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5th SAGA Biennial Conference and Exhibition
- Conference date: 29 Sep 1997 - 30 Sep 1997
- Location: Swakopmund, Namibia
- Published: 29 September 1997
1 - 20 of 52 results
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Geophysical Contributions To Exploration For Gold In The Archaean Of Tanzania, East Africa
By K. GeerthsenHistorically gold mining was an important contributor to the economy of
Tanzania. With the advent of better political and fiscal regimes exploration for
gold in the Archaean greenstones of the Tanzanian Craton is booming. Gold
mineralization is structurally controlled and often associated with sulphide and
alteration minerals. Geophysical surveys are mainly used to improve geological
and structural mapping and to locate associated sulphide mineralization or
alteration minerals. Three case histories are presented to describe the use of both
airborne and ground surveys during exploration.
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Borehole Radar At A Surface Ventersdorp Contact Reef Site
Authors D.R. Vogt, J.C. Trickett and E.C. WedepohlBorehole radar trials were conducted at a surface geophysical test site at Durban
Roodepoort Deep. The trials included single hole reflection, single hole
directional and cross hole surveys. Results from a representative sample of
surveys show that borehole radar is capable of mapping the economically
important Ventersdorp Contact Reef (VCR) and is an exciting addition to the
toolbox of the high-resolution geophysicist. The potential for increasing the
information gained from borehole radar surveys through processing is
demonstrated.
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Geophysical Response Over A Kimberlite
More LessFollowing a combined airborne frequency domain EM and aeromagnetic survey, a
prominent circular anomaly was observed and interpreted as a possible kimberlite
pipe. On drill investigation this interpretation proved correct. A ground
geophysical program consisting of magnetics, gravity, frequency and time
domain EM, radiometrics and physical property measurements was embarked
upon. All the above techniques gave an unambiguous response from the
kimberlite pipe except for the radiometric technique.
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Lithostructural Mapping Of The Rössing Area, Namibia - An Integrated Approach Using Hlgh-Resolution Geophysical Data, Landsat Data And Aerial Photographs
Authors Helen Anderson and Colin NashThis paper presents results of an integrated appraisal of recent high-resolution
aeromagnetic, radiometric and remote sensing data over the Rössing mine area
and lower Khan Gorge region of Namibia. Image processing and interpretation of
the high resolution geophysical data provide a fresh overview of the complex
structure within the Damara orogenic belt, irrespective of regolith cover.
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Exploration For Massive Sulphide Mineralisation Along The Eastern Extension Of The Matchless Amphibolite Belt [Sysmin Area 6- Aem Survey]
Authors S.J. du Plessis, P.J.A. Lombaard and A.H. MatthewsAerodata Botswana carried out a Questem Time domain EM survey along the eastern extension of the Matchless Amphibolite Belt (MAB) on behalf of the Geological Survey of Namibia. Data
acquisition was completed during November 1995. The area was denoted ‘Area 6’ and formed part of the Sysmin-funded exploration projects in Namibia (funded by the European Union).
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A Case Study Aimed At Locating And Delineating Further Ore Bodies In The Soil Covered Areas Southwest Of The Rosh Pinah Mine
Authors B. Corner, L. de Munnink, D.J. Alchin, C. Naude and G. WepenerThe Rosh Pinah Pb-Zn mine is situated in the southwestern part of Namibia. The
deposit is stratabound in arkosic and quartzitic sediments of the Rosh Pinah
Formation which forms part of the late Proterozoic - early Palaeozoic Gariep Belt.
The sulphide ore is mostly disseminated comprising sphalerite, galena and pyrite.
A square 2km grid was geophysically surveyed to the south of the existing ore
fields, with the objectives of locating further ore bodies and providing guidelines
for future exploration.
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Improved Reservoir Characterization Incorporating Time-Lapse Seismic And Production Data
Authors Xuri Huang and Geoffrey A. KingToday, geostatistical reservoir characterization from 3D seismic volumes provides most static descriptions for reservoir models. These models can be improved by integrating the dynamic data in the reservoir description process. 3-D time-lapse seismic surveys have been proposed to relate time dependent changes in seismic attributes to the flow processes in the reservoir. This paper presents a new approach to reservoir characterization by integrating time-lapse seismic and production data. The issues involved in the integration will be examined and demonstrated using a case study over a turbidite sheet sand reservoir in the Gulf of Mexico. Seismic data from the base survey were combined with log and production data to build an initial reservoir model which was run forward to the time of a second monitor seismic survey. Dynamic history matching further improved the model. The output from this simulation was then
converted to a synthetic monitor seismic survey using Gassmann’s equations and a simple convolutional approach. The quantitative combined seismic and production history-matching methodology was then tested. It constrains the modeling process to match the production history and simultaneously minimize the differences between the synthetic and real 3-t) seismic time-lapse data. This new systematic approach provides us with a quantitative time-lapse seismic analysis and reservoir characterization tool which has the potential to improve reservoir management.
