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13th SAGA Biennial Conference & Exhibition
- Conference date: 06 Oct 2013 - 09 Oct 2013
- Location: Kruger National Park, South Africa
- Published: 06 October 2013
81 - 100 of 108 results
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Large-scale Automatic Generation Of Hydrological Input From Resistivities And Boreholes
Authors N. Foged, A. Vest Chrsitiansen and P. Aabye MarkerThis study presents a semi-automatic sequential hydrogeophysical inversion method for the integration of resistivity data and lithological borehole information into groundwater models in sedimentary areas. Large scale airborne geophysical EM-surveys play an increasingly important part in the geological mapping of the subsurface especially in a hydrogeological context. Airborne EM surveys provide valuable information of the geological structures and the lateral heterogeneity than boreholes cannot reveal due to the spatial scarcity (Jørgensen et al., 2003). However, boreholes play a key role in linking the resistivity to the different lithological and hydrological classes. Today, geologists and hydrogeologists in most cases interpret AEM-derived electrical resistivity distributions manually along with borehole observations within the context of a given geological setting. Due to the discrepancy between hydrological and geophysical parameter spaces the challenge is to translate the electrical resistivity distribution into hydrogeological classes. Our results suggest that a competitive groundwater model can be constructed from densely sampled resistivity models from AEM surveys together with borehole information, using the procedure outlined below.
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3d Geological Modelling Using Geological And Aem Data
Authors J. Deparis, B. Vittecoq, T. Jaouen, F. Lacquement and J. Perrinyears. Nevertheless, geological, hydrological or hydrogeological data are often very scarce, and appropriate approach have to be found in order to improve the hydrogeological functioning of these islands. Groundwater resource is closely correlated with the permeability of rocks, and, in basaltic volcanic islands, permeability is usually considered to be negatively correlated with age formations. Younger lavas have higher permeability than older one’s often highly weathered (Custodio et al., 1988). Existing hydrogeological models of basaltic volcanic islands describe large-scale systems at the volcano or island scale, and two end models have been described. The Hawaiian model (Meinzer, 1930) considers a low-lying basal aquifer linked to inland dike-impounded and perched aquifers overlying impervious layers and/or confined by dykes (Tabasaki and Mink, 1983). The Canary Islands model (Custodio, 1975; Custodio et al., 1988) considers a continuous and isotropic basal aquifer, and a decrease of hydraulic conductivity with the age of the volcanic rocks. Nevertheless, those models are consistent with Young Island (< 5 Ma) and seem not fully appropriate for older island such as Mayotte Island.
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Results Of Recent Ground-based Geophysical Surveys In The Okavango Delta
Quasi-3D inversions of an extensive helicopter time-domain electromagnetic (HTEM) data set acquired across the Okavango Delta (OD; Figures 1 and 2) yield 3- to 4-layer electrical resistivity models that include (1) a shallow resistive layer of dry and fresh-water-saturated sands, (2) an intermediate-depth conductive layer of intercalated saline-water-saturated sands and clay, and (3) a relatively deep resistive layer of fresh-water-saturated sands/gravels and/or crystalline basement. The upper resistive layer clearly represents unconsolidated sediments in the current alluvial fan-like environment, whereas the intermediate conductive layer likely represents sediments deposited under earlier lacustrine conditions. The top part of the deeper resistive layer has an intriguing fan shape, centred about the entrance to the main part of the delta (Figures 2e and f). If the fan-shaped portion of the deeper resistive layer comprises fresh-water-saturated gravels/sands, it would be evidence for a paleo-alluvial fan, which we refer to as the Paleo-Okavango Delta in the following text.
