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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
1 - 50 of 78 results
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Geophysical Methods Applied to Mining at Ultra-depth: First Resuits of the DEEPMINE Programme
More LessThe main objective of the first year of the DEEPMINE Programme has been to assess rigorously the
capability of the industry to mme safely and profitably at ultra-depth (3 to 5 km). Geophysical methods
provide signijïcant input to two technology elements, namely the delineation and definition of geological
structures, and the management of seismicity. The resuits of the first year of the Programme are reviewed.
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Interpretation of the Western Ultra Deep Levels 3-D Seismic Survey
Authors M.A.S. Gibson, S.J. Jolley and A.C. BarnicoatAnglogold has used 3D seismic surveys to assist in planning operations over several gold
mines in the Archaean Witwatersrand seidmentaiy basin. Mine planning benefits from the
ability of 3D seismic data to accurately image the subsurface, preventing mis-placed
development from incorrect geological models. The WUDL survey is the largest mineral 3D
survey in southern Africa at nearly 300km2. It imaged much of the Carletonville Goldfield~
located 80km southwest of Johannesburg, from Deelkraal G.M. in the west, to Driefontein
G.M. in the east. Results included the geological structure of economic targets such as the
Ventersdorp Contact Reef (VCR), a coherent model for the development of the region, and
some indication of the prospectivity of unmined ground.
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Recent Innovations in Geophysics Techniques for Detecting-Mineral Sand Deposits
Authors Dr. Desmond J. FitzGerald and John BrettVarious geophysical techniques have been used over the last five years in the exploration of
mineral sand deposits in the Murray Basin, North-West Victoria, Australia. Radiometrics,
magnerics and graviry have featured prominently. Amongst the larger efforts spent exploring
in the Murray Basin, a pro-geophysics and anti-geophysics experience has emerged. An
analysis of what is needed to make successful use of geophysics is given and warnings about
the pitfalis are also given.
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Application of Scalar CSAMT Surveying in the Search for Massive Sulphide Mineralisation at Nkomati MSB Mine, Mpumalanga, South Africa
Authors M.M. Nyoni and J.R. BishopThe Nkomati MSB Mi, a joint venture between Anglovaal Mining and Anglo American,
exploits a magmatic Ni-Cu-Co-PGE massive sulphide deposit associated with the Uitkomst
Complex, in Mpumalanga, South Africa. The Uitkomst Complex is a linear, northwest-southeast
trending, layered mafic-ultramafic body, intruded into the Transvaal Sediments. It hosts
disseminated and massive sulphide mineralisation. An exploration program was undertaken to
search for further zones of massive sulphide mineralisation associated with the complex.
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Satellite Radar Imagery, Geological And Geophysical Data For Anomaly Detection In Mineral Exploration
More LessIn areas of dense vegetation common multispectral sensor systems such as Landsat TM
are often of limited use for geological and structural mapping.
Radar imagery, such as ERS-1/2 and Radarsat, differ in many ways from products
obtained from multispectral sensor platforms. Acquiring data in the microwave portion of
the electromagnetic spectrum, radar systems are highly sensitive to surface roughness
and geomorphology due to the acquisition geometry. Geomorphological variations can
thus cause different backscatter results. Morphological ridges are therefore supposed to
show higher backscatter effects than flat ground, resulting in brighter areas on the
image. A similar scenario applies to any other prominent tectonic elements such as
faults. Specialfilter techniques allow the enhancement of those areas.
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Reducing the Risk in Gas Field Development A Case Study of the E-M field Offshore South Africa
Authors Kathy Bardwell, Alex Storer and Roy KellyThe use of long reach, sub-horizontal wells in the development of structurally complex
oil and gas fields has become an increasingly common practice. This paper describes
some ways in which the reservoir related risks associated with drilling such wells can
be reduced using a variety of geological and geophysical techniques. The E-M gas
field, offshore South Africa is presented as a case study.
