K&uuml / &ccedil / &uuml / k Menderes River Basin located in western Turkey has been facing continuous groundwater level decreases for the past 30 years. In dry periods, irrigation demand is completely met by pumping from groundwater system, which reduces water levels significantly. This provides enough storage to be recharged in wet seasons when streams are running. However, increased runoff in wet season are not utilized neither for irrigation nor for recharge and lost to the Aegean Sea without being infiltrated. Hence, surface artificial recharge methods can be useful to collect excess water in recharge basins, thus allowing infiltration to increase groundwater storage in wet seasons to be later utilized in dry seasons. A 2-D groundwater model is set up by using SEEP/W software. The material functions and parameters used in the model for saturated/ unsaturated conditions are taken from previous studies. Calibration was done to check the accuracy of input data and to control the validity of model. The amount of excess water that will be collected in recharge basins was estimated from flood frequency analysis. Concerning different probabilities, different scenarios were simulated to observe the increase in groundwater levels. Simulation results suggest that significant increase in groundwater storage could be achieved by applying artificial recharge methods. In addition to recharge basins, to reinforce the effect of artificial recharge, simulations were repeated with the addition of an underground dam at downstream side of the basin. Simulation results indicate that the increase in groundwater storage is not sufficient to warrant construction of the underground dam.

Temporal variations in gas hydrate related geochemical signatures under different deposition conditions are the primary purposes of this study. Accreted wedge located offshore Southwestern Taiwan receives high terrigenous river materials, 100 MT/yr, at present time. It is not clear how seep environment varied during the past glacial. A 25 meters long piston core was taken offshore Southwestern Taiwan on r/v Marion DuFresne. Short piston cores and box cores were also taken on r/v OR-1. Samples were analyzed for pore water dissolved sulfide, sulfate, methane, chloride, del O18, calcium, magnesium, alkalinity, pH, and sediment AVS, pyrite, inorganic carbon, del O- 18, C13. Changes in deposition environment play a major role in the study area. Three stages of geochemical processes are identified in the 25 meters long core, interchange between reduce and oxic depositional environments, with reducing condition in the top 10 m, oxic in between 10-20 meter and reducing below the 20 meter. High concentrations of dissolved sulfide, rapid sulfate depletion, increase of methane, decrease of calcium were found in pore water in the top 10 m of sediments together with high concentrations of pyrite, relatively higher proportion of coarsegrained sediment. Concentrations of pyrite were very low in sediments between 15 to 20 meters but increased rapidly from 20 to 25 meters with a maximum concentration at 400 umol/g. Chloride concentrations also increased to a maximum concentration of 630 mM at 20 m. The rapid increase of chloride indicated gas hydrate formation at this depth. Authigenic carbonate nodules were found in sediments below 20 m. The carbonate content also increased rapidly beneath this depth. Stable isotopic carbon composition of the carbonate varied rapidly beneath 20 m with a low at -28 per mil. The existence of oxic/reducing alterations indicates that methane seep may vary in the past in the study area.

