Geosciences, School of 2019-09-20T07:24:22Z 2019-09-20T07:24:22Z Subsurface seismic imaging in the presence of multiply scattered waves Lomas, Angus 2019-09-19T10:18:44Z 2019-11-26T00:00:00Z Subsurface seismic imaging in the presence of multiply scattered waves Lomas, Angus The aim of seismic imaging is to produce maps indicative of spatial variations in properties of the Earth's subsurface. To create such images geophysicists use seismograms of energy measured over time by receivers at fixed observation points. These seismograms partially sample the seismic wavefield and are used to estimate the interactions between the seismic wavefield and the subsurface heterogeneity. However, because observation points are limited spatially the true interactions are unknown so approximations must be made to estimate these interactions. Conventional methods make the assumption that seismic waves observed in the seismograms reflect or diffract at most once from the subsurface heterogeneity (the so-called Born approximation). This assumption allows a low-wave-number smoothly varying estimate of the subsurface velocity structure to be used to back extrapolate the observed seismic wavefield to points inside the subsurface { producing an estimate of the subsurface seismic wavefield. However, this approximation can lead to inaccuracies when the seismograms contain energy that has reflected or diffracted more than once. In this thesis we create a suite of methods that offer a solution to this problem in a variety of scenarios. The majority of this thesis focuses on a set of novel techniques called Marchenko methods. These enable us to project seismograms to points in the subsurface - creating seismograms as though they had been measured at each subsurface point - while accounting for many of the complex, multiply-reflected seismic wave interactions that take place in the real Earth's subsurface. As a result images created using Marchenko imaging contain a reduction in artifacts that usually contaminate subsurface images due to multiply-reflected seismic waves. This thesis has four main aims which are addressed in consecutive chapters: (1) To introduce Marchenko methods with the minimum amount of mathematics required to understand how the methods iterate to a solution, and to provide a well-commented, easily editable MATLAB code package for demonstration and training purposes. (2) The second aim is to understand the application of Marchenko methods in a three-dimensional world, that is to say we investigate the implications of three-dimensional data, subsurface structures, wavefields and acquisition geometries on the results of Marchenko redatuming and imaging. (3) In a third set of results we aim to incorporate the additional wavefield sampling of vertical seismic profile (VSP) data (measured in boreholes in the Earth's subsurface) into Marchenko imaging with the emphasis being on improving imaging of vertical and near vertical subsurface interfaces. (4) The aim of the final set of results is to use multiply scattered (particularly duplex waves) as well as primary (singly scattered) waves to image the subsurface, again to improve imaging of vertical or near vertical interfaces but this time only using surface seismic data. Overall the results of this thesis demonstrate the effectiveness of Marchenko methods to redatum and image accurately when only low-wavenumber smoothly varying estimates of the subsurface velocity structure are available. We demonstrate the applicability of the methods to three-dimensional problems and a means to include VSP data into the method. Finally, we also redefine the conditions used to create subsurface images, allowing us to image using singly and multiply scattered seismic waves. 2019-11-26T00:00:00Z Assessing ecological resilience to human induced environmental change in shallow lakes Ives, Stephen Christopher 2019-09-19T10:04:41Z 2019-11-26T00:00:00Z Assessing ecological resilience to human induced environmental change in shallow lakes Ives, Stephen Christopher Sudden unpredictable changes in ecosystems are an increasing source of concern because of their inherent unpredictability and the difficulties involved in restoration. Our understanding of the changes that occur across different trophic levels and the form of this change is lacking. This is especially true of large shallow lakes, where characteristics such as fetch and depth are close to theoretical boundary values for hysteretic behaviour. The development of reliable indicators capable of predicting these changes has been the focus of much research in recent years. The success of these early warning indicators (EWIs) has so far been mixed. There remain many unknowns about how they perform under a wide variety of conditions and parameters. Future climate change is predicted to have a wide range of impacts through the interaction of combined pressures, making the understanding of EWIs and the in-lake processes that occur during regime shifts imperative. Loch Leven, Scotland, UK, is a large shallow lake with a history of eutrophication, research and management and as such is an ideal study site to better understand resilience and regime shifts under a range of interacting stressors. The objectives of this research are to: (1) analyse long term data to identify the occurrence of common tipping points within the chemical (water column nutrient concentrations) and biological (macrophytes, phytoplankton, zooplankton) components of the loch, then test these tipping points using five statistical early warning indicators (EWIs) across multiple rolling window sizes; and (2) quantify the changes in lake ecology using a before/after analysis and testing for non-linearity, combined with modelling using the aquatic ecosystem process model PCLake to determine the level of resilience following a regime shift during recovery from eutrophication; (3) using PCLake, examine the sensitivity of Loch Leven to regime shifts in the face of predicted environmental change (e.g. climate change, nutrient pollution). Statistical analysis identified tipping points across all trophic levels included, from physical and chemical variables through to apex predators. The success of EWIs in predicting the tipping points was highly dependent on the number of EWIs used, with window size having a smaller impact. The 45% window size had the highest overall accuracy across all EWIs but only detected 16.5% more tipping points than the window size with the lowest overall accuracy. Differences between individual EWI performance and usage of them as a group was substantial with a 29.7% increase between the two. In both individual and group use of EWIs, false positives (early warning without a tipping point) were more common than true positives (tipping point preceded by EWI), creating significant doubts about their reliability as management tools. Significant change was seen across multiple variables and trophic levels in the before/after analysis following sudden recovery from eutrophication, with most variables also showing evidence of non-linear change. Modelling of responses to nutrient loading for chlorophyll, zooplankton and macrophytes, under states from before and after the shift, indicate hysteresis and thus the presence of feedback mechanisms. The modelling of responses to nutrient loading and predicted climate change in temperature and precipitation demonstrated that increases in temperature and decreases in summer precipitation individually had large impacts on chlorophyll and zooplankton at medium to high phosphorus (P) loads. However, modelling of the combined effects of these changes resulted in the highest lake chlorophyll concentrations of all tested scenarios. At low P loads higher temperatures and increased winter precipitation had the greatest impact on system resilience with a lower Critical Nutrient Load (CNL). The difference between chlorophyll and zooplankton as opposed to macrophytes was in the presence of a lower CNL for the increased winter precipitation-only scenarios which was not seen in the macrophytes. This highlights the potential role of high winter inputs potentially loaded with particulate matter in reducing resilience at lower P loads. This research has highlighted the vulnerability and low resilience of Loch Leven to environmental change. The presence of multiple tipping points and high levels of EWI activity show a high level of flexibility in the system. Coupled with the occurrence of widespread trophic change during a sudden recovery and a small level of hysteresis and high levels of sensitivity to climate change, the low levels of resilience become clear. The impact of lake-specific characteristics such as moderate depth, large fetch and a heterogeneous bed morphology is particularly evident in the limitations on macrophyte cover and the reliance on zooplankton to determine the hysteresis offset (amount of phosphorus (P) loading between the two CNL). The presence of these characteristics can be used to identify other lakes vulnerable to change. Improving the predictive capabilities of resilience indicators such as EWIs, and better understanding of the ecological changes that occur during non-linear change in response to recovery and climate change, can help target relevant ecosystem components for preventative management. These actions may become necessary under even the most conservative estimates of environmental change. 2019-11-26T00:00:00Z Big cat biogeography, morphology and climate change Cooper, David Matthew 2019-09-19T10:00:37Z 2019-11-26T00:00:00Z Big cat biogeography, morphology and climate change Cooper, David Matthew This thesis investigates the long term biogeographic factors which may have contributed towards subspecific variation within modern populations of the lion (Panthera leo) and tiger (Panthera tigris). Present day morphological variation in both species is examined in light of this biogeographical modelling, existing genetic research, and further analysis of the effects of phenotypic plasticity. This research provides a spatially explicit view of the recent evolutionary history of the lion and tiger, and examines their morphology using an unprecedentedly large sample of specimens, from both captive and wild populations, and with known geographic origins. These strands of investigation are important, as it can corroborate, or challenge existing phylogenetic and morphological studies, thereby lending support to or challenging the validity of subpecific taxonomy, management units and clines. This thesis is organised around two main research themes: 1) Biogeographical range shifts in the tiger and lion: Species distribution modelling techniques, incorporating palaeoclimatic models form the basis of understanding the changing ranges of the lion and tiger through glacial/interglacial and pluvial/interpluvial cycles of the Late Pleistocene and Holocene. Models are augmented with palaeoenvironmental evidence including fossil records, rock art and historical records and evidence of large scale stochastic events. Standard species distribution models are built upon by creating Global Environmental Zones and Strata, which aids in their descriptive power. 