This thesis examines the sensitivity of the marine controlled source electromagnetic (CSEM) method to sub-seafloor resistivity structure, with a focus on gas hydrate and free gas occurrences. Different analysis techniques are applied with progressive sophistication to a series of studies based on simulated and measured data sets. CSEM data are modelled in time domain for one-dimensional models with gas hydrate, free gas and/or permafrost occurrences. Linearized and non-linear inversion methods are considered to infer subsurface models from CSEM data. One study applies forward modelling and singular value decomposition to estimate uncertainties for permafrost models of the Beaufort Sea. This simulation study analyzes the resolution of the CSEM data for shallow water depth which is a challenging case because the electromagnetic signature of the air-water boundary may mask the sub-seafloor response. The results reveal a blind window as a function of water depth in which the CSEM data are insensitive to the sub-seafloor structure. However, the CSEM data are sensitive to the top and the bottom of the permafrost with increasing uncertainties with depth. The next study applies non-linear Bayesian inversion to CSEM data acquired in 2005/2006 on the Northern Cascadia margin to investigate sub-seafloor resistivity structure related to gas hydrate deposits and cold vents. Bayesian inversion provides a rigorous approach to estimate model parameters and uncertainties by probabilistically sampling of the parameter space. The resulting probability density function is interpreted here in terms of posterior median models, marginal and joint marginal probability densities for model parameters and credibility intervals. The Bayesian information criterion is applied to determine the amount of structure (number of layers) that can be resolved by the data. The parameter space is sampled with the Metropolis-Hastings algorithm in principal-component space. Non-linear, probabilistic inversion allows the analysis of unknown acquisition parameters such as time delays between receiver and transmitter clocks or unknown source amplitude. The estimated posterior median models and credibility intervals from Bayesian CSEM inversion are compared to reflection seismic data to provide a more complete geological interpretation. The CSEM data on the Northern Cascadia margin generally reveal a 1 to 3 layer sediment structure. Inversion results at the landward edge of the gas hydrate stability zone indicate a sediment unconformity as well as several potential cold vents which were previously unknown. The resistivities generally increase upslope due to sediment erosion along the slope. Inversion results on the middle slope infer several vent systems close to well-known Bullseye vent in agreement with ongoing interdisciplinary observations. Finally, a trans-dimensional (trans-D) Bayesian inversion is applied to CSEM data acquired in 2012 in the German North Sea to investigate possible free gas occurrences. Trans-D inversion treats the number of layers as an additional unknown sampled probabilistically in the inversion. %over the parameter space by evaluating probabilistically the transition to a higher or lower number of interfaces. Parallel tempering is applied to increase sampling efficiency and completeness. Inversion results for the German North Sea yield resistivities at the seafloor which are typical for marine deposits, while resistivities at greater depth increase slightly and can be correlated with a transition from fine-grained marine deposits (Holocene age) to coarse-grained, glacial sediments (Pleistocene age), which is observed in a sediment core. The depths of layer interfaces estimated from CSEM inversion match the seismic reflector related to the contrast between the two depositional environments. The CSEM survey targeted a strong, phase-reversed, inclined seismic reflector within the glacial sediments, potentially indicating free gas. While interface-depth estimates from CSEM inversion do not correlate closely with this reflector, resistivities are generally elevated above the strong seismic amplitudes and the thickness of the resistive layer follows the trend of the inclined reflector. However, the uncertainties of deeper interface depth estimates increase significantly and overlap with the targeted reflector at some of the measurement sites. Relatively low resistivities of a third layer correlate with sediments of late-Miocene origin with a high gamma-ray count indicating an increased amount of fine-grained sediments with organic material. The interface at the bottom of the third layer has wide uncertainties which relates to the penetration limit of the CSEM array. / Graduate

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No description available.

Arc magmatism is sustained by the complex interactions between the subducting slab, the overriding plate, and the mantle wedge. Partial melting of mantle peridotite is achieved by fluid-induced flux melting and decompression melting due to upward flow. The distribution of melting is sensitive to temperature, the pattern of flow, and the pressure in the mantle wedge. The arc front is the surface manifestation of partial melting in the mantle wedge and is characterized by a narrow chain of active volcanoes that migrate in time. The conventional interpretation is that changes in slab dip angle lead to changes in the arc front position relative to the trench. We explore an alternative hypothesis: evolution of the overlying plate, specifically thickening of the arc root, causes arc front migration. We investigate the effects of varying crustal morphology and viscous decoupling of the shallow slab-mantle interface on melt production using 2D numerical models involving a stationary overriding plate, a subducting plate with prescribed motion, and a dynamic mantle wedge. Melt production is quantified using a hydrous melting parameterization. We conclude: 1) Localized lithospheric thickening shifts the locus of melt production trenchward while thinning shifts melting landward. 2) Inclined LAB topography modulates the asthenospheric flow field, producing a narrow, well-defined arc front. 3) Thickening of the overriding plate exerts increased torque on the slab, favoring shallowing of the dip angle. 4) Viscous decoupling produces a cold, stagnant forearc mantle but promotes arc front melting due to reduction in the radius of corner flow, leading to higher temperatures at the coupling/decoupling transition.