Ce travail de thèse traite de l’optimisation des analyses et des prétraitements des images radar pour la détection des nappes d'huile en domaine océanique (communément appelés "oil slicks" en anglais) ainsi que la localisation des sources de suintements d’huiles d'origine naturelle ("oil seeps") sur le plancher océanique. Moyens, méthodes et difficultés des divers traitements y sont exposés. Il se compose des trois axes de recherche distincts expliqués et détaillés ci-dessous :1- Une approche stochastique pour le prétraitement et l'amélioration des images radar en bande C afin de détecter automatiquement les nappes d'huile.2- Une approche stochastique utilisant une grande quantité d'images radar pour évaluer l'influence de la vitesse du vent et les différents modes de l'instrument (SAR) pour l'optimisation de la détection des nappes d'hydrocarbures.3- La localisation précise de la source des émissions d'hydrocarbures marins à l'aide d'un nouveau modèle de dérive verticale, appliqué au Golfe du Mexique (USA).En premier, nous nous sommes intéressés à l'optimisation des prétraitements et l'amélioration des images radar en bande C par des méthodes stochastiques. La méthodologie proposée comprend trois niveaux de traitement: prétraitement, seuillage et nettoyage binaire. Le premier niveau s’attèle à corriger l'hétérogénéité de la luminosité dans les images radar due à la réflexion non lambertienne du signal radar sur la surface de la mer. Le deuxième niveau consiste en une étape de seuillage qui vise à produire des objets noirs aussi proches que possible de l'ensemble de données d'apprentissage manuellement élaborées. Le troisième niveau, quant à lui, vise à nettoyer les images binaires de sortie des résidus de bruit. Plusieurs méthodes de prétraitement et de nettoyage ont été testées et évaluées par un moteur de qualification qui compare les objets détectés automatiquement avec les zones des objets noirs détectées manuellement. Par la suite, nous nous sommes penchés sur l'évaluation de l'influence de la vitesse du vent et des modes de l'instrument sur la détection des nappes d'hydrocarbures sur les images radar en utilisant une approche stochastique. Cette étude a été dictée par le besoin de définir les conditions météorologiques à même de permettre une détection optimale des nappes d’huiles, à partir des images radar. L’objectif a été de déterminer l’intervalle de vitesse du vent qui optimise la détection des nappes d'huiles dans toutes les images radar utilisant du BigData et une approche stochastique. Ce travail a également été une occasion de nous intéresser aux propriétés des modes d'acquisition radar employés dans la détection des nappes d'huile. Ainsi, un ordre de performance de 5 modes est établi (IW, APP, PRI, IMP et WSM) et montre que le mode IW (Sentinel-1), avec la meilleure résolution spatiale (supérieure à 5x20m), est la plus approprié pour détecter une nappe d'huile à forte vitesse du vent. Enfin, nous nous sommes focalisés sur l'estimation de la localisation des sources de pétrole naturel marin à l'aide d'un nouveau modèle de dérive verticale. Les manifestations de suintements d'hydrocarbures sur la surface de la mer sont généralement décalées de leur source sur les fonds marins de plusieurs centaines de mètres ou même de kilomètres. Ce décalage est fonction de la vitesse ascensionnelle et des courants marins le long de la colonne d'eau. Dans cette étude, le diamètre des gouttelettes ne nous est pas connu à priori. Pour combler ce manque d’information, on a appliqué une nouvelle méthode appelée «le chemin des sources». Si ces trois études peuvent être prises chacune indépendamment des autres, elles sont solidement interconnectées et complémentaires. Elles forment une sorte de processus allant de l'optimisation de la détection d’une nappe (les moyens et les outils les plus adéquats pour une meilleure détection) jusqu’à la localisation de sa source sur le plancher océanique / This thesis deals with the preprocessing of radar images and their optimization for the analyzes in order to detect natural marine oil slicks (Sea surface Outbreak/SSO) as well as better determine their source location at the Sea Floor Source (SFS). We explained herein means, methods and difficulties encountered. This thesis consists of the following three distinct research axes represented by three submitted papers :1- A stochastic approach for pre-processing and improvement of C-band radar images to automatically detect oil slicks;2- A stochastic approach using a large quantity of radar images to evaluate the influence of wind speed and the different modes of the instrument (SAR) on the delectability of marine oil slicks ;3- Accurate location of the Sea Floor Source of marine hydrocarbon emissions using a new vertical drift model within the water column, applied to the northern Gulf of Mexico (southern USA).So first, we focused on the optimization of pre-processing and the improvement of C-band radar images by stochastic methods to automatically detect oil slicks. The proposed methodology includes three processing levels : preprocessing, thresholding, and binary cleaning. The first level consists of correcting the heterogeneity of the luminosity in the radar images resulting from the non-Lambertian reflection of the radar signal on the sea surface. The second level