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Applications of wavelet transforms in aeromagnetic data processing
Authors T.A. Ridsdill-Smith, M.C. Dentith and R.D. ListWavelet transforms are a powerful new tool in aeromagnetic data processing. The wavelet transform preserves both spatial and frequency information about a signal allowing us to design a range of spatially-varying filters that act on the wavelet coefficients. Two methods are outlined in this paper. The first, using the continuous wavelet transform, is used to construct 1D and radially-symmetric 2D linear filters with spatially-dependent frequency responses. An application of this is the level to variable-surface upward continuation operator. The second method uses the considerably more efficient discrete wavelet transform to generate a range of 1D derivatives with locally adaptive noise reduction. Both methods provide robust and efficient new frameworks for designing filters that are impractical to implement using conventional space or frequency domain techniques.
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An Application of Aeromagnetic Imagery and Borehole Geophysics in Regional Groundwater Investigations at Palla Road & Khurutshe, Botswana
Authors Neil Andersen, Luc Antoine, Leif CarIson and Comfort MoIosiwaThe National Water Master Plan study, NWMP, (DWA, 1991 and 1992) proposed that
the future water supply to the south-east and central Botswana should use sulface water
from dams along the Motloutse and Shashe rivers. To convey the water to the demand
areas, the North South Carrier (NSC) was proposed. The NSC runs through the Palla
Road and Khurutshe areas which have major groundwater potential which are also being
developed to be used as a strategic backup supply to these dams. The groundwater
resources are encountered in the Ntane Sandstone in Upper Karoo formations.
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The Y2K Geomagnetic Problem
Authors S.R. McMullan, P. Geo and S.L. NiccolIn the oncoming peak of Solar Cycle 23 sunspot activity, the diurnal variations in the Earth‘s
magnetic fleld may increase twenty fold. This will increase the noise component in high
resolution aeromagnetic surveys, which degrades the utility of magnetic surveys for detailed
geological and structural mapping.
Diurnal data from the peak of the previous Cycle 22 in March 1989 were added to 3-D
magnetic model data to simulate the effect of increased diurnal noise. Severe line-based and
random noise is evident in the magnetic grids, which cannot be removed using conventional
tie line/flight line levelling and filtering techniques.
Gradient measurements are inherently diurnal-free, and the anomalous magnetic field can be
calculated using the Hilbert transform. The resulting image is much improved and is free
from high frequency and line-based noise caused by the diurnal. Gradient measurements are
therefore the optimal solution to reduce the effect of increasing diurnal noise in Cycle 23.
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Enhancement of Shallow Magnetic Signals using Seperation Filtering and Texture Filtering
Authors Mike Dentith, Duncan R. Cowan, Linda A. Tompkins and Sheila CowanSeparation filtering of magnetic data can be extended from simple regional/residual
separation to the general multilayer case it is theoretically possible to design filters to
deconvolve the effects of causative sources occurring around a particular level. The problem
posed is non-linear since the field equations contain terms relating to source width and
thickness as well as source depth.
Separation filtering becomes very difficult when there is considerable overlap in the spectra
of individual depth ensembles. The degree of separation achieved depends on the depth
difference and the spectral b/B ratio, the ratio of the amplitudes of the shallow and deep
ensembles. A high b/B ratio is needed if the fîltering objective is to deconvolve the effects of
shallow sources with minimum contamination by deeper sources.
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The LSDARC Method of Seismic Refractor Analysis, and the Resolution of Lateral Variations in Refraction Velocity
More LessThe LSDARC method of seismic refraction analysis uses both the reciprocal and a least-squares
difference method to optimize estimates of time-depth terms. The difference method estimates
differences between time-depth terms at adjacent receivers in which errors in source timing are
eliminated, thus permitting the effects of such timing errors to be minimized. Time-depth terms
need only be defined at some receiver locations, and the gaps are filed in by the difference
method, thereby allowing flexibility in survey design. Refractor velocities are estimated by
subtracting time-depth values for each source and receiver from the first-break times. This
procedure projects each source and receiver on to the refractor surface. A damped least-squares
inversion procedure is then used to calculate and apply corrections to the times for each shot
gather to make all times appear to be produced by a single shot placed on the refractor surface
at one end of the profile. Refractor velocities can be estimated from the corrected times using
distance windows that are independent of the length of the recording spread, and which can be
adjusted to get a preferred trade-off between variance in velocity and window length. An
alternative method of velocity determination involves preparing slowness profiles in opposite
directions, and using a model for refraction across a dipping interface to compute true refractor
velocities. The advantage of the alternative approach is that refractor velocities can be reliably
determined without knowledge of the time-depth terms. The main features of the method are
illustrated using a hammer seismic survey undertaken near Ladismith in the Western Cape.
