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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 - 50 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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New Heat Flow Map Of Southern Africa
Authors M.Q.W. Jones and L.A.G. AntoineThis paper presents a compilation of heat flow data that have been accumulated in southern
Africa during the past 60 years in the form of a map showing the variation of heat flow within
the tectonic framework of the subcontinent. The data base comprises approximately 190
published and 60 unpublished values resulting in an up-to-date heat flow map. The map shows
up important relationships between thermal regime and tectonics.
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Gravity And Geoid Investigations Of The Kaapvaal Craton
By S.J. WebbThis study forms part of the Kaapvaal Craton research project. The gravity and
geoid data of southern Africa are evaluated in light of the tectosphere model. This
is accomplished by parameterizing the geology by age and tectonic history. The
seismic data can be related to the resulting geoid model by evaluating the
influence of density on both data sets. The area of the Kaapvaal Craton seismic
experiment will initially be evaluated, but this study can be extended world wide.
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Relative Moment Tensor Inversion Applied To Clusters Of Mining Induced Seismic Events: Stability Tests
More LessA relative moment tensor inversion technique, developed by Dahm (1996) for
seismic sources belonging to a small source region, is applied to clusters of
mining induced seismic events. The sensitivity of the output moment tensor
components to random noise applied to the input amplitudes is tested.
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Preliminary Estimates Of Seismic Hazard In Eastern And Southern Africa
Authors A. Kijko, G. Graham and C.R. RandallMost seismic hazard assessment procedures rely on the identification of seismic source
zones and the accurate determination of their parameters, such as the Gutenberg - Richter b
value, activity rate, maximum possible earthquake and completeness of the observed data.
In Africa in particular this type of assessment can become very subjective as the available
earthquake catalogues are incomplete and seismogenic areas or structure are not well
defined. A new seismic hazard analysis approach which does not rely on the existence of
source zones is described here. Through this approach an estimation of the seismic hazard
taking cognisance of incomplete data, at individual sites is possible.
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A New Software Package To Process The Data Obtained By The South African National Seismograph Network
More LessThis paper describes the seismicity of South Africa and introduces the reader to
the South African National Seismological Network, analysis method, publication
of data and plans for the future. The emphasis of this paper is on the SEISAN
earthquake analysis software which was recently implemented by the Council for
Geoscience for analysis of the data obtained by the National Network.
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Contributions To The Ims As Part Of The Nuclear Test Ban Treaty: A Primary Seismological Station For Southern Africa
Authors M.R.G. Smith, G. Graham and D. RoblinA group of scientists formed during the United Nations Conference on Disarmament
initiated an international network of seismological stations as part of an experimental
monitoring system. The expectations of this system are to be able to register
explosions, especially nuclear ones, and thus to act as a monitoring device for a future
comprehensive test ban treaty. South Africa is one of the 60 countries involved in
such an experiment, and supports stations in Boshof , Sutherland and Antarctica.
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A Comparison Between Robust M-Estimation And Adaptive Lp-Norm Techniques In Determining The Best Apparent Resistivity Versus Frequency Curves In Mt Soundings
Authors J. Prinsloo, E.H. Stettler and A. KijkoData collected at a magnetotelluric sounding station are usually severely affected
by noise. The result is that impedance estimates do not fall on a single point but
are scattered. Statistical manipulation of the data is necessary to reduce the
scattered points to the most probable value. Although least squares regression
provides a simple way of doing this it can lead to erroneous results since outliers
ensure that the residuals are not normally distributed. Robust M-estimation deals
with outliers by applying a weight function, determined by normalized values of
the residuals, to the least squares regression formula. Contrary to this the adaptive
Lp norm technique uses the quality of the data to determine the best norm value
for minimizing the error function. The results of tests with synthetic data show
that the adaptive Lp norm technique yields the best results.
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Back-Calibration Of Regional Airborne Gamma-Ray Data Sets From Namibia - Results And Experiences
Authors D. EberIe, D.G. Hutchins and R. WackerleDuring 1992/93 and 1995/96 select portions of existing airborne gamma-ray data
comprising 210 000 line 1cm flown in west central and southern Namibia were
reprocessed. Available data represented 21 different surveys collected over a
period of about twenty years with a variety of gamma-ray spectrometers, spectral
windows and survey parameters. The compilation involved the interactive digital
recovery of 42 000 km of original analogue chart traces. Preliminary digital grids
of each radioelement were compiled, verified and used to select representative
sites for ground measurements within each survey block. Results obtained from
the ground program were used to back-calibrate the airborne data and convert
airborne measurements into equivalent ground concentrations of potassium,
thorium and uranium. The survey blocks were merged providing a digital master
grid. The merged data are available as either contour maps or as digital grids
conform in scale and coordinate system with the regional airborne magnetic series
of Namibia.