consists of a thresholding step which aims to produce dark objects as close as possible to the manually developed training data set. The third level consists of cleaning the output binary images from the noise residuals. Several preprocessing and cleaning methods have been tested and evaluated by a qualification engine that compares the objects automatically detected with the manual detection. Then, we focus in a second chapter in the evaluation of the influence of wind speed and instrument modes on the detection of oil slicks from radar images by using a stochastic approach. This study was dictated by the need to define the meteorological conditions capable for an optimal detection of oil slicks, from the radar images. The objective was to determine the wind speed range which optimizes the detection of oil slicks in all radar images using BigData and a stochastic approach. This work was also an opportunity to investigate the properties of the radar acquisition modes used in the detection of oil slicks. Thus, a 5-mode performance order is established (IW, APP, PRI, IMP and WSM) and shows that the IW (Sentinel-1) mode, with the best spatial resolution (greater than 5x20m) detects oil slicks at high wind speed. Finally, we focused on estimating the location of marine natural oil seeps sources using a new vertical drift model, applied in the Gulf of Mexico. Thus, we have developed a new method for detecting the source of oil seeps from natural sources on the seafloor according to the vertical drift model. Occurrences of oil seeps on the sea surface are generally offset from their sources on the seabed by several hundred meters or even kilometers. This deflection is dependent on the upward velocity of the oil and marine currents along the water column. In this study, the diameter of the droplets is not known to us a priori. To fill this gap, a new method called "the sources path" was applied herein that propose the Sea Floor Source taking into account the droplet size and the vertical drift within the water column before their Sea surface Outbreak (SSO).If these three studies can be taken independently of each other, they are firmly interconnected and complementary. They form a sort of process ranging from the optimization of the detection of an oil slick (the most appropriate means and tools for better detection) to the location of its source on the seafloor

Introdução: A Apneia Obstrutiva do Sono (AOS) é uma das entidades clínicas mais encontradas na população, sendo caracterizada por obstruções parcial ou total recorrente da via aérea durante o sono, levando a hipóxia intermitente e fragmentação do sono. Suas consequências envolvem sonolência excessiva, risco de acidentes de trabalho e de trânsito, déficits cognitivos e doenças cardiovasculares. Os potenciais evocados auditivos avaliam a atividade neuroelétrica na via auditiva, fornecendo uma medida objetiva do funcionamento do sistema auditivo. Visto que as alterações cognitivas na AOS podem ser múltiplas, espera-se que os potenciais evocados auditivos de longa latência (PEALL) possam ser utilizados como uma ferramenta de avaliação nos pacientes com AOS, mostrando uma melhor compreensão dos prejuízos da AOS e auxiliando no tratamento. Objetivo: Caracterizar os potenciais evocados auditivos de longa latência em indivíduos portadores de AOS de grau leve, moderado e grave, correlacionando os achados eletrofisiológicos com a severidade do distúrbio do sono, além de comparar com os resultados obtidos em indivíduos sem AOS. Método: Participaram deste estudo 59 indivíduos, na faixa etária de 22 a 54 anos (média de 34,87 anos) e de ambos os gêneros (52 homens e 07 mulheres). Os voluntários foram divididos em quatro grupos: sem AOS, com AOS leve, com AOS moderada e com AOS grave. Todos os participantes realizaram a polissonografia (PSG) para fornecer o grau da severidade da AOS, mesmo os voluntários sem queixa para AOS, para se descartar a possibilidade de apresentar a doença. Os indivíduos também foram submetidos à avaliação audiológica convencional. Para a obtenção dos componentes P1, N1, P2, N2 e P3 do PEALL, foram utilizados dois estímulos acústicos: fala (sílabas /ba/ e /da/), e tone-burst (frequências de 1000 Hz e 2000Hz), ambos na intensidade de 75dBnNA. Foram analisadas as latências (em ms) dos componentes P1, N1, P2, N2 e P3 e as amplitudes (em uv) P1- N1, P2-N2 e P3. Resultados: No PEALL com estímulo fala, o grupo grave apresentou a menor latência no N2 (p-valor < 0,001) e para o P3, todos os grupos apresentaram maior latência na orelha direita (p-valor=0,016). Houve diferença estatisticamente significante para amplitude do P3 (p-valor=0,016), com maior amplitude para orelha esquerda em todos os grupos. No PEALL com estímulo tone burst, apenas as latências dos componentes P2 e N2 apresentaram diferença estatisticamente significante (p-valor=0,010 e p-valor= 0,010, respectivamente), com menor latência para o grupo com AOS grave. Nestes mesmos componentes, houve diferença entre as orelhas. Foi encontrada diferença na amplitude P2-N2, com menor amplitude para o grupo com AOS grave (p-valor < 0,001). Conclusão: Indivíduos com AOS apresentam um aumento progressivo da latência do componente P3 dos PEALL conforme aumento da severidade da doença para o estímulo fala, bem como uma diminuição da amplitude P2-N2 