarding surface geology and potentially economic mineralisation. In the process, a number of conventional practices have been adopted, largely regarding calibration and its supporting assumptions. The science of radiation protection looks at the quantification of radiological exposure to human beings, the assessment of the health impacts of these exposures and the development of strategies to minimise these exposures. As a result, the underlying methodologies and supporting assumptions differ, in some cases greatly, from those made in geophysical surveying, potentially resulting in misunderstandings between experts in the two fields. Airborne surveying on its own cannot generally directly quantify the radiological dose to human beings on the ground, although it can be used, given certain assumptions, to estimate ground level exposure rates. It can, however, play a vital role in the location, identification and, to some extent the characterisation of ground sources of radioactivity which could lead to increased radiological doses to the public. The extensive aerial coverage provided by the airborne survey method also offers a unique opportunity for the mapping of potentially radiologically significant sources. While it has traditionally been applied to the mapping of naturally occurring radioactive materials, modern multi-channel airborne spectrometers may also be applied to the search for and mapping of sources containing artificial radionuclides.
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The Use of VTEM Data in Geological Mapping and Mineral Exploration in North-eastern Namibia.
Authors M. Combrinck, W. J. Botha and D. HutchinsIn June 2008 a 3310 line-km airborne geophysical survey was flown in north-eastern Namibia for the Geological Survey of Namibia. Time domain electromagnetic data (dB/dt and B-field) as well as magnetic data were acquired with the VTEM system. The survey area is covered with sand and no other detailed information on the sub-surface geology is available. The objectives of the survey were to delineate sub-surface geological units and trends, and to identify possible exploration targets. The TDEM data were processed to obtain three-dimensional conductivity-depth information, decay constant maps and discrete anomaly picks from both the B-field and dB/dt data sets. Together with the magnetic total field data and its derived products (analytical signal, vertical and horizontal derivatives), these were used to delineate geological structures and classify geological units based on their regional setting and physical properties. In addition, targets with exploration potential were identified. The data and some interpretation results are presented in this paper, illustrating the value added to regional mapping and greenfields exploration with airborne TDEM surveys.
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Determination of Earth Gravity Field Spherical Harmonic Coefficients Using SLR data
More LessSatellite orbital perturbations resulting from the disturbing-potential (difference between spherical gravitational potential and aspherical geopotential) are in addition to being one of the major sources of orbital perturbations, important indicators of geophysical signals. Determining accurate coefficient values of Earth gravity models is a complex process, usually achieved by combining satellite and terrestrial data, which allow models of high degree and order to be developed. Recently we have developed software that estimates zonal coefficients 2,020,0JJ− as well as and These different coefficients have special applications within interesting scientific questions 21C21.Sranging from relativistic tests of post-Newtonian parameters to fixing the rotation axis of the reference frame. With the development of new models using satellite data (GRACE, GOCE etc.) higher levels of accuracy should be reached, allowing increased potential for more accurate estimates of solve-for parameters of interest, such as SLR station position, other orbital perturbations, geophysical processes and relativistic tests. The new models should still be validated utilising independent techniques such as Satellite Laser Ranging, where particularly the coefficients of lower degree and order affect geodetic satellite orbits. In addition, the ability to estimate gravity coefficients allows determination of trends in the coefficients and validates the software’s force models. A brief description of the method used for the spherical harmonic expansion coefficient is given as well as some results using SLR data to LAGEOS 1 and LAGEOS 2.
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Products of Space Geodesy and Links to Earth Science and Astronomy
More LessProducts of Space Geodesy such as Earth Orientation Parameters, polar motion and precise positioning, as well as various applications of these products are of importance to both Earth Sciences and Astronomy. Global geodetic site positions and velocities find application in geodynamics, on a local and global scale, including plate mobility and tectonics, on time scales varying from sub-seconds to millions of years. Precise orbit determination provides information on Earth’s gravity field and changing shape, it also allows calibration of satellite platforms which measure ocean and ice levels. Nuisance parameters such as observable delays due to the ionosphere and atmosphere provide additional information about total electron content and integrated water vapour. Global reference frames on Earth, and celestial reference frames in the universe are maintained and linked to one another, providing unique positioning capability on Earth and in space. Accuracies and precision of techniques and instrumentation are such now that even relativistic parameters can be tested to new levels. Future developments are very promising as the scientific community moves towards implementation of the Global Geodetic Observing System (GGOS), where sub-cm accuracy on global scales is the objective, requiring a factor ten improvement in measurement accuracies and improved models in terms of factors which affect measuring accuracies. The different Space Geodesy techniques will be discussed briefly with examples of their applications, their links to astronomy and the earth sciences, as well as current and future developments, and South Africa’s role currently and in the near future.