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The Hentiesbaai Range For Airborne Gamma Spectrometer Calibration
More LessGammaspectrometers used in geophysical prospection have to be calibrated i.e. the measured window-countingrates in the potassium-,uranium- and thorium- windows have to be converted into potassium-, uranium-, and thorium-concentrations of the soil or rock underneath of the detector. In case of small portable spectrometers which can be put directly to the surface of a socalled calibrationpad with a large diameter compared to the diameter and height of the detector crystal this can be done using concrete-pads of one or two meters diameter and 30 cm thicknis and exactly known concentrations of the above mentioned three radioelements (Lovborg 1984), (Grasty 1987, 1991).
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Equivalent Layer Gridding Of Geophysical Data
More LessAny magnetic or gravity data can be modelled using a surface layer which
produces an equivalent anomaly to that produced by the real subsurface geology.
Once such a layer has been estimated, then the field at any position in space can
be simply calculated. This method of interpolation has the advantage that the
variation of the field with distance is not some arbitrary function such as 1 /rn, but
is a natural function of the magnetic or gravity field itself. To avoid the
interpolated field oscillating in in amplitude in regions far from known data
points, the depth of the equivalent layer is varied according to the spacing
between the known data points. The disadvantage of this method is that it involves
the inversion of large matrices.
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Geomagnetic Activity At Low Latitudes: Periodicity And Predictability
Authors G.J. Kühn and P.R. SutcliffeThe sun has progressed past the minimum of the
current 11-year sunspot cycle and solar activity,
which is the energy source of terrestrial magnetic
disturbance, is on the increase. One can therefore
expect that human technological activities
affected by geomagnetic disturbance will be
increasingly affected during the next 3 to 7 years.
In geophysical surveying this effect can be
beneficial (e.g. magneto telluric surveying which
relies on the occurrence of natural geomagnetic
disturbance as a signal input) or detrimental
(aero-magnetic surveying which is adversely
affected by natural magnetic disturbance -
especially at the resolution currently being pursued).
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Progress Toward A Regional Geomagnetic Field Model Embracing The Sq Variation
More LessGlobal and regional geomagnetic field models give the components of the
geomagnetic field as functions of position and epoch. However, the only
temporal variation generally catered for in these models is the secular variation.
There is an increasing need amongst certain users for models able to provide
shorter term temporal variations, such as the solar quiet day (Sq) variation. In this
paper we give a brief review of the Sq variation and describe how artificial neural
networks (ANNs) are being used to develop a geomagnetic daily variation model
for the Southern African region.
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An Analysis Of The Natural Fluctuations Of The Geomagnetic Field
Authors D.M. Seabrook and L.A.G. AntoineThe noise in high-resolution aeromagnetic surveys
originate from many sources, for example, from the
instrumentation, navigational errors and the natural
time-varying magnetic field. These inherent noise
components, in the survey’s noise budget, are
summarised in Table 1. It is dear from this table
that the noise from temporal variations are
significant. The effect of the time varying field can
be removed by subtracting the field recorded at a
base station. A common alternative practice is to
include this noise in the tie-line levelling
procedure. Past experiences and experiments are
incongruent as to which is the most appropriate
technique.
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Images Of Pollution Geophysical Mapping In Environmental Monitoring
More LessHigh quality geophysical imaging of the surface and subsurface is an important
mapping component of site investigation exercises carried out to fulfil the
requirements of Environmental Management Programmes. This is particularly so
when evaluating the potential for, or the presence of, groundwater contamination
caused by acid-mine-drainage or leachate escape from waste disposal sites. Rapid
reconnaissance geophysical mapping is used to screen relatively large areas and
draw up “environmental hazard” maps, whose comprehensive coverage ensures
that all major potential leakage routes or pollution centres have been located for
follow-up investigation by more costly, direct sampling methods.
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Integrated Geophysical Approach To Groundwater Exploration On The Lebowa Granite Suite
Authors M. Combrinck and W.J. BothaIn 1996 an integrated groundwater research project
was launched by the University of Pretoria’s
Geology Department. This project is funded by the
Foundation for Research Development (FRD) and
supported by the Department of Water Affairs and
Forestry (DWA&F). The objective of this project
is to produce a sustainable, clean water supply in
rural areas. This is done in two phases:
• Exploration for new or additional groundwater
sources
• Using GIS to design a management plan
(including aspects such as pollution, balance
between recharge and pumping and future land
use planning)
Different disciplines incorporated in the study are
geology, engineering geology, structural geology,
satellite imagery and geophysics.
From this project it became apparent that the
successful siting of boreholes for groundwater
development was crucial. This led to the current
project which concentrates on improving borehole
siting techniques.
