Marine seismic exploration is a method employed by the hydrocarbon industry to find geological structures in the sub-surface with the potential to contain trapped hydrocarbons. A source of seismic energy is towed behind a ship. The energy produced by the source propagates as a sound wave through the sea into the sub-surface. Within the sub-surface the energy is reflected, refracted and diffracted. The ship also tows an array of hydrophones behind the seismic source, and these are used to measure the wavefield. If the source signal is known, then the received signal at each hydrophone can be deconvolved for the source signal to obtain the impulse response of the earth between the source and the hydrophone. These impulse responses can highlight some of the structures in the subsurface. Maps of the subsurface built up from these impulse responses are then interpreted to estimate the locations of trapped hydrocarbons. The most commonly used seismic source is the seismic air gun, which is a canister containing highly compressed air. The air is released into the sea, forming an oscillating bubble. There are two methods used by industry to determine the signal produced by an air gun or air gun array: (1) modelling, and (2) extrapolation from near-field measurements. Traditionally, industry uses the first method. With broader bandwidth data that are being recovered in data processing by removing the sea-surface reflection at the source and receiver (source and receiver ghosts), it has been found that modelling is inferior to extrapolation from near field measurements, although industry has been slow to adopt the second method. Despite this change, modelling remains a valuable tool in the design of air gun arrays, where designs can be optimised by adjusting parameters of the array and using modelling to determine the wavefield of each variation of the array. The aim of this work is to develop methods which can improve on current air gun bubble modelling. In this thesis I develop a novel artificial boundary condition for use in finite volume simulations of oscillating bubbles. The purpose of the work is an improvement to the modelling of seismic air gun bubbles. However, the techniques presented in this thesis are not limited to air gun bubbles, but are applicable to any oscillating bubbles, or indeed any fluid dynamics problem which is spherical in nature, close to spherically symmetric, and produces flow speeds of low (< 0:1) Mach number some distance from the region of interest. The boundary condition is based on an existing approximation to the motion around a spherical bubble, which is derived from the asymptotic solution to the motion in the far field. It is applied as follows: (1) use the solution on the domain boundary to calculate the approximate solution external to the domain; (2) use the approximate external solution to calculate spatial derivatives of properties on the domain boundary, due to the external solution, and (3) use the spatial derivatives to describe characteristic waves incoming to the domain. I develop a finite volume scheme in which I apply this boundary condition. I present the results of one- and two-dimensional of simulations using this scheme, and demonstrate the efficacy of this boundary condition. The boundary condition performs well, allowing finite volume simulations of bubbles to be carried out for long run-times (5 105 time steps with a CFL number of 0:8) on highly truncated domains, in which the boundary condition may be applied within 0:1% of the maximum bubble radius. Conservation errors due to the boundary condition are found to be of the order of 0:1% after 105 time steps. One- and two-dimensional results show a third-order convergence rate of errors due to the boundary condition as the domain is enlarged. The one- and two-dimensional simulations of air gun bubbles I present are, to my knowledge, the first finite volume simulations of air gun bubbles carried out, and the first air gun bubble simulations in which the contents of the bubble are not considered to be homogeneous. Two-dimensional results show non-spherical aspects of air gun bubbles, which may be incorporated into models used by industry. The model captures surface instabilities, bubble translation and deformation due to gravity, and the formation of jets due to asymmetries on collapse. The results indicate that bubble surfaces are unstable throughout collapse. These phenomena are shown to increase the damping of bubble oscillations. The results of the two-dimensional air gun modelling highlight the potential value of my artificial boundary condition, and also the aspects of my computational scheme which require improvement. I extend the numerical scheme to include viscous effects, which I show to have limited impact on the signals emitted by air gun bubbles, although the influence of a boundary layer around the bubble is significant, causing an 18% reduction in rise rates. I extend the scheme to include the effects of the sea surface, and present results which show the impact of the reflection from the sea surface (the ghost wave) on the bubble. This extension shows the reflection of the ghost wave off the bubble, which provides a novel explanation of some of the higher frequencies present in measurements. This extension further increases the practical value of my contribution, and further demonstrates the ability of the boundary condition to handle asymmetrical flow features.

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Thesis (M. Sc.)--University of Alberta, 2009. / Title from PDF file main screen (viewed on Feb. 19, 2010). A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Master of Science in Geophysics, Department of Physics, University of Alberta. Includes bibliographical references.

Seismic reflection (P- and SH-wave), ground-penetrating radar, correlative drilling, and age dating data provide evidence of neotectonic deformation along the Caborn (CF) and Hovey Lake (HLF) faults, in the Wabash Valley fault system (WVFS). The WVFS is a series of high-angle normal faults located primarily in southern Indiana and Illinois. Since their formation, these faults have likely been transpressionally reactivated in the contemporary E-W-oriented compressive stress state. The WVFS has experienced large prehistoric earthquakes, but only moderate historic and contemporary seismicity; therefore, the seismic potential in this region is poorly defined. The bedrock expressions of the CF and HLF were imaged with seismic reflection data (P- and SH-wave). Higher resolution analyses were performed with seismic (SHwave) and ground-penetrating radar surveys to characterize structure that may extend into the overlying Quaternary sediments. Anomalous features were cored to verify structure, and to collect datable material. The CF and HLF are interpreted to extend into the uppermost five meters of sediment and to displace horizons dated to 19,740 and 31,000 years before present, respectively. Displacement along the HLF is interpreted to extend 2-3 meters above the associated age date. These structures represent the only known primary coseismic deformation of the Late Quaternary within the WVFS.