para estímulo tone-burst no grupo com AOS grave quando comparada com os demais grupos, sugerindo interferência do grau de severidade da doença nos PEALL. De uma maneira geral, indivíduos com AOS grave apresentaram latências prolongadas e amplitudes reduzidas no PEALL quando comparados com indivíduos sem AOS / Introduction: The Obstructive Sleep Apnea (OSA) is one of the most common disease found in the population, characterized by partial or full airways obstruction during the sleep, leading to hypoxia and frequent awakenings. Its consequences are daytime sleepless, risk of work and car accidents, cognitive deficit and cardiovascular disease. The auditory evoked potential evaluates the neuroelectric activity in the auditory pathway, giving an objective measure about the auditory system function. Since the OSA cognitive dysfunctions can be multiples, it is expected that the Long Latency Auditory Evoked Potential (LLAEP) could be used as an evaluation tool in the patients with OSA, showing a better comprehension of its injuries and helping on the treatment. Objective: To characterize the long latency auditory evoked potentials in individuals with mild, moderate and severe OSA, comparing the results with the severity of the disease, and also comparing the results with individuals without OSA. Method: 59 individuals participated of this research, with ages bettwen 22 and 54 years old (average 34.87) and from both genders (52 men and 7 women). The volunteers were divided in four groups: without OSA, with mild OSA, moderate OSA and with severe OSA. All participants performed the polysomnography (PSG) for classifying the OSA severity, even the volunteers without OSA, to guarantee the inexistence of the disease. They were also submitted to a conventional audiology evaluation. To obtain the components P1, N1, P2, N2 and P3 in LLEAP, it was used two acoustic stimulus: speech (syllables / ba / e / da /), and tone-burst (frequencies of 1000 Hz and 2000 Hz), both with 75 dBnNA intensity. It were analyzed the latencies (in ms) of the components P1, N1, P2, N2 and P3 and the amplitudes (in uv) P1-N1, P2-N2 and P3 .Results: In the LLEAP with speech stimulus, the severe group presented the lowest latency in N2 (p-value < 0.001) and for P3, all groups had delay of latency in the right ear (p-value = 0.016). There was a significantly difference for P3 amplitude (p-value = 0.016), with higher amplitude for the left ear in all groups. In the LLAEP with tone burst stimulus, only the P2 and N2 latencies presented a significantly difference (P-value = 0.010 and P-value = 0.010, respectively), with lower latency for the group with severe OSA. In these same components, there was difference between the ears. A difference in the P2-N2 amplitude was found, with a lower amplitude for the group with severe OSA (p-value < 0.001). Conclusion: Individuals with OSA showed a progressive increase in the latency according to the severity of the disease in the component P3 in LLAEP for the speech stimulus, as well as lower amplitude in P2-N2 with tone burst stimulus for the severe OSA group, suggesting interference of the disease severity degree in the LLAEP. In general, individuals with severe OSA presented increased latencies and lower amplitudes in the LLAEP when compared to individuals without OSA

Using the NEPTUNE-Canada cable-linked ocean-floor observatory we plan continuous, real-time monitoring of the gas hydrate-associated, “Bullseye” cold vent offshore Vancouver Island. Our group inferred the presence of a massive gas hydrate deposit there, based on the significant resistivity anomaly in our controlled-source electromagnetic (CSEM) dataset, as well as anomalously heightened shear moduli, from seafloor compliance data. This interpretation was confirmed by drilling by IODP expedition 311 (site U1328), which indicated a 40 m thick gas hydrate layer near the surface. Sporadic venting and variations in blanking in yearly single-channel seismic surveys suggest the system is evolving in time. We are preparing two stationary semi-permanent imaging experiments: a CSEM and a seafloor compliance installation. These are designed not only to assess the extent of the gas hydrate deposit, but also for long-term monitoring of the gas hydrate/free gas system. The principle of the CSEM experiment is to input a particular electromagnetic signal at a transmitter (TX) dipole on the seafloor, and to record the phase and amplitude of the response at several seafloor receiver (RX) dipoles, at various TX-RX separations. The data are sensitive to the underlying resistivity, which is increased when conductive pore water is displaced by electrically-insulating gas hydrate. The experiment is controlled onshore, and can be expanded to include a downhole TX. Repeated soundings at this site, over several years, will allow measurement of minute changes in resistivity as a function of depth, and by inference, changes in gas hydrate or underlying free gas distribution. Similarly, the displacement of pore fluids by solid gas hydrate will affect elastic parameters. Thus, seafloor compliance data, the transfer function between pressure and seafloor displacement time series, most sensitive to shear modulus as a function of depth, will be gathered continuously to monitor the evolution of the gas hydrate distribution.