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Characterisation of Mining-induced and Natural Seismicity
More LessPreliminary analyses of seismic records of mining-induced earthquakes from the
Kaapvaal Craton experiment indicate that these data are of sufficiently high quality
and large quantity to enable significant improvements of the crustal velocity structure
of southern Africa through the routine identification of seismic phases. Principal
event parameters, such as origin time, location, depth and local magnitude, are
collated from the bulletins published by the Council for Geoscience (Graham et al.,
1997) and from individual mine databases from gold mining companies throughout
South Africa. The waveform data for events with a magnitude greater than 2.0 are
extracted from the Kaapvaal database. This database was acquired through the
deployment of a 54-station, SO-site network of broadband seismic stations distributed
across the southern African region. as part of the multinational project “The anatomy
of an Archean craton” (Carlson et al., 1996). The restricted time of this deployment
is from April 1997 until June 1999. Careful identification of phases across and
around Kaapvaal craton are fundamental in deriving more representative velocity
models of the crust and upper mantle of the southern African region. Results from
other current seismic research, including receiver function and two-station surface
wave dispersion studies, will also be used to assist refinement of the S wave velocity
models for the crust and upper mantle (James et al, 1998). The event location data
provided by the mines are accurate to a few hundred meters, providing a unique
opportunity of combining very accurate locations with waveform data from the
regional Kaapvaal network. This will enable an improved velocity model of the crust
and upper mantle to be calculated and used in the future for locating events.
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Peak Particle Velocities in Various Geotechnical Areas in South African Gold Mines
More LessMore than 50 per cent of fatalities occurring in the mining industry are rock related
accidents. Of these, slightly more than half are associated with rockfalls, whilst the
remainder are a consequence of the failure of dynamically loaded rock during seismic
events which result in rockbursts.
One of the tasks of the DEEPMINE program was to expand on existing stope support
expertise, and to develop support criteria for effective stope support in ultra deep
level mining. This primary output was achieved by four main research thrusts, one of
which was to determine the peak particle velocities for different geotechnical areas.
This paper describes the methodology and assumptions underlying the technique used
to establish peak particle velocities across various reefs and their dependence on
depth. In addition, an outline of the main research findings is given.
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Seismic Velocities Measured in a Mine tunnel at Thabazimbi, South Africa
Authors Cedric Wright, Jeanne Walls and Donizeti de Jesus CarneiroAnalysis of the refracted arrivals on a seismic reflection profile recorded along the wall of a
tunnel al an iron mine near Thabazimbi, South Africa, shows significant variation in P-wave
velocity in dolomite in and away from the de-stressed zone. Very low seismic velocities away
from the tunnel wall are associated with alcoves or ‘cubbies' involving offsets in the wall of up
to 10 m. Large variations in seismic velocity resolved over distances less than 15 m with
signals of wavelength around 6-9 m are attributed to variations in the fracture systems in the
de-stressed zone. The results show that refraction static processing of ‘in-mine’ seismic
reflection profiles is as important as in surface surveys, since the frequency content of deep
mine seismic energy is higher than on surface, and can assist in analysis of stress regimes
within in-mine developments.
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Comparing Transmitter Waveforms for Airborne EM Systems
Authors Steve Lynch, Peter B. Leggatt and Braam du PlooyRecently there has been considerable discussion in geophysical journals regarding
the optimum waveforms for airborne electromagnetic (AEM) systems. The
approaches taken have varied from data processing, to theoretical modeling, to
hardware modifications. This paper uses real data from the SPECTREM system to
compare different waveforms. SPECTREM is a full-duty time domain AEM system
which has operated successfully since 1989. The data used in the paper actually led
to the discovery of the Konuto mine in the Canadian Shield.
