Browsing by Author "Johansen, Geir A."

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    Entropic Thresholding Methods in Reconstruction of Capacitance Tomography Data
    (IEEE, 1996) Mwambela, Alfred; Isaksen, O.; Johansen, Geir A.
    High quality reconstruction of capacitance tomography data is of vital importance for the extraction of quantitative information from such systems. The use of such soft-field sensor systems and the LBP reconstruction algorithm produces distortions in the reconstructed tomogram data (i.e. the true intensity values and their location in the tomogram under consideration are corrupted or altered). The main problem with LBP reconstruction algorithm is a smearing effect of sharp transitions in the dielectric constants. Several authors have suggested different approaches for improving the quality of the reconstructed tomogram. The quality of the generated tomograms could be improved by including a thresholding procedure in the reconstruction process for the purpose of minimising these distortions. Here, the use of global entropic thresholding methods in the reconstruction process of capacitance tomography data is reported. The results obtained are compared to the previously published thresholding methods of Xie et al. (1992) and an Implicit Model based reconstruction. It is concluded that distortions in the reconstructed capacitance tomograms can be minimised by using a thresholding procedure. It can be seen that the evaluated global entropic thresholding methods without supervision cannot be used to produce reasonably accurate results when it comes to multicomponent flow imaging. The smearing effect of the LBP reconstruction algorithms blur the phase transition making it hard for autothresholding, particularly for small objects. This effect is more pronounced when the two components under investigation have a small difference in permitivity
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    Multiphase Flow Component Volume Fraction Measurement: Experimental Evaluation of Entropic Thresholding Methods Using an Electrical Capacitance Tomography System
    (2001) Mwambela, Alfred; Johansen, Geir A.
    A laboratory experiment to measure component volume fractions in a multiphase flow of an oil/gas/water mixture, where water is the discontinuous phase, has been conducted using a measurement system based on electrical capacitance tomography. The image reconstruction algorithm of the electrical capacitance tomography system has been modified to incorporate entropic thresholding methods, which facilitate accurate information extraction from the tomograms. Evaluation results show that this approach, using only one thresholding method, fails to meet the desired requirements over the full component volume fraction range, is flow regime dependent and is highly dependent on the concentration of the components in the mixture. However, some of the algorithms developed are complementary with the conventional algorithms over the full component volume fraction range. Integration of complementary algorithms improves the overall measurement results to within the desired requirements for operational purposes in the oil industry.
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    The Use of Entropic Thresholding Methods in Reconstruction of Capacitance Tomography Data
    (Elsevier, 1997) Mwambela, Alfred; Isaksen, Ø.; Johansen, Geir A.
    High-quality reconstruction of capacitance tomography data is of vital importance to ensure quantitative information from such systems. Several methods have been tested, such as linear back projection (LBP), representing a fast and crude method, and model-based reconstruction (MOR) which significantly improves the results, although at higher computational cost. The main problem of using LBP is the smoothing effect on sharp transitions in the dielectric constant distribution. The reconstruction quality could be improved by including a thresholding procedure in the reconstruction process. In this work, entropic thresholding methods are used. These methods are compared to previously published thresholding techniques such as Xie's method and an implicit model-based reconstruction method. A computer simulated eight-electrode capacitance tomography system has been used for the evaluation.