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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
61 - 78 of 78 results
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Regolith electrical structures associated with kimberlite dykes - an example from the Archaean Yilgarn Craton, Western Australia.
Authors T. Munday and J. SumptonThe regolith - electrical structures associated with a kimberlite located in the Yilgarn
Craton, Western Australia are examined. The kimberlite is hosted by a deeply
weathered granitic host. The saprolite associated with the granite and the kimberlites
are conductive as defined from downhole geophysical logging. The kimberlite shows
as a conductor in ground EM (SIROTEM and GEM-2 systems) surrounded by a
resistive host. It also shows a small conductive response in 56Khz DIGHEM data but
is not apparent in lower frequency data or in SALTMAP. The geometry of the latter
suggests that any response of the kimberlite is likely to be lost in a coupling of
transmitter with conductive wedges of granitic saprolite to the north and south of the
blow. Stitched sections of 1D LEI’s for SIROTEM, DIGHEM and SALTMAP show
similar trends and suggest that the regolith electrical structure is effectively modelled
as a layered earth, with some exceptions. An AEM “response map” in this area is
essentially a map of the variation of regolith/thickness and conductivity.
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Fluvial-Deltaic Architecture of the Orange River Delta, Examples from the Block 2A, AK 3D Seismic Survey
Authors J.B. Aldrich and T.B. BergeAfrica has three aerially extensive, thick, deltaic systems on it’s West coast. Of these the
Congo and Niger Deltas have major commercial hydrocarbon resources. The Orange
River delta is less-explored but has already had gas and oil discoveries that indicate it’s
potential. These discoveries indicate that at least 3 hydrocarbon systems exist in the
area; in graben deposits of the early rift succession. e.g. AJ-1 well (oil), Kudu-type late
rift (gas) and in the Albian-Aptian drift succession (gas). The deepwater portion of the
delta has yet to be drilled so its potential is yet to be assessed.
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An Integrated Approach to Prospect Risk Reduction in Block 9, Bredasdorp Basin, South Africa
Authors D.S. Macgregor, K.A. Bardwell and J.H. JacksonExploration drilling is underway in the Mossgas-operated portion of Block 9 in the
Bredasdorp basin, South Africa. The main prospective reservoir is an Early
Cretaceous shallow marine sand developed at the top of a syn-rift sequence (the
“Upper Shallow Marine” or USM). The remaining exploration potential of the block
has been assessed to lie in structural (one-sea!) traps deep in the basin, where
reservoir quality becomes a concern, and in subtle (poly-sea!) traps closer to
established infrastructure. Most of the proven gas-bearing traps in this region are
structural, although many structural crests are bald, with reservoir developed on the
flanks only.
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Using the Nature of the Misfit Surface to Improve the Inversion of Gravity Data
More LessKnowledge of the form of the misfit surface can be used to facilitate the inversion of geophysical
model parameters which have a non-linear response. Local quadratic models are fit to regions of
the misfit surface to enable estimates of the optima! model parameters, then least-squares
inversion is used to 'tidy‘ the result. The form of the misfit surface can also be used to check for
the presence of minima nearby to that which the inversion has just converged.
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Compression of Geophysical Data by means of an Equivalent Layer
More LessThe efficient storage of geophysical data, particularly if it is to be transferred across networks
such as the Internet, is of major importance. Suitable compression of data can cut data
transmission costs and times dramatically. The equivalent layer compression (ELC) technique,
instead of storing the magnetic or gravity data itself stores a model whose geophysical response
is used to generate that data. This can result in compressed file sizes of the order of less than 10%
of the original file size, better than can be obtained with commercial compression packages.
While the compression operation is computationally demanding, the decompression is relatively
simple, and the technique allows portions of the dataset to be decompressed separately.
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The use of Wavelet Transforms to constrain the Solutions Generated by Euler Deconvolution
Authors G.R.J. Cooper and R.J. DurrheimEuler deconvolution is routinely used in the interpretation of aeromagnetic data, but the method
has a tendency to produce vast numbers of solutions, all corresponding to slightly different
geological geometries. Wavelet transforms can be used to give information on the scales and
locations of features in the data, and this information can be used to determine improved
parameters for the Euler deconvolution. This results in output that is easier to interpret and which
reflects the true geology more accurately.
