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11th SAGA Biennial Technical Meeting and Exhibition
- Conference date: 16 Sep 2009 - 18 Sep 2009
- Location: Swaziland, South Africa
- Published: 16 September 2009
151 - 157 of 157 results
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Comparison Between Time- and Frequency-Domain Induced Polarisation Parameters
Authors M. van Schoor, L.P. Marè and C.J.S. FouriePhysical properties of rock samples from exploration or mining sites are often needed to assist in the planning of geophysical surveys or in the interpretation of geophysical results. For example, the output from a physical property study may be used in numerical model studies aimed at assessing the applicability or performance of a geophysical technique. Physical properties are also useful for constraining model parameters during processing and in the interpretation of field data. Electrical property data are often recorded in the time-domain, yielding resistivity and chargeability values; however, a frequency-domain, or even complex resistivity approach, may also be followed. The latter approach, for example, would produce resistivity magnitude and phase angle outputs. It is shown that both the commonly used standard time-domain chargeability parameters and the (single-frequency) complex resistivity phase angle parameter could produce misleading estimates of the polarisable nature of mineralised rocks and of the contrast between different rock types. It is further shown that a multi-frequency (spectral) approach can be used to avoid this pitfall; similarly, the calculation of different time-domain induced polarisation (IP) parameters, (using different definitions) may provide better insight into the polarisable nature of rock samples than a single, arbitrary chargeability value.
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Tree Root Mapping with Ground Penetrating Radar
Authors M. van Schoor and C. ColvinIn this paper, the application of ground penetrating radar (GPR) for the mapping of near surface tree roots is demonstrated. GPR enables tree roots to be mapped in a non-destructive and cost-effective manner and is therefore a useful prospecting tool in a variety of diverse study fields, including geohydrology, ecology and civil engineering. Case study examples are presented of how GPR can play a role in detecting and quantifying subsurface biomass and also how GPR can be used to discriminate between subsurface roots and man-made utilities.
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Increased Accuracy in Mineral and Hydrogeophysical Modelling of HTEM Data via Detailed Description of System Transfer Function and Constrained Inversion
Authors Andrea Viezzoli, Anders Vest Christiansen and Esben AukenThis paper aims at providing more insight into the parameters that need to be modelled during inversion of Helicopter TEM data for accurate modelling, both for hydrogeophysical and exploration applications. We use synthetic data to show in details the effect, both in data and in model space, of the low pass filters present in any system, and of waveform repetition. Low pass filters affect the shallow to intermediate part of the model, whereas the waveform repetition the deeper part. Results show how filters and waveform are parameters, like frame altitude, Tx-Rx timing and so on, that need to be taken into account and modelled correctly during inversion of HTEM data. We then present an application of this approach on real VTEM data from an exploration survey. The results from constrained inversion of the VTEM, compared with borehole information and with other modelling methodologies, show its validity.
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Geomagnetic Field Strength Recorded in Iron Age Ceramics from Southern Africa
The Earth’s dipole magnetic field has decreased in intensity by 5 to 10 percent over the last 150 years. To understand more about the nature of this decline, it is desirable to have a longer record, extending back in time beyond the data available from modern magnetic observatories. Firing of archeologic objects (e.g. ceramics and hut floors) raises temperatures above the Curie temperatures of their constituent magnetic minerals. Paleointensity analyses of fired artefacts-- commonly known as archeomagnetism—can provide longer term geomagnetic records. Such studies are well developed at North Hemisphere sites, but there are no extensive records from the Southern Hemisphere for the millennium prior to the era of observatory recording. There is an extensive record of Iron Age ceramics from southern Africa, with associated burnt structures that has the potential to provide a Southern Hemisphere geomagnetic field record. We discuss our efforts to test the potential of this record using rock magnetic and paleointensity analyses of Iron age test samples. Rock magnetic results to date suggest that Iron age Iron age ceramics contain magnetic minerals of appropriate composition and size to retain a high resolution record of past field intensity. Data from preliminary paleointensity experiments further support this conclusion.
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Overview and Preliminary Results of a Magnetotelluric Experiment Across the Southern Barberton Greenstone Belt
Authors U. Weckmann, A. Nube, X. Chen, O. Ritter and M. deWitThe Kaapvaal Craton of South Africa is the oldest well-preserved continental fragment on the planet and thus is a key area for studies of geodynamic processes of the early Earth. One major controversy concerns the importance of plate tectonic processes in Archean times and the time when this may have been first initiated. Several potential ancient suture zones have been identified at surface within the Barberton greenstone Belt, and these provide an ideal natural laboratory to test for possible subsurface remnants of mid-Archean plate tectonic processes on lithospheric scale. Within the framework of the German-South African geo-scientific research initiative Inkaba yeAfrica a high resolution magnetotelluric (MT) field experiment, ELIBABA, was carried out in April/May 2009 in the Barberton/Badplaas area, eastern Mpumalanga, RSA. An 110 km long profile and two complementary shorter 70 km long profiles with almost 100 MT sites provide a good areal coverage of the Barberton suture and its complex geology. Preliminary two-dimensional inversion models show images of the crustal electrical conductivity distribution which will be compared to existing shallow geophysical results.
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The Good, the Bad, and the Ugly: Airborne EM and the Regolith
By C. WijnsEM signals may be highly influenced by the regolith in weathered terrain, to the point of obscuring geologic targets at deeper levels. In time-domain data, this influence can extend from the earliest measurement times into the mid-range time gates. The degree of interference by the regolith varies according to the terrain, and is mostly influenced by the longevity and variability of weathering. Examples from different parts of Africa illustrate the extent to which long-lived regolith dominates the signal of airborne EM surveys.
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Interpretation of Laboratory Measured Data: New Information on Pore Structure and Anisotropy using Relating to the IP Effect
Authors V. Zadorozhnaya and L.P. MaréMembrane polarization is a fundamental phenomenon of geophysical methods such as induced polarization (IP) method and frequency domain induced polarization method (FD IP). Determination of the characteristic parameters of induced polarization is required for studying physical properties of rocks. Mathematical modeling of a little known model of IP referred to as “induced polarization caused by constrictivity of pores” was conducted. Diffusion equations with specified boundary conditions that are different for current on- and off-times were used in the model. It was shown that membrane polarization occurs in all types of rocks if surface areas and transfer numbers are different for connected pores. During the polarization process all contacts between pores of different transfer numbers will be blocked and the electrical current will flow through the remaining canals. The new algorithm was tested on laboratory measurements. Several samples were selected: shale, mudstone, tillite, hematite, lava and manganese ore. Each theoretical model includes pores of more than 40 sizes sporadically distributed in the sample. The output of this stage of work is the pore size distribution in the sample, anisotropy and the relative amount of pores able to transport ions. It was shown that the size (pore radii) of pores can be different even when the porosity of samples is the same. The prevalent radii of investigated samples varied from 10 μm up to 1 μm. Laboratory data showed good agreement with theory and provided new information on the pore structure of rocks. Mathematical modeling provides reliable information of pores space of rocks, their dynamic porosity and permeability and transportation, especially of contaminant compounds
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