An investigation is presented of the behavior of time-reversal focusing in soils. Initial numerical models demonstrate time-reversal focusing to be effective in elastic media, including when a large number of scattering objects were present in the medium. When scattering objects are present, time-reversal focusing demonstrates superior focusing ability when compared to other excitation methods such as uniform excitation or time-delay focusing. Multiple experimental investigations of experimental time-reversal focusing performed in sand evaluate time-reversal focusing effectiveness when multiple near-surface scattering objects are present in the medium. Experimental results demonstrate that time-reversal focusing is effective in the experimental context as well as the numerical models. Further experiments examine time-reversal focusing in more extreme cases where the entire ballistic wave is blocked, and the only energy reaching the focus point is reflected from scattering objects in the medium. A comparison to other focusing methods demonstrates that under these conditions, most focusing attempts with traditional methods will fail completely while time-reversal focusing does not. Additional configurations of time-reversal focusing examine its effectiveness when scattering is caused by an asymmetrical surface layers. The impact of an asymmetrical or non-uniform excitation array is also examined for time-reversal focusing in the presence of scattering objects. An investigation of the effects of scattering object geometry on focusing resolution in time-reversal focusing is also presented. Scattering object field density is found to have a strong, but diminishing effect on focusing resolution as the scattering object field density increased. Loss of surface wave energy available for focusing due to mode-conversion is found to be correlated with the density of the scattering object field. The impact of the weak non-linear nature of the soil on time-reversal focusing is examined through a study of time-reversal focusing behavior for a variety of amplitudes that generate different levels of non-linearity in the soil. This study of nonlinearity is coupled with a study of the impact of noise on time-reversal focusing. It appears that both non-linearity and noise have an impact on time-reversal focusing effectiveness. Further, the loss from these mechanisms seems to be interrelated. Noise seems to enhance non-linear loss in the soil.

Thesis (MA)--University of Stellenbosch, 2002. / ENGLISH ABSTRACT: This is a study of the growmg importance of Non Governmental Organisations (NGOs) in global governance. Global governance is defined as a complex and dynamic process that deals with issues that go beyond the capacity of national governments, that is distinguished from global government because of an absence of a central authority that can ensure compliance and the presence of a wide range of actors including non-state actors. The theory of Complex Multilateralism captures the role of NGOs and NGO coalitions well. Using this theory as a theoretical framework, this study focuses on two recent transnational NGO campaigns (The International Campaign to Ban Landmines [fCBL) and the NGOs against Arms Trade) to determine why some NGO campaigns are more successful than others. The theory of Complex Multilateralism in combination with extensive information on different types of NGOs and their activities on national and international levels, makes it possible to identify criteria that can determine success. These criteria are: a realistic goal, the issue area (type, number, salience and techniques used to frame the issue), government and intergovernmental organisation (lGO) commitment, access to IGOs, extensive expertise, effective use of the media, effective use of information technology, activity variance, leadership, persuasive and influential spokespersons, membership and funds. These criteria are described, defined and then applied to the above-mentioned transnational NGO campaigns. The main finding was that the ICBL was the more successful of the two campaigns because it had more of the criteria for success. In this case the most important reasons for success is: a realistic goal, the focus on a single issue and the effective framing of the issue, significant government commitment as well as good leadership. Although, the criteria that were developed can hardly be universalised, they do provide a useful starting point for further research into this important field in International Studies. / AFRIKAANSE OPSOMMING: Hierdie werkstuk bestudeer die toenemende belangrikheid van Nie-Regerings Organisasies (NROs) in "global governance" Laasgenoemde konsep word gedefinieer as 'n komplekse en dinamiese proses wat behels die hantering van vraagstukke wat buite die beheer is van 'n enkele staat, wat onderskei word van 'n wereld regering as gevolg van die afwesigheid van sentrale gesag en die aanwesigheid van 'n wye reeks van akteurs of rolbekleers insluitente nie-staatlike rolbekleers. Komplekse Multilateralisme bied 'n goeie teoretiese begrip van die rol van NROs en NRO koalisies in hierdie proses. Hierdie studie maak gebruik van Komplekse Multilateralisme as 'n teoretiese raamwerk om te fokus op twee onlangse transnasionale NRO veldtogte ( Die internasionale veldtog om landmyne te verban en die NROs teen wapenhandel) en sodoende te bepaal hoekom sekere NRO veldtogte meer geslaag is as ander. Reedsgenoemde teorie in kombinasie met inligting oor verskillende tipes NROs en hul aktiwiteite op nasionale en internasionale vlakke maak dit moontlik om kriteria vir 'n suksesvolle NRO veldtog te identifiseer. Hierdie kriteria bestaan uit die volgende: 'n realistiese doel, die aard van die vraagstuk (tipe, hoeveelheid, "salience", en tegnieke wat gebruik is om die vraagstuk te formuleer, toewyding van regerings en tussen-regerings-organisasies, toegang tot tussenregerings -organisasies, veelsydige kundigheid, effektiewe gebruik van die media, effektiewe gebruik van inligtingstegnologie, verskeidenheid van aktiwiteite, leierskap, oorredende en invloedryke segspersone, lidmaatskap en fondse. Die bevinding is dat die internasionale veldtog om landmyne te verban die meer geslaagde veldtog is. Die belangriskste redes hiervoor is: 'n realistiese doel, die fokus op enkele vraagstukke en die effektiewe formulering van die vraagstuk, die toegewydheid van baie regerings, sowel as goeie leierskap. Alhoewel die kriteria wat in die studie ontwikkel is nie veralgemeen kan word nie dien dit as 'n nuttige basis vir vedere navorsing oor hierdie belangrike tema in die veld van Internasionale Studie.

This thesis presents a robust method for estimating the relaxations of a metallic object from its electromagnetic induction (EMI) response. The EMI response of a metallic object can be accurately modeled by a sum of real decaying exponentials. However, it is diffcult to obtain the model parameters from measurements when the number of exponentials in the sum is unknown or the terms are strongly correlated. Traditionally, the time constants and residues are estimated by nonlinear iterative search that often leads to unsatisfactory results. In this thesis, a constrained linear method of estimating the parameters is formulated by enumerating the relaxation parameter space and imposing a nonnegative constraint on the parameters. The resulting algorithm does not depend on a good initial guess to converge to a solution. Using tests on synthetic data and laboratory measurement of known targets the proposed method is shown to provide accurate and stable estimates of the model parameters.

The objective of the proposed research is to develop structural changes to the design and application of electromagnetic (EM) sensing models to more efficiently and accurately invert EM measurements to extract parameters for applications such as landmine detection. Two different acquisition modalities are addressed in this research: ground-penetrating radar (GPR) and electromagnetic induction (EMI) sensors. The models needed for practical three-dimensional (3D) spatial imaging typically become impractically large, with up to seven dimensions of parameters that need to be extracted. These parameters include, but are not limited to target type, 3D location, and 3D orientation. The new special structures for these models exploit properties such as shift invariance and tensor representation, which can be combined with strategic inversion techniques, including the Fast Fourier Transform and semidefinite programming. The structures dramatically reduce the amount of computation and can eliminate the need to store up to five dimensions of parameters while still accurately estimating them.