Two-dimensional waveform tomography of the Queen Charlotte Basin of Western Canada and the Seattle fault zone

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Waveform tomography
Traveltime tomography
Visco-acoustic modelling
Visco-elastic modelling
Geological interpretation

Two-dimensional frequency domain visco-acoustic waveform tomography is applied to limited-offset marine seismic reflection data from the Queen Charlotte sedimentary Basin of western Canada, and from the Seattle Fault Zone in Puget Sound, Washington. It was possible to obtain high resolution P-wave velocity and attenuation images of the subsurface, and to practically evaluate the effectiveness of the visco-acoustic waveform tomography method. A specific data preconditioning and inversion strategy is developed to recover models to a depth of 1.2 to 1.3 km. The preconditioning of the data converts the field data to a form similar to that predicted by the acoustic waveform modelling algorithm. A multiscale inversion strategy was designed to mitigate non-linearity issues and to improve the estimation of attenuation. The starting velocity model is derived from first arrival traveltime tomography and the starting attenuation model is a homogeneous Q p -value. Four seismic lines in the Queen Charlotte Basin are imaged, and the recovered velocity models aid in interpreting shallow structures such as Quaternary strata and Pliocene faulting. The joint interpretation of the velocity and attenuation models enables the identification of siltstone, shales, the presence of hydrocarbons and seafloor pockmarks. The shallowmost basement rocks are interpreted to be volcanic. Using a section of the seismic data across the Seattle Fault Zone, synthetic visco-acoustic and visco-elastic modelling was used to verify the effectiveness of applying visco-acoustic waveform tomography to visco-elastic data. The results show that visco-acoustic waveform tomography of marine seismic reflection data is reliable when high velocity gradients are absent from the model. Finally, an interpretation is provided for the inversion results across the Seattle Fault Zone. The inverted velocity and attenuation models enable the identification of glacial and post-glacial Pleistocene, Tertiary sedimentary rocks, and Eocene volcanic rocks. Several north-dipping shallow thrust faults, anticlines and a syncline are identified across the Seattle uplift and the Seattle Fault Zone. The orientation of the faults are consistent with the interpretations of the Seattle Fault Zone as either a fault propagating fold with a forelimb breakthrough, or as the leading edge of a triangle zone within a passive roof duplex.

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Andrew Calvert
Science: Department of Earth Sciences
Thesis type: 
(Thesis) Ph.D.