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Enhancing Circular Features in Potential Field Data using a Generalised Radial Derivative Filter
More LessThe enhancement of potential field anomalies over Kimberlite pipes and meteorite impact sites is of considerable interest, and much work has been done on this subject. This abstract suggests a modification to the generalised derivative to make it act in a radial manner, thus making it useful for the enhancement of near circular features. The filter is applied to gravity data from South Africa.
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A Solution-space Approach to the Euler Deconvolution of Map Data
More LessThe application of Euler deconvolution is a common first step in many potential field interpretation projects, however the method is well known for producing large numbers of 'sprays' of solutions which can make the interpretation more difficult. A solution-space approach is taken here, wherein the half-space is discretised into cubes and the number of possible solutions to Eulers' equation that have a particular structural index is counted. This number is a measure of the probability that a potential field source with that structural index is present at that location. The method is demonstrated on gravity data from South Africa.
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The Uranium Boom: a Challenge to Geophysical Exploration
By B. CornerThis paper discusses the present and predicted global need for uranium, and the geophysical exploration challenges for the location, mapping, and evaluation of the new deposits needed to address the projected supply shortfall. In 2008, approximately 60% of the world’s uranium production was from Canada, Australia and Kazakhstan. Africa contributed 18.5 %, derived, in order of supply, from Namibia, Niger and South Africa (WNA, 2009). The major requirement for uranium stems from the needs of both developed and rapidly developing countries who do not necessarily have uranium resources of their own but who have expansive nuclear power generation plans, in particular China and India. The potential for Africa to become a leading supplier is immense. The major challenge thus posed to the geosciences is for the refining of resources to proven reserves, and for the location and evaluation of new deposits. In this respect geophysics has played, and continues to play, a leading role in every aspect of the nuclear fuel cycle, including: the direct exploration for uranium, mapping under cover to ever increasing depths, borehole logging for ore reserve evaluation, ore sorting on mines, environmental monitoring, and nuclear waste disposal. In this presentation, some of the main geophysical advances in uranium exploration technology, and the geophysical challenges to uranium exploration, are addressed.
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Radon Emanometry Case Studies in Namibia: the Spitzkoppe and Tumas Uranium deposits
Authors B. Corner, Dr. V. Osiyuk, S. Lytvyniuk, A. Kuchmin and D. VerranThe RadonXTM technique uses the principle of adsorption of radon, emanating from buried uranium mineralization, onto activated charcoal. The charcoal is contained within a cartridge which is fitted into the base of an inverted cup, and is buried in the ground for a period of approximately 10 days. The technique differs from alpha-sensitive radon detection systems in that it measures the gamma radiation arising from the daughter products of the adsorbed radon, namely 214Bi and 214Pb. Two case studies are presented, dealing with the detection of partially buried shallow uranium deposits, using the RadonXTM radon emanometry technique. The case studies are sited over the Spitzkoppe and Tumas deposits in the Central Zone of Namibia, comprising uranium mineralization contained in shallow calcretized palaeo-valley gravels buried under a thin cover of calcrete or gypcrete duricrust. The results show that both deposits have clear RadonXTM target anomaly signatures, in large part correlating with the higher grade areas. The technique is thus an effective and essential adjunct to gamma spectrometric surveys in surrounding areas of cover. Although these deposits are shallow, with a depth extent of up to ~20 m, case studies in other areas show an excellent depth of penetration, of 100m or more, under favourable permeability conditions.