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The TEMPEST AEM Systëm and the Walford Creek Prospect
Authors R. Lane, A. Green, C. Golding, M. Owers, C. Plunkett, P. Pik, D. Sattel and R. ThornA new airborne electromagnetic system has been developed by the CRC for Australian
Mineral Exploration Technologies and is operated by World Geoscience Corporation.
TEMPEST operates in the “INPUT” configuration with a towed bird and a transmitter
located on the aircraft. It utilizes a 25 Hz square-wave transmitter with a 50% duty cycle
and a variable switching ramp. The system has been designed to measure a wide
bandwidth (25Hz - 3ZSkRz) EM response for both geological mapping and recognition
of conductive targets.
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Drillhole Electromagnetic Methods at the Nkomati MSB Mme, Mpumalanga, South Africa
Authors J.P. Bishop, M.M. Nyoni and S.J. du PlessisThe Nkomati MSB Mine, a nickel-copper mine with PGE credits, is located in the Uitkomst Complex,
a layered mafic/ultramafic body which forms a linear trough-shaped intrusive into sediments of the
Transvaal Sequence. it is jointly owned by Anglovaal Mining (Avmin) and Anglo American and is
managed by Avmin. The ore is dense, magnetic and highly conductive and a number of geophysical
techniques, both surface and drillhole, are being used to help explore for more ore-grade
mineralisation both in and adjacent to the complex. This paper presents a comparison of results from
three drillhole methods: time domain drillhole electromagnetics (TD-DHEM), frequency domain
DHEM (FD-DHEM} and drillhole magnetometric resistivity (DHMMR). The data were collected
from a section across the complex where the holes intersected sulphides from both the shallower and
lower grade ‘Main Mineralised Zone’ (7i4MZ) and the deeper ore-grade ‘Massive Sulphide Body’
(MSB). All of the surveyed holes were vertical and thus only axial data could be meaningfully collected.
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Using an induction coil sensor to indirectly measure the B-field response in the bandwidth of the transient electromagnetic method
Authors R.S. Smith and A.P. AnnanIn time-domain systems, the traditional sensor used in transient electromagnetic (EM) systems
is an induction coil. This sensor measures a voltage response proportional to the time rate of
change of the magnetic field in the EM bandwidth. By simply integrating the digitized output
voltage from the induction coil, it is possible to obtain an indirect measurement of the magnetic
field in the same bandwidth.
The simple integration methodology is validated by showing that there is good agreement
between: a) synthetic voltage data integrated to a magnetic field, and b) synthetic magnetic-field
data calculated directly. Further experimental work shows that airborne EM magnetic-field data
collected with an induction coil correspond quite closely to that measured using a SQUID
magnetometer.
Comparison of measured voltage and magnetic-field data show that the two sets of profiles have
quite different characteristics. The magnetic-field data is better for identifying, discriminating
and interpreting good conductors, while suppressing the less conductive targets.
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Intrabed multiple removal: theory and practice
Authors Marina S. Biryulina and Gennady A. RyzhikovA novel approach to removal of intrabed multiples is
suggested. Based upon a Source-Signature Invariant
Inversion (hereafter SoSI-inversion), a relevant strategy
is reduced to a few sequential steps, each of those exploits
a technique similar to the predictive deconvolution. A
total number of steps is controlled by related number of
strong reflectors, which cause intrabed reverberations.
Due to linearity the codes are very fast, while a proper
regularization makes them fairly robust. A few examples
of processing synthetic/real marine data with the codes
in a fully automatic mode are exposed.
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Offset Weighting for Multiple Suppression in AVO Analysis
Authors R.A. Sutherland and P. ManuelContamination of primary reflections by remnant multiple reflections continues to
degrade the results of rock property estimation from Amplitude Versus Offset (A VO)
analysis of seismic data. Conventional stacked seismic sections benefit from the
application of near and far trace tapering prior to stacking, but this would not seem
applicable in AVO analysis. Presented here is a means whereby offset weighting
schemes can be incorporated into A VO analyses, resulting in improved multiple
suppression. The weighting is inserted into the Aki and Richards approximation to the
Zoeppritz equations, and thus can be used with any method which attempts to
minimise the error between the observed and predicted seismic amplitudes. The
incorporation of this weighting into the GEOSTACK method of Smith and Gidlow is
demonstrated, together with a real data example showing the multiple suppression
achieved by this approach. The combination of a suitable multiple suppressing
weighting scheme and a least squares minimisation approach produces a more robust
method of AVO processing in the presence of multiples or any other noise.