Introdução: A Apneia Obstrutiva do Sono (AOS) é uma das entidades clínicas mais encontradas na população, sendo caracterizada por obstruções parcial ou total recorrente da via aérea durante o sono, levando a hipóxia intermitente e fragmentação do sono. Suas consequências envolvem sonolência excessiva, risco de acidentes de trabalho e de trânsito, déficits cognitivos e doenças cardiovasculares. Os potenciais evocados auditivos avaliam a atividade neuroelétrica na via auditiva, fornecendo uma medida objetiva do funcionamento do sistema auditivo. Visto que as alterações cognitivas na AOS podem ser múltiplas, espera-se que os potenciais evocados auditivos de longa latência (PEALL) possam ser utilizados como uma ferramenta de avaliação nos pacientes com AOS, mostrando uma melhor compreensão dos prejuízos da AOS e auxiliando no tratamento. Objetivo: Caracterizar os potenciais evocados auditivos de longa latência em indivíduos portadores de AOS de grau leve, moderado e grave, correlacionando os achados eletrofisiológicos com a severidade do distúrbio do sono, além de comparar com os resultados obtidos em indivíduos sem AOS. Método: Participaram deste estudo 59 indivíduos, na faixa etária de 22 a 54 anos (média de 34,87 anos) e de ambos os gêneros (52 homens e 07 mulheres). Os voluntários foram divididos em quatro grupos: sem AOS, com AOS leve, com AOS moderada e com AOS grave. Todos os participantes realizaram a polissonografia (PSG) para fornecer o grau da severidade da AOS, mesmo os voluntários sem queixa para AOS, para se descartar a possibilidade de apresentar a doença. Os indivíduos também foram submetidos à avaliação audiológica convencional. Para a obtenção dos componentes P1, N1, P2, N2 e P3 do PEALL, foram utilizados dois estímulos acústicos: fala (sílabas /ba/ e /da/), e tone-burst (frequências de 1000 Hz e 2000Hz), ambos na intensidade de 75dBnNA. Foram analisadas as latências (em ms) dos componentes P1, N1, P2, N2 e P3 e as amplitudes (em uv) P1- N1, P2-N2 e P3. Resultados: No PEALL com estímulo fala, o grupo grave apresentou a menor latência no N2 (p-valor < 0,001) e para o P3, todos os grupos apresentaram maior latência na orelha direita (p-valor=0,016). Houve diferença estatisticamente significante para amplitude do P3 (p-valor=0,016), com maior amplitude para orelha esquerda em todos os grupos. No PEALL com estímulo tone burst, apenas as latências dos componentes P2 e N2 apresentaram diferença estatisticamente significante (p-valor=0,010 e p-valor= 0,010, respectivamente), com menor latência para o grupo com AOS grave. Nestes mesmos componentes, houve diferença entre as orelhas. Foi encontrada diferença na amplitude P2-N2, com menor amplitude para o grupo com AOS grave (p-valor < 0,001). Conclusão: Indivíduos com AOS apresentam um aumento progressivo da latência do componente P3 dos PEALL conforme aumento da severidade da doença para o estímulo fala, bem como uma diminuição da amplitude P2-N2 para estímulo tone-burst no grupo com AOS grave quando comparada com os demais grupos, sugerindo interferência do grau de severidade da doença nos PEALL. De uma maneira geral, indivíduos com AOS grave apresentaram latências prolongadas e amplitudes reduzidas no PEALL quando comparados com indivíduos sem AOS / Introduction: The Obstructive Sleep Apnea (OSA) is one of the most common disease found in the population, characterized by partial or full airways obstruction during the sleep, leading to hypoxia and frequent awakenings. Its consequences are daytime sleepless, risk of work and car accidents, cognitive deficit and cardiovascular disease. The auditory evoked potential evaluates the neuroelectric activity in the auditory pathway, giving an objective measure about the auditory system function. Since the OSA cognitive dysfunctions can be multiples, it is expected that the Long Latency Auditory Evoked Potential (LLAEP) could be used as an evaluation tool in the patients with OSA, showing a better comprehension of its injuries and helping on the treatment. Objective: To characterize the long latency auditory evoked potentials in individuals with mild, moderate and severe OSA, comparing the results with the severity