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A Radon Emanometry Case Study of the Rössing South Deposit, Namibia
Authors B. Corner, H. Sinclair and D. VerranThe RadonXTM technique uses the principle of adsorption of radon, emanating from buried uranium mineralization, onto activated charcoal. The charcoal is contained within a cartridge which is fitted into the base of an inverted cup, and is buried in the ground for a period of approximately 10 days. The technique differs from alpha-sensitive radon detection systems in that it measures the gamma radiation arising from the daughter products of the adsorbed radon, namely 214Bi and 214Pb. The present case study shows that RadonXTM has an improved sensitivity compared to an alpha detection system, in which cups are buried for a period of 30 days. A case study is presented, in which the RadonXTM radon emanometry technique is sited over the Rössing South uranium deposit. The mineralization occurs under a cover varying between 20 and 80 m, within rocks of the Damara Supergroup and associated leucogranites, in the Central Zone of Namibia. The results show that the deposit, blind in terms of a radiometric signature, has a clear RadonXTM target anomaly signature, making the technique an effective and essential exploration tool in areas of deep cover. An excellent depth of penetration, of 100m or more has been quantified elsewhere in the Central Zone, under favourable permeability conditions.
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Quantifying the Shared History of Species and Landforms to Decipher Late Cenozoic Co-evolution of Landscape and Biodiversity Across the Kalahari Plateau
Authors F.P.D Cotterill, S. Goodier, C. O’Ryan and M. J. de WitEpisodic uplift and denudation of the Kalahari Plateau, the vast portion of High Africa centred on the Botswanian-Zambezian region, was initiated over 100 mya, a time span that coincides with a long term trend of global cooling from the Late Cretaceous onward. Yet, the origin and subsequent erosion history of this plateau remains poorly known, despite more than 100 years of study (e.g. E. Suess, 1904). The limiting factor in this debate is precise and accurate dating of both the ancient and recent episodic history of Kalahari landscape changes, dates that can then structure tests for controls on epeirogeny and climate change. Geomorphology in particular faces daunting challenges of tracking the evolution of landforms and the formative events that delimit their tenures. These challenges characterize the landscapes of the vast top-surface of the Kalahari plateau. First, because the Kalahari’s veneer –sediments and duricrusts - comprises a condensed sequence that lacks strata suitable for dating, and second, because its gentle topography reflects numerous ancient amalgamated erosion surfaces and a complex history of palaeo-drainage dynamics. Here, we describe results of a new approach to date recent events that affected the Kalahari surface, using detailed studies of the region’s biodiversity and biogeography. We apply methods and concepts of phylogeography to species with distributions confined closely to the landforms we wish to date. We term these species “biotic indicators of landscape evolution” because informative signals of landscape history are preserved in their genomes. We characterise their gene trees to resolve where and when changes modified the distributions of these indicator species. Dating of molecular clocks – on gene trees - confers temporal precision on these evolutionary events that acted on both biota and landform. A synthesis of such biogeographical data can quantify when landforms (e.g. rivers) changed; while phylogeographic studies quantify when these formative events occurred; when populations diverged; and/or experienced demographic expansions. All these signals quantify events that altered landscapes. Relative sensitivity to changes in a landform varies greatly between different species, so their respective phylogeography exhibits corresponding differences in what they can tell us about where and when landscapes changed. We concentrate on aquatic organisms, especially fishes, because they are especially informative biotic indicators, given their sensitivity to habitat changes. Anomalies in fish distributions often reflect relatively subtle changes to drainage systems. We find that Late Cenozoic drainage evolution across the Kalahari Plateau reflects profound genetic structuring of extant fish faunas in its wetlands today. This testifies to river captures and tenures of palaeo-lakes. Conceptualizing this region’s river networks and depocentres as a wetland archipelago provides a valuable framework to synthesize patterns of biotic and landscape evolution. This confers explanatory resolution on geomorphological reconstructions of its history. Phylogeographic studies of selected species are revealing where and when these drainage systems changed. The combined evidence points to the underlying causes of this landscape evolution against which patterns of climate change may be tested.