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Geophysical Methods Applied To Mining At Ultra-Depth: First Resuits Of The Deepmine Programme
Authors D.J. Gouws, G.J. Kühn and E.H. StettlerOne of the major problems in aeromagnetic data processing is the elimination of the effects of time dependent external magnetic field variations (daily variation, magnetic storms and ULF pulsations) from the aircraft survey data. Not only do these phenomena contaminate the survey data, but magnetic storms may also result in unproductive time while the aircraft is kept on the ground during the active phase of these storms. The use of data from a fixed base station has distinct problems. Accurate time synchronisation between the aircraft and base station
recordings is usually required and direct subtraction of the base station data from the aircraft data can in fact result in further deterioration of the aircraft data - e.g. if the base station is located on a conductivity anomaly its recordings may suffer frequency dependent phase shifts and amplitude changes.
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SQUID Airborne Gravity Gradiometer and Magnetometer Systems for Base Metal Exploration
By S.J. LoweSQUID technology increases the sensitivity of geophysical instruments by several orders of
magnitude so those targets previously transparent to geophysical methods may now be
explored. Superconducting technology has enabled the design of airborne gravity
gradiometers, sensitive enough for geophysical prospecting. The application of SQUID
gravity gradiometers and magnetometers to base metal exploration was investigated using
forward models of the gravity and magnetic response over an existing Nickel deposit in
Africa. A thin, ribbon-like massive sulphide body produces very subtle magnetic and
gravitational anomalies at normal airborne exploration attitudes, which may only be detected
using SQUID instruments.
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Compiling Regional Compilations using a New, Automated Grid Stitching Method
Authors S. Cheesman, I. MacLeod, G. Hollyer and R. TinnionRegional compilations of gridded geophysical data from disparate individual surveys are playing
an ever more important role in resource exploration. A key processing step in such compilations
is the merging of overlapping grids to create a single grid. Traditional methods of connecting
grids together can produce smooth final products but the process is time-consuming and has
difficulty with differences that involve both long and short wavelength errors. A novel,
completely automated method addresses several main challenges, such as determining how to
select a path along which overlapping grids can be joined The technique uses Fourier analysis
to deconstruct the errors along a suture path into a sum of functions with different spatial
wavelengths, and applies corrections that propagate smoothly into the grids by a distance
proportional to the individual wavelengths. The result is an almost seamless grid that minimizes
distortion from the correction process.
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The Kaapvaal Craton Broadband Seismic Experiment: Summary of Results and Ongoing Projects of the Wits University Group
A network of fifty-four broad-band seismometers that forms part of the international Kaapvaal Craton programme (Carison et al, 1996) was deployed at over eighty separate locations across southern Africa from April 1997 until April 1999. The seismology group at the University of the Witwatersrand was responsible for operating and maintaining part of the network, and has developed new facilities and research initiatives in applying broadband seismology to the study of earth structure and regional seismicity.
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Hydrocarbon Potential of the Tertiary Rift Basins of Kenya
Authors Ego J. K. and Ng’enoh D.K.Studies incorporating geology and geophysics in the Tertiary Kenya rift have given new
insights into the potential productivity of the Neogene sediments in the Kenya rift. Since
Neogene period syn and Post rift siliciclastic sediments have been deposited in the basins
punctuated by volcanic pulses. The most important of this is the Tambach Formation which is
deposited on the basement, it consists of potential good arkosic sandstone reservoirs and
organic rich shales with TOC upto 3%. East of the Tambach Formation the Miocene
Ngorora lacustrine shales provide mature source rock with potential of generating
hydrocarbons. Intermittent volcanic eruption masks the sediments in the Kerio trough sub basin.