Geochemical processes associated with gas hydrate formation lead to the growth of iron sulphides which have a geophysically-measurable magnetic signature. Detailed magnetic investigation, complemented by petrological observations, were undertaken on cores from a permafrost setting, the Mackenzie Delta (Canadian Northwest Territories) Mallik region, and two marine settings, IODP Expedition 311 cores from the Cascadia margin off Vancouver Island and the Indian National Gas Hydrate Program Expedition 1 from the Bengal Fan. Stratigraphic profiles of the fine scale variations in bulk magnetic measurements correspond to changes in lithology, grain size and pore fluid geochemistry which can be correlated on local to regional scales. The lowest values of magnetic susceptibility are observed where iron has been reduced to paramagnetic pyrite, formed in settings with high methane and sulphate or sulphide flux, such as at methane vents. High magnetic susceptibility values are observed in sediments which contain detrital magnetite, for example from glacial deposits, which has survived diagenesis. Other high magnetic susceptibility values are observed in sediments in which the ferrimagnetic iron-sulphide minerals greigite or smythite have been diagenetically introduced. These minerals are mostly found outside the sediments which host gas hydrate. The mineral textures and compositions indicate rapid disequilibrium crystallization. The unique physical and geochemical properties of the environments where gas hydrates form, including the availability of methane to fuel microbiological activity and the concentration of pore water solutes during gas hydrate formation, lead to iron sulphide precipitation from solute-rich brines. Magnetic surveying techniques help delineate anomalies related to gas hydrate deposits and the diagenesis of magnetic iron minerals related to their formation. Detailed core logging measurements and laboratory analyses of magnetic properties provide direct ties to original lithology, petrophysical properties and diagenesis caused by gas hydrate formation.

Among the three common gas hydrate structures, structure-H (sH) hydrate has been regarded as forming only in the laboratory since it was first reported in 1987. However, natural gas hydrate samples obtained from the Cascadia margin showed that sH hydrate can form naturally. Not only was the sH hydrate found in natural samples, but it was also discovered that n-alkanes such as n-pentane and n-hexane, considered to have too large molecular size to be sH hydrate formers, can act as co-guests of sH hydrates in mixtures with other sH hydrate formers. In this study, thermodynamic measurements and spectroscopic analysis of powder X-ray diffraction and 13C solid-state NMR methods, were performed for synthetic hydrate samples in order to identify the accommodation of n-alkanes with five or more carbon atoms. In addition, some new hydrate guests were found to form sH hydrates. From the present results, it is clear that, so far, our understanding of gas hydrates and guest enclathration needs to be revised and expanded in order to explain new findings.

Using cryogenic SEM, we investigated the physical states of gas-hydrate-bearing samples recovered by drill core from several localities including the SE India margin (NGHP Expedition 01), Cascadia margin (IODP Leg 311), Gulf of Mexico (RV Marion Dufresne 2002), and Mackenzie River Delta (Mallik site, well 5L-38). Core material with a significant fraction of preserved hydrate has only been obtained for cryogenic SEM investigation from relatively few sites worldwide to date, yet certain consistent textural characteristics, as well as some clear differences between sites have been observed. Gas hydrate in cores recovered from Cascadia, Gulf of Mexico, and Mallik often occurs as a dense substrate with typical grain size of 30 to as large as 200 μm. The hydrate often contains a significant fraction of isolated macropores that are typically 5–100 μm in diameter and occupy 10-30 vol. % of the domain. In fine-grained sediment sections of marine samples, gas hydrate commonly forms small pods or lenses with clay platelets oriented sub-parallel around them, or as thin veins 50 to several hundred microns in thickness. In some sections, hydrate grains are delineated by a NaCl-bearing selvage that forms thin rinds along hydrate grain exteriors, presumably produced by salt exclusion during original hydrate formation. Preliminary assessment of India NGHP-01 samples shows some regions consistent with the observations described above, as well as other regions dominated by highly faceted crystals that line the walls or interior of cavities where the hydrate grows unimpeded. Here, we focus on gas hydrate grain morphology and microstructures, pore characteristics and distribution, and the nature of the hydrate/sediment grain contacts of the recovered samples, comparing them to each other and to laboratory-produced gas hydrates grown under known conditions.

