This thesis work is directed at estimating gas hydrate and free gas distribution and saturation in local structures in the Ulleung Basin, East Sea offshore Korea. The estimates are obtained from a 2-D multi-channel seismic (MCS) reflection profile from the basin. Firstly, structures of locally focused upwelling fluid and gas flow were imaged using time-migrated sections and seismic attributes, and secondly seismic velocities were obtained to estimate gas hydrate and free gas saturations. The structures investigated are up to 1 km across, and are characterized by reduced reflectivity (‘blank zones’) and pulled-up sediment reflectors on the seismic sections. Throughout the study, a comparison is made between the blank zones and areas outside them where not much gas hydrate or gas is expected, to examine their peculiar characteristics as related to the formation of gas hydrate and underlying free gas. The regional depth of possible occurrence of gas hydrate and free gas is determined by predicting the base of the gas hydrate stability zone (BGHSZ) from sediment properties and heat flow estimates calibrated by a few bottom-simulating reflectors (BSRs) from outside the analyzed seismic section. A large number of normal moveout (stacking) velocity profiles were obtained within and outside the blank zones. Interval velocities were then derived by applying the commonly used unconstrained Dix equation as well as by applying constraints to inversion using regularized linear inversion and non-linear Bayesian inversion. The latter method fully explores the uncertainty of the interval velocity estimates. Compared to areas outside the blank zones, the velocities within the blank zones are up to 30% larger at about 30 m above the BGHSZ and up to 65% smaller immediately below the BGHSZ. The velocity increase implies a gas hydrate saturation of 10-40% of the pore space. The velocity decrease implies a free gas saturation of 1-4% of the pore space. Their detailed distribution within individual structures cannot be resolved. Reflector pull-up in time sections in the hydrate zone allows an independent velocity estimate, assuming the pull-up is solely a velocity effect. The implied velocity is much higher than the interval velocity estimates, so there also must be physical deformation. The heat flow estimated depth of the BGHSZ is in good agreement with the transition from gas hydrate to free gas as inferred from seismic velocities. The general conclusion of the thesis work is that a variety of careful analyses of MCS data that characterize the seismic signal and estimate the seismic velocity structure can provide insight into gas hydrate and free gas occurrences. The large amounts of gas hydrate and free gas associated with the blank zones inferred by this study should draw special attention to future energy and climate effects in this area and other similar regions.

Over a decade ago, it was discovered that the earth’s natural seismic wavefields, propagating as seismic noise, can be processed using correlation methods to produce surface waves, similar to those generated by earthquakes. This discovery represents a paradigm shift in seismology and has led to several tomographic studies of earth structures, at different scales and resolutions, in previously difficult-to-study areas around the world. This PhD dissertation presents research results on multi-scale and multi-purpose applications of ambient seismic noise wavefields under three topics: (1) Imaging of sedimentary basins and sub-basin structures in eastern and western Canada using ambient seismic noise, (2) Combining measurements from ambient seismic noise with earthquake datasets for imaging crustal and mantle structures, and (3) Temporal variation in cultural seismic noise and noise correlation functions (NCFs) during the COVID-19 lockdown in Canada. The first topic involved imaging the sedimentary basins in eastern and western Canada using shear wave velocities derived from ambient noise group velocities. The results show that the basins are characterized by varying depths, with maximums along the studied cross-sections in excess of 10 km, in eastern and western Canada. Characteristics of accreted terranes in eastern and western Canada are also revealed in the results. A seismically distinct basement is imaged in eastern Canada and is interpreted to be a vestige of the western African crust trapped beneath eastern Canada at the opening of the Atlantic Ocean. In western Canada, the 3D variation of the Moho and sedimentary basin depths is imaged. The thickest sediments in eastern Canada are found beneath the Queen Charlotte, Williston and the Alberta Deep basins, while the Moho is the deepest beneath the Williston basin and parts of Alberta basin and northern British Columbia. For the second topic, I worked on improving the seismological methodology to construct broadband (period from 2 to 220 s) dispersion curves by combining the dispersion measurements derived from ambient seismic noise with those from earthquakes. The broadband dispersion curves allow for imaging earth structures spanning the shallow crust to the upper mantle. For the third topic, I used ambient seismic data from the earlier stages of the COVID-19 pandemic to study the temporal variation of seismic power spectra and the potential impacts of COVID-19 lockdown on ambient NCFs in four cities in eastern and western Canada. The results show mean seismic power drops of 24% and 17% during the lockdown in eastern Canada, near Montreal and Ottawa respectively and reductions of 27% and 17% near Victoria and Sidney respectively. NCF signal quality within the secondary microseism band reached maximum before the lockdown, minimum during lockdown and at intermediate levels during the gradual reopening phase for the western Canada station pair. / Graduate