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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 - 50 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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