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Evidence For A Paleo-okavango Delta And Overlying Mega-lake From Airborne Tem Data
Authors J.E. Podgorski, A.G. Green, W.K.H. Kinzelbach, T. Kalscheuer, E. Auken and T. NgwisanyiThe near juxtaposition of the Makgadikgadi Basin, the world's largest saltpan complex, with the Okavango Delta, one of the planet's largest inland deltas (technically an alluvial megafan), has intrigued explorers and scientists since the middle 19th century. It was also clear from early observations that the Makgadikgadi Basin once contained a huge lake, popularly referred to as Paleo Lake Makgadikgadi. Through a contract with a commercial company, the Botswana Department of Geological Survey has acquired a helicopter time-domain electromagnetic (HTEM) data set across the entire Okavango Delta. The HTEM data were of extremely high quality, mainly due to very low noise levels (there are no large power lines and only a small number of settlements and tourist lodges in the delta) and significant contrasts between the electrically resistive units (i.e., dry and fresh-water-saturated sand and basement) and electrically conductive ones (i.e., saline-water-saturated sand and clay). Our inversions of these data returned electrical resistivity models containing three principal layers: (i) an upper heterogeneous layer that is largely resistive, (ii) an intermediate conductive layer, and (iii) a lower resistive layer. According to borehole logs and groundwater sampling, the electrically conductive layer is due to a combination of clay and saline-water-saturated sediments. We interpret these sediments to have belonged to Paleo Lake Makgadikgadi, which would have extended into the region presently occupied by the Okavango Delta. The total area of Paleo Lake Makgadikgadi would therefore have exceeded 90,000km2, larger than Earth's most extensive fresh-water body today, Lake Superior. Our HTEM data also provide evidence for a southeast-dipping paleo megadelta or megafan beneath the Okavango Delta’s upper fan and the Paleo Lake Makgadikgadi sediments. Seismic refraction/reflection experiments and boreholes suggest that this feature is composed of fresh-water sediments, probably representing a Paleo-Okavango Delta resting on top of basement rock. If this interpretation is correct, then the clay layers of the Paleo Lake Makgadikgadi sediments act as an aquitard protecting the fresh-water sediments of the paleo-delta from salt water contamination above.
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Induced Polarisation Effect Associated With Acidic Groundwater In Witwatersrand Gold Mining Areas
Authors D.P. De Villiers, S. Webb, I. Weiersbye and M. JonesAcidic mine water is generated in situ in the hanging and foot-walls of mines. In the gold mines of the Witwatersrand the oxidation of pyrite generates acid. This acid infiltrates groundwater and is and will continue to be a major environmental hazard specifically in and around the mines. Non-invasive geophysical techniques make it possible to map the extent and the flow pattern of acidic water. Induced polarisation (IP) in conjunction with direct current resistivity will be used to map the extent of acidic mine water at a bioremediation program near the Vaal River in the Northwest Province. Data will be collected where the survey can be repeated in order to monitor the time dependence of the phenomenon. Ways of separating the environmentally-associated IP signal from electromagnetic coupling and cultural noise will also be investigated.
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Estimating The Location And Depth Of Contacts From Aeromagnetic Data
More LessThe Tilt-depth method is commonly used with pole-reduced aeromagnetic data to obtain the depth to the upper surface of vertical contacts. Unfortunately the method involves determining the distance between contours of the Tilt angle, which is both computationally intensive and often ambiguous. A new method is introduced here which does not use contours at all, enabling the depths to be simply read from the output image. If there is no remanent magnetisation present then the method also indicates when the contact is dipping.
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Evaluation Of Two-dimensional Electrical Resistivity Models Using Image Processing Method
Authors K.S. Ishola, M.N.M. Nawawi, K. Abdullah, K.A.N. Adiat and A. AbdulrahmanTwo-dimensional inversion results obtained using three electrode arrays, the Wenner, Schlumberger and dipole-dipole synthetic data sets were combined using Algorithm librarian of an image processing program PCI Geomatica widely used for remotely sensed data. The hybrid or combined images which were obtained using, the maximum, minimum, median and average values of the produced inverse models were compared. Image registration was applied in order to establish correspondence and integration between the images. All the images resolved and recovered the essential features of the true models. The images produced bear considerable likeness to the true models. An evaluation of the accuracy of the seven images for the four geologic models was assessed using four criteria via an error measure and the recovered mean resistivity values of the predicted blocks from the true blocks. The results show that the images of maximum approach give the least estimated mean absolute error. Also, the mean resistivity values of the blocks are closest to the true blocks for all the images of maximum method. Thus, it is corroborated that when inverse models are combined, most reliable and detailed information about the geologic models is than using individual inverse models.
