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1st SAGA Biennial Conference and Exhibition
- Conference date: 29 Jun 1989 - 30 Jun 1989
- Location: Johannesburg, South Africa
- Published: 29 June 1989
1 - 20 of 49 results
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On The Agulhas Magsat Anomaly: Implications For Continental Breakup Of Gondwana
Authors L.A.G. Antoine and A.B. MoyesThe Agulhas anomaly is a prominant intense satellite magnetic anomaly situated off the southeastern coast of Africa. The anomaly is clearly identifiable on all published satellite magnetic anomely maps (Regen et al., 1975; Langel et al., 1982; Hastings, 1982; Hahn et al., 1984; Yanagisawa and Kono, 1985; Arkani-Hamed and Strangway, 1985; Kuhn and Zaaiman, 1986). Implicit in the publication of a global satellite magnetic anomaly map is that the data represent magnetic responses from magnetic mineral concentrations originating within the crust and/or upper-mantle.
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New Heatflow Estimates For Australia And South Africa
By F.W. ArnottGeothermal constraints for mapping and evolution studies of the lithosphere, fission track dating and in exploration studies of oil maturation
and exothermic deposits are limited by the distribution of heatflow data.
This is due to the time and oost required to collect data and the absence
of deep drill holes in areas where new heet flow data is required. Heet
flow data is only available for 109 sites in Australia (Cull 1982) and 112
sites in South Africa (Gough 1963; Carte and van Rooyen 1969; Jones 1987;
1988). These are mostly in mineralized terranes.
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The Bumbeni Line - An Aligned Group Of N.E. Trending Bodies In Northern Natal
More LessA characteristic of the African crust are numerous NE-SW trending
lineaments along which volcanic and sub-volcanic ring complexes, basalt
flows and acideous plutons predominantly occur.
One such lineament is the Bumbeni lineament (Fig. 1) which consists
of numerous igneous complexes and 13 can presently be identified (fig.
2). The majority of the complexes are buried and situated along a line
stretching from Sodwana Bay (northeastern Natal coast) through the
Hiuhiuwe and Umfolozi Game Reserves towards the southern boundary of
the Kaapvaal Craton (Fig. 1). The only exposed volcanic member along
the lineament is the Bumbeni Complex (Fig. 1) which can geophysicafly
be subdivided into two separate bodies namely Complex A, consisting of
acideous plutons and a ring dyke complex called Complex B.
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Seismic Stratigraphy: An Integrated Aproach To Predicting Hydrocarbon Plays
By G. BeamishThe post-rift Cretaceous sequences of the Bredasdorp Basin, offshore
South Africa, have been rigorously analyzed using the unified
application of the latest seismic-stratigraphic, sequence-stratigraphic
and depositional systems concepts. Utilizing 101 seismic profiles,
totalling 5600km, 10 cyclic depositional sequences have been identified
bounded by 9 Type 1 and 2 Type 2 unconformities.
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The Trans-Witwatersrand Basin Deep Seismic Reflection Profile:<br>Implications For Horizontal Tectonics In The<br>Early Archean Basement
Authors W.P. Colliston and W.U. ReimoldThe Trans-Witwatersrand Basin Deep Seismic Reflection profile (Geol.
Survey of Rep. of S. Africa, open file) adds a new dimension to the
Vredefort controversy (hypothesis: astrobleme, internally derived
catastrophe, tectonic origin). In principie, such a profile could
resolve this uncertainty, but the resolution is as yet not sufficient to
remove all ambiguity.
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The Beattie Anomaly And Its Significance For Crustal Evolution Within The Gondwana Framework
By B. CornerIn this study magnetic data from southern Africa and adjacent areas in
Antarctica are compared within a Gondwana framework. The results show
striking similarities in regional structural style, as interpreted from
these data, between the Namaqua-Natal Belt and the area to the east of the
present-day Lebombo margin of the Kaapvaal craton (western Dronning Maud
Land, Antarctica).
