Geophysics Unit

Pretoria, South Africa

Geophysics Unit

Pretoria, South Africa
Time filter
Source Type

Nyabeze P.K.,Geophysics Unit | Venter J.S.,Environmetal Geoscience Unit | Olivier J.,Unisa | Motlakeng T.R.,Environmetal Geoscience Unit
Proceedings of the 3rd IASTED African Conference on Water Resource Management, AfricaWRM 2010 | Year: 2010

The Siloam hot spring is located on the Siloam fault in the northern Limpopo Province of South Africa. Geophysical data were collected over the aquifer hosting the spring with the aim of determining its physical characteristics. Regional airborne magnetic data provided information about structural features on a larger scale, allowing for the identification of number of dykes and faults, including the Siloam fault. Ground magnetic and electromagnetic surveys were collected in the area around the Siloam hot spring. The increased resolution of the ground magnetic data made it possible to delineate two east-west striking dykes, one to the north of the hot spring and one to the south. The dykes created impermeable barriers of the aquifer. Results of the electromagnetic survey showed that the hot spring is located on a very conductive zone that is roughly a 150 m wide. Combining the results of the ground and airborne geophysics gave a width of 150 m and a lateral extent of approximately 11500 m for the aquifer. The high conductivities associated with the spring suggest that the spring water may be mineralized. The impact of the thermal water on the environment and depth characteristics of the aquifer will need to be studied further.

Scheiber-Enslin S.E.,Geophysics Unit | Scheiber-Enslin S.E.,University of Witwatersrand | Ebbing J.,University of Kiel | Webb S.J.,University of Witwatersrand
Geophysical Journal International | Year: 2016

A 3-D density model of the crust and upper mantle beneath the Karoo basin is presented here. The model is constrained using potential field, borehole and seismic data. Uplift of the basin by the end of the Cretaceous has resulted in an unusually high plateau (>1000 m) covering a large portion of South Africa. Isostatic studies show the topography is largely compensated by changes in Moho depths (~35 km on-craton and >45 km off-craton) and changes in lithospheric mantle densities between the Kaapvaal Craton and surrounding regions (~50 kg m-3 increase from on- to off-craton). This density contrast is determined by inverted satellite gravity and gravity gradient data. The highest topography along the edge of the plateau (>1200 m) and a strong Bouguer gravity low over Lesotho, however, can only be explained by a buoyant asthenosphere with a density decrease of around 40 kg m-3. © 2016 The Authors.

Scheiber-Enslin S.E.,Geophysics Unit | Scheiber-Enslin S.E.,University of Witwatersrand | Webb S.J.,University of Witwatersrand | Ebbing J.,University of Kiel
South African Journal of Geology | Year: 2014

Recent interest in the main Karoo Basin of South Africa has been sparked by the possibility of extensive shale gas resources. Historical reflection seismic, petroleum exploration wells and regional magnetic data are used to better understand the distribution and geometry of dolerite intrusions within the basin that could have impacted the shale reservoir. The lowest concentration of dolerites are found in a region stretching from the southwest to the southeastern part of the basin around the town of Graaff-Reinet. These intrusions are confined to the Beaufort Group, -1000 m shallower than the shale reservoir. In the southeastern Karoo around Queenstown, 5 to 30 km wide saucer-shaped sills extend down to -800 m, with dips of between 2° and 8°. Further south, dolerite sheets around Somerset-East extend for over 150 km at dips of between 3° and 13°, and are imaged down to ~5 km. These dips appear to increase closer to the Cape Fold Belt in the south, although there is no correlation between the southern edge of these dolerites (i.e., the dolerite line) and the dip of sediments due to folding. Magnetic data are useful shale gas exploration to detect shallower (<200 m) dykes that can extend to reservoir depth and are not visible on seismic data. Karoo dykes are usually between 1 and 10 m thick and are shown to often be beyond the resolution of the regional aeromagnetic data. Integrated studies using seismic and higher resolution magnetic data are therefore necessary to better understand the complex dolerite network of the Karoo. © 2014 December Geological Society of South Africa.