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The Relationship between Lamé’s Constants, 2 and µ, and the Fluid Factor in AVO Analysis of Seismic Data
By George SmithDescription of Lamé’s Constants
The idea of using Lamé’s constants in AVO analysis was first propounded by Goodway et al
(1997). Lamé’s constants are elastic moduli which have been found useful in the description of the elastic
behaviour of materials. The modulus p is the shear modulus or rigidity, but .... is generally held to
have no physical meaning, being a parameter substituted into equations describing elastic
behaviour to make them simpler.
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Prestack Time Migration as a Crucial Step in Resolving AVO Anomalies in the Synrift Sandstones of the Bredasdorp Basin, Offshore South Africa: A Case History
Authors Donald L. Bryson, Louis R. Castro, Thomas R. Jensen and Leslie O. NiemiAmplitude versus offset methods have been demonstrated as highly useful tools to
suggest the presence of hydrocarbons in the sandstones of the Bredasdorp Basin, both
within the drift sequence (e.g. Oribi Field) and within the synrift sequence on the
northern flank of the basin (e.g. F-A Field). The E-OI well, drilled near the southern
flank of the basin in Block 9, found a 15 m oil column in the shallow marine
sandstones at the top of the preserved synrift section. The sand lies immediately
below the lAti major unconformity and the sand itself is eroded up dip forming a
wedge trap, with overlying shale providing the seal Since this truncation follows a
regional structural trend, a sizable trap could exist containing several hundred
MMBOIP. With only 2D seismic available, one dip and one strike line through the
discovery well were reprocessed to evaluate the potential for using AVO in the synrift
section on the southern flank of the basin. Initially, the data were not migrated. Only
the strike line indicated a weak anomaly within the sand unit and misties between the
two lines and with the well created an interpretation dilemma. To resolve the matter,
prestack time migration was performed in the offset domain. Results were improved
with anomalies evident on both lines and ties more acceptable. With reflection energy
better imaged, AVO responses from both the dip and strike lines clearly show the
possible limits of the oil-bearing sand and even suggest the ability to interpret the oil
water contact. The method will be used in reprocessing additional 2D lines to map
the extent of the hydrocarbon-bearing sand within this subcropping wedge as well as
several other similar synrift discoveries in the central and southern portions of the basin.
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Gabon - Technical Challenges in Seismic Data Analysis
Authors R. Godfrey, R. Goto, C. Trowell, J. Navarro and M. AitchinsonGabon is currently undergoing an unprecedented surge in seismic exploration with large 3D surveys being shot covering the shallow, deep and ultra-deep water portion of the continental shelf Prior to these 3D surveys, large regional 2D programs have been shot and interpreted during the previous 10 years. For an excellent introduction to the challenges facing seismic exploration in West Africa, refer to the special section in the May 1999 edition of The Leading Edge and in particular the article by Bray and Lawrence (1999).
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Helicopter Assisted Regional Gravity Survey of Northern Botswana
Authors S.R. McMullan, D.I. Koosimile, W.H. McLellan and B. SizibaA regional gravity survey was carried out in northern Botswana by Poseidon Geophysics
(Pty) Limited on behalf of the Geological Survey Department, Ministry of Minerals,
Energy and Water Affairs, Republic of Botswana. The main objective of the survey was to
provide regional gravity coverage to improve the geological understanding of northern
Botswana, as an incentive to private sector mineral exploration. A total of 4003 stations
at a station spacing of 7.5km were established during the period October 8, 1998, to May
24, 1999. The results from the survey are currently being used by various exploration
concerns to guide their exploration programs.
