The possibility of commercial natural gas production from gas hydrates has been tested by researchers and industry for more than ten years. Depressurisation of gas hydrates in porous and permeable sandstones has successfully produced water and natural gas. However long term sustainable production is still elusive. Catastrophic sand production into the wellbore has terminated at least three of the significant depressurisation trials including the 2013 trial at the Daini-Atsumi knoll, Nankai Trough, offshore Japan. Sand production is generally thought to be the result of mechanical and hydrodynamic instability, however it appears the failure mechanism is not the same for all reservoirs and the location of reservoir porosity and pressure on the normal compression line for sands could be a controlling factor. Sand production in reservoirs at shallow depths and low confining stresses (less than 10 MPa) are likely to be influenced by fluid flow effects like those described by the Shields (1936) diagram. The relative density of the formation may also affect the nature of the sand production in these reservoirs. The Daini-Atsumi knoll is a structural high on the outer ridge of the Kumano forearc basin, offshore Japan. Hydrate saturations of 50 to 80 % occur within three geological units of the Middle Pleistocene Ogasa group. This group is made up of deep water sediments including sediment gravity flow deposits distinguished by alternating silt and sand layers. The presence of these alternating layers could have influenced the sand production seen during the trial. This reservoir heterogeneity at the 2013 Daini-Atsumi knoll gas hydrate production trial site was characterised using the descriptions of geological units, analogues and statistical techniques. Scenarios of this heterogeneity were tested in a high pressure plane-strain sand production apparatus. The results of these tests suggest the boundary shear stress of the fluid on the grains is a significant control on sand production for the Daini-Atsumi Knoll reservoir and the layering and grainsize structure of the sediments encourages sand production. Relative density of the sediments appears to impact the nature of the sand production where denser sediments show more localised movement. These results indicate that even minor weaknesses in sand control devices will result in uncontrollable sand production rates from the Daini-Atsumi Knoll gas hydrate reservoir. Managing the fluid flow rate in the reservoir and selectively completing coarser grained zones at the base of sand layers could help limit sand production in future trials.

This thesis explores topics related to the formation and development of the large-scale structure in the Universe, with the focus being to compute properties of the evolved non-linear density field in an approximate way. The first three chapters form an introduction: Chapter 1 contains the theoretical basis of modern cosmology, Chapter 2 discusses the role of N-body simulations in the study of structure formation and Chapter 3 considers the phenomenological halo model. In Chapter 4 a novel method of computing the matter power spectrum is developed. This method uses the halo model directly to make accurate predictions for the matter spectrum. This is achieved by fitting parameters of the model to spectra from accurate simulations. The final predictions are good to 5% up to k = 10 hMpc-1 across a range of cosmological models at z = 0, however accuracy degrades at higher redshift and at quasi-linear scales. Chapter 5 is dedicated to a new method of rescaling a halo catalogue that has previously been generated from a simulation of a specific cosmological model to a different model; a gross rescaling of the simulation box size and redshift label takes place, then individual halo positions are modified in accord with the large scale displacement field and their internal structure is altered. The final power spectrum of haloes can be matched at the 5% level up to k = 1 hMpc-1, as can the spectrum of particles within haloes reconstituted directly from the rescaled catalogues. Chapter 6 applies the methods of the previous two chapters to modified gravity models. This is done in as general a way possible but tests are restricted to f(R) type models, which have a scale-dependent linear growth rate as well as having 'chameleon screening' - by which modifications to gravity are screened within some haloes. Taking these effects into account leads to predictions of the matter spectrum at the 5% level and rescaled halo distributions that are accurate to 5% in both real and redshift space. For the spectrum of halo particles it is demonstrated that accurate results may be obtained by taking the enhanced gravity in some haloes into account.

