Masters Degrees (Physics)

Permanent URI for this collectionhttps://hdl.handle.net/10413/6604

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An investigation in the use of satellite data to develop a geomagnetic secular variation model over Southern Africa.
(2011) Nahayo, Emmanuel.; Alport, Michael J.; Kotzé, Pieter B.
Time variations of the geomagnetic field can be classified into two main categories of internal and external origin with respect to the surface of the Earth. It has been found that the variations that take place on longer time scales (~ 1 year and longer) are commonly known as secular variation (SV) and are of internal origin. There is a need to develop SV models using satellite data as the use of ground data is not always possible with many limitations including the limited data points and lack of data over ocean areas that are not easily accessible. Two regional geomagnetic field modelling techniques namely polynomial surface modelling (PolyM) and Spherical Cap Harmonic Analysis (SCHA) were applied to CHAMP satellite data recorded between 2001 and 2005 to investigate the use of satellite data to develop a geomagnetic SV model over southern Africa. The restricted area of investigation is between 10° and 40° South in latitude and between 10° and 40° East in longitude. The resulting regional models of this investigation were validated against the two widely used global field models IGRF 10 and CHAOS using the available ground survey data obtained during the same period over southern Africa. The results suggest that the regional field models can be derived based entirely on satellite data. However, the regional SV models can be improved by combining both high quality satellite and ground survey data, since they lack the high quality of a global field model like CHAOS.
Measurement and validation of waterlines and surface currents using surf-zone video imaging.
(2001) Naicker, Jaysen.; Alport, Michael J.
The continuous monitoring of beaches and structures near, or in the surf-zone, has become a vital task from an environmental and economic standpoint. Specifically, the Durban beaches are continually nourished with sand that is dredged south of the Durban Harbour mouth. Constant monitoring is essential to control the erosion and accretion of these beaches. Currently, infrequent and labour-intensive manual surveys are being used to fulfil this task. This dissertation describes the techniques used to process and validate surf-zone video images to extract information, which is equivalent but more frequent and cost effective to that obtained using manual surveying methods. The emphasis is on the extraction of accurate hourly waterlines by analysing video images obtained from Coastal Imaging stations located at Addington Beach and North Beach in Durban, South Africa and the measurement of rip currents in the surf-zone from video images together with the validation of these results. The implementation of a neural network incorporating non-local image pixel data is more reliable than previously used methods, for example, grayscale thresholding or the delta-discriminator. Grayscale thresholding relies on the difference between the water and the sand pixel intensities. The delta-discriminator incorporates hue-saturation-lightness (HSL) values to improve the discrimination between water and sand. These methods could not, however, overcome the problem of varying seasonal and diurnal light intensities and require the prior choice of threshold values. The use of non-local data, such as the average RGB-values, in addition to individual pixel values as inputs to a neural network is shown to give better results in changing ambient lighting conditions. Wavelet and Fourier analysis of the temporal nature of the breaking waves in the surf-zone is also used to calculate more accurate waterlines. These extracted waterlines are validated by data collected from manual surveys. The use of Digital Correlation Image Velocimetry (DCIV) has been proven as a successful method in tracing velocity flow fields in a fluid medium. Typically, the medium is populated with distinct seeds that are tracked through successive video frames using a cross-correlation technique. It is shown that DCIV can be used in surf-zone images to measure surface rip currents by tracking the structure of the advected foam. This technique was validated by simultaneously tracking a large number of yellow corks scattered on the water surface.
Measurement of soil in sugar cane using non-destructive techniques.
(2001) Padayachee, Thavashen.; Alport, Michael J.
The soil being delivered with sugar cane consignments. from the cane fields to the factories, has been a recent cause for concern in the South African sugar industry. The soil impurities increase the wear of processing machinery reduce extraction efficiency and represents an unnecessary transport of material. The cost due to soil was estimated at R63 million (about US $8 million) over the 1996/97-season. The need to reduce costs, due to the unwanted soil component, has been given a high priority. Ashing is currently used by the sugar industry to estimate the amount of soil in cane. Although simple to implement, the method is destructive, requires long processing times and limited to small sample sizes. In fecent times, non-destructive techniques have become more prominent in industry. Hence, the decision to apply such techniques to the soil in cane-problem. This dissertation describes an experimental investigation into Dual-Energy Transmission (DuET) and X-ray lmaging for quantifying the amount of soil in cane. DuET can determine the relative concentrations of the components of a binary mixture by measuring the transmission of low- and high-energy gamma photons through the mixture. The principle of DuET was successfully demonstrated with aqueous solutions of ferric chloride. Experimentally-determined mass attenuation coefficients of water and ferric chloride were compared to theoretical values. DuET was then applied to dried, shredded sugar cane spiked with various amounts of soil. Results showed large variations in the predicted soil concentrations. These variations were attributed to radiation scatter and incomplete volume sampling by the radioactive source. However, new experimental arrangements are expected to improve the technique: initial test results are given of a sample holder that continuously rotates a sample up and down through the source-detector axis. An alternative approach to processmg DuET-spectra, using the discrete wavelet transform coupled with an artificial neural network, is also introduced. X-ray Imaging was the second technique investigated. A literature survey revealed that this technique had not previously been applied to the soil in cane-problem. The present work constituted an initial investigation to determine the feasibility of applying X-ray imaging to measure the amount of soil in cane. The soil/cane-samples, that were used for DuET, were imaged us ing a commercial mammography unit, and the resulting radiographs were analysed using image processing techniques. Although the results are promising, a more comprehensive investigation is foreseen.