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Helicopter-Borne Electromagnetic Groundwater Exploration Survey In The Kuiseb Dune Area, Central Namib Desert
Authors B. Siemon and K.-P. SengpielAs part of a groundwater exploration project, BGR conducted an aerogeophysical survey in the Kuiseb Dune area, southeast of Walvis Bay, Namibia, in 1992. The helicopter-borne electromagnetic system (DIGHEM III) provided three frequency HEM data, which were inverted into the parameters of homogeneous or layered half-space models. The resistivity data revealed a network of palaeo channels of the Kuiseb River beneath the sand cover, and several freshwater lenses along the coast.
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Adding Value To Airborne Geophysical Data: Gis Explorations Of An Environmental Problem
Authors Henk Coetzee and Stéphane ChevrelGold Mining activities have had a significant effect on the environment of the
Witwatersrand and the surrounding downstream areas. There are numerous and
diverse data describing these effects, but to date little effort bas been made to
integrate these. Furthermore, few studies have been done which look at the
environmental impact of mining on a regional scale. In this study we have
combined various available datasets to produce an assessment of the pollution and
pollution risk for a portion of the West Rand.
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Regolith Mapping Using The Spectrem Aem System
By C.T. ArcherThe problem of conductive regolith is one which confronts earth scientists in many of the world’s prospecting environments. Electromagnetic responses of target conductors located beneath
this conductive cover may be attenuated in amplitude and altered in shape by the influence of the overlying cover, leading to a reduction in the accuracy of the interpretation applied to the data. It is therefore important that an attempt be made to understand and minimise the effects of conductive overburden on electromagnetic prospecting.
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Delineation Of Oil And Gas Fields In The Bredasdorp Basin, Offshore South Africa, Using Seismic Attribute Mapping, Seismic Inversion And Avo <br>Techniques
By K.R. BartonThe E-BD oil and the E-CE oil and gas accumulations occur as separate reservoirs
consisting of amalgamated deep-marine sandstones. Large areas of these reservoirs
are close to or below seismic tuning thickness. A suite of geophysical techniques
have been applied to a 3D seismic data volume covering these fields in order to
determine their geometries. The techniques employed include seismic amplitude
mapping, seismic correlation, seismic inversion and AVO processing. These
methods were evaluated and used to assist the interpretation, often complementing
each other. In this way, geological models for sandstone distribution have been
derived which explain the lack of communication between hydrocarbon-bearing and
water-bearing sandstones, and the nature of the reservoir facies encountered in
boreholes. This has led to a better assessment of the in-place hydrocarbon volumes
and upside potential of these fields.
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Geophysical Survey Over The Elizabeth Bay Mine, Namibia
Authors J. Blume, H. Lütjen and C.C. PretoriusIt was decided that geophysics could play a significant role in mapping the
diamondiferous aeolian deposits at Elizabeth Bay. The emphasis was to derive
more from the geophysics than determining orebody thickness and the occurrence
of day, but also to extract information on the degree of cementation of the
deposit to assist mine geologists and engineers in mine planning. A detailed
electrical survey was carried out, followed by borehole logging and seismic
refraction work. Accurate geophysical models of the orebody were achieved.
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How To Find A Water Wellfield : Geophysical Mapping For A Platinum Mine Water Supply
More LessAt start-up in 1993, the PPRust Platinum Mine water supply of over 2 million
cubic metres per annum was entirely sourced from water wellfields located over
mafic Bushveld rocks up to 9km north of the mine. The aquifers supporting this
supply were mapped, drilled and evaluated in a geophysically-driven programme
compressed into a 5 month period during mid 1992.
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Application Of Image Processing Techniques To Airborne Gamma-Ray Spectrometric Data
By Henk CoetzeeAirborne gamma-ray data are commonly collected as an add-on in magnetic surveys,
at little additional cost. Data are presented in a similar format to magnetic data, and
processed from raw line data to finished maps and images using similar algorithms. A
gamma-ray spectrometric measurement is however fundamentally difficult to a
magnetic field measurement, and therefore requires different treatment and a different
approach. Image processing techniques commonly applied to remotely sensed visible
and infra-red reflection data may be more suitable for the enhancement of radiometric
anomalies.
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The Development Of A Gis System And Database For The Management Of Geophysical Data And Interpretations For The Geophysics Division Of The Council For Geoscience
More LessGeographic Information Systems (GIS) and
databases have played a vital role in the
management of various types of data throughout
industry and the world. In every area of life, there
is normally some form of information involved,
but at some point, the efficient management of
that information must be questioned. It is for this
purpose that GIS systems and databases play a
vital role. Not only is it necessary to be able to
store the data efficiently, but it is also necessary
to be able to retrieve the data in as short a period
of time as possible, while obtaining the maximum
amount of information about that data.