An efficient air transport system is an important part of social and economic development of Southern African Development Community (SADC). Efficient intra-SADC air service connections enhance regional integration, access to the global economy, international tourism and contribute towards the vision to establish the African Economic Community by 2034. SADC, in July 1998, embarked on liberalisation of the regional civil aviation sector in order to enhance the efficiency of air transport services. In the United States of America and European Union, the liberalisation of air transport has transformed civil aviation networks. The fragmentation of air service connections on the intra-SADC network in the midst of the liberalisation process is symptomatic of a poor implementation strategy coupled with air transport market imperfections. The purpose of this thesis is to examine, understand and explain the factors that influence the disintegration of the intra-SADC air transport network .The aim is to identify how regional air transport services can be transformed to meet the social and economic demands of the region. This research adopts network theory, as the conceptual framework of the investigation. Assuming a graph approaching maximal connection as the sought after state of affairs for SADC; this study benchmarked the post liberalisation network structure to the regional economic communities of ASEAN and MERCOSUR. The aim of the benchmarking is to identify the extend of the differences in air transport network in those two regions, resulting from the policies adopted and to establish how the SADC policies may be improved and implemented more efficiently. The findings of the study are that, in comparison to the two developing regions, SADC’s liberalisation measures have failed. The study developed and evaluated an econometric model which analysed demand patterns on the intra-SADC passenger air transport network. Although low levels of passenger demand seem to characterise the majority of SADC city-pairs, the study identified nodes with sufficient demand to justify direct connections which would in turn reduce network fragmentation. This research also establishes that the absence of a realistic detailed roadmap, an ill-defined programme of action and inadequate resources contributed to the failure of SADC’s liberalisation strategy. In its final sections, this study proposes an ideal demand-driven network configuration and offers specific recommendations to SADC member states for that network to be functional. The proposed network improves network connectivity from the current poor levels, where a connectivity measure of 15% suggests underdevelopment, to levels over 40%. The study however, acknowledges that air transport liberalisation does not necessarily guarantee equitable distribution of network efficiency in developing regions. There are communities that cannot sustain commercially viable air service connections without economic subvention, probably in the form of the Public Service Obligation (PSO) programme adopted in the EU.

The search for a theory of quantum gravity has persisted through the last century. Although many beautiful theories such as string theory and loop quantum gravity have been proposed, experimental evidence to support or refute these theories has been difficult to obtain. Searching for Lorentz invariance violation (LIV) is one of a limited number of experimental tests which can be used to search for evidence of quantum gravity since new physics may only be observable at energies well beyond those present in the most energetic astrophysical objects, which are far greater than the energies accessible in a terrestrial laboratory. One method of searching for LIV is to look for energy-dependent time delays in the arrival of high-energy photons from distant astrophysical sources. We search for Lorentz invariance violation (LIV) using VERITAS, an imaging atmospheric Cherenkov telescope (IACT) located in southern Arizona. Significant TeV gamma ray flaring activity was detected from the blazar Markarian 421 on the night of February 17, 2010 (MJD 55244), which presented a good opportunity to search for delays in the energetic emission. We demonstrate the performance of two different dispersion estimation algorithms and apply these algorithms to our data to search for LIV. We find that while the emission from Markarian 421 contains significant variability, a necessary condition for an LIV detection, the presence of a constant background flux severely limits our sensitivity. We expect our findings to be useful for guiding future LIV studies, especially those using IACT data. In the latter part of this work we discuss the alignment of ground-based gamma-ray telescopes and present a digital autocollimator which will be used in the alignment system of a next-generation IACT. The configuration of our autocollimator enables measurement of the angle formed between the planar surface of a distant reflector and the line of sight over a range of ±0.126° with a precision better than 5 arcsec. We present a detailed description of the instrument and its data acquisition software that was used during laboratory testing.

Coadjoint orbits of Lie algebras come naturally imbued with a symplectic two-form allowing for the construction of dynamical actions. Consideration of the coadjoint orbit action for the Kac-Moody algebra leads to the Wess-Zumino-Witten model with a gauge-field coupling. Likewise, the same type of coadjoint orbit construction for the Virasoro algebra gives Polyakov’s 2D quantum gravity action with a coupling to a coadjoint element, D, interpreted as a component of a field named the diffeomorphism field. Gauge fields are commonly given dynamics through the Yang-Mills action and, since the diffeomorphism field appears analogously through the coadjoint orbit construction, it is interesting to pursue a dynamical action for D. This thesis reviews the motivation for the diffeomorphism field as a dynamical field and presents results on its dynamics obtained through projective connections. Through the use of the projective connection of Thomas and Whitehead, it will be shown that the diffeomorphism field naturally gains dynamics. Results on the analysis of this dynamical theory in two-dimensional Minkowski background will be presented.