Pharos : pluri-director, high-resolution, analyser of radiometric properties of soil cores.
(2001) Pitout, Richard.; Alport, Michael J.
The core-logger has been designed for the high-resolution radiometric analysis of soil cores using multiple detectors. This device allows for the automation of the measuring process and eliminates the need to dissect the cores. The design is aimed at measuring soil-cores with a 10cm radius and a length of 1m and allows for up to 4 detectors to be mounted on the measuring platform. Currently a combination of Bismuth-German.te (BGO) and C.esiwn-Iodide (CsI) detectors are used. The core logger required a good spatial resolution of - 1 cm. This has been difficult to obtain and has required extensive investigation. The shielding configurations were varied and the effect of background radiation was looked at in detail to determine an optimal construction. A secondary objective has been the complete measurement of a single core in 24 hours. This has also been difficult to achieve because the low activity of natural radiation in the core samples needs longer measuring times. The BGO detectors were used as a more efficient detector (than, e.g. CsI) which helped to reduce the required measuring time. Measured spectra have been analysed to determine the activity concentrations of the specific radionuclides of interest: 232Th, 238U, 40K and 137Cs. These activity profiles of the measured cores provide information that can then be used to radiometrically fingerprint the sample to determine soil characteristics such as grain size and mineral content. However, because the actual resolution of the system ( ~3cm) is greater than the typical core slice (~ 2cm), the radiometric information in a specific core-slice contains contributions from its adjacent slices. This folding or convolution of the measured spectra can be undone using a deconvolution method. which was examined and commented on.
A study of plasma source ion implantation.
(1993) Thomas, Kim.; Alport, Michael J.
The work described in this thesis is an analysis of the Plasma Source Ion Implantation (PSII) process. A metal target is placed within a plasma, and pulsed to a high negative potential (10 - 50 kV). The electrons in the plasma close to the target are then repelled very rapidly, leaving an area of uniform positive charge. This causes an electric field to be set up between the plasma and the metal target. The ions close to the target are then accelerated towards the target by the electric field. The ions reach the target at high velocities, and implant deeply into the metal (-5 x 10-8 m), and form nitrides, which pin dislocations within the metal's atomic structure. The strength of the metal is therefore increased, and other properties such as the corrosion resistance of the metal are also improved. Metals that have undergone the PSII process have widely diverse applications. For example, in the motor industry, ion implanted metal punches last much longer than nitrided punches, while in the medical industry ion implanted metals are used for artificial limbs. A combination of a number of different analytic, numerical and simulation models are used to describe the PSII process, including the plasma behaviour and final nitrogen implantation profile in the metal target after the application of the voltage pulse. In all cases, a specific attempt has been made to realistically describe as closely as possible, the actual experimental arrangement at the University of Natal. For example: a waveform with a fast rise time, short plateau and exponential decay was used; the nitrogen plasma was more realistically described by a two species fluid to account for the measured N+, N; mix; and finally, the actual atomic composition for 304 stainless steel was used in the TAMIX particle simulation. This work thus models the whole PSII process, and could form the basis of future studies for the optimisation of the process.
Two-dimensional plasma sheath observations in plasma source ion implantation.
(1996) Meyer, Kevin Alan.; Alport, Michael J.
Plasma Source Ion Implantation (PSII) is the process of implanting high energy ions [10-50 keV] into metallic targets, by pulsing them negatively whilst immersed in a background plasma. PSII achieves surface hardening, and increased wear and corrosion resistance. Numerous papers have been published describing numerical simulations and models of the PSII process, most of which have been limited to one dimension. This thesis presents the results of work carried out III the Plasma Processing Laboratory at the University of Natal, Durban, during 1994-1995. In particular, measurements of two-dimensional plasma sheath effects due to spherical and complex shaped targets are compared with a particle-in-cell simulation code. The simulation results are used to define a relationship between the plasma potential of the sheath edge and the saturation currents. Thus allowing for the saturation currents to be used to trace sheath evolution. These results are compared with the experimental measurements from the spherical target. Results from the rectangular and complex saw-tooth targets show a lack of sheath conformality. The ion saturation currents were susceptible to electron swamping, which occured in localised regions associated with target structure. It is thought that secondary electrons ejected from the target are focused and accelerated by the high target potential into these regions, where they swamp the ion current.

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Browsing Masters Degrees (Physics) by Author "Alport, Michael J."