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Normalization of the Gravity Anomaly Pattern of Africa reveals Deep Geology
Authors C.M. Doucouré and M.J. de WitAvailable land-based gravity data and geopotential data model infills are used, together with a Digital
Elevation Model, to produce a Bouguer gravity anomaly map of Africa. The pattern of Bouguer gravity
anomalies is an intricate interference of contributions reflecting density heterogeneities on a wide spectrum of
scales. These contributions relate to the various structures and underlying geodynamic processes within the
African lithosphere and underlyting mantle. A selective normalization process is used to isolate some of these
contributions. The end product is a gravity anomaly map of Africa that can be directly correlated with geology
and interpreted in terms of crustal structures. We will present the results of this work.
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Interpretation of Usangu Basin gravity data in Tanzania by Means of Hartley Transform
By I.M. MarobheThe gravity data of the Usangu sedimentary basin in southern west Tanzania is analyzed
using Hartley transform. The Hartley transform is found to accomplish spectral analysis that
could be achieved using the Fourier transform.
The spectral depths determined from the radial spectrum reveal an average depth of the
regional component of about 5 km, intermediate depth of 2.0 km and shallow depths of 1 km.
Enhancement of the gravity data by calculating second vertical derivatives and downward
continued maps give results that enhance shallow structures. The upward continued and the
matched filtered maps also give maps that enhance regional anomalies under the expense of
shallow structures.
The filtering of the gravity data has revealed that the sediments are overlying basement of
different lithologies.
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Borehole Radar at an Underground Ventersdorp Reef site
Authors J.C. Trickett, I. Mason and F. StevensonSince long-range GPR probing from mine excavations can be problematic because of
significant clutter noise, recent experimentation has moved towards the application of GPR
from the relatively “clean” environment of boreholes. This technique, known as Borehole
Radar (BHR) was tested successfully for the first time in an underground deep level gold
mine in March of 1999. The Ventersdorp Contact Reef (VCR) is imaged with sub-metre
precision over ranges of up to ~ 45 m away from the borehole. Although still in its
developmental stage, BHR holds excellent promise as a high-resolution tactical tool for
providing geological information ahead of mining.
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Environmental Monitoring in The Mining Industry Integrating Remote Sensing and Ground Geophysical Data
More LessPollution associated with the mining of Witwatersrand gold-bearing conglomerates may be
mapped and monitored using hyperspectral scanner imagery. Analysis of such data allows the
identification of the type and abundance of oxides, clays and polluting sulphates. Vegetation
types associated with acid mine drainage can also be identified.
In searching potential pollution in surface and ground waters thermal data may be used to map
near-surface aquifers, “eyes “, and seepage points close to riverbanks. Thermal imagery has also
been useful in locating dykes and sometimes depicts near-surface groundwater flow across these.
Furthermore, this kind of data may be useful in locating potential sinkhole sites in dolomite areas.
A very good correlation is observed between thermal and resistivity data. Where follow-up high
resolution resistivity surveys have been carried out to better define the cross-section of a
pollution plume, for optimal siting of pollution monitoring boreholes, Thermal anomalies can be
linked to highly conductive ground The resistivity adds value in being able to differentiate between diffuse plumes over weathered bedrock and confined plumes running along faults or dykes. Use of the integrazed data products include:
• baseline study database for existing mining/pollution areas
• suitability studies for slimes dam and reduction plant siting
• slimes dam and rock dump pollution and stability studies
• baseline environmental studies for potential new mining sites
• optimal placement of monitoring boreholes or sampling point's
• remediation effectiveness studies
• groundwater recharge potential and boreholes siting
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Geophysical Studies of the Highbury Meteorite Impact Structure, Zimbabwe
Authors T. Gumede, D.J. Robertson and S. MasterThe 20-1cm-diameter Highbury meteorite impact structure in northern Zimbabwe, centred on 30°
06’55’ ‘E, 17° 03 ‘53” S, is developed in Paleoproterozoic metasedimentary country rocks of the c.