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A Geophysical Case Study Of A Nickel Sulphide Occurrence Situated In The Western Portion Of The Bushveld Complex
Authors B. Corner, E.C. Wedepohl and R.J. DurrheimIn this study, a nickel sulphide occurrence, situated in the western
Transvaal some 150 km to the northwest of Pretoria, was investigated
geophysically. The geophysical surveys were conducted as part of field
schools held, in 1985 and 1986, for the third-year and honours students of
the Department of Geophysics, University of the Witwatersrand.
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Seismic Raytracing Of An Overturn Structure
By G.E. DavisonSeismic exploration of the Witwatersrand Basin has greatly increased our
understanding of the structures within the basin over the last several
years. In most of our surveys, the seismic data have been sufficiently clear to determine the structures within the basin and the position of the
Central Rand Group. However, in several areas around the basin, complex
structures (such as overturn structures) exist. Where seismic profile lines
cross these structures, the seismic data become very confused, and
interpretation becomes very difficult. This is especially true of overturn
structures. In an attempt to better understand what happens seismically at
overturn structures, 2-D and 3-D raytracing were performed on a computer
model of an overturn structure within the Witwatersrand Basin. This
experiment was also an attempt to image such a structure.
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Seismic Raytracing As An Aid To Seismic Interpretation
By G.E. DavisonOften times, seismic profiles run oblique to structures, causing sideswipe
and confusing reflector patterns. Seismic Raytracing can be very helpful in
distinguishing structures that are profiled at an oblique angle. The
procedure would be to create an interpretive model, raytrace it, create a
synthetic seismogram from the raytracing data, and compare it to the actual
seismic data to see if the model is indeed representative of the actual
structure.
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Pleistocene Estuary Of The Orange River
Authors R.W. Day, A.J. Franzsen and J. RogersPalaeochannels are widely distributed along the inner
and middle shelf of the west coast of Southern Africa, but
of particular interest is a set of channels off the southern
Namibian coast (Fig 1). This article describes the mapping
of these channels using seismic records, and offers a model
for their formation.
Two types of seismic data are presented - one using
Boomer (Marine Diamond Corporation) and the other using a
lower frequency airgun (Geological Survey of South Africa).
The Boomer penetrated 20 metres or more into the inshore
sediment wedge and its records often show details within the
Cretaceous and Tertiary bedrock (Fig 2). The airgun
generally achieved better penetration into bedrock but at
the expense of resolution. For both surveys, Decca Navigator
was used for position fixing (Foster 1973; Birch 1979).
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A New Approach To Velocity Filtering
More LessThere are two very important considerations when deciding on or evaluating
velocity filtering (also known as dip filtering) of reflection seismic shot records:
1. Are relative amplitude strengths preserved ?
2. Is there sufficient discrimination on the basis of slope
between primary reflections and coherent noise ?
The first issue is very important when any AVO (amplitude variation with
offset) processing is done, and the second issue determines how effective
the filtering process will be.
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Coherent Noise On Marine Seismic Data - Its Causes And Cures
Authors A.T. Dippenaar and P.R. LevittDuring recent years SOEKOR bas been plagued by linear coherent noise on
stacked seismic sections. The dips of these coherent events are mainly in
the negative direction with velocities varying between 2200 and 3500 m/s.
Many theories have been forwarded to explain the origin of these linear
events and just as many solutions offered to solve the problem. SOEKOR's
increased reliance on seismic stratigraphic (as opposed to structural)
interpretation, which requires good signal-to-noise and high resolution
seismic sections, has made it imperative that solutions for this problem
be found.
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Numerical Modelling Of Dc Resistivity Problems
More LessNumerical methods for solving the DC geoelectrical problem can
be divided into two groups : domain methods and boundary
methods. In domain methods the whole of the domain is
discretized while in the boundary methods only the boundaries
of domains are discretized.Under domain methods one would find
amongst others the finite difference method and the finite
element method. A popular boundary technique is the boundary
element method.
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Significance Of Seismically Reflective Layering Within The Basement To The Witwatersrand Basin
Authors R.J. Durrheim, B. Corner and G.R.J. CooperDuring the past decade deep seismic reflection profiling in
Europe, north America and Australia has produced new, insights into
continental structure and evolution. However, very few profiles have been
acquired over Archaean rocks. There are several fundamental guestions
regarding the evolution of the earth which seismic studies of the Archaean
crust may help to answer. These include:
- Have tectonic styles evolved with time?