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The Application Of Moment Tensor Inversion Methods To Fracture Monitoring Of Civil Engineering Materials
Authors L. Linzer, T. Schumacher and L. MhamdiA moment tensor inversion (MTI) code written to compute source mechanisms from mining-induced seismicity data is now being used in the laboratory in a civil engineering research environment. Quantitative seismology methods designed for geological environments are being tested with the aim of developing techniques to assess and monitor fracture processes in concrete structures such as bridges. In this paper, the aspects of the MTI code that make it applicable to performing inversions on acoustic emission data recorded by networks of uniaxial sensors are highlighted, and the results of some laboratory experiments where the MTI code has been applied to both rock and concrete are summarised. It is also demonstrated how the MTI code is being used to verify numerical models so that the predicted behaviour corresponds to actual fracture processes.
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Topographic Effects On Magnetic Data: Data Reduction And Application To The Southern Andes
More LessDespite the many advances on magnetic surveying technology and semi-automated modelling and interpretation routines, topographic effects on the data are normally neglected; usually due to the assumption that magnetic data collected on a surface parallel to the ground will not suffer from terrain effects. However, on areas of substantial topographic relief like the Andes, large magnetic anomalies induced by the terrain can be common and of similar amplitude than the anomalies generated by the geological target of interest. These effects are mostly dependent on the orientation of the effective magnetic field versus the topographic slopes, and the large susceptibility contrast between air and the ground. We present a combined 3D forward and inverse modelling technique to reduce magnetic data from topographic effects. The algorithm is applied on a dataset at the Southern Andes, where topographic relief is in excess of 4000 m and the observed magnetic anomalies showed direct correlation with topography, but field mapping did not identify any major faults at the location of the magnetic lineaments. Although the computation could possibly be automated, it is recommended to apply it carefully on a case by case basis and with proper geological control.
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In-situ And Laboratory Determination Of Thermal Properties Of Tar Sands In Eastern Dahomey Basin Southwestern Nigeria
Authors M.A. Oladunjoye, A. Adefehinti and O.A. SanuadeThermal properties of tar sands are of paramount importance in the design of commercial recovery operation that involves heating of the tar sands as well as in the research activities that preceded field applications. Estimation of thermal properties such as thermal resistivity, thermal conductivity, thermal diffusivity, thermal effusivity and volumetric specific heat is of great importance in bitumen recovery from tar sands where heat transfer takes place through injection of thermal energy into the tar sand formation. This research work therefore aims at determining thermal properties of tar sands in the Eastern Dahomey basin Southwestern, Nigeria with a view to having more information in planning for thermal recovery of the tar sands. Twelve locations were established along the tar sands belt in order to measure its thermal properties. A KD2 Pro analyzer was used to measure the thermal properties. In laboratory, thermal properties for samples from each location were measured and physical properties that influenced them such as grain size distribution, percentage by weight of bitumen content, moisture content, bulk density, porosity and dry density of the samples were determined. Thermal conductivity, specific heat and bulk density were used to calculate the thermal effusivity of the tar sands. Positive correlations was observed between the field and laboratory measurements values of the thermal resistivity, thermal conductivity, thermal diffusivity, thermal effusivity and volumetric specific heat with R-values 0.86, 0.85, 0.81, 0.78 and 0.49 respectively. It was observed that the thermal properties of the tar sands determined have close similarities with those reported on Athabasca tar sand Canada, therefore the thermal recovery process used in the Athabasca basin could also be employed in the Eastern Dahomey basin Nigeria. Therefore, thermal properties of tar sands are quite essential in thermal recovery of bitumen so as to minimize wasting of thermal energy and resources.
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High Resolution Aeromagnetic Data Interpretation For Lineaments And Iron Ore Mapping Of Ogbomosho Area, South-western Nigeria
Authors M.A. Oladunjoye, A.I. Olayinka, Y.A. Mustapha and M.A. Adabanijaard continuation filter were grouped into NE-SW and NW-SE, respectively. The depth to the top of the deepest magnetic source is about 3.5 Km. The pattern of the high positive anomaly (47.788–86.331nT) shows that Ajase BIF is structurally controlled with approximately NE-SW trend. This mode of occurrence was ascertained by the result of Euler deconvolution using structural index of 1.0 that proves the confinement of Ajase iron ore mineralization to veins.