of the disease, and also comparing the results with individuals without OSA. Method: 59 individuals participated of this research, with ages bettwen 22 and 54 years old (average 34.87) and from both genders (52 men and 7 women). The volunteers were divided in four groups: without OSA, with mild OSA, moderate OSA and with severe OSA. All participants performed the polysomnography (PSG) for classifying the OSA severity, even the volunteers without OSA, to guarantee the inexistence of the disease. They were also submitted to a conventional audiology evaluation. To obtain the components P1, N1, P2, N2 and P3 in LLEAP, it was used two acoustic stimulus: speech (syllables / ba / e / da /), and tone-burst (frequencies of 1000 Hz and 2000 Hz), both with 75 dBnNA intensity. It were analyzed the latencies (in ms) of the components P1, N1, P2, N2 and P3 and the amplitudes (in uv) P1-N1, P2-N2 and P3 .Results: In the LLEAP with speech stimulus, the severe group presented the lowest latency in N2 (p-value < 0.001) and for P3, all groups had delay of latency in the right ear (p-value = 0.016). There was a significantly difference for P3 amplitude (p-value = 0.016), with higher amplitude for the left ear in all groups. In the LLAEP with tone burst stimulus, only the P2 and N2 latencies presented a significantly difference (P-value = 0.010 and P-value = 0.010, respectively), with lower latency for the group with severe OSA. In these same components, there was difference between the ears. A difference in the P2-N2 amplitude was found, with a lower amplitude for the group with severe OSA (p-value < 0.001). Conclusion: Individuals with OSA showed a progressive increase in the latency according to the severity of the disease in the component P3 in LLAEP for the speech stimulus, as well as lower amplitude in P2-N2 with tone burst stimulus for the severe OSA group, suggesting interference of the disease severity degree in the LLAEP. In general, individuals with severe OSA presented increased latencies and lower amplitudes in the LLAEP when compared to individuals without OSA

The devastating levee failures associated with hurricanes Katrina and Rita, and the more recent Midwest flooding, placed a spotlight on the importance of levees and our dependence on them to protect life and property. In response to levee failures associated with the hurricanes, Congress passed the Water Resources Development Act of 2007 which established a National Committee on Levee Safety. The committee was charged with developing recommendations for a National Levee Safety Program. The Secretary of the Army was charged with the establishment and maintenance of a National Levee Database. The National Levee Database is a critical tool in assessing and improving the safety of the nation's levees. However, the NLD data model, established in 2007, lacked a structure to store seepage and slope stability analyses – vital information for assessing the safety of a levee. In response, the Levee Analyst was developed in 2008 by Dr. Norm Jones and Jeffrey Handy. The Levee Analysis Data Model was designed to provide a central location, compatible with the National Levee Database, for storing large amounts of levee seepage and slope stability analytical data. The original Levee Analyst geoprocessing tools were created to assist users in populating, managing, and analyzing Levee Analyst geodatabase data. In an effort to enhance the Levee Analyst and provide greater accessibility to levee data, this research expanded the Levee Analyst to include modifications to the data model and additional geoprocessing tools that archive GeoStudio SEEP/W and SLOPE/W simulations as well as export the entire Levee Analyst database to Google Earth. Case studies were performed to demonstrate the new geoprocessing tools' capabilities and the compatibility between the National Levee Database and the Levee Analyst database. A number of levee breaches were simulated to prototype the enhancement of the Levee Analyst to include additional feature classes, tables, and geoprocessing tools. This enhancement would allow Levee Analyst to manage, edit, and export two-dimensional levee breach scenarios.