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Proposal for a Lunar Geotechnical GIS
By L. CroukampThe Lunar Mapping and Modelling Project (LMMP), (Cohen, et al, 2008) is a NASA led initiative with the aim to create a single, consistent, uniform, intuitive and easy to use NASA portal that accesses lunar mapping and modelling products, tools and media.
The USGS started with the foundation for such a system, known as PIGWAD (Planetary Interactive GIS on the web analysable Database). It is clear that the nations of the world are going back to the moon in what is termed “The new Race for the Moon”. The USA Government committed itself to billions of dollars for the Constellation programme which is the programme aimed at returning humans to the moon. The proposal currently in planning stages is to have unmanned and manned rover missions to the moon by 2012, the beginnings of an outpost by 2015, and the construction of a manned moon base by 2020. To prepare for the ambitious international plans for advancing lunar research vast amounts of geological and topographical data need to be collected. Geotechnical properties of the lunar surface must be determined for rover mobility, and will also be required to determine construction methods and materials for a permanent moon base. The GIS proposed in this project will contribute substantially to building the base of relevant data required to enhance and support future missions, target generation for mining of materials for the purpose of propellants, oxygen, construction materials and the planning of excursions as well as the construction of a permanent moon base. Envisaged data layers to be collected include but are not limited to: Geology, Topography, Slope angle, Slope aspect, Surface roughness, Excavatibility, Bearing capacity, Soil density, etc. Once the abovementioned data, and other data have been collected and geo-referenced in a coherent GIS it would be possible to do GIS analysis of the various datasets, either separately or in combination. Maps of resources could be created, as well as the most cost-effective paths for excursions, rover missions and travel paths between bases and sampling points. The inherent structure of a GIS allows several permutations of looking at available data.
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O- and H-isotope Study of the Cretaceous Koegel Fontein Complex
Koegel Fontein is a 30 km diameter anorogenic igneous complex that intruded Namaqualand gneisses during the initial phase of break-up of Africa and South America. It is notable for the presence of quartz porphyry dykes that formed from low-δ18O magmas. Many of the igneous rocks show petrographic evidence for alteration and have had their δ18O values lowered by fluid-rock interaction at high temperatures. Partial melting and/or assimilation of this material produced the low- δ18O magmas. Whole-rock δD and δ18O values indicate equilibrium with meteoritc water with a δ18O value around -10 ‰, which is abnormally negative given the relatively low latitude at the time of intrusion. A combination of an elevated volcanic edifice and the ‘continental effect’ is the most likely explanation.
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Geophysics and the Deep Structure of the Barberton Belt: A Review
Authors Johan de Beer and Edgar StettlerThe Barberton Greenstone Belt has received world-wide attention as one of the classical areas for studying early Archaean rocks and their environments. However, the Barberton Belt has received little attention from the geophysical community. In 1970 Burley and others made the first attempt to estimate the depth extent of the belt using the gravity anomalies in Swaziland. In a somewhat oversimplified analysis they deduced that the gravity anomaly over the belt can be produced by a flat, steep-sided body, outcropping at the surface and extending to a maximum depth of 3.2 km. In 1975 Darracott compiled a Bouguer gravity anomaly map of the area, which showed that the mafic and ultramafic rocks comprising the Onverwacht Group are associated with a positive gravity anomaly of 20 to 30 mgal. Interpretation of this anomaly indicated that the Barberton Greenstone Belt has a probable depth extent of 3 to 4 km, with the possibility of depth-extents reaching 6 km beneath the deeply infolded sediments of the Fig Tree and Moodies Groups in the vicinity of Barberton. In 1988 De Beer, Stettler and others in a study using the DC resistivity and gravity methods showed that the rocks in the Barberton Belt and the surrounding granitoid terrain have distinctive resistivity and density properties. The models based on the integrated data set established that the depth extent of the greenstone belt does not exceed 8 km. The maximum depth extent is always more than 4 km on all profiles modelled. The authors also found large areas of granitic terrain underlain by greenstone material. The three independent studies basically reached the same conclusions. With better coverage and larger data sets, the estimates for the maximum depth extent increased. The results are in agreement with similar studies in other parts of the world. Invariably the geophysically determined depth extent is much less than the geologically determined stratigraphic thickness.
