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6th SAGA Biennial Conference and Exhibition
- Conference date: 28 Sep 1999 - 01 Oct 1999
- Location: Cape Town, South Africa
- Published: 28 September 1999
41 - 60 of 78 results
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Neogene Structural Evolution of the Southern Zone Bibans Chain (Algeria)
Authors H.L. Kheidri and R.S. ZazounThe area under study is located in the external zones of the Maghreb ides fold belt. This represents the southern section of the Alpine origin. It is bounded from the North by the Bibans chain, which constitutes the “para autochthonous” formations, that consists of Mesozoic limestones-marls series. From the South by the Meso-Cenozoic “foreland” shaley limestones. The whole area is unconformably overlaid by a sandy-shales miocene facies.
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The many facets of integration in E&P information systems
Authors D. Fakorede and R. GrasOperators are pursuing integration in different ways, and in all the components of
information systems. Integration can be achieved at the levels of data, processes and
people, and facilitated by technology. On each of these levels integration adds value to
E&P businesses in different ways. With increasing availability of enabling and
competitive technologies, fully integrated working environments are emerging as the
common place in the E&P industry.
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INTERFEROMETRY An integrated approach to achieving high resolution, wide swath bathymetrical measurements
By H.F. TurckA number of Cape Town based, West Coast, marine diamond mining companies have
been established in the last few years. The requirement to service these companies has
resulted in the rapid growth in the knowledge and techniques required to identify
prospective marine diamond placer targets on the inner shelf
A decade ago most bathymetrical surveys used single echo sounders coupled to a
heave swell compensator. Subsequent to this, multibeam swath systems were
introduced to the marine mineral, diamond, oil and gas exploration and engineering
markets as a new survey technique to gather sonar data in swaths allowing more
density and greater range.
Although multibeam swath systems are a huge step forward in technology, their
usefulness is limited at far ranges where beam spreading occurs. For this reason,
Interferometry was developed as an alternative technique of acquiring bathymetry
soundings providing a greater data density and range than a conventional multibeam
swath system, thereby decreasing survey time and cost.
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SEABIRD and REDAS: New Geophysical Tools for Offshore Mining Exploration
Authors J. Adamy, D. Cathie and C. CerradaThe investigation of the seabed is a major concern for the offshore mining, cable installation and
dredging companies. SAGE proposes two new tools based on geo-resistivity and seismic refraction
methods to address these requirements. While acoustic based methods (subbottom profiler, seismic
reflection) give structural information of the upper part of the seabed, geo-resistivizy and seismic
refraction give a measurement of the resistivity and of the seismic velocity for the investigated part of the
seabei The measurements can be correlated to in-situ geotechnical data such as coring, CPT,
vibrocores, etc...
In this paper, the REDAS and SEABIRD techniques will be presented and typical data will be shown,
validated by ground truthing with cores and CPT data.
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Equivalent Layer Processing of Geophysical data
More LessProcessing of magnetic and gravity data to remove noise or enhance features of interest is a
common requirement. Vertical continuation (to and from both flat surfaces and surfaces of
variable elevation) and the calculation of the vertical gradient are performed here using an
equivalent layer rather than the usual frequency domain methods. The first step is to invert the
data to give the density contrast of a thin surface layer of cylinders which produces the same
anomaly as the observed data profile. The layer’s geophysical response can then be calculated at
other locations, to yield upward continued and vertical gradient data. The equivalent layer
method does not impose many of the stringent requirements on the data that fast Fourier
transform methods do, namely data stationarity, equal data sampling intervals, or the need for 2ⁿ
data points.
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Optimising field acquisition of shallow seismic reflection and avoiding common pitfalls
Authors A.R. Vorster and W.J. BothaHigh resolution shallow seismic reflection (ssr) data acquisition parameters can often only
be determined in the field. Although a good idea of the required parameters can be
determined in the office, there are a few factors that are site dependant which can
considerably influence the acquisition parameters. It is often very difficult, and in some cases
even impossible, to rectify problems associated with the wrong acquisition parameters during
processing.