Geophysics is a field which is certainly no
exception to this fact.
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Fractal Convergence Properties Of Geophysical Inversion
More LessOne serious problem with most inversion schemes is that they are liable to
converge to local minima - that is they reach a set of model parameters that
provides a better fit to the data than the starting guess, but they do not reach the
set of parameters that would provide the best fit possible. The set of initial model
parameters that converge to a particular minima of the misfit surface for magnetic
data is studied, and found to be fractal. The principle is demonstrated for the
situation where there are two minima available for the model to converge to.
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The Use Of Wavelets In The Filtering Of Geophysical Data
Authors G.R.J. Cooper, M.Q.W. Jones and D. SeabrookWavelets have many advantages over conventional Fourier transforms when it
comes to the filtering of geophysical data. Wavelet transforms give information
about the energy present in a signal at different frequencies and at different
positions. This can be used to filter data effectively. In addition, the wavelet used
can be selected for the task required - a wavelet that is well localised in space can
be used for edge detection, for example. The applications of wavelet based
filtering to magnetic data are demonstrated.
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Structural Insights Gained From A Comparison Of Offshore Namibia Satellite Gravity Data With Onshore Magnetic And Gravity Data
More LessOffshore Namibia satellite gravity data are juxtaposed against the aeromagnetic
data and Bouguer gravity data of southern Africa in order to elucidate structural
and lithological features within the offshore magnetic basement. A large circular
gravity high) interpreted to be of similar origin to the Erongoberg, is seen some
50km southwest of Walvis Bay. The most striking result of this study is that the
interpreted Mesozoic age, narrow, coast-parallel intrusions, associated with
extension, have been faulted along the offshore continuations of early Palaeozoic -
late Proterozoic structures, suggesting that these structures were active until late
Mesozoic times. An important implication is that these structures provided
potential pathways for major drainage systems and hence the focus of major
offshore sedimentation.
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Dighem Resistivity Techniques Applied To The Detection Of Intrusive Dykes Within Coal Seams
More LessThe DIGHEMv airborne HEM system has been
widely used:
1 to aid geological mapping,
1 for groundwater exploration,
• for the detection of kimberlites,
• as an alteration mapping tool in gold
prospecting
• and also to find and delineate diamondiferous
gravels.
To our knowledge, this system has not previously
been applied for the detection of dykes
transecting coal seams.
The DIGEMv system has been used at the Avmin
- Forzando Coal mine in order to detect non
magnetically susceptible dykes. The survey was
carried out by Geodass Pty Ltd in conjunction
with DIGHEM/ CGG.
The Forzando Coal mine is located in the Bethal
district in Mpumalanga.
Dykes intruding into coal cause major mining
problems and can also reduce the coal reserves by
burning of the coal seams in the vicinity of the
dyke.
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The Bushveld In A Single Pulse?
By A. RiceComputational modelling of freezing magma chambers bas suggested that
inclusion of the effect of suspended crystal bad (which increases the effective
density) serves to split the magma chamber up into a number of stratifications,
each stratification a layer of magma of significant depth, but each layer varying
in composition. This suggestion has been verified by reviewing the experience of
other disciplines (e.g., industry, limnology, etc) whose own efforts indicate the
size of these layers are amenable to quantitative estimates.
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The Use Of Geophysical Techniques To Site High Yielding Water Supply Boreholes
Authors Peter Rosewarne and Ted MillsGeological and climatic conditions in South
Africa are not generally conducive to the
formation of large scale aquifers capable of
sustaining high yielding boreholes. Over 80% of
the country is underlain by old consolidated rocks
with negligible primary porosity and 75% of the
country receives <250 mm/a of rainfall. With this
information as background it is not surprising
that, although groundwater is found almost
everywhere in the country, most boreholes have
sustainable yields of «51/s. It is also not
surprising that groundwater supplies only about
13% of the total water consumption in South
Africa, the remainder coming from surface water
sources. This ratio is reversed in many European
countries.
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An Assessment Of The Current Radio Tomography Imaging Capabilties - Results Of A 2-D Computer Simulation Study
Authors A.M. van Schoor, E.C. WedepohL and D.R. VogtIn this computer simulation study, the current RT technology as developed at CSIR,
Miningtek, is benchmarked. The basic principles, strengths and weaknesses of the
technique are highlighted and its applicability to the mapping of small-scale features
that disrupt the mining of ore bodies is assessed. It is found that RT can potentially
solve some of the typical problems encountered in South African gold, platinum and
base metal mines. In addition, areas where the current RT technology needs to be
improved are identified. Also, ways in which to reduce RT survey time and cost are
investigated.
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