Saline aquifers and depleted hydrocarbon fields situated beneath the North Sea are currently being proposed as storage repositories for anthropogenic CO2 captured from point source emitters in the UK and mainland Europe. Two experimental sites are already operating successfully offshore Norway: Sleipner since 1996 and Snøhvit since 2007, collectively storing several million tonnes of CO2/year in the sub-surface. Despite the apparent success of these current projects, one of the major public and scientific concerns is the ability of storage sites to retain CO2 on the millennial timescales required for CO2 plume stabilisation and dissolution. Some areas of the North Sea are also known to contain palaeo-gas seepage pathways within overburden sediments that overlie deeper hydrocarbon reservoirs (e.g. Witch Ground Graben). These areas either need to be avoided for CO2 storage or rigorously assessed in terms of leakage risk. Since the Sleipner storage site lies within such a province, this thesis delivers a detailed evaluation of the Nordland Group overburden and a critical assessment of its long-term sealing capability for CO2. From interpretation and detailed mapping of a baseline 3D seismic dataset (acquired before CO2 injection operations commenced in 1996), we have identified numerous palaeo-migration pathways and high-amplitude seismic anomalies within the Nordland Group overburden sediments deposited above the Sleipner CO2 storage site. We attributed these features to thermogenic or biogenic gas migration, accumulation and bio-degradation over geological time. We also mapped a complex network of sand-filled, glacial channels and tunnel valleys distributed within a few hundred metres below seabed and highlighted their significance as potential fluid migration networks and/or secondary storage containment for leaking CO2. Of further significance, we confirmed that these overburden features also create spatial density variations that impact on the accuracy of seismic time-depth conversions, resulting in the probability of topographic distortions being propagated into seismic interpretations and models. To the best of our knowledge no such detailed mapping of the Nordland Group overburden at Sleipner has been undertaken previously. To determine whether the top layer of the CO2 plume at Sleipner might encounter these relict pathways as it ascends and migrates laterally beneath the caprock, we evaluated the critical column heights required for a CO2 accumulation to enter such a pathway under a range of storage conditions for a CH4/CO2/brine system; assuming that these pathways currently contain methane gas. Risking scenarios were based on a range of phase saturation, pressure, temperature, density, viscosity, interfacial tension and wettability conditions likely to be encountered at depths commensurate with the caprock at Sleipner. We concluded that given certain conditions at the caprock, CO2 could leak more easily into palaeo-migration pathways than CH4 (i.e. at lower entry pressures and therefore smaller column heights), assuming that brine densities and, most importantly, pore radii have not changed significantly over geological time (i.e. no cementation or dissolution has taken place). To further understand the dynamic significance of these palaeo-migration pathways, channels and tunnel valleys, including their ability to form inter-connected leakage/migration networks, we constructed a series of high-resolution 3D models of the Sleipner storage site and overburden, then used stochastic basin modelling and simulation techniques to investigate the processes involved during the introduction of CO2 into the storage site over a prolonged time period. Models were populated with geological, stratigraphic and structural information derived from our seismic interpretation. Flow simulations were calibrated to published data and matched to the present-day plume distribution. The absence of observational reservoir pressure and temperature data from Sleipner introduces significant uncertainty to model outcomes with respect to CO2 density and column height estimates and to surmount this difficulty we constrained the caprock temperature to CO2 density estimates obtained from the most recent gravity data observations at Sleipner. We concluded that the overburden heterogeneity is significant and palaeo-migration pathways, highpermeability channels and tunnel valleys at Sleipner may become potential migration pathways for CO2 as the plume continues to spread laterally over the coming decade, but the possible storage response is difficult to quantify given the absence of sufficient overburden rock property information and accurate pressure and temperature data for the storage site. The overall conclusion from this work is that insufficient information was collected within the Sleipner area prior to storage site development and too many significant studies which should have been performed as a pre-requisite (e.g. obtaining a caprock sample for laboratory testing of potential seal capacity), were actually performed some years after CO2 injection operations had already commenced. The pressure and temperature conditions at the caprock depth for the Sleipner storage site are also marginal in terms of maintaining CO2 above critical point conditions in dense phase and thus maximising storage efficiency. Most significantly, no rigorous overburden mapping and risking was performed for Sleipner (such as the work described in this thesis), thus the fact that no leakage has been detected at Sleipner is more due to good fortune than following best practices. Hopefully, our work has highlighted these key deficiencies so that future CO2 storage site feasibility and development studies will be performed more diligently.