To ensure that high resolution data is captured walk-away noise tests and/or a small test
survey are normally conducted before starting a full survey. The results of such a test survey
are studied carefully to ensure that the various pitfalls are avoided asfar as possible and that
the optimum acquisition parameters are selected.
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Selected Applications of the Transient EM Method in the Time Domain (TDEM) to Geological Mapping and the Delineation of Shallow Objects
More LessThe transient EM method in the time domain (TDEM) was developed in Russia by V. Sidorov and V. Tikshaev (Saratov) who, for the first time in 1969, conducted field measurements of transient EM processes in a case, where the distance between a transmitter and a receiver was considerably shorter than the depth of investigations. Simultaneously, in the sixties, G. Obukhov in Moscow and A. Kaufman in Novosibirsk developed the theoretical foundations of TDEM sounding. During the first fifteen years the TDEM method was used generally for the search and locating of oil and gas fields located at depth and, in the time range of 0.1 to 10 seconds. Special equipment, which allowed sounding in the range of milliseconds, was used for
modeling of electromagnetic processes in electrolytic baths. The recorded time range of TDEM processes was thus from 1 millisecond to more than 10 seconds.
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A New Helicopter Time Domain AEM Survey system An Overview
Authors I.J. Kane, P.S. Klinkert and T.B. HageSpectrem Air as part of the Anglo American Geophysical department has developed a
new electro-magnetic system (ExplorHem) for airborne surveying by helicopter. The
design of the system is based on similar principles as applied on Spectrem Air’s DC3
fixed wing aircraft system (Klinkert et al paper). This system operates using time
domain, on-time principles. The helicopter unit is nearing its completion and will make
it possible to explore rugged and remote terrain, which has previously been
inaccessible when using fixed wing airborne .systems. Furthermore ExplorHem is an
ideal tool for detailed follow-up operations on surveys carried Out by the fixed wing
system. Spatially detailed surveys are obtained from the helicopter unit, which is
capable of very high resolution conductivity mapping and sounding. The system is
particularly suited for the detection of base metal deposits and kimberlites under thick,
conductive overburden and can also be used in the search for groundwater supplies.
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The Spectrem 2000 AEM System
More LessThe Spectrem2000 AEM system was commissioned on 1 September 1999 after a major
overhaul of the original Spectrem II system.
The Spectrem2000 system is an on-time (as opposed to pulse system in the case of
GeotemDeep, MegaTEM, Questem45o etc) and has an RMS dipole moment of 480 000
Am2 (approx. 300 000 Am2 for MegaTEM) and a peak to peak current of 2000 Amps for
a peak Power Disiipation of 2 MegaWatts, with a rise time of 160 micro-seconds, 100%
duty cycle square wave transmitter.
An on-board 210 SHP Sundstrand Turbine APU generates electrical power for the
Spectrem2000 system.
The receiver systems have been upgraded to allow for selectable operation at variable
base frequencies 25 to 90 Hz, and massively parallel DSP’s and Pentium 333 processors
(96 in all) are utilized in a streaming , and real time processed on-board acquisition
system. Signals are digitized to 24 bit resolution and transformed to STEP response real
time. Noise levels are down from 1000 ppm (Late time STEP response) to about 200 ppm.
New algorithms for Spheric rejection, Low Frequency and Power Line rejection and real
time Conducitivity Depth Imaging have also been developed.
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Possibilities of TEM for oil-gas deposit prospecting in conditions of East Siberia
Authors G.M. Trigubovich, Y.G. Soloveichik, M.E. Royak, V.S. Surkov and V.S. MoiseevThe electroprospecting by transient electromagnetic method (TEM) for oil
prospecting in conditions of East Siberia in the near future is oriented to use areal
observation system. Areal measurement can be made as with the fixed source, as with
using coaxial source and receiver loops. As known, the observations system with fixed
source allows to supply many measurements with minimum expenditures. However
there is an opinion that the observation system with measurements distant from
source, has the worse resolution, than the system with a coaxial disposition of source
and receiver loops. Let’s try get outside of this problem some sides.