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Denudation Rates for Karoo Dolerites in Southern Africa Determined with Cosmogenic Noble Gas Nuclides
Authors J.E. Decker, S. Niedermann and M.J. de WitSouthern Africa displays an anomalously elevated landscape with a modal elevation of approximately 1000 m above sea level, even though the region is not associated with convergent plate boundaries or active rifting. The origin and evolution of this region’s topography may be constrained by determining the rates of surface processes on intermediate timescales (103 – 106 yr) with in situ-produced cosmogenic nuclides. Here we present minimum exposure ages and maximum denudation rates for Karoo dolerite surfaces across southern Africa, based on the cosmogenic noble gas inventories (3He, 21Ne) of pyroxene separates. Our minimum exposure ages for vertical and inclined scarp faces vary from ~20 to ~380 ka. Maximum denudation rates for horizontal surfaces are consistently low, and generally less than 3-4 m/Myr. These low denudation rates are compatible with a currently stable landscape, and suggest that southern Africa’s topography developed under a significantly different tectonic and/or climatic regime.
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Large Scale Thrusting Across the Kango Inlier, Cape Fold Belt, South Africa.
Authors M.J. de Wit and S. MooreStructural mapping and analyses of the Table Mountain Group in the Cape Fold Belt along both the northern and the southern margins flanking the Kango Inlier, 10km west of Calitzdorp, reveals km-scale thrust stacks (duplexes) with more than 70% horizontal shortening along the southern boundary between the Huisriver Formation of the Kango Supergroup and the Peninsula Formation of the Cape Supergroup. In addition, 100-1000m scale folding is documented within the overturned Baviaanskloof Formation to the north of the northern boundary of the Kango Inlier in the Gydo Formation along the Gamkapoort section in the Hell. Strain analysis along this section provides evidence that the extensive thrusting predates the ubiquitous upright folding in the Swartberg Mountain range, suggesting that Peninsula Formation allochthons were emplaced across the Kango inlier early during the deformation history of the Cape orogeny.
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The International Continental Scientific Drilling program (ICDP) and South Africa: Steps Towards a Deep Drill-hole Through Earth's Oldest Suture Zone & Establishing a Centre of Drilling Technology in Mpumalanga
More LessThere has long been ad-hoc bilateral international co-operation in a number of scientific drilling projects. However, a multi-national continental drilling program, comparable to the successful Ocean Drilling Program (now Integrated Ocean Drilling Program), was established in the mid 1990s after a conference on the establishment of an International Continental Scientific Drilling Program. Since then the ICDP has grown into a ‘mega-project’ institution. The ICDP comprises 21 members. These include 19 countries, the UNESCO and Schlumberger Services Inc. as a Corporate Affiliate. The youngest members are New Zealand and France, who joined in spring 2009. South Africa has been a member since 2006. The scientific aims of the ICDP are interdisciplinary and attempt to cover a broad spectrum of contemporary Earth Sciences in order to discuss how scientific drilling could complement on-going geo-scientific studies, and make it possible to address fundamental, unresolved questions critically relevant to both societal needs and an improved understanding of the Earth and its Lithosphere. ICDP operates across on all continents, including Antarctica, and has more than 50 active drilling projects and proposals for new drilling projects that fall within the general realms of : Climate Change and Global Environment; Impact Structures; Geobiosphere and Early Life; Volcanic Systems and Thermal Regimes; Mantle Plumes and Rifting; Active Faulting, Collision Zones and Convergent Margins; Natural Resources; Deep Earth Observatories. Technology and capacity building spin-offs are huge. Member countries can participate in any of the ongoing/planned drilling projects, and benefit from numerous short-courses and workshops, advice and technical equipment. They may design also their own deep drilling projects and submit proposals to ICDP for funding thereof, and apply for technical assistance. Project proposals are internationally scrutinized by a Scientific Advisory Group (SAG) that makes recommendations to the ICDP Board. I have now been on the SAG as the RSA representative for 3 years. To encourage more active participation of South African researchers and industry scientists, I would like to take this opportunity to share some of my SAG experiences and convey some of the excitement of ongoing drilling projects that range from deep drilling into volcanic magma chambers, active calderas, active faults (San Andreas), subduction zones, lakes for climate and biodiversity changes, impact sites, deep biosphere, biomarkers of ancient life, geothermal energy, deep mines (RSA, USA), hotspot tracks, and others, under very varied conditions from the tropics to the poles. A group of local and international scientists have recently submitted a proposal for a deep (5-10 km) drill-hole in Mpumalanga, and to build around this a rural college for drilling and related S&T. Phase One of this proposal has been accepted by the ICDP Board, and we will host an exploratory workshop soon. I will outline the aims and timelines of this project; ways to get involved, and solicit new ideas.