Scheiber-Enslin S.E.,Geophysics Unit | Ebbing J.,University of Witwatersrand | Webb S.J.,University of Kiel
South African Journal of Geology | Year: 2015

Here we present a comprehensive depth and thickness map of the main Karoo and Cape Basins using borehole and reflection seismic data. The depth to the Whitehall Formation, which is the focus of current shale gas interest within the Karoo, is also mapped. Change: The deepest part of the basin is in the south, along the northern boundary of the Cape Fold Belt (∼4000 m in the southwest Karoo and -5000 m in the southeast; ∼5500 to 6000 m sediment thickness). The Whitehill Formation along this boundary reaches a depth of ∼3000 m in the southwest and ∼4000 m in the southeast. Limited borehole data in the southeastern Karoo show a broad deepening of the basin here compared to the southwestern Karoo. In the southeast near East London faulting has resulted in deepening of the basin close to the coast, with the Whitehill Formation deepening to over ∼5000 km. Seismic and borehole data show that the Cape Supergroup pinches out below the Karoo Basin around Beaufort West and Graaff-Reinet in the southern Karoo (32.6°S for the Bokkeveld and 32.4°S for the Table Mountain Group). The Cape Supergroup reaches thicknesses of around 4 km in the south. The gravity effect of these sediments does not account for the Cape Isostatic Anomaly (CIA) in the southern part of the Karoo Basin near Willowmore and Steytlerville, i.e., an ∼45 mGal Bouguer gravity low. A refraction seismic profile over the anomaly shows this region is associated with a large volume of low velocity/density shallow sediments (4.5 m/s2, 2500 kg/m1), as well as a low velocity/density anomaly associateti with a normal fault and the Klein Winterhoek Thrust Fault (5.5 m/s2, 2650 kg/nr1). These low density shallow sediments are explained by uplift of Karoo and Cape sediments of ∼2 km or greater that is evident on Soekor reflection seismic data. This deformation has brought lower density shales (1800 to 2650 kg/m3) of the Reca Group closer to the surface. These shallower features along witli a deeper lower crust in this region (6.5 m/s2, 2900 kg/nr1) are interpreted to account for the CIA. © 2015 September Geological Society of South Africa.

Finn C.A.,U.S. Geological Survey | Bedrosian P.A.,U.S. Geological Survey | Cole J.C.,Geophysics Unit | Khoza T.D.,University of Witwatersrand | Webb S.J.,University of Witwatersrand
Precambrian Research | Year: 2015

Geophysical models image the 3D geometry of the mafic portion of the Bushveld Complex north of the Thabazimbi-Murchison Lineament (TML), critical for understanding the origin of the world's largest layered mafic intrusion and platinum group element deposits. The combination of the gravity and magnetic data with recent seismic, MT, borehole and rock property measurements powerfully constrains the models. The intrusion north of the TML is generally shallowly buried (generally <1500. m) with a modeled area of ~160. km. ×. ~125. km. The modeled thicknesses are not well constrained but vary from ~<1000 to >12,000. m, averaging ~4000. m. A feeder, suggested by a large modeled thickness (>10,000. m) and funnel shape, for Lower Zone magmas could have originated near the intersection of NS and NE trending TML faults under Mokopane. The TML has been thought to be the feeder zone for the entire Bushveld Complex but the identification of local feeders and/or dikes in the TML in the models is complicated by uncertainties on the syn- and post-Bushveld deformation history. However, modeled moderately thick high density material near the intersection of faults within the central and western TML may represent feeders for parts of the Bushveld Complex if deformation was minimal. The correspondence of flat, high resistivity and density regions reflect the sill-like geometry of the Bushveld Complex without evidence for feeders north of Mokopane. Magnetotelluric models indicate that the Transvaal sedimentary basin underlies much of the Bushveld Complex north of the TML, further than previously thought and important because the degree of reaction and assimilation of the Transvaal rocks with the mafic magmas resulted in a variety of mineralization zones. © 2015 Published by Elsevier B.V.

Cole J.,Geophysics Unit | Cole J.,University of Witwatersrand | Webb S.J.,University of Witwatersrand | Finn C.A.,U.S. Geological Survey
Journal of African Earth Sciences | Year: 2014