In North America, deepening economic integration under free trade has led to the formation of several cross-border regions between Canada and the United States and such regions have become a significant focus for public policy research in Canada. A key question is whether, as a result of increased economic integration, there are tendencies towards policy and institutional convergence within cross-border regions; especially in areas viewed as critical in determining competitive economic advantage. One such area is employment relations. However, relatively little research has focused on how, or even if, employment relations are changing within cross-border regions. Previous studies comparing differences and similarities in employment relations between Canada and the United States have tended to focus on one of three scales: the nation, the firm, or the individual workplace. Here, the focus is on employment relations within a cross-border region. Such regions often share similar economic and social characteristics. Thus, we might expect that if cross-national employment relations are becoming more similar due to deepening economic integration this would manifest most clearly at this scale. The empirical focus is the forest products industry in the cross-border region of Cascadia, comprised of British Columbia, Washington state, and Oregon. Employment relations are compared across three components of the forest products industry: pulp and paper, solid wood processing, and logging. Data are organized around case studies of each component and focus on employment, wages, and productivity; the restructuring of firms and ownership; the labour movement; work practices, training, and the reproduction of the labour force. The dissertation concludes that employment relations in the pulp and paper and logging industries in Cascadia are becoming more similar cross-nationally, while those in solid wood processing are increasingly differentiated cross-nationally. Moreover, it concludes that employment relations in British Columbia’s solid wood processing and pulp and paper industry are becoming more similar, while employment relations in the PNW solid wood processing and pulp and paper industries are increasingly differentiated. The dissertation contributes to broader debates in economic geography by examining the tensions between national and sub-national political economic actors contribute to the production of scale and territory. / Thesis (Ph.D, Geography) -- Queen's University, 2010-05-28 11:48:30.745

New relative sea-level (RSL) observations dating from the late Pleistocene and early Holocene, during and after the collapse of the Cordilleran ice-sheet (CIS), are provided for two regions in southern coastal British Columbia. They record the glacial isostatic adjustment (GIA) response of the Earth to the changing surface load of the waning CIS. The data provide a new RSL curve for Sechelt, on the mainland coast north of Vancouver, and extend and revise a previously constructed curve for Barkley Sound on the west coast of Vancouver Island. The observations create a new profile of RSL curves oriented southwest-northeast across Vancouver Island and the Strait of Georgia. A previously-defined profile of RSL curves is oriented northwest-southeast profile along the east coast of Vancouver Island. The two profiles intersect in the central Strait of Georgia. The new RSL curves sample different parts of the Cascadia Subduction Zone (CSZ) and provide constraints on the history of the CIS. The Juan de Fuca plate subducts beneath the North American plate in roughly the same southwest to northeast direction as the RSL profile. GIA modelling of the RSL observations along this profile may indicate spatial variations related to the structure of the Cascadia Subduction Zone (CSZ). The CIS flowed roughly from northeast to southwest over the regions of interest. RSL observations along this path indicate how sea-level change differed with distance from the edge of the ice-sheet towards its centre. The CIS model of James et al. (2009b) is refined to fit observed sea levels while applying glacial geological constraints to regional ice sheet advance and retreat. Sea level in Barkley Sound dropped from greater than 27 m elevation before 15 cal kyr BP to -46 m below present around 12 cal kyr BP. At Sechelt, sea level closely follows the same trend as in the central Strait of Georgia, dropping from over 150 m before 14 cal kyr BP and falling past present levels after 12.4 cal kyr BP to a poorly constrained lowstand between 12 and 9 cal kyr BP. The initial crustal uplift rate near Sechelt was at least 85 mm/yr, comparable to that of the central Strait of Georgia. The sea-level observations are best fit with predictions employing an Earth model with a 60-km effective lithosphere thickness and asthenospheric viscosity and thickness of 4 × 1019 Pa s and 380 km, respectively. The transition zone and lower mantle viscosities are based on the VM2 Earth model (Peltier 2002). Sea level in Barkley Sound fell quickly (15-30 mm/yr), and observed sea level is best fit with the same asthenospheric viscosity, but with a thinner 30-km thick lithosphere, consistent with the regional tectonic structure. Revisions to the ice model are consistent with radiocarbon constraints on ice sheet history and provide good agreement with the observed sea-level history for the study regions as well as RSL histories previously described for the Strait of Georgia and southern Vancouver Island. / Graduate / 0372