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Monte Carlo simulation of airborne gamma-ray spectrometers
By Henk CoetzeeGamma-ray spectra have been simulated using Monte Carlo simulation, applied to
simplified models of real survey conditions. Monte Carlo methods can accurately mimic
the stochastic nature of radioactive decay, transport and detection processes, allowing the
method to be used for the production of synthetic data for testing noise reduction techniques. Simulated spectra may also be applied to calibration and potentially used for direct
forward modelling of spectrometer data.
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Deep Water Seismic Data Processing
By P.M. GidiowIn the quest to meet the increasing global demand for oil and gas, new technologies are developing which allow the search to expand to environments previously considered hostile, such
as deep water. A notable trend is towards increased activity in the deep waters off West Africa, where the lure of high yields helps to offset the high costs. Nevertheless, high drilling costs mean that the industry seeks to extract ever greater amounts of information from the seismic data. Today, typical 3-D surveys in this region cover five times the area and are completed in a half the time taken ten years ago.
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Geologic Signatures Revealed Directly from 3D Seismic Data using the Coherence Cube Methodology
Authors Tony Rebec, Laura Evins and Richard JakemanApplication of the coherence cube methodology has provided improved understanding of structural and stratigraphic details of the subsurface, leading to revised 3-D geologie models for the exploration and production of hydrocarbons throughout the world. Presented here is a description of the Coherence Cube methodology and various new applications with examples from major
oil fields. It is intended as both an introduction to the technology, and to demonstrate the power of its use as a complementary volume to the conventional seismic volume for 3-D seismic interpretation on the workstation. This technology reveals important geologie information that may be totally overlooked using conventional techniques.
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Use of 3D Seismic in Offshore Mining of Gold and Diamond Deposits
Authors T.B. Berge and S.L. CheletteCoherence CubeTM processing of 3D seismic data has recently become a useful method
in the Oil and Gas exploration business for revealing stratigraphic details of target
reservoirs. We suggest that this technology might be borrowed from the oil industry and
effectively applied to high-grade prospective areas for dredge mining of gold and
diamonds in the shallow fluvial-deltaic deposits of the Orange River Delta.
The fluvial dynamics of these relatively denser clasts suggest they should be more
concentrated along meander cut-banks, in point-bars, and in proximal parts of deltas.
Such stratigraphic detail can be readily seen on flattened time-slices of Coherence
CubeTM images generated from 3D seismic volumes.
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3-D seismic modelling using Lithoprobe AGT’92 high resolution seismic wide angle reflection and refraction data in Sudbury, Ontario, Canada
Authors Lingxiu Jiao and Wooil M. MoonA 3-D seismic velocity structure of the Sudbury Structure, Ontario, Canada, hosting one of the world’s largest Ni-Cu mineral deposits, was computed and imaged using the 1992 Lithoprobe
Abitibi-Grenville Transect (AGT) high resolution seismic wide angle reflection and refraction data. One of the objectives of the 1992 Lithoprobe high resolution seismic experiment was for providing physical constraints in the investigation of the origin of the enigmatic meteorite Sudbury impact structure (~ 1.85Ga) (Clowes, 1989). In this study, the 3-D weighted back-projection tomography algorithm (Hole, 1992) and 3-D simple seismic ray tracing modelling technique were applied to reconstruct and reveal the complex crustal structure.
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Crustal Structure of Southern Africa: Results from Receiver Functions and Surface Wave Inversions
Authors T.K. Nguuri, C. Wright, D. James and J. GoreAn 82-station broadband array with an aperture of 1800 1cm was deployed in Southern Africa in April 1997 (Carison et al., 1996) to record local and teleseismic earthquakes for two years. Teleseismic data have been processed using receiver function methods (Ammon, 1990) and two-station phase and group velocities (Herrman, 1973) have been estimated from both teleseismic and regional events.