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What’S Wrong With Shallow Seismic Refraction Methodology?
By C. WrightThe methods currently used to analyse shallow seismic refraction data are inefficient in their use
of arrival times for determining shallow velocity structure. A strategy for making more effective
use of the data is as follows. Firstly, depths to bedrock or a sub-weathering layer are estimated
by a combination of two approaches: the reciprocal (or generalised reciprocal) method and a
least-squares difference method. Seismic velocities in each layer are then calculated by
constructing a single ‘static-corrected’ travel-time versus distance relation. This is accomplished
using a least-squares inversion procedure to correct for baseline differences between shots that
have common recording locations, and time reversal to make all seismic energy appear to travel
in one direction. The resulting travel-time distance relation is then smoothed to give a preferred
velocity solution as a function of position. Hammer seismic surveys provide examples of the
effectivenessof the new methodology, and assist in formulating simple guidelines for maximising
the resolution of seismic refraction surveys while minimising field costs.
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Seismic Onshore/Offshore Investigations In The Area Of The Damara Igneous Province
Authors K. Bauer, N. Fechner and A. SchulzeContinental rifting and the Early Cretaceous opening of the South Atlantic formed
the passive, volcanic margin off Namibia. Just prior to the onset of sea floor
spreading a Large Igneous Province emplaced including voluminous extrusives,
continental flood basalts and subvolcanic ring complexes. Multichannel seismic
reflection and wide-angle data were collected to explore the deep structure of the
entire continent-ocean transition. This contribution presents preliminary results
from P-velocity modelling and inversion.
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Namosex: Namibian On-/Offshore Seismic Experiment - Geological Background And Targets
Authors K. Weber, M. Raab, K. Hinz, D. Eberle, J. Fritsch, H. Meyer, S. Neben, Chr. Reichert, K. Bauer, A. Schulze, R. Auerbach, B. Baier, N. Fechner and H. MillerThe continental margins of Namibia and Argentina belong to the Large ligneous Provinces (LIPs) of the earth (Hinz et al. 1994). These volcanic types of continental margins are
characterized by volcanic structures that are 5 to 10 km thick and approximately 100 km wide and lower crustal seismic Vp-velocities of 7,2 - 7,6 km/s. The hitherto known very large volcanic units of the south Atlantic continental margins were possibly formed within a relatively short time span of only a few million years.
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The Western Deep Levels Gold Mine 3-D Seismic Survey
More LessThe past 15 years have witnessed the extensive application of the seismic reflection technique to mineral acquisition and production ventures in Southern Africa. (Pretorius et al, 1994). Recently, 3-D surveys have been used successfully in mine planning operations. Several surveys have now been acquired by different mining finance houses (Campbell and Crotty, 1990, de Wet and Hall, 1994, and Tucker et al, 1994. The geological environment in which these surveys have been conducted differs rnarkedly from hydrocarbon surveys. This paper examines the application of 3-D to mine planning using the Western Deep Levels survey as an example.
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Geophysics - Your Key To Success In Africa
Authors D.B. Farquhar-Smith, C. Ecorcheville and S. Kilty“One of the principal problems which faces the geologist in Botswana is that
exposure of pre-Tertiary formations is restricted to about 25 per cent of the
country” (Carney, 1994). In fact the Kalahari sands cover about 1 million square
kilometres of Central Africa extending from the northern Cape Province of South
Africa to the Zaire River in Central Africa. The problem of mapping the geology
through significant cover is restricted to neither Botswana, nor other countries
throughout Africa, it is prevalent on other continents, such as south western USA,
Brazil and the Yilgarn of Western Australia.
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Innovation In Geophysical Gridding Techniques: Intrepid Perspective
Authors D. FitzGerald, P. Dart and M. SchneiderA wide variety of numerical procedures in potential field geophysics require data
modelled on a regular grid. However, airborne data tend to be highly sampled
along the flight line and sparsely sampled in the perpendicular direction,
A gridding method commonly called ‘bi-cubic spline’ is widely used in potential
field geophysics. Standard bi-cubic spline methods used on aeromagnetic data
produce artefacts when a geological feature’s ‘line of strike’ is not perpendicular
to the direction of the acquisition line. This method has a tendency to break up
thin elongated magnetic anomalies, at an oblique angle, into a series of bulls eye
artifacts. A method of finding local anomalies and their strike along lines based
upon minimum variance principles reduces these effects. This technique has
significant impact on the quality of output grids.
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Recent Magnetometer And Spectrometer Experiments Using An Airborne Microlight Platform
Authors E.H. Stettler, M.E. Hauger, H. Coetzee, T. Grace, R.J. de Meijer, J. Limburg, P.J. Külper and P. CoIeFrom the need to perform geophysical surveys cheaper and faster the Geophysics
Division started experimenting with a remotely piloted drone with a magnetometer
and GPS. Since this technology is still relatively underdeveloped the experiments
branched out to using a Streak Shadow microlight plane with modern
experimental fluxgate magnetometer and lightweight 512-channel gamma-ray
spectrometer coupled to a real-time GPS system. Ultra high-density surveys can
therefore be flown at greatly reduced cost. An airborne TDEM system is currently
also being developed.