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Gravity and Magnetic Surveys of the South African Offshore
More LessA substantial number of magnetic and gravity surveys have been carried Out over the last
forty years around the South African coast, the main participants being SOEKOR, the
Council for Geoscience, and De Beers Marine.
Since 1994, an increased interest by international companies in petroleum ventures in the
country’s territorial waters has resulted in the necessity of re-evaluating the existing gravity
and magnetic data.
Maps on the poster show the areas covered by the various gravity and magnetic surveys. The
vintage, mode of acquisition, number of lines, line spacing, total distance of lines traversed,
and status of each survey in terms of the extent to which the data have been processed and
interpreted is tabulated. Image compilations of areas around the South African coast are
shown, generated by juxtaposing SOEKOR ‘s gravity and magnetic data against other data
sets.
Individual case histories show the use of gravity and magnetics with respect to aiding seismic
interpretations in:
• Determining gross structure and depth to basement of sedimentary basins.
• Identifying sills and/or volcanics within sedimentary sequences.
• Distinguishing between carbonate reefs and dolerite dykes.
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The Block 11a Gas Opportunity - Offshore South Africa
By J. TurnerBlock 11a (1270km² / 314,000 acres) is dominated by a synrift (pre-Valanginian) structural high, the crest of which is located in the north-central part of the block. Two gas fields have been
discovered within Block 11a, on and adjacent to this feature, the Superior High. To date 12 wells have been drilled, three of which encountered gas at commercial production rates. A total of 2,625 line kilometers of 2D seismic data have been acquired between 1977 and 1990.
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Interpretation of Combined Onshore and Offshore Magnetic and Gravity Data Sets of Namibia
More LessA number of excellent magnetic and gravity data sets
are available covering continental Namibia as well as
its offshore regions. These include:
• continental aeromagnetic coverage, including the
U most recently acquired data - • continental Bouguer and Free Air data
• offshore aeromagnetic data, recently acquired by
Namcor, extending along the entire Namibian
[j coast and roughly 250km out to sea
• offshore satellite-derived Bouguer and Free Air
U • extensive shiptrack magnetic data
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Geomagnetic investigations over old waste deposits. Problems with measurements and interpretation - 2 case studies
More LessA geophysical field survey consists generally of three steps:
1. establishing a contract
2. survey
3. interpretation
Experience bas shown that a successful interpretation of the measurements suffer from insufficient data due to the financial constraints as defined by the customer. During recent years
geophysical investigations of old waste deposits gained importance - resulting in extensive experience regarding effective survey planning.
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Electrical and Magnetic Investigations of Deep Aquifers in north Matabele land, Zimbabwe
Authors Torleif Dahlin, Daniel Glatz, Nina Persson, Patience Gwaze and Richard OwenGeophysical investigations are being carried Out in North Matabeleland, Zimbabwe, with
the aim of finding suitable aquifers to provide the city of Bulawayo with water. In this
part of the study an attempt has been made to map the hydrogeology in an area around
the railway siding Sawmills.
The investigated area is covered by basalt that is mostly weathered on the surface.
Outside the Umguza River valley Kalahari Sands overlie the basalt. The basalt rests on
the Upper Karoo sandstone, which is the target aquifer. (MacDonald 1970;
Stagman 1978).
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Integrated Seismic and Electromagnetic Methods applied to Deep Mining
By T. BerheAs most exploration and mining activities are getting deeper there is a need for
geophysical techniques that can image with better resolution over a large distance range.
Integration of geophysical methods using different physical parameters can reduce
ambiguity of interpretation by providing coincident bounds on these parameters, thus
providing more detailed geological interpretations.
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Wavefield Analysis in borehole Seismic methods with mining Applications 2-D Offset-VSP and Crosshole Borehole Data
By M.K. ShokeBorehole seismic methods (offset-VSP and cross-borehole) have been applied in response to the need to obtain high resolution seismic images of the subsurface of the earth. These methods have been applied for mapping geological structures associated with hydrocarbon accumulates, and are now being developed as tools to map geological structures of interest in mining.
These structures include ore bodies, faults, reefs and potholes.
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