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The Role Of Fugro Tempest Aem And Falcon Agg Surveying In Stratiform Cu And Iocg Exploration In Zambia
Authors A.N. Christensen, R. Van Buren, T. Whiting and D. BurrowsIn this paper we present the successful application of fit-for-purpose airborne geophysics on two copper exploration projects in Zambia: the use of TEMPEST® time-domain Airborne Electro-Magnetic (AEM) surveying over the strati-form Cu Kansanshi project, and the use of FALCON® Airborne Gravity Gradiometry (AGG) on the Iron Oxide Copper Gold (IOCG) exploration project at Mumbwa.
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Preliminary Design Parameters For A Htem System Dedicated To Overburden Mapping
Authors R. Caron, C. Samson, M. Chouteau and M. BatesA helicopter-borne transient electromagnetic (HTEM) survey system is being designed with characteristics suitable to map glaciolacustrine overburden overlying Precambrian bedrock for the purpose of correcting airborne gravity measurements for lateral variations in overburden thickness. The HTEM system parameters such as transmitter power and waveform are optimized for overburden mapping, by using forward modelling and inversion. This paper will discuss some inversion results that test the capability of the system at resolving the thickness of an overburden under multi-layer geological scenarios.
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Reducing The Dependence Of The Analytic Signal Amplitude Of Aeromagnetic Data On The Source Vector Direction
More LessThe analytic signal amplitude (As) is a commonly used edge detection filter for aeromagnetic data. For profile data its response is independent of the source magnetisation vector direction, but this is not the case for map data. A modified As is inroduced here which has a much reduced dependance on the source magnetisation vector direction. Because it uses second order derivatives the method is sensitive to noise. The modified As is demonstrated on synthetic datasets and on an aeromagnetic dataset from South Africa.
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Displaced Extension Of Slave Diamond Corridor - Geophysical Evidence
More LessThe Slave “Diamond Corridor” is a north northwest trend that appears to have controlled the emplacement of the most significantly diamondiferous kimberlites of the Slave craton of northern Canada and any extension of the corridor is prime diamond exploration country. A northern extension of the corridor into the Lena West diamond area of the Northwest Territories that includes the diamondiferous Darnley Bay and Dharma kimberlites requires major left-lateral displacements (~350km) that are not generally recognised and are not identified in the surface geology. Major faults are recognised by Zolnai in the petroleum literature and there is geomorphological and geophysical evidence to support them. Euler deconvolution analysis of regional airborne magnetic surveys show little in the near surface platform rocks but strong linear breaks that coincide with other evidence of faulting in the deeper basement rocks Discovery of an economic diamond field in a corridor south of Darnley Bay striking parallel to the Slave “Diamond Corridor” will support continuity of the corridor and the need for a major displacement.
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Investigating Magnetic Source Depths In The Soutpansberg Basin, South Africa
Authors P.K. Nyabeze, M.P. Dube and O. GwavavaThe Limpopo Province of South Africa has thermal manifestations in the form of hot springs. The Soutpansberg Basin hosts several hot springs and is located in the north-eastern part of the Limpopo Province. Research was undertaken in order to understand the geothermal potential of the area. Airborne magnetic data was analyzed. Methods that were used to determine magnetic source depths were Radial Averaged Power Spectrum, Euler De-convolution and the Tilt Derivative. The magnetic source depth, at which crustal rocks reach a temperature where spontaneous magnetization is lost, is assumed to be the heat source depth and is called the Curie depth. Mapping of Curie depths is based on the estimation of the depth to the bottom of magnetic bodies. The depth to the bottom of magnetic sources in the vicinity of thermal manifestations was found to be in the range 2 km to 5 km. The deepest zones at 8 km to 10 km depths are located to the west and east of the basin. The deep seated north-south oriented structures could be related to the East Africa Rift fracture system. Depth solutions that were obtained using the Radial Average Power Spectrum were comparable to Euler depth solutions. The Tilt Derivative results confirmed the presence of shallower source depths below areas that had thermal manifestations. Results of the investigation have shown that regions with shallow Curie isotherm depths are potential targets for geothermal investigations.