2.0-1.8 Ga Magondi Belt [1,2) (Fig. 1). The undeformed Munwa Granophyre, believed to be an
impact melt-rock, intrusive info shocked, brecciated country rocks, hos a zirco U-Pb age of 1034±1 3
Ma (21. Ground gravity, magnetic, and resistivity studies of the Highbury structures were carried out
in 1995 [3). The gravity study indicates a central high (with a large offset anomaly coinciding with
the granophyre) surrounded by concentric gravity lows. This is interpreted to be the result of a
central! uplift cored by denser material from the underlying Chinhoyi greenstone belt. Magnetic
studies show that a prominent magnetic anomaly is centred on the Munwa Granophyre, while other
anomalies are due to dipping mafic dykes. Resistivity studies show the limited extend of the Munwa
Granophyre, and support its interpretation as a downwardly injected impact melt.
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A Study of Geoid, Topography and Volcanism in East Africa: Implications for Continental Rifting
Authors A. Tessema and L.A.G. AntoineThe combined investigation of topographic swells, volcanism and undulations of the geoid are key to understand the processes of continental rifting. Cenozoic flood basalt provinces,
plateau uplift, geoid warping and the development of the East African Rift (EAR) are linked to dynamic processes in the earth’s mantle. We integrate undulations of the geoid with the
topographic data to explain the cause of anomalous topography and continental extension in eastern Africa. The geoid derived from EGM96 model and the topography derived from ETOPO5 are used in this study.
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Alternative Approach to Groundwater Exploration on the Nebo Granites
Authors M. Combrinck, W.J. Botha, G. Ngaisiue and B.C. HavemannIn 1996 an integrated groundwater research project was launched by the University of
Pretoria ‘s Geology Department. This project is funded by the FRD and supported by the
Department of Water Affairs and Forestry. The purpose of the project is to improve
community water supply in areas where the usual techniques didn't deliver the success
required, and to design a management program for such resources. To assist in the
above project, this research program, funded by the Water Research Commission,
Council Geoscience and Geodass, was initiated in 1997.
The main objective of the research was to determine if a regional exploration technique
could be applied to groundwater exploration with greater success than the previously
applied crisis management approach.
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Initial Data Results from the South African National Geophysical Test Site at Roodekraal
Authors S.J. Webb, D. Morgan and V. MashabaThe SAGA geophysical test site was established in 1995 in order to provide the
southern African geophysical community with a weil-characterized and
constrained site to be used for the calibration and testing of a wide varieiy of
geophysical equipment. A permanent grid, roughly 400 X 600 m, was laid Out
and surveyed in a gently sloping valley on the farm Roodekraal 454 JQ,
approximately 11 km southeast of Potchefstroom. The site is located in the
Roodekraal volcanic complex, a pre-Vredefort age complex (Le. > 2.0 Ga.), with
a variety of volcanic rocks. The copper mineralisation has been constrained by
drilling and appears to lie in lens shaped areas dominantly located in the
amygdaloidal units. The test site is located directly over one of these lenses.
The various volcanic units are expected to provide geophysically mappable
variations of physical properties and the mineralised zone should be a
challenging electrical target. To date a variety of geophysical data have been
collected and they show well defined anomalies, which should encourage the
further testing of a wide variety of equipment and collection of more data.
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A study of the Khoy Geothermal Field as an interesting Geological Area
Authors M. Ali Riahi and R. NazarianSeveral authors (Cagniard, 1953; Smith, 1961; Vozoif, 1972; Pedersen, 1982; Giroux, et al, 1997) have put forward their attempts to introduce, compile and model magnetotellurics methods to interpret geological features. In this context, in north-west part of Iran in Khoy district of Azarbayjan province about 30 km aerial distance from Turkey borders there are evidences of a high temperature gradient due to some interesting geologic phenomenon such as Tertiary volcanoes of Zagros belt orogeny and its related faulting system. The direct evidence of such a high temperature gradient is hot water springs of about 60 0C.
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No Pain - No Gain: Challenges in Reprocessing a Countrywide Airborne Data Set of Tanzania of over 1 Million Line km
Authors C. Gram, T. Grace, E. Stettler and J. MarkramIn 1996, digital archives of the countrywide airborne survey of Tanzania flown by
Geosurvey International (GmbH) were rediscovered. Previously, only paper contour
maps or digitized derivatives thereof were available. The Council for Geoscience
(South Africa) was contracted to reprocess the digital data to current standards. The
reprocessing of this legacy data was a great challenge. Some problems with the data
could not be rectified, however the quality of the reprocessed data is excellent. This
reprocessed airborne countrywide data will stimulate further exploration in Tanzania.