In this thesis we accomplish two goals: We construct a two dimensional conformal field theory (CFT), in the form of a Liouville theory, in the near horizon limit for three and four dimensions black holes. The near horizon CFT assumes the two dimensional black hole solutions that were first introduced by Christensen and Fulling (1977 Phys. Rev. D 15 2088-104) and later expanded to a greater class of black holes via Robinson and Wilczek (2005 Phys. Rev. Lett. 95 011303). The two dimensions black holes admit a $Diff(S^1)$ or Witt subalgebra, which upon quantization in the horizon limit becomes Virasoro with calculable central charge. These charges and lowest Virasoro eigen-modes reproduce the correct Bekenstein-Hawking entropy of the four and three dimensions black holes via the Cardy formula (Bl"ote et al 1986 Phys. Rev. Lett. 56 742; Cardy 1986 Nucl. Phys. B 270 186). Furthermore, the two dimensions CFT's energy momentum tensor is anomalous, i.e. its trace is nonzero. However, In the horizon limit the energy momentum tensor becomes holomorphic equaling the Hawking flux of the four and three dimensions black holes. This encoding of both entropy and temperature provides a uniformity in the calculation of black hole thermodynamics and statistical quantities for the non local effective action approach. We also show that the near horizon regime of a Kerr-Newman-$AdS$ ($KNAdS$) black hole, given by its two dimensional analogue a la Robinson and Wilczek, is asymptotically $AdS_2$ and dual to a one dimensional quantum conformal field theory (CFT). The $s$-wave contribution of the resulting CFT's energy-momentum-tensor together with the asymptotic symmetries, generate a centrally extended Virasoro algebra, whose central charge reproduces the Bekenstein-Hawking entropy via Cardy's Formula. Our derived central charge also agrees with the near extremal Kerr/CFT Correspondence in the appropriate limits. We also compute the Hawking temperature of the $KNAdS$ black hole by coupling its Robinson and Wilczek two dimensional analogue (RW2DA) to conformal matter.

Knowing what goes on in the upper atmosphere (∼80-140 km) is very important to the space science community. There are several competing forces that influence the temperature and densities of neutral molecules in that region. OPAL (Optical Profiling of the Atmospheric Limb) is funded by the National Science Foundation (NSF) to measure the temperature there using light from oxygen molecules (∼760 nm). To accomplish this,OPAL is built into a CubeSat (a satellite the size of a loaf of bread) to be launched from the International Space Station (ISS) at an altitude of about 400 km. This vantage point is needed to see the light that is absorbed before it makes it to the ground, so a satellite is the optimal choice. Similar to looking at a tennis ball in your hand and trying to see the details of the yellow fuzz fibers on the outer edges of the ball, OPAL is trying to see the light emitted from oxygen at the outer edge of the atmosphere (also called the limb). In order to see how well OPAL can detect space weather signatures affecting the oxygen emissions a suite of models are made to simulate its output. This suite is made of: simulating the flight path of CubeSat, modeling where the OPAL instrument is looking, and how the oxygen light changes with where the instrument is looking. Because we are currently in a solar minimum, the occurrence of solar storms and geomagnetic storms are considered rare events. This allows for the concentrating on detecting gravity waves in this region and the minimum values of detecting them with this developed model.

Gravity Recovery and Climate Experiment (GRACE) mission has amplified the knowledge of both static and time-variable part of the Earth’s gravity field. Currently, GRACE maps the Earth’s gravity field with a near-global coverage and over a five year period, which makes it possible to apply statistical analysis techniques to the data. The objective of this study is to analyse the most dominant spatial and temporal variability of the Earth’s gravity field observed by GRACE using a combination of analytical and statistical methods such as Harmonic Analysis (HA) and Principal Component Analysis (PCA). The HA is used to gain general information of the variability whereas the PCA is used to find the most dominant spatial and temporal variability components without having to introduce any presetting. The latter is an important property that allows for the detection of anomalous or a-periodic behaviour that will be useful for the study of various geophysical processes such as the effect from earthquakes. The analyses are performed for the whole globe as well as for the regional areas of: Sumatra- Andaman, Australia, Africa, Antarctica, South America, Arctic, Greenland, South Asia, North America and Central Europe. On a global scale the most dominant temporal variation is an annual signal followed by a linear trend. Similar results mostly associated to changing land hydrology and/or snow cover are obtained for most regional areas except over the Arctic and Antarctic where the secular trend is the prevailing temporal variability. / Apart from these well-known signals, this contribution also demonstrates that the PCA is able to reveal longer periodic and a-periodic signal. A prominent example for the latter is the gravity signal of the Sumatra-Andaman earthquake in late 2004. In an attempt to isolate these signals, linear trend and annual signal are removed from the original data and the PCA is once again applied to the reduced data. For a complete overview of these results the most dominant PCA modes for the global and regional gravity field solutions are presented and discussed.

