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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
21 - 40 of 52 results
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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published and 60 unpublished values resulting in an up-to-date heat flow map.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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