This thesis focuses on the concept of Single Well Imaging (SWI) in which a seismic source and receivers are deployed in a borehole to investigate the surrounding geology. The Uniwell project (1997-1999) was the first attempt to develop the SWI method; it used a fluid-coupled downhole source, which unfortunately generated high amplitude guided waves in the borehole which obscured all other useful information. Initial research work detailed in this thesis focused on removing the high amplitude guided waves, known as tube waves. Two-step source signature deconvolution using first the recorded source signature, and then the tube-wave reflected from the bottom of the well, succeeded in compressing the tube wave. The results were not consistent across all receivers, but there is enough correlation to identify a P-wave. Further work concentrates on using a new technique called Empirical Mode Decomposition to separate the tube-wave mode from the data. This identifies three dominant modes and a possible body wave arrival, but the results are ambiguous due to the inability of the decomposition to focus on the narrow bandwidth of interest. The source signature deconvolution technique can also be used to process real-time vertical seismic profiling (VSP) data down-hole, during pauses in drilling, in what is referred to as a Seismic-While-Drilling (SWD) setup. Results show that the technique is versatile and robust, giving 1 ms precision on first-break picking even in very noisy data. I also apply the technique to normal VSP data to improve both the resolution and the signal-to-noise ratio. A major effort in this thesis is to consider the effect of a clamped downhole source to overcome the tube-wave problem, using a magnetostrictive source. Earlier work established that the use of a reaction mass tended to excite resonances in the tool which caused the transducer to break. A new design for the source was developed in cooperation with colleagues which utilises a hydraulic amplifier design and a low power coded waveform driving method exploiting the time-bandwidth product to extract the signal from the noise. My results show that as the run time increases the resolution improves. With a run length of 80s it is possible to resolve the signal transmitted 50 cm through a granite formation. This analysis led to a revised design of the source to improve its efficiency. I have used finite difference modelling, with a variable grid technique, to compare an ideal explosive source with an ideal clamped source. The fluid-coupled source emits high amplitude tube waves which virtually obscure the body wave, whereas the clamped source emits a clearly identifiable P-wave along with lower amplitude tube waves. This clearly illustrates the advantage of an ideal clamped source. To model the source more accurately the idealwavelet is replaced by the respective recorded source signatures, and the data is then processed by cross correlation with the appropriate signature. The results show that the coded waveform approaches the resolution of the ideal wavelet very well, with all major events being visible. However, the fluid-coupled source performs very poorly with only the highest amplitude tube-wave visible. This work illustrates that by replacing a fluid-coupled source by a clamped source driven by a coded waveform, and by processing the data using cross correlation or signature deconvolution, it is possible to minimise or eliminate tube-wave noise from a SWI survey. It is hoped that the results outlined here will provide the basis for a new SWI method than can be used to prolong the supply of North Sea oil.

The Gorringe Bank region is located at the eastern end of the Azores-Gibraltar plate boundary, which plate kinematic studies show to progressively change from extension at the Azores, through pure right lateral strike slip at the Gloria fault to compression at Gibraltar. The region is dominated by high relief (4-5 km), highly deformed, uncompensated, ENE-WSW trending seamounts and intervening abyssal plains with basin sediment thicknesses in excess of 4 km and minimal surface deformation. Gorringe Bank, which was formed by overthrusting of the African plate upon the Eurasian plate at about 10 Ma along the plate boundary, is supported in part by exure of the Eurasian plate, as indicated by pre-loading sediments and basement to the north which are tilted towards Gorringe Bank. Broken plate models show the Eurasian plate to have an elastic thickness of about 35 km which is in agreement with that expected for the crustal age (130-135 Ma) at the time of loading. Coral Patch Ridge was formed by a combination of thrust faulting and whole crustal buckling resulting from the past 20 Ma compression and was partially uplifted before deposition of an olistostrome in the Middle Miocene. Recent compressional deformation is distributed over a wide region, as indicated by the dispersed shallow seismicity and has a trend which rotates from approximately N45oE to N70oE from west to east across the region, near perpendicular to westward verging plate motion vectors. The majority of extensional and strike-slip deformation is explained by a regional strike-slip strain ellipse model, including an antithetic NNE-SSW strike slip fault between Gettysburg and Hirondelle seamounts which marks the boundary between the Eastern and Western Horseshoe Basins. Isostatic models for the Madeira-Tore Rise, which initially formed at the Mid Atlantic Ridge, give an elastic thickness of approximately 15 km indicating that significant material was added to the Rise as it moved away from the Ridge.