2) Phenotypic variation within modern populations: Morphology is examined using an unprecedentedly large sample of linear craniometric measurements of lion and tiger specimens. The effect of phenotypic plasticity is examined by comparing the vastly different environmental conditions found between captive and wild lion and tigers. This preliminary analysis supports the interpretation of wild variation by separating the influences of life history, from evolutionary history. Geo-referenced specimens of wild lions and tigers are used to examine geographical patterns and potential environmental influences on modern day variation, in light of the biogeographical modelling and phenotypic analysis. Core Findings: Whilst interglacial conditions have largely benefitted the population extent of the tiger, allowing northward and westward dispersal, the opposite is true in the lion where combined interglacial and interpluvial conditions have decreased their potential range. The biogeographical modelling broadly supports contiguous potential populations of both species through changing climatic cycles, except where sea level change has isolated the Sunda populations of the tiger. Such broad scale analysis may not fully account for narrow, yet enduring barriers and conduits to dispersal such as rivers. Significant phenotypic plasticity is found within lion and tiger skull morphology, which is likely related to differences in the mechanical properties of diet. Morphological variation between wild populations is largely determined by clinal size differences in the tiger, although the similarly sized Amur and Indian populations are separable on multiple skull parameters which likely relate to phenotypic plasticity. Clinal size variation occurs less strongly in the lion with no step change in size or shape between the northern and southern subspecies. The Asian lion population shows shape differences from populations in Africa, likely due to a combination of environmental effects on skull plasticity, and the recent population bottleneck of the Asian population. 2019-11-26T00:00:00Z Frequency-dependent anisotropy in partially saturated rocks Jin, Zhaoyu 2019-09-17T09:23:41Z 2019-11-26T00:00:00Z Frequency-dependent anisotropy in partially saturated rocks Jin, Zhaoyu Seismic wave propagation through fractured rocks is greatly influenced by the fracture system and fluid content. This has become increasingly apparent from recent laboratory measurements and field observations which have shown evidence that seismic anisotropy varies systematically with frequency and fluid saturation. Exploiting this behaviour for reservoir characterization relies on the availability of adequate theoretical rock physics models. Many previous theories on frequency-dependent anisotropy have been limited to the single fluid assumption, despite the fact that almost all reservoirs are partially saturated. This thesis focuses on improving the understanding of multiphase saturation effects on anisotropic seismic wave response in reservoirs containing aligned fractures. I do this by theoretically deriving expressions for the anisotropic frequency-dependent elastic constants. These depend on the relative mobilities of the saturating fluids and the coupled impact of ‘squirt flow’ and ‘patch saturation’ effects, which have previously been considered independently, on anisotropic seismic wave propagation. The effect of relative permeability is pronounced; fluid mobility can be lower in partially saturated rocks compared to the fully saturated case, and this can lead to a stiffening which dominates compressibility effects. This can result in unexpected non-monotonic relationships between moduli and water saturation, complicating attempts to invert saturation from seismic data. I test the theory against laboratory measurements carried out at the National Oceanography Centre, Southampton (NOCS) of S-wave splitting and P-wave anisotropy in synthetic fractured sandstones saturated by mixtures of both water/air and brine/CO2. I demonstrate that both squirt and patch mechanisms are significant for the interpretation of saturation effects on frequency-dependent anisotropy. I use the theory to predict the impact of anisotropic dispersion on azimuthal P-wave reflections based on an sand model with transverse isotropy with horizontal axis of symmetry (HTI) overlaid by shale with transverse isotropy with vertical axis of symmetry (VTI). Although VTI in the overburden does not lead to azimuthal anisotropy, its effect on angle dependence could significantly affect the azimuthal Amplitude-Versus-Offset (AVO) responses at far offsets. I show a modest effect on the amplitude and large effect on the phase, the latter of which could even be mistaken for azimuthal velocity variations. I present a Bayesian inversion based on a forward modelling technique aimed at recovering water saturation, fracture density and fracture length of an HTI sand. The results show potential of using frequency-dependent azimuthal AVO for the discrimination of large-scale fractures from micro-scale cracks. 2019-11-26T00:00:00Z