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Architecture of the Onverwacht Suite, Barberton Mountain Land, South Africa: New Stratigraphic Nomenclature and Revised Models
More LessSynthesis of new and existing data identifies at least seven major shear zones and a major unconformity that separate the rocks of the southern Barberton greenstone belt into seven complexes with different complex geologic histories. The stratigraphic nomenclature presently applied to these sequences (e.g. formations) cannot incorporate these complexities. This requires changing the status of the traditional ‘Formations’ of the Onverwacht Group of the Barberton greenstone belt into ‘complexes’. The complexes are referred to collectively as the Onverwacht Suite. The total age range and thickness of the suite is about 120 million years and 15 km, respectively, but the precise age ranges of each complex and their original thicknesses remain unknown. All the complexes include significant volumes of intrusions into their volcanic and volcanoclastic host rocks. The ages of the intrusions of the different complexes are different. In some complexes the intrusions and volcanic rocks have similar if not identical geochemistry, and are co-magmatic. Others provide ‘stiching’ ages between complexes. Six of the complexes have ophiolite-like affinities that suggest the complexes formed predominantly in oceanic spreading- and intraoceanic arc- environments with water depths of 2-4km. The complexes are tectonically stacked and the original spatial relationships between them can only be inferred in most cases. At least one complex was uplifted by 2-4 km between about 3470 and 3460 Ma, giving a minimum uplift rate of some 2-4 mm/yr, similar to that in modern subduction zones and during emplacement of Phanerozoic oceanic crust as ophiolite complexes. Overlying coarse clastic sediments overlap in age with about 15 myrs of felsic-intermediate igneous activity dated between 3460 to 3445 Ma, that, in turn, relate in age to the intrusion of the tonalite-trondjemite plutons to the south of the greenstone belt. These sediments may therefore represent a sequence deposited in a tectonic basin associated with subduction and obduction processes. The emergence of the deep water complex above sealevel and the onset of subaerial clastic sedimentation (> 3455 Ma) probably best represents the start of obduction of the lowermost Onverwacht complexes across the older complexes (>3.5 Ga) that are now only preserved as remnants in the surrounding granitoid terranes. Timing of a second major episode of subduction/accretion, dominated by folding, thrusting and exhumation of the Barberton greenstone belt, occurred between 3.230 -3.140 Ma. Early minimum cooling rates during more than 18 km unroofing were in the order of 1.5-5 mm/yr, similar to those recorded in modern orogenic belts. Oblique convergence ended in collision and strike-slip displacements around 3.1 Ga. The deposition of the Moodies Group sandstones and conglomerates has been linked to this period of deformation, again at rates comparable to those measured in modern orogenic belts and transcurrent plate boundaries. Thus within all crustal regimes for which reasonably robust rates of tectonic processes can be determined, these rates are similar to within an order of magnitude as those encountered today. By 3.0 Ga the region was part of a rigid peneplained continent.
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Imaging Techniques for Full Tensor Gravity Gradiometry Data
Authors Jade L. Dickinson, J.R. Brewster, J.W. Robinson and C.A. MurphyFull Tensor Gravity Gradiometry has previously been demonstrated to be an effective tool in both the minerals and oil and gas industries. Full tensor data is a richer source of information than standard single tensor gravity. This extra information can be used by tensor imaging techniques to enhance target anomaly definition.