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Regional Interpretation Of The Airborne Magnetic Data From Northern And Central Namibia
Authors D. EberIe and R. WackerleInterpretation results of refined regional airborne magnetic survey data
over northern and central Namibia are reported. Significant new findings
with regard to the regional geology, tectonic pattern and geodynamic are:
• Seven major crustal domains have been outlined on behalf of their
magnetic anomaly pattern.
• Four directional magnetic lineament sets occur in the region: NW/SE,
NE/SW and to a minor degree E/W, N/S. The latter seem generally to
be longer in extent than the former.
• Magnetization along the southern rim of the Angola craton is extremely
high as modelling has shown. Serpentinized mafic rocks are strongly
supposed to be the magnetic source material.
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Regional Interpretation Of The Airborne Magnetic Data From Southern Namibia
Authors D. EberIe and R. WackerleInterpretation results of refined regional airborne magnetic survey data
over southern Namibia are reported. Significant new findings with regard
to the regional geology and crustal evolution are:
(i) Three major crustal domains have been identified using the magnetic
pattern. These are from northeast to southwest the Kalahari craton,
the Sinclair and Namaland domains.
(ii) The Namaland domain clearly shows indications of intracontinental
rifting given by the Karas magnetic trough trending NWISE over a
distance of more than 300 1cm.
(iii) Three directional magnetic lineament sets are observed in the region:
NW/SE, SW/NE and E/W. Magnetic linear features are particularly
abundant in the Sinclair domain and the Vioolsdrif sub-domain,
which is the southernmost part of the Namaland domain.
(iv) Numerous intersections of magnetic linears have been identified and
as such are locations with promising potential for hydrothermal
mineralization.
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Aeromagnetic Gradiometry And Compensation
More LessAlthough the concepts of aeromagnetic gradiometry are not new, it is only in the past several years, with a handful of gradient-equipped aircraft becoming operational, that the benefits are being realized. The main applications to date are the following:
• In a total field survey, a lateral gradiometer provides additional information about the field between the lines, leading to higher resolution surveys for a given line spacing, or conversely,
the equivalent resolution can be obtained with wider line spacing.
• The lateral gradiometer signal is free from diurnal variations and combined with a longitudinal gradient signal, van be used to calculate a total field that is largely diurnal free, thus leading to the possibility of fewer tie lines being needed for levelling.
• Using measured lateral and longitudinal gradient, a single profile can identify 2-D structures at strike angles other than perpendicular to the flight track, and for such structures, their depth van be accurately calculated using methods such as Werner deconvolution. This technique can be useful in the early redesign or reorientation of a survey with very little flown data.
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Assessment Of Seismic Monitoring In A Deep Level Scattered Gold Mining Environment
Authors R. Ebrahim Trollope and S.N. GlazerSeismic monitoring over the Klerksdorp goldfield has been undertaken since 1971
and over the past three decades development has progressed to the point that
forecasting of large potentially damaging events have become a reality. Three
failure processes have been identified namely, fracturing ahead of the face,
localized structural failure and massive structural failure. In order to improve the
forecasting ability, events recorded from each of these processes will have to be
separated and methods developed to identify massive structural failure.
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Using Broadband Seismology To Resolve The Structure Of The Crust And Uppermost Mantle Of The Kaapvaal Craton
Authors T.K. Nguuri and C. WrightThis paper summaries ongoing research and some preliminary results as part of
current seismological studies of the crust and upper mantle at the Bernard Price
Institute of Geophysical Research. The research project aims at solving problems in
crustal evolution and geodynamics in southern Africa. Anisotropy will be quantified
using Moho mode converted phases and shear waves from mining-induced and
teleseismic earthquakes. Stacking can be applied at each station to mining tremors
with repetitive sources and forms the basis of a pilot study of improvement to the
identification of seismic phases at short distances. The events to be stacked are
carefully selected depending on epicentral distance, azimuth and data quality. The
traces are then aligned by cross correlation, linear shifting and summing. The weak P
arrival becomes distinct in the stacked trace. Other phases such as PmP, S, SmS, can
then be more easily identified. A One-D ray tracing (assuming a flat earth model)
modelling is employed to aid in picking the different phases.
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3-Dimensional Calculations Of Transport Processes Associated With Convection In Freezing Magma Chambers And Attendant Hydrothermal Circulation In Country Rock
Authors A. Rice, A. Botha, K. Harrison, J.M. Moore, P. Clayton, S. Panagou and K. WatkinsReported here are some initial results of a computational effort to conduct finite
element modelling of the formation of mineral deposits. This includes the
convection in a freezing magma chamber and attending hydrothermal circulation
in surrounding country rock. Although 2-D calculations will be presented for
comparison, we have performed the first 3-D calculations of magma chamber
cooling and the response of the surrounding country rock.
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