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Q-values For P And S Waves In South Sinai And South Gulf Of Suez Region, Egypt
More LessThe quality factor Q has been estimated using spectral amplitudes of P and S waves from earthquakes recorded by the seismic network of Egyptian National Seismological Network (ENSN) in south Sinai and north Red Sea region. The earthquakes recorded at nine stations – DHA, NUB, TR1, TR2, KAT, SH2, GRB, HRG and SFG have been used. The spectral amplitude ratios have been calculated between 2 - 20 Hz and single station spectral ratio method has been applied for this purpose. The results show that the quality factors for both P and S waves (Qp and Qs) increase as a function of frequency according to law Q = Q0fn. By averaging the Q- Value obtained at all stations we calculated the average attenuation laws: Qp = (13.15± 0.76) f0.95± 0.19 and Qs = (20.05± 0.79) f1.03±0.04 for P and S waves respectively. The QS /QP ratio for KAT station at lower frequencies is less than 1, whereas at HRG and SH2 stations QS /QP ratio is are greater than 1. These relations are useful for the estimation of source parameters of earthquakes and simulation of earthquake strong ground motions.
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Smart Exploration With No Return
By A. GuirouGuelb Moghrein has been classified as an Iron-Oxide Copper Gold (IOCG) deposit; however, the complexity of this deposit gives this classification significant ambiguity. IOCG is a holistic deposit description, which confirms very little about genetic process. Guelb Moghrein hosts two mineralized ferromagnesian carbonate (FMC) lenses called Occidental and Oriental, each of which contains mineralisation at around 1-2% Cu and 1g/t Au (Garman, 2012). Exploration for a similar type of deposit in Akjoujt has little success despite the effort, smartness and multidisciplinary sciences used in the project.
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High Resolution Reflection Seismic Mapping Of Shallow Coal Seams
Authors S.B. Mngadi and R.J. DurrheimSubsidence and collapse of unmapped shallow coal mine workings poses a risk to the public and hampers the development of valuable property. A high-resolution reflection seismic survey was conducted to determine whether it is possible to map the extent of the mine workings. Two 94 m profiles (tied to boreholes) were surveyed using a sledgehammer source. Processing was optimized to image the shallow reflections. The refraction seismic models and stacked time sections were compared and integrated with the borehole data to produce a 2-D geological model. It was concluded that high-resolution shallow reflection seismics could be successfully used to map the extent of the old mine workings provided adjustments are made to the acquisition and processing parameters.
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Groundwater Prospect Within The Basement Complex Of South-western Nigeria Using Vertical Electrical Soundings
Authors S.A. Oke, O. Ologe and G. Van TonderGeophysical investigations involving the use of Vertical Electrical Soundings was conducted across the basement complex rocks with the aim of determining its geoelectric parameters and hydrologic significance. A total of eight (8) vertical electrical soundings were carried out using the Schlumberger array configuration, with half the current electrode separation (AB/2) varying from 1m to 100m. VES were conducted cross the whole study area to obtain information about the subsurface lithologies, overburden thickness and possible basement structures that could harbour the accumulation of groundwater. The geo-electric parameter results obtained from the sounding curves revealed 3-layer, 4-layer and 5-layer earth models, respectively. The 3-layer model has 75% of occurrence, while the 4- layer has (12.5%) and (12.5%) for the 5-layer models. This shows that the subsurface layers can be characterised into topsoil, clay /sandy-clay, weathered/fractured layer, and the fresh bedrock. The weathered and/or the fractured basement are the aquifer types delineated across the area. Depth to bedrock ranges from 0.2m to 8.5m across the study area. On the basis of subsurface parameters and further interpretations of the VES data, assisted in the zoning the study area into moderate and low groundwater potential zones. The investigation revealed that basement depressions mostly constitute high groundwater potential zones in the study area. The study, envisaged to provide a reliable guide on the groundwater potential in the area, is expected to assist in future development of groundwater resources to sustain the water requirements of the community.
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