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Gravity Gradiometry – Today and Tomorrow
Authors Dan DiFrancesco, Thomas Meyer, Asbjorn Christensen and Desmond FitzGeraldGravity gradiometry is coming of age as a standard exploration process. The acceptance and scope of airborne surveys is on the rise, with success stories published and documented. A renewed interest in marine surveys for hydrocarbons is also occurring. New sensor and system developments are nearing a point where they may be ready for field tests and commercial deployment. It is accurate to say that the state of gravity gradiometry is healthy in today’s commercial marketplace. As good as this is, there remain a number of challenges and opportunities for full utilization of gravity gradiometry as a tool for the explorationist. A number of questions and concerns need to be addressed ranging from sensor performance to operational efficiency to data handling to educating users. While these challenges might seem daunting, the future looks bright for gradiometry as innovation and acceptance continue to grow. In fact, the time seems right to ask some thought-provoking “What if” questions: Is the era of gradiometry just beginning to dawn? How will the future unfold for this capability? What is the optimal exploration system? What if multiple gravity components could be measured at the same time (i.e. scalar gravity, 2nd order tensors, and 3rd order tensors)? Are the physical limits of measurement already being met? What if data were available without limits throughout the world?
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Methane on the Move: Quantifying Natural Greenhouse Gas Emissions over Geological Time in the Orange Basin, South Africa
Authors Rolando di Primio, Brian Horsfield, Zahie Anka and Gesa KuhlmannIn the assessment of past, present and future global climate the contribution of natural greenhouse gas emissions has been largely disregarded. Our studies in the Orange Basin, South Africa indicate a highly variable rate of methane emission from the basin to the hydro- and atmosphere through geologic time. In order to understand the interactions between sedimentary basin evolution in time and space and Earth´s climate a global assessment of thermogenic methane emissions is required.
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Stress Patterns in the Cape Fold Belt
By R.N. DomoneyRocks outcropping north of the Worcester Fault near Worcester, South Africa, belong to the Malmesbury Group and the Peninsula Formation of the Table Mountain Group. These exposures were studied to get an idea of their structural evolution. Flattening strains produced the planar fabrics in the rocks, with flattening being the dominant deformation mechanism resulting from successive compressive events. This flattening was responsible for the preferred attitude of the rocks and the compressional crenulation cleavages. The compression direction varied from NE-SW to NW-SE during the formation of the composite foliations during the Saldanian Orogeny. These foliations were subsequently crenulated during the Cape Orogeny. In general, NW-SE directed compressional stresses produced the deformation in the Peninsula Formation, where σ1 is oriented NW-SE. Extension direction is perpendicular to this, i.e. NE-SW. For the Malmesbury Group of rocks, NW- directed compressive stresses produced the fabrics in these rocks, with the extension direction being approximately perpendicular to this compressive direction. In general, it would appear as if the regional stress field varied somewhat in its orientation, and the structures in the Malmesbury rocks developed as a relatively continuous sequence of events within a geologically short period of time. It is also very clear that the later Cape Orogeny had a profound effect on the Malmesbury basement rocks, with stress orientations of the former influencing those of the latter.
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Significantly Increasing TEM Survey Performance by Modifying Frequency Content of Transmitter Waveforms
More LessTEM surveys typically operate with a simple transmitter waveform, such as a 50% duty cycle alternating square wave. The frequency of transmission and the duration of measurement is decided before the survey or, frequently in the case of ground TEM surveys, adapted during the survey by the operator to suit the conditions. With some sensor types in particular, achieving good quality data throughout all parts of the decay is difficult. Additionally, all surveys can be complicated by the presence of external sources of interference such as power transmission lines. We argue that significant improvements can be made by optimising the frequency content of the transmitter waveform. Additionally, in the case of ground surveys, the duration of an individual reading can be controlled in order to achieve rapid production and desired data quality. Variables are the EM noise spectrum (a function of the sensor and environment) and the conductivity of terrain. These change along the TEM profile and best results are obtained by regular re-evaluation in light of the target sought. There are several options available to modify the effective frequency content of a TEM transmitter waveform. For a conventional square wave transmitter, an irregular pattern of polarity reversals can be transmitted. Another method is to use two or more base frequencies sequentially. The survey can be automated and data can be combined automatically into a single decay with optimal signal-to-noise ratio over the entire decay.