- Is the type of upper crust related to the type of lower crust ?
- Why does Proterozoic crust seem to have a greater tendency to be covered
by Phanerozoic sediments than Archaean crust?
- Has there been significant overprinting of early tectonic features in the
lower crust ?
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Seismic Velocity Structure Of The Central Kaapvaal Craton
Authors R.J. Durrheim and R.W.E. GreenDuring the 1950’s several investigations made use of
Witwatersrand mine tremors in order to study the seismic velocity structure
of the Kaapvaal Craton, and derived simple one- or two-layered models
[Wilmore et al.., 1952; Gane et al., 1956; Hales and Sacks, 1959]. In this
study we take advantage of the advances in seismograph design, signal
processing and seismic interpretation techniques to derive a substantially
more detailed crustal model.
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Non-Hyperbolic Moveout Correction Of Reflection Seismic Records
More LessFor a horizontally layered earth the exact two-way travel time equation
is:
T2 = T02 + x2 / V2 + C3 X4 + C4 X5 + - - - (1).
(Taner and Koehler, 1969).
T = two-way time from source to receiver; T0 = vertical two-way time;
X = source to receiver distance (offset); V, c3, c4, = constants
which depend on the interval velocities (V1) thicknesses of the
individual layers (Fig. 1).
The terms in X4 and X5 are small for low and medium angles of
incidence, and these terms are normally ignored in seimic processing.
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Vertical Seismic Profiles - The Heard Rock Environment
By A.M. FRANCISIt is now some years since the first Vertical Seismic Profile (VSP) data
was acquired in South Africa in the so-called “hard rock” mineral
exploration environnent. In the intervening period to date, although use
has been made of the technique, most noticeably for seismic correlation,
the VSP in hard rock has not been utilized to quite the same extent as in
the oil industry.
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Preliminary Results Of Potential Field Studies Of The Kaapvaal Craton - Kheis Subprovince - Gordonia Subprovince Transitions
Authors K. Geerthsen and M.J. MaherAs part of the National Geophysics Programme (NCpP) a study of the geo-
physical signature of the transition from the Kaapvaal Craton to the Kheis
Province and from the Kheis Province to the Nemaqua Metamorphic Complex was
undertaken. Previous geophysical work on these features includes the
modelling of long gravity profiles by De Beer and Meyer (1984), a detailed
gravity survey by Maher (1980) and natural remanent magnetisation (NRM)
studies by Onstott et. al. (1986).
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New Developments In Underground Microgravity
Authors Christian Gram and Dr. Uwe CastenUnderground gravity observations in deep coal mines using the conventional
gravity meter LaCoste-Romberg Model D which has been adapted to the fire
damp regulations, can be accurate to ± 3 ugal. For underground
determination of the vertical gradient of gravity the LaCoste-Romberg meter
is used together with a specially designed measuring tower. Using this
equipment an accuracy in tower gradient observations of ± 30E was obtained.
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A Geophysical Case Study Of A Copper-Zinc Sulphide Deposit In The Northern Cape
Authors E. Groenewald, E. Gurria, J. King, V. Nooko and C. RedekoppThe 1989 Wits University Geophysics Field School was held at
Copperton, 60km southwest of Prieska, northern Cape. With the
permission of Anglovaal Ltd., an intensive geophysical study was
carried out over the annex to the main sulphide ore body. The
geology of the deposit is described in detail by Wagener and Van
Schaikwyk (1986). At the site of the Annex sulphide body, the host
rocks are overlain by a thin layer of tillite of the Dwyka Formation.
Calcrete is locally developed. On the basis of previous work
(Anglovaal Ltd.), two lines on the existing survey grid ware chosen
for detailed study. Various techniques were used, and instrument
responses were compared over these two lines, each 500m long and 200m
apart. We present data from one of these lines (Fig. 1). The
techniques used ware EM (Geonics EM-34, Crone Pulse EM, Scintrex
Genie), time domain IP (gradient array), SP, resistivity sounding
(Schlumberger array) and profiling (gradient array), gravity,
magnetics and hammer refraction seismics. The results and a
qualitative interpretation follow.
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