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9th SAGA Biennial Technical Meeting and Exhibition
- Conference date: 14 Sep 2005 - 16 Sep 2005
- Location: Cape Town, South Africa
- Published: 14 September 2005
21 - 40 of 66 results
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The applicability of various seismic offset reflectivity models
By G. SmithThe variation of the amplitudes of seismic reflection data as a function of angle of incidence or offset (AVO) is a
technique that is widely applied in exploration for hydrocarbons and the characterization of reservoirs. Many
approximations of the Zoeppritz equations have been proposed, and another one is added here to add to the general
confusion. A simple method of comparing them is used here to compare two approximations with the full solution.
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The fluid factor angle applied to the Sable oil field area (South Africa)
More LessThe study was aimed at exploring the robustness of seismic attribute displays and the application of different fluid
factor angles to delineate the differences between the E-BD channel complex and E-BD2 sandstone, in the region of
the Sable Oil field, Bredasdorp Basin, offshore South Africa. Seismic attribute slices generated from stacked
amplitudes, fluid factor, intercept AVO and gradient AVO were sliced at 13At1 and E-BD top horizons, which were
interpreted as the base and top of the E-BD sandstones respectively.
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The application of the continuous wavelet transform to potential field data
By G. CooperThe continuous wavelet transform (CWT) has been used with much success in the analysis of non-stationary time
series. It has been used much less frequently in the interpretation of magnetic or gravity data, although several
approaches have been tried. A simple method of obtaining location and depth estimates of potential field sources is
suggested here. For gravity data the method uses wavelets based on the integer-order horizontal derivatives of the
gravity anomaly from a point source.
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Processing of gravity data using gravity wavelets
By A. ChatuDuring the last decade, many new geophysical data processing methods have been developed. Gravity wavelet
is a new method in geophysical processing that is drawing attention of most potential field workers. It has
ample applications in processing and interpretation of potential field data.
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Three-dimensional gravity model of the Witwatersrand Basin and Vredefort impact Structure
By S. CoomberThe Archaean Witwatersrand basin is, economically, one of South Africa’s most important geological feature.
Approximately 40% of the gold ever mined globally comes from the Central Rand Group (contained in the
Witwatersrand Supergroup) making it a unique, mammoth resource. As a result, the Witwatersrand basin has become
the most extensively studied basin in the world.
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Ibhubesi Field, South Africa: is cross-plotting of AVO attributes a reliable / suitable technique to predict gas directly
By M. MbandeziThis project investigates the use of crossplot analysis for interpreting 3D seismic data to detect and map the distribution
of gas-sands in the iBhubesi gas field on the west coast of South Africa. Forest Oil international provided us with
stacked P R , P-wave reflectivity and S R , S-wave reflectivity, and inverted P and S wave 3-D seismic and borehole log
data, in 2003.
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Adding value to old seismic data through re-processing
By M. GibsonGold Fields has an extensive archive of 2D seismic lines acquired in the Witwatersrand Basin during the 1980’s, as
well as more recent 3D seismic surveys at the Oryx (1992), Driefontein (1995) and Leeudoorn (Kloof 7 shaft, 1994)
mines. Recent projects at Oryx and Kloof needed updated structural interpretations of borehole and seismic data.
At Oryx, a new 2D seismic acquisition was considered to tie new boreholes.
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Case studies on Kirchhoff and wave equation anisotropic PSDM
By S. Grion3D prestack depth migration (3D PSDM) is the tool of choice for accurate structural imaging. This paper addresses
the following key issues in 3D PSDM: (1) the relative merits of Kirchhoff and Wave Equation Migration (WEM)
schemes and (2) the use of anisotropy and the importance of well calibration.
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Angle gathers for wave equation pre stack depth migration
More LessIt is now recognized that shot-record wave-equation migration is the best method for imaging in complex media.
However, it is also recognized that producing one output image is not enough. Velocity analysis can only be done
when a gather is produced, as well as AVO analysis or multiple attenuation.
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Imaging deep geology using full tensor gravity gradiometry inversion: K-2, deepwater Gulf of Mexico
By P. HoughtonA common misunderstanding in the oil industry is that Full Tensor Gravity Gradient (FTG) data
is only of use when resolving shallow sections of the earth model. This paper will highlight the
ability of FTG to resolve deep as well as shallow geological targets by showing examples from
an imaging study of the pre-salt K-2 Field, Deep Water Gulf of Mexico. Combined Pre Stacked
Depth Migration (PreSDM) and gravity gradient inversion was used to resolve the base of salt at
a depth in excess of 7 ½ kms.
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Borehole radar EM wave simulation for mining applications using a parallel implementation of the 3-D FDTD algorithm
More LessA borehole radar is a high resolution electromagnetic (EM) sensor for detecting electrical discontinuities in
rock formations. Often the host rock medium through which the wave propagates as well as the target ore-body
have complex electromagnetic properties. Modelling EM wave propagation in 3-D can be used to better understand
the physics of observed responses. It can provide useful insight into the interpretation of real data sets, as well as
show the effects that drilling fluid (mud), water and air in the borehole will have on the coupling from the antenna
to the host rock medium.