Gravity models reveal the 3-D extent of the mafic component of the Bushveld Complex, critical for understanding the origin of the world's largest layered mafic intrusion and largest source of platinum-group elements (PGEs). New density information, broadband seismic data, borehole data and geological discoveries have improved the constraints on the gravity modelling. Furthermore, all of the models published up to now have been done in two or 2.5 dimensions which is not well suited to modelling the complex geometry of the Bushveld intrusion. Constrained three dimensional modelling takes into account effects of variations in geometry and geophysical properties of lithologies, providing better fits to the shape and amplitude of calculated fields. Gravity data reveal subsurface density contrasts to great depths and the significant density contrast between the mafic rocks of the Bushveld Complex and the surrounding granites and sediments, as well as contrasts across the crust-mantle boundary, make gravity modelling ideal for constraining the 3D geometry of the Bushveld Complex.The aim of this paper is to demonstrate the effect of the new constraints and use of full three dimensional modelling on gravity models of the Bushveld intrusion. We remodel previously published models using full three dimensional potential field modelling software to test the existing conceptual models in an equally conceptual way. Including the measured thicker crust underneath the Bushveld Complex necessitates the presence of dense material in the central area between the eastern and western lobes. The simplest way to achieve this is to model the Rustenburg Layered Suite as a single connected intrusion. This is similar to the first geometries suggested for the Bushveld Complex. In addition to these findings, variations in the lower crust and mantle densities also contribute to models of this scale and have to be considered. © 2014 Elsevier Ltd.

Scheiber-Enslin S.,Geophysics Unit | Scheiber-Enslin S.,University of Witwatersrand | Ebbing J.,University of Kiel | Webb S.J.,University of Witwatersrand
Tectonophysics | Year: 2014

The source of the Beattie Magnetic Anomaly (BMA) still remains unclear, with several competing hypotheses. Here we add a piece to the puzzle by investigating available potential field data over the anomaly. Filtered magnetic data show the BMA as part of a group of linear magnetic anomalies. As the linear anomaly north of the BMA is associated with exposed supracrustals, migmatites and shear zones within the Natal thrust terranes we assume a similar source for the BMA. This source geometry, constrained by seismic and MT data, fits potential field data over the BMA and other magnetic linear anomalies in the south-central and south-western Karoo. In these models the bodies deepen from ~5km towards the south, with horizontal extents of ~20-60km and vertical extents of ~10-15km. Densities range from 2800 to 2940kg/m3 and magnetic susceptibilities from 10 to 100×10-3 SI. These magnetic susceptibilities are higher than field values from supracrustal rocks (10-60×10-3 SI) but could be due to the fact that no remanent magnetisation was included in the model. The lithologies associated with the different linear anomalies vary as is evident from varying anomaly amplitudes. The strong signal of the BMA is linked to high magnetic susceptibility granulite facies supracrustals (~10-50×10-3 SI) as seen in the Antarctic, where the mobile belt continued during Gondwana times. © 2014 Elsevier B.V.

Madi K.,University Of Fort Hare | Nyabeze P.K.,Geophysics Unit | Gwavava O.,University Of Fort Hare | Sekiba M.,Geophysics Unit | Zhao B.,WorleyParsons Group Inc
Acta Geophysica | Year: 2016

Finding productive boreholes in the Karoo fractured aquifers has never been an easy task. Fractured Karoo aquifers in the neotectonic zones in the Eastern Cape Province can be targeted for groundwater exploration. The Polile Tshisa hot spring is located in a seismo-tectonic region beset by neotectonics. Hot springs are indicative of circulation of groundwater at great depths along fault zones, and accordingly of neotectonics. The characterisation of hot springs by means of magnetic and electromagnetic methods can help infer the occurrence of structures which are favourable for groundwater potential. The Polile Tshisa hot spring is characterised by faults, fractures, and dolerite dykes. All these structures make the hot spring a good case study for groundwater exploration. © 2016 Institute of Geophysics, Polish Academy of Sciences.

Sakala E.,Geophysics Unit | Tessema A.,Geophysics Unit | Nyabeze P.K.,Geophysics Unit
International Journal of Modelling and Simulation | Year: 2014

The study area is located in the northern Limpopo Province of South Africa. The aim of the study was to identify groundwater targets that could assist in improving the quality of life of rural communities. Airborne magnetic data was interpreted to identify dykes, lineaments and magnetic sources that could control groundwater occurrences. The length, parallelism of magnetic lineaments in some parts of the area suggests emplacement under tensional stress field along pre-existing zones of weakness. Lineaments extracted from the airborne magnetic data and satellite imagery data were superimposed on drainage lines to investigate the relative importance of structural features controlling the distribution of surface water and groundwater. In addition, normalized difference vegetation index (NDVI) was used in identifying areas of vegetation banding, which enable inference of fracture zones and high moisture content in the soil. The study shows that the northern and central eastern parts of the study area are more prospective for groundwater occurrence, while the southern and south-western parts of the project area are dry with no surface manifestation of groundwater. Integration of lineaments derived from aeromagnetic data and Landsat imagery as well with the NDVI was able to identify areas with a potential for groundwater occurrence at a regional scale.

Loading Geophysics Unit collaborators
Loading Geophysics Unit collaborators