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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Geophysics

Supervisor

Molnar, Sheri

Abstract

Passive seismic methods use background low frequency ambient vibrations produced by natural and artificial sources. Advancements that enable the utilization of passive seismic signals are required in several areas, including understanding the wave content of the recorded passive seismic wavefield, and seismic site characterization and near-surface seismic tomography. This thesis utilizes case studies to tackle aspects of these three passive seismic research areas. My first case study determines the possible wave type contributions, where the microtremor horizontal to vertical spectral ratios (MHVSRs) were quantified in terms of their peak broadness and their fitness using mathematical functions. We developed an approach that determines the predominant wave types contributing to the microtremor wavefield using particle motion plots of cross-correlated microtremor recordings. Furthermore, the impact of a Rayleigh wave ellipticity assumption of the forward amplification model for MHVSR inversion to estimate the shear wave velocity (VS) depth profile was also evaluated. It is found that incorrect wave contribution assumptions lead to up to 30% error in prediction of soil thickness and bedrock velocity. My second case study closes some of the gaps in seismic site characterization in eastern Canada, where we applied both non-invasive (passive and active source seismic) and invasive methods to characterize 172 sites in Essex County, southwestern Ontario. The findings indicate that the soils in Essex County are softer than Montreal, but stiffer than other regions in eastern Canada. We also found greater soil thickness in northern and eastern Essex County, which could make it more susceptible to seismic damage in the case of an earthquake. By combining non-invasive and invasive techniques, we developed region-specific relationships between important seismic site characterization parameters. The third case study of this thesis advances the use of passive seismic recordings for ambient noise tomography to develop a three-dimensional (3D) velocity model of an area of mineral exploration interest near Marathon, Ontario. The novel aspect was to extract surface wave dispersion estimates by cross-correlating passive seismic recordings to achieve a data-driven high-resolution 3D velocity model, which is useful to improve mineral generation models and identify exploration targets.

Summary for Lay Audience

Passive seismic (background) noise of extremely low amplitude vibrations has always existed. By the 2000s, humans had worked out how to use these recorded background vibrations across several sites to analyze the subsurface Earth structure beneath them. This thesis focuses on advancing the use of passive seismic recordings for understanding Earth structure in both: the near surface (upper 100’s of meters), impacting our seismic hazard understanding and thereby seismic engineering design and mitigation; and at the deeper depths (upper 10’s of kilometers), impacting mineral exploration. In my first case study, I developed new methods that use only one or a few seismometers to try to better understand the seismic wave types that contribute to the background seismic wavefield because no one really understands what those wave types are. I illustrated that when creating a layered Earth model to fit the recorded passive seismic data, assuming the incorrect wave contributions can result in a 30% error in predicting of the bedrock velocity and subsurface soil thickness. In my second study, I used a combination of passive and active-source-generated seismic techniques to define subsurface ground conditions that are essential to ground motion prediction for Essex County, which is the southernmost in Ontario. The post-glacial sediments in Essex County are comparatively stiff and most similar to those in Montreal and Charlevoix, while the post-glacial sediments in Ottawa are the softest. This study closes a spatial gap in our understanding of measured seismic site conditions in eastern Canada. I developed region-specific relationships for the key seismic site parameters and showed how using these new region-specific relationships improves seismic hazard mapping. In my third case study, a passive seismic tomography method was utilized to develop a three-dimensional (3D) velocity model of a mineral exploration area in Ontario. Our 3D model is able to identify low and high velocity rocks, along with their associated geologic structure and potential faults. This tomography method is the only environment-friendly geophysical method able to image the upper 1000’s of meters, providing the 3D image of subsurface ground conditions to reinterpret the mineral generation model or target new mineral exploration areas.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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