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The integration of gravity data with seismological results from the Kaapvaal Craton Seismic Experiment: an assessment of isostatic balance
More LessRecently, the collection of broadband seismic data was completed marking the end of this
phase of the multinational “The anatomy of an Archean craton” project (Carison et al.
1996) (Kaapvaal Craton project). During the two-year project over 80 individual sites were
occupied by broadband seismometers. For the initial year 54 sites were occupied, 23 of
these were then moved to new sites for the second year of operation. Additional sites were
occupied as some sites were unsuitable. Preliminary moho depths have been calculated for
many of these sites (James et al. 1998). These crustal thickness data are used to assess the
isostatic balance of the crust. A simple model of crustal thickness based on Airy isostasy
(i.e. topography as the only bad) is calculated and compared with preliminary receiver
function results. This comparison reveals that the crustal thickness variations are far
greater than can be explained with topographic variations. Most of the cratonic regions
appear to be in isostatic balance. An enigmatic region is that of the Bushveld, which
appears to have thickened crust that is not in isostatic balance. These results support the
proposal that this thickening is due to the Bushveld being continuos from west to east, thus
causing crustal flexure (Cawthorn et al.1998).
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The upper mantle and transition zone below and in areas adjacent to the Kaapvaal craton from P- and S- wave arrivals
Authors R.E. Simon and C. WrightThe upper mantle and transition zone below and in areas adjacent to the Kaapvaal craton is investigated using travel times, amplitudes and waveforms of regional and teleseismic earthquakes
at distances less than 40°. The data are used to study possible discontinuities and lateral heterogeneities in the upper-most 800km of the mantle, and are from six events recorded by the Kaapvaal craton broadband seismic network, anatomy of eighty broadband station that form part of the international Kaapvaal craton programme (Carison et al., 1996). The seismometers were deployed at locations across southern Africa from April 1997 until April 1999, with about fifty stations operating at any one time.
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New geological and petroleum insights gained from geophysical imaging along South Africa’s western margin
More LessThis presentation will illustrate how new, until now hidden, messages can be interpreted
from gravimetric, magnetic and conventional 2-D seismic reflection data imagery along the
western continental margin of South Africa (Fig.1). The advent of satellite derived offshore
gravity together with sea floor and land topography has opened up a refreshing way of
looking at our continental margins, and has re-emphasised the link between the latter and
their continental interiors and oceanic regions. Reprocessing of old magnetic records, and
the integration of land and marine data sets, is also rapidly expanding our understanding of
basement architecture. Expanding knowledge of sand-rich deep-water canyon-fan ~systems
(largely as the result of pioneering petroleum exploration and 3-D seismic surveys), has given
new insight into the predictive interpretation of 2-D seismic data in frontier areas where no
boreholes or 3-D seismic data exist. The direct effects that hydrocarbons may have on
seismic data have helped reduce exploration risks world -wide. For example, the ‘chimney’
effect that natural gas escaping through the rock column can have on seismic data may point
to exploration fairways for gas in the Orange Basin (Fig.1). This is particularly the case
when viewed together with gas shows, surface gas seeps, ‘bright spots’, AVO anomalies, etc.
in the same vicinity, or along the predicted migration routes away from known source rock
‘kitchens’. The increasing attention given to the deep-water portions of margins has led to
the recognition of possible gas hydrate zones in many areas. This margin is no exception.
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Geophysical Investigation of the Marble Hall Fragment of the Bushveld Complex
Authors Mr. O.K.T. Babayeju, Prof. W.J. Botha and Prof. S.A. de WaalThe geophysical investigation of the Marble Hall Fragment is part of a research project on the
Marble Hall Fragment jointly funded by the Foundation for Research and Development (FRD)
and ISCOR. The Marble Hall Fragment is the folded complex of sediments of the Transvaal System that
surrounds the town of Marble Hall and is situated approximately 160 kilometers due north-east
of Pretoria. The area covered by this investigation lies between longitudes 29° 10’ 0011E and 29°
25’ 00”E; and latitudes 24° 50” 00”S and 25° 06’ 30”S respectively.
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