In this thesis I address several topics concerning the interaction of convection and density stratification in oceans and lakes. I present experimental and theoretical investigations of the interaction between a localized buoyancy source and a heat flux through a horizontal boundary, and of the interactions between salt fingers and intermittent turbulence or shear. ¶ An extensive series of laboratory experiments were used to quantify the stratification and circulation that result from the combined presence of a localized buoyancy source and a heat flux through a horizontal boundary. Previous studies found that convection in the form of a turbulent buoyant plume tends to produce a stable density stratification, whereas the distributed flux from a horizontal boundary tends to force vigorous overturning and to produce well-mixed layers. A new result of this thesis is that a steady density profile, consisting of a mixed layer and a stratified layer, can exist when the plume buoyancy flux is greater than the distributed flux. When the two fluxes originate from the same boundary, the steady state involves a balance between the rate at which the mixed layer deepens due to entrainment on the one hand and vertical advection of the stratified water far from the plume (due to the volume flux acquired by entrainment) on the other hand. There is a monotonic relationship between the normalized mixed layer depth and flux ratio R (boundary flux/plume flux) for 0 < R > 1, and the whole tank overturns for R > 1. The stable density gradient in the stratified region is primarily due to the buoyancy from the plume and for R > 0 there is a small increase in the gradient due to entrainment of buoyancy from the mixed layer. For the case of fluxes from a plume located at one boundary and a uniform heat flux from the opposite boundary the shape of the density profile is that given by Baines & Turner (1969), with the gradient reduced by a factor (1 + R) due to the heating. Thus, when R < - 1 there is no stratified region and the whole water column overturns. When 0 > R > - 1, the constant depth of the convecting layer is determined by the Monin-Obukhov scale in the outflow from the plume. ¶ One application of these laboratory experiments is to surface cooling in lakes and reservoirs that have shallow sidearms. During prolonged periods of atmospheric cooling, gravity currents can form in these sidearms and as the currents descend into the deeper waters they are analogous to isolated plumes. This can result in stratification at the base of a lake and an upwelling of cold water. Away from the shallow regions, surface cooling leads to a mixed surface layer. The depth of this layer will be steady when the rate of upwelling balances the rate at which the mixed layer deepens by turbulent entrainment. A series of laboratory experiments designed to model the depth distribution of a lake with a shallow sidearm showed that the steady depth of the mixed layer depended on the ratio of the area of the shallow region to the area of the deep region. Significant stratification resulted only when the reservoir had shallow regions that account for more than 50 % of the surface area. The depth of the surface mixed layer also depended on the ratio of the depths of the shallow and deep regions and no significant stratification forms if this ratio is greater than 0.5. These results are in good agreement with observations of circulation and stratification during long periods of winter cooling from Chaffey reservoir, Australia. Theoretical time scales are also developed to predict the minimum duration of atmospheric cooling that can lead the development of stratification. ¶ In the second part of this thesis, I report a series of laboratory experiments which are designed to investigate the fine structure and buoyancy fluxes that result from salt finger convection in the presence of shear and intermittent turbulence. We find that, when salt finger convection in deep linear gradients is superposed with a depth-dependent spatially periodic shear, variations in the density profile develop on the same wavelength as the shear. The laboratory experiments presented in this thesis were carried out in a continuous density gradient with a spatially periodic shear produced by exciting a low-frequency baroclinic mode of vertical wavelength 60 mm. The density gradient consisted of opposing salt and sugar gradients favourable to salt fingers (an analogue to the oceanic heat/salt system). Where the shearing was large the salt finger buoyancy fluxes were small. Changes in salinity gradient due to the resulting flux divergence were self-amplifying until a steady state was reached in which the spatial variations in the ratio of salt and sugar gradients were such that the flux divergence vanished. Thus, along with reducing the mean salt finger buoyancy flux, a spatially