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Improved interpretation of borehole radar data through interactive 3D forward modeling
By D. VogtBorehole radar is being applied to delineate tabular orebodies in South African gold and platinum mines. The radar
is omni-directional in azimuth, leading to an ambiguity in the direction to a reflector. One way of resolving the
directional ambiguity is through the use of a priori information. The regional dip and strike of the orebody are
known, and can be used as a first approximation for the position of the reflector. A program called Fresco has been
developed in which a candidate orebody can be manipulated, and its radar response can be modelled in real time.
The model can then be manipulated until the modelled response agrees with the measured response.
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Borehole radar delineation of the UG2 reef at Modikwa Platinum Mine
By P. du PisaniThe effectiveness of imaging the major platinum bearing reefs of the Bushveld Complex with borehole radar has
been proven under the PlatMine Research Collaborative. In this paper the technology was applied, at Modikwa
Platinum Mine, as a value adding exploration tool to assess two mining challenges on the UG2 reef.
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Application of the multi-electrode resistivity method to surface-water/ground-water interaction as applied to vegetation controls
By L. SoltauThe South African National Water Act of 1998 makes provision to have a groundwater reserve set aside to cater for
basic needs and ecologic use of water. Groundwater/surface water interaction is of import to define this groundwater
reserve. The distribution of vegetation around the Langebaan Lagoon is expected to be at least in part dependant on
groundwater conditions.
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Surface nuclear magnetic resonance (SNMR) sounding technique applied to primary aquifers in Southern Africa
By R. MeyerSurface Nuclear Magnetic Resonance sounding (SNMR) is the only geophysical method that allows the direct noninvasive
determination of both the volume of exploitable groundwater and its depth distribution. Furthermore, the
technique is unique in that it can discriminate between the sizes of the water bearing pores. Since 1999 a number of
exploratory SNMR surveys have been conducted in Botswana, Namibia and South Africa.
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Electro-kinetic soundings with a TDEM instrument
More LessIn 2003 the newly developed theory and an algorithm for the calculation of the EKE effect was presented at SAGA.
Since that time the progress in further developing the method of electrokinetic sounding was as follows:
1. New software for the mathematical modeling and interpretation of EKE in Matlab was written. It allows improved
visualization, more accurate and faster calculation of the EKE effect in a multilayered media.
2. The instrument TSICL-5, intended to the TDEM method was adjusted to allow for the additional measurement of the
EKE effect. Supplementary to the basic receiver an external amplifier with an input connection designed for a matched
impedance of the ground electrodes or porous pots for the electric signal was built and fitted to the instrument. An external
trigger device fitted to the receiver converts the geophone pulse to run the software. The software was also altered
to display the recorded seismo-kinetic event. The set up of the receiver is done automatically (calibration) before the
actual measurements take place.
3. Laboratory and fields testing of the instrument and software has showed not only a principal possibility of the registration
of EKE, but very stable and repeatable signals.
4. The EKE soundings were recorded along a profile (approximately 200 m in length) on the farm Horseback, close to
Cullinan. This profile was repeated twice. Along one part of the profile no considerable EKE signals were recorded but
the opposite end shows very strong EKE signals with amplitudes of the order of a few millivolts. This signal is very
well correlated from sounding to sounding. The strong signals coincide, according to the farmer to a local spring located
in this area. The interpretation of collected data was done using the newly developed software. Field result
showed that the EKE anomaly is associated with water saturated sediments located at a shallow depth of 3 -4m. The
predicted permeability of the water saturated layer is to be ~20-30 D.
The obtained results show that the EKE method can be successfully used for searching for groundwater at very
shallow depths. It is very important because most of the other geophysical methods cannot be used for this type of hydro
geological problem.
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In-mine electrical resistance tomography (ERT)
More LessIn-mine, tunnel-to-tunnel electrical resistance tomography (ERT) has, in recent years, been a topic of research at
CSIR Mining Technology. This novel and unconventional application of ERT is aimed at detecting and delineating
geological features that affect the continuity of the tabular platinum reefs of the Bushveld Complex ahead of
mining. In this paper, the contrast between conventional ERT and the in-mine approach is highlighted. Some of the
key challenges associated with in-mine ERT are discussed, together with how these challenges are overcome in an
effort to optimise the application for routine in-mine use. Two proof-of-concept case studies are also included.
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Development of a combined resistivity and induced polarization instrument for the laboratory environment
By L. MareAs part of the expansion of the physical properties database of the Council for Geoscience to include resistivity and
induced polarization (IP) data, a demonstration model of the Swedish built ABEM RIPS3 was obtained in September
1996. Right from the onset, using the ABEM RIPS3 produced several problems.
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Application of an improved S-layer differential transform on TDEM sounding data to delineate an iron ore deposit
By M. CombrinckThe S-layer differential transform, based on the late time approximation of a thin conductive sheet, was applied to
Time Domain Electromagnetic (TDEM) sounding data acquired in order to delineate an iron ore prospect.
Although giving very good results on synthetic data, the S-layer differential transform is extremely sensitive to
noise in real data. This is due to the fact that the method requires two successive numerical differentiations which is
an extremely unstable procedure.
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Numerical integration for reducing degree of generated noise
By S. LephotoInterpretation of TDEM sounding data is commonly done using the S-layer differential transform. This method is
based on the thin sheet model. In this method, the first and second time derivates of the magnetic field are solved
simultaneously for the conductance (S) and depth (d). Taking the derivatives of S to d then yields the conductivity
of an equivalent S-layer and depth d.
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