- Home
- Conferences
- Conference Proceedings
- Conferences
6th SAGA Biennial Conference and Exhibition
- Conference date: 28 Sep 1999 - 01 Oct 1999
- Location: Cape Town, South Africa
- Published: 28 September 1999
21 - 40 of 78 results
-
-
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.
-
-
-
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.
-
-
-
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).
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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).
-
-
-
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.
-
-
-
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.
-
-
-
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.
-
-
-
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.
-