Examples will be presented to illustrate improvements in performance.
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Case Study: Dighem Survey for Non-magnetic Dyke Detection; Dorstfontein Colliery, Total Coal RSA
Authors S.J. du Plessis and R. JoubertA Dighem airborne geophysical survey (simultaneous acquisition of airborne magnetic and electromagnetic data) was carried out for Total Coal South Africa from September 14 to October 23, 2007. Three blocks in the Secunda area were flown, namely: Dorstfontein, Boschmanskop-Tumelo and Forzando West. This paper deals with the Dorstfontein colliery survey only. The Dorstfontein mining area is located immediately east of the town of Kriel in Mpumalanga, South Africa. The purpose of the survey was to detect both magnetic and non-magnetic dykes. The data quality from the Dighem survey is very good and much improved on previous similar surveys in the 1990s and in 2000. This is due to improvements in technology, especially electronics. The Dighem survey successfully delineated known magnetic and non-magnetic dykes in the Dorstfontein area, as well as probable and possible dykes. Some 60% of the known and probable and possible dykes were detected by the magnetic method and 40% of the probable and possible detected dykes were non-magnetic. Using two flight directions at right angles to each other proved to be important, since if only one flight direction was used, some of the dykes may not have been detected.
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The Physics of the Ground Loop
Authors Braam du Plooy and John BellA ground loop is often used both as a model to explain the basics of electromagnetic exploration and as a tool in the calibration of AEM systems. A thorough understanding of the underlying physics is essential. During the calibration procedure of an AEM system, the electrical current in the ground loop was measured and recorded. A mathematical model in the form of a differential equation was stated for the current in the ground loop. The solution of the differential equation was calculated allowing for the particular transmitter current wave shape. There is good qualitative conformity between the measured current and modelled current. Quantitative agreement of the current amplitude depends on an accurate knowledge of the geometries of the problem.
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AfricaArray: Aims, Achievements and Future Activities
Authors R.J. DurrheimPHGM Dirks, A.A. Nyblade, G. Graham and S.J. Webb and MQW JonesAfricaArray (www.africaarray.org) is a long-term programme to build geosciences capacity in Africa in support of the mineral, petroleum, groundwater and environmental sectors. It seeks to do this by establishing effective Africa-based education and research programmes and observational networks, initially focused on geophysics, but planned to include other geoscience disciplines. The seismic network comprises 37 broadband stations in 16 countries as well as three temporary networks. More than 20 papers reporting on research findings have been published or are in press. Five students have graduated with MSc or PhD degrees, and 26 are registered.
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Observational Study to Mitigate Seismic Risks in Mines:a New Japanese - South African Collaborative Project
Mining-induced earthquakes pose a hazard to workers in deep South African mines, while natural earthquakes pose a hazard to people living close to plate boundaries. We introduce a 5-year Japanese-South African collaborative project entitled "Observational study to mitigate seismic risks in mines". The principal investigators are H. Ogasawara (Japan) and RJ Durrheim (South Africa). The project, which seeks to develop human and instrumental capacity in South Africa, will build on previous studies carried out by Japanese and South African seismologists and rock engineers in deep gold mines. This knowledge will be used in efforts to upgrade seismic hazard assessment schemes and to mitigate the seismic risks in deep mines. The knowledge is also relevant to the study of the mechanisms that generate tectonic earthquakes. The project was conditionally approved in April 2009 by the Japan Science and Technology Agency (JST), an external agency of the Ministry of Education, Culture, Sports, Science and Technology, and the Japan International Cooperation Agency (JICA), an external agency of the Ministry of Foreign Affairs. It is anticipated that the agreement between the Japanese and South African governments will be concluded by the end of the 2009 financial year and that research work will commence in 2010.
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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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