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Three Component Downhole Magnetic Surveys. A Case Study of the Namaqualand Copper District
Authors C. Elstadt and R. van RensburgThe three-component downhole magnetic probe from IFG Corporation was originally
designed as a borehole deviation survey tool. However, because of the accuracy and
sensitivity of the sensors, it quickly evolved into a multi-purpose geophysical exploration
tool that can supply information for the three components of the Earth‘s magnetic field,
the magnetization of the surrounding rock and the deviation of the borehole path during
a single run.
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Magnetic, radiometric, electromagnetic and near-infrared profiling with an ultralight airborne platform
Authors E.H. Stettler, H. Coetzee, M.E. Hauger, T. Grace, P. Cole and P.J. KülperOver the past three years the Geophysics Division and co-workers have developed an
ultra light airborne mapping facility. It consists of a Streak Shadow ultralight aircraft which
records the following data sets namely a) total field magnetic, b) 512 channel gamma-ray
spectrometric, c) pulsed, wide-band time domain electromagnetic, and d) 512 channel near
infra-red reflected light.
The purpose of this facility is to geophysically map areas identified for exploration,
mapping or environmental monitoring, in unprecedented detail, much more cost effectively
than ground surveys or helicopter borne surveys. Routine survey specifications for the
Streak Shadow are 50m flight line spacing, flying 50m above surface (terrain and weather
conditions permitting).
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Estimating Source Locations From Potential Field Data
By Neil PendockA new technique is presented for solving the inverse problem of estimating the locations of a set of sources which generate a potential field from a set of observations of that field. Assuming a simple forward model, we derive an equation which is a non-linear transformation of the observed data values but is linear in a set of parameters from which the source positions may easily be extracted. The new method is tested on theoretical data sets and gives excellent results for an isolated source and good results on data generated by several interfering sources.
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Modern Deformation Processes in the Earth’s Crust and Acoustic Noises in Boreholes
Authors B.P. Dyakonov, A.K. Troyanov and M.N. TeterinaMeasurement of acoustic noises of rocks in borehole allow to receive essentially new
information about modern dynamic processes in geological medium. Maximum
amplitudes of seismoacoustic signals in boreholes are being observed in the zones of
present-day active micromovements and timed spatially to the zones of faults, areas of
crushing and fracturing of rocks. The acoustic noises amplitude-frequency
composition depends on the physical-mechanical properties of rocks. Daily and semi
diurnal periods in temporal variations of acoustic noises, that are registered in pits
and boreholes, are caused by the influence of the Earth ‘s tides.
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The Measurements of Seismoacoustic Emission and Helium Concentrations in Boreholes within Tectonic Zones
Authors A.K. Troyanov, A.K. Yurkov and V.I. YtkinMeasurements of seismoacoustic emission (SAE) of rocks in borehole allow to receive essentially new information about modem dynamic processes in geological medium. Zones of rocks
breaking and jointing are as a rule marked by anomalous level of SAE amplitudes. Dynamically active tectonic fractures in space around a borehole are also distinguished by the same SAE
anomalies. Inereased helium concentra tion in subsurface water is connected with disjunctive disturbances in the Earth’s crust, especially with deep faults. In this case there are marked only perme able disturbances, along which filtration of subsurface water may take place. Measurements of SAE and helium con centrations in boreholes of the Urals and Northem Kazakhstan are used to study the tectonic of ore fields. Interpretation of measurements of SAE and helium concentrations aflows to characterize a tectonic disturbance according to its dynamic activity and its permeability.
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New Geophysical Method for Investigations in Oil Boreholes
Authors Yu.G. Astrakhantsev, V.V. Dryagin and A.K. TroyanovNew digital instrumentation for conducting three-component measurements of
acoustic noises in boreholes was developed. The possibilities of the technique in
determining the character of oil-gas-water saturation of collectors, not opened by
perforation, were shown. Some results of the use of the instrumentation and technique
at oil deposits were considered.
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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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