varying shear can also lead to the formation of density structure. ¶ In the ocean intermittent turbulence can occur in isolated patches in salt finger-favourable regions. I present new results from laboratory experiments in which a partially mixed patch was produced in deep linear concentration gradients favourable to salt finger convection. Salt fingers give rise to an “up gradient” flux of buoyancy which can reduce the density gradient in a partially mixed patch. This can then lead to overturning convection of the partially mixed patch if a) the ratio of T and S gradients, R\rho =aTz/_ /betaSz, is near one, b) if turbulence results in a nearly well-mixed patch and c) the patch thickness is large enough that convective eddies are able to transport T and S faster than salt fingers. Once overturning occurs, subsequent turbulent entrainment can lead to growth of the patch thickness. Experimental results agree well with the theoretical prediction that h= \surd 8h B/N2 t, where h is the patch thickness, t is time, h is the mixing efficiency of turbulent entrainment, B is the buoyancy flux of the salt fingers and N is the buoyancy frequency of the ambient gradient region. This thickening is in contrast to the collapse that a partially mixed patch would experience due to lateral intrusion in a very wide tank. In regions of the ocean that contain salt fingers there is the possibility that, after a period of initial collapse, an intrusion could enter a regime where the rate of collapse in the vertical is balanced by the growth rate due to turbulent entrainment from the salt fingers buoyancy flux, thus tending to maintain the rate of lateral spread. ¶ A further series of laboratory experiments quantified the buoyancy fluxes that result from salt fingers and intermittent turbulence. A continuous density gradient, favourable to salt finger convection, was stirred intermittently by an array of vertical rods that move horizontally back and forth along the tank at a constant velocity. Previous experiments had found that continuous turbulence destroys any salt fingers present because the dissipation of turbulent kinetic energy occurs at scales that are generally smaller than salt fingers widths. However, when turbulence is present only intermittently, the salt fingers may have time to grow between turbulent events and so contribute to the vertical diffusivities of heat and salt. We conclude that the vertical buoyancy flux of salt fingers is strongly dependent upon the intermittency of the turbulence, and equilibrium fluxes are only achieved if the time between turbulent events is much greater than the e-folding time of the salt fingers. When these results are applied to an oceanographic setting, the effect of intermittent turbulence, occurring more 5% of the time, is to reduce the effective eddy diffusivity due to salt fingers below equilibrium salt finger values, so that at R\rho > > 2 the eddy diffusivity is due only to turbulence. The time averaged salt fingers fluxes are not significantly reduced by intermittent turbulence when R\rho > 2 or if the intermittence occurs less than 2% of the time, and so may contribute significant diapycnal fluxes in many parts of the ocean.

This thesis studies the scattering properties of different types of imperfections in large Arctic and Antarctic ice sheets. Such irregularities include cracks, pressure ridges and both open and refrozen leads. The scattering by a transition region between sea ice and a very thick ice shelf, for example as is found in the Ross Sea in Antarctica, is also treated. Methods of solution are based on applications of Green�s theorem to the appropriate situation, which leads to either a single integral equation or a pair of coupled integral equations to be solved at the boundary between the ice and the sea water. Those equations over a finite interval are solved using numerical quadrature, while those over semi-infinite ranges are solved using the Wiener-Hopf method. Results calculated using different techniques are able to be checked against each other, giving us great confidence in their accuracy. In particular, the scattering by three ice sheets of different thicknesses is confirmed analytically by mode-matching coupled with the residue calculus technique. The scattering by the single irregularities is investigated partly for its own sake, and partly with the aim of using it to treat the scattering when large numbers of features are included in a single ice sheet. The principal objective of doing this is to observe the change in the general amounts of reflection and transmission as the background ice thickness is changed. There is enough variation in our results for us to conclude that there is definite potential for using the change in an incident wave spectrum after